# Is there a website that shows equivalent views through different telescopes?

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I'm trying to buy a beginners telescope. I have a Celestron TravelScope, but would like something better. When I look at other models, many users have posted images from their viewfinders. Is there a site that collates all of these, to give an idea of what I can expect from a given scope (e.g. is Mars just a red dot, can I make out Saturn's rings, what's the detail on the moon like, etc)?

Deep sky watch has an article that covers this

http://www.deepskywatch.com/Articles/what-can-i-see-through-telescope.html

It includes sample image of what can be seen in, for example, a medium telescope (150mm objective) with 180x magnification

Not a website, but Stellarium allows to show the view through different telescopes / different eye pieces or the view area of different sensors or through binoculars.

It comes with a selection of fairly usual eye pieces, telescopes and sensors and barlow lenses (if any), but you can always add your own choice to each of these categories so that you can also preview exactly of what you want to compare, if you know exactly what to compare.

The astronomy.tools is a great site for visual and imaging "expectations".

On Deep Sky Archive you can browse different views of the same object with different instruments and magnification; here you won't find any planet.

As of 2020 Deep Sky Archive no longer collects user observation, but it is available Taivaanvahti observation database. On this latest observation database you can search for Category "Solar System", 'saturn' on "Free text search" and browse the images, looking for those taken with a telescope (many of the photos are captured by smartphone). For each observation, under "Technical information" section, you can read the instrument used to take that image.

## Is there a website that shows equivalent views through different telescopes? - Astronomy

Anyone starting out in astronomy now and buying their first telescope will probably find that it comes with the ability to point at any object in the sky with the touch of a button. This GoTo ability, as it is known, is a relatively new phenomenon, certainly on cheaper/ smaller telescopes, and follows the increased power and reduction in cost of computers generally.

Before the age of GoTo many telescope mounts were fitted with setting circles which, once correctly aligned, allowed the observer to locate any object by moving the telescope until the correct RA (Right Ascension) and Dec (Declination) was displayed on the circles. RA and Dec being the equivalent in space of longitude and latitude on earth. For an explanation of RA and Dec see Paul Abels’s tutorial here and for different types of telescope mounts his tutorial here.

But what if you don’t have GoTo or setting circles, how do you find your target? Well there is a time honoured way of bagging your quarry known as Star Hopping. Simply put you “hop” from star pattern to star pattern using maps or star chars until you reach your target.

##### Image orientation

Star Hopping is not difficult and can be very rewarding, but it does take practice and what is essential is that you know the orientation of the image in your eyepiece - is north up, south up, east to the right or west to the right - or some combination of these? Without this information it is impossible to relate the eyepiece view to the star chart that you are using.

So why this confusion with image orientation? After all with a normal pair of binoculars you will see an image you are familiar with. Top will be at the top and anything to the right will appear on the right. You will see the same as a naked eye view but magnified.

So why is it different in a telescope? The simple answer is because different telescopes types, such as reflector, refractor or Cassegrain have different optical configurations. Of course it is possible to add extra lenses or mirrors to turn an image the “right way up” but any extra glass will reduce to some extent the light getting through to your eye (or camera) and frequently in astronomy light is at a premium. And, of course, what is the right way up in space anyway? This strange eyepiece orientation can be very confusing for beginners and even if your telescope is equipped with GoTo it is worthwhile understanding why the orientation is the way it is.

When we look at the night sky north is not overhead but towards the north celestial pole, the point about which the sky appears to revolve. This is very close to the aptly named Pole Star (alpha Ursae Minoris). South is then in the direction exactly opposite to north. So when you are observing and you want to find north just nudge the telescope towards Polaris and north is the direction in which new stars enter the field of view. The east / west line is then at 90 degrees to this north / south direction. But which direction along that line is west and which direction is east? Luckily there is an easy way to find out.

Because the Earth spins from west to east it gives the impression that the sky rotates from east to west. With the telescope drive (if you have one) switched off east is always the direction in which new stars enter the field of view. Unfortunately there is another slight complication because the direction in which new stars enter the field depends upon where exactly the telescope is pointing. Most people are familiar with photographs of star trails showing stars wheeling around the pole. All these stars are moving westwards but the ones above the pole are moving in exactly the opposite direction to those below the pole (Figure 1). So, if you are observing near to the pole and your telescope is pointing just below it, west will be in the opposite direction to if it is pointed just above it.

Confusing? Yes, possibly, but really only if you are trying to relate directions in the sky to those on the ground. If you are using an equatorially mounted telescope aligned on the pole the situation is somewhat easier as the position of the cardinal points in the eyepiece remain fixed as the telescope is moved around the sky - important for long exposure photography.

With an altazimuth mount, however, such as a Dobsonian or a simple tripod, these positions move as the telescope is moved, an effect known as field rotation. It might be thought from this that an equatorial mount is vastly superior to an altazimuth mount but unfortunately life is not so simple and whereas the eyepiece of a Dobsonian telescope can always be at a very convenient position, that of an equatorially mounted Newtonian, certainly if observing near the pole, is usually close to being the most inconvenient position possible.

Although some of the above may sound complicated, the two simple rules to remember for finding celestial directions in an eyepiece are that north is always the direction in which new stars appear when the telescope is nudged towards Polaris and east is always the direction in which new stars enter the field in a stationary telescope.

To consider how eyepiece image orientation is affected by telescope type we will consider the view of the galaxy pair M81 and M82 in Ursa Major as seen through different telescopes (for a basic description of common telescope types see the article by Paul Abel.

In Figure 2 there are 3 possible image orientations. The first, on the left, can be regarded as a terrestrial image and is known as an erect image. It is what would be seen using traditional binoculars i.e. binoculars that would be used for daytime terrestrial viewing where everything is the correct way up and nothing is reversed. In this case, looking at this galaxy pair north would be up and west to the right. This is also the orientation that a small “birding” refractor would give – it would be confusing to see birds upside down and light is not normally at a premium when bird watching.

An astronomical refractor however would not normally show this orientation unless it was fitted with an erecting diagonal. Instead the image would be as seen in the middle figure, known as an inverted image. This is also the view that a traditional Newtonian reflector would give and it can be seen that it is identical to the first image except that it has been rotated through 180 degrees. South is now at the top and west to the left. In the final figure here (right hand side) the orientation is as seen through a refractor or Cassegrain telescope fitted with a star diagonal. Here, no amount of image rotation will give the view as shown in the other figures as it is a mirror image. North is up, as in the first view, but west is to the left.

Before the advent of charting software, observers whose telescopes were equipped with diagonals giving a mirror image and who wanted to relate their eyepiece view to a star map, were advised to turn the map over and shine a light through it from behind, thereby viewing the map as a mirror image. I’m not sure if anyone ever had success with this procedure, I’ve never tried it, but users of star diagonals will surely be relieved to know that modern software has made the procedure redundant and charts can now be printed in any format necessary.

A mirror image is what occurs in any optical system that reflects light an odd number of times so removing the star diagonal from a refractor or Cassegrain would give a standard inverted image as seen in a Newtonian. Although this would make the image easier to interpret, it would also involve gymnastics and neck stretching in getting your eye to the eyepiece if the telescope was pointed at any significant angle above horizontal. This is easy to imagine by looking at the working end of the refractor in Figure 3.

Many beginners are confused by the fact that on star maps (and on Planispheres) east and west are reversed compared to their positions on normal terrestrial maps. The explanation is that whereas terrestrial maps are viewed from above - with you looking down on them - star maps are held above your head and so are looked at from underneath. This reverses right and left or, more accurately, east and west.

##### Finder scopes

Most telescopes are fitted with a small finder scope mounted parallel and optically aligned (collimated) with the main telescope tube (Figure 4). Finders are rather like one half of a pair of binoculars and their purpose is to give a wider field view of the sky than would be had through the main telescope, thus helping in finding targets or showing you exactly where your telescope is pointed. Often though, certainly on small refractors, the size of the finders is too small to be useful and it is much easier to simply put a low power eyepiece in the main telescope. To be of any serious use a finder should have a size around 10X50, that is 10X magnification with a lens diameter of 50mm.

My small refractor (70mm f/6) gives a field of view of almost 4 degree when fitted with a 24mm eyepiece (see Fig 3). This is 8 times the diameter of the full Moon and the same field as my 15X70 binoculars. It does not need a finder scope. However, what can be extremely useful on any telescope is a zero power device such as a red dot finder. Looking through it will show you a naked eye view of the sky with the addition of a red dot superimposed, telling you exactly where your telescope is pointed (Fig 3 right hand side). Usually the intensity of the dot can be adjusted so you can reduce the brightness when you are fully dark adapted and also, in some cases, the dot can be made to flash.

Similar, but slightly more sophisticated than a red dot finder, is a Telrad. This is another zero power finder that shows you a naked eye view of the sky, but instead of just projecting a small dot, projects three concentric rings, 0.5, 2.0 and 4.0 degrees in diameter. Some charting software e.g. the deep-sky program MegaStar, allows you to add Telrad circles to your finder charts in addition to eyepiece circles. The author’s 12-inch (30cm) Newtonian (Figure 5) is fitted with 2 Telrads (with dew shields) along with a traditional finder. Often, with a good star chart and a Telrad, no other finder is needed. When I visited the 60-inch Hale telescope on Mount Wilson a few years ago I was intrigued to see a Telrad fitted next to the eyepiece holder!

##### Conclusion

Knowing which direction is which in your telescope may seem complicated at first but with practice it soon becomes second nature. This knowledge and a suitable means of aiming the telescope will enable you to find your way around the sky and discover the many interesting sights visible in even a small scope.

--------------------------------------------
Stewart L. Moore is a former Director of the BAA Deep Sky Section. He is a keen visual observer who prefers using a pencil and paper to a CCD and computer.

## Is there a website that shows equivalent views through different telescopes? - Astronomy

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## Simulated Telescope Planet Images

The purpose of this page is to provide you with some idea the view to expect from various size telescopes at various magnifications. For this example we will consider a view of the planet Saturn. First we will show approximately how Saturn might look through three different telescopes operating at magnifications within their useful capability. Then we will show what to expect as a small telescope is pushed beyond its useful magnification range.

Important: It must be remembered that the images on this page are simulated (the image of Saturn is one I took through my Celestron CG-11 scope however). No monitor can cover the brightness range that the telescope (or the human eye for that matter) can see. Thus, do not expect telescope images to appear exactly as those depicted here. The proper calibration and maintenance of the technologies we use will allow us to see an image of the planet's likeness but not an exact account. The main purpose of this page is to provide an approximate comparison of the relative performance of various sized telescopes. Actual views through a real telescope depend on many factors telescope (and eyepiece) quality and seeing conditions are the two most important determining factors. The simulated images below assume a decent quality eyepiece is being used, one with an apparent field of view of about 50 degrees (in other words, similar to a typical Plossl eyepiece). If nothing else, this page will help to show why any astronomer will tell you that "department store" telescopes (such as Tasco, Jason, Bushnell, etc) cannot show quality images at the preposterous magnification capability claimed by these instruments!

If the calibration bar below does not show 16 distinct shades of gray, the images on this page will not display in an ideal manner!

## Saturn through an 11" scope at 400x (simulated)

This is the reference image. This image of Saturn was taken through a Celestron CG-11 telescope. This is about how Saturn looks through the scope when it is at about 400x magnification on a night of good seeing and with the scope thermally stable.

## Saturn through a 5" scope at 200x (simulated)

This image simulates how Saturn would look through a 5 inch telescope operating at about 200x magnification. Note that the image is the same brightness as the image in the 11 inch scope above, however Saturn appears quite a bit smaller. Because the smaller scope gathers less light, the image size (magnification) must be reduced in order to hold the same brightness as the larger scope.

## Saturn through a 3" scope at 100x (simulated)

This image simulates how Saturn would look through a smaller telescope such as a typical 80mm scope (roughly 3 inch) at about 100x. The image is the same brightness as that of the 11 inch scope but it is obviously a lot smaller. As before, because the smaller scope gathers less light, the image must be a lot smaller to hold the same brightness level as that of the larger scope.

## Saturn through 5" scope at 400x (simulated)

This image shows what Saturn might look like with a 5 inch scope pushed to 400x. Note that the image is quite a bit dimmer than that of the 11 inch scope. This is because the smaller scope gathers less light, and this means that the image will be a lot less bright because the smaller amount of available light has to be spread through the larger image. Note: if you have a high quality refractor (like Astro Physics, Takahashi, TeleVue, etc.) the image will almost certainly appear better than that shown here. The image above would be typical of a 5" Schmidt Cassegrain telescope or a Newtonian Reflector scope.

## Saturn through a 3" scope at 400x (simulated)

This image shows what happens when a smaller scope is pushed way beyond its useful magnification range. Here we show what Saturn might look like through a small scope when the magnification is at about 400x, and this is quite a bit less than the 675x claimed by some low end telescope makers (Tasco, Bushnell, Jason, etc)! The image above is so dim that little or no detail can be seen. Would you want to look at Saturn if it looked like this? This assumes that you could even keep the image centered in the narrow field low quality eyepieces that come with low end scopes.

## Using A Star Chart at the Telescope

A standard atlas for serious telescope users is Sky Atlas 2000.0 by Wil Tirion and Roger W. Sinnott. It covers the celestial sphere in 26 big charts that plot a total of 81,000 stars (to as faint as magnitude 8.5) and 2,700 other objects. The smaller Pocket Atlas, with stars to magnitude 7.6, is an excellent low-cost starter and all you'll ever need for binoculars or a telescope of 3-inch aperture or less.

Zoomed-in star charts may look terribly complex at first. But step back for a minute, squint your eyes, and look at only the brighter stars. These form the same, familiar constellation patterns as on a naked-eye map.

Hunting for a good deal on a first telescope for yourself or someone who you care about? Or are you looking for a fancier upgrade? Learn about the differences between reflectors, refractors, and compound telescopes to get a telescope that best fits your needs. Discover why (and if) you should care about aperture, magnification, and resolution and find out whether bigger is always better. We recommend several telescopes under $100 and even some compact travel-scopes that would fit in your carry-on. You’ll also find articles about your telescope’s most essential ingredient: its eyepiece. A good eyepiece can help you make the most out of your scope, and we’ll tell you how to pick the right one. Looking for a more visual approach? Our series of telescope-tutorial videos has got you covered. No matter what type of telescope you’re looking for, browse the articles below before you buy to find the best telescope for you. ## 6. Griffith Observatory Photo credit: Dave Sizer via Flickr • Location: Los Angeles, California • Score: 4.5/5 on TripAdvisor, 65% Excellent Reviews, Certificate of Excellence • Website:griffithobservatory.org Colonel Griffith J. Griffith left funds in his will to build a public observatory in Los Angeles because he believed in the transformative power of observation. Since opening in 1935, Griffith Observatory has fulfilled his vision by offering public telescope viewing through the historic Zeiss telescope, historic coelostat (solar telescope), and portable telescopes on the lawn. Free public telescopes are available each evening the Observatory is open and skies are clear with knowledgeable volunteers available to help guide visitors. If you want more guidance, public star parties are held monthly at Griffith Observatory the dates vary throughout the month, can be found on the Griffith Observatory website, and are free to attend. ## Publicly Accessible Telescope Viewing Shows offered Tuesday and Friday nights. Viewings still available with planetarium presentations.$3 per person

### Arizona

The WIYN telescope at sunset. [NOAO/AURA/NSF]

Tucson
Kitt Peak National Observatory Nightly Observing Program
520-318-8726

Every night of the year except July 15-September 1

This program offers views through a 20-inch and two 16-inch reflecting telescopes, and includes an introductory presentation and a light dinner. Visitors also have binoculars to use while not looking through the telescopes.

$48 adults,$44 seniors 62 and over, students, and current military (with ID). Group rates available.

Every night of the year except July 15-September 1. Provided transportation departs at 4 pm.

This program offers views through a 36-inch research telescope. The program includes transportation to the site from the University of Arizona campus in Tucson, dinner, and an introductory presentation.

$120 per person Monday through Friday during the regular school semester Telescope viewing offered. Free admission Wednesday through Saturday, excluding holidays Viewing through a 16-inch telescope offered. Free admission ### Arkansas Third Wednesday of every month Stargazing through a 14-inch reflector at the University's observatory Free admission ### California Third clear Saturday of every month Stargazing through telescopes provided by amateur astronomers, plus lessons on how to navigate the night sky. Free admission Stargazing is available through a single telescope every Friday night that the planetarium is open.$5 adults, $2.50 children and students Every Friday Views through 16-inch telescope offered Free admission,$2 parking fee

One Saturday a month

Several telescopes available, provided by Los Angeles Astronomical Society and the Los Angeles Sidewalk Astronomers.

The telescope domes at Chabot Space and Science Center. [Conrad Jung]

Every Friday and Saturday evening, every Saturday and Sunday afternoon

Nightime and daytime viewing through several telescopes, including a new 36-inch reflector.

Available most nights of the year

The largest telescope in the world when it was dedicated in 1908, this century-old instrument is now dedicated to public viewing for groups of 1-25. A session director and telescope operator are included as part of the fee.

$1,700 full night Every other Friday night, late June through mid-September After talks by two research astronomers and a presentation on the history of Lick Observatory, visitors look through the 36-inch Great Lick Refractor and the 40-inch Nickel Telescope.$5 per person

Every other Saturday night, late June through mid-September

Concert and a talk by a research astronomer is followed by viewing through the 36-inch Great Lick Refractor and, conditions permitting, the 40-inch Nickel Telescope.

$35-$150 per person, depending on package

Little Thompson Observatory

Every third Friday night

Viewing through an 18-inch telescope, on the grounds of Berthoud High School.

Every Friday the University of Colorado is in session

Viewing through 16-inch, 18-inch, and smaller telescopes.

Every Tuesday and Thursday

Programs include talks, tours of the historic observatory, and viewing through the 20-inch Alvan Clark refractor, installed in 1894.

Admission is $3 for adults,$2 for children

Every Friday and Saturday night during summer

Viewing through a 30-inch reflector, the largest public telescope in Colorado

Recommended donation is $5 for adults,$3 for children

### Connecticut

One night per month during spring and fall sessions

Viewing through a 20-inch reflector at the University's campus observatory.

Viewing through 24-inch reflector, 20-inch Alvan Clark refractor, others on the campus of Wesleyan University

### District of Columbia

Programs offered every Wednesday and Saturday from 1 - 3 pm through a 16-inch telescope, which monitors the sun and the phases of Venus.

### Florida

Every Friday and Saturday evening.

Viewing through a 24-inch Ritchey-Cretien telescope.

Free admission, although there is a charge for the planetarium and IMAX movies.

Every Wednesday, Friday, and Saturday

Public viewing through the Broward Community College Observatory.

First Friday of every month

Viewing through two Meade telescopes is offered.

Every Friday and Saturday evening

Viewing through a 10-inch telescope offered.

Three Fridays a month during the summer.

Viewing is offered through several telescopes before the planetarium show.

### Georgia

Every Thursday and Friday

Public viewing through a 36-inch reflector.

One or two nights per week

Star parties offer views through a 16-inch telescope and other instruments at several parks in the Columbus area.

Free admission, although the parks may require a parking fee.

Every Friday night.

Several telescopes are avaliable for viewing.

Every Friday evening during the school year

Viewing through a Cassegrain telescope offered.

### Hawaii

First and third Fridays of each month

Telescope viewing is avaliable after Planetarium show.

$6 adults,$4 ages 4-12 cash only.

Available every night of the year

The visitor center for the telescopes at Hawaii's Mauna Kea, including the giant twin Keck Telescopes, offers views through small telescopes until 10 p.m. the program includes a documentary on the mountain's cultural and research heritage.

### Idaho

Every Friday and Saturday evening, mid-March through October

Idaho's largest public observatory offers viewing through several telescopes.

$3 ages 6 and up, plus$5 per vehicle park admission

Second Saturday of every month

Viewing through a 24-inch telescope (fully wheelchair-accessible).

### Illinois

Once a month during the summer

Telescope viewings and stargazing, accompanied by American Indian stories.

### Indiana

Most Friday and Saturday evenings

Telescope viewing is avaliable after a planetarium show.

$3 adults$2 children, students and seniors $7 families of five or less Thursday nights during fall and spring semesters A variety of telescopes McCollom Science Hall, UNI campus Free admission At least one Saturday night per month, March-November A variety of telescopes at Palisades State Park ### Kansas Every Saturday night, May-October A variety of telescopes, including a 30-inch reflector Recommended donation$6 adults, $4 children First and third Thursday of the month during fall, spring semesters On the campus of Washburn University Free admission Every Friday and Saturday Varying astronomy presentations followed by 16-inch telescope viewing$5 adults, $3 children 6-12, children under 6 free ### Kentucky Open March - October 7 days a week, 5 days/week in November, 4 days/week in December. Viewings through four telescopes and one Hydrogen-Alpha refractor. Free admission ### Louisiana Every Friday and Saturday Public viewing through several telescopes. Free admission ### Maine Customer Request Telescope viewing and star parties for private parties, offering viewing through six telescopes of up to six inches.$175 per star party

Friday and Saturday evenings, September–April

Viewing through an 8-inch refractor built Alvan Clark & Sons offered.

### Maryland

Every other Friday

Star parties offer views through a 14-inch telescope.

### Massachusetts

Fridays from early March to late September, 8:30-10 p.m.

Public viewing through an 11-inch reflector atop the museum's parking garage.

### Michigan

One or two Fridays per month Public viewing is offered through a 16-inch reflector and several smaller instruments at this facility on the University of Michigan campus. Open houses are also held in conjunction with special public lectures and planetarium shows.

Shows offered Friday evenings, 8:30-10 p.m.

Public viewing is offered through a 6-inch refractor in the Institute's public observatory.

Two Saturdays per month during warmer weather

Hosted by a local astronomy group, this University of Michigan facility offers viewing through a 24-inch telescope, plus those set up by the astronomy club.

One Friday and Saturday per warmer month

Public Observation Nights offer views through a 24-inch telescope, with additional, smaller telescopes avaliable.

$2 for children,$2.50 for students and seniors, $3 for adults ### Minnesota Varying dates, depending on season and sky events A variety of telescopes at Baylor Regional Park$5 per vehicle parking fee

Every Friday during fall and spring semesters

Public viewing atop the physics building on campus

Every other Thursday

Viewing through a 16-inch telescope

### Mississippi

Second Friday of every month

Viewing through a variety of telescopes

Call for rates

### Missouri

Every Wednesday

A 16-inch telescope, atop the Astronomy and Physics Building, University of Missouri-Columbia

Thursday nights during Central Methodist University terms

A historic Alvan Clark & Sons reflecting telescope

One Saturday night per month, March-November

A variety of telescopes at North Campus, near the Fine Arts Building

Every Friday night

A variety of telescopes at Broemmelsiek Park

### Montana

One Friday night each month during summer

At Gallatin Regional Park, hosted by Southwest Montana Astronomical Society.

Viewing offered through 8-, 12.5-, and 14-inch telescopes every Saturday night

First Friday of every month.

Public viewing through 14- and 22-inch telescopes

### New Hampshire

First and second Friday of the month

Viewing through several telescopes

First and third Satury of every month

Viewing through several telescopes

Fridays during the school term

On the campus of Dartmouth University

### New Jersey

Every Friday

Two informal talks are followed by public viewing through 24-inch and 10-inch reflectors.

### New Mexico

Every Friday during the fall and spring semesters

Public viewing through a 14-inch telescope

Recommended donation is $5 for adults,$3 for children

Friday and Saturday evenings in April and October Tuesday, Friday, and Saturday evenings May through September.

The programs begin with staff presentations on archaeoastronomy and cultural history, followed by telescope viewing.

### North Carolina

Second Friday of each month

Viewing through several telescopes after public talk and tour of Pisgah Astronomical Research Institute, a radio observatory in the Pisgah National Forest

$20 adults,$10 children 14 and under, $15 students and seniors One Saturday per month Viewing through several telescopes at Jordan Lake State Recreation Area, south of Chapel Hill Free admission First and third Saturdays of every month Public viewing through several telescopes after the Center's 8 p.m. planetarium show. Free admission The Cincinnati Observatory Center. [Cincinnati Observatory Center] The oldest professional observatory in the United States offers views through two historic telescopes on its astronomy nights, including a 16-inch refractor built by Alvan Clark and Sons and an 11-inch refractor that was built in 1842. Tickets include a tour of the Observatory and an astronomy lecture. Thursday shows, suggested donation of$4 per person, Friday shows are $6 for adults,$4 for children. Not available first Thursday and Friday of each month, reservations required for all programs.

Every Friday night, except July

Star parties include viewing through the 32-inch Schottland Telescope, which is the Observatory's primary research and teaching telescope, and smaller instruments. Other features include lectures and Observatory tours.

$7 adults,$5 children 3-17, $5 seniors (ages 62 and over) (advance purchase only if space is available, tickets are available at the door for$2 more)

Brochure

Every Friday evening.

Viewing through two 10-inch and one 6-inch telescope.

First Friday of every month

Viewing through a 16-inch and two 10-inch telescopes offered.

### Oklahoma

Every Wednesday when the university is in session

### Oregon

Every Friday and Saturday, late May through September

Public viewing through a 32-inch and other telescopes

Recommended donation is $5 per person One Saturday per month Viewing through 24- and 13-inch telescopes Free admission One Saturday per month, plus other dates Viewing through a variety of telescopes at both Rooster Rock State Park (east of Portland) and L.L. “Stub” Stewart State Park (west)$5 per vehicle park entry fee

Various nights, depending on season

Viewing through several telescopes

$6 adults,$4 children 12 and under

### Pennsylvania

One Saturday per month, see website for dates.

An astronomy lecture and planetarium show are followed by skywatching through the society's telescopes.

$2 donation recommended for adults Mondays and Wednesdays throught the school year 10-inch telescope viewing avaliable after planetarium show. Free admission The Philadelphia skyline stands behind one of the Franklin Institute telescopes. Second Thursday of every month Viewing through five different telescopes offered.$5 admission, free for Franklin Institute members
The Philadelphia skyline stands behind one of the Franklin Institute telescopes.

Once a month throughout the year

Viewing through the club's telescopes offered and star charts avaliable.

### Rhode Island

Every Friday

Viewing through 16-inch reflector, other telescopes

Recommended donation $1 Every Saturday Viewing through four telescopes, including 16-inch reflector, 8-inch Clark refractor Free admission Ladd Observatory, Providence Every Tuesday On the campus of Brown University Free admission Every Wednesday Viewing through a 16-inch reflector, on the campus of Community College of Rhode Island Free admission ### South Carolina Every Saturday after planetarium show Viewing through several telescopes on the campus of University of South Carolina Free admission Every Monday On the campus of University of South Carolina Aiken Free admission One or more Saturdays per month Viewing through several telescopes Free admission ### Tennessee About once a month Star Parties offer views through several telescopes, and visitors are welcome to bring their own. Free admission ### Texas Wednesday, Friday, and Saturday nights when the University is in session Viewing through a 16-inch telescope atop RLM Hall on Wednesday nights, and through a 1930s-vintage 9-inch telescope atop Painter Hall on Friday and Saturday nights. Free admission$3 parking in University garages after 6 p.m.

Open one Saturday a month for public star parties

Public viewing is offered through 16-inch and 12.5-inch reflectors at this site, which is operated by the Austin Astronomical Society in conjunction with the Canyon of the Eagles Lodge and Nature Reserve in the Texas Hill Country.

Free admission for lodge and reserve guests

Every Saturday, 3-11 p.m.

Public viewing through 36-inch, 18-inch, and 14-inch reflecting telescopes operated by the Houston Museum of Natural History on the grounds of Brazos Bend State Park.

$5 per person, plus park admission fee Visitors attend a McDonald Observatory star party. [Frank Cianciolo] Every Tuesday, Friday, and Saturday In addition to naked-eye tours of the night sky, the star parties offer views through a 22-inch telescope, a 16-inch telescope, and several smaller instruments.$12 adults, $8 children 6-12,$40 family (five or more). $60 per person for 107" viewing. Also available as part of a combination ticket that includes daytime Observatory tours. 432-426-3640 Dates and starting times vary check the web site for schedules McDonald Observatory offers views through the 107-inch Harlan J. Smith and 82-inch Otto Struve telescopes one or more nights per month. Programs include dinner and a presentation by a research astronomer. The Observatory also offers viewing through the 36-inch research telescope, but this program does not include the presentation are dinner. Reservations are generally required several months in advance. Not suitable for children under 10.$75 per person for the 107-inch and 82-inch telescopes, $50 for the 36-inch telescope 432-426-3640 Once a month during the summer Star Parties offer viewings through several telescopes and interaction with Fort Worth Astronomical Society members. Free admission Full Moon over Bryce Canyon [National Park Service] 1-3 nights per week, depending on the season Telescope viewing and multimedia presentations. Other astronomy programs include solar viewing three times per week and an annual star party.$25 per vehicle park admission fee

Every clear Wednesday evening

On the roof of the South Physics Building

### Vermont

A few warm-weather nights during the year

Viewing through several telescopes

### Virginia

First and third Friday of each month (except holidays)

Established in 1885, the Observatory offers views through a 26-inch refractor built by Alvan Clark and Sons and a modern 10-inch reflector. The program includes a tour of the Observatory and audio-visual presentations.

Twice yearly, in April and October

This University of Virginia observing station offers views through 40-inch and 31-inch reflectors. Advance tickets are required.

Once a month, April - December

NRAO volunteers provide telescopes for celestial viewing.

Every Friday night during the summer

Viewing through several telescopes offered.

Daily access offers viewing through four telescopes and two sun-spotting devices.

### Washington

Every Wednesday through Sunday, April through September

Public viewing through a 24.5-inch and other telescopes daytime programs available as well, offering telescopic views of bright stars or planets. Operated by Washington State Parks

A winter evening at Washington State University's Jewett Observatory.

Approximately once a month

Viewing through 12-inch Alvan Clark & Sons telescope.

### Wisconsin

Every Wednesday during the summer. First and third Wednesdays of each month year-round.

Observatory open for public viewing.

Three days a week, during the school year

Viewing through a 0.4-meter telescope offered.

## 6.1 Telescopes

There are three basic components of a modern system for measuring radiation from astronomical sources. First, there is a telescope , which serves as a “bucket” for collecting visible light (or radiation at other wavelengths, as shown in (Figure 6.2). Just as you can catch more rain with a garbage can than with a coffee cup, large telescopes gather much more light than your eye can. Second, there is an instrument attached to the telescope that sorts the incoming radiation by wavelength. Sometimes the sorting is fairly crude. For example, we might simply want to separate blue light from red light so that we can determine the temperature of a star. But at other times, we want to see individual spectral lines to determine what an object is made of, or to measure its speed (as explained in the Radiation and Spectra chapter). Third, we need some type of detector , a device that senses the radiation in the wavelength regions we have chosen and permanently records the observations.

The history of the development of astronomical telescopes is about how new technologies have been applied to improve the efficiency of these three basic components: the telescopes, the wavelength-sorting device, and the detectors. Let’s first look at the development of the telescope.

Many ancient cultures built special sites for observing the sky (Figure 6.3). At these ancient observatories, they could measure the positions of celestial objects, mostly to keep track of time and date. Many of these ancient observatories had religious and ritual functions as well. The eye was the only device available to gather light, all of the colors in the light were observed at once, and the only permanent record of the observations was made by human beings writing down or sketching what they saw.

While Hans Lippershey , Zaccharias Janssen , and Jacob Metius are all credited with the invention of the telescope around 1608—applying for patents within weeks of each other—it was Galileo who, in 1610, used this simple tube with lenses (which he called a spyglass) to observe the sky and gather more light than his eyes alone could. Even his small telescope—used over many nights—revolutionized ideas about the nature of the planets and the position of Earth.

### How Telescopes Work

Telescopes have come a long way since Galileo’s time. Now they tend to be huge devices the most expensive cost hundreds of millions to billions of dollars. (To provide some reference point, however, keep in mind that just renovating college football stadiums typically costs hundreds of millions of dollars—with the most expensive recent renovation, at Texas A&M University’s Kyle Field, costing \$450 million.) The reason astronomers keep building bigger and bigger telescopes is that celestial objects—such as planets, stars, and galaxies—send much more light to Earth than any human eye (with its tiny opening) can catch, and bigger telescopes can detect fainter objects. If you have ever watched the stars with a group of friends, you know that there’s plenty of starlight to go around each of you can see each of the stars. If a thousand more people were watching, each of them would also catch a bit of each star’s light. Yet, as far as you are concerned, the light not shining into your eye is wasted. It would be great if some of this “wasted” light could also be captured and brought to your eye. This is precisely what a telescope does.

The most important functions of a telescope are (1) to collect the faint light from an astronomical source and (2) to focus all the light into a point or an image. Most objects of interest to astronomers are extremely faint: the more light we can collect, the better we can study such objects. (And remember, even though we are focusing on visible light first, there are many telescopes that collect other kinds of electromagnetic radiation.)

Telescopes that collect visible radiation use a lens or mirror to gather the light. Other types of telescopes may use collecting devices that look very different from the lenses and mirrors with which we are familiar, but they serve the same function. In all types of telescopes, the light-gathering ability is determined by the area of the device acting as the light-gathering “bucket.” Since most telescopes have mirrors or lenses, we can compare their light-gathering power by comparing the apertures , or diameters, of the opening through which light travels or reflects.

The amount of light a telescope can collect increases with the size of the aperture. A telescope with a mirror that is 4 meters in diameter can collect 16 times as much light as a telescope that is 1 meter in diameter. (The diameter is squared because the area of a circle equals πd 2 /4, where d is the diameter of the circle.)

### Example 6.1

#### Solution

the area of a 1-m telescope is

and the area of a 4-m telescope is

After the telescope forms an image, we need some way to detect and record it so that we can measure, reproduce, and analyze the image in various ways. Before the nineteenth century, astronomers simply viewed images with their eyes and wrote descriptions of what they saw. This was very inefficient and did not lead to a very reliable long-term record you know from crime shows on television that eyewitness accounts are often inaccurate.

In the nineteenth century, the use of photography became widespread. In those days, photographs were a chemical record of an image on a specially treated glass plate. Today, the image is generally detected with sensors similar to those in digital cameras, recorded electronically, and stored in computers. This permanent record can then be used for detailed and quantitative studies. Professional astronomers rarely look through the large telescopes that they use for their research.

### Formation of an Image by a Lens or a Mirror

Whether or not you wear glasses, you see the world through lenses they are key elements of your eyes. A lens is a transparent piece of material that bends the rays of light passing through it. If the light rays are parallel as they enter, the lens brings them together in one place to form an image (Figure 6.4). If the curvatures of the lens surfaces are just right, all parallel rays of light (say, from a star) are bent, or refracted, in such a way that they converge toward a point, called the focus of the lens. At the focus, an image of the light source appears. In the case of parallel light rays, the distance from the lens to the location where the light rays focus, or image, behind the lens is called the focal length of the lens.

As you look at Figure 6.4, you may ask why two rays of light from the same star would be parallel to each other. After all, if you draw a picture of star shining in all directions, the rays of light coming from the star don’t look parallel at all. But remember that the stars (and other astronomical objects) are all extremely far away. By the time the few rays of light pointed toward us actually arrive at Earth, they are, for all practical purposes, parallel to each other. Put another way, any rays that were not parallel to the ones pointed at Earth are now heading in some very different direction in the universe.

To view the image formed by the lens in a telescope, we use an additional lens called an eyepiece . The eyepiece focuses the image at a distance that is either directly viewable by a human or at a convenient place for a detector. Using different eyepieces, we can change the magnification (or size) of the image and also redirect the light to a more accessible location. Stars look like points of light, and magnifying them makes little difference, but the image of a planet or a galaxy, which has structure, can often benefit from being magnified.

Many people, when thinking of a telescope, picture a long tube with a large glass lens at one end. This design, which uses a lens as its main optical element to form an image, as we have been discussing, is known as a refractor (Figure 6.5), and a telescope based on this design is called a refracting telescope . Galileo’s telescopes were refractors, as are today’s binoculars and field glasses. However, there is a limit to the size of a refracting telescope. The largest one ever built was a 49-inch refractor built for the Paris 1900 Exposition, and it was dismantled after the Exposition. Currently, the largest refracting telescope is the 40-inch refractor at Yerkes Observatory in Wisconsin.

One problem with a refracting telescope is that the light must pass through the lens of a refractor. That means the glass must be perfect all the way through, and it has proven very difficult to make large pieces of glass without flaws and bubbles in them. Also, optical properties of transparent materials change a little bit with the wavelengths (or colors) of light, so there is some additional distortion, known as chromatic aberration . Each wavelength focuses at a slightly different spot, causing the image to appear blurry.

In addition, since the light must pass through the lens, the lens can only be supported around its edges (just like the frames of our eyeglasses). The force of gravity will cause a large lens to sag and distort the path of the light rays as they pass through it. Finally, because the light passes through it, both sides of the lens must be manufactured to precisely the right shape in order to produce a sharp image.

A different type of telescope uses a concave primary mirror as its main optical element. The mirror is curved like the inner surface of a sphere, and it reflects light in order to form an image (Figure 6.5). Telescope mirrors are coated with a shiny metal, usually silver, aluminum, or, occasionally, gold, to make them highly reflective. If the mirror has the correct shape, all parallel rays are reflected back to the same point, the focus of the mirror. Thus, images are produced by a mirror exactly as they are by a lens.

Telescopes designed with mirrors avoid the problems of refracting telescopes. Because the light is reflected from the front surface only, flaws and bubbles within the glass do not affect the path of the light. In a telescope designed with mirrors, only the front surface has to be manufactured to a precise shape, and the mirror can be supported from the back. For these reasons, most astronomical telescopes today (both amateur and professional) use a mirror rather than a lens to form an image this type of telescope is called a reflecting telescope . The first successful reflecting telescope was built by Isaac Newton in 1668.

In a reflecting telescope, the concave mirror is placed at the bottom of a tube or open framework. The mirror reflects the light back up the tube to form an image near the front end at a location called the prime focus . The image can be observed at the prime focus, or additional mirrors can intercept the light and redirect it to a position where the observer can view it more easily (Figure 6.6). Since an astronomer at the prime focus can block much of the light coming to the main mirror, the use of a small secondary mirror allows more light to get through the system.

### Making Connections

If the astronomy course you are taking whets your appetite for exploring the sky further, you may be thinking about buying your own telescope. Many excellent amateur telescopes are available, and some research is required to find the best model for your needs. Some good sources of information about personal telescopes are the two popular US magazines aimed at amateur astronomers: Sky & Telescope and Astronomy. Both carry regular articles with advice, reviews, and advertisements from reputable telescope dealers.

Some of the factors that determine which telescope is right for you depend upon your preferences:

• Will you be setting up the telescope in one place and leaving it there, or do you want an instrument that is portable and can come with you on outdoor excursions? How portable should it be, in terms of size and weight?
• Do you want to observe the sky with your eyes only, or do you want to take photographs? (Long-exposure photography, for example, requires a good clock drive to turn your telescope to compensate for Earth’s rotation.)
• What types of objects will you be observing? Are you interested primarily in comets, planets, star clusters, or galaxies, or do you want to observe all kinds of celestial sights?

You may not know the answers to some of these questions yet. For this reason, you may want to “test-drive” some telescopes first. Most communities have amateur astronomy clubs that sponsor star parties open to the public. The members of those clubs often know a lot about telescopes and can share their ideas with you. Your instructor may know where the nearest amateur astronomy club meets or, to find a club near you, use the websites suggested in Appendix B.

Furthermore, you may already have an instrument like a telescope at home (or have access to one through a relative or friend). Many amateur astronomers recommend starting your survey of the sky with a good pair of binoculars. These are easily carried around and can show you many objects not visible (or clear) to the unaided eye.

When you are ready to purchase a telescope, you might find the following ideas useful: