Talking Telescopes Yahoo Group FAQ

From SkyInsight

Jump to: navigation, search

Send this article to a friend


Telescope Selection

What telescope should I buy

Q: I am looking to purchase a new telescope, either a refractor or > a reflector. But, I am not too sure which would be best for me, as I am getting started (re-started) in astronomy.

A [Geoff Gaherty]:

A good first step would be to track down your local astronomy club. They usually hold regular "star parties" where members bring out their scopes to observe together, and they are an excellent opportunity to look through a variety of scopes and get a better idea of what you might want to buy. Here are two sources of club information:

Another step is to do some "homework". Phil Harrington's excellent book, Star Ware (Wiley) is a guide to currently available telescopes. Here are some online articles you may find helpful too:

For more general information on amateur astronomy, I'd recommend NightWatch by Terence Dickinson (Firefly). This puts everything in context in easy-to-read form, and includes some specific recommendations on telescopes.

Q: I have wanted a telescope for years, but I know NOTHING about them, really. The only thing I DO know, is there are two basic types, refractor, and reflector.

A [Geoff Gaherty]: Actually there are _three_ types. The third is catadioptric, which combines lenses and mirrors to make a more compact package.

Q: I know one of them makes things upside down, and one does not. I can't remember which is which.

A [Geoff Gaherty]: In fact _all_ telescopes turn things upside down (except for a couple of oddball types)'s just that the images in refractors and catadioptrics can be more easily turned right side up with a device called an erecting diagonal.

Q: I was wanting to get a GOOD telescope for sky AND terrestial veiwing, so I was wondering if anyone could tell me some GOOD telescopes to look at?

A [Geoff Gaherty]: Since you want to use your scope for terrestrial as well as astronomical viewing, you'll be better off with a refractor. A reflector can't easily have its image turned right side up, and most catadioptrics have too narrow a field of view for most terrestrial subjects.

Q: I know if you buy cheap you buy twice, so I am assuming the 100 dollar ones are junk, right?

A [Geoff Gaherty]: Absolutely! You should expect to pay at least twice that for a decent telescope.

Your best source for a telescope is a store which specializes in selling telescopes, because the salespeople there usually know their subject well and can give you honest reliable advice. Check your Yellow Pages under "Telescopes" to see if there are any stores near you. If not, there are a number of long-established reliable online dealers, such as Anacortes, Astronomics, Hands On Optics, Oceanside Photo and Telescopes, and Orion, to mention just a few. My personal favorite is Orion, because of their excellent web site, which I find particularly helpful for beginners, and their good selection of telescopes of all kinds. Conflict of interest alert: I'm currently writing articles for Orion in exchange for review equipment, but I was a regular customer of theirs for seven years before I started working for them.

A particularly useful scope for people who want both astronomical and terrestrial use at a low price is the Synta 80mm f/5 refractor, available from several sources:

It is available on various mounts which favor either terrestrial or celestial observation. An interesting variant is the Celestron StarSeeker 80mm GoTo Refractor, which is mounted so that it is useful for both forms of observation:

I'd consider this scope the bare minimum for your requirements. If you're prepared to spend more money, there are plenty of choices; you pretty much get what you pay for. I own a couple of Orion's ED refractors, 80mm and 100mm (bought with my own money), and find them absolutely superb for both terrestrial and astronomical use.

The only warning I'd give about refractors is that, if astronomy becomes your primary interest, their small aperture lenses will become a frustration to you. For astronomy, aperture rules, and you'll be much more satisfied with a larger reflector scope. Alas, it won't serve well for terrestrial use, though. That's why many of us end up with more than one telescope!

Telescope for Astrophotography

Deep Space Telescope

Planetary Telescope

Finder Scopes

[Ed Hitchcock]

From lengthy discussion on finder preference on the Talking Telescopes forum, here is a list of the main options for finders used by backyard astronomers:

  1. straight-through finder/two eyed method: use straight-through finder and keep both eyes open. With a little practice you can superimpose the cross-hairs of the finder eye on the "real sky" image in the other eye. Very efficient, but some people find the position awkward, or just never get used to it.
  2. Unit finder and RACI (Right Angle Correct Image): use a unit finder (Rigel quickfinder works a bit better on a dob than the telrad, for weight and shape reasons) to get in the right ballpark, and the RA finder to pinpoint the target. This is a bit more comfortable, generally, than the straight-through method, assuming you can mount the unit finder near the end of the tube, but requires a 2-step operation.
  3. unit finder only: some people find that using only a telrad or quickfinder they can get close enough that they can go straight to the eyepiece. This is generally less true in light-polluted areas.
  4. Right Angle finder only: some people find they can bag their target using only an RA or RACI finder. Personally, I find it frustrating, so I take my hat off to those who do it regularly.

There is a new category of finder - the green laser finder. This is a bracket that holds a green laser, that shows you where the scope is pointing. It works much like a unit finder, but you don't have to look through it, just along the beam. It is generally bad etiquette to use a green laser at star parties.

So I will add to the list:

  1. Green laser finder and RA: Use the laser to get in the ballpark, and an RA or RACI finder to pinpoint your target.
  2. Green laser only: See #3 unit finder only.
So there you have it in a nutshell, find the method that works for you. Depending on the scope and the conditions, I use any of the first three methods.

Telescope Maintenance


Collimating a refractor

Q: If there are not sets of screws around the primary lens cell, there may be some kind of adjustment in the attachment of the cell to the tube.

A: [Geoff Gaherty] No, the vast majority of small refractors have no means of adjusting collimation...they are essentially "factory collimated". For decades a major selling point for refractors over reflectors has been that they never need collimation. Until Synta introduced collimatable cells on their larger refractors a few years ago, I think Meade‘s were the only adjustable mass market refractors. Even the legendary Unitrons of yesteryear and the Tele Vues and Vixens of today are not user collimatable.
Most inexpensive refractors are not made to be collimated. Some people have had some success by adjusting the angle of the focuser, which usually is attached by three screws.

Search for messages with "collimation" and "refractor"

Laser Collimators

Q: I am wondering if the general ATM public believes that laser collimators are worth the time, trouble and expense - time & trouble to first collimate the collimator, and the expense of the higher-end, tighter-fitting models.

A: [Phil Harrington] I‘ll throw in my $0.02 and say that I‘m not wildly enthusiastic about laser collimators. I‘ve seen some that claim to be highly accurate, but in reality, are off by a considerable amount. Problem is the machining isn‘t accurate enough. In these cases, you could actually see the laser dot trace out a small circle on the primary mirror as you rotated the collimator in the focusing mount.
Personally, I use a sight tube and Cheshire eyepiece 95% of the time. I also have an autocollimator, but find that I have a pretty eye with the others, so it‘s just used for checking. And, yes, you pretty much get what you pay for with this stuff.

[Laird Scott] A laser collimator works well if the telescope‘s focuser is collimated -- otherwise, the beam‘s return path will be in error and collimation won‘t be as good as a simple peep-hole collimator would provide. Rather than center mark my secondary, I used another procedure. I carefully collimated the telescope in a conventional, non-laser, fashion. Then, I installed the laser collimator and adjusted focuser collimation until the beam was exactly centered on the primary. Then I compared normal laser to cheshire collimation, and readjusted the focuser until the two methods agreed. It took a few tries. Finally, I barlowed the laser for a very quick, easy, and accurate method of collimation.

[Geoff Gaherty] I used a laser collimator for a while after I got my Starmaster 11" f/4.3 Newtonian. Then I bought a set of Tectron tools and discovered that, if used carefully and consistently, they were much more accurate than the laser. A good laser can be useful for quick in-the-field checks of a scope that has been previously collimated with traditional tools like sight tube and Cheshire, but can‘t by itself guarantee collimation. I used the Kendrick 2" laser, which seems well made. I‘ve never used the Barlowed laser technique, so can‘t comment on that.

Search for messages with "laser" and "collimator"

Cleaning Optics


Coma, aberration, astigmatism

[John Bambury]

  • Coma is an aberration which "in a newtonian" makes stars appear like little comets or badminton shuttles with their tails all pointing away from the centre of the FOV. Refocussing will not compensate for Coma.
  • Field curvature is another aberration people often mistake for coma which is where the centre of the FOV is in focus and the edge is out of focus. Refocussing will bring the edge of field into focus and take the centre out of focus.
  • Astigmatism is probably the most common aberration mistaken for coma and it makes stars appear as little "slits" as you go away from the centre of the FOV. When you move the focuser from one side of focus to the other the "slits" change orientation by 90 degrees.

[Geoff Gaherty]

No aberration:     Astigmatism:        Coma:
    ***                ///              \|/
    ***                ///              -*-
    ***                ///              /|\

Planetary viewing

In the August 2004 Sky and Telescope, Gary Seronik compared what he considers to be the best planetary eyepieces available:

  • Tele Vue Plossl
  • Vernonscope Brandon
  • Clave Plossl
  • TMB Super Monocentric

Gary reported that all of the eyepieces offered excellent resolution and are among the best planetary eyepieces available. For all practical purposes Gary found no difference between the set.

Ron B[ee]: For picking up planetary detail, for me it‘s the Tele Vue Nagler zoom and the TMB SuperMono, in my opinion ;-). I also like TV Plossl but they don‘t have short FL for good reasons.

Q: What short focal length 4-9mm eyepieces do you recommend for high power planetary and other work? Can you recommend Ortho or Plossl? Do I really need expensive modern eyepieces for high power work?

A: [Geoff Gaherty] Both the Plössl and orthoscopic designs work quite well with today‘s short focus telescopes, if they are well designed. You have to remember that "Plössl" and "orthoscopic" are very old terms for very old (19th century) designs, and are often interpreted in very different ways by modern optical designers. In short focal lengths I can recommend Tele Vue Plössls, the various orthoscopics imported from Japan by University Optics, Antares, and possibly others, and the Japanese 5-element "Plössls" imported by Antares (Elite), Celestron (Ultima), and Orion (Ultrascopic), and possibly others. However, these designs all suffer from extremely short eye relief, generally slightly less than their focal length. In the old days we accepted this and learned to live with it, or else combined longer focal length eyepieces with Barlow lenses to increase the eye relief. Nowadays there are designs incorporating built-in Barlows, such as Tele Vue‘s Radians and their various clones. These offer short effective focal lengths with long (20mm) eye relief, and are extremely comfortable to use, though they tend to show any dirt on the eye lens. However, they are quite expensive and physically large and heavy.


Digital setting circles - What are they?

[Geoff Gaherty] Digital setting circles consist of two parts: encoders on the two axes of the telescope, and a small hand control containing a computer. You tell the computer the date and time, and it suggests a couple of stars to orient itself. You point the telescope at the two stars one after the other, and the computer will then give you instructions to point at any object in its database. Usually it does this with a couple of readouts. You move the telescope until both readouts are zero, and the scope should then be pointing at the selected object. A GoTo telescope works exactly the same way, except that the pointing is done by motors, rather than by the operator manually moving the scope. The main advantages of digital setting circles over GoTo is that they are less expensive, consume far less battery power, and are silent in operation.


[Geoff Gaherty] The folks who find objects by starhopping usually don‘t know or care about the coordinates of the object. They‘re only interested in its position relative to known stars.

To take an analogy, suppose you want to visit my house. I can give you its longitude and latitude, and you can find it using a GPS receiver. Or I can give you directions how to find it starting from a known point, such as: start at the corner of Eglinton Avenue and Yonge Street (in Toronto), walk six blocks west and then two blocks south. Similarly, to find Comet Machholz tonight: start at the Pleiades cluster (in Taurus) and sweep about 4 degrees (half a finder field) to the northwest.

The analogy of finding places in a new city is very similar to the problems the beginning stargazer faces with finding objects in the sky. The trick is to first learn some landmarks, and then locate things relative to those landmarks. When I moved to Toronto, I used subway stops as my reference points; when I started observing, I used the Big Dipper to identify some bright stars, and then those bright stars to find fainter objects in between. In both cases I was initially overwhelmed and bewildered, but after a few months I got my bearings, and soon I could find anything. For me (and probably for most people) starhopping comes naturally because I use maps all the time to find my way around my city and other places. Mostly I don‘t know or care what longitude or latitude things are at, though nowadays people who use GPS a lot are more comfortable in that system than with maps.