This page last modified 2006 August 18
Evaluating a Telescope in the Store
(To evaluate binoculars, click here.)
(Or, if it's arrived boxed, most of this is also applicable to what you can do in your home)
There is no substitute for taking an astronomical telescope under the stars,
but this is too often not possible. Here are some pointers for things to look
for "in store". It is your choice how you spend your money, but it is
my strong opinion that you should be as aware as possible of any shortcomings
before you buy. Almost all telescopes exhibit some of the "features"
that these tests will reveal, but they are often present in greater number and
severity in cheap telescopes. Many can be remedied – see the
tutorial on upgrading a 60mm refractor.
Remember: Anything that is merely irritating in the shop will most likely become infuriating under the stars.
If the salesperson will not permit you to
perform any of these tests, ask yourself why. Does the answer you get imply
concealment, ignorance, reasonable caution, a pressured salesperson..?
- Appearance. Is the telescope properly set up? An
incorrectly assembled telescope is indicative of the knowledge of the seller.
- General. Anything slack or loose will get worse, not better, with
use and transport. Ask the salesperson to give the telescope a good shake
(unless it's a Dob with a sling-cell for the primary). If there is any objection
to shaking it, explain that the telescope will be shaken when travelling over
rough roads to your dark observing site.
- What is the build quality like?
- Any rattles?
- Optics in general. The main optic (primary mirror or objective
lens) is often quite good, but you need to test the optical system overall.
Focus the telescope on as distant an object as possible with as high contrast as
possible. A television antenna against a bright sky is my favourite. Look at
the quality of the image.
- Can you achieve focus with each of the eyepieces when they are fully inserted into the focuser drawtube or star diagonal?
- Does the telescope "snap" to an easily-defined best focus, or is there a small range where the target object seems to be almost-but-not-quite focused?
- Does the image get "mushy" or fuzzy at the edge of the field of view?
- Is there a lot of false colour? This will be intrusive when viewing the Moon and bright planets.
- Does a line that is straight when it is in the centre of the field of view become curved as it moves across the field?
- Eyepieces.
- Does the field of view seem narrow? If so, be aware that this makes it more difficult to find an object, especially for a child (the usual recipient of this sort of telescope).
- Can you get a sharp focus with the highest power eyepiece? (The highest power one is the one with the shortest focal length. This should be printed on the eyepiece, e.g. "6mm".)
- Does the image get darker at the edge of the field of view with a low-power eyepiece? (The lowest power one is the one with the longest focal length. This should be printed on the eyepiece, e.g. "20mm".)
- Do the eyepieces have 24.5mm (0.965") diameter barrels? The standard sizes for astronomical telescopes are 31.8mm (1.25") and 50.8mm (2"), which permit wide-angle views that are not possible with the 24.5mm ones. The 24.5mm ones will therefore probably not be transferable if you upgrade your telescope in the future.
- Focuser.
- Is its motion smooth? Any jerky bits make focusing difficult.
- Is it too tight or too loose? A loose focuser doesn't hold focus; a tight one is difficult to focus.
- Is there a lot of backlash in the mechanism? "Backlash" is the amount of play in the knob before the focuser moves. It should be minimal.
- Is there any play in the focuser tube? Perceptible play is an indication of a poorly made focuser. Manufacturers try to conceal it by over-tightening and by putting excessive thick grease into the mechanism.
- Is there a lot of grease on the focuser tube? This is often used to disguise poor engineering (see above). It also gets transferred, by the user's fingers, to optical components, where it is a nuisance to remove.
- Finderscope. Get the sales person to remove the objective ("big
end") lens (it usually unscrews anticlockwise).
- Is there an aperture stop (a disc with a hole in it) close to the end of the tube? Measure the diameter of the hole. This is the true aperture. This aperture stop is introduced to disguise poor quality lens. It also reduces image brightness, often to that of a young person's dark-adapted eye. This is not a "good thing".
- Is the finder secure in its mounting? Wobble = observer frustration.
- Is the finder mounting secure on the main telescope? Wobble = observer frustration.
- Are the finder adjustment screws easy to use? Anything that is fiddly in the shop will be more so in the dark with cold fingers.
- Barlow and or Image Erector Lenses. These are of almost universally
poor quality in cheap telescopes; try them out.
- Can you get a sharply focused image?
- Do they introduce false colour?
- Does the field of view become ridiculously small?
- Altazimuth Mount. Check it for rattles, slackness, etc. The main
offender is the altitude stabiliser/adjuster/slow-motion device. With the
highest magnification possible, look at a distant object and give the telescope
a sharp tap.
- Does the image shiver briefly before stabilising or does it oscillate wildly for a few seconds? Any vibration will be much more intrusive under the stars.
- Is there any play in the motions of the telescope?
- Does it stay where it is pointed? If it doesn't, this will become very frustrating when you are finding or tracking an object.
Also perform these tests with a camera tripod if you are intending to use it with a telescope. - Equatorial Mount. Check it for rattles, slackness, etc. With the
highest magnification possible, look at a distant object and give the telescope
a sharp tap.
- Does the image shiver briefly before stabilising or does it oscillate wildly for a few seconds? Any vibration will be much more intrusive under the stars.
- Is there any play in the motions of the telescope?
- Do the slow-motions operate smoothly, with no tight spots?
- Does the image vibrate excessively when you move the telescope in right ascension? If so, observing an object at high magnification will be extremely frustrating.
- Can the slow motion handles foul (interfere with) any part of the mount?
- Does it stay where it is pointed? If it doesn't, this will become very frustrating when you are finding or tracking an object.
- Tripod. Check for any slackness. If it has extensible legs, test it
with legs both extended and retracted. It will be more stable with legs
retracted, but this may make for an uncomfortable viewing position, especially
for objects near the zenith with a refractor.
- Can you achieve a comfortable viewing position for objects near the horizon and near the zenith?
- Can the tripod be knocked over (e.g. by your domestic pet, or by you stumbling about in the dark) with the telescope on it?
- Does the telescope foul (interfere with) the tripod legs at any part of its motion?
- If there is an accessory tray, are the wing-nuts and bolts going to be difficult to manipulate in the cold and dark? Most second-hand telescopes of this type have some of these nuts/bolts missing, indicating ease of loss.
- If it is a table-top tripod, how easy is it to accidentally move it when using the telescope?
Also perform these tests with a camera tripod if you are intending to use it with a telescope. - Computer Database. Some cheap telescopes now come computerised.
These seem very attractive, but do perform the tests outlined above, where
applicable. It's well-nigh impossible to test the pointing accuracy in the shop,
but do ask the salesperson how many, and which, of the objects in the computer
database are visible in the telescope. If he says, "All," ask him to
put it in writing.
