Telescope Accessories

By Robin Gill on August 20th, 2021 at 7:29am

So far, we’ve been discussing the basic telescope assembly itself. It is time to wander into the wilderness of telescope accessories. Since I already did a post on eyepieces, I’ll concentrate on non-eyepiece accessories.

First, you have to be able to find stuff. Many scopes come with a small telescope, called a finder, attached and parallel to the axis of the telescope. They are typically specified like a binocular – something like “6x30” or “8x50”, where the first number is the magnification, and the 2nd number is the aperture of the objective of the finder. Recall from our earlier discussion that aperture is related to light gathering power, and larger means that you can see fainter in your finder. This is an advantage if you are planning to use the telescope for looking at faint deep sky objects. You will need to develop the skills of star hopping and recognizing faint star fields.

Note that in many scopes, you can upgrade accessories like finders. The manufacturer may not be generous with their standard equipment. Finders come with crosshairs and rings with the ability to adjust alignment. A finder that doesn’t point the same place as the main optics is not much help.

Another type of finder is the “zero-power” finder. It has no magnification or primary optic. It works by projecting a target - red concentric rings or a dot – on the sky. These are often seen on entry-level computer-controlled scopes these days. They figure all you need to be able to find is alignment stars to set up the control, and the control will do the rest. True… but more on this later.

Focusers and adapters are used at the eyepiece end of the scope. As mentioned in our eyepiece piece, 2” and 1.25” OD are the common eyepiece sizes. Adapters allow you to interchange these. Schmidt-Cass (S-C) and refractors often use right angle adapters for more comfortable viewing. These also hold eyepieces and may need adaptors. S-C scopes have various back-end accessories for photography, and there are also focal reducers which reduce the f/10 or f/11 light cone to something faster for photography.

To go along with your eyepieces, you can buy filters to enhance your viewing. These thread into the end of the eyepiece opposite the eye lens. There are several types – but all are used to enhance contrast. There are color filters which can be useful in detailed planetary viewing. These days, light pollution filters are popular – they help enhance contrast of deep sky objects by blocking some pollution wavelengths and passing those of nebulas. “Oxygen III” filters are a particular example that are designed specifically for planetary nebulas which emit this wavelength. Another handy filter for viewing the moon is a neutral density filter. It just reduces the overall brightness of the moon – which can be painfully bright through a large telescope.

Refractors and S-C scopes have a problem with dew on the front-end optic – the corrector place or the objective lens. A lens hood or dew shield is recommended to discourage dew formation. A heated “dew zapper” can warm the air around the optic just enough to keep dew away.

If you are planning to travel with your scope away from the 120v comforts of home, you will need portable power for running your mount, your computer control, your dew zapper, and any other electrical toys you need to run to observe. Portable power stations – rechargeable battery packs – are commonly available. Make sure the voltage output accommodates your needs. These days many also have USB outlets for charging your phones, tablets, etc.

OK. Time to discuss the computer-controlled elephant in the room. Computer controls are features that allow the user to set up the telescope, align it to a couple of known stars, and then use a hand-held remote to bring up a catalog of objects and go to them. I will state my personal bias up front. Your author is a believer that there is value in learning the sky, and in learning to find things. It is kind of like paying your dues. OK. I have stated my opinion. I won’t belabor it. I use a go-to telescope at our observatory. But I also would not trade my years of star-hopping my way through the Messier catalog.

These days many computer-controlled mounts come with GPS so that you don’t have to enter your coordinates or time. But if not, you need to enter these. The telescope has to know exactly where it is in the Einsteinian space-time continuum. Then, you need to orient it to one or two or three known stars in the sky to give the scope its bearings. (Exact details depend on your mount and control design). If your scope has an equatorial mount, this does not take the place of polar alignment. It is in addition to it. Many scopes can also be run by serial or USB from an app on a phone, tablet, or laptop. This is particularly helpful if you are an advanced observer going after more obscure targets that were not pre-programmed into your scope.

OK – so what is the downside? As mentioned previously, computer control is sexy, and electronics are relatively cheap. It has become depressingly common to see computer controls on really crappy telescopes – poor optics and poor mechanical design and execution. You get an excited new observer with this high-tech telescope, and they cannot find anything. They don’t know their guide stars, they don’t know how to look through a scope, and their mount will not stay pointed toward anything. Major turn-off.