Chapter 6: Caring for and Collimating your Telescope

All you need to know about protecting and cleaning your telescope, collimating Newtonian and ReflectorTelescopes, and the do-nots!!

Some Telescope Care Tips
A telescope's greatest nightmare is the possibility of having an over passionate owner who constantly attempts to keep their telescope optics as speck clean as possible. Fact is, cleaning telescope optics should only be done when necessary (after you use your tube as a human cannon launcher or a trip down to a coal mine), and so for the rest of us, here's some advice below.

*WARNING* If you own an air spaced reflector, please contact your manufacturer for specific advice on cleaning. Do NOT remove refractive lenses from the lens holder as there are very few individuals who have the experience and equipment to properly handle these complex lenses. This advice also applies towards most complex eyepieces. If you doubt your abilities in any way, then you're probably right. Cleaning that is done properly will not degrade the optical performance of your telescope.

Cassegrains - Since most reflectors and Cassegrain telescopes are closed systems, it is not necessary to clean the inner lens surfaces. Cleaning the field (R1 or outer) lens may be done with the lens in the cell attached to the optical tube. If you need to, remove the lens cell from the tube. It is better to leave the lens retaining ring in its place, and progess with the lens in the cell. Try not to make any adjustments to the retaining ring. It is normal to hear a slight rattling sound if you shake a reflector objective in its cell since the factory set spacing accommodates expansion of the lens due to typical environmental temperature changes.

Newtonians - Most Newtonian telescope mirrors are held within a Mirror Cell which can be removed in one piece from the telescope. Pay careful attention to the orientation of the lens/mirror cell within the tube. You may also wish to mark the tube and lens cell where they meet with a dab of liquid paper/whiteout. Just be careful that your cleaning procedure doesn't remove the index marks. Set up the optics onto a clean well-padded platform, with the surface of a mirror or lens to be cleaned upright. After that, loosen the retainers on the lip of mirror and remove them, while taking extra precaution not to tilt or rotate the mirror.

Collimating Telescopes

Collimation is the process of aligning your telescope's mirrors so they work with each other to provide properly focussed light to your eyepiece. It is a fairly important process for your viewing experience, as if your telescope isn't properly collimated, you will observe images that are out of focus.

Collimating Newtonian Telescopes
Although collimation in Newtonian telescopes is NOT an absolute necessity, correct alignment will greatly improve the quality of the image you see, as well as your ability to resolve images.

Aperture size largely determines the frequency of collimation that is necessary. For example, a 10 inch Newtonian may require collimation every time it is transported, whereas an 8 inch scope will require collimation only after 3-4 times of viewing. A 2 inch scope might not need any collimation at all!

Alignment Example

In the image above, you'll see two Airy discs. If the rings are symmetrical around the Airy disc, you're good to go and the telescope is properly collimated. If it doesn't, and you need to collimate your telescope, keep reading.

Use a 35mm film canistere as a collimating tool
You will now need a collimating tool. If you don't have one, you can easily make one out of a plastic 35mm film canister, and you should be able to find them at photo development studios, or your local grocery store. Drill or punch a small pinhole - about 2-5mm in diameter – in the exact center of the lid. Then, cut off the bottom of the canister.

This device will keep your eye centered directly on the focuser tube. If the canister is the correct diameter, it will fit nicely into the focus tube, acting as a peephole. Insert the canister in place of a regular eyepiece.

A Telescope that needs Collimation

Alternatively, you can check your collimation by centering a bright star through an eyepiece with high magnification (e.g. 50x). In a misaligned scope, the star is out of focus and will appear as a doughnut shape with a dark center. If your telescope is misaligned, this dark patch will be perfectly centralised within the bright ring. This is actually the shadow cast by the secondary mirror, and you may also see the spider veins (as in the figure below). When the optics are in need of alignment the dark patch will be off center.

Collimating a Reflector Telescope

Collimating a Reflector Telescope – A Guide to Set-Up and Alignment

(with Adjustable Objective-Lens Cell)


Collimation – in Reflector telescopes – is the process of aligning the lenses of your telescope. This ensures that the light that the lenses collect are able to focus on the right spot on the back of your telescope. This ensures that your eyepieces are able to function correctly.

Collimation is a simple process, despite its reputation as being tedious. Here's how to get your reflector's collimation sorted:

Remove the dew cap on the front of your telescope – then look through the scope. You'll see your lenses held in a threaded cell. The cell will be held in place by 3 pairs (6 individual) screws spaced 120 degrees apart (Fig. a).

There will be two kinds of screws. The first are larger Phillip head screws (cross-head), and they hold both ends of the cell to each other.

The smaller, buried Allen screws push against a ledge within the tube, allowing it to tilt slightly against the tension provided by the Phillip's screws.

So how do I know if I need to collimate my telescope?

The easiest way to collimate your reflector telescope uses Polaris (Fig. b) – the North Star, and an eyepiece.

The North Star is the outermost star in the handle of the Little Dipper, located in the constellation Ursa Minor.

In order to collimate your telescope, you will have to alternately loosen and tighten each set of screws until the image you see through the telescope is properly centred.

With this method, ensure that your telescope isn't polar aligned – in fact, if possible, try to intentionally point your telescope mount's head due east or west.

Now then, here's the process.

Firstly, use the lowest powered eyepiece you own – it will be the one with the largest number (or longest focal length) in mm. Use this eyepiece to acquire – or spot – Polaris, and place Polarins in the center of your eyepiece view.

Secondly, switch to an eyepiece of a higher power, and keep the image centered.

If your telescope is in focus, the star image you will see – known as an Airy disc - will have a bright point in its center, and fainter rings that move outwards in sequence. Refer to the image (Fig. c.) for examples of images through a collimated and off-collimated image. As you will see, an off-collimated image will have its brightest point off center.

So, I'm going to need to collimate my reflector. How do I do this?

If you find that your telescope requires collimation, start by taking out your star diagonal and look at your off-focus image. This will allow you to gauge the deflection of the lenses.

You will now have to loosen the pair of screws – slightly – on the side of the cell you spotted the deflection through the star diagonal.

To do this, you will have to:

Firstly, slacken the Allen head screws., then tighten the Phillip's head screws against them once more.

Secondly, re-check the image through your star diagonal, and move the image to the centre of any eyepiece you own.

Lastly, If you find that the image you see is even more off-center (i.e. it seems worse), repeat step one, but in the opposite direction. Alternatively, slacken the other two Allen screws slightly.

You will now have to keep repeating steps 1 – 3, until you have a perfectly collimated telescope!

Once you have a round, perfectly-centered star image (Airy Disc), you're all set!

It helps if you get a friend to help you with collimating your reflector telescope. Have your friend adjust the screws according to your directions while you look through an eyepiece.

Chapter 7: Solar Observations and a How-To guide on making a solar filter