Telescopes and buying guide: astronomy equipment

You are probably reading this because you have finally decided that listening the Patrick Moore in the middle of the night and trying to point out the occasional planet or constellation to some disinterested friends is not enough for you. You want to buy some toys!

This page is a rough introduction to the basic elements of astronomy kit. They can be summerised as follows:

Telescopes

More properly termed "OTA" - which is an acronym for "optical tube assembly". This phrase sums it up pretty well. There is a lot more to think about than just a chuffing great mirror or lens. The OTA needs an optical element, but it also requires a mirror cell or lens cell to hold the glass. It needs methods for collimation. It needs something to hold the eyepeices or cameras and focus them. It also has to have a dovetail or similiar for holding it to the mount. Its essentially a mechanical device. And it needs to be very mechanically sound. If the mirror rattles around each time you slew to a different part of the sky, then you are in for a rough ride.

Telescopes, as you probably know, are divided roughly into reflectors and refractors. The common feature being that they all use a combination of optical elements (lens and mirrors) to bring the distant light to a common focus.

Refractors use lenses. They are generally available in two types these days. One is inexpensive and called a achromatic refractor. An achromat is not able to focus all the different colours of light together. They are colour corrected to a certain extent, but you are going to get fringing around objects in the violet end of the colours. However, they are not bad. Especially compared to telescopes of a bygone age. Remember all those pictures of huge long telescopes? The general point there was to reduce the distortion of colours as light passes through the lens. Technical astronomer types call this distortion "Chromatic abberation".

There exists a special breed of refractors called ApoChromatic Refractors. These are FULLY colour corrected. They also attract a price ticket up in the silly end of the scale. However looking at, say, Saturn, in an 'apo' (that's a trendy term) has been known to make grown men weep. Apos also tend to cause very bad cases of that terrible astronomer aliment, ICC - Inflamation of the Credit Card.

Reflectors are the type of telescope many of us end up buying after we have escaped unscathed from the apochromatic part of the telescope shop. Reflectors use a big mirror to focus the light. Mirrors are much easier and cheaper to make than lenses. Thus we get more bang for our bucks. "Bangs" in astronomy parlence generally translates to "inches". I could go and buy an 8 inch reflector for 1000 pounds. I could buy, if I was mad, an 8 inch apochromatic refractor for 15,000 pounds. With my cheap reflector I get a few more optical aberations, but a lot less poeple irritating me at star parties.

The first "serious" telescope I purchased was a 500 8inch F5 newtonian reflector of far eastern origin. I will say this now: I have never regretted this choice. You need to pitch your telescope purchase at the right price point. You may quickly outgrow a Tasco or Bushnell telescope, but some on the Meade or Celestron level will last you much longer.

Mounts

Telescope mounts never fail to amuse me. The reason is simple. Everybody who is indoctrinated into the black art of astronomy will never fail, given the slightest prompting, to tell you that the mount is the most important part of the telescope. Whats so funny? My question is, why does nobody draw attention to this in the adverts and sales pitches?

Well, let me put it in black and white. The astronomers ARE right. The mount is critical. The best OTA on a poor mount will be unusable. A mediocore tube on a decent mount will sing. If you venture into astrophotography, the mount becomes ever more important.

There are fundamentally two types of mount: Alt/Azimuth and Equatorial. The Alt/Az works in a familiar up/down and left/right motion. The sky, however, does not do this. It goes round and round. Usually around Polaris, the North star. Which means an Alt/Az mount is a bloody stupid design for a telescope mount. This is why we have equatorial mounts. An EQ mount will track the sky by moving one axis very slowly (the RA Axis). An alt/az mount must do with tracking with 2 axis. A direct consquence of this is the image rotating in the EP or camera. This makes Alt/Az mounts useless for taking pictures. Happily, and Alt Az mount can be made into an equatorial by addition of a wedge. This tilts the base up to point at the north star. However, decent wedges are expensive, so you'll be better of getting something designed as a EQ mount in the first place.

The critical factor mentioned above is the "very slowly track" business. To make a mechanical device track the rotation of the heavens smoothly enough to take pretty pictures is virtually impossible. All telescope mounts need to have electronic training and optical guiding system to be capable of long exposure astrophotography. Even the really really expensive ones! Almost every mount ever made will exhibit something called Periodic Error. This is a repeating error in the tracking that must be corrected.

The final magic of a mount is GOTO. Almost everybody has a GOTO mount these days. A few button presses (or, more usually these days, a few mouse clicks) slews the scope to the required target without that mucking about with red lights, damp star charts and stiff necks. GOTO scopes vary a lot... Don't expect perfect performance from the word go. Such things usually require a lot of learning and a bit of tweaking.

There is a special type of Alt-az mount. The Dobsonian mount. A Dob (trendy term) is a manual alt/az mount with no tracking. I've never understood the fasination myself, but a lot of poeple get a lot of pleasure from them.

The best advice is buy the best mount you can afford. The best amateur mounts in the world are probably the Paramounts. Other more socially priced offerings are available from Losmandy. A good mount will stay with you for years. The cheap mounts that come with the likes of Tasco Telescopes are not really worth the nasty cast aluminum they are made from.

There is another word of caution - cast aluminum. Many mounts are made from this nasty material - the problem? Well, the manufacturers tap holes in the castings. Then they put stainless steel bolts in them. After a few screws and unscrews your nice tapped hole will be stripped because the casting is very soft.

Any mount that is of oriental origin can usually be improved by taking the whole thing to bits, cleaning off the nasty sticky glue they use as grease, and lubricating it with white lithium grease.

Eyepieces

The amount of money an otherwise normal sane astronomer seems capable of spending on eyepieces is always rather shocking. The eyepiece is the bit of metal with some lenses in which you use to magnify the image in the telescope and view it with your eyeball (which of course contains its own lenses, just to complicate matters).

I only own 3 eyepieces, and I rarely use them, so I'm not really qualified to comment.

Filters

Some astronomers are dreadful horders of filters. They complusively purchase filters whenever the opertunity arises. They are out of control! Filters are thin pieces of glass in a threaded holder that can be fitted to the end of your eyepiece or camera. They filter (hence the name) the incoming light. Some frequencies are blocked, and some are allowed to pass.

Broadband pass filters, such as light pollution filters and colour filters allow a fairly wide spectrum of light through. Narrowband filters (eg Hydrogen Alpha) let almost no light through, expect a very particular frequency. Narrow filters are more useful for astrophotography. Many imagers use a combination of emission line filters to make fantastic weird looking colour images - see here for some great results.

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