Rick's b.log - entry 2015/08/22

Telescope

As you can see from this image, the optics were kind of manky:

This summer I decided that a telescope with dirty optics was no use at all, so I stripped it down until it was an empty tube and a pile of bits.

It was a reasonably decent budget telescope, proper glass at one end and a proper mirror at the other. None of this plastic rubbish you get in supermarket telescopes, or that cheap reflector I saw once that had a shiny plastic "mirror" (!).

The metal parts were cleaned using a damp rag, the mirrors and glass were cleaned using little towels intended for cleaning glasses. They did a great job.

After a time to let everything dry, the telescope was carefully reassembled, taking care to line up the optics so everything appeared to be correctly centred (this task was performed visually).

That part of the electric pole from the first picture? Here it is again:

And here is a picture looking down the scope itself, and you can see how far away the electric pole is.

Reverse angle, looking at the telescope.

For those unfamiliar with the principle of telescopes, the most basic telescope is a long tube. The light enters one end and is focused on an eyepiece at the other end. This, known as a "refractor", is the sort of telescope that you get in supermarkets and unless you are spending over three digits, the quality and optics will be degrees of awful. If the telescope has plastic lenses, it will suffer badly from chromatic aberration - that is when a white star will appear to be red on one side and blue on the other. In really bad (toy store) telescopes, the optics may be so bad that the viewer literally sees three distinct stars (a red one, a white one, a blue one) where only one star exists in reality. If you plan to get a child a telescope, only buy a supermarket model if you intend to put them off of pursuing astronomy as a hobby. And note well - run away from any telescope sold on the power of its magnification. I think mine does something like 20×. There is no way a small telescope that claims to achieve 600× will be any good for anything - the way to get better magnification is... a bigger hole at the front to let more light in and a longer focal length. Without that, any claims of epic magnification are pointless.

This telescope is slightly different. This, called a Newtonian Reflector (guess who invented it) accepts the light in the front. The light entering is bounced off of a mirror at the back of the telescope, focussed towards a smaller mirror in the centre of the glass lens (that explains the black lump with the screw in it). This is another mirror that bounces the light sideways out to the eyepiece. It can be tricky when you first use this type of telescope as you look into the side of it, which means there is a disconnect between where the telescope is pointing and where you are supposed to aim it. There is another disconnect in that this optical arrangement flips the image (which is why the electric pole is upside down, that is how it appears through the eyepiece). To make things easier, it is usual to have a small "finder scope" attached (you can see this at the upper right of the telescope). This is a cheap, simple, lowish magnification refractor with a cross-hair lens. Simply find what you are looking for in the finder scope, make sure it is in the centre of the cross hairs, then look through the telescope proper.

The reflector has benefits. Firstly it demands good optics and properly constructed mirrors, so staying away from eBay tat, even the less expensive models (such as mine) ought to give fairly clear images. The first stage of focussing is performed by a curved mirror, not a lens, which means that chromatic problems do not occur. Because the "image" is folded back on itself and out of the side, the telescope can be much more compact than a comparable refractor. And finally because you look at it from the side (and near the top), you can mount the telescope at a height that suits you. Some people with refractors have been known to take them off of the mountings and lie on a deck chair and just hold them up to look through them (if you are free-handing a telescope, you might start to understand why 600× is a very very bad idea). Note that reflectors can suffer from coma (especially under f5 or so), where stars can appear to have a comet tail, though lenses can be designed to minimise this effect. Additionally, the light collection ability is reduced due to the lump in the middle of the lens. That said, most telescopes are a set of compromises.

There are other types of telescope - a Cassegrain uses two mirrors to act like a cross between a reflector and a refractor (the light is bounced off of the back, off of a smaller mirror in the front, and instead of going sideways, it passes out of the back), and the Dobsonian which is a sort of reflector, only really really big (typically 8 inch and upwards - they are known as "light buckets"), and dozens of other variations on the theme.

If you are looking for a telescope to get started with, I am probably the last person to offer advice, but I will dispense some advice that really ought to be obvious. Your best telescope is never going to be the twenty inch computer-guided Dobsonian with nitrogen-cooled CCD imager and incredible resolving power that causes proximity orgasms even in people that mistake it for some sort of missile launcher. No. Your best telescope is going to be the one you take out and use the most. Huge telescopes need management. Possibly alignment and other sorts of maintenance. Computer guidance will need calibration. Imagers need computers. It's a huge hassle. Sure, you'll take your huge telescope outside and get some incredible photos to throw at Facebook (and risk triggering a planet-wide proximity orgasm), but after a couple of weeks? Meh. Unless you have this stuff fixed in a garden shed with removable roof, you are going to get bored of it. Setting it up, lugging it around. You'll probably even give up going to local astronomical meets as somebody is bound to turn up with an even more epic telescope than yours. So start off with something more reasonable. Have an idea of what you want to do (and don't say photography, nothing land based will ever be able to match a photo taken by Hubble, so don't think it will). We have incredible aids nowadays - with a basic Android phone and Google Sky Map, I can just hold my phone up and it'll tell me what I'm looking at. Another lame reason not to bother learning the constellations...

That said, I can put the telescope on the back seat of the car. I can pick it up and carry it all around the land to find the bit of sky that interests me. The mounting isn't ideal (it is attached to a camera tripod) but I can usefully adjust the legs to cater for uneven ground. And I can set the telescope to a height where I just need to slouch stand and look through the eyepiece.

And see sunspots (with a special Sun lens - NEVER look at the sun using a regular lens or you'll burn out your eye before you know what happened):

Even with the difficulties of holding a mobile phone up to the eyepiece, it is possible to get an idea of how the moon looks. I find it more interesting to look at partial moons as the light throws up some interesting shadows.

Finally, here is a first for me. I saw, just as the last light of day was fading, a bright star close to the moon. I pointed my telescope at it and saw an oblong smear. Tweaking the focus, I was like "no way...".

Yes way.

Let me make that a little bit bigger for you:

That's the first time that I have ever seen Saturn look, well, like it does in the science textbooks. The rings were just the right angle to look impressive. And, okay, the top picture is fairly accurate, it was a tiny tiny quantum Saturn inside my telescope, but then that's about what one should expect with such a model. I bet a huge Dobsonian would offer a much more impressive view, but only if I bothered to set it up and look...

Lightning

Your comments:

Please note that while I check this page every so often, I am not able to control what users write; therefore I disclaim all liability for unpleasant and/or infringing and/or defamatory material. Undesired content will be removed as soon as it is noticed. By leaving a comment, you agree not to post material that is illegal or in bad taste, and you should be aware that the time and your IP address are both recorded, should it be necessary to find out who you are. Oh, and don't bother trying to inline HTML. I'm not that stupid! ☺ ADDING COMMENTS DOES NOT WORK IF READING TRANSLATED VERSIONS.
 
You can now follow comment additions with the comment RSS feed. This is distinct from the b.log RSS feed, so you can subscribe to one or both as you wish.

No comments yet...

Add a comment (v0.11) [help?] . . . try the comment feed!
		Your name
		Your email	(optional)
		Validation	Are you real? Please type 71399 backwards.
		Your comment