I got an astronomy education, but I have never had my own telescope. There always was something between me and the sky: lack of time, money or other similar life problems. But finally I got to the point where I can get a decent one to enjoy the sky. There are quite a lot of people out there who is looking for their first scope, I hope my research will save them some time, because it is really not an easy task to pick a telescope.
First of all I decided how much I can afford to pay for a telescope. Also after doing some initial research I learnt that there is no ideal telescope, it is always a trade off, so I had to decide what is important to me. Because I have never had a telescope there will be some learning involved, so I decided to put astrophotography away for now — there will be enough stuff to pick up without it, but I definitely want to move to it later.
I like reflectors more, than refractors, because they are cheaper, so I decided not to bother with refractors and get a mirror based scope. The most important characteristic of a telescope is a mirror diameter. Larger diameter allows to capture more light and thus see dimmer objects. Also you should keep in mind, that mirrors are round, and amount of captured light is determined by mirror area. Area of a circle is determined by a square diameter, so twice bigger diameter means 4 times more light gathered, that’s why 8″ -> 10″ mirror upgrade almost doubles the amount of captured light and 10″ -> 12″ upgrade increases captured light by ~ 50%.
Beside scope itself some money should be spent on accessories like filters, eyepieces and on telescope mount. As I mentioned I want to move to astrophotography later, so I want to get the most basic mount for now and upgrade it later. I haven’t done research on eyepieces yet, I will write a post once I learn more about them. So far the best fit for my criteria is 10″ Dobsonian reflector, which looks something like this:
XT10 Telescope. Image from company7.com
Tags: astronomy, Stargazing, choosing a telescope, telescope, dobsonian, reflector
July 9, 2009 at 11:26 am |
For viewing, a 10″ dobsonian is a great bang for the buck scope, but it really won’t work for astrophotography without some unconventional solutions (at least for deep sky stuff – you can do lunar and planetary imaging).
July 9, 2009 at 12:47 pm |
What is the main reason for this, Ed? I was going to upgrade a mount later and hook up a camera for taking pictures.
July 9, 2009 at 11:45 pm |
Well, in terms of tracking with a dob, you’ve (as far as I know) got two main options.
The first option is to get some sort of motorized system to move the telescope axes and then track that way. While this can work, long exposure astrophotography with this method will have field rotation since the scope is alt-az mounted.
The second option is to get a motorized equitorial platform for dobsonians. I’ve seen a few of these, but my impression is they aren’t common and I don’t know how well they work. The look sort of like large bathroom scales that you place your dob on. It then sort of moves with the sky. I’m not sure how well these work and I think you could have some real problems getting one aligned well enough for photographic work (my impression is that they are mostly to keep a dob owner from having to track the sky all the time by hand). Heck, good quality alignment is hard enough with a standard equitorial mount.
In terms of remounting in some other way, I’m not sure there’s any standard way of remounting a dob with a more traditional equitorial mount of some kind, whereas many other scopes, you can take the optical tube assembly and remount it on whatever you want. I would suspect that the constructions of dobs pretty much eliminates that possibility (though it’s certainly possible I’ve just never come across it – I’m a realatively new stargazer).
With brighter stuff (the moon and planets, mainly), the camera takes a bunch of short exposures. Since the exposure time is a fraction of a second, the scope doesn’t need to track at all in order to get the image. With deep sky imaging, it’s all about the amount of time you can put your scope on target and keep it there. Longer exposures equal more data (though there are limits to that) and for most things with an 8 or 10 inch scope, you really want to be taking 1-2 minute or even longer images. If your scope cannot precisely track over that period of time (or if your alignment is bad) then you’ll have trailing stars and your images won’t look very good.
I’ve also just realized there is a third option, which would be for you to observe precisely at either the north or south pole. Then, any alt-az mounted scope will track perfectly with the spin of the Earth. :)
July 10, 2009 at 10:23 am |
Thank you for such a detailed reply, Ed, it will help other people a lot too. I also did a little research after your previous comment and so far found only two problems for astrophotography with dob telescope:
1) Tracking the object
2) Coma aberration
Tracking the object is a mount responsibility, so I still hope find/build some kind of mount which will do a good tracking for the scope. Telescope itself is a tube at the end of the day and it shouldn’t matter what kind of mount do you place it on. Here, I even found a link for such monster mounts:
http://www.equatorialplatforms.com/about.our.platforms.html
which makes me think that it is at least somewhat possible :-)
Coma aberration is a defect of mostly this particular telescope type and there is not much I can do about it, but for f/5 scope it should not be a huge problem. Every scope has it’s weaknesses anyways, mine will be coma :-) Also I think it could be corrected with some kind of lenses if really needed.
July 16, 2009 at 8:28 pm
It’s certainly possible to correct coma – there are a number of different products to do that, but none of them are cheap. In terms of remounting a dob on anything other than an equitorial platform, I’ve never heard of it being done and without some osrt of significant strengthening of the tube, I can’t really imagine it either.The images from those platforms are pretty impressive too, but that said, for the same cost, you could get a brand new telescope and mount that would be better suited to astrophotography. Then you’d have two scopes! One to image and one to view visually.
July 16, 2009 at 8:33 pm |
Hey Ed,
I got my first scope already!
http://sciencearound.com/2009/07/11/choosing-a-telescope-quest-ends/
I decided to go for a small refractor as it appeared to be very costly to upgrade Dobsonian for astrophotography.
Small refractor is almost maintenance free and suits well for taking pictures. I will get a bigger scope some time later and will use refractor as a guide scope, so no wasting money here ;-)