Monday, October 31, 2005

Buying a Telescope

Someone asked for advice on how to buy a telescope and what to consider in the process, so I thought I would help out and write up a little guide based on the few things I discovered as I went through my own investigations, and using the lessons I learned to best advantage.

I don't know everything about them, but perhaps I know a little. I will pass on that little bit.

Rule # 1 Do NOT buy a cheap telescope.
It is a waste of money. Generally speaking, if you are planning to spend less than $600, then just get a nice pair of binoculars, and use those. You will find it a lot easier to see things, and find things. Cheaper scopes tend to shake a lot, and you can never focus them, and the lenses are not accurate enough to capture any detail, etc. They will drive you crazy when you try to find things and focus on them. Just don't bother. Honestly.
There are two big brand names in telescopes. Celestron and Meade. Then there are a lot of smaller name brands. From what I have heard and read, these two are roughly equal in quality and price.
Mine is a Celestron Celestar 8. Essentially the common misconception about telescopes is that the bigger they are, the more they magnify the image. This is false. (Why is it that so much in this world is always counter-intuitive??). The magnification is entirely produced by the eyepiece. However, the more you magnify an image, the more light you need to supply in order to see it. And that is what the telescope does. It captures the light and focuses it into the eyepiece. A larger diameter telescope captures more light, therefore it allows you to use a more powerful eyepiece to magnify the image more to see more detail. With most telescopes, there is a small telescope, called a 'finder scope' attached to the side. This allows you to easily find what you are looking for and 'aim' the main scope. These obviously have to be exactly synchronized with the main scope. Most sessions start with synchronizing them by pointing them are terrestrial targets. Something not moving.

There are 3 main different types of telescope to get. There is the classic style, called a Refractor, which is long and narrow, and has one main lens at one end and the eyepiece at the other. Think Captain Bly. These are either cheap and useless models, or they are impractically large and far too expensive, and awkward to use. Pass.
A Newtonian (also called a reflector telescope) is a large, long tube with an open hole at one end, and a mirror at the other end. The eyepiece is sticking out the side near the front. The light comes in the open hole, travels down the tube to the mirror, then bounces back up into a prism which reflects it sideways out through the eyepiece. This type is typically less expensive, and therefore allows you to get a larger diameter for the same price. However, they are bulky, and the open end leads to dust inside which intereferes with quality.
Then there is the Schmidt-Cassegrain style. This is the one that most people get when they get even a little serious about it. This has a shorter, fatter cylinder, with a glass lens on the front, and a solid center section in the middle of that lens. The light comes in that lens, travels down the tube, bounces back from the sealed mirror at the back, up into another mirror embedded in that center section in the front lens, and then bounces back again to the bottom again, this time focused through the center through a prism and an eyepiece. By bouncing the light twice, it gives the effective power of a telescope three times as long, without having to actually be that long, It is much more compact. (for sound engineers, this concept is the optical equivalent to a 'folded horn' speaker enclosure)

In all cases, the key in terms of the 'power' of a telescope is the diameter. A 4" or 5" diameter is fairly small and will only capture limited light. You will see the moon. You might see a couple of planets. You will not see any DSO's (Deep Space Objects). Very little of interest. These are essentially toys. They are meant to appeal to well-intentioned parents to buy for their kids for Christmas. Usually the kids try to set it up, get frustrated, leave it, and never go back to it. Save your money. Pass. Then there are the 8" size. These are good. You can capture quite a lot with that. DSO's, planets, nebulas, clusters, lots of stars, details on planets and their moons, etc. (instead of just a white dot) Then there are 10" size. These will capture slightly more things. Nebulas are easier to see, etc. Then it goes into 14", 18" 21", etc. These are for institutions. They are tens of thousands of dollars typically.

Weight is a significant consideration. An 8" and it's stand will weigh somewhere between 60 and a hundred pounds to carry outside to set up and use. A 10" and it's stand may be 140 lbs or more, It is considerably more effort. Anything larger than that is a two-man job and a serious bit of work Like picking up and carrying a big-screen TV. You will want those to be permanently mounted in a private observatory. That's a whole different level of expense and commitment then.....

So really, practically speaking, the best size is either an 8" or 10". Preferably an 8". You must have a motorized one. If it has no motor, then walk away. It is useless. Consider that when you are focused on an object in the sky with any serious magnification at all, the Earth is turning. That star or planet will scroll out of your field of vision in roughly 2 seconds. Perhaps less. That makes it , for all practical purposes, literally IMPOSSIBLE to find anything. It always takes more than a few seconds to find what you are looking for. Trust me on this. And when things are constantly sliding around on you at the same time, you WILL want to give up. As for motors, there are two kinds. 1) A right-ascencion motor (RA motor). This is the one that tracks the Earth's movement, and counters for it. It allows an object to stay in the scope. You have to start by orienting the scope to the north star, and then the RA motor is aligned with the direction of spin of the Earth. (that is a little trickier than it This is a basic requirement on any decent scope. 2) A Declination motor. This is optional. This allows for up and down motorized movement of the scope in the 90 degree opposite direction as the RA motor. (north/south) The combination of both motors allows you to have those digital finder systems, where you punch in the coordinates RA XX.X, DE YY.Y, and it finds your star for you. That's nice. I don't have that on mine. I just have the RA motor. The rest I do myself.

Things to look for: A good STURDY stand. I cannot emphasize this strongly enough. If the stand is not rock solid, then the slightest breeze will blow on your scope and shake the image and make it blurry and ruin all your hard work finding things. A vertical adjustment. I didn't get this and now I find it is sometimes hard to get into position under the scope to see things. That would have helped a lot. Optics. Extremely important. Basically, if you get Celestron or Meade, you're ok. They both have excellent optics. I cannot say about any other brands.

Prices: Mine, with an 8" Schmidt-Cassegrain Celestron, with an RA motor and a sturdy stand, but no vertical adjustment, was about $1199. Then with the extra eyepiece, and a barlow lens (this is a 2X multiplier) , and a couple of filters, etc. It got to about $1500 or $1600 I think. That's a very basic price for a decent telescope. If you want one with the digital finder system (both motors, etc), in the same size, then you are looking at roughly $2,000 to about $5,000. The 10" will be significantly more. Mostly, they like to sell the ones with the digital finder systems. More money for them, of course. But I frankly wasn't into it to the point that I wanted to spend that much money. $1500 was my limit. For the same model with the digital finder system at that time, it was $3300. And then the extra eyepieces and filters, etc. Way too much for me for the degree I wanted to get involved. I don't regret that now. It's been great to have fun with once in a while, but I don't use it nearly enough to make it worthwhile for that.

One cautionary note: The moon is very bright when looking through a telescope. Use colored filters, or risk damaging your eyes permanently. Another hint, take the scope outside a couple hours before you want to start using it. Let it aclimatize to the outside air first. Otherwise you will get fog on the lens and mirror and it will be impossible to focus. Also, turn off the lights in the house. Get your eyes used to the dark. One more, take a compass. It's handy when aligning it to exact north.

One more thing, you will need to be able to find the north star by yourself. To do this, find the big dipper. Take the two outer stars on the cup part of the dipper. Draw a line between them and extend the line out straight, above the cup. That line points directly at the north star. That is the one star that will not move. Everything in the sky rotates around that. You will need to align your scope with that.

Another thing, all the planets are on the same ecliptic plane as the sun and the moon. They all rise where the sun and the moon does, and set where the sun and the moon do. Draw a mental line across the sky from where the sun rises to where it sets. When looking for planets, look along that line.

Oh one last thing - get a software program. There are lots. Some are cheap, some are expensive. In this case, the cheap ones are probably good enough. Save your money for the scope. I think the one I use is Observatory Gold. It works nicely, and wasn't too much money. Plan your viewing ahead of time with that, and use it to know exactly where to look for what you want. It's fun.


At 12/11/2005 8:28 PM, Anonymous RobertG said...

If you own a Palm device, I would recommend the Planetarium for Palm.
This is amazingly good software. The manual includes detailed instructions for connecting the Palm device to a telescope.

At 6/08/2006 9:59 AM, Anonymous igor said...

may I chip in my advice?

if you are just starting your astronomy hobby - I recommend you rent/borrow your first telescope.

thus you can get your hands on better instument than when coughing up the full sum - so you'd be in good position to try and see whether you like it (stargazing thru 'scope) at all.

the common grievance is that in reality what you can see thru 'scope with your own eyes is far much less spectacular than Hubble pictures from NASA website.

i'd bet 90% of all first-time telescopes rot in basement after being used a couple of times. (sigh, I did that mistake myself...)

At 6/10/2006 11:10 AM, Blogger Val Serrie said...

I guess my earlier long response didn't make it on to here.

Igor, I agree with you. Most people who get a telescope lose interest in it quickly when they find out that it is not what they expected. Saturn, Jupiter, and the moon are interesting to look at. Everything else is just white dots with no detail.
Only cameras can 'see' color, because they have long exposure times. So if you think you are going to see the Eagle Nebula in full color, forget it.
Also, the viewfinder only allows a very limited field of vision. Not like a TV screen or a page in Astronomy magazine.

My telescope sits in my study, looking like a prop for the "thinking man's study.". I don't think I've actually taken it out and set it up in about 4 years now.


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