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Your First Telescope
"How big is too big?"

by
Ed Ting


Avoiding "Aperture Fever"

Audiophiles have a saying that goes something like this: The stereo system which reveals the most music to you is the one you use the most.

For most of us, that's our car stereo.

Astronomy is a lot like that. The probability that a telescope will be used is inversely proportional to its size. This seems to apply to just about everyone, regardless of experience.

I've carried on a correspondence with a fellow astronomer. He has an 18" "Luxo-Dob", I have a tiny TeleVue Ranger. Our conversations tend to go something like this:

Me: So, did you see Saturn last night?
Him: No, it was too cold out to go observing.
Me: Oh.
Him: But my 18" dob blows your puny little 2.7" refractor out of the water, you teeny dweeb!

OK, so maybe it doesn't go exactly like that, but you get the idea. Sure, he's got me whipped on aperture, but I got in 2.7" worth of observing that night, and he got in zero.

Little scopes get used more often, and thus show you more. Your Luggability Tolerance may be different from mine, however, and that's where visiting public star parties becomes an invaluable experience.

The star parties also come in handy when I DO want to look through a big scope. I just look through someone else's. This way, I get my share of "big gun" observing time and I don't have to deal with the hassle of set up and break down.

One more issue on this topic, and then we'll move on. Large aperture scopes are more easily affected by sky glow (the light pollution thrown up into the sky by civilization.) Sure, a 12" scope will gather more light than a 6", but the 12" will also gather a lot more sky glow, giving a yellowish, brown tint to your images. In severe cases, you won't be able to see anything through the brown, muddy glow.

In other words, there is an aperture limit, at any given observing site, beyond which you will not be making any further gains. You'll just be capturing more sky glow. In most suburban back yards and driveways, that limit is right around 6". In my driveway in rural New Hampshire, that limit is closer to 10". If you have access to some really dark skies, count yourself lucky. A large aperture scope can richly reward when you take it there.

Update, 12/99: I have recently learned to dodge this "bulk" issue with larger telescopes by placing them on rolling platforms with lockable castors. Go to your local lumber yard/hardware store and get 3/4"-1" plywood and four castors. Don't skimp on the quality of the castors; get the good ones. Also remember to use lock washers or the nuts will eventually work themselves loose.

I can leave the 20" Obsession fully assembled in my garage, and when I want to use it I just roll the whole thing outside. I can be observing in less than 5 minutes. There is something rather nice about kicking a 20" Dob out the door and observing with it within a couple of minutes while your friends are still assembling their small equatorial mounts. This works so well, I built platforms for the rest of my scopes as well.

[Beginning] - [Next Page - Don't forget some good Eyepieces]



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