Expert: Tom Whiting - 11/7/2010

Question
I have been doing some research to determine what scope I should buy.  My first impression was that a larger mirrror will collect more light, provide me with higher magnification ability and still give me a good field of view.  I am confused due to a recent article that I read which basically says that since the focal length on a large mirror scope, say 12 inches, is around f5 it is not as good a tool for viewing deep space objects. One should consider f10 or higher for this.  Sounds like a smaller mirror would be more practical but what about my original premise. I am confused. Thanks for your time.

Answer
Hi Walt,
Yes, a larger mirror provides more light and a 12 inch is our desirable minimum size for seeing detail in the Deep Sky Objects... (BUT not necessarily more magnification). With Deep Sky objects, you're after the maximum light gathering ability, and not magnification.
No, the article is incorrect... in fact, most of us like the f5 focal ratio with our 12 inch scopes, because that gives us a nice wide field
of view (say, almost 1 degree at a low power 40 mm eyepiece).
If you had a 12 inch f10, then you'd have double the magnification which is nice, BUT you have an enormous sacrifice of field of view, probably only 0.2 or 0.3 degree with same 40 mm eyepiece. AND its even a smaller FOV with even higher power eyepieces!  Since most DSO's are in the order of 1/2 degree or even bigger (Andromeda Galaxy is a full 3 degrees across) FOV is much more important than magnification for DSO's. Keep in mind, with any telescope (or microscope) when you go up in magnification, you lose 3 things; light, detail, and FOV... the image is dimmer, it's not quite as sharp, and the FOV shrinks dramatically. (Just the optical laws of physics).  So we who are into DSO's prefer to maximize the 'other 3' at a sacrifice of only one factor, namely magnification.
People into lunar/planetary detail prefer the reverse, so they like the 12 inch f10, where they can achieve super-magnification on a very very small spot in the sky, like 0.1 degree across... to magnify the smallest details on the moon and planets. So they prefer the longer focal ratios like f10 or bigger. The longer focal length (120 inches in this example), gives them double the magnication with that same 40 mmm eyepiece, but at a horrible cost of "the other 3" factors.  
The nice thing with the 12 inch f5, you can insert an aperture stop (also called an off-axis aperture mask) say, down to a 3 inch hole, so you've converted your 12 inch f5 with a 60 inch focal length, to a 3 inch f20 (60/3) thereby increasing resolution by 200% and thus enjoying the best of both worlds.  You can always "reduce" your aperture size with an aperture stop, but the guy with say a 4 inch f15 refractor used on say planets, has no way to "increase" his size or light gathering power.  Personally, I prefer the large mirror aperture with the f4.5 focal ratio, then use an aperture stop for the planets and splitting very tight double stars. In fact, I've never used any other type of scope for serious observing!
And if you are fairly new to astronomy, take 6 months to learn the night sky and then you won't need all that GO-TO and computerized motor crap... for an extra thousand dollars or so. And put the extra money into an even bigger mirror! Clock drives and equatorial mounts are only for those that want to do photography thru the scope, and that's another whole bag of worms, plus another extra $2000 or more.
(And who out there can beat the Hubble, or the twin Keck 10 meter scopes)? No one.
In fact, if your pretty new to astronomy, it may be worth your while to punch up our club website, and punch on "Tom Whiting's Sound Advice for the Novice Astronomer", at
http://www.velocity.net/~bwhiting
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA