Telescope Reviews: Which is best ?

This is one of those questions that comes up all the time and seems hard to answer until you start really shooting a lot of targets.

Unlike a zoom camera lens, every telescope has a fixed aperture (which largely determines necessary exposure length) and a fixed focal length (which largely determines field of view/magnification). For now, let's just worry about field of view.

For a given camera, each telescope will produce a particular field of view. By trial and error, I've found, personally, that any target object which fills between 10% and 100% of the field of view seems pleasing. Any larger object "won't fit" and any smaller object "seems too small to see detail".

So, if you want to own "just one telescope", then you should start to think about the sizes of the objects that you'd like to shoot, and then choose the focal length very carefully. So what size range tends to be universally useful?

I've found (just a rule of thumb, I urge you to do your own research) that there are a ton of fun objects to shoot that are between 1.5deg square and 10arcmin square.

There are a small handful of objects that are larger than 1.5deg; M31, North America Nebula, Veil Nebula, The Rosette Nebula, The Pleiades. There are others, obviously, but the number is small.

There are a huge number of objects that are smaller than 10', also. However, getting enough magnification to make a 10'x10' square turn into enough pixels on your image means a long focal length, and a long focal length will either mean slow focal ratio, huge heavy optics, or both. All of that adds up to "not really a useful set of beginner targets".

How about between these extremes? Well, this is the realm of a *lot* of "famous" targets: M42 on the big end. M51, M63, M64 on the small end. Lots of M16, M17, M20, M27, and M33 spread in the middle for a big happy feast of deep sky pie. You could spend years in a place with pristine skies plumbing the depths of space for objects this size before you really came up for air, looked around, and realized that you needed to go larger or smaller to get something new accomplished.

So, how do you figure out what focal length will put you in the sweet 1.5deg to 10' size range? Here's the formula to calculate your FOV (in degrees) with a given scope/camera:

( 57.3 / focal length ) * size of "CCD" = Field of view in degrees

Note "CCD" refers to whatever you're using for a "chip".
35mm film can be thought of as a "CCD" that's 36mmx24mm for purposes of this calculation.

Why 57.3? It's just 180/pi.

So, say you are interested in shooting 35mm film with a 8" f/10 SCT. The SCT has a focal length of 2000mm (8" = 200mm * 10).

( 57.3 / 2000mm ) * 36mm = 1.03deg
( 57.3 / 2000mm ) * 24mm = 0.68deg = 41'

So an 8" SCT will produce a 1deg by 0.68deg field on 35mm film.

This combination will tend to be a little on the tight side for FOV; a little tougher to guide well to start off, and the field a little restrictive for targets on the large side; M42 (which is about 1deg on a side) will spill off the edges of the field.

Tweak the focal length around until you find one that works well with the camera you've selected, then find a commercially available scope that has something near that focal length, shoot, and enjoy.

So I've totally ignored focal *ratio* to this point. Put it this way. Once you've centered in on a focal length, a larger scope of that focal length will have a faster focal ratio, which means that it will require shorter exposures to achieve the same result. So a 5" f/8 scope has the same focal length (1000mm), and thus *magnification* as an 8" f/5 scope. But *exposure length* is related to the square of the f/ratio, so the f/8 scope (= 64 "exposure units", a fictional unit that I'm using here to demonstrate the exposure length difference) will require almost 3 times the exposure length of the f/5 scope (= 25 "exposure units"). Same magnification, but the 5" scope will require 3x longer exposure than the 8" scope. Makes sense, right?

But of course the 8" scope will be bigger and heavier (but not longer) than the 5", which may or may not cause problems when trying to mount the scope in such a way that it's stable enough for photography.

So:
1) choose a camera (figure out how big its sensor is)
2) figure out what focal length you'd like
3) Choose a telescope with about that focal length that has a focal ratio that you like but that's not too heavy for your mount
4) fun, profit, sleepless nights under the stars.

Good luck.

Best,
Jimbo

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Order of the Unblinking Eye

G-11 D70 ST4 8"f/5 AT66ED (Rig)

I used to shoot film.