What is considered to be a "good focal length" for a Reflector Telescope?

Question

What is considered a good focal length on a Reflector type telescope? Does it have any relation to the optical diameter (ie. 114mm/4.5", 130mm/5", 150mm/6")

Example:

(OPDia 114mm/4.5" - FocalLength 500mm/20")

VERSUS

(OPDia 130mm/5" - FocalLength 1000mm/39")

VERSUS

(OPDia 150mm/6" - FocalLength 1400mm/55")

Eyepieces being equal, using Moon or Orion's belt as reference...which would be better? (I hope that's a better summary question.)

Answer 1

As Tina says, you buy telescopes by aperture (what you seem to be calling, incorrectly, "optical diameter"), not focal length. All things being equal, the one with the largest aperture should be the best.

However, when I see a 150mm reflector with a focal length of 1400mm, alarm bells go off in my head! I know of only one telescope with these specifications, the notoriously bad eBay 6" "short-tube" reflector. It appears on eBay (and possibly other places) under various brand names, but it always has a 1400mm focal length. DO NOT BUY THIS TELESCOPE!!!! It is one of the worst designs ever made and marketed. In fact, don't buy _any_ telescope from eBay unless you really know what you're doing; most of the scopes there are total junk.

Here are a few web pages with good information on beginner's telescopes:

http://www.gaherty.ca/tme/TME0702_Buying...

http://www.scopereviews.com/begin.html

http://observers.org/beginner/j.r.f.begi...

For more advanced information, read Phil Harrington's Star Ware, 4th edition (Wiley).

You'll get the greatest value for your money with a Newtonian reflector on a Dobsonian mount, such as these:

http://www.telescope.com/control/categor...

http://www.skywatchertelescope.net/swtin...

Buy from a store which specializes in telescopes and astronomy, either locally or online; don't buy from department stores, discount stores or eBay as mostly what they sell is junk. Find your local astronomy club and try out different telescopes at one of their star parties:

http://www.skyandtelescope.com/community...

Answer 2

Be aware that if you go below f5 you will definitely need coma corrector or baader mpcc. No way around this. All newtonian reflectors will have Coma (distorted stars) at the edge of field, getting worse the lower the f-number goes. This adds cost. Also below f5 it collimation becomes very very tedious. This will do your head in if you are not used to reflectors.

I find f5-f6 to be perfect + it is helpful when looking at planets & moon.

Whatever you do make sure that the telescope you go for has parabolic mirrors - some cheaper makes on fleabay use spherical mirrors which will hugely degrade performance.

Answer 3

If you divide the aperture into the focal length, that will give you the f-number or focal ratio.  The focal length is a general indication of power or magnification, but the longer focal lengths come at a cost of light gathering ability.

The focal ratio usually dictates what kind of observing the scope is best at.  Low focal ratios, (fast telescopes), are good for deep sky because they have a high light gathering to magnification ratio.  Big focal ratios are the opposite.  I'll calculate them for you.

First: f-4.4

Second: f-7.7

Third: f-9.3.

Many people buy reflectors up to f-11.  But these are really better for things like observing the moon and planets -- relatively bright objects that you can magnify without any significant loss of image.

If you are more into the deep sky observing like nebulae and galaxies, then you would want something between an f-4 and a f-6.  The bigger aperture is always more desirable, but not as good for deep sky if it has a big f-number.  An f-9 at 1400 mm is probably better than an f-7 at 1000 mm.  To some degree, you can adapt for different types of observing through the choice of eyepiece.  Now if that bigger scope was an f-6, I'd have no trouble choosing.  But I'm more of a deep sky person.

Answer 4

you buy telescopes by aperture, not focal length.

slower telescopes can have higher optical quality. faster telescopes are either very expensive or have mediocre optical quality. fast scopes require very precise collimation and fancy eyepieces for the best results. slow scopes require neither.

slow telescopes are physically larger than fast ones. i remember seeing pictures of a 10" f/12 scope somebody had made for planetary observing. it was a monster!

what, exactly, are you looking for? since you've already looked through other astronomer's telescopes, you must have some idea?

Answer 5

Personally, I like a 60-inch focal length.  It's long enough that you can get high power views without having to resort to Barlows and extremely short focal length eyepieces, and it's short enough that you can get decent wide field views without having to resort to extrememly long focal length eyepieces.

A Dobson type scope with a 60-inch focal length puts the eyepiece at my eye level when the scope is pointed at the zenith, so I never have to use a ladder to reach the eyepiece.  In fact, I can view most of the sky while seated.