What are the advantages/disadvantages of a fast fo

I have used a variety of telescopes for years, but I have yet to get into astro-photography, and I am not sure that I fully appreciate the advantages of a fast vs. longer focal length scope. Granted, the same eyepiece will yield a wider field of view with a shorter length scope, and an a 20-inch obsession at f15 would require a 20+ foot high platform for the observer. But, short of the purely mechanical issues,and the assumed qualitative advantages of very long focal length scopes; What are the arguments for and against a fast scope?

Enrique Barrio said:

I have used a variety of telescopes for years, but I have yet to get into astro-photography, and I am not sure that I fully appreciate the advantages of a fast vs. longer focal length scope. Granted, the same eyepiece will yield a wider field of view with a shorter length scope, and an a 20-inch obsession at f15 would require a 20+ foot high platform for the observer. But, short of the purely mechanical issues,and the assumed qualitative advantages of very long focal length scopes; What are the arguments for and against a fast scope?

Enrique:

I think that you have pretty much caught the essentials of the differences. A fast scope is more compact, is capable of wider fields of view, larger, brighter exit pupils. Faster optics are more difficult to manufacture and they are tricker to align/collimate. They also have more inharent aberrations and require more sophisticated eyepieces to achieve views that are sharp off-axis.

Each design has it's limits. F/4 Newtonians are relatively common, F/4 refractors are quite scarce.

My experience paints this picture: A fast Newtonian (F/4), with good quality optics, with careful attention to collimation, with eyepieces designed to operate at F/4, with a Coma corrector (Paracorr etc) can provide some very good views, not only widefield views but also high magnification views. In a smaller scope, it a lot of expense and effort but in a larger scope it makes for a more comfortable to use scope, (no ladder maybe) and the wider, brighter field of view.

I find it a worthwhile trade off.

Jon Isaacs

Enrique,

I think you won't really appreciate the differences of a short scope until you try it. Don't under estimate the value of a wide-field view.

I was out last night panning the skies with a hand-held 80mm F5 using a 24mm Pan. You can't do that with a long focal length. The views are really different than one gets through a long focal length scope. More like a binocular view only more flexible. Everything is framed much better, relative sizes of objects becomes obvious, and the position of one object with respect to another is much easier to see. There were some objects, like M26, that I have easily spotted with the short tube but have been elusive when I searched using my F12.5 refractor. There's lots to see with the shorty including M11, M26, M27, M33, M31-32-110, double cluster, Auriga clusters, and everything else in between. The Teapot was too low in the wash from the city lights last night but in the summer it is a great place to wander around.

I took the short scope out because transparency was forecasted to be good but there was turbulence and seeing wasn't expected to be good. There was also some wind gusting and my long refractor can get the shakes under those conditions. It was a good night for the short scope and I enjoyed it thoroughly rather than fighting with the big scope for less-than-satifying viewing.

Under better conditions, I would have had both scopes out and stayed all night.

clear skies,
dan

As the owner of an 8"f/8 Eq Newt, I can state with much certainty that finding objects is much easier in a fast scope. Once I've found something, my scope is wonderful, but my scope pointed straight up with the mount working against me and my neck hurting from contorting to get my eye to the finderscope, forget it! That is why my next scope is a 12" f/6 dob (under construction)! The benefits come with planets, It would take a very expensive refractor to equal my views, and I get to use my 12mm where as the other guys are stabbing themselves with their 3 or 4mm eyepieces. Oh yes, Collimation is a breeze as well.
Sean

I think there is one other consideration to account for in my mind it jumps right out when I am thinking about a fast scope vs slower one. Given equal apeture the mount needed to operate a slower scope is more substantial(read heavier) and usually not as easy to move around, so the fast scope has the advantage of being suitable to a smaller mount and easier set up. I do realize this may or may not be a pro/con arguement for some of you, but I think it needs to be pointed out.

depth of focus can become microscopic in fast scopes.... try kicking back and watching planets in an f12 refractor some time.... your eyepieces cooperate easily, and focus is a breeze