Fast Scope V Slow Scope?

[Dom1961]

What is the difference between fast and slow scopes and what are they each better for?

[nightfisher]

this is the focal length, a slow scope is considered better for planets and solar system, a fast scope better for deep sky

[damnut]

Hi There,

Fast and slow relates to the relationship between aperture  and focal length; this is called focal ratio and it is this that is fast or slow.  anything lower than f6 is considered fast. anything with f6 and higher is considered slow.

The faster the optical system, the more demanding it will be on eyepieces and therefore sometimes (but not essential) you need to spend more to get the same quality of view.   (don't worry about this).

Slower scopes are often stated as having more contrast and detail on planets etc and this is somewhat true but more aperture will actually give you more detail on the best nights. You really only need to be concerned about scope speed for astrophotography.

[triton1]

As what damnut said above but with a fast scope you do get a wider field of view but why they make a mini dob that you can put on a table top at f4 beggars belief.A eyepiece would cost as much as the scope!.I use an f5 and the bst explorers work ok but slowly I am upgrading.The best all rounder I would think is a 150 or 200p dob at f6, the sponsors of this site at top of page have them in.stock at reasonable prices.You could always ring them and the advice they give will be sound.

[LukeSkywatcher]

this is the focal length, a slow scope is considered better for planets and solar system, a fast scope better for deep sky

I have to say that my 90mm (F11) refractor scope gave me the best views of the planets ever. My F5 scopes give me great views of the same planets (because they have bigger apertures also), and they give outstanding views of DSO's.

Curious question here about my 8SE. I was under the impression that it was an f5 scope. Online it says the focal ratio is "10".

Can anyone explain?. I know a bit about the technical side of the optics and in such a short scope the light coming in bounces twice before coming to focus and giving the scope a fl of about 2032mm?.

I'm puzzled now.

Maybe the double bounce of light makes it an F5?

[Owmuchonomy]

As already quoted f ratio is focal length divided by aperture. Your 8 SE is therefore close to f (ratio) 10.

[rowan46]

This explains it better than I can Paul http://starizona.com/acb/basics/equip_telescopes_scts.aspx

[ghostdance]

why they make a mini dob that you can put on a table top at f4 beggars belief...

Perhaps a little off topic but a word for those little guys! The cheeky impertinent and fast 'table-tops'...

My first scope was the mini dob table-top Orion Starblast 4.5" F4 Why? Well, lessee....ulltra easy to use for a novice (so easy that I'd be set up for viewing within minutes *every chance* I got) with very quick cool-down, light, portable, stowable, nice wideish view but still a good intro to Jupiter and Saturn, and of course the Moon, no faffing about with a tripod, pretty good optics which married well with Vixen NPLs ( hardly costly glass!)...oh, and cute as all get out. This little mini-dob was imho a great introduction to telescope stargazing, taught me lots...

Sure, you had to find a stable base if you didn't want to be sitting on the floor ( I certainly didn't) but that was a heck of a lot easier than affording a wobble-free tripod/mount, and the focuser was not the best... but that was all part of the fun! And I honestly think had I gone for the usual 200p (despite that nice hassle free base, and to be sure a fine scope) that I would have thought twice about lifting it out and down the stairs as often as I picked up the baby table-top Starblast...

The defence rests.

:)

[Avocette]

Curious question here about my 8SE. I was under the impression that it was an f5 scope. Online it says the focal ratio is "10".

Can anyone explain?. I know a bit about the technical side of the optics and in such a short scope the light coming in bounces twice before coming to focus and giving the scope a fl of about 2032mm

The focal length of a Schmidt/Cassegrain or a Maksutov/Cassegrain is much longer than simply the (folded) light path would measure. The secondary mirror is a convex one so the apparent length of the light path from the eyepiece via the secondary to the primary mirror is several times the physical distance. With my SkyMax 150 Pro, the physical distance from the primary mirror to the secondary is not much more than 300mm but the convex nature of the secondary makes the eyepiece to secondary mirror distance amplified by around five times so the total focal length is 5x300 for EP to secondary + 1x300 for secondary to primary adding up to a total of 1800mm.

[LukeSkywatcher]

This explains it better than I can Paul http://starizona.com/acb/basics/equip_telescopes_scts.aspx

"For observers who want to be able to do a little of everything, from observing to photographing, SCTs are a great all-around telescope.  They are very portable (at least in 11" and smaller sizes), and easy to setup and use.  They often have the latest and greatest features in terms of computer systems and other high-tech gadgetry.  If your only interest is in visual observing, you can get a larger-aperture Dobsonian for the price.  But most observers have varied interests and that makes the Schmidt-Cassegrain a very popular choice."

That's my scope. It does a bit of everything very well. Not tried imaging with it yet. Probably never will.

Cheers

Paul