Fast Telescope...? :s

[collangm]

Newbie question obviously, but what is actually meant when someone says I would need a 'fast' telescope?

Gary

[libraryman]

i am no expert but generally speaking telescopes with a focal ratio of less than f/6-f/5 would be considered fast, i use a fast Reflector of f4.4 which gives a widefeild of veiw, the higher the ratio the smaller the feild of veiw is generally the the result for a given set of eyepeices etc.

perhaps other comments would give you a better idea.

Ray

[airprox]

To add to what Ray has said, and again, I'm no expert, to determine the focal ratio, divide the focal length by the aperture. So my ST120 has a focal length of 500mm and aperture of 120mm, so 500/120=f5.

Hope that helps a little.

[Luke]

Hi Gary,

"fast" telescopes can be important if you want to do some deep sky imaging. A fast telescope will need a lot less exposure time than a slow telescope.

If you have a fast F5 telescope, in theory it takes only a quarter of the time to take a photo that a slow F10 telescope takes.

I'm still fairly new to imaging but if I started again I would start with a fast scope. I tried with a slow F10 scope first and it was a lot harder, I needed longer for exposures, I had a narrower field of view so any inaccuracies in tracking the object during the exposure were more of a problem. Because the images were not exposed for long enough, I had more problems with noise, etc.

[Ad Astra]

The idea of 'fast' and 'slow' originally came from photography. A lens with a focal ratio of less than f/4 was considered "fast" because it required less exposure time for the same photo as a "slower" f/8 or f/11 lens.

Telescopes can be thought of a being no more than giant, fancy large aperture lenses for the camera when we do astrophotography and so we also say "slow" and "fast" with regards to scopes.

Rule of Thumb: Less than f/5 = Fast.... f/6 - f/10 = mid-range.... over f/10 = Slow

Long, slow refractors of f/11 to f/20 used to be much more common, and fast scopes of less than f/8 were very rare until new glass technology and multi-element apo and other fancy lens designs allowed fast refractors. Fast reflectors lagged even further behind, because grinding a very fast mirror produced lots of coma (optical distortion that gets worse toward the edge of the star field). Modern 'corrected' designs offer very fast scopes with very flat fields - at VERY expensive prices! Celestron's HD-EDGE, Meade's ACF, and offerings by Planewave, and others show this.

Hope that helps!

Dan

[collangm]

Thanks Dan. I had been considering buying a Meade LX90 ACF SCT type scope but the jury seems to be out on whether the scope and mount are suitable for both DSO and planetary imaging/viewing. Do you have any experience of this scope or similar?

Gary

[brianb]

I had been considering buying a Meade LX90 ACF SCT type scope but the jury seems to be out on whether the scope and mount are suitable for both DSO and planetary imaging/viewing.

The 8" or 10" are fine for visual work (planetary & DSO) or imaging planets with a webcam. The mount is not adequate for long exposure DSO imaging, even with a wedge. The 12" is too heavy for the mount, the LX200 mount is considerably sturdier (and a lot heavier).

[xboxdevil]

One other thing to bear in mind is that the light cone in a fast scope is steeper than that in a slow scope and needs bending back more in the eyepiece, so in general faster scopes need better quality eyepieces and have a tighter focus point than slow scopes.

Another consideration is that the collimation sweet spot on a fast scope is much smaller than than on a slow scope, no big deal when you know how to collimate, but could be a pain if you don't!

Good luck with whatever you decide to buy.

[dph1nm]

When talking about CCD imaging, the correlation of fast/slow with optical f-ratio is somewhat tenuous, as pixel size can be an over-riding factor. So an f10 scope with a CCD with 9um pixels is the same 'speed' as an f5 scope with 4.5um pixels. So which f-ratio is 'fast'!?

NigelM