Focal length, ratio+fast scopes

[matt c]

Hi there can someone explain to me what is meant by a fast scope, I know how to work out the focal ratio but what does a focal ratio of 5 actually mean in practical terms. Does it just simply mean the scope focus's quicker or does it have an effect on resolving power.

And also does a longer focal lenghth give better resolving power. I'd greatly appreciate anyones help on this thanks people :)

[Cornelius Varley]

The focal ratio the focal lenght of the telescope divided by the aperture.

For example : a 150mm f5 telescope would have a focal length of 750mm and a 150mm f10 would have a focal length of 1500mm. A telescope with a focal ratio of f5 would be considered a fast telescope and the f10 telescope slow. The faster telescope will have a lower magnification for a given eyepiece than the slower one. Fastrer telescopes are more suit to wide field astronomy and slower ones more suited to planetary and lunar observing. The resolving power of a telescope is related to the aperture so both would have the same resolution.

Peter

[matt c]

ok thanks:) i understand now, for some reason i thought focal length effected resolving power cheers:)

[SteveP]

Matt

That's a good question and one that seems to create a lot of confusion.

In simple terms, for imaging purposes first, a faster FR results in shorter exposure times and is therefore regarded as a good thing. For visual purposes however, a faster FR, say below f/6, puts greater emphasis on the quality of the eyepieces due to the steepness of the light cone within the scope. This can be considered a bad thing in the sense that it requires more expensive EP's for visual purposes. Hence the need for some compromise in those scopes which might double up as an imaging scope or observing scope. THe other effect of having such a steep light cone in a fast scope is that the focusing range becomes very critical, ie it's a little more difficult to maintain focus and small movements to the focuser have a large impact on the focus of the image. This can be problematic whether the scope is used for imaging or visual purposes

The FR has little to do with the 'magnification power' of the scope nor the field of view. Both of these are a function of the scope's focal length, not its focal ratio - combined with the EP's parameters

If you want to read more, have a look at the Tele Vue site which has a good description on the maths behind EP design

HTH

Steve

[matt c]

Yeah definately helped thanks steve it's just when you first start out it seems the only important thing is aperture but it becomes more complicated than that the more you learn and look into it. thanks again :)

[ottUp]

A few other miscellaneous points on the differences:

Fast scopes:

• Shorter exposure times for imaging (this is the origin of the word "fast")
  
• Less depth of field, so smaller range for focuser movement (focus seems to "snap in")
  
• Higher chromatic aberration in achromatic refractors
  
• With certain optical designs and certain eyepieces, more chance of various kinds of distortion at edge of field
  
• Physically shorter for given aperture (for dobsonian reflectors, this means not having to stand on a ladder when pointing straight up)
  
• Shorter focal length for given aperture means lower magnification with given set of eyepieces compared to slow scope
  

Slow scopes:

• Longer exposure times for imaging (origin of word "slow")
  
• More depth of field = more focuser movement, location of "perfect" focus less obvious
  
• Lower chromatic aberration in achromatic refractors
  
• Less field distortion, more forgiving of various eyepiece designs
  
• Physically longer for given aperture (can make mounting more challenging, can place eyepiece very high or very low for certain designs)
  
• Longer focal length for given aperture means higher magnification with given set of eyepieces compared to fast scope
  

A handy rule of thumb: a scope's maximum useable magnification is achieved with an eyepiece whose focal length is half the scope's f/ratio. So an f/10 scope will give maximum useable magnification with a 5mm eyepiece, an f/6 scope with a 3mm eyepiece. (This is just a trick of algebra by taking the formula for f/ratio and the formula for magnification and letting all the terms cancel.)

Except for the possibility of aberrations, no difference to the resolving power. Resolving power is a function of the diameter of the objective. (Interesting note: "diameter", not "aperture" although for a single scope this is the same thing. This is why multiple scopes - usually radio scopes - can be combined for higher resolution - the "diameter" of the simulated scope is the distance between them, while the "aperture" is only the sum of their apertures.)

Regards

- Richard

[matt c]

thanks aswell richard :)