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Focal Ratio Question

Started by randallrp, 05/06/2002 01:59AM
Posted 05/06/2002 01:59AM Opening Post
Hello All

I have been ready lots of posts, most recently another excellent one by Mark R. and I am wondering about how focal ratio (FR) relates to deep sky performance. I undestand the effects of the central obstruction as it relates to the MCT vs MN. and also understand how the FR relates to field of view.

However what I am wondering is does a shorter FR scope perform better on deep sky because it "sees the light" faster or better than a longer FR or is it mainly because a wider field of view issue.

Thanks
Randy
Posted 05/06/2002 04:26AM #1
A shorter focal ratio will yield a wider FOV(field of view)A long focal length will be a smaller FOV at higher power.Short focal is usually better for deep sky because you don't need high power as much as you do aperture.Many deep sky objects don't need lots of magnification.You can get some nice views at low power of deep sky objects.But a short focal length scope is usually not going to take a lot of power unless it is high end.There is no better view of deep sky objects than a large aperture-short focal length newtonian or dob.For planets and high power,a long focal length is usually better.hope this helps.
Posted 05/06/2002 07:36AM #2
It is just a lot easier to design a fast scope to be capable of wider true fields of view. A slow scope can sometimes match, but the backend situation can get out of hand, e.g. you can get about a 2 degree field with a 6" f/5 apochromat using 30mm Ultima 1.25" eyepiece. A 6" f/9, on the other hand, would require a 2" 55mm Plossl or some such to get into that neighborhood. A 6" f/15 would require a 90mm/52degree eyepiece with a barrel no smaller than about 3.3" in diameter. Of course most reflecting designs are incapable of illuminating the required 80mm-wide image plane at f/15.

The speed thing is just derived from photography since the prime focus image plane of a faster scope is served up at a lower magnification than that of a slower scope and hence is brighter. Visually and for afocal photography, there is no brightness advantage one way or the other once magnification is equalized with appropriate eyepieces.
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Mike