Re: [APML]: Guiding Error Question

[Chuck Vaughn <aa6g@aa6g.org>]

charles@jon.cjfny.org wrote:
 
> Does anyone know the maximum error (in seconds) before stars appear
> elongated on film for various focal lengths? I'm interested specifically
> in 300mm and 180mm.

It's difficult to give an exact answer to this question because it depends
on what is the smallest star diameter that those lenses can deliver. It
also depends on how many times during the exposure you exceed the tolerance.

I can tell you what an absolutely safe number would be based on 12um stars
being the smallest star diameter I've seen using an H-alpha filter and
Tech Pan. The number could be larger though.

4126/FL (mm) = Star size in arcseconds for 20um stars and 12um stars would
be 60% of that.

(4126/300)*.6 = 8.25"

(4126/180)*.6 = 13.8"

You need to guide to at least half the size of the smallest stars to
be sure they are round so you're looking at 4 and 7 arcseconds respectively.

Chuck  <aa6g@aa6g.org>