Focal Length value for platesolve and FITS header ?

[Tom°Dresden]

Hello,

there are 2 different platesolve tools: One for polar alignment, and one for main scope platesolve.
Is it right that polar alignment don't use the focal length value set in SC setup, it is a separate platesolve tool and don't need a focal length ?
SC setup focal length is used for main scope platesolve. BTW, I saw that the new SC internal platesolve seems to be very tolerant, I get a fast solve even with 50mm focal lenght in setup and a 300mm scope.

There is a focal length written into FITS header of captured images, where does this value come from ?
If I do a platesolve, then it is the exact value detected, and if I do no platesolve, it is taken from value in SC platesolve setup,
is that right ?

Thanks and CS
Thomas

[admin]

Hi Thomas,

you are right that the focal length setting is *not* used for the polar alignment plate solving. Really, having the focal length is a way to speed up plate solving by limiting the amount of searching needed, but for polar alignment the search is already limited to a small area around the pole, so using a focal length would reduce maybe from 10ms to 2ms, but no-one would notice the difference!

SharpCap's internal plate solver is also (as you spotted) pretty forgiving on this - it only uses the focal length to cut out the search for very fine scales (small fields of view), so it will practically ignore the focal length unless you are using a small field of view and also set a short focal length.

The FOCALLEN header is taken from plate solve data if available (in which case it will be exact) - otherwise it comes from either the ASCOM telescope mount driver or the SharpCap manual focal length setting.

cheers,

Robin