Hey.

Does anyone know, how much if any - effect focus has on the curvature in CCDInspektor2.1.6?

We are two that, have a discousing about that. Maybe we are talking about two different things, but does a bad FWHM affect the curvature messure in CCDInspektor and is so how much?

We have a picture of Epsilon Cephei, with a FWHM of 5.05" to 5.45" taken with a Canon EOS 1000D on a 8" f/5 newton with Baader Comacorrector.
From this picture we get a curvature of 12.8%, one says that the high FWHM is the reason for this. The other is sayning, that no matter what FWHM (within reason) the curvature will be the same..

Any insights?

Link to CCDInspektor Screenshoot:
http://epsilon.flight.dk/Pictures/0016/Curvature.JPG
Link to MaxIm DDP prossed picture (without sharpen):
http://epsilon.flight.dk/Pictures/0016/DDP.jpg

Defocus is a non-linear function applied to the star image as produced by the optics, the atmosphere, and the tracking errors. In general the defocus function will be combined quadratically with the point spread function (PSF) to produce a larger FWHM.

Say, for example, that the defocus is a gaussian blur function with FWHM of 2 arcseconds.

Now, let's say that in perfect focus, the system produces a 20% curvature with the center FWHM of 4 arcseconds, as measured by CCDInspector. Center FWHM is 4", then edge FWHM is 4.8".

What will the curvature be after the defocus of 2 arcseconds?

Center FWHM after defocus = Sqrt(4*4 + 2*2) = 4.5 arcseconds
Edge FWHM after defocus = Sqrt(4.8*4.8 + 2*2) = 5.2 arcsecs

So, after the defocus, the curvature is now: (5.2-4.5)/4.5 = 15%

In other words, defocus will lower the apparent curvature measured by CCDI.
[To simplify things, this discussion ignores other non-linear aberrations that can interact with defocus to produce somewhat different behavior]

Regards,

-Paul

This message has been edited. Last edited by: Paul Kanevsky,

Damm, then the bright guy (not me) is right..

Thanks for the clear up.