Astroart Forum

Patrick wrote: ↑

14 Aug 2022, 02:32

Hello,
1) I am familiar with the Bahtinov technique but am looking for descriptions of how to integrate this into Astroart processing?

2)Also looking for descriptions of how to integrate focusing on a spectral line into Astroart processing - especially finding the width of a line?
Regards,
Patrick.

Autofocus

This procedure finds automatically the best focus position. Two procedures are available: "subframe" where a single star is measured, and "full frame" where several stars are measured at a time. Both methods share some options:

Positions. The number of positions to find the best focus. Usually it's an odd number. For example, if you set "7" the program will search: three positions back, the current one, and three ahead.

Step. The step between positions, this value depends by the range of your focuser (some have a range 0..5000 up to 0..50000). Usually 1/500 of the range is a good starting value. The best value is the one which gives slightly defocused stars at the extreme positions.

Binning. The binning mode when taking focus images. If possible set a value which gives focused stars of 3-4 pixels, this will optimize the S/N ratio and the download time.

Method. The algorithm to estimate the focus from the star. If you know in advance that for a good focus you can get stars of 2-5 pixels (i.e. setting an higher binning) then the method "Small disc" is usually the best. For slightly bigger stars (5..8 pixels), use the method "Large disc". If you prefer a generic algorithm then it's also available the FWHM (full width half maximum) and the HFD (half flux diameter, see: Wikipedia). Since the best method depends on your instrumentation, a quick initial test with all methods is recommended. Every focus method produces a measure from 0 to 1000, which is relative, and cannot be used to compare methods.

The subframe focus window contains two extra methods to focus, one for the Bahtinov Mask on a bright star, see Wikipedia, and one for a spectrum reference line, where the width of the line is used to estimate focus.

Exposures. How many exposures per position. This is used only in the "subframe" procedure when the seeing is not good. If the seeing is good, choose a value from 1 to 3, if it's bad, 3 to 5.