Dithering and multi-star guiding

[John Engstrom]

I noticed an odd behavior when I switched from guiding with a 120mm guide scope to using an OAG on my 550mm refractor.  Due to pixel scale I had my dithering set to 10 pixels on my 120mm guide scope.  Then when I changed to use the OAG (almost identical pixel size on the guide camera as my imaging camera) I changed the dither to 20 pixels.

The behavior I noticed was that every 4th or so dither I would lose all of my multi-guide stars and just be guiding on the main guide star.  It's not like I would lose one or two of those multi-guide stars at a time (as could be explained by those stars moving to the edge of the OAG where their misshapeness becomes so extreme that they are no longer recognized as stars).  I would lose all of them.

I finally tracked it down to the Search Region setting under the Guiding tab of Advanced Settings.  I had it set to 15 which was fine when I was dithering 10 pixels on my 120mm guide scope.  But when I upped the dithering pixels in my image capture software to 20 then I would eventually get a dither larger than the Search Region and the multi-stars went away.

As a software developer I know that things are not as simple as them seem, but my question is would it be possible to automatically look for the multi-guide stars after a dither in the direction and magnitude of the dither?  That seems to be the behavior for the main guide star.  I can see how the multi-guide stars would fall down a different code path that would be difficult to flag as having a dither that just happened.  But I didn't see anything in the documentation on the Search Region or in the pop-up help on having it set to be larger than your dither for multi-star guiding.

And I'm also certainly open to the possibility that my conclusion about Search Region is not correct and somehow there's some other relationship at work.  If so I'd love to know some more about how the multi-stars are reselected after a dither.  Thanks.

[bw_m...@earthlink.net]

Hi John.  I think your conclusion is probably right – I’ll take a look at it and see what needs to be done.

 

Thanks,

Bruce

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[John Engstrom]

Thanks Bruce.  I appreciate you looking into it.

[Bruce Waddington]

I'll fix this in the next dev release - thanks again for raising the issue.

https://github.com/OpenPHDGuiding/phd2/issues/1044

Bruce

[John Engstrom]

Bruce, thanks for letting me know.