diffraction spike rotation in altaz mounts?

[Astronomist]

Hi everyone

I am posting this here as I thought it would be inappropriate for the imaging forum, since I am not seeking advice, I am simply curious. 

in the planewave direct drive mounts, and other observatory class Alt AZ mounts, how is the rotation of the diffraction spikes averted? I get that a field derotator can be used, but this cannot compensate for the rotation of the diffraction spikes relative to the image. surely this would limit the sub length for these scopes dramatically?

clear skies,

Astronomist

 

[symmetal]

Yes, this is usually solved with either using curved spider vanes or fitting curved masks over existing straight vanes to spread the diffraction spikes throughout the image.  This topic on the PI forum gives some examples.

Alan

[vlaiv]

Another way to handle would be to not handle them at all.

Sure, in single exposure - you would get broader diffraction spikes, but with sigma clip stacking of multiple exposures - same thing would happen that happens with satellite airplane trails in the images - what is there in one sub but not the rest would be discarded as invalid signal so one would actually loose long diffraction spikes and perhaps only keep their "stems" as those won't rotate much after alignment. They will probably "merge" into halo what would otherwise form from curved spiders.

[Astronomist]

I see, the diffraction spikes sort of become one large diffraction disk, so presumably when one sees an  image taken with a planewave direct drive mount (or similar) that has diffraction spikes the imager was using a polar wedge.

thanks 

Astronomist