What's an acceptable FWHM for a guide scope?

[irtuk]

Hey.

What is an acceptable FWHM for a guide scope? I am using a 50mm guide with an altair camera and I can get about 5ish. feels like its too high but the scope is not the greatest.

Ed.

 

[terryf]

I'm not sure that the FWHM matters all that much. The guiding routines work by figuring out the centroid of the star (or stars in multi-star guiding), and the view seems to be that precise focus is not necessary as a bit of defocus won't alter the calculation of the centroid. I've even seen it said that defocussing slightly is beneficial, and I can sort of see the logic to that. If all the light from a star was not dispersed in any way, its light would pure falling on one pixel (unlikely, but just go with me in this) and the centroid could only ever be at the centre of that pixel irrespective of where it was actually hitting it. However, if the light is smeared around a bit over multiple pixels, then the calculation can locate the centroid to sub-pixel accuracy as long as the smearing is done in a repeatable way. If that is true, then a high FWHM figure shouldn't be a problem I'd have thought. Where I live, seeing is the main limitation on guiding anyway so an APO wouldn't help me, but multi-star guiding seems to help significantly with that. As an example, I use a 30 mm SvBony guide scope (120 mm FL) and I manage to get sub-arc second guiding on my AZ-EQ5GT - mind you I don't actually know what FWHM that scope achieves.

Terry

[The Lazy Astronomer]

I've also seen that precise focus is not required for a guide scope, as such l never bother refocusing my guide scope and pay no particular attention to the fwhm reported by phd2.

Typical guiding rms is around 0.6ish" with an eq6r pro, and stars are tight and round on 5min narrowband subs so I'm happy with it.