Simple Autoguider Question

[Meng Lee]

Hi,


Just to confirm a thought. I seem to have the idea that when the pixels of the autoguider camera are optimally sampled, eg Starshoot autoguider/ QH5 at around 540mm focal length which gives sampling of 2 arcsec/pixel, that will be the best autoguiding resolution.

If the focal length is increased, and the pixels become undersampled, seeing comes in significantly as well, then autoguiding may even be worse.

Correct right?

[anat]

I don't think so. You will get better resolution when increasing the focal length. However, a long focal length is not needed right now because CCDs and software can do sub-pixel guiding. But "how long is long enough?" is the matter of trials and errors

Anat

[rcj]

yes indeed it is. Actually, one will not need to have it undersampled very much. Certain autoguiders work with sub-pixel correction, and in this case, one can work even with Borg50s or the Efinders from SBIG. When seeing is bad, it is better to set the autoguider to correct for fluctuations less often than needed, say 3-4 seconds once, rather than 0.5-2 seconds once, which in effect, it will be "chasing the seeing" instead of correcting mount errors.

clear sky again!

[Meng Lee]

I don't think so. You will get better resolution when increasing the focal length. However, a long focal length is not needed right now because CCDs and software can do sub-pixel guiding. But "how long is long enough?" is the matter of trials and errors

Anat

Thanks Remus and Anat,

I think you do not really have higher resolution with longer focal length. There is a lower bound. If you ignore atmospheric limitations, the lower bound is called Nyquist sampling. This is easily visualised as using enough pixels so that the Airy Disk of the star is properly, digitally reproduced.

I think I can conclude that, to get best autoguiding results, optimally sample the autoguider sensor, use also subpixel autoguiding then let the exposure be sufficient so as not to chase the seeing. Then if the imaging system is at the optimal image scale/ sampling, the stars will turn out to be good. (Provided the mount is good.)

Yah, seems to be clear sky but there is some haze.

[rcj]

yes. certainly a text book answer! but it is clear skies now....do something!