Phd stars

[pjphilli]

Hi
When using PHD I usually see a selection of stars to guide from and when selected the S/N ratio of the star is displayed. My question: Is there an optimum S/N that I should select or just the highest one?
Cheers Peter

[mithrandir]

It doesn't have to be the best SNR. PHD is pretty forgiving.

Absolutely pinpoint stars (e.g. single pixels) are not good. Apart from anything else, you might be guiding on a hot pixel.

Craig recommends slightly defocusing so your guide star covers a few pixels, for instance 2x2 or 3x3. This reduces any effect that "seeing" has on the guide star.

For an idea of the size it can cope with, PHD will guide on Mars through an Orion ST80 with an Atik 16ic camera.

[multiweb]

Any star will do faint or bright as long as its core is not saturated. That's why defocusing helps.

[pjphilli]

Thanks Guys - I did a bit of experimenting last night and firstly I tried a pretty bright star with the star profile showing a flat top - presumably the core was saturated. This guided reasonably well but a fainter star with a nice pointed star profile guided better. Also some cloud came over and I was surprised that PHD just kept on guiding. Cheers Peter

[mithrandir]

If you wanted to use that bright star, you could reduce the exposure time until you lost the flat top.