Pixel size for guiding

[Guest]

Hi all,

I have a question, I am looking at a new guide camera the QHY5Lll which has a pixel size of 3.75 micron square, I currently use a. Meade DSI l colour which has a pixel size of 9 x7 microns.

My question is when guiding with the DSI I use the min motion setting in PHD at 0.15 pixel before a guide command is sent to the mount and this works very well, so with a much smaller pixel should I set this at around 0.45 to get the same guiding results, I am assuming that 0.45 of the smaller pixel is more or less the same distance as 0.15 of the larger pixel

Have I understood this correctly

MM

[davew]

Here is a calculator for this very question - http://www.ccdware.com/resources/autoguidercalcv4.cfm

Dave

[Leveye]

This is great stuff. Please let us know how your guiding sessions go using this calculator. Does anyone think that slightly defocusing your guide scope helps in guiding? 

[Guest]

This is great stuff. Please let us know how your guiding sessions go using this calculator. Does anyone think that slightly defocusing your guide scope helps in guiding?

Yes it does help, as the guiding corrections are more accurate this way, I always slightly due focus the guide star

Regards

MM

[davew]

If memory serves well then I think PHD likes a slightly out of focus star where as Maxim doesn't particularly. Different programs will probably prefer levels of smear.

Try it out. Takes a few minutes.

Dave.

[glappkaeft]

To provide another datapoint the manual for the Lacerta MGen2 stand alone autoguider suggests that you should defocus if your pixel scale is to high, i.e. the stars are too small for the built in sub-pixel guiding algorithm to do its job properly.  

[IanL]

This may help:

http://stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/

The min motion change sounds about right.  Don't forget you may also need to reduce your calibration step size with smaller pixels to avoid PHD completing the calibration in too few steps. To be safe, it should take at least 7 steps in each direction to calibrate; somewhere between 10 and 20 steps is usually reliable.

Defocusing a small amount can help in some circumstances, e.g. if you have a very bright star that is saturating.  You shouldn't defocus if that reduces the SNR of the guide star to an unacceptable level as PHD will have trouble staying locked on to the star.