WDM camera exposure control question

[Peter B]

Just a quick question regarding setting the exposure for my ZWO ASI120MM.  Do I need to set the exposure duration in the camera dialog as well as on the main PHD panel?  Ie if the main panel shows 2 sec and the camera dialogue shows 1 sec what actual exposure am I getting.  I noticed in PHD LAB it is reporting an exposure every 3.2 secs when I have PHD set on a 2 sec exposure.

Thanks,

Peter

[Andy Galasso]

Peter,

The way PHD2 works with WDM Webcams is this: when you connect to the camera, the camera starts exposing frames at the camera's exposure duration specified in the camera dialog. When PHD2 is ready for a new frame, it accumulates (sums) camera frames for the duration selected on the main window (but always at least one frame).  For example, if the camera is set to 100ms, and the exposure duration on the main window is 2s, PHD will accumulate 100ms camera frames for 2 sec, approximately 20 frames.

If you prefer to use the ASI120MM's long exposure capability directly without accumulating frames in PHD2, you could connect to the camera using the ASI120MM ASCOM driver. In that case PHD2 will do single exposures of the duration specified on the main window.

Andy

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[Peter B]

Thanks for clarifying that Andy.  Do you know if one method yields better results than the other?  I am trying to end up with an exposure long enough to smooth out seeing and get a bright enough star without saturating.  I've also realised there is quite a bigh SNR difference between the dimmer guide stars.  I've been fixing the exposure at 2 sec in both PHD and the Cam Dialogue and then varying gain to get a FWHM plot with a pointy top.  Unfortunately I then need to redo darks for the new gain.  If I go the ASCOM route can I still change the camera gain once it is connected?

Peter

[Andy Galasso]

On Friday, April 18, 2014 7:54:31 PM UTC-4, Peter B wrote:

Thanks for clarifying that Andy.  Do you know if one method yields better results than the other?  I am trying to end up with an exposure long enough to smooth out seeing and get a bright enough star without saturating.  I've also realised there is quite a bigh SNR difference between the dimmer guide stars.  I've been fixing the exposure at 2 sec in both PHD and the Cam Dialogue and then varying gain to get a FWHM plot with a pointy top.  Unfortunately I then need to redo darks for the new gain.

I do not know which method gives the best results. Maybe someone else who has guided with an ASI120MM can weigh in. If nobody here responds, you could try asking on the start-labs-astronomy-software yahoo group?

 

  If I go the ASCOM route can I still change the camera gain once it is connected?

Yes, the ASI ASCOM Driver camera setup window has a gain control (though you have to set it before connecting which is a bit awkward.)

Andy

[Peter B]

Looks like I'll stick with the WDM driver so I can access gain on the fly.

[Peter B]

Andy,

I think I've settled on having the camera exposure at 1 sec and setting PHD up for 2 sec to give a stack of 2 images.  This seems to give a better SNR than setting both at 2 sec and I'm finding it easier to get an unsaturated star this way.

I now have a question re darks - I assume the dark should be for a 1 sec camera exposure rather than a 2 sec PHD stack?  How does PHD treat this combination when I click the "take dark" button?

Peter

[Andy Galasso]

Peter,

Dark exposures are done exactly the same as exposures when looping or guiding. I don't think you should need to change anything when taking darks. It's probably best to leave the camera settings the same for both darks and normal exposures.

Andy

[Peter B]

OK - thanks for clarifying that.

Peter