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Old 07-02-2019, 09:36 AM
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Merlin66 (Ken)
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Join Date: Oct 2005
Location: Junortoun Vic
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Actual camera exposure times in PHD2???

I've been using PHD/ PHD2 for many years and somehow always thought the "Camera exposure duration" control was just that -if 0.5 sec was chosen the the camera "shutter" was opened for 0.5 sec and then the resulting "image" at the end of 0.5 sec analysed by PHD to apply the necessary corrections.
Thinking about it, I couldn't really see how this could be correct - if you changed to a 4 sec "Camera exposure duration" then many stars would end up over exposed and unusable.
I've been unable to find out exactly which real camera exposure PHD2 uses.

Then I came across a Ytube PHD tutorial, which says that the "camera exposure control" only sets the interval between PHD2 image analysis - nothing to do with the actual exposures from the camera.

Somehow makes some sense - but if correct, what controls the exposure/ frame rate etc. of the images PHD2 uses????

EDIT:
OK, found the https://openphdguiding.org/man-dev/A...d_settings.htm

This states:
'Auto Exposure' - these are the settings that control Auto exposure time.
Min Exposure - the minimum exposure time. PHD2 will not set the exposure time less than this value, even if the guide star SNR is higher than the target SNR value. If the min exposure time is set too low, you are likely to chase seeing effects and thereby get poor guiding results. Users of AO units will usually set this to a lower value, since rapid small corrections are often desirable with an AO.
Max Exposure - the maximum exposure time. Before a guide star is selected, PHD2 will set the exposure time to the maximum value. Once a guide star is selected, PHD2 will then incrementally decrease the exposure time until the desired SNR is reached.
Target SNR - this is the average SNR value that PHD2 will attempt to achieve by adjusting the exposure time. SNR can fluctuate from frame to frame even with a fixed exposure duration, so be sure to account for that when choosing a target SNR value. PHD2 will reject frames when SNR drops below 3.0. The default value of 6.0 should provide enough of a cushion to prevent fluctuations from causing the SNR to go below 3.0.
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