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Old 07-02-2019, 10:36 AM
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Merlin66 (Ken)
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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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Old 07-02-2019, 05:36 PM
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The_bluester (Paul)
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My observations would have been that the exposure time does actually set the exposure time, there is a second setting in the camera tab that sets the time between guide frames, presumably to prevent over commanding the mount with lots of guide pulses if you are using short exposures.
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Old 07-02-2019, 06:02 PM
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Merlin66 (Ken)
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Paul,
Yes there is ambiguity.....
The camera settings define as far as I see set the limits , not the actual exposures....
The camera exposure settings also seem to define the interval between the PHD analysis cycles...
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Old 07-02-2019, 07:35 PM
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Yes, the Time lapse setting appears to be used to extend the time between guide corrections for very short exposures or guiding with a video based camera.


By what I could see when I had the SSAG it seemed that the longer exposure setting was resulting in correspondingly longer guide exposures. That one had the indicator LED on the camera body.
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Old 07-02-2019, 07:43 PM
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Paul,
The issue/ concern I had was that there was no record or notification of the actual exposures being used by PHD2.
You set the “camera exposure” interval and then leave it in the hands of the PHD SNR modeling......
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Old 07-02-2019, 07:58 PM
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There are two situations: camera in trigger mode and camera in streaming mode. Most drivers operate in trigger mode in which case the exposure time is what it says (provided the driver behaves). Then there is a wait while the image downloads and guiding takes place. Some drivers introduce unwanted latency before triggering the image on top of that.
Streaming mode is more complicated. The driver sends a stream of images at whatever frame rate is set and PHD2 picks up and stacks as many as it can in the interval set by the exposure time.
Recently the ZWO driver in PHD2 switched from video to trigger mode as video was causing issues from trailing
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Old 07-02-2019, 08:26 PM
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Ken,
Help me....
Where is this documented?
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Old 07-02-2019, 09:15 PM
kens (Ken)
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I found out by working on the code: https://github.com/OpenPHDGuiding/phd2
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Old 07-02-2019, 09:50 PM
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Hmm,
Not ready accessible info for mere mortals....
OK.
Then for "standard" cameras the "camera exposure control" setting is the effective exposure used by PHD (plus or minus)
I would have thought it would have been more clearly stated/ available within the manuals.
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Old 08-02-2019, 09:05 AM
kens (Ken)
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Unfortunately it is driver dependent and there are over 30 different camera drivers, counting ASCOM and INDI as one each.
The documentation at https://openphdguiding.org/manual/?s...Star_Selection is about all most people need to know.
If you are having issues then you can post at the support forum and the folks there are only too happy to help out. That can and does involve delving into the code to investigate. That's what happened with the recent ZWO issue where some people reported trailing whilst calibrating and during backlash compensation. Unfortunately for me it occurred just as I was setting up the Touptek driver for Linux and had set it to work in video mode to overcome a 600ms lag in taking a frame in trigger mode in the vendor supplied driver. So due to the know issue with video mode we reverted it to trigger mode and we asked the vendor to see if they could fix their driver. To be fair to the vendor, it could be a function of my specific camera hardware (no longer a current model) but nobody has a suite of all the vendor's cameras to test if that's the case.
Same with the documentation. If you think it could be improved you can highlight that on the forum and it will go on the todo list. Or anyone can make the changes themselves and submit them. Any reasonable request usually gets dealt with pretty quickly which is quite impressive given that its all done voluntarily.
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Old 08-02-2019, 09:39 AM
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Ken,
Thanks for that.
Yes, I agree the PHD2 guys are very helpful. They implemented the spectroscopic slit guiding following my request.

The reason behind my questioning the exposure control was that in spectroscopy we generally don't have an option of guiding on fainter stars if the target star is too bright. It would sometimes be easier to reduce the exposure to give "better" results. I then worry about chasing seeing at 0.5sec etc. exposures.
(I have to say I have resorted to guiding on the secondary reflection from the slit plate to over come this.)
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Old 08-02-2019, 09:49 AM
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I must download the development version with the change to the ZWO cameras, I did not realise it was working in video mode. I will be interested to see the difference in star shapes, they are always going to be ugly with an OAG at the edge of field in a SCT scope but hopefully they look a bit nicer in snapshot mode.

Unfortunately getting the develpment version has been an issue, I tried to download it late last night but NBN has utterly borked our fixed wireless connection and we have been "Enjoying" speeds that only dial up users would envy for most of the last month, with another two weeks to go before the next scheduled "Maintenance" that will hopefully fix it.

That is after they required us to be home all afternoon last Friday so they could visit to investigate while internally they cancelled the ticket without bothering to tell us that no one was going to show up.
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Old 08-02-2019, 11:23 AM
kens (Ken)
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Quote:
Originally Posted by Merlin66 View Post
Ken,
Thanks for that.
Yes, I agree the PHD2 guys are very helpful. They implemented the spectroscopic slit guiding following my request.

The reason behind my questioning the exposure control was that in spectroscopy we generally don't have an option of guiding on fainter stars if the target star is too bright. It would sometimes be easier to reduce the exposure to give "better" results. I then worry about chasing seeing at 0.5sec etc. exposures.
(I have to say I have resorted to guiding on the secondary reflection from the slit plate to over come this.)
What you could try then is to lower your gain (if possible) and if still too bright then reduce your exposure time. The use the Z-filter algorithm and apply a generous exposure factor. I designed this filter with fast sampling rates in mind (my mount is an Avalon and it works best with low latency/frequent corrections). As a "proper" frequency domain filter the z-filter lets you choose the cutoff frequency via the exposure factor to suppress high frequency noise from seeing. It is calibrated (roughly) so that the response is equivalent to the hysteresis algo with default settings operating with an exposure time of exposure factor x exposure time (or more correctly the sampling rate). You can tune the exposure factor to optimise removal of PE on the RA axis vs chasing the seeing. On the dec axis you can be quite aggressive with the exposure factor since there is no periodic error. This has the beneficial side effects of reducing the number of correction reversals and, in some situations, causing the dec guide graph to be offset from the axis when the corrections just balance out the PA drift. This results in almost no dec reversals and related backlash issues.
The MinMo setting also operates differently. In other algos the MinMo applies to the input deviations and acts as a crude filter. In the Z-filter is applies to the corrections so that miniscule corrections are not sent to the mount.
I'm currently developing the "image guiding" algorithm which may be helpful when the guide star is saturated. Still early days though and initial testing is not encouraging. The algorithm is fine but performance with sparse, noisy images is not good as it guides on the fixed pattern noise rather than the star.
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