Missing images during Live Stack with dithering

[Jean-Francois]

Hello Robin,

I tested yesterday the first time the image dithering during the Live Stack with the 4.2.15217 beta.
It was working with short exposure time (5 to 10 seconds). But it was no more fine with 30 or 60 seconds exposure.
I used "Dither only (no guiding) using ASCOM mount pulse guide".

What I remarked ?

With 60 s exposure and the camera in Force Still image ON.
Start the Live Stack during an exposure of 60s. The image arrived and added in the stack.
The next images were a sequence with one 60 s image added in the stack, one 60 s image not added and several short 2 s exposure during the dithering. The 60 s image was not added and the status mentioned it as "waiting during dithering".

It is annoying to lost the half of the 60 s exposure when SharpCap seems to wait during some dithering.

I tested different settings of the "dithering" parameters, but without success.
Can you give me some parameters values that should work ?

Regards,
Jean-Francois

[admin]

Hi Jean-Francois,

sorry to hear about the lost data. Unfortunately I think this is likely to be the sort of problem that needs digging into to understand why it is happening. My first guess is that code that is supposed to spot a frame where the capture time of the frame overlaps with the time when the dithering was happening is being triggered incorrectly. If that was the case then you would likely find entries in the SharpCap main log along the lines of

Code: Select all

Pausing stacking to ignore frame : <some reason message>

If you have those lines, knowing the reason would help pin down what might be happening.

thanks,

Robin

[Jean-Francois]

Hello Robin,

I have the telescope outside with my EAGLE computer ... so the log file is not easy to retrieve when the weather does not allow to open the telescope cover. But I can try to reproduce the problem with other camera and my desktop computer.

Concerning the dithering .. for my test, it was set after each frame.

Regards,
Jean-Francois

[admin]

Hi,

I am running on hunches and instinct here, but I wonder if this is all down to exact timings being just right (just wrong really) causing the problem. Imagine that SharpCap requests a 60s frame from the camera after dithering, but the camera maybe only exposes for 59.8s... Nobody would normally notice, but if the image transfer to SharpCap is fast then it's possible that SharpCap would calculate an exposure window for the frame (based on the time it arrived and the presumed exposure) that starts before the frame was requested (and perhaps even before the dither finished).

I will add some margin to that check as that is something I can fairly easily do on a global basis - trying to track the exact frame request time would require changes for each brand of camera and also wouldn't be possible for cameras running in a true video mode.

If you do get hold of a log, ping it over, as it will either confirm I am on the right track or hopefully point me in a new direction.

cheers,

Robin

[Jean-Francois]

Hello Robin,

here the log file of the observation night.

Log_2026-10-05T19_52_59-10180.zip

(729.95 KiB) Downloaded 2 times

Regards,
Jean-Francois

[Jean-Francois]

Hello,

here a new log from this evening with other computer and camera (now on my desktop computer).

At the beginning with 5s, 10s and 20s ... no problem.
But with 30s ... again the half of the images is not used.
Strange this time (I did not remember to see the same before) ... some capture was longer 34-35s and other was shorter ~ 22s.

Log_2026-10-08T20_58_36-17672.zip

(26.49 KiB) Downloaded 1 time

Regards,
Jean-Francois

[admin]

Hi Jean-Francois,

interestingly there are QHY camera related issues in both of these logs - I am wondering if these are behind what is going on - perhaps the frames are not being lost in the live stacking process, but are not being delivered from the camera at all.

In the second log where you were testing, towards the end we see this associated with the end of every dither

Code: Select all

Warning	21:25:40.239930	#29	QHY camera failed to produce live frame of 2000ms. Waited 24001ms and now giving up and resetting live view	

My guess is that the camera is not reacting well to the change in exposure when the dither is over and SharpCap ends up waiting for a timeout before trying again to get the next frame, hence the loss of nearly 30s of imaging time.

In the first log, we instead see this sort of message

Code: Select all

Warning	19:54:30.476817	#46	GetQHYCCDSingleFrame gave an error, have already processed 4 frames successfully. This may be a one off glitch, if lots of frames are lost try closing and reopening camera.

This also indicates a failure to get a frame from the camera, but this time with the camera operating in still mode - again this would be lost exposure time at the camera level, rather than the live stacking rejecting the frame I think.

The control of QHY cameras with longer exposures and the camera in live mode is not really satisfactory - there is often an issue where the camera tries to buffer two frames before sending one to the software, which is a real problem for longer exposures. This can sometimes be fixed (some cameras) by turning the DDR setting to off - that might help for the case where the camera is in live view mode.

Normally I would suggest though that for longer exposures the camera is run with the 'force still mode' setting set, but that is giving the second problem shown above

I can't suggest much else to help at the moment as I don't have enough information - I am going to add some more logging so that we know more about the exposure length of the missing frame in the still mode case and in the case of frames being dropped due to overlap with the dithering time. I will also try testing with the various QHY cameras I have here, but unfortunately the behaviour of various QHY models in this sort of regard is far less uniform than the behaviour of other brands, so I suspect I may not see the same problem

cheers,

Robin

[Jean-Francois]

Hello Robin,

OK, good that you found some point to investigate.
The first log was with a QHY268 camera and my EAGLE computer.
The second log with my QHY600 camera and my desktop computer.
I can do some test with the still mode and other cameras.
Until today, I did not used the dithering. But I think that other people are using it.

Regards,
Jean-Francois