Dithering and Live stacking unsuccessful

[dykky]

Sorry if this is a double post
I am trying to do unguided dithering with EZEUS2-ASCOM using Sharpcap4.1 with C11 (focal length 1740mm). However, even if I start live stacking after plate solving, I am unable to stack after the first dithering. I have set it to 20 seconds exposure per frame and dithering every 10 frames. Which setting is wrong?

[admin]

Hi,

the first place to start would be to turn down the max dither step size in the SharpCap guiding settings - the fact that it seems to work until the first dither may indicate that the dither is too large and is changing the field of view completely (or almost completely). You could check this by keeping an eye on the mount RA/Dec before and after dithering to see how far the mount is being moved, and how that compares to your field of view.

cheers,

Robin

[dykky]

Thank you for your prompt reply. I'd like to try it right away, but unfortunately I can't see the stars tonight. By the way, I think the maximum order step size was 20 by default, but should I try to implement it by reducing it to the minimum of about 1? The focal length is 1760mm and I use UranusC. I'm looking forward to the next sunny night. Thank you very much

[admin]

Hi,

yes, try going down to 1 and see if that fixes the problem, from there you can see if a higher value is workable and maybe gives usable dithering. When you are using the unguided dithering, the number specifies the maximum length of time for the dither move, so lower values are probably in order than when using PHD2 (the number then is the arcsecond distance).

cheers,

Robin

[dykky]

Thank you for your kind suggestion. I tried lowering the maximum dither step to 1, but the result was the same, and the stack stopped after the first dither (dithering once every 5 images). Currently, I'm using planetarium software called Stella Shot, and shooting after plate solving with SharpCap. When the live stack stops, the telescope tracking of Stella Shot seems to stop as well, and I have to restart it manually at this time. Is the stack failure with SharpCap a compatibility issue with the link with Stella Shot? Should I consider the planetarium software I use to introduce the first target? Are there any solutions?

[admin]

Hi,

OK, thanks for trying - it sounds like your mount is doing something unexpected during or after the dither movement, which is then stopping the stacking procedure from working. Maybe it is moving much further than expected, or maybe it is stopping tracking (which would tie in with the planetarium program issue). I do suspect that whatever is happening is coming from the mount ASCOM driver changing state and the planetarium reporting it, rather than the planetarium causing any problem.

The next step in troubleshooting this would be for you to turn on the option to Log addititional information about 'Mount hardware communication' in the SharpCap logging settings. Do this when you open SharpCap, before opening the camera or connecting to the mount. Once you have turned on the logging, run a minimal test that makes the problem happen, then close the camera and send me the full content of the SharpCap log (use Help->Show Log then either 'Copy to Clipboard' or 'Locate on Disk').

The additional logging will record all the communications between SharpCap and the ASCOM driver, including the regular requests for status updates on whether the mount is tracking, what the co-ordinates are, etc. I can analyse that and hopefully spot if anything unusual is going on.

thanks,

Robin

[dykky]

Thank you as always for your valuable teachings.
I shot with a 20-second exposure Livestack, dithering every 5 shots, with a maximum dither amount of 1, but after the third dithering, stacking stopped. Looking at the Sharpcap coordinate data at this time, the right ascension and declination did not move until the second time, but by the third time, the right ascension(RA) was off by 18 seconds. The UranusC screen is 0.4°x0.2° (24''x12''), so if it moves by 18'', it is highly likely to go outside the field of view of the UranusC. Could this be the problem?
I tried to save the log from this time, but I saved it incorrectly and was unable to save it. I will try again and record the log. However, it has been raining a lot recently in the area where I live, so I have not been able to take my telescope outside. I will try to save the log again on the next clear night, so I would appreciate your guidance when I do so.

[admin]

Hi,

thanks for recording this information. Looking at the data, I think you have become mixed up between minutes of arc (1/60th of a degree) and seconds (1/3600th). The FOV of your camera at 0.4x0.2 degrees is 24x12 arc minutes. The offset after the dither is 18 seconds of RA, which we have to multiply by 15 (because RA is measured in hours, not degrees) to get 270 arc seconds or 4.5 arc minutes.

Looked at like that, the offset is still quite big (up to 1/3 of the field of view, depending on the direction), but isn't enough to push things completely out of view, so I'm not sure if stacking would be expected to fail at that point... If there were plenty of stars being detected then I would probably expect it to continue, but if the star count is low then maybe it would stop due to not enough stars to align.

Another thing that I should mention is that you can check the logs for information about the dithering movements - look for lines in the log containing the text

Code: Select all

Random dither of (<some numbers>) arcsec 

If you can find those lines in the log of a new session (or just look in the SharpCap log folder on your PC and find them from a previous session - SharpCap keeps old log files for up to a mount) then please post them here so that I can check the values are sensible.

thanks,

Robin

[dykky]

Thank you for your prompt and courteous replies.

I found the log file, so I have pasted it below. (It's too long to send, so I've extracted the logs before and after "Random duther of...")

Shooting data
2025-6-1
20:06 Installed M3 from Arctools (with Stellashot3), and performed platesolving with Sharpcap.
20:07 Started first stacking (10 second exposure, dithering every 5 images)
20:09 Started second stacking
20:09 Started third stacking,
20:09 15 images were stacked, but stacking became impossible immediately after that.

I apologize for the inconvenience, but I would appreciate it if you could analyze the log file.

Log file below file.

[admin]

Hi,

thanks for sharing the log - we can pull some interesting figures out of it...

At 20:03:36.674350, the RA value is read back as 13.723545755527399
Then, at 20:07:43.463791, SharpCap decides to dither :

Code: Select all

Info   	20:07:43.463791	#17	Random dither of (-0.93,-0.20) arcsec; rates are {X=0.0025068447, Y=0.0033424597}, pulse guide times are (-0.103,-0.017)s

So, SharpCap has picked dither amounts of -0.93 arc sec (RA) and -0.2 arcsec in Dec. The guide rates are in degrees per second, so for the RA dither component, we have 0.93 / 3600 = 0.0002583 degrees, then divide that by 0.0025068447 to get a movement time of 0.103s or 103ms in the west direction (due to minus sign).

So, then we see the pulse guide command being sent (OK, 102ms - probably just rounding of the number being different)

Code: Select all

Verbose	20:07:43.472137	#34	Mount : SharpCap.MountProxy.ASCOM.IMountInterceptionProxy.PulseGuide :: Starting PulseGuide(West, 102)

Soon after, SharpCap re-reads the RA value

Code: Select all

Verbose	20:07:44.037651	#26	Mount : SharpCap.MountProxy.ASCOM.IMountInterceptionProxy.get_RightAscension :: Completed (returned 13.723123880527398) get_RightAscension()

So, now we have 13.723123.... - that's a change of -0.0004227 hours of RA, or multiplying by 15, -0.00634 degrees. Multiplying again by 3600 to convert to arc seconds, we get -22.8 arc seconds. Not what we expected - larger by a factor of 25 or so and I can't see a reason for that.

If we look through the same area of the log and track the declination values and the N/S guiding movement, we see the pulse guide movement command to the south, but oddly we don't see any change in the RA values being read back from the ASCOM driver by SharpCap (in fact the RA stays the same throughout the log). Note that throughout the log, SharpCap is also reading back the 'Target' values for RA/Dec - these can be ignored as we are not expecting the target co-ordinates to be changed by the dithering movements.

So, we have a few facts that have emerged...

* The pulse guide commands being sent by SharpCap are very brief - 10s to 100s of milliseconds, and should give movements of under 1 arc second at each dither according to the guide rates reported by the ASCOM driver.

* The movements we are seeing in RA are much bigger than expected (25x in the case of the one looked at in detail). There is no obvious cause for this

* There are no movements in DEC due to the dec guide pulses, which is odd

* The total difference between the RA at the start and the end of that log section is only 0.0003 hours of RA or ~16 arc seconds. That's only about 2% of your field of view in the smaller direction (0.2 degrees), so should not account for the image moving significantly.

Unfortunately this doesn't help explain the problems - there is no good reason for the stacking to stop because of these dither movements even though the mount is not really doing what it is supposed to.

If you turn off dithering, does the stacking continue without errors for a long time (significantly longer than it takes for the dither related problem to happen?) I have seen cases where a mount can slip (clutch not able to hold the load), which would move the view without changing the RA/Dec co-ordinates. Something like that could be happening at random and just seem to happen when the dithering happens?

cheers,

Robin