Dithering in Live Stack - phase shift

[admin]

Hi,

OK, not the align turned off then. I can see from the screenshot that you have been saving the raw frames when stacking - if you have a look at those, are they all 'good', or do some of them have double vision?

cheers,

Robin

[Oberon510]

Hi Robin,

Each frame image is clean. Frames 1-4 are aligned and 5-8 are shifted slightly per the dither step.

The dither is doing what it is designed to do and the frames aren't captured when that happens (per design) but the alignment processing in SharpCap or PixInsight seems to not handle it.

At this stage I just capture the raw frames in SharpCap and process them in PixInsight. Does the alignment in SharpCap keep the rawframes aligned - if not then my apologies and I should be looking at PixInsight processing and not bothering you.

Thanks,
Ken

[admin]

Hi Ken,

I think it is PI that you need to look into - the alignment in SharpCap does not align the raw frames, only the data that goes into the live stacking. Maybe your PI setup has not got alignment switched on?

cheers,

Robin

[Oberon510]

Hi,

I did another Live Stack with dithering last night and just saved the image in Sharpcap. Any thoughts on why I am getting multiple star points on the image. The stack is 30 x 30s with a PHD2 dither after every 4 frames and just stretched. (Settings were no capture while dithering)

The mount was polar aligned with SharpCap (Excellent) before the session and after an initial PA with Polemaster the day before.

Any thoughts on this. I doubt changing the settling time would help.

Ken

[Oberon510]

This may be a clearer view

[Xee]

hey,

I'm having the same issue, only its much more subtle. I do live stacking with long exposures (just for the fun of it), i stack my raw images later.

I like the visual aspect of it, and helps me keep an eye on what's going on, but im having the same exact issue.

Im using an OAG, and phd with multi star guiding. My PA error is between 0.1 and 1 arc min, and my guiding total RMS is between 0.4-0.9.

Over time i see drifting in one direction, i cant figure out what the issue is.

Drift.PNG (52.3 KiB) Viewed 1509 times

Xee

[admin]

Hi,

first of all, it's worth looking at the raw frames that you have saved to check that there really is some noticeable drift between them - if they all seem to be well aligned then it could be a problem with the software side of things in live stacking wrongly detecting the drift.

If it is a real drift with stars moving between frames, then you have to start thinking about what could cause a drift on the main camera while the guide camera is steady.

The first thing to do is to check that your OAG is rock solid. The one I bought - I think it might have been Orion USA or one of the other brands that are basically from the same Chinese factory - was very wobbly, and I had to drill and tap several extra threaded holes and put grub screws into them to even make it passably steady. Otherwise the weight of the guide camera/cable or even just the OAG itself is sufficient to move the prism gradually which means that the guiding is correcting for the flex in the OAG rather than drift of the mount.

It's also possible that the focuser of the main scope could be flexing as the monut rotates, but that is much less likely than the OAG since the focuser is much heavier duty.

Finally, make sure that the star you are guiding on is close to your field of view - if it is too far off this can cause issues if you are imaging near the pole.

cheers,

Robin

[Xee]

Hi robin,

I think everything is actually working well. There are no trails or any issues with the capture itself.

I think the issue here is with the dithering, while it does seem to be random i do believe it slightly favors a certain direction, which incrementally adds up.

I say this because the stacking artifacts at the edges, always seem to be getting bigger on one side than the other, as opposed to them being random in all directions.

If you can give me a test scenario, and what type of logs you need I would be glad to provide it

Regards,

Xee

[admin]

Hi Xee,

I think you will need to look in the PHD2 logs - in PHD, do 'Help', 'Open Log Folder' and then you can investigate the log files. If you search inside the log files for

Code: Select all

dither:

You should find the lines where PHD records the dither movements each time it is asked to dither - you can see if those show any sort of bias perhaps.

If they look 'correct' - ie no obvious offset in any direction then you will need to dig into the possibility of hardware shift between the OAG and the main camera - with both running, poke the OAG or gently pull on the camera cable to see how much the OAG image moves and does it affect the main camera or not.

cheers,

Robin