Dithering in Live Stack - phase shift

[Oberon510]

Anyone using dithering with PHD2 in Live Stacking (Guiding options) and getting a large shift in the view - causing object shift. I am using PixInsight to align and integrate but it seems to be not handling the alignment.

See the attached images -first is from 8 exposures and the second from 15.

Any thought on this - am I missing an option I need to select?

Thanks,
Ken

[ChrisR Oz]

Hi Ken,

I use PHD2 with SharpCap and dithering, and it works like a charm. Sure, dithering does move the image by some random amounts to spread the data across the camera to even out non-uniformities in the sensor, which is the point of dithering of course, but SharpCap aligns these back well, once you have the settings working. Pixinsight is a different story, which I don’t have experience with. Strange that your 15 frames aligned, but the 8 did not. Seems to be an issue in Pixinsight setup.

Or, are we seeing movement (I.e. the dither) integrated into an accepted frame? Do you use the raw frame save option “Save stacked” and check the option to not save frames while dithering, I.e. when not guiding? In practice, on the Guiding tab in Live Stack, check all the options and specify how many frames between dithers.

Cheers, Chris.

[admin]

Hi,

yes, probably worth having a look at the individual frames if you have them to see if there are any anomalies like a frame with double stars or a big jump at some point between two frames. The jump here seems quite large for dithering (obviously depends on your settings).

cheers,

Robin

[Oberon510]

Hi Chris - thanks for the suggestions and I will have a look the settings once the rains pass in a few days and I get the mount back out. It looks like it did take an image during the dither process as frames 9/10 have a jump and 12/13 also - nothing up to frame 8. I was sure that I told it not to take an image during the dither but maybe I need a delay after the dither so this does not happen during the next exposure.

Hi Robin - the frames look clean so no movement during the dither that is caught in the image. It just shows that frame 9 & 10 are out of phase and the same for 12/13 (that is as far as I have looked at the 96 frames). The dither does return the frame to the correct position after it so it looks like it is capturing during the dither. I need to check that setting per Chris's comment.

Thanks,
Ken

[Oberon510]

Last question here on SharpCap Max Dither step setting in Guiding (default of 20).

If my Guidescope to Imaging scope is 210/2032mm and cameras are 2.4/4.78 pixel sizes and imaging scales are 2.3542857/.4845866 giving a 4.857748 pixel move on imager from 1 pixel move on guider
then
If my rough calculation of 3 time star size of ~25 pixels should I set the Max Dither step size in SharpCap as 5?

I hope this is clear and I understand that PHD2 will use the max dither size in a random calculation (from 0 up to max dither size)

Thanks,
Ken

[admin]

Hi Ken,

that looks right to me - the SharpCap value (which is passed to PHD) will be in terms of the pixel size of the guide scope.

cheers,

Robin

[Oberon510]

Thanks Robin - understood on the "applies to the guidescope pixels". If correct it should help with my phase issue - reduce from ~92 to ~25 pixels range on the imaging scope. I will have to review it on my next live stack.

[Oberon510]

My newly calculated dither is much less than the previous setting bit the images are still out of phase after each dither (i.e shifted location).

I took a stack of 8 images. 1-4 are aligned, 5 is after first dither and is shifted compared to 1-4 but aligned with 6-8.

To me this indicates that it is not a settling time issue with SharpCap but probably with PHD2 and the mount.

Any thoughts or suggestions on this?

Admin - has anyone ever suggested an option in the Guiding section of Live Stacks to have a dither time of either "every X image" or "X times exposure time" as well as choosing a set time? I kind of like the idea that dithering could be set to happen after say every 4 images and I don't have to change this when I have different exposure time stacks.

[admin]

Hi,

sorry not to suggest this earlier, but can you check that you have 'Align Frames' turned on in the live stacking settings? That being turned off would explain your issues with doubling stars etc.

You can now choose to dither every X frames - just make sure you are using the latest SharpCap 4 (rather than 3.2 or an old verison of SharpCap4)

cheers,

Robin

[Oberon510]

Hi,

Thanks and yes I had the align frames selected - per dummy live stack attached.

I thought I may be able to resolve it in PixInsight processing but it's image processing and integration didn't help. See attached from an 8 frame stack.

I may just have to turn it off.

Ken