Stacking centered on comet nucleus

[DrBobAZ]

Hi Robin

Went back and checked, and you were right, the FWHM filter comes back online when you exit comet alignment mode.

Bob

[abillionnewstars]

For anyone reading this, the comet stacking works well!
Thank you Robin!!

Give it a try on what's left of ZTF when the moon goes away or when the next comet comes around (hopefully not too distant in the future...).

Stephan

[rac19]

I have used the Comet Alignment setting for the first time tonight, I finally got a clear night with C/2022 E3 at a reasonable Alt in the Southern Hemisphere. I have recently removed my wedge, being content to operate in Alt/Az mode with frame rotation being dealt with during stacking. I think that I may need to use the wedge for Live Stacking and Comet alignment though. I have my doubts that the comet image is being "de-rotated". The star trails are curved. Is this correct? I can attach an image if required.

EDIT: Image attached, limited post processing. The radius of the star trails is "interesting". I am not sure whether the comet is de-rotated or not. The question for me is whether or not I need to go back to using the wedge.

[admin]

Hi,

stacking in comet mode does not de-rotate the image, so you will need to track on an EQ mount (or with a wedge) rather than ALtAz.

The reason for this is that the comet stacking uses just a single point to align (the nucleus of the comet), so cannot work out a rotation for it.

I'm afraid I hadn't thought that one through when designing this feature, so it didn't even occur to me until seeing your post

If I do get a version that stacks the stars *and* the comet then I would be able to fix this because I could get the rotation from the stars...

cheers,

Robin

[Jean-Francois]

Hello,

I did yesterday (the first imaging since November) some tests on the comet with the new stacking feature.
My setup has 2 telescopes mounted on the same mount (Newton 10" with reducer to F#2.9 and a RASA8" F#2).
I take at the same time a live stack with 2 sessions of SharpCap. One connected to the QHY-268 camera, the other to the QHY-174 camera.

The stacking on the comet nucleus was good working (after tuning the values) with the RASA and the QHY-268 camera.
It was not possible to find the pair of value for the Newton/QHY-174 camera. I try during 20 minutes (it was OK ... SharpCap was stacking the comet on the other telescope ) all the values between min and max.

Here some of the results (note some images with no or minimal processing). No dark or flat correction with the RASA/268, only dark subtraction with the Newton/174.

Here a sub region of the RASA/268 image: (I will process the raw again while the box was still centered on the comet).

C2022E3-ZTF_90frames_900s_RASA8_QHY264_L.jpg (151.67 KiB) Viewed 2599 times

Here the resulting of the live stacking on the stars with the Newton:

C2022E3-ZTF_40frames_1200s_N10-F2.9_QHY174_L.jpg (140.33 KiB) Viewed 2599 times

Here the result after processing the same raws with Prism software (with image shift calculated after plate solve and comet direction vector):

C2022E3-ZTF_40frames_1200s_N10-F2.9_QHY174_L_Prism-Stack.jpg (406.91 KiB) Viewed 2599 times

I will now start a second session ... 2 nights with clear sky is rare in my region.

Regards,
Jean-Francois

[admin]

Hi Jean-Francois,

glad that you had some success - do you have any of the individual frames (4 or 5 would do) from the ones that failed to align? I can test them to see if I can work out if anything can be improved.

cheers,

Robin

[Jean-Francois]

Hello Robin,

I do now some tests this evening. This time only with the Newton telescope.
It works now ... I have no idea why not yesterday.
I will prepare a summary and send it tomorrow.

Now I do other tests ... I remark that the SynScan driver allows very, very low rate.
I calculated the RA and DEC rate necessary for compensating the movement of the comet (0.0015x and 0.0086x).
I use my SHG_Script for commanding the telescope. The MoveAxis command allows to move in RA and in DEC at the same time.
And ... it works

By the way ... if you have some time, can you please describe a little the changes of some functions with the new ASCOM in the version 4.1 ?
I can not use my SHG_Script with the version 4.1. An error appears saying that something like that "AxisRate expected, got int()".

Regards,
Jean-Francois

[rac19]

Hi,

stacking in comet mode does not de-rotate the image, so you will need to track on an EQ mount (or with a wedge) rather than ALtAz.

The reason for this is that the comet stacking uses just a single point to align (the nucleus of the comet), so cannot work out a rotation for it.

I'm afraid I hadn't thought that one through when designing this feature, so it didn't even occur to me until seeing your post

If I do get a version that stacks the stars *and* the comet then I would be able to fix this because I could get the rotation from the stars...

cheers,

Robin

Thanks Robin,

I didn't think about it myself, until I saw the curved star trails emerging. I reinstated the wedge this morning. I was thinking of doing this anyway because, while the Evolution mount has questionable precision for long exposures, the wedge does help with the framing of larger DSOs. It looks like comets are another reason to use a wedge, even when stacking shorter exposures.

Best regards,
Richard

[Jean-Francois]

Hello,

So here the summary of last night.

This time the live centering on the comet nucleus works with the Newton images.

Here 10 minutes (60 x 10s) ... AAAccchhh!! again forget to uncheck the box showing the target.

C2022E3-ZTF_60frames_600s_N10-F2.9_QHY174_L_SharpCap.jpg (155.89 KiB) Viewed 2561 times

OK, second try this time 20 minutes (40 x 30s) (without the box ):

C2022E3-ZTF_40frames_1200s_N10-F2.9_QHY174_L_SharpCap.jpg (150.66 KiB) Viewed 2561 times

I used the automatic dark and flat subtraction in SharpCap. But my flat is not very good ... the right side of the image is over corrected.

After that I did an interesting test ... the idea was that my mount follows the movement of the comet (not more the star, by the way note that I have no autoguiding). The ASCOM driver from my EQ8 mount allows an intermediate rate. The test was to set the mount to the displacement of the comet.
The SynScan software does not allow to set a custom rate on the axes and in SharpCap it is only possible to use the predefined rate (typ. the same as the hand controller or the SynScan driver). With SharpCap Pro, it is possible to program a small application ... I did it for my sun observation with a spectrometer (SHG imaging). So yesterday evening, I tested my script for the comet imaging.
But first ... in which direction shall move the axis ? ... With the software Prsim, I load the actual orbit parameters, on the 9.2.2023 the comet displacement was 8.78 "/min in the direction 190.1°, yesterday it was 7.83"/min.
After simple calculation, the mount movement should be in RA 0.0017 "/min and in DEC 0.0096 "/min ... but in + or - direction ? In addition the DEC direction change depending on the side of the pier.

Here the result (20 x 30s) ... the mount was moving in RA minus = 0.0017 "/min and in DEC minus = 0.0096 "/min.
Note that the values change permanently. But for this image, it was necessary to disable all the automatic centering in SharpCap.

C2022E3-ZTF_20frames_600s_N10-F2.9_QHY174_L_MoveAxis.jpg (180.18 KiB) Viewed 2561 times

Maybe that one day I will write a small program for the comet tracking (open loop without autoguiding).

One bonus image ... here with a small piece of a plane.

C2023E3ZTF_with_plane.jpg (133.21 KiB) Viewed 2561 times

Good weekend,
Jean-Francois

[FXModels]

I have been trying to track on a comet but I just get double exposures like I am looking at it crosseyed with two frames, then triple exposures with three rames etc...

No matter what I set the ignore stars filter size to it doesnt work. No matter what I set the detecton threshold to it doesnt work. It just makes the comet fatter and and adds another copy of the comet into the frame.

I have Align Frames and Align on Comet checked. I have Ignore Stars Filter size at 50 which was the default and it didnt work. Then I set it to
15 with no difference.
I have detection threshold at 0.10 to no effect, 0.05 to no effect and now .20 just to see what it does. No difference.

Its just not working. Comet is 2023 A3 Tsuchenshan-ATLAS. Its the fuzziest thing in the image.

Is this feature broken in the latest release?