Help needed resolving tracking or guiding setup

[Acceptedplanet]

Hi

I am new to astrophotography and am having some problems figuring out what is wrong with my current setup;

Edge HD 8" 0.7 reducer

AVX mount

Skywatcher Evoguide 50mm guide scope (may replace with OAG at some point, but know people do use guide scopes too with this setup)

ZWO 120 mm-s as guide camera

ZWO 533 main camera

Stellarmate and Kstars

 

Decided to take Bodes galaxy.

I did polar alignments using pole master camera and then did starsence alignment, was then able to do plate solving within few arch secs. so all seemed good.

I then made sure guide camera and main camera were best I could focus (still room from improvement)  and started stellarmate internal guiding.

The guiding seem to be good as RMS was low ra and dec, seemed to be guiding with 2 arc sec (much better the a 4/5 arch secs few weeks ago trying to polar align manually). 

I took about 1.5 hrs of 180sec sub exposures and each one looked good in kstars with little sign of star trail on each sub. I would of expected to have seen some if the guiding was not working.

So was happy, but on trying to stack the images using pixinsight the image turned out bad with massive star trailing. I also noticed the tracking does not seem to be keeping up with the target (bodes galaxy in these images) with it drifting over time when I did blink process. I assumed pixinsight would still be able to matchup stars and  align but it does not seem to work. The star drift must be too much.

Maybe someone experienced looking at these results will know straight away thats causing it or I have missed a step? Could it be guiding picking up hot pixels and not acutely tracking as expected? 

[Aramcheck]

Did you do Pixinsight's StarAlignment process before integrating (stacking)?

Cheers
Ivor

[Acceptedplanet]

9 minutes ago, Aramcheck said:

Did you do Pixinsight's StarAlignment process before integrating (stacking)?

Cheers
Ivor

Yea have tried both WBPP and doing it old way with StarAlignment and the Image Integration to see where it's going wrong. Not sure if the algorithm is having problem because the stars are so soft in the images. 

[ONIKKINEN]

Looks like the trailing is mostly in declination.

Could mean polar alignment issues resulting in DEC drift or differential flexure of the guidescope and the imaging scope. I would guess a bit of both. Your tripod could sink or something other in the mount could "settle" after polar alignment, taking you out of what you thought was good polar alignment. If you have guide logs you can look through, you can see what the actual accuracy of your polar alignment was. Dont know if that's something you can do with your guiding (stellarmate internal?) but i would hope there is a log of some sort written by the software. The differential flexure thing is also quite likely since you are imaging with an SCT where the primary mirror can move or "flop" around in the scope without you being aware of it. Guiding through a separate guidescope means the guidestar stays in the right position on the guidescope, but the imaging scope drifts away. The fix for that would be an OAG. The total amount of drift is not an issue, as long as you have the sensor area to spare for it and the drift is not apparent in a single sub.

The soft stars thing is probably what leads to the failed alignment. If no stars are detected, or worse: hot pixels are detected as stars then the alignment will fail like it did with your shot. How many stars does PI report from the subs?

[Acceptedplanet]

10 minutes ago, ONIKKINEN said:

Looks like the trailing is mostly in declination.

Could mean polar alignment issues resulting in DEC drift or differential flexure of the guidescope and the imaging scope. I would guess a bit of both. Your tripod could sink or something other in the mount could "settle" after polar alignment, taking you out of what you thought was good polar alignment. If you have guide logs you can look through, you can see what the actual accuracy of your polar alignment was. Dont know if that's something you can do with your guiding (stellarmate internal?) but i would hope there is a log of some sort written by the software. The differential flexure thing is also quite likely since you are imaging with an SCT where the primary mirror can move or "flop" around in the scope without you being aware of it. Guiding through a separate guidescope means the guidestar stays in the right position on the guidescope, but the imaging scope drifts away. The fix for that would be an OAG. The total amount of drift is not an issue, as long as you have the sensor area to spare for it and the drift is not apparent in a single sub.

The soft stars thing is probably what leads to the failed alignment. If no stars are detected, or worse: hot pixels are detected as stars then the alignment will fail like it did with your shot. How many stars does PI report from the subs?

Many thanks, Yes that makes sense as could be as you say it polar aligned at start but then settles out of place, on moving to the target to be imaged. I think I need to get it on more solid foundation as maybe they are sinking a bit due to weight and centre of gravity over time shifting, hence affecting the dec. I will try to record the guiding next time and then can see more info. 

Pixinsights seems to be finding lots of stars, but still not aligning them, here is from one sub

Reading FITS image: 16-bit integers, 1 channel(s), 3008x3008 pixels: done

58 FITS keywords extracted

Structure map: done

Detecting stars: done

121 stars found.

Matching stars:

88 putative star pair matches.

Performing RANSAC: done

77 star pair matches in 38 RANSAC iterations.

 

[Aramcheck]

By default StarAlignment uses PSF fits. Under "Star Detection" you could try changing the 'Compute PSF fits' from "Distortion Correction only" to "Never" & ensure that the Registration Mode is "Projective Transformation"

Juan Conejero suggests this where stars aren't focused in this thread: https://pixinsight.com/forum/index.php?threads/star-alignment-fails.13598/

Cheers
Ivor

[newbie alert]

If the guiding wasn't flagging up any errors but stars have drift in them then that's differential flexture.. I do notice that some of your stars seem to have  a miscollimation , but others seem ok.. what spacing are you using?

[Acceptedplanet]

1 hour ago, Aramcheck said:

By default StarAlignment uses PSF fits. Under "Star Detection" you could try changing the 'Compute PSF fits' from "Distortion Correction only" to "Never" & ensure that the Registration Mode is "Projective Transformation"

Juan Conejero suggests this where stars aren't focused in this thread: https://pixinsight.com/forum/index.php?threads/star-alignment-fails.13598/

Cheers
Ivor

Thank for the link, I tried those setting but still not aligned, so played around with RANSAC tolerance and nose reduction and seems to have helped. 

[Acceptedplanet]

18 minutes ago, newbie alert said:

If the guiding wasn't flagging up any errors but stars have drift in them then that's differential flexture.. I do notice that some of your stars seem to have  a miscollimation , but others seem ok.. what spacing are you using?

Yea my backfocus should be 105 as I have the 0.7 reducer. I have Celestron focus unit. Was having fun trying to get automatic focus working in stellarmate , might try using my Bahtinov mask next time

[newbie alert]

8 hours ago, Acceptedplanet said:

Yea my backfocus should be 105 as I have the 0.7 reducer. I have Celestron focus unit. Was having fun trying to get automatic focus working in stellarmate , might try using my Bahtinov mask next time

With a HD?  The standard XLT is quoted as 105 with the .63 reducer, can't remember what Kurt ( astroquest 1) set his up as ...

Just looked it up and he's using 108.5 on his...Im not using the spec'd backfocus on mine as 105 didn't work for me ( XLT with .63 reducer)

 

 

 

[bosun21]

14 hours ago, ONIKKINEN said:

since you are imaging with an SCT where the primary mirror can move or "flop" around in the scope

The Edge HD has mirror lock screws to avoid the mirror moving around. Whether the OP is using these is another matter.

[newbie alert]

38 minutes ago, bosun21 said:

The Edge HD has mirror lock screws to avoid the mirror moving around. Whether the OP is using these is another matter.

Mirror locks don't  totally eliminate mirror flop or mirror shift...

[michael8554]

The subs have "round" stars, with some tilt and coma.

So no Diff Flex.

I wouldn't worry about the slight Dec drift over 90 minutes, since guiding seems to be coping.

The stacking error suggests a wrong mode like comet stacking, but obviously the galaxy isn't moving with respect to the stars like a comet would.

Back focus for an 8" Edge HD is 133.5mm without the 0.7FR, 105mm with the FR:

https://www.innovationsforesight.com/support/celestron-edgehd-back-focus-tolerance/

Michael

[newbie alert]

1 hour ago, michael8554 said:

 

Back focus for an 8" Edge HD is 133.5mm without the 0.7FR, 105mm with the FR:

https://www.innovationsforesight.com/support/celestron-edgehd-back-focus-tolerance/

Michael

Is it??

That's the specified backfocus, don't mean that it is..

[michael8554]

You've lost me there I'm afraid.

Michael