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Help request for star shape diagnosis [Deep Sky] Acquisition techniques · patrice_so · ... · 8 · 493 · 5

patrice_so 7.87
patrice_so
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Dear fellow astroimagers, 

A few weeks ago (not to say months or "ages"), I changed my main imaging camera. I swapped my trusted ASI294MC Pro for a Touptek 2600mm. I ran some test and the camera appears to work fine. However, I have persisting star shape issues since I move to this camera. In short, I have unsatisfactory stars accross the field, but with a pattern that does not clearly suggest bad guiding, bad collimation, bad BF or tilt. I am bit lost here and I would like to gather some qualified opinions. 

Please find the abberation inspector frames of two 600s subframes with my Ha and Oiii filters. 
image.png
Both were taken tonight a few minutes appart, with my 200/800 f4 newtonian, my touptek 2600mm, an OAG with an ASI120mm, all of that on my EQ6r Pro. Note that guiding was below 0.7'' at anytime (tonight sky is not better than that but that's ok). With good conditions, I get down to 0.35''. 

Any insight would be much apprectiated. 

EDIT 1 : I replaced the mosaic by one where the Ha and the Oiii have the same orientation. 
EDIT 2: Here is a mosaic of a 1 hours stack, nothing done except gradient extract. 
image.png

CS

Patrice
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astrospaceguide 2.41
Josh Jones
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Hello:

I think the best thing to do would get a very star saturated field here.  It would provide a better/clearer picture of what might be happening here if there were more stars in the images.   It's pretty hard to estimate here with only 6 to 7 stars in a field, and each filter is taken of different target so can't really compare.
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AstroTrucker 6.22
Tim Ray
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Hard to say, your examples are 180 degrees out of phase. One looks like Oiii the other Ha which would account for the difference in star size and signal strength. Smaller stars appear to have same amount of deformation in both mosaics…

I personally would let BXT chew on them which should yield acceptable results…

CS Tim
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andreatax 9.89
andrea tasselli
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600s images is NOT the way to go to pinpoint issues with the imaging train and neither is using NB filters. Try 5s images with no guiding and L filter and see what fwhm you get. Looking at the image it looks like you have mis-collimation but it is highly subjective.
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andresch 1.91
Christian Andres
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Another culprit could be vibrations introduced by the fan of the camera cooling. I've had this issue with an ASI 2600mc on a 150/750 newtonian. Taking a few exposures with the cooling turned off revealed the issue in my case.
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Paulinho 5.01
Paul Larkin
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Hi, Patrice.
I had a similar issue with my EdgeHD8 and ZWO ASI2600MM.  Whether or not it's the same problem, you could go with @andresch and test for vibration - there are lots of threads in CloudyNights about this, at least for the ZWO ASI2600MM - fan vibration has been an oft-cited issue.  Not sure about the Touptek, but if the only change you have made is the camera, then it is likely something related to that change.

It's quite easy and it costs nothing to test - no fiddling with tilt, spacers, etc. 

Take some subs with the fan ON and some others with the fan OFF.

Here is an example of the difference it made for me.  Equipment is EdgeHD8, ZWO ASI2600MM, Antlia 3nm filters, ZWO filter wheel, ZWO OAG with ASI174MM mini guide camera.
Guiding was on for both of these tests: 300s (Ha filter).  I actually did several to confirm, including at shorter exposures with L filter etc.



Fan ON.  Misshapen stars (this is a crop, but it was across the whole field of view).
image.png

Fan OFF.  Lovely round stars (notwithstanding that collimation was not spot on).
image.png

I ended up replacing the fan and the problem was solved.  Some folks add rubber bushes, but I found they were not needed.

I put in a 50mm Fractal Fan which I bought on Amazon.  This fan was recommended by a couple of folks on CloudyNights.

Problem solved.  After some accurate collimation, I am very happy with the result.

I hope that helps, Patrice. 

All the best.
Paul
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Gondola 8.11
Tony Gondola
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As Andrea pointed out, 600 sec. subs are unnecessarily long. There's simply no reason to go that long on a 2600 sensor. All you really gain is more time for issues like this to become apparent. Your lowest FWHM will come with the shortest subs. It can go even lower with a well culled set of short subs.
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ScottBadger 7.63
Scott Badger
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Something else to check…..you can have decent guiding according to total RMS, but still get misshapen stars if RA and DEC RMS are significantly different. I ran into that when my mount developed a new periodic error. PHD2’s PPEC algorithm tames most of it.

Cheers,
Scott
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patrice_so 7.87
patrice_so
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Dear all, 

Many thanks for your support. 

I narrow down the problem to a faulty spacer that was introduced some tilt in the train and that also affected the BF. Swapping is for another one, unfortunately thicker, helped a lot, as I now have a regular slightly excessive BF distance pattern. I will order some space to remove entirely the issue. At the moment, BlurX is enough to fix the issue. That will do for the clear nights we have for christmas. 

image.png

Clear skies, 

Patrice
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