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Conflicting tilt measurements results

Pleiades Astrophoto PixInsight Equipment Tilt Coma Collimation
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Eyecon avatar
Eyecon avatar
Hello,
I'm struggling to fine tune the last of the edge star aberrations in my ONTC 10" F4 Newtonian after countless attempts at tilt and coma adjustments. I use both Skywave collimation tool and Hocus Focus to adjust coma and tilt respectively. 

My collimation scores on SKW are excellent and visually, stars near the center of the sensor are perfect. Also after several iteration of using NINA's hocus focus to check for tilt, I'm within or very near CFZ for all corners of the sensor. The trouble is, I still see elongated stars towards the top of the image. ASTAP and Pixinsigth image analysis indicates a mild to sever tilt towards the top of the sensor but Hocus Focus seems to think there is slight tilt left to right.


Below are some results. To my eyes, I'd like to believe ASTAP/Pixinsight as the star shapes seem to follow the tilt suggested by these tools. My question is: should I attempt to fix the top to down tilt or should I believe what Hocus Focus is telling me?

Equipment: IMX 571 Mono camera, Antlia 3nm SHO filters, TS GPU Comma corrector, TS ONTC 10" F4

Here's the NINA Hocus Focus latest aberration inspector run (tested 2-3 times to eliminate measurement noise):


This is what ASTAP shows of an exposure on the same target within a few minutes of finishing the autofocus run (several consecutive images shows mild tilt but all in the same direction):

Same Image in Pixinsight Eccentricity analysis (several consecutive images shows similar patterns):


This is the mosiac of the corners from pixinsight...not terrible but I can see more aberrations in the top tiles but not so much near the center and bottom:


top row only:

bottom row only:
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Oskari Nikkinen avatar
Oskari Nikkinen avatar
As a newtonian imager i am cautious when attempting to fix these kinds of problems, so that i dont make things worse by chasing the tilt unicorn. Personally i would be satisfied with your tilt measurements here, but i am on the lazier side with these things.

BlurXT will make that very small issue of the top row vanish with a click. Not as good as not having tilt at all, but i wager nobody could tell the difference in a processed image unless you told them about the tilt.

One thing i should ask, how is your thermal management? The scope needs to be at ambient temperature to not have tube currents which would also appear as an elongation of sorts, possibly making tilt measurements inaccurate.

Thermal management is something i struggle with every time i image, because my scope is stored indoors and outside temperatures are usually 20-30 degrees c lower. In my case a mirror fan is not optional, with an 8" scope. As it happens i will soon also have the 10" ONTC you are using, and will have to sort out the fan thing with that too.
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Eyecon avatar
Eyecon avatar
Oskari Nikkinen:
As a newtonian imager i am cautious when attempting to fix these kinds of problems, so that i dont make things worse by chasing the tilt unicorn. Personally i would be satisfied with your tilt measurements here, but i am on the lazier side with these things.

BlurXT will make that very small issue of the top row vanish with a click. Not as good as not having tilt at all, but i wager nobody could tell the difference in a processed image unless you told them about the tilt.

One thing i should ask, how is your thermal management? The scope needs to be at ambient temperature to not have tube currents which would also appear as an elongation of sorts, possibly making tilt measurements inaccurate.

Thermal management is something i struggle with every time i image, because my scope is stored indoors and outside temperatures are usually 20-30 degrees c lower. In my case a mirror fan is not optional, with an 8" scope. As it happens i will soon also have the 10" ONTC you are using, and will have to sort out the fan thing with that too.

Good advise, to your point about BXT, I'm just trying to get the source images to be as balanced as possible to give BXT the best chance at deconvolving the image.

About thermal management, I have a semi permanant setup and I usually start the night by checking collimation and exploring different targets to see what looks best for the night. All the test results above are towards the second half of the night when the temperature has stabilized. The telescope was also well isolated from day time temperatures ( I cover the whole pier in an aluminzed mylar blanket to avoid excessive heat abosrbtion during the day) and left open to the air for an hour or so before I  started the evening. I also have a weather station near by to monitor temperature drops...all in all and in comparison to the beginning of the evening, I don't see much change in aberrations due to thermal changes.
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andrea tasselli avatar
andrea tasselli avatar
You have a slight outward tilt at the top right corner, pretty mild mind you.
Eyecon avatar
Eyecon avatar
andrea tasselli:
You have a slight outward tilt at the top right corner, pretty mild mind you.

Yes that's what the static image analysis results are also saying but I'm not sure why focus curve analysis is saying otherwise. Given that Hocus Focus is not telling me much at this point, do you think this is something worth pursuing using ASTAP/CCDInspector type tools?

How do you know that the tilt is outwards (i.e. sensor is too far I assume)? This is exactly why I rely on HF vs the other tools because I can never tell what direction to make the adjustment.
andrea tasselli avatar
andrea tasselli avatar
Yes that's what the static image analysis results are also saying but I'm not sure why focus curve analysis is saying otherwise. Given that Hocus Focus is not telling me much at this point, do you think this is something worth pursuing using ASTAP/CCDInspector type tools?

How do you know that the tilt is outwards (i.e. sensor is too far I assume)? This is exactly why I rely on HF vs the other tools because I can never tell what direction to make the adjustment.


I never use any of those tools, I do it interactively adjusting the tilt with the TCU if I feel it is worth the effort. Tangential elongation (coma-like) are indication of outward (too far) tilt while radial elongation (astigmatic line) indicates the sensor is too close, IMO. This assuming that you are well collimated, which might not be obvious.
Josh Jones avatar
Josh Jones avatar
So the problem here I think is that Hocus Focus has a poor fit, so I wouldn't trust it.  My guess here is there just aren't enough stars in the detection to determine.  that's why the poor fit.  

ASTAP single frame analysis is probably fine since it's using focused stars.  A nice test here is save a hocus focus run, then goto ASTAP and the stack inspector.  You can use those saved images from your Hocus Focus run and load here into the ASTAP -> Inspector section and analyse.  It will show star count and how quickly it drops off in out of focus images. 

You really need to have super milky way targets rich in stars to get good fits in Hocus Focus and best results.

The pixinsight eccentricity map shows similar to ASTAP, I would trust those, work on that top tilt…  work in small movements, see if it gets better in a the right movements as you do.
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Eyecon avatar
Eyecon avatar
Josh Jones:
So the problem here I think is that Hocus Focus has a poor fit, so I wouldn't trust it.  My guess here is there just aren't enough stars in the detection to determine.  that's why the poor fit.  

ASTAP single frame analysis is probably fine since it's using focused stars.  A nice test here is save a hocus focus run, then goto ASTAP and the stack inspector.  You can use those saved images from your Hocus Focus run and load here into the ASTAP -> Inspector section and analyse.  It will show star count and how quickly it drops off in out of focus images. 

You really need to have super milky way targets rich in stars to get good fits in Hocus Focus and best results.

The pixinsight eccentricity map shows similar to ASTAP, I would trust those, work on that top tilt...  work in small movements, see if it gets better in a the right movements as you do.

I understand what you mean but unfortunately this is not the case here. I noticed that HF fits get significantly worse the closer I get to a good tilt. This has been my observation over several attempts...I can get very close (.05 degrees), beyond that the sensor model fits are extremely poor and I can't even visually see the tilt on the sensor diagram to tell what direction I need to make adjustments. At this point I rely on the sensor model image( the heat map image at the top of the anlysis results) to tell what's going on. In this case and many other attempts, the image shows a relatively even but round curvature even though as you see, several other tools indicate a tilt in a specific direction.


The target above was a very dense star field in the mikly way near NGC7380 and the annotated HF images shows a very good coverage of recognized stars across the entire image. I also usually do exactly what you suggested: run ASTAP  Image inspector on the "final" fits image stored in the HF run folder and that's when I notice a conflict in results.

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Eyecon avatar
Eyecon avatar
andrea tasselli:
Yes that's what the static image analysis results are also saying but I'm not sure why focus curve analysis is saying otherwise. Given that Hocus Focus is not telling me much at this point, do you think this is something worth pursuing using ASTAP/CCDInspector type tools?

How do you know that the tilt is outwards (i.e. sensor is too far I assume)? This is exactly why I rely on HF vs the other tools because I can never tell what direction to make the adjustment.


I never use any of those tools, I do it interactively adjusting the tilt with the TCU if I feel it is worth the effort. Tangential elongation (coma-like) are indication of outward (too far) tilt while radial elongation (astigmatic line) indicates the sensor is too close, IMO. This assuming that you are well collimated, which might not be obvious.

Ah yes, that makes sense. Yes collimation is supposed to be spot on at least according to Skywave and also visual inspection of the focused and defocused stars around the center.
Josh Jones avatar
Josh Jones avatar
Josh Jones:
So the problem here I think is that Hocus Focus has a poor fit, so I wouldn't trust it.  My guess here is there just aren't enough stars in the detection to determine.  that's why the poor fit.  

ASTAP single frame analysis is probably fine since it's using focused stars.  A nice test here is save a hocus focus run, then goto ASTAP and the stack inspector.  You can use those saved images from your Hocus Focus run and load here into the ASTAP -> Inspector section and analyse.  It will show star count and how quickly it drops off in out of focus images. 

You really need to have super milky way targets rich in stars to get good fits in Hocus Focus and best results.

The pixinsight eccentricity map shows similar to ASTAP, I would trust those, work on that top tilt...  work in small movements, see if it gets better in a the right movements as you do.

I understand what you mean but unfortunately this is not the case here. I noticed that HF fits get significantly worse the closer I get to a good tilt. This has been my observation over several attempts...I can get very close (.05 degrees), beyond that the sensor model fits are extremely poor and I can't even visually see the tilt on the sensor diagram to tell what direction I need to make adjustments. At this point I rely on the sensor model image( the heat map image at the top of the anlysis results) to tell what's going on. In this case and many other attempts, the image shows a relatively even but round curvature even though as you see, several other tools indicate a tilt in a specific direction.


The target above was a very dense star field in the mikly way near NGC7380 and the annotated HF images shows a very good coverage of recognized stars across the entire image. I also usually do exactly what you suggested: run ASTAP  Image inspector on the "final" fits image stored in the HF run folder and that's when I notice a conflict in results.


I think it says 138 stars in this image though, that's extremely low...  a good focused images should detect over 1000 stars.  Of course I don't know all the setup, but just my guess as to why HF isn't producing same results here is just star detection setup in HF is askew or something.