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An idea for inspecting spacing/tilt issues

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#1 AstroGabe

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Posted 26 December 2020 - 10:11 PM

I've been working on my Astroimaging Planner for a while now, but have detoured a bit into automatic image grading.  While working on that, I have some functions that will extract the properties of each star in a subframe.  These include FWHM, eccentricity, ellipsoid angle, local sky background, SNR, and other moment properties of the star shape.  I'm interested in using that data to uncover more insights about my imaging systems.  I've already found a lot of interesting effects with the almost 5k subs that I've stored over the years. 

For my imaging rigs, I feel like some of them could be optimized, but it's all very ad-hoc.  Tonight, I shared here on CN an analysis frame of one of the Takahashi Epsilon 160EDs raw subs that was shared there (impressive scope!).  This analysis uses some of the tools I'm writing.  It bins stars into a resampled grid on the chip to determine bulk FWHM/eccentricity stats, etc.  Here's the view of that plot showing what appears to be tilt.  The red bars indicate the direction of the ellipticity, and the length is proportional to the ellipticity.  The size of the dot at each grid site is proportional to the aggregated FWHM for that bin.

 

test_resampled_asi2600_eps160.jpg

 

That got me thinking about some of the methods to determine the best spacing between a flattener and the chip.  One is to inspect the radial/azimuthal direction of the ellipticity when the spacing is too short/long.  Another that I heard recommended by Roland Christen is to focus on a star in the middle, then focus on a star at the edge of the frame.  The relative difference between focus position indicates whether you need to increase/decrease the chip-flattener spacing. 

 

So, naturally, I want to automate this and thought that Voyager's Dragscript would be the perfect partner for it.  It's incredibly easy to setup a script for this with Voyager. 

Screen Shot 2020-12-26 at 9.08.44 PM.jpg

 

 

The plan, shown in this example, is to get 16 frames, spaced by 50 microns (set by hand to my specific system depending on CFZ, etc.), and inspect each binned FWHM/ellipticity/theta to see if the effect that can reliably be measured and implemented.  In the end, I think this process should have an outcome of:
- `increase spacing by 0.8mm` to address flattener/chip spacing issues
- `add spacing in the 60 degree direction by 0.1mm` to address tilt in the imaging train

Winter in my area is notorious for bad weather for astronomy.  I'm hoping to try this in the next few months to see if it'll work.


Edited by AstroGabe, 27 December 2020 - 06:02 PM.

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#2 bugbit

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Posted 26 December 2020 - 11:13 PM

It sounds very interesting. Please keep us posted


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#3 spokeshave

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Posted 27 December 2020 - 08:46 AM

Brilliant. I have never understood why other software will go to the trouble to measure eccentricity and then throw out the data regarding the orientation of the eccentricity. I always thought that an eccentricity map just as you have made would be an essential image analysis tool yet no one that I know of does it. I sure hope you make your software available to others when it is done. I would gladly pay for it.

 

Tim


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#4 Rasfahan

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Posted 27 December 2020 - 01:43 PM

This sounds (and looks) like an amazing toolkit. Please keep us posted.


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#5 lucam

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Posted 27 December 2020 - 04:42 PM

This is excellent, Gabe. I don't if you know if you are familiar with diffusion tensor imaging in MRI but do you are doing here is very similar to that technique. Given this information, you may want to consider modeling the best fit to a combination of field curvature, tilt, and center focus spot position, akin to CCD Inspector. 

 

Very nice work and am going to follow your project with great interest.

 

Luca


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#6 AstroGabe

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Posted 27 December 2020 - 05:34 PM

Thanks everyone for the encouragement.  

 

Here are some more examples from other frames

Here's one with obvious spacer issues from years ago (more on this below):

m8_test.jpg

 

Another with wind issues resulting in elongation in one direction:

m31_test.jpg

 

 



#7 AstroGabe

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Posted 27 December 2020 - 05:35 PM

Luca, I worked in Medical Physics/Medical Imaging in a past life, but worked on the CT/DSA side of things.  I'll look into DTI.

 

I'm assuming there's continuity across the distorted stars due to the aberration, so that would allow the elongation to be modeled as a vector field.  We can break these vector fields down into divergence and curl terms which indicate inward/outward spacing.

 

Vector field with divergence, indicating flattener spacing is too close:

Screen Shot 2020-12-27 at 4.32.39 PM.jpg

Vector field with curl, indicating flattener spacing is too far:

Screen Shot 2020-12-27 at 4.32.48 PM.jpg


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#8 prefetch

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Posted 27 December 2020 - 06:02 PM

i think this is great.  i'm excited about what you working on!

 

having had to deal with my own spacing/tilt issues i recently discovered the pixinsight tools that are applicable - but they are not exactly easy to use while your fingers are on the dial in realtime so to speak.

 

additionally, they don't give an explicit indication of vector.

 

is your intention for your software to be a "realtime" tool like CCD inspector?

 

RJHDKFv.png


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#9 AstroGabe

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Posted 28 December 2020 - 05:34 PM

I don't think it'd be a realtime application, at least not the first version.  It'd be something similar to the planner I've written, and maybe even integrated into that app

https://www.cloudyni...maging-planner/

 

That app is all open source, so people are free to contribute, etc.

 

Right now, what I have written is in a notebook, but the plan would be to feed it one or multiple fits files for the analysis, and a summary of tilt, field curvature, astigmatism, etc. would be provided.

 

Gabe


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