PSFEstimation script

[mschuster]

Thank you Sergio,

I agree with Rob.

I didn't envision focusing with the scripts, I think they won't be efficient. Does FocusMax work with DLSR?

Mike

[Astrocava]

well, if using an OSC you have to debayer the frames first...

Yes, that's sound reasonable, the stars of a non-debayered image are not the best to perform this task. 

for focusing, you might be better off using the various focus quality metrics built into BackyardEOS. if you have a bright enough star available you can actually use live view to achieve focus. in addition the latest version can also understand a bahtinov mask diffraction pattern and give you feedback about the focus.

also there's some freeware for windows called "bahtinov grabber" that can take a (user defined) screenshot, and given the focal length of your instrument can determine if you are in the critical focus zone.

I'm now using live view for focusing in APT, but I want more precision and be more seeing independent. APT implements bahtinov grabber,  I will try to make a mask and use it.

if you want to try this script then maybe the best thing to do is just set the camera to take JPGs while trying to focus?

Yes, that's a nice idea. Thanks

Thank you Sergio,

I agree with Rob.

I didn't envision focusing with the scripts, I think they won't be efficient. Does FocusMax work with DLSR?

Mike

The problem with FocusMax is that you need Maxim to control the camera, and that's expensive.

[mschuster]

Sergio,

The B-mask is worth trying, but it might not be accurate on OSC, I don't know. The B-mask diffraction pattern may be less legible on a bayer array.

I printed the mask pattern on a sheet of mylar, that did not work, it was not accurate. I am not sure why, maybe its flexibility.

The metal mask from spike-a.com works wonderfully for focusing both wideband and narrowband. It is what I use for focusing my f/5 setup.

It is seeing insensitive and accurate. A 10 micron change in focus is visible with the mask (measured with my Atlas digital focuser).

Based on a phase-detect principle, both the direction of defocus and its magnitude are measurable from a single B-mask exposure, unlike contrast-detect used by FocusMax for example which requires multiple exposures.

Also the B-mask gives information across the entire frame. With a set of reasonably bright stars across the frame (e.g. an open cluster), focal surface curvature and tilt is also made visible.

Regards,
Mike

[antonio]

Hello Mike, thanks for the very useful script! It was not immediate to figure out what did you mean by "eccentricity", but I understand that the standard geometry definition for the ellipse applies. I also realized that, if one wants, your script can even provide the same information as CCD Inspector: one just needs to perform a best fit of the measured function FWHMm(i,j) with the mathematical expression of FWHM for a tilted plate and a miscollimated optics. The least-square fits would return the optical center on the plate (xc, yc), the curvature of the focal plane and the tilt angles in the x and y directions (or even a single tilt angle and its direction in the x-y frame).
Regards,

Antonio

[mschuster]

Thank you, Antonio.

Yes, the standard geometry definition for the ellipse applies to eccentricity. I included formulas and a table in SubframeSelector documentation relating eccentricity, aspect ratio and a flatness measure. Stars with eccentricity values less than 0.42 to 0.44 look mostly round. Much larger than that and they start to look elongated.

Presenting focal surface parameters like CCD Inspector would be very helpful. Spatial FWHM variations may be due to various causes unrelated to focal surface curvature/tilt (e.g. astigmatism, coma, field rotation, guiding errors, flexure, etc). These other causes can be a source of confusion.

My setup suffers from tilt, verified by refocusing along the tilt axis. Here is a plot. The tilt runs diagonally, the upper left is inside of focus, the center is in focus, the lower right is outside of focus. I have replaced the entire imaging train but the tilt remains nearly the same. So I think it may be due to optical misalignment and/or sensor tilt.



In hopes of solving this tilt problem, I bought one of these and am now awaiting delivery of adapters.

Regards,
Mike

[antonio]

Mike, I agree with you: the parameters summarizing "field correction" in CCD Inspector can be misleading sometime as in the cases you mentioned. For sure, the 2D maps of the FWHM spatial dependence are most truthful as "real data": from these maps one can optimize his scope setup without knowing the exact field curvature.
The map of your ccd FWHM does show clear indication of a tilt effect, but I wonder whether this is clearly discernible by visual inspection of the stars in the image: after all you go from 0.96 to 1.06 pixels which does not seem so much. Just curious!
Regards,

Antonio

[mschuster]

Hi Antonio,

You are right. I have to admit that looking at the raw frames the tilt is not discernible, well maybe only very, very slightly.

But with a Bahtinov mask on a test exposure, the tilt is annoyingly obvious. The diffraction patterns show a rather large portion of the sensor outside the critical focus zone. It's something I want to fix if possible.

Regards,
Mike

[mschuster]

With some work and PixInsight's help with measuring stars, I think I have finally eliminated my setup's sensor tilt and temperature dependent focus drift.

Here is an map of a 40 minute light without the tilt fix, the tilt runs from upper-left to lower-right:



Here is a map of a 40 minute light with the tilt fix, nearly no tilt:



Here is a plot comparing FWHM's of short 20 second exposures with no guiding to the FWHM's of long 40 minute guided exposures. One short was captured immediate prior to starting each long exposure. FWHM's are nearly the same with nearly no focus drift:



Regards,
Mike

[pfile]

nice work - how did you eliminate the focus drift? i thought this was purely a function of the expansion of materials under a change in temperature.

i have a long way to go with my reflectors… currently far from properly collimated and/or tilt free.

rob

[mschuster]

nice work - how did you eliminate the focus drift? i thought this was purely a function of the expansion of materials under a change in temperature.

Hi Rob,

Thank you. My setup doesn't truly eliminate focus drift, rather it compensates for the drift by using active focusing. It works just like an Optec TCF focuser, but uses a different focuser, thermistor, and my own drivers and modeling software. Challenges were good modeling for a mobile setup in varying weather conditions, and image shift when the focuser makes small mechanical movements.

Regards,
Mike

[Juan Conejero]

If PixInsight Magazine were still alive this would make for a very nice article. Really good stuff.

[Geoff]

If PixInsight Magazine were still alive this would make for a very nice article. Really good stuff.

A stand-alone article perhaps?

[georg.viehoever]

The idea behind PixInsight Magazine - high quality well redacted articles - still has a lot of appeal for me. But ultimately, any magazine has to feed its editors (i.e. earn money). I wonder if it is possible to build a business case for such a PI centered product, maybe a PI magazine, a PI book, a PI (meta) blog, a PI newletter, ... . Any opinions?

Georg

[mschuster]

Thank you. I have been playing more with the data:

Having used Pixinsight to measure stars and write the measurements to .csv files, I used Mathematica to plot smoothed histograms of star FWHMs. One histogram for each integration, with combined FWHM measurements from all of its subframes.

My setup at 4.2 arcsec per pixel is undersampled, so FWHMs can't be much smaller than about one pixel. But how close to one pixel can they be in good seeing?

Here is the progression over the past year and a half in three charts, each with just four histograms to make them more legible. Since this past August, the histograms have shifted leftward and become more peaky. This of course means that FWHMs are getting smaller and more equal across the frame. This improvement is due to better focus and better collimation.

The charts together have data for almost 300 forty minute subs, almost 200 hours of exposure.

Regards,
Mike