New script for testing the on-axis optical quality of telescopes

[mcgillca]

Dear Mike,

This is a fantastic script - I tried it on Saturday. It worked really well (other than the result - some collimation to do... ).

One question: my focuser is not adjusted well at the moment and the focus positions are not repeatable. I captured images about 12mm either side of the focus position, but there is an error in that of about 0.1mm.

How sensitive are the results to the exact locations of the images?

Thank you very much for a wonderful tool.

Colin

[mschuster]

Thank you so much Colin! I am very glad that it worked!

Focus distance on either side should be equal to within 5%. The script automatically adapts to differing focus distances. So you are well within the allowed range.

Thanks,
Mike

[madrid sky]

Dear Mike.

First off, congrats on your script. I have two questions:

1) I am planning to use your script to create a PSF that models my optical train and feed that model to Pixinsight for deconvolution purposes. How do you think I could do this?
2) I guess the 180º filp your script does it automatically. I just have to provide extra and intra focal images. How do you get 5% max difference in focus between extra and intra? (Maybe measuring PSF external diameter?)

Thank you in advance,
Sam

[mschuster]

Hi Sam,

1) The script’s PSF estimate is likely too optimistic for deconvolution purposes. The estimate characterizes “on-axis optics only” and does not account for several aspects that typically increase Strehl diameter on target captures such as atmospheric turbulence, focus drift, sensor tilt, off-axis aberrations, frame registration interpolation, and light scatter.

2) The script’s Exposure tab provides helpful tools. First estimate a target defocus diameter, then repeatedly adjust focus and measure the result until defocus diameter is within tolerance. The script does the 180º flipping.

Thanks,
Mike

[madrid sky]

Ok, Mike.

Then... how would you recommend to me to do a PSF estimation for deconvolution purposes in Pix Insight? I have an artificial star and I aim mainly to high resolution lunar/planetary work.

Let me know, please.

Thank you in advance,
Sam

[mschuster]

Sam,

The DynamicPSF process. Expose a star field in typical conditions, then use DynamicPSF to export a PSF given a selection of stars. See DynamicPSF documentation, Juan's posts in this thread, and other forum posts and resource tutorials.

Thanks,
Mike

[TobiasLindemann]

Mike,
this is a excellent script. I have tried it last week on my TSA102 and it worked perfect. Thank you very much.

Greetings
Tobias

[mschuster]

Thank you Tobias! It is really great to see your results presented so nicely! I am also pleased that the script worked with a flattener/reducer, I did not do such a test.

Best regards,
Mike

[tommy_nawratil]

hello Mike,

congrats, just stumbled upon your script - amazing, this is a variety of Roddier test popular in France, they work a lot on it.
I tried this years ago, but couldn't come to reliable results with the software available at that time.
Switched to the use of a Twyman-Green interferometer as I do optical testing regularly now.
As soon as I have some time I'll work through your script and then we can compare the results to interferometry,
if you didn't do that already.

Or maybe you are even able to use one of the real star tests in my blog with your script and compare the results.
http://interferometrie.blogspot.co.at/

There are many entries that have both star test and measurement.

respect and best regards,
Tommy

[mschuster]

Thank you Tommy,

I am reading your blog, excellent work, a really great resource for anyone interested in optical testing!

The script provides an estimate whose accuracy is ultimately limited by atmospheric turbulence and algorithmic approximations. Long combined exposures help to average out atmospheric turbulence, the plot below shows an example of variability in estimated Strehl ratio for different total exposure times with my FSQ-106EDX in metropolitan observing conditions. Long exposures (at least 100 seconds in both intra and extra-focal stacks) are required for reasonable estimates.



In each column, the white line corresponds to the median of the Strehl ratio estimates, the bottom and top of the box correspond to the 25% and 75% quantiles, and the lower and upper fence correspond to the minimum and maximum estimates.

The script is also limited by algorithmic approximations. On tests with known aberrations (and no atmospheric turbulence) generated by numerical integration of the Rayleigh-Sommerfeld diffraction integral, relative accuracy in wavefront RMS error ranged from 5% to 20% depending on the optic and set of aberrations simulated. For more information, see sections 1.9, 5, and 6 in the script's documentation. For more accurate results, interferometric testing is necessary. I have not done such testing, anything you could do for comparison would be very helpful!

The script requires bias-subtracted frames as input (as well as detector specifications), so unfortunately I can't use images clipped from your blog.

Thanks,
Mike

[andrewluck]

Thanks for this incredibly useful tool Mike.

I ran it against my FSQ-85 and the results are revealing. Up until now, Takahashi have been claiming my problem is down to tilt in the optical train. This gives me some useful results to feed back.

Andrew

[mschuster]

Thank you Andrew,

FYI: my first new FSQ-106 arrived decollimated, fairly good on-axis, but bad off-axis. I sent Tak a frame, Tak asked me to send it back, I did, guessing it would take a trip to Japan and a while to fix. Two days later a new one arrived unannounced. Very good service.

Second one was vastly better, but a small tilt was evident across the frame made easily visible with a Bahtinov mask, and also consistently measureable in SubframeSelector's FWHM plots. I installed a hardware detilting device in the optical train to solve this problem. I have never been able to figure out the cause of the tilt, could be the detector itself or maybe a cumulative tilt from all the various parts.

Mike

[mschuster]

WavefrontEstimator Version 1.19 soon to be released as an update.

• Fixed a bug in the Bayer RGB detector support code, the script now supports other non-Bayer CFA type detectors also.

• Supports a wider range of defocused image diameters, from 32 to 320 pixels.

• Tested on a Nikon D800 camera with a Zeiss f/2 lens with expected relative error in the estimated wavefront in the 10 to 20% range.

• Includes several other usability improvements.

Thanks,
Mike