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

[mschuster]

WavefrontEstimator Version 1.19: Script for testing the on-axis optical quality of telescopes.

WavefrontEstimator estimates the on-axis optical wavefront of a telescope from long-exposure defocused stellar images. Exposure differences between an intra-focal image and an inverted (rotated by 180°) extra-focal image of the same bright star reflect local changes in the curvature of the wavefront. WavefrontEstimator measures the defocused image exposure differences, reconstructs the wavefront, and provides a diagnosis of wavefront aberrations. WavefrontEstimator relies on long combined exposures of at least 100 seconds and 100,000 e- to average out the effects of atmospheric turbulence and provide sufficient signal-to-noise ratio. The telescope must be in thermal equilibrium.

The estimated wavefront defines a map of optical phase on the aperture plane, normalized to zero mean phase. Non-zero estimates correspond to wavefront aberrations and lower image quality.

WavefrontEstimator resolves wavefront deformations or corrugations at a maximum spatial frequency that depends on the degree of defocus, the aperture diameter and focal length of the telescope, and the observation wavelength. The maximum corrugation spatial frequency measured is denoted the corrugation resolution. Typical values range from 10 to 15 cycles per aperture diameter.

Call for help: If you would like to help test this script, please let me know!

For more information and examples, please see the script's documentation.

Thanks,
Mike Schuster

[pfile]

yes i would be interested in trying this - however, will it work with a reflector that has a secondary obstruction? the defocused stars will look like donuts rather than the pure circular diffraction pattern (is there a name for that?)

thanks

rob

[mschuster]

Hi Rob,

Yes, the script should work with a secondary obstruction as long as it is circular and centrally located (i.e. the usual case). The documentation PDF has examples of these ring shaped Fresnel diffractions in section 6.

What is the aperture diameter, focal length, and obstruction diameter (an estimate)? I can run more tests given this info.

Thanks,
Mike

[Juan Conejero]

Hi Mike,

Thank you so much on behalf of me and all PixInsight users, and congratulations on an excellent design, programming and documentation work.

[pfile]

Hi Rob,

Yes, the script should work with a secondary obstruction as long as it is circular and centrally located (i.e. the usual case). The documentation PDF has examples of these ring shaped Fresnel diffractions in section 6.

What is the aperture diameter, focal length, and obstruction diameter (an estimate)? I can run more tests given this info.

Thanks,
Mike

Hi Mike, the OTA is a Ceravolo 300 in f/9 configuration, so 2700mm focal length, 300mm aperture and 165mm obstruction. i would probably wait until the moon is back in the evening sky before getting the test images, so it will be a little while before i can experiment with this.

thanks,

rob

[mschuster]

Hi Rob,

Thanks. Moonlight is OK. Please make sure the telescope is in thermal equilibrium. This is very important. I forgot to mention this requirement in the documentation. You have an excellent site with good seeing and stable temperatures which will make testing easier.

Assuming PL 16803, I set gain 1.4 e-/DN, pixel size 9 micron, defocus exposure 45,000 e-. With corrugation resolution set to 10 in the exposure estimator tool, required defocus distance is 8.8 mm. Is your focuser capable of moving both inside and outside at least this much from the position of best focus?

Assuming a separate guide scope (i.e. tracking system is not compromised by defocusing like it is on my setup), you should be able to use long exposures. At 100 seconds the estimated target star magnitude is 5.1 with an R, G, or B filter, or 6.3 with an L filter. The script wants ~100,000 e- total or more, so you need at least two intra-focal frames and two extra-focal frames. These magnitudes are estimates, you should double check that the entire defocused image lies within the linear operating region of the detector. You also will also need a master bias for calibration. No other processing is necessary (i.e. no flat fielding and no other pre-processing).

I will run synthetic tests.

Thanks,
Mike

[pfile]

Hi Mike - yes, PL 16803 is correct. we have an optec focuser which has a pretty long throw so i think we can easily reach 8.8mm inside and outside of focus. i will check. i don't remember what the step size is.

we are using an external guidescope so we can do long exposures while defocused.

normally we use RBI pre flash - do you think this should be turned off for the purposes of this script?

thanks,

rob

[mschuster]

Ok good.

RBI pre flash should be fine. I say leave it ON. As long as the defocused image is well-exposed (i.e. ~50% full-well) the shot noise in the image will easily overwhelm RBI pre flash leakage noise (if any).

Thanks,
Mike

[mmirot]

Wow,

Quite a tool.

[jkmorse]

Mike,

Hell of a script.  I will test with my secondary system (Takahashi e130D and STT8300) next time I am out under the stars, hopefully around Sept 12.  For your test purposes, would it also be helpful for me to test with my CDK12.5 and F16M?

Best,

Jim
 

[jkmorse]

Mike,

I am assuming that you need to be able to get an adequately defocused image on both sides of focus.  I ask because with my e130D/STT8300 combo, I have very little inside focus to work with. 

Thanks,

Jim

[mschuster]

Hi Jim,

Yes, you will need to image on both sides of focus. The pics below show example parameters for both setups with 5 second L filter exposures. You may not be able to get +/- 2.3mm on the Tak. If not, maybe +/- 0.75 mm is possible, with corrugation resolution set to 8? Tests on the CDK would be very helpful also.

When you capture a frame, if you can run PI while you are capturing, you might want to first double check that it is not over- or under-exposed, then do a bias-subtract using ImageCalibration, and finally run the Exposure measurement tool. It will measure defocus diameter and exposure. Use this to double check your focus position and exposure time. It may take a couple iterations. Take care to get inside and outside focus diameters as equal as possible (within a few %). This all can be a bit of a pain, especially the first time or if focusing manually.

You will need at least 100 seconds total on each side of focus. I like to capture more, like 300 seconds each side. It is critical to capture enough to average out seeing effects. Otherwise the script will confuse image intensity variations due to seeing with optical aberrations and give garbage as a result.

Thanks,
Mike

Takahashi:




Planewave:

[pfile]

i have to remember to work on this tonight. the weather at SRO has been really bad for a month due to a huge fire burning just to the south...

rob

edit: phooey, looks like it's going to be cloudy tonight...

[rbotero]

Hi Mike

Have you taken 1.15 down from your Dropbox?  I tested with 1.12 last night and got very good results.  Was struggling a little bit at the beginning since I fed the script uncalibrated frames to start with and it would not converge.  Fixed that with a resized version of my master bias and it all worked fine.

I'm testing an (very old 1991) Astro-Physics 152mm f/7.5 refractor working at f/8.5 or so with a Moravian G4-16000.

Roberto

[mschuster]

Hi Roberto,

New version 1.15 is now available in Dropbox. Testing continues for a few more days, and then it will be released as an official PI update.

Thank you so much for your post, I am really glad to hear the script worked OK!

Mike