SHG flats, a suggestion

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Spectral Joe
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SHG flats, a suggestion

Post by Spectral Joe » Tue Aug 09, 2016 4:05 am

"Flatting out" all of the unwanted features in SHG images is a vexing problem. Various means are used, Fourier transforms are effective but using directional filters with them has issues, and the transform itself can cause artifacts that are as bad as the ones being eliminated. Flats using light sources other than the sun often fall short because the illumination source function (etendue) is different from the sun's. Synthetic flats generated from a continuum area of the spectrum are quite effective, but are a lot of work and are still not quite perfect. I have tried all of these, and more, to varying effect, and have settled on a different approach. A reverse Galilean telescope is inserted between the objective and the input optics, this both enlarges the solar image and shifts it farther from the objective, without changing the ray trajectories. Any artifacts caused by dust or other stuff on the SHG optics still appear as they would in a normal image. To give a uniform illumination source I tune away from the line in use to a nearby continuum spot. Any irregularities in the continuum disk are washed out by the defocussing caused by the image shift. In the 3 frame montage below the raw image is on the left, the flat is in the center and the raw divided by flat is on the right. No other processing has been done to these yet.
Montage larger.jpg
Montage larger.jpg (72.86 KiB) Viewed 861 times
The sharp vertical line is where two linear CCD chips are butted together to form one long one. Virtually all of the spatial artifacts in the raw image are eliminated.
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Merlin66
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Re: SHG flats, a suggestion

Post by Merlin66 » Tue Aug 09, 2016 6:30 am

Joe,
Sounds like a possible solution.
Any issues with adding additional optics within the train (the reversed Galilean optics) - I assume between the telescope objective and the entrance slit?
Does the final size of the OOF flat influence the registration of any motes etc.
(I know you say ""....without changing the ray trajectories. Any artifacts caused by dust or other stuff on the SHG optics still appear as they would in a normal image"" - just confirming)
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Re: SHG flats, a suggestion

Post by marktownley » Tue Aug 09, 2016 8:42 am

Well the before and after speak for themselves. I like the logic :)
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Re: SHG flats, a suggestion

Post by Spectral Joe » Wed Aug 10, 2016 2:32 pm

Ken

There are no unwanted effects from adding the extra optics, they are mounted on a motor driven positioner and are located just before the focus of the objective. In my instrument there are additional optics between the prime focus and slit that are part of the scanning system, these new lenses are between those and the objective. Keeping the ray paths the same is important because the stripes in the final image are shadows of "stuff" in the optical path, and if the ray paths change these shadows will shift in the final flat image and will not subtract out properly. A previous attempt using just a negative lens was only partially successful because of this. The example shown is the most basic one step process for this, it is adequate for most uses but a more elaborate two step process gives a flat that preserves the original intensity profile exactly. In that process a copy of the original flat is smoothed with a 50 pixel wide gaussian blur and the smoothed (low pass filtered) image is used to correct the main (full resolution) flat. The extra step takes a few minutes but gives a slightly better result.
Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.
Not blind yet, either!
Light pollution? I only observe the Sun, magnitude -26.74. Pollute that!

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