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CaH and continuum SHG images (resolution upgrade)

Posted: Tue Jan 11, 2022 1:25 am
by thesmiths
We recently upgraded our 300mm focal length f5.6 spectroheliography (see viewtopic.php?f=8&t=34153). The new version uses a 30mm x 30mm 2400 l/mm Edmund Optics grating and a 10 micron wide, 3mm long Thorlabs slit. We also increased the camera lens focal length from a 135mm to 150mm (an enlarger lens, focused with a bellows). We kept the same camera (ASI 290MM). These changes improved both the spatial and spectral resolution.

Here is a mosaic of CaH spectral images with a spectral (pixel) shift from zero to +16 (0, 2, 4, … 16). This means the images range from the centre of the CaH line (top left) to what would be essentially a "continuum" image (bottom right).

CaH SHG mosaic, 7 Jan 2022
CaH SHG mosaic, 7 Jan 2022
111919_clahe_0_16.jpg (295.42 KiB) Viewed 150 times

Here is a link to the zero pixel shift image taken with the previous setup: download/file.php?id=61774

Here is a link to some images from Pedro taken on the same day: http://pedroreastrophotography.com/SUN_ ... mosaic.jpg

The image posted above is best viewed in its own tab to compare to Pedro's images: download/file.php?id=62960

The upgraded version of the SHG produces better spatial resolution and has increased contrast than before. Comparing to Pedro's white light, CaK and H-alpha images, our series of CaH SHG images show details which are visible in all three imaging modes. By that I mean, normal CaK filter images do not show the fine structure seen in the CaH SHG images, but only in the H-alpha etalon images (presumably because of the narrower bandwidth). The SHG continuum images also show more fine structure than a normal continuum filter image.

We also took images at the H-alpha wavelength but the 54mm diameter aperture of the imaging optics did not give particularly impressive results so I will not include them here. The small optics does seem to work nicely for Calcium images though. There were also not particularly impressive prominences visible that day.

A Meade #47 violet filter was used in front of the camera to reduce background skylight. The exposure time was 5ms, the gain was 97 (16%), the average FPS was 199 and the scanning rate was 16x sidereal. Imaging was done through the 2nd floor bedroom window (necessary during the UK winter). The mount was a Vixen GP2 with an EQ Mod compatible motor upgrade (necessary for good scanning). Software used was FireCapture v2.7beta for data acquisition (1600x200 pixel ROI), home-built SHG code for image reconstruction from a 16-bit SER file, and Photoshop Elements 15 for sharpening and filtering of the CLAHE image and mosaic creation.

Re: CaH and continuum SHG images (resolution upgrade)

Posted: Tue Jan 11, 2022 6:31 am
by marktownley
Excellent results! What does the pixel shift correspond to in terms of angstroms / wavelength off CaH centre line?

Re: CaH and continuum SHG images (resolution upgrade)

Posted: Tue Jan 11, 2022 8:11 am
by Montana
Wow!! these are absolutely fantastic!! :bow :bow :bow

Alexandra

Re: CaH and continuum SHG images (resolution upgrade)

Posted: Tue Jan 11, 2022 5:57 pm
by thesmiths
marktownley wrote: Tue Jan 11, 2022 6:31 am What does the pixel shift correspond to in terms of angstroms / wavelength off CaH centre line?
Using Ken's SimSpec SHG spreadsheet, I calculate that the dispersion is 0.08 angstroms/pixel. The spectral resolution is calculated to be 0.3 angstrom.

I have included below a screen shot of a frame from the SER video file from this dataset. All the images in the mosaic above are generated from this single SER file. The spectral lines are horizontal; the Sun's full diameter corresponds to a width of approximately 1400 pixels. This particular frame corresponds to a line that cuts horizontally through the two white patches in the lower third of the mosaic images and a pair of sunspots (the scan was done in DEC). Sunspots look dark across all wavelengths while other features may only appear in the spectral line itself.

The very sharp vertical line near the left is a defect in the slit and is automatically removed by the SHG reconstruction software. The height of the video is 200 pixels so the maximum pixel shift of +16 is an 8% vertical offset from the centre of the CaH line.

It's almost magical that a video file of incomprehensible lines can be turned into a pretty mosaic of solar images.

Single frame of CaH scan
Single frame of CaH scan
calcium H SHG video.JPG (51.49 KiB) Viewed 106 times

Re: CaH and continuum SHG images (resolution upgrade)

Posted: Wed Jan 12, 2022 7:51 am
by christian viladrich
Very nice ! Thanks for sharing.

Re: CaH and continuum SHG images (resolution upgrade)

Posted: Wed Jan 12, 2022 8:00 am
by Merlin66
Doug,
Thanks for reminding us of the data that can be collected from each SER video.
Again one of the strengths of the SHG.