CaK 600mm FL full-disk SHG image -- with stacking!

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thesmiths
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CaK 600mm FL full-disk SHG image -- with stacking!

Post by thesmiths »

I decided to revisit an older SHG setup and to install one of the new long chrome-on-quartz stilts in order to image around the Calcium lines. This way, I could keep my 106mm aperture 700mm FL telescope with the 2400 l/mm grating reserved for the H-beta to H-alpha range of wavelengths.

The 400nm part of the spectrum seems to have "special needs". Although my 106mm APO refractor does very well at 400nm, it's probably not really necessary in most cases to use such a large aperture (my 40mm CaK PST does pretty well). In addition, I found that the 2400 l/mm grating gives contrast that is a too high in most cases: the white areas are so intense, they get burned out. I therefore chose to use a 1200 l/mm grating (50mm x 50mm) to reduce the contrast, and this seems to have turned out quite well.

The photo below shows the equipment setup. The "telescope" is a vintage 300mm f5.6 telephoto lens from Prinz Galaxy (made in Japan). I added a 2x M42 extender from APS (made in Japan). This makes the "telescope" 600mm FL at f11. The telephoto lens was very cheap and it actually is not rated very highly for sharpness. However, it is very light, has a nice focuser and a built in tripod attachment (which also rotates, which is very convenient).

SHG using camera lenses for all three optics. 1200 l/mm grating.
SHG using camera lenses for all three optics. 1200 l/mm grating.
1404.jpg (192.91 KiB) Viewed 1138 times

The collimator is a lovely Vintage "Jupiter-11" 135mm f4 M39 lens (made in the USSR). The camera lens is a vintage 150mm f4.5 enlarger lens from "Phago Anastigmat" (made in Japan), mounted on a BPM bellows (made in England). These two lenses give a magnification of 150/135. Therefore, the Sun's diameter will be approximately 6.1mm at the sensor. This means the image fits nicely on the 7.4mm width of a 178MM camera (the collimator focal length was chosen to achieve a good fit).

The whole setup is mounted onto a pair of intersecting Vixen-type dovetail bars (the main one made by ADM). Compared to my main "foamboard" SHG, this one has larger optics: lens diameter 33mm (vs 25mm) and a grating of 50mm (vs 30mm). I notice there was better illumination of the slit and less "vignetting". An even longer focal length than 600mm could probably be used effectively.

The weather was not as good as last week, which had very clear skies. There were very thin high clouds that diminished image quality. I therefore decided to attempt some stacking to see if this improved things. I took 6 images (all taken in the same scan direction), cropped with so they were of equal size and put them through AutoStakkert. The output looked a bit better than the individual frames and I was then able to use imppg to sharpen. With a bit of Photoshop, the final image is shown below:

CaK SHG image -- 6 images stacked
CaK SHG image -- 6 images stacked
210622_AS_imppg_big.jpg (255.97 KiB) Viewed 1130 times
CaK SHG image colourised
CaK SHG image colourised
210622_AS_imppg_small_colour.jpg (145.62 KiB) Viewed 1138 times

Some nice filaments are visible (normally not seen with CaK filters), as are a few prominences (these were pulled out from the same stacked image). I think the contrast -- lower than the SHG images taken with a higher l/mm grating but higher than with a CaK filter -- is a nice balance.

PC screen during imaging
PC screen during imaging
calcium-K-capture.jpg (130.93 KiB) Viewed 1138 times

On the acquisition side, I attach a screen shot of the capture session. EQ Mod is used to position and scan. The ROI is used to focus on just the line of interest (in this case, Calcium K). I found the focus was quite stable, so while waiting (unsuccessfully) for the light clouds to pass, I actually ran the instrument using Remote Desktop from inside the house. Our home built SHG software v3.2 was used to process the images.

I should also mention that the key parts of the setup were covered to block out ambient light. Also, a #47 Meade violet filter and an Astronomik L1 UV/IR filter was placed in front of the camera to reduce unwanted stray light.

See this post for full-disk CaK and H-alpha images taken on the same day (June 21): viewtopic.php?t=36727. I think the SHG image is kind of a mix between the images taken with the two traditional filters.

See this SHG image we took last week for an example of CaK taken with a 2400 l/mm grating: download/file.php?id=68636 With the much higher spectral contrast, some details in the bright areas are lost. Also, I think the comparison suggests that the stacking (even with just 6 images) probably improved the image sharpness slightly.


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CaK 600mm FL full-disk SHG image -- with stacking!

Post by thesmiths »

I used the pixel shift feature in our software to create a series of images from 0 to +11 pixel shift and created a mosaic.

CaK SHG mosaic: 0 shift top left, +11 bottom right
CaK SHG mosaic: 0 shift top left, +11 bottom right
151109_shift_0_11_clahe_small.jpg (365.88 KiB) Viewed 1115 times

When I compare to Stuart Green's CaK filter image from the same day (https://www.flickr.com/photos/solarcarb ... 163380338/), it seems to me that the filter image looks like the SHG image shifted by about 4 pixels.

Using Ken Harrison's SimSpec v1.3 spreadsheet, I find the current setup has a spectral resolution of 0.55 angstroms (vs 0.22 angstroms with the 2400 l/mm grating). The current dispersion is 0.133 angstroms per pixel.


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Re: CaK 600mm FL full-disk SHG image -- with stacking!

Post by nicosolcy »

I have no idea how your equipment works, but I find the results spectacular, with the CaK SHG mosaic taking the cake. Thanks for sharing this!


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Re: CaK 600mm FL full-disk SHG image -- with stacking!

Post by ChrisHalpha2017 »

woaww ; very impressive spectroscope !! thats very good!! and your results in the UV are really good :)

looking forward to see more :)

Chris


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Re: CaK 600mm FL full-disk SHG image -- with stacking!

Post by marktownley »

These are brilliant!

Here's mine with filters a couple of hours after yours.

ImageCaK-FD-40mm-f15-zwo183mm-bw by Mark Townley, on Flickr


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Re: CaK 600mm FL full-disk SHG image -- with stacking!

Post by fulvio.mete »

Hi, Douglas:
I missed this post: my congrats for your interesting results!
Fulvio


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Re: CaK 600mm FL full-disk SHG image -- with stacking!

Post by thesmiths »

Here is another exercise in stacking SHG images. Using the newest version of our SHG software that partially automates several functions (cropping, mirror flip and rotation), we produced 43 images at a pixel shift of -4. We then stacked these using AutoStakkert and let the program chose the best 50%. We then used imppg and Photoshop.

CaK SHG, pixel shift -4, best 50% of 43 frames stacked.
CaK SHG, pixel shift -4, best 50% of 43 frames stacked.
210622-stack43-shift--4.jpg (177.75 KiB) Viewed 980 times

Comparing to Mark's narrowband CaK filter above, it seems to me that one of the main differences is not just the bandwidth but positioning of the filter with respect to the minimum of the absorption line.


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Re: CaK 600mm FL full-disk SHG image -- with stacking!

Post by thesmiths »

Just to complete the picture, so to speak, I redid the first CaK stack (at the centre of the line) using the same procedure as the second one (at -4 pixels from the centre of the line): 50% of 43 frames stacked, imppg, Photoshop. I then assembled the two SHG images along with Mark's CaK filter image into a mosaic. Much easier to see them all in a row for comparison purposes.

CaK SHG (centre of the line); CaK SHG (-4 pixel shift); CaK filter (Mark T). All on June 21.
CaK SHG (centre of the line); CaK SHG (-4 pixel shift); CaK filter (Mark T). All on June 21.
210622-stack43-shift=0,--4.jpg (611.33 KiB) Viewed 974 times


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Re: CaK 600mm FL full-disk SHG image -- with stacking!

Post by marktownley »

Excellent comparison DOuglas!


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Re: CaK 600mm FL full-disk SHG image -- with stacking!

Post by Montana »

That's a great comparison but why have all the filaments and proms disappeared in the middle image?

Alexandra


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Re: CaK 600mm FL full-disk SHG image -- with stacking!

Post by thesmiths »

Montana wrote: Wed Jul 06, 2022 6:40 am why have all the filaments and proms disappeared in the middle image?
That's a very good question. I guess it shows that they are only visible at the very middle (darkest) part of the Calcium lines. (By the way, I don't see any significant difference between K and H -- the H line is somewhat narrower and generally not as dark so probably a little easier for narrowband work.)

I did another experiment to see what the best way is to mimic a CaK filter: I stacked a range of pixel shifts (wavelength ranges) from -5 to +5 using AS3. It's a little different than just images stacked at -4 pixel shift, but not that different. (There is little more noise since it's taken from a single video file rather than from a number of them.)

CaK stack from -5 to +5 pixel shift
CaK stack from -5 to +5 pixel shift
151109_shift=-5to+5_clahe.jpg (197.78 KiB) Viewed 931 times


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Re: CaK 600mm FL full-disk SHG image -- with stacking!

Post by thesmiths »

I had a look at Christian Viladrich's analysis of some CaK filters http://www.astrosurf.com/viladrich/astr ... lters.html. There is likely an effect due to both the bandwidth (FWHM) and the offset (delta CWL). In the case of the Barr Associates filter, bandwidth was measured as 2.6 and the offset as -0.7 angstrom. (As I recall, Mark T.'s calcium filter is a combination of a Lunt CaK and another filter or two.)

CaK filter vs Calcium K line
CaK filter vs Calcium K line
Barr-30April2022-spectrum-Solex-125mm-colim-1245mm-imager-ASI290.jpg (56.68 KiB) Viewed 919 times

The SHG image (shown above) which shows filaments and prominences are with a bandwidth of around 0.5 angstrom, centred very closely to the minimum of the CaK line. A shift of 4 pixels also represents approximately 0.5 angstrom.


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Re: CaK 600mm FL full-disk SHG image -- with stacking!

Post by marktownley »

thesmiths wrote: Wed Jul 06, 2022 3:23 pm (As I recall, Mark T.'s calcium filter is a combination of a Lunt CaK and another filter or two.)
Mine is 2x coronado CaK PST filters

I need to try and nudge my filter CWL a little more red ;)


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