Full-disk SHG images with a Canon 300mm lens

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thesmiths
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Full-disk SHG images with a Canon 300mm lens

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Christian Buil on his new YouTube channel has been demonstrating that it's possible to use DSLR lenses attached to the Solex SHG instead of a telescope. This has the advantage of giving a short focal length so it is easier to squeeze a full solar disk onto the relatively short imaging slit. I happen to have a 300mm f4 Canon lens, which will allow fitting the full disk onto my short 3mm Thorlabs slit.

I decided to build this setup in an entirely new way by attaching the Canon lens to a 14" ADM dovetail bar. The 10 micron slit and 100mm f4 collimator lens were attached to the lens using a Geoptik adapter. The camera was attached to a second dovetail bar that was shorter and lighter than the main ADM bar. The camera lens was a 135mm f5.6 enlarger lens mounted on a BPM bellows. The camera was a ZWO 290MM. The grating used was my 50mm x 50mm 1800l/mm ruled grating from Thorlabs. The grating was mounted on an ADM dovetail plate adapter. There were a lot of degrees of freedom so distances and angles could be easily adjusted. By luck, everything was at the same height.

Canon 300mm SHG
Canon 300mm SHG
1194.jpg (243.73 KiB) Viewed 899 times
SHG mounted on Vixen dovetail bars
SHG mounted on Vixen dovetail bars
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#29 red filter and UV-IR filter, 72mm
#29 red filter and UV-IR filter, 72mm
1189.jpg (265.22 KiB) Viewed 899 times

On the front of the Canon lens, I attached a 72mm #29 red filter and a UV-IR filter (both from B+W) in order to reduce the energy on the slit. On the front of the camera lens, I put a 25A red filter to reduce stray light. The filters on the front of the Canon lens allowed around 90% of the H-alpha light to pass through in a relatively narrow window. The filters also reduced the f-ratio slightly to around 4.5.

The large amount of H-alpha light (and the fairly large grating) allowed for extremely short exposures: less than 300 microseconds at a gain of zero and 1x1 binning. In theory, I could have used a frame rate of almost 1000 fps (limited by the electronics) but the mount I used (a small Vixen GP2) would only allow scanning up to 32x sidereal rate. I therefore targeted around 400 fps and an acquisition time of around 6 seconds (of which on the Sun would be 3.75 seconds).

In previous experiments, I noticed I could leave the top of the box open when aimed at the Sun since the light from the Sun was so intense and overpowered most of the stray light. So on the first attempt, I decided to build no box at all around this instrument, only a piece of black foamboard behind the Canon lens to prevent direct sunlight from reflecting off the grating (this also acted as a very good solar finder).

I still need to work on a better way to focus the Canon lens (it's not really designed for manual focusing). Also, the results should improve using my Edmund Optics 30mm x 30mm 2400 l/mm grating. Nevertheless, the first attempt was very promising. Below is a mosaic of images taken at the centre of the hydrogen alpha spectral line and wavelength shifted by 1 pixel in each image. Pixel shift of zero is in the top left and pixel shift of 11 is in the bottom right. The shift of zero corresponds to a normal H-alpha image and by a shift of 11, the image is essentially a continuum image.

SHG images, H-alpha to +11 pixel shift
SHG images, H-alpha to +11 pixel shift
103033_shift=0_11.jpg (614.41 KiB) Viewed 899 times

This data was taken on 28 Aug 2021 so can be compared to the following H-alpha and white light images taken one day earlier: viewtopic.php?f=4&t=33152


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Re: Full-disk SHG images with a Canon 300mm lens

Post by Montana »

Wow!! amazing shots and lovely sequence, I am sure you will soon overcome the focus issues :bow :bow :bow

Alexandra


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Re: Full-disk SHG images with a Canon 300mm lens

Post by marktownley »

Very good indeed!


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Update on Full-disk SHG images with a Canon 300mm lens

Post by thesmiths »

The preliminary experiment above was promising so I sought to improve the focusing by adding a short Pentax helical focuser behind the Geoptik adapter. In this way, the Canon lens focuser could act as a coarse focus and the helical focuser a fine focus. I also flipped the setup so the grating was pointed downward instead of upward towards the sky. This significantly reduced the amount of stray light, even without adding any covering on the setup.

slightly improved setup
slightly improved setup
1222.jpg (358.78 KiB) Viewed 823 times
I decided to keep the 50mmx50mm 1800 l/mm grating for the time being. The red filter on the camera is now a 1.25 inch 25A at the nosepiece of the ASI 290MM (rather a 49mm red filter on the front of the 135mm f5.6 enlarger lens). The idea is the I plan to use a #47 1.25 inch violet filter on camera when imaging in Calcium.

H-alpha at 300mm
H-alpha at 300mm
131150-small.jpg (189.71 KiB) Viewed 814 times

Here is a quick test done at a scan rate of 32x on my cute little Vixen GP2 mount. The exposure time was 0.46 ms at a gain zero. The average FPS was 388, with a ROI of 1600x100 and 1x1 binning. The image was formed by combining the High Contrast and Protuberance PNG files. The image can be compared to an etalon H-alpha (DS Lunt 60): viewtopic.php?f=4&t=33245

To see if we can acquire images even faster, we moved the SHG to a HEQ5 Pro, which could scan faster than the Vixen GP2. We increased the FPS to 774 and acquired five scans in 16 seconds at the setting "5" of the SynScan handset (which is supposed to be 64x sidereal rate). The exposure in the image below was 0.54ms at gain zero.

64x scan, 774 fps
64x scan, 774 fps
135708_shift=0_clahe.jpg (279.53 KiB) Viewed 818 times

The image processing software only does a geometric correction on one of the scans (in this case, the one on the far right) and since the correction is incorrect for scans in the reverse direction, the 2nd and 4th scans look slightly distorted. The images also look rotated by 90 degrees from the large image above since we scan in DEC rather than RA.


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Re: Full-disk SHG images with a Canon 300mm lens

Post by Montana »

Wow!! this really is an incredible image :bow :hamster:

Alexandra


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