Improving our SHG imaging optics

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thesmiths
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Improving our SHG imaging optics

Post by thesmiths »

This is post describes some recent upgrades we are undertaking to further improve our SHG imaging.

We have been using two distinct setups recently: one with a 106mm aperture and one with 80mm. The larger aperture has given higher resolution images (particularly with H-alpha) but suffered from not being quite able to keep good focus across the whole disk. This telescope used a modified Solex design that had 25mm diameter optics (100mm f4 for both the collimator and camera lens) and a 30mm x 30mm grating. See, for example, this image where the bottom is not very sharp.

H-alpha SHG image, 106mm aperture, stack of 6 of 24.
H-alpha SHG image, 106mm aperture, stack of 6 of 24.
140722-Ha-3-700mm-6-24_small.jpg (323.52 KiB) Viewed 1255 times

Our 80mm telescope used a different design with 33mm diameter optics (135mm f4 collimator and 150mm f4.5 camera lens) and a 50mm x 50mm grating. This could produce good focus across the whole disk, as shown in this image:

H-alpha SHG image, 80mm aperture, stack of 8 of 26.
H-alpha SHG image, 80mm aperture, stack of 8 of 26.
150722_Ha_480_2400_8-26.jpg (290.05 KiB) Viewed 1255 times

What we decided to do is to combine the two instruments into one in order to see if we could get the best of both worlds. The following image is not particularly good quality due to poor seeing, but shows what we had hoped to demonstrate: the larger aperture, longer focal length telescope, using the larger SHG optics, and able to achieve good focus across the whole solar disk:

H-alpha and H-beta SHG images, 106mm aperture.
H-alpha and H-beta SHG images, 106mm aperture.
240722-Ha-Hb-106mm-2400g.jpg (658.59 KiB) Viewed 1255 times

As this experiment turned out so well, we've decided to improve things further by upgrading both the collimator and camera lens to 135mm f3.5 optics (38.6mm diameter). This should further improve issues like vignetting that caused the slit to be unevenly illuminated. The upgrade should be done in a few days and we hope good viewing conditions return soon after.


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Re: Improving our SHG imaging optics

Post by Montana »

Perfection :bow :hamster: you really are raising the bar, well done :bow :hamster:

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Re: Improving our SHG imaging optics

Post by thesmiths »

I'll show here some photos of our upgraded SHG. There's been no real chance to take any solar images yet due to weather constraints, but so far testing has shown it is performing up to expectations.

106mm APO refractor with upgraded SHG on the back.
106mm APO refractor with upgraded SHG on the back.
1430.jpg (308.08 KiB) Viewed 1213 times
Rear view of the SHG optics. Both lenses are 135mm f3.5. Everything is mounted on Vixen style dovetail bars.
Rear view of the SHG optics. Both lenses are 135mm f3.5. Everything is mounted on Vixen style dovetail bars.
1432.jpg (490.88 KiB) Viewed 1213 times
Closeup of the SHG optics. The grating is mounted on a rotating disk which is attached to a dovetail clamp so it is removable and swappable.
Closeup of the SHG optics. The grating is mounted on a rotating disk which is attached to a dovetail clamp so it is removable and swappable.
1434.jpg (487.08 KiB) Viewed 1213 times
Diffraction grating (in this case 2400 l/mm holographic, 50mm x 50mm), mounted on a dovetail clamp.
Diffraction grating (in this case 2400 l/mm holographic, 50mm x 50mm), mounted on a dovetail clamp.
1428.jpg (207.89 KiB) Viewed 1213 times
Top view of the SHG optics (slightly updated with a tilt corrector in front of the slit and a helical focuser at the camera).
Top view of the SHG optics (slightly updated with a tilt corrector in front of the slit and a helical focuser at the camera).
1550-1.jpg (203.51 KiB) Viewed 985 times
Attachment of the SHG to the telescope with a pair of ADM Vixen-style dovetail extensions.
Attachment of the SHG to the telescope with a pair of ADM Vixen-style dovetail extensions.
1548.jpg (344.14 KiB) Viewed 985 times
View of the SHG with its stray light cover on. It is made from black foamboard, black cardboard and blackout curtain material (black inside and white outside). It slides onto the end of the camera dovetail. The piece of white foamboard behind the camera is a sunshade to prevent the camera from overheating.
View of the SHG with its stray light cover on. It is made from black foamboard, black cardboard and blackout curtain material (black inside and white outside). It slides onto the end of the camera dovetail. The piece of white foamboard behind the camera is a sunshade to prevent the camera from overheating.
1551.jpg (306.04 KiB) Viewed 982 times

The collimator lens is a 135mm f3.5 Asahi Pentax SLR M42 camera lens. The camera lens is a 135mm f3.5 M42 macro bellow lens from Wallace Heaton (made in Japan). The design is an evolution of our previous setups, in this case optimised to give better illumination of our long slits, in order to achieve improved full-disk, high-resolution scans.

By the nature of the way the optics is mounted on the dovetail bars, the scanning must be done in DEC (rather than RA). In general, mounts seem to scan better in DEC but scanning in RA allows scans in equatorial bands to generally be at the same focus (as usual, there are compromises).

I should mention that there is a cover made of foamboard, black cardboard and blackout curtain fabric which is put over the SHG to block out ambient light. If truly dark conditions are required, I can also put a black plastic bag over the normal cover, but that's typically not necessary.
Last edited by thesmiths on Wed Sep 28, 2022 1:19 pm, edited 3 times in total.


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Re: Improving our SHG imaging optics

Post by hopskipson »

Bravo on your captures and SHG! I'm new to SHG imaing, so what is your secret to getting such a smooth limb?


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Re: Improving our SHG imaging optics

Post by thesmiths »

The rough limb is due to two factors: (1) uneven movement of the mount; (2) air turbulence. The smoothness of the mount movement can be improved with better mount balance (or a higher-end mount). The air turbulence issue can be improved by increasing the scan speed (and of course imaging when there is less turbulence).


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