First attempt at 140mm aperture SHG in H-alpha

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thesmiths
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First attempt at 140mm aperture SHG in H-alpha

Post by thesmiths »

Our 106mm aperture SHG is working nicely. So the natural human instinct is to try to go even bigger and see how far it can be pushed. Here is the first attempt at using a 140mm aperture, 800mm focal length refractor in H-alpha. The results are somewhat encouraging and I'm sure it can be made to work better with some modifications.

The large refractor was one I have had for many years, the Vixen NA 140ssf :https://www.firstlightoptics.com/user/vixen_review.pdf. It's not a bad telescope for narrowband deep sky work. For such a large aperture it is fairly light and it has a quite flat field due to two lenses at the rear (essentially a built-in flattener/reducer). The doublet at the front leads to some chromatic aberration but that's not really important for narrowband. The focuser is very strong and there is a reasonable amount of back focus.

For a first test, I attached our foamboard SHG to the back of the telescope using some M42 extensions tubes and a 2-inch telescope adapter. I added a 2-inch UV/IR filter from GSO to the front to act as an ERF. This UV/IR filter is not particularly wideband (unlike the Astronomik L1) but for this purpose it worked well. I was going to use a Baader 35nm H-alpha filter but I could not find one anywhere in stock. But the GSO filter appears to have prevented any damage to the chrome / chrome oxide slit. Here is a photo of the SHG on the telescope:

Vixen 140mm 800mm refractor with foamboard SHG
Vixen 140mm 800mm refractor with foamboard SHG
1564.jpg (743.16 KiB) Viewed 237 times

Our foamboard SHG uses 100mm f4 optics and a 30mm x 30mm grating. Even with our 720mm refractor, this wasn't able to give good edge-to-edge image quality, which was the reason why we increased the size of the optics to 135mm f3.5 and a 50mm x 50mm grating. As expected, only the centre of the scan had good quality:

(Almost) full-disk H-alpha solar image at 140mm aperture, 800mm focal length. Stack of 6 frames.
(Almost) full-disk H-alpha solar image at 140mm aperture, 800mm focal length. Stack of 6 frames.
111022-Ha-140mm-800mm-stack-6-full.jpg (245.11 KiB) Viewed 237 times

The foamboard SHG uses a 178MM camera, which is has a 7.4mm width chip, slightly too small for this application. A 100% crop of centre region shows quite reasonable quality:

Crop of the centre horizontal region. Scan direction is horizontal (RA).
Crop of the centre horizontal region. Scan direction is horizontal (RA).
111022-Ha-140mm-800mm-stack-6-crop.jpg (1.07 MiB) Viewed 237 times

I only had one hour to test since that's all the direct sunlight I get this time of year. I scanned in RA instead of DEC, and I could tell the mount would have preferred DEC scanning, especially with a bigger telescope. But I could tell in the FireCapture preview and by looking at the SER files that the resolution was quite good, just not over the full disk. I also didn't have time to make very many scans to stack.

For a first test with non-optimised equipment, the results were encouraging. I may have to increase to optics to something like 200mm f4 if I want to get really good results. I'll have to think if this extra effort is warranted. The current 106mm aperture seems to give good results most of the time and probably the 140mm aperture would not always be useable (and would only be for H-alpha, while the 106mm can be used nicely for Calcium, with a bit of an aperture mask).


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Re: First attempt at 140mm aperture SHG in H-alpha

Post by highfnum »

sheesh!


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Re: First attempt at 140mm aperture SHG in H-alpha

Post by Alun_H »

Fantastic spectroheliograms Douglas.

If you are still after a Baader 35nm filter ENS optics have one in stock,unmounted 50.8 https://ensoptical.co.uk/index.php?rout ... ct_id=8113

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Re: First attempt at 140mm aperture SHG in H-alpha

Post by Montana »

What a delight :bow :bow :bow :hamster:

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Re: First attempt at 140mm aperture SHG in H-alpha

Post by thesmiths »

Alun_H wrote: Thu Oct 13, 2022 8:02 am If you are still after a Baader 35nm filter . . .
Yes, I saw that one but I thought unmounted would be too much trouble. Rupert at Astrograph said he could order one for me but it would take a few weeks. He said he doesn't keep them in stock this time of year because it seems to be very much a solar item. I'm not sure if this is true, but he said the material is the same as for the front mounted Baader H-alpha ERF.

The "narrow-band" UV/IR from GSO (which looks visibly a bit reddish) seems to work quite well for this purpose. I had previously used this telescope without any filter and it damaged the chrome oxide film after awhile under the bright summer sun. This time, I did not notice any damage.


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Re: First attempt at 140mm aperture SHG in H-alpha

Post by Alun_H »

I purchased one of these filters and can confirm it is the same coatings as the full aperture ERF,red colour on one side and mirror finish on the other.
I was thinking it would drop into a 2" filter cell but I was wrong but I did manage to mount it in an old Canon dslr T mount adapter which was an ideal fit to the focuser tube on my 80mm to allow the filter to be used as an internal ERF which I use for my Lunt 50 mod,three screws at 120 degree spacing help hold the assembly in place and also lock in a tilt.

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Lunt 50THa solar scope
Lunt CaK B12 module
SW Evostar 72ED for full disk imaging
80mm f/7 achromat used with pressure tune module off the Lunt with Lunt 100mm ERF
Bresser 90 f/10 achromat used for CaK & white light imaging & PST mod with Lunt 100mm ERF
Coronado SM40 (pre Meade) and BF10 filter set + SMII 60 richview DS etalon
Skywatcher EQ6 Pro and Ioptron CEM60
Then there is all the night kit which is far too long a list to write up here! :lol:
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Re: First attempt at 140mm aperture SHG in H-alpha

Post by marktownley »

Impressive Douglas.

Have you done a side by side of imahes with the 140 and the 100 to compare the difference in resolution?


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Re: First attempt at 140mm aperture SHG in H-alpha

Post by thesmiths »

marktownley wrote: Fri Oct 14, 2022 5:28 am Have you done a side by side of images with the 140 and the 100 to compare the difference in resolution?
The 140mm setup is still in development. I'm thinking that it might be possible to swap all the optics from the 106mm telescope to really judge the difference. I'm not sure that the potential improvement in performance will be worth the effort so I'm hesitating on doing this. I could already tell that the seeing effects were greater with the larger diameter.

There is also a technical reason why bigger has a drawback. With more pixels to capture, the scan speed needs to be slowed down, which then tends to increase the 1/f noise. This is essentially also a seeing problem.

But I could tell by looking at a static spectral line that there were more details (wiggles) which suggested to me that the resolution was theoretically higher. But turning this into a good image depends on a number of other factors.


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