Ca-H images taken with a 1200 l/mm grating and 300mm Canon lens

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thesmiths
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Ca-H images taken with a 1200 l/mm grating and 300mm Canon lens

Post by thesmiths »

I decided to do some SHG measurements with a 1200 l/mm ruled grating. The idea was to use a lower power diffraction grating than previously (1800 l/mm and 2400 l/mm) to reduce the dispersion, thereby increasing the "bandwidth" to make them appear more similar to conventional Calcium Lunt-type filters.

The experimental setup was essentially like our most recent measurements. The "telescope" was a Canon 300mm f4 lens with a Hoya 72mm NDX4 neutral density filter on the front to reduce the energy arriving at the slit. The collimator was a Pentax 100mm f4 bellows lens and the camera lens was a 135mm f5.6 enlarger lens. Both of the lenses were mounted on bellows to focus. The Canon lenses had a M42 helical focus attached to improve focusing precision. The diffraction grating was a 50mmx50mm 1200 l/mm Thorlabs ruled grating, blazed for 500nm. The slit was a 10 micron wide and 3mm long air gap, also from Thorlabs.

SHG 300mm Canon setup
SHG 300mm Canon setup
1259.jpg (298.45 KiB) Viewed 778 times

Note that there is a protective plastic cover on the diffraction grating in this photo (I usually keep the cover on when not doing imaging). There is some black out curtain material attached to the back to reduce stray light and I usually lay a piece of the same material on top during imaging. Note also the cute new Astro-Gadget stepper motors that I recently installed on my Vixen GP2 mount. The new motors allow faster and more flexible scanning and also interface to EQMOD.

Here is a single image taken with an ASI 178MM camera at 4ms exposure, gain of 141, 246 fps, 1x1 binning (taken through an open bedroom window since during winter months I don't really have much unobstructed views from the ground due to trees).

Calcium H, 1200 l/mm grating, 2 Nov 2021
Calcium H, 1200 l/mm grating, 2 Nov 2021
131248_shift=0_clahe_a.jpg (104.91 KiB) Viewed 778 times

Below that same data is processed with the pixel shift feature of our software from 0 to 7 pixels. As the pixel shift increases, the image looks increasingly like an image taken with a Baader K-line filter.

Calcium H, 1200 l/mm, 0 to 7 pixel shift
Calcium H, 1200 l/mm, 0 to 7 pixel shift
131248_shift=0-7_clahe.jpg (272.81 KiB) Viewed 778 times

My conclusion is for the rather wide Calcium lines, the reduced bandwidth is somewhat more "aesthetic" compared to the higher contrast that results from using a more powerful diffraction grating. See our previous post for Ca-K images taken with a 1800 l/mm grating:

viewtopic.php?f=8&t=33283

This post shows the earlier experimental setup and some images taken in H-alpha at 1800 l/mm (H-alpha looks considerably better at 2400 l/mm though):

viewtopic.php?p=306081#p306081


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Re: Ca-H images taken with a 1200 l/mm grating and 300mm Canon lens

Post by Montana »

Superb images, well done :bow :hamster: I am trying to place on the spectrum what my CaK PST looks like :)

Alexandra


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Re: Ca-H images taken with a 1200 l/mm grating and 300mm Canon lens

Post by thesmiths »

I should have put my CaK PST on right after to compare. Next time I'll do that and maybe reduce the aperture to 40mm to make a more accurate comparison.


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Re: Ca-H images taken with a 1200 l/mm grating and 300mm Canon lens

Post by marktownley »

Very nice indeed. I like the sequence showing going off band.


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