New foam board spectroheliograph results

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New foam board spectroheliograph results

Post by thesmiths »

In my previous post viewtopic.php?f=8&t=32136, I described the construction and initial testing of our foam board spectroheliograph (SHG). One of the things I learned was how important the mount was -- not a unique discovery in astrophotography. In particular, good polar alignment, the option for solar tracking rate, and finer control of the slewing rate were missing elements in the "first light" tests.

Another issue was a better optimisation of the frame rate of the camera vs the slew rate of the mount, which is typically a multiple of the sidereal rate (SR). The relationship between the diameter of the Sun on the camera sensor in pixels [let's call it DIA], the slew rate [R in multiples of SR)] of the mount, and the frame rate of the camera [FPS] is DIA = FPS x 120 / R. This is because (at 1x SR) the Sun takes approximately 120 sec to traverse a particular point in space. As an example, if the diameter of the Sun is 2000 pixels wide and we use an FPS of 100, we need a slew rate of R = 6 in order to be neither over nor under sampling. In general, we would prefer to over sample to some degree -- the downside to excessive over sampling being the image will take longer to collect (which can have observational consequences) and the issue of too large a data file. Christian Buil on the SolEx website http://www.astrosurf.com/solex/sol-ex-o ... on-en.html suggests a slew rate to give an under/over sampling between 0.9 to 1.5.

Since the slew rate of a mount is typically only in particular multiples of SR, we need to vary the FPS in order to match the scanning rate chosen. The following shows the slew rates of the SynScan (Sky-Watcher) HEQ5 mount. This table shows what the "Rate" number on the hand controller corresponds to in SR:

slew rate
slew rate
slew speed.JPG (36.57 KiB) Viewed 684 times

The realistic choices for a high scan rate are either 8x and 16x, which give a scan time of around 16 sec and 8 sec (allowing for a bit of extra time on either side of the Sun's diameter to avoid clipping the image). Using the example of a 2000 pixel wide solar diameter and a scan rate of 8x, we need an FPS of 160 FPS to achieve an over sampling factor of 1.2 (and 320 FPS at 16x).

SHG on HEQ5 with 80mm f6 APO
SHG on HEQ5 with 80mm f6 APO
1069.jpg (453.29 KiB) Viewed 684 times

With these rough constraints in mind, we embarked on our "second light" imaging using the foam board SHG, mounted this time on an HEQ5 to provide improved solar tracking and high-speed scanning. We used the same F6 480mm FL telescope, Lunt Herschel wedge, and ASI 178MM camera as last time. We chose to use FireCapture 2.7 this time (vs ASICap previously) because of the excellent usage of DRAM buffering to allow very high FPS.

After finding the Sun, adjusting focus, locating the CaK line, and setting the ROI at 2200 x 200 pixels, we adjusted the shutter speed and USB Traffic control to allow an FPS of 281. Starting with the slit over the maximum width of the Sun (in order to avoid clipping of the full disk), we took four full scans in DEC at 16x using the SynScan hand controller. This resulted in an SER file of 12,430 frames (5.1GB at 8 bit depth). We decide to throw this huge file at the SLiM program by Wah!, selecting a 4 pixel bandwidth. After calculating for a very long time, the program produced a very nice output showing the four and a half back and forth scans:

four cak scans
four cak scans
160553-four.jpg (60.84 KiB) Viewed 684 times

We then selected one of the scans and cropped it in Photoshop Elements, applied some filtering and levels, and adjusted for what turned out to be about a 10% over scan (which caused the raw images to look slightly oval):

SHG CaK
SHG CaK
553_2.jpg (137.04 KiB) Viewed 684 times

For comparison, here is a CaK PST image taken immediately after the SHG image:

PST CaK
PST CaK
cak-pst-2.jpg (151.74 KiB) Viewed 684 times

Seeing condition were not ideal (imaging was done in the late afternoon) and this may have adversely affected the SHG more than PST image since the time to take an image was 7.5 seconds (SHG) vs 10 ms (PST). Also, only one full SHG frame was processed, rather than 250/1000 frames stacked for the final PST image. I did try to stack all four SHG scans; however, while the noise did decrease slightly, the registration between the images was not very good so overall the image did not look as sharp. Nevertheless, some progress was made and the result is, I believe, an improvement over the "first light" attempt. There remains room for improvement in both spectral and spatial resolution.


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Montana
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Re: New foam board spectroheliograph results

Post by Montana »

That's a terrific image, well done, you will soon get it better than the PST :)
:bow
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Re: New foam board spectroheliograph results

Post by thesmiths »

Montana wrote: Thu Jun 10, 2021 2:11 pm That's a terrific image, well done, you will soon get it better than the PST :)
The CaK PST is such an amazing little telescope, that will be hard to beat. I'm not even quite sure what makes it so good. I heard something about it using the same H-alpha PST collimator in its filter train? My impression is the CaK PST is better than the Lunt CaK filter on the back of a normal telescope.

On the other hand, the SHG should be able to beat a H-alpha PST in most respects.


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