Improved SHG Ca-K results

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Improved SHG Ca-K results

Post by thesmiths »

The second attempt at "spectroheliography" (SHG) went even better than the first. The major changes were: the front objective, changed from a 100mm f4 to 200mm f4 (ie 50mm diameter) Pentax lens; increased exposure gain to improve contrast inside the Ca-K line; smaller region of interest (ROI) to boost frames rates. Here is the new objective on the 1200 l/mm grism spectrometer. It actually is a little more balanced now (since there is a 150mm f4 Pentax lens at the camera end).
SHG 200mm
SHG 200mm
SHG-200mm.jpg (118.23 KiB) Viewed 5819 times
I've added a video here of one of the scans. It was taken with the ASI 120MM at 2ms exposure, 53 gain, 256 fps. The ROI is 816 x 128 pixels. The temporal and spatial resolution have been reduced for presentation. The Ca-K line is horizontal. The white features inside the thick dark Ca-K absorption line are the areas of the Sun's surface that appear bright in Ca-K images. The occasional thick black vertical lines that span all spectral lines are sunspots.
CaK 2a.mp4
CaK video
(1.26 MiB) Downloaded 1294 times
Using Wah's SpecLineMerge program, we produced images at 1 pixel and 10 pixel bandwidth directly in the centre of the Ca-K line. The spectrometer has theoretical resolution of 9.2 pm/pixel so 1 pixel BW should be around 0.1 angstroms and 10 pixel BW should be around 1 angstrom.
CaK BW1
CaK BW1
CaK_030514_155004_3_002b.jpg (96.05 KiB) Viewed 5819 times
CaK BW10
CaK BW10
CaK_030514_155004_3_004b.jpg (89.9 KiB) Viewed 5819 times
For comparison here is a Ca-K PST image posted on this site by Smerral. His was taken on May 2, ours on May 3. I reduced the size and removed the colour to make comparison easier.
CaK PST
CaK PST
smerral-may2b.jpg (79.1 KiB) Viewed 5819 times
The PST has an objective of 40mm diameter and BW of around 2 angstroms, so it is not dissimilar to the instrument characteristics of our second, 10 pixel BW, image. Of course with a PST, one can take hundreds of full disk images in the time it takes to make a single scan of the Sun with a SHG and these can be stacked to greatly improve signal to noise. The trade off is that the single SHG video contains all the information to look at various centre wavelengths at any bandwidth you like.

I'm pleased to see that with our second attempt the images at least start to resemble traditional Ca-K images taken with normal solar telescopes. I would also note that the larger 1 angstrom BW makes the images look more "normal" (I would imagine that a 2 angstrom BW would look even more "normal"). I guess this is because that is what we are all used to looking at.

Camera note: the higher the fps, the more spatial resolution across the Sun's surface while scanning. However, on my last video attempt, I tried to push it to 625 fps . The video became unstable and started to miss a lot of frames. After completing the video, the instability increased -- and then the ASI 120MM stopped recording altogether and appears comatose (not quite dead but not responding). I may have pushed it too hard. It worked fine up to 300 fps. I will send it back to ZWO and they will have a look. I'm sure 625 fps is outside the anticipated design usage of their (very good) planetary camera. So I guess my fault for trying to push the envelope.

Note to Wah: I did a bit of work to improve the subtraction of SHG slit artefacts, following the guidelines in your "SHG imaging & processing" document. I found a slightly different approach that worked better for me. I generated a 10BW off-band image (ie not directly on the Ca-K line) and created a "master flat" from this. Since there were no bright white areas, this made the process much easier (and I could, as you suggest, use "lighten" as for H-alpha images). This "master flat" I could blend with any image created from the same video.


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Re: Improved SHG Ca-K results

Post by swisswalter »

Hi Douglas

fantastic results and what a fine way to generate a master flat


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Re: Improved SHG Ca-K results

Post by Wah »

Your alternative approach is better than mine, and the result is also very good!

BTW, how do you calculate the bandwidth/pixel? It seems too high resolution for me :roll: :roll: :roll:

I just downloaded your video and captured the image @ 100% size.
By comparing the lines in this website:
http://fermi.jhuapl.edu/liege/s07_0014.html

I got the result is about 0.26A/px


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Re: Improved SHG Ca-K results

Post by marktownley »

Way cool Douglas!


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Re: Improved SHG Ca-K results

Post by DSobserver »

hey

really nice! :bow

I'm still fighting with my SHG for the focus...

How do you manage it between the telescope focus, collimator focus and camera focus?


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Re: Improved SHG Ca-K results

Post by Wah »

Focus spectral lines first, then solar image.


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Re: Improved SHG Ca-K results

Post by p_zetner »

Super result! Your in-line grism arrangement is quite unique and makes for a nice, compact instrument.

I have to admit, though, that it's unlikely you are achieving 0.1 Angstrom bandpass. Multiplying the width of the single pixel in a slice by the (reciprocal) linear dispersion will give you 0.1 Angstrom as a number but, depending on your slit width etc, that pixel will have spectral contributions from neighbouring pixels as well. It would be interesting for you to test the bandpass directly.

Cheers.
Peter.


EDIT: Oh, I just now see that Wah has calculated your bandpass by comparison with the Liege solar spectrum. His number of 0.26 A/px sounds more like it.


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Re: Improved SHG Ca-K results

Post by thesmiths »

To Wah: You're right, I made an error in the theoretical dispersion calculation (using SimSpec). It should have been 20.2 pm per pixel. So you are quite right, the bandpass you calculated is more like it. So the the two images are approximately 0.2 angstrom and 2 angstrom BW. The video on the post has been downsized for presentation but the original video was 816x128 pixels and total of 1432 frames. I guess you took this into account in your calculation. Thanks for figuring out the error!


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Re: Improved SHG Ca-K results

Post by thesmiths »

To DSobserver: Focusing all the different elements is a challenge and the most important thing to make it all work properly.

Step 1: focus the collimating lens on the slit. There is no need for a front optical element (telescope) for this. I point the slit at a compact florescent light bulb and attach a camera lens set at infinity focus and use a DSLR with Liveview (10x magnification). In this step, you should also rotate the slit so it is aligned with the grating. After adjusting this, it's important not to move the slit or change the collimating focus.

Step 2: with the actual acquisition camera mounted, focus on the spectral lines in question. This is fairly easy as the lines have very high contrast. But the camera lens ideally needs to have a fairly good helical focuser.

Step 3: focus the front lens (telescope) so it focuses the Sun on the slit. This is the most difficult focusing job. The only thing you have to focus on, as far I can see, is the actual edge of the Sun as you really cannot make out any real surface details. In fact, the first step here is to make sure you even have the unfocused Sun on the slit. Then move the blurry Sun around and find an edge when the Sun's image is at maximum width (ie taking a cross section approximately through the centre). Trying to focus on the Sun's edge is a lot harder than focusing on a sunspot but I don't see any other way.

By the way, it's useful first to calculate approximately what percentage of the slit will be covered by the image of the Sun. According to my calculations, you can take the focal length of the front lens in mm and divide by 107 to get the size of the Sun in mm on the slit. So, for example, with 200mm focal length lens, the (maximum) size of the image will be 1.9mm, which will comfortably fit onto a 3mm long slit.


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Re: Improved SHG Ca-K results

Post by DSobserver »

wah, thesmiths,

:bow

Wish to get sunny days soon now....


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Re: Improved SHG Ca-K results

Post by Peter Williams »

Great follow up and adjustments to your amazing instrument, the results certainly have improved. Very impressive.


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Re: Improved SHG Ca-K results

Post by thesmiths »

To Peter: I really like the inline grism approach for SHG. I actually built a very nice reflecting grating spectrometer which I can use with a 1800 l/mm grating or even an Echelle grating. That one gives really high resolution (I could detect Doppler rotation when used in 2nd order) but it is fairly big and heavy so it is difficult to actually use on a telescope mount. I realised you have to design things to work on a mount, not on a benchtop. The idea of the inline grism for SHG came to me when I saw the Sheylak ALPY at Astrofest this year. The APLY is gaining tremendous traction in the night-time spectroscopy world because you can attach it so easily to a telescope. It doesn't have the highest resolution but the fact you can so easily adapt it to any existing optics and mount makes it a killer device. So my goal was to make a "Solar Alpy".

With regards to my broken ASI 120MM: Sam at ZWO says while I send my broken one back, he can send me to test the new USB 3.0 version that will come out shortly. He says it can do 72 fps at full frame (1280x960), double the current USB 2.0. So I'm assuming at small ROI it can easily do 500 fps. I now just have to figure out how to make my Vixen mount scan faster. Or maybe it's easier to simply put a rotating mirror or prism near the front.


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Re: Improved SHG Ca-K results

Post by solarchat »

you are way ahead of me on this stuff but I think you might be referring to the LHIRES III as the ALPY 600 cannot be mounted to a telescope. I use them both in outreach. The LHIRES III has a two inch barrel that goes into any scope and a guide camera port built in but the ALPY 600 has only a small window in the case that lets the sunlight through.
This is both of ten side by side at a recent event...

the ALPY 600 is on the left. Do they have another ALPY model that I don't know about?

IMG_1125.JPG


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Re: Improved SHG Ca-K results

Post by thesmiths »

The instrument on the left is, I believe, what they call the Lhires Lite:
Lite
Lite
LhiresLite_400x320.gif (38.14 KiB) Viewed 5766 times
The Alpy 600 looks like this:
alpy
alpy
fsq80ed_2.jpg (112.12 KiB) Viewed 5766 times
Tthe ALPY is essentially a souped-up Paton Hawksley spectroscope, with a small grism, a small collimator and a camera lens. You can actually take off the camera lens at the back and use it visually, just like the Paton Hawksley spectroscope. In fact it uses their grism. It just costs 10 times more.

In fact, those photos sort of show the motivation for developing a grism SHG. What I had originally built was more or less the same size and capability of the Lhires III. But as you can see, it is a pretty chunky piece of metal and generates a lot of torque at the back of a telescope. The ALPY, on the other hand, particularly without the guiding module, does not add much weight at all.


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Re: Improved SHG Ca-K results

Post by solarchat »

well, Ill be danged.. you're right. I guess I was calling it the wrong thing..:)


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Re: Improved SHG Ca-K results

Post by Merlin66 »

For "serious" (night time) astronomical spectroscopy you'll always need a slit, a reference lamp and a good guiding arrangement.
The ALPY (and the Elliott CCDSpec etc.) are good for what they are but will never do the same hi-res job as a Spectra-L200 or Lhires III.


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Re: Improved SHG Ca-K results

Post by thesmiths »

Although this is a bit off topic for a solar site, the Alpy does have a slit and optional guiding and calibration modules. The only meaningful difference vs the Lhires III is the lower dispersion, which may or may not be important, depending on what you are trying to resolve.


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Re: Improved SHG Ca-K results

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thesmiths wrote:To DSobserver: Focusing all the different elements is a challenge and the most important thing to make it all work properly.

Step 1: focus the collimating lens on the slit. There is no need for a front optical element (telescope) for this. I point the slit at a compact florescent light bulb and attach a camera lens set at infinity focus and use a DSLR with Liveview (10x magnification). In this step, you should also rotate the slit so it is aligned with the grating. After adjusting this, it's important not to move the slit or change the collimating focus.

Step 2: with the actual acquisition camera mounted, focus on the spectral lines in question. This is fairly easy as the lines have very high contrast. But the camera lens ideally needs to have a fairly good helical focuser.

Step 3: focus the front lens (telescope) so it focuses the Sun on the slit. This is the most difficult focusing job. The only thing you have to focus on, as far I can see, is the actual edge of the Sun as you really cannot make out any real surface details. In fact, the first step here is to make sure you even have the unfocused Sun on the slit. Then move the blurry Sun around and find an edge when the Sun's image is at maximum width (ie taking a cross section approximately through the centre). Trying to focus on the Sun's edge is a lot harder than focusing on a sunspot but I don't see any other way.

By the way, it's useful first to calculate approximately what percentage of the slit will be covered by the image of the Sun. According to my calculations, you can take the focal length of the front lens in mm and divide by 107 to get the size of the Sun in mm on the slit. So, for example, with 200mm focal length lens, the (maximum) size of the image will be 1.9mm, which will comfortably fit onto a 3mm long slit.
One more question : when you move from cak to Ha wavelength, do you also need to do again the focus? In that case you only do it with the acquisition lens, right?


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Re: Improved SHG Ca-K results

Post by thesmiths »

That is a very interesting question. In theory, you might have to do all three again as the optical elements are unlikely to behave exactly the same from violet to deep red. However, in practice, I would leave the step (1) focusing unchanged. I don't think the small change in collimation at different wavelengths will make a significant difference. Also, there is no way to really readjust the collimation when the SHG is already set up to image. So I would recommend not to mess with this focus. In fact, probably better just to adjust it well once and then try not to ever change it again.

You will almost certainly have to readjust the step (2) focus. In fact, expect the lines to be way out of focus if you move all the way from violet to red. But this focusing is very easy since the spectral lines are extremely distinct.

The step (3) focusing will probably need a little adjustment, but I suspect not very much. All lenses will have a little chromatic aberration. This is, as I said, the most difficult focusing job but it is exactly equivalent to trying to focus a telescope on a planet (or the Sun!).


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Re: Improved SHG Ca-K results

Post by DSobserver »

I see...

and any advices regarding smile effect? :evil:


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Re: Improved SHG Ca-K results

Post by thesmiths »

I've never been exactly sure what causes the smile. For some reason, in all my most recent spectroscopes that use transmission gratings, there is never any smile at all. But I always saw it with reflection gratings.

In any case, all the software post-processing programs seen to deal with smile.


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Re: Improved SHG Ca-K results

Post by Merlin66 »

I find "smile" more of an issue in Littrow designs - due to the off-axis beam through the collimator. Aggravated by long slit aberrations. Certainly more noticeable in prism designs - that's the nature of the beast.
The straightest lines I get are with a Classical design when everything is on axis.

Buil's IRIS or John's BASS software will correct for smile (and tilt and slant)


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Re: Improved SHG Ca-K results

Post by fulvio.mete »

The results seems to be very interesting, Thesmiths.
My advice is not to use a photo lens to send the solar image to the spectroscope, for the loss of contrast due to the several internal optics.
It's very compact instrument, similar and probably less in weight and in size to my UPS.

http://www.lightfrominfinity.org/UPS/UPS.htm


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Re: Improved SHG Ca-K results

Post by fulvio.mete »

I find "smile" more of an issue in Littrow designs - due to the off-axis beam through the collimator. Aggravated by long slit aberrations. Certainly more noticeable in prism designs - that's the nature of the beast.
The straightest lines I get are with a Classical design when everything is on axis.
Hi, Ken:
I tried to solve this problem putting the slit and the eyepiece holder off axis, and tunable.
With this , in my four Hires Littrow solar spectroscopes I get quite perfect straight lines in spectra, with no smile at all.

Fulvio


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