Spectroheliograph: First light.

[Merlin66]

Very impressive!

I'm not sure cooling the slit will enhance the results, but it will be interesting to see your results.

What fps you you anticipate getting??

What software do you use to prepare the spectroheliograms?

Ken

 

[skybadger]

Are they third-party demo CCD boards that you are hooking into ?

 

[Icosahedron]

Ken,

I process my images first with my own destriping algorithm and then with GIMP.

My aim is to capture at least as many lines in the time it takes for the sun's image to drift across the slit as the diameter of the image measured in pixels. With the new CCD this will not be possible but I've a simple solution: Instead of bringing the mount to a halt I'll let it track at a slow rate thereby extending the drift time.

Skybadger,

There is nothing special about the boards. I'm using the example circuits obtained from the respective data sheets for the CCD and signal processor.

 

[Merlin66]

Icosahedron,

Thanks for the update, understood.

What's the fastest frame rate you can obtain from the sensor?

 

[skybadger]

Cheers for that, have you a link to your normal supplier ?

 

[Icosahedron]

Ken,

The prototype camera managed 21 frames (lines) per second at H-alpha wavelength. Anyway, it's history now. I've discovered that by managing the communications differently, I get a significant increase in data throughput.

Skybadger,

I was able to source everything I needed from AliExpress. One CCD had a scratched window.

[Merlin66]

Icosahedron,

Thanks for the update.

 

[Icosahedron]

At last the cloud cover offered an opportunity to test my new camera in combination with the narrowest slit that I can achieve. Not enough light was reaching the camera and I had to resort to maximum gain and a long exposure to obtain an image. The camera was also powered by the same unregulated 12V supply used for the cooler fan and pump; hence the noise.

There is one aspect about the sensor that I'm not happy with. It employs two CCD registers, one for odd and the other for even numbered pixels. The characteristics of the two registers are not identical and this produces line artifacts regardless of double correlated sampling. I'm hoping to reduce it with a software solution. This stretched image shows the effect:

On a positive note I'm more than happy with the limb. With the low sun at this time of year I anticipated poor seeing. Time will tell but for now I'm blaming the slit cooler.

Here is the full image with lots of slit dust and tree parts obscuring the view. I didn't have time to check and adjust the tilt of the sensor in relation to the Fraunhofer line as the wavelength varies from left to right:

[Merlin66]

I think you are achieving spectacular results with your set-up.

Unfortunately I’m a little concerned about the single array achieving your goal.

As you know the Doppler shift due to solar rotation ( and any Doppler associated with filaments and proms) will always be an issue taking  spectroheliograms and the spectral spread may be too much for the single array.

Reducing the dispersion may help? I look forward to seeing your continued development of your SHG. You’re certainly at the cutting edge.

The current CMOS cameras have good QE, fast frame rates and probably an easier solution. 😐 

[Icosahedron]

Thanks Ken, the camera is actually my main interest in this project. It has provided me with months of entertainment under the cloud over here. Regarding dispersion, the result that I achieve certainly will influence my choice of a larger grating. I can also revert back to the original camera with larger pixels if need be.

I've been playing with a formula to reduce the CCD register mismatch. An offset is calculated using a second degree polynomial which is then added to the values from the "dark" register. I've tested this with a LED at the slit position first with no gain and then with full gain at 1/10th of the exposure. In both cases the result is promising:

[Merlin66]

I'm sure with your dedication and rigour you'll find a solution.

 

[Merlin66]

Just a hint to new SHG users.....
How do you know if your slit gap is at the focus of the telescope?
Normal method is to check the top/ bottom edges of the solar spectrum being recorded. When in focus, they should appear tight and crisp.
A quick way to get you close....
Use a finder to look through the objective and check that the slit gap appears in clean focus.
Helps during initial set-up.

[Icosahedron]

Nothing was going to stop me today from finding out if a slightly wider slit would provide enough light to the camera. It did indeed despite relentless cloud and I managed to capture the same number of lines as the image width in pixels during the transit. The grating is least efficient at the red end of the spectrum and higher frame rates will be possible at shorter wavelengths. I'll measure the slit width for future reference.

My destriping algorithm made a valiant attempt to remove the cloud but did reveal diffraction limited detail:

 

First light is truly over now and I'm making this my last post. Thanks for the interest and suggestions. Here is an action shot from today:

[Merlin66]

Keep us posted on the status of your SHG.

You have achieved a lot with this project.

Look forward to seeing your future successes.

 

[Icosahedron]

OK, I'll continue to update this thread with further results once observing conditions improve.

Have now added a third coefficient to the formula addressing the CCD register mismatch eliminating it entirely.

I incorrectly assumed that the centre pixel of the array is located at the centre of the ic package. It is actually 2.2mm off centre and I consequently misaligned the optics which is the cause of the vertical banding on the right.

[Icosahedron]

I combined one of my old slit assemblies with three neon bulbs to allow me to align and calibrate the shg while the sun takes a break:

This is the most intense emission line at 585.25 nm. Detail of the bulb elements is visible due to the wide slit and the bulbs not situated at the focal point:
 
As Ken pointed out the curved spectral line is not the result of misalignment. The grating equation is only valid for the on-axis centre of the slit. A straight spectral line is essential if one of a narrow bandwidth is to be imaged which to me is the whole point of a shg. I thought of two ways to address this: A prism also produces curved spectral lines but in the opposite direction. Combining a prism with the grating will cancel the effect but it involves having to relocate the grating as well as sourcing a suitable low dispersion prism. The second option is to introduce a tilted lens in front of the image sensor. This will make the correction adjustable and I happen to have a few slr camera close-up lenses available to experiment with; my next step.

 

[Icosahedron]

This is the result obtained with a tilted lens in front of the sensor showing the straightened spectral line. I tilted the lens by hand and went a little too far introducing a slight curve in the opposite direction. A stepper motor will give finer control. The lens is a 1 dioptre meniscus type. As expected the focal length of the camera is reduced somewhat:

Hopefully this is the final piece of hardware required for this project:

[Merlin66]

You're certainly trying very hard to accommodate the linear CCD array.

I respect your decision and hope you find a workable solution.

With "conventional" sensors used in the SHG, life is easier - the processing software can correct for tilt/ slant/ smile......

Edited January 14, 2021 by Merlin66

[Icosahedron]

Hi Ken,

I'm not up to date with the capabilities of all the latest commercial cameras but here is a comparison between mine and ZWO cameras:

My camera: USB 1.1, 16-bit, 3600 x 1 pixels, 21.5 fps
ZWO ASI6200MM (mono): USB 3.0, 16-bit, 3840 x 2160 pixels, 14.38 fps
ZWO ASI2600MM (mono): USB 3.0, 16-bit, 3840 x 2160 pixels, 6.71 fps

Frame rate determines resolution in this application so I'm better off with what I have even though it is antiquated.

The result above for my camera is from the last attempt. A USB 2.0 link further increases the frame rate by a few percent, IMO not worth the trouble. The time it takes to read the array and transmit the data is insignificant compared to the exposure time at the red end of the spectrum. The Arduino DUE does take advantage of USB 2.0 but I'm using a USB 1.1 webcam as a solar finder on the same link which limits it to full speed.

Edited January 15, 2021 by Icosahedron

[Merlin66]

John,

I use both the ASI 174MM (1936 x 1216, 5.86 micron) and the ASI 1600MM ( 4656 x 3520, 3.8 micron) USB3 cameras.

With the ASI 174 I select an ROI (1936 x 120 ) to suit the target line and full height of the solar spectrum, for CaK would use an exposure around 0.6ms. I have achieved frame rates >>100fps, but to reduce the file sizes ( full frame, 3Gb (!!!), ROI 500 Mb) and maintain a reasonable resolution I currently set the frame rate to 60fps.

Using x2 sidereal scanning rate, 21 sec for full disk scan, gives me 1400 frames in SER.

One issue I do have with the CMOS cameras is resonance banding......

 

Edited January 15, 2021 by Merlin66

[Icosahedron]

Not possessing an astro camera I was unaware that it is possible to specify a custom frame resolution. Definitely an option to pursue then. Thanks for the suggestion.

[Merlin66]

I use FireCapture to acquire my SER videos. It allows you to set any ROI, frame rate, etc. etc.

http://www.firecapture.de/

 

 

Edited January 16, 2021 by Merlin66

[Icosahedron]

An hour long observing opportunity finally arrived and here is the result:

This is the first time that I attempted to capture the Calcium K line and I was surprised at its low intensity. Maximum camera gain was used combined with a long exposure time resulting in poor resolution. In hindsight I should have sacrificed pixel depth as 16 bits really is overkill. Anyway, a bigger grating will address this as well as the vignetting present. The choice of grating is now limited to 1800 lpmm:

I was keen to check the effect of the decurve lens and cooling of the slit and revisited the narrower Sodium line at 589.6 nm to compare. Much better this time round:

The session ended as clouds started to roll in. Now to get rid of the dust causing the image artifacts.

 

[Merlin66]

I'm speechless!!!

Your results are absolutely amazing!

You have pulled out some serious detail both in H alpha and CaK....... you put my meagre efforts to shame.

Excellent results, keep them coming.

 

[Icosahedron]

I've decided against upgrading to a bigger grating. The west to east axis of the sun passes through the centre of the optical system and is not subjected to vignetting whereas the north to south axis is. A bigger grating might reduce this but will not eliminate it. The amount of vignetting can be calculated making it possible for it to be neutralised in the north/south dimension. I've found this paper on the subject very informative. Eliminating vignetting produces a flat field image to which uniform vignetting can be applied. I've reprocessed my last H-alpha image using this approach:

I still require a higher frame rate at the violet end. Finally got round to investigate and determine the optimum slit width and will next aim for 1/3 to 1/2.5 the Airy disk diameter which is about the thickness of aluminium foil. The good news is that I know the current width is less than this. I used Al foil first to set the gap and then made it narrower as it appeared quite wide. Also, as mentioned before, I'm prepared to sacrifice pixel depth.