Icosahedron

Yes, that is correct (13->2). I'm using a spectroheliograph with a theoretical spectral resolution of 0.1 Angstrom. I'm riddled with doubt now. Consulting Rowland's Solar Spectrum Wavelengths today I noted that there is a Fe I line at 373.48 nm. At the time I was on my feet and only armed with the wavelength of H13. I calibrated on the centre of Calcium K, did the hop from there under electronic control and recorded the line at the destination. Next time I'll confirm the presence of the Fe line first.

Yesterday I was rewarded with the best atmospheric seeing so far this year so I dared an attempt at H13 (373.44 nm). Fairly pleased with the result.

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.

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.

Yellow, violet, black, = 47 ohm, not 470 ohm (yellow, violet, brown). This might lead to overheating or overloading of your PSU. Edit: Sorry, I made a mistake, it is a four colour band resistor. Yellow, violet, black, black is indeed 470 ohm.

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.

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.

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:

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.

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.

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:

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:

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:

Ancient technology indeed.

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.

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.

No, I'm still not in a position to give it another go. Over the last week I've built the new camera and got it working. Currently giving it a soak test while still waiting for a few non-critical components: Next step is making a new slit. I'm now convinced that the movement of the lines was due to thermal convection currents inside the unit and will in future cool the slit assembly while observing: I've abandoned the idea of replacing the Arduino DUE with a Teensy 4.1. Unless I'm mistaken Teensyduino does not support Arduino's SerialUSB object and their Serial object is fixed at full USB speed. In this application the DUE outperforms the Teensy.

I would use an air conditioner louvre motor😉.

The slit length is 45mm, more than twice the required length as I find it easier to make than a short one. So far I've simply set the width to the minimum possible. The second set didn't quite match the quality of the first which is why I left it a bit wider. Vignetting is present and fortunately can easily be removed during processing. Not contemplating upgrading the grating as yet. Today I received the linear CCD array (5000 x 7µm x 7µm pixels) that I'd prefer to use. I don't expect image qaulity to improve as a result, but the spectral bandwidth will be halved. The imaging system will have to be sped up hence the mention of a Teensy in the excitement of my first post. The prototype camera board is a mess being subjected to experimentation. I've already prepared a new board that can accommodate either sensor via solder links.

Merlin66: Thanks for the information. I'll start with checking the effect of the slit assembly temperature at the next opportunity.

Had another session with a clear sky and a gusty wind. The curve as seen here in the telluric lines indicates that the optical components are not in alignment. This misalignment becomes evident when viewing lines with a narrow bandwidth as can be seen further down. The image sensor requires leveling as the focus on the left differs from the right. Newton's rings can also be seen on the left. The maximum camera exposure time limit has been increased in software and the slit assembly replaced with one that permits easy cleaning. This time I tried a slightly wider slit which has harmed resolution. I decided not to spend time resetting it and pressed on with other checks. My next step was ascertaining whether focus could be reached at the other end of the spectrum. Then, thanks to the grating equation and interpolating focus settings, I made quick visits to H-beta and one of the Sodium lines. This was rapidly followed by checking H-alpha again to confirm that thin cloud wasn't rolling in. The curve in the H-beta and Sodium lines can clearly be seen. The lines also appear to have moved relative to the sensor in the Y-dimension. The structure is stiffer in the Y-dimension with a smaller area exposed to the wind and if the wind was causing it to flex I would expect it to be predominantly in the X-dimension as the wind gusts show. Totally perplexed at the moment. Any ideas?

Hi Gina, Please forgive me if I appear to be rude but I'm trying to help and save you from future disappointment. I think you must accept the fact that by nature this 3D-printed material will never ever be waterproof on a molecular level. Effectively you are mounting a dome on a sponge with waterproof material between the two. Over time the housing will absorb water which then evaporates both to the outside and inside where it condenses when the temperature drops. I would suggest you mount the parts in a waterproof outside junction box designed for the purpose. By all means, 3D print the parts you require inside and outside of this enclosure like the cover for the dome. I worked for a company that developed a product that despite all their efforts failed to remain waterproof right to the bitter end. My suggestion that they approach NASA with their product to be used as an endless water supply on Mars was not appreciated.

In the above diagram camera refers to a lens. The mount is part of an abandoned project from years ago and it's very satisfying finally putting it to use. I'll be moving it in and out of the house as it's on wheels and fits through a standard door frame. Operation will be as follows: The line scan camera runs continuously while the mount tracks the sun, allowing the wavelength and focus at that wavelength to be set. By analysing the scanned line (lit length and position within line), tracking can be corrected in RA and DEC. When instructed it then moves ahead of the sun, stops tracking and an image is acquired. Once calibrated it should be a fairly rapid automated process.

😁, thanks, now I can relax.

And finally...