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About Icosahedron

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  1. 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:
  2. 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
  3. 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.
  4. 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.
  5. 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
  6. 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:
  7. 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 tw
  8. 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.
  9. 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
  10. 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 f
  11. 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
  12. Ancient technology indeed.
  13. 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.
  14. 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 process
  15. 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 i
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