Imaging

This page is here to describe my imaging workflow and document the findings a made while capturing. My setups can be found here.

Solar Halpha

All of my solar images are captured using lucky imaging, where the exposure times are low enough to freeze turbulence in the atmosphere to capture images that are sharper then the seeing would usually permit.

I usually try to keep the exposure time lower then 4ms and only use gain when i can't increase the exposure time any further. When setting the exposure and gain i try to fill the histogram as much as possible while not over- or under-exposing the image.

Of course there are some cases in which these constraints are not the best and the exact exposure time and gain depend on the seeing. For example when imaging prominences, were overexposing the surface is not as important as capturing the faintest details possible, i use a higher gain.

It's important to keep capture time shorter then about 20 seconds so that the dynamic motion of the suns surface does not blur the final image. Normally 1000-3000 images are sufficient to get enough "lucky" frames.

The software i use on my Linux software is called vidoxide.

Step by Step

1. connect camera to telescope and pc
2. start vidoxide
3. connect camera to vidoxide
4. set capture output directory in vidoxide
5. set gain to 0 and exposure time to something sensible like 2-3ms
6. center the sun using a solar finder or the shadow from parts of the telescope (e.g. telescope-clamps) until it can be seen on the screen
7. first focus pass to get the solar limb into focus (this does not have to be perfect)
8. move to the center of the sun
9. open the preview/processing window in vidoxide (it's really useful to see the faint details while capturing or checking the tuning)
10. tune the etalon(s) to have as uniform a contrast over the whole image as possible using the preview windows stretch-histogram and gamma features to get a better preview of what you are doing
11. find what you want to image on the sun and frame it
    1. if it's full disk using the full disk setup
       1. the goal is to get about 80% histogram fill
       2. depending on the seeing increase the exposure to at a maximum of 3-4ms (most of the time i try to stay around 2ms to be save)
       3. remember that in this case longer exposure times are not as bad because of the smaller aperture telescope
       4. only now use gain
    2. if it's a prominence using the closeup single stack setup
       1. remember that having the surface overexposed does not matter
       2. the goal is to set the exposure and gain to get the faintest details into view while keeping the brightest parts of the prominence just below being overexposed
       3. depending on the seeing increase the exposure to at a maximum of 3-4ms (most of the time i try to stay around 2ms to be save)
       4. only now use gain
    3. if it's the surface using the closeup single stack setup
       1. the goal is to get about 80% histogram fill
       2. depending on the seeing increase the exposure to at a maximum of 3-4ms (most of the time i try to stay around 2ms to be save)
       3. only now use gain
    4. if it's the surface and a prominence over the solar limb (fila-prom) using the closeup double stack setup
       1. the goal is also to get to about 60% histogram fill
       2. go back to the center of the sun and find a spot with as little features as possible
       3. depending on the seeing increase the exposure to at a maximum of 3-4ms (most of the time i try to stay around 2ms to be save)
       4. only now use gain
       5. move telescope focus out by a bit until the preview looks blurred
       6. capture a 2000 flat .ser file
       7. focus the telescope (this still does not have to be perfect)
       8. try to get about 80% histogram fill setting gain and exposure as before
12. focus the telescope exactly (this might be hard in bad seeing, keep trying and wait for moments of good seeing)
13. if you are doing a time-laps
    1. connect the mount to the computer and vidoxide
    2. center the feature you want to image
    3. set the guiding anchor on the feature
    4. calibrate guiding in vidoxide
    5. center the feature you want to image
    6. start guiding
    7. capture between 1500 and 3000 frames
    8. stack and check the result in a quick processing pass
    9. set vidoxide to capture in a loop with a delay of 0 to 5 seconds between captures
    10. set vidoxide to take between 1000 and 1500 frames per capture
    11. start capturing the time-laps
    12. periodically check if computer memory is full and that guiding didn't fail
14. if you are doing a single image
    1. capture between 1500 and 3000 frames
    2. do this about 5 times
    3. move to the next object remembering to correct the histogram fill
15. if you are doing a full disk image
    1. capture between 1500 and 3000 frames
    2. with my setup i have to take make mosaic from to images to capture the full disk so i immediately move to the other have of the sun and capture another 1500 to 3000 frames