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Narrow-band solar imaging is both challenging and rewarding. Looking through one of these marvels is amazing but, taking a picture to show others is beyond cool.
Many (like myself) start with the family camera and then, after a little success, move on to dedicated astronomical instruments. No matter which way you choose to share the sun, remember my motto - it's a hobby, keep it fun!
Capturing the Sun
Single-Shot Astronomical CCD Cameras (SBIG, Apogee, FLI, QSI)
The
main advantage of dedicated astronomical CCD cameras is that they offer the
ability to
image the entire disk of the sun with just one shot. Large
chip sizes are available that allow you to fit the native solar image
size to the array.
Even though this information is specific to one camera, the steps can be applied to just about any single frame camera available.
Start by focusing the sun in a low power eyepiece. Tune the hydrogen-alpha filter to see the greatest amount of detail. In the camera control software, use the fastest focus capture setting to center the sun on the chip. With SBIG's CCDOps it's called full-low, auto update. Once centered, switch to full-high (unbinned), auto capture mode to find focus on the limb or disk.
Next, grab either a single or multiple full resolution captures. Shutter speeds vary between .001 and .040 seconds without the use of a neutral density filter or at .1 and .3 with one. Save as an uncompressed TIFF file. Pick the one that has the sharpest focus and open it in PhotoShop.
Live Feed Video Cameras (Lumenera, DMK)
Despite the small chip sizes of video and webcams, they offer you the ability to capture many frames over a short period of time. This allows you to stack them together to reduce the effects of the atmosphere. Real time viewing is a major plus.
Once the sun is tuned in Ha and in focus,
make these adjustments:
Exposure to lower half of the scale - Exposure is the
shutter speed in ms
Gain to the lowest possible setting - Gain can add noise
very quickly
Gamma low to middle scale - Controls the greyness of the
overall image
Contrast to 1 (or default) - Controls
the deepness of black
Brightness to low to middle - Controls the deepness of white
Use the Exposure slider to control the overall initial brightness of the shot. Use Gamma and Brightness together to brighten space and expose prominences. Lower Exposure and Gamma to darken the disk. Contrast and Gain are not moved.
Limit the capture time to several seconds (~150 frames). Save the captured frames as an AVI.
Stacking Solar Images with Registax
Registax works very well with solar avi captures and can be fast to
process.
Open the avi file and the frame list box at the
top. Settings:
FFT (Fast Fourier Transform) box to on
Select a large enough alignment box
to cover your target area.
Single or multi alignment point
Scroll down the frame list and find a good frame in the middle of the list. This will set the alignments initial comparison frame. Find a dark spot on the disk or inside a prominence to align on. Click in the middle of your chosen alignment feature.
Once clicked, the FFT display will show how much
movement is expected in the alignment process. A tight, round display
will move little, while an elongated FFT will move too much and cause
alignment errors. Choose another alignment area if the FFT is not
perfect. Click align.
Select the Frame List box and deselect all frames. Reselect only the
clearest frames and click Limit then Optimize and Stack. Once the
Wavelets screen loads, choose a higher number (4,5) wavelet and slide it to
the right. Click reset to try again. Try a combination of two wavelets
if the seeing is good. Save the file as a tiff and open in PhotoShop.
Comparison for Solar Imaging
| Live Feed Video
Can be
Inexpensive Cons |
Single Shot CCD Large chip sizes for whole disk
Cons |
Processing the Sun in Adobe PhotoShop or GIMP
For the disk, I’ll first process it with Filter | Sharpen | Unsharp Mask and get it as clear as possible without causing artifacts to appear. The radius is set to 1.0 pixel and the threshold to zero. It's pretty simple, open Unsharp Mask and move the slider.
Next, under Image | Adjustments | Levels, I’ll adjust the mid-tone point slider to darken the disk detail and then the white point slider to brighten any active regions. Don’t worry about the prominences at this point.
Once you’ve got it where you like it, use the wand tool (press W) to select the disk and copy the image. Click directly on the edge (limb) of the Sun. Right mouse click on the Sun and click Select Inverse. Next select Edit | Copy. I usually set the wand tolerance (at the top of the page) to 10 to capture as much of the limb as possible. It sometimes helps click Select | feather and feather the highlighted disk. Set the feather radius to 1.0 pixel.
I’ll open another copy of the raw image and process the prominences the same way. You’ll probably only use the white level slider. The disk will be completely overexposed but it makes no difference. Get them as sharp and bright as possible.
Adding Color
For the color, I switch to Image | Mode | RGB color mode and open the Image | Adjustments | Curves function. Use the drop down list and adjust each Red, Green and Blue channel individually. Click and drag the Red channel up and left, Green down and right. Blue down and far right. Apply.
Go back and play with the Image | Adjustments | Hue and Saturation or Image | Adjustments | Color Balance and try different settings.
You’ll have to experiment with the settings to find a color you like but that’s half the fun!
Once you're done, send them off to NASA APOD or SpaceWeather for the world to enjoy!
