Photographing the sun without the correct equipment is not recommended.
However, using a Coronado PST Solarscope highly detailed Hydrogen Alpha images of the sun can be taken using normal DSLR camera or preferably a mono CCD camera.
My setup for solar photography is a Coronado PST solarscope mounted on a Acuter Merlin motorised mount with the photographs captured with a Canon 7D fitted to the back of a Baader Hyperion 8-24mm zoom eyepiece using a threaded adaptor. To ensure that the focus and tuning of the PST is as good as possible I connect the Canon 7D to a laptop so I can monitor the live view from the camera while I’m adjusting focus on the solarscope.
Alternatively I use a Imaging Source DMK41 USB mono CCD camera fitted with a short (less than 20mm long) nosepiece. It is important to remove the C/CS thread adaptor from the camera body and to make sure that the nose piece can slide fully into the PST eye piece tube so that the DMK41 camera housing touches the top of the eye piece tube. If the DMK41 camera housing doesn’t touch the top of the eye piece tube it will be difficult, if not impossible, to achieve focus without the aid of a barlow extension tube or Powermate.
As a quick ‘grab and go’ setup the PST and Merlin mount is ideal, you can be viewing the sun within a few minutes and it is stable enough for photography.
Your first image taken through this PST solarscope is very likely to be similar to mine.
It will be very RED
and to be honest I thought it was a reasonable image until you see what the experts can produce.
It is also worth knowing that the sun is approximately 108 times larger than the earth so the prominence at 3 o’clock in the image above is larger than the diameter of the earth!
The Hydrogen-alpha frequency is only captured by the red channel of a DSLR camera, the green and blue channels only muddy the final photograph as you can see in the image above. For this reason the seasoned solar imagers use monochrome CCD cameras and process a captured video through a programme like Registax to get the highly detailed images that can be post processed to introduce the colour.
For DSLR solar imaging it is very important to use the histogram during the image capture. The histogram will allow you to assess the signal on each of the R, G and B channels and adjust the exposure to maximise the signal on the red channel without overexposing it. If the red channel is overexposed all of the detail in the disk will be lost even if the prominences show up well.
One or two images can be used to make the final composite image. To process the captured image(s) it should be split into individual colour channels and normally the blue and green channels are discarded.
The red channels of the images can then be processed, as grey scale images, to produce two images, the one highlighting the surface detail can be processed using the wavelet function in Registax or PixInsight to enhance all the detail in the image.
and the second one to highlight the prominences.
The surface detail of the disk can then be pasted accurately over the prominence image to produce a composite grey scale image.
To re-colour the image it needs to be converted back to a RGB colour image and then using colour balance and hue/saturation controls the false colour is re-introduced. Layers should be used for these operations, if your software supports them, so you can go back and adjust them if you want to tweak the final image.
This image was taken at the same time as the image shown earlier but processed ‘correctly’. To remove the black line around the edge of the disk you can adjust the selection slightly by contracting the selection and then smooth and feather the edge before pasting over the proms image.
Typical values for colour balance are:
- Red +30
- Green -20
- Blue -40
- Red +20
- Green +0
- Blue -60
- Red +50
- Green +10
- Blue -100
make sure ‘Preserve Luminosity’ is checked during the colour balance operation
Depending on how you have applied the colour balance you might find you need to adjust the Hue/Saturation to finish the image but typically it should be desaturated by about -30.
To compensate for the inversion of the image orientation through the PST I flip the image on the horizontal and vertical axes and then apply a -40 degree rotation to the image.
Since posting this blog I’ve progressed to a Lyra Optic 4″ f/11 refractor modified for Solar H-a imaging and observing