The Calcium Connection


In 2006 as part of an overhaul of my entire imaging and viewing setup, I invested in a pair of solar telescopes, one being the Coronado PST Hydrogen Alpha, and the other being a Coronado PST CaK. Much has been written about these scopes in a variety of reviews, but I thought for this article I would cover the basics of imaging with the PST CaK.


As with the PST H-Alpha, with CaK light, we are effectively dealing with a monochromatic light source so a black and white CCD for imaging will in general give the best results. However, the standard Phillips Toucam Pro (and successors) can and will give excellent results. The key difference between the single shot colour cameras and B&W sensors being that the colour sensors utilize a Bayer Matrix on the CCD to derive the colour information, which impacts the overall resolution of the image. With H-A and Calcium line images effectively not utilizing most of the colour elements (H-A for example is a narrowband red light source), you effectively lose about 66% of the incoming light (not quite correct in most cameras, as bleed does occur).


Focusing and imaging with the CaK PST are largely similar to the processes with the hydrogen alpha model, as the focuser mechanism is the same. However, there is one notable difference. As the image is a lot dimmer, the gain/exposure times for the camera need should be adjusted higher to take this in to account.  This is a simple process using the camera control panel in the imaging software (in my case K3CCD), and my tip here is to focus with an overexposed image, until the limb becomes sharp, and then drop the gain down, until you start to see the tell-tale super-granule features pop out on the solar disc. Once again, features such as sunspots can aid in the focusing process, but even the relatively large super granule features may be used to fine tweak the focus.


With the Toucam attached to the PST CaK, again the preview will show (with no focal reducers or Barlow lenses) approximately 1/3rd to 1/4 of the solar disc. For full disc imaging, I will set the frame rate and gain to suit the sky conditions, but these are typically a lot higher than for H-Alpha imaging at about 1/25th to 1/100th second and the gain at anywhere between 30-70%. I typically split the Sun into 4 panes, and take 5-600 frames of video per pane, attempting where possible to get some overlap with a surface feature between them, making the post processing and compositing in to a full disc image easier. 600 frames at 5-10FPS on my Toucam is a minimum on a good clear day. On a better day, I tend to aim for anywhere up to 2000 frames of video per pane.


Wavelet processing in K3CCD, along with some contrast and brightness adjustment, typically will be more aggressive than with the PST Hydrogen Alpha images to get the best detail/contrast, though this does depend on how good the final stacked image is. As the images, especially now at solar minimum can be rather lacking in features for tracking/stacking


If no spots or features are visible I tend to set the tracking box in Registax a bit larger than a prominent super granule, and just let Registax run through the images automatically. If the sky conditions are poor, I will manually go through each frame, removing ones, which are not so clear, though this can be a time consuming process.


Once the disc has been imaged, I then up the shutter speed and increase the gain quite substantially on the camera, and start hunting around the limb (after a minor refocus) for prominences.


Typically again with the prominences, I will capture about 5-600 frames for stacking, and then attempt to select the prominence feature as the tracking object for stacking. This can sometimes yield inconsistent results, as the prominences are quite faint and subtle, and sometimes results in the tracking box losing them. In these cases, I tend to increase the tracking box size in Registax and aim for the limb curve against the black sky background as a guide reference.


The PST CaK does lend itself to other imaging methods such as afocal imaging, though more care has to be taken in shielding stray light from the eyepiece. Digital SLR imaging as with hydrogen alpha can also be used, but I find the webcam/dedicated astronomical camera option to be the best for seeing and capturing the Sun in this interesting frequency of light.



Nick Howes