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Coronado Calcium-K PST

Posted by Robert Piekiel   01/17/2006 12:00AM


A few months ago, S&T did a review of the Coronado 70mm CaK. It described the scope as being very nice for imaging, but the extreme blue-end-of-the-spectrum CaK light was very dim for visual use. About a year prior to that, the Coronado Ha PST was reviewed in S&T as well, with the opinion that the scope was very suitable for visual use, despite a “sweet spot” in the center of the field. It could also be used photographically, but it didn’t have the resolution of its bigger brothers, such as the Solar Max 40, 70, or 90. So the PST is suppose to lend itself for quick-and-easy visual use (you just plop it on a table with its little tripod and have a look at the sun any time you want), but what about the CaK version that is just now leaving the doors of the Coronado factory? What can you expect out of this small-aperture specialty scope that inherently operates at a frequency of light that may not be easily visible to the human eye?

Other than a few test models, I received what is probably one of the first production-run versions of the CaK PST, as the factory just began shipping them in January 2006. Since I work for an outdoor-ed center, and solar programs are part of our routine, I not only could use the little CaK at my home, but also take it to a public event for evaluation. (Did I mention that I also have a Ha PST and have used it regularly for quite some time?)

The little CaK PST looks just like the Ha model except for a blue band on the body instead of a tuning collar (The CaK model has NO tuning collar, only a focus knob). It comes with a 20 mm Kellner eyepiece. This gives a magnification of 20x, as the scope itself has a focal length of 400mm. (The Ha model also has a 400mm focal length, but is typically supplied with a 12mm Kellner eyepiece) You can purchase the factory mini-tripod for it, or attach it to anything of your own in that it mounts on a standard ¼-inch tripod mounting bolt. (It is essentially identical to the Ha PST in these respects).

Setting the scope out on a clear day gave me my first look through at the sun in CaK light. Due to un-cooperative weather, I also had many opportunities to view the sun behind various thin cloud layers, and as a result of that, I would STRONGLY recommend that your first use of the CaK PST be with a perfectly clear, cloud-free sky. Why is that? Because the image is DIM. Dimmer than any other kind of conventional viewing you’ve probably ever done.

The CaK solar disk will appear about ½ the size of the image that the stock Ha PST gives due to the 20 mm eyepiece it is supplied with. That’s probably a good thing, as too much magnification will dim the view even more. To get an idea of what the image of the sun will look like, try to imagine a black piece of construction paper with a small circle of transparent, dark blue paint applied to it. Now look at this black paper in a shaded room and try to see the blue spot. I could also describe the color of the image as similar to the dim, purple glow of a blacklight, but a bit more blue.

The instructions that come with the CaK PST say that you need to get your eyes accustomed to seeing the image, and that about hits the nail on the head. They also suggest using a dark cloth to cover your head to keep stray light out of your eye, and I can say that that’s also a must, or at the very least cupped hands around your eye. ANY AMOUNT OF STRY LIGHT GETTING TO YOUR EYE WILL PREVENT YOU FROM SEEING THIS IMAGE AT ALL. It’s almost like having to get your eyes dark-adapted to see the CaK light. Once you know what to look for, it gets easier, but it can be challenging at first.

Focusing the dim image also takes some care, and is next to impossible if there’s a bit of cloud cover. That’s why I say do your first viewing in a crystal-clear sky. Once you get all this down (seeing it, focusing it, etc) the next step is trying to discern details in the visual image. This is also challenging, but the white spots resulting from the sun’s magnetic field “twists” can be viewed if you’re diligent. Increasing the magnification by changing eyepieces will only dim the image more, so it’s a trade-off between a small dim image and a larger, dimmer one.

Can everybody see this CaK light? I don’t know about everybody, but my 17 year old daughter can see it, my 81 year old mother can see it (with some “training”), my 54 year old wife can see it, as well as several middle-aged folks that I have had the opportunity to show off the unit to. A few other children could NOT see it, but I suspect that this was simply due to their inexperience at looking through a telescope.

What about using a wide-field eyepiece, in place of the supplied Kellner? The answer: It won’t really benefit you much. The Kellner works fine, simply because the light cone coming from the PST simply isn’t wide enough to fill the edges of a wide-field eyepiece. (The same is true for the Ha model) This is one case where a good-quality NARROW-FIELD eyepiece will do just as well, if not better, than a wide-field (expensive) one.

Now to imaging with the CaK PST. I don’t personally own a dedicated CCD imager, but instead do most of my astrophotography with my Nikon Coolpix-4500 digital camera. By coupling such a camera to the supplied eyepiece (It has a removable rubber eyeguard) in the afocal style, you can get very acceptable images of the “blue” sun, as the camera’s detectors are much more sensitive to CaK light than our own eyes. The trick is to get it FOCUSED properly, as you will be attempting to view the image on your camera’s LCD screen (or laptop screen if you have a CCD device that hooks to a computer) while in broad daylight. This again means bring out the dark cloth to put over your head and screen in order to see the image. Once you can view the image on your screen and focus it, you will be able to get a very respectable image of the CaK sun with exposures of 1 second or less. Consider using a tracking mount here for maximum sharpness.

If you wish to use a more sophisticated imager, Coronado lists a few recommended CCD imagers in their instruction manual for the CaK PST. I could say that the trick is to use one with the imaging chip itself close enough to the imager’s barrel end that it gets close enough to the focus of the system. A CCD device with the chip set way back up inside it simply won’t work. They also recommend taking images in the monochrome mode of the imager if it’s a (single-shot) color unit, as the monochrome mode will use all the pixels in a more efficient way to handle the dim image.

All in all, I would say that the $499.00 CaK PST is a highly-specialized telescope that fills a niche for die-hard solar viewing fans, but is probably not on the “must-have” list of a more casual observer on a limited budget. Personally, I’m glad I bought one, and makes for a very educational instrument in my public astronomy programs, but it does take some time in getting used to. Happy Observing, Bob Piekiel

Click here for more about this subject. -Ed.