Product Review: The Coronado PST
Jack Kramer
Despite my good intentions to spend more time on solar observing, white light solar work just didn't hold my interest for long. But given the opportunity to look through someone's scope with a hydrogen alpha filter ... well, that's a different story. The sun assumes a whole new dimension when you can view prominences, flares, and plages - the really active aspects of the solar disk.
Normal glass or Mylar solar filters ("continuum filters") yield what is called a white light or integrated light image. They allow us to safely observe the sun's photosphere. By passing many different wavelengths of light, it's about what we could see if we were able to look directly at the sun. Without getting too technical, hydrogen alpha (Ha) is a very narrow emission line that permits us to see the amazing features that dance in the sun's tenuous outer layer, the chromosphere. A Hydrogen alpha filter passes only those wavelengths that correspond to the center of the red Ha spectral line at 6562.8 angstroms. To show good detail, a hydrogen-alpha filter must perform with a sub-angstrom bandpass.
Hydrogen alpha filters require a very high level of precision. While costly, they're just about mandatory for serious solar observers. Solar prominences are fascinating, but are the rest of us fascinated enough to shell out $2000, $3000, or more for a filter? Coronado Instruments has probably done the most to popularize Ha observation, but equipment from Daystar and Solarscope tends to be more highly regarded by the real aficionados. Their stuff is also pricier - Solarscope's filters start at over $4000. Then when Coronado introduced the Personal Solar Telescope - the PST - with a price of $499, it became the hydrogen alpha instrument within reach of Everyman. It's a dedicated 40mm telescope containing an integral Ha filter with a bandpass of <1.0.
Since I didn't want to commit three times the money for the next step up the Coronado line (Solarmax 40), the PST fit the bill. I was aware that the view could not be as good as in the higher priced rigs, but I felt that if I could at least see prominences, it would be money well spent. When I first got the PST, tuned it in on the sun, and a prominence snapped into view, it was "YESSSS...money well spent!"
All told, the PST delivers on what it promises. It appears solidly built, with almost all metal parts. A 1/4-20 screw hole beneath the focuser housing allows easy mounting on a camera tripod. I've mounted it piggyback on my GP-C102 refractor. The clock drive makes observing easier, and I can switch between Ha in the PST and a white light view in the 4". (Although I pretty much ignore the white light image nowadays.) The PST also has a neat little built-in sun-finder, which also helps in lining up the bigger refractor.
Although I'm satisfied with the PST, it does have its limitations. First of all, it's only a 40mm f/10 telescope; that means it won't give the resolution and magnification you may have seen in Ha filters mated to larger scopes. The internal focusing mechanism operates by turning a small knob under the eyepiece housing. A 12mm Kellner eyepiece is included with the PST. I had heard that not all eyepieces will come to focus, and did discover that Radians and a Meade 26mm Super Plossl end up just shy of a sharp focus. But this seems to be a backfocus issue because pulling them slightly out of the focuser tube allows a sharp focus. A 19mm Panoptic, 12.5mm UO Ortho, and 6.7mm Meade Ultra Wide work just fine. It's also important not to overpower the little scope in an effort to get a closer view. Not only does a high magnification image lose sharpness, but the daytime atmospheric unsteadiness is working against you. (Solar observing is best done early or late in the day when the atmosphere is steadier.) In addition, some users have reported a "sweet spot" where images are sharper at a certain point in the field, which varies from one PST to another. So far, I haven't found any such sweet spot.
The bandpass of the PST favors prominences over surface details, although you can still see some surface activity. A tuning ring on the tube allows a slight adjustment of the spectral response of the etalon filter, which can be affected by things as minor as barometric pressure. According to Coronado, you can achieve a better balance of features by stacking their SolarMax 40 filter on the PST. This narrows the bandpass to <.6. The cost becomes considerably higher than the basic PST alone, and I question whether there would be much to gain in view of the inherent limitations of the PST design.
One concern (hopefully unfounded) is what will happen to Coronado Instruments now that they have been acquired by Meade, which has a spotty quality reputation and a tendency to compromise good designs with some sub-standard components. I decided to get a PST before Meade does a lot of tinkering with it. Product changes have apparently taken place already. Specs on the Astronomics web site say the PST has a thread-on metal lens cover, but the one I received has a snap-on plastic cover. That's no big deal, but I'd have some qualms if they were to announce a "new, lower-priced" PST ... possibly using a lot more plastic.
The danger is that it's easy to get hooked on observing in the hydrogen alpha band. Then you begin to think about what more competent (and expensive) equipment could show...
Published in the July 2005 issue of the NightTimes
