Based on three additional observing sessions with the Cak PST, here some further thoughts and observations, taken from the update I've posted on my website. (www.jimrichberg.com/astronomy)
Eyepieces matter a lot with this scope! My 7-22mm Apogee zoom nearly had me convinced that the scope was defective, since I was seeing a pronounced dark circle in the central 50% of the field of view at 15mm or less focal length, much like the shadow of the secondary mirror in a reflector when an excessively low powered eyepiece is used. My Nikon 9-21 zoom didn't display this central darkening to the same degree-- though it was still noticeable-- and this eyepiece also produced a brighter image with more discernable detail. (No surprise here-- see my H-a solar zoom report Zoom eyepiece review http://www.jimrichberg.com/astronomy/zoom_eyepiece_solar_review.htm ) I tested these two zooms, the stock 20mm Kellner, a 14mm series 4000 Meade Ultrawide (which worked surprisingly well, given the number of elements inside it), a 10.5mm orthoscopic, and the Nagler 3-6mm zoom. The Nagler, Nikon, and 10.5mm ortho-- all known high light transmission eyepieces-- seemed to be the best performers, and the margin of superiority seemed greater than in Hydrogen-alpha solar viewing. (I've also heard that the Cemax Plossls are much stronger performers compared to standard Plossls in CaK light.)
Eyepieces of 15mm focal length or more seem to work the best. Anything shorter than this gives up too much image brightness, in my opinion, and seems linked to this central shadowing issue noted above. The Nagler zoom and the 10.5mm ortho were just working with too little light to be fully effective, even when observing a part of the solar disc away from the obstructed region of the FOV. The Nikon zoom showed plage features that I couldn't spot in the Apogee zoom until I'd first found them in the Nikon, but even its' performance began to drop off below 15mm focal length settings. In this respect, the supplied 20mm K eyepiece looks like a better choice than on first blush.
The most aggressive shrouding/shielding of ambient light attainable is not overkill with this scope-- though you may end up passing out from heat exhaustion in the Summertime!
My maximum shrouded hat with its 3-4 layers black nylon cloth was barely adequate, and I'll probably supplement it with something thicker when its time for CaK viewing. However, eyepiece fogging is already a problem with my current setup-- and this is in sub-freezing temperatures! As a "field expedient" measure, I was surprised at how well simply cupping both hands tightly around an aftermarket eyepiece cup worked. Not as good as a shrouded hat-- not by a long shot-- but it was probably two-thirds as useful in heightening contrast and bringing out features. You need a good bellows-style eyepiece cup to make this work-- the stock ones on most eyepieces are not big enough.
Practice and "time on target"-- preferably seated-- help considerably in coaxing out visual detail. This was not a surprise, since the same principles apply in nocturnal observing, but the impact seems amplified here-- probably because you start out by seeing little beyond a dim bluish orb, and work up from there.
Several of my sessions were marked by numerous active regions in CaK light, and I could see hints of granulation as well. (Of course, if I increased the magnification to bring out the granulation, the drop in brightness killed it.) Observing from a seated position helped a lot, too. (Again, not a surprise.)
At the end of the day, the image remains dim, though. Dim enough that getting the focus exactly right could be an issue, even when using something as linear and stark as the limb of the sun. Even if you do everything you can to relax your body and maximize the contrast of the image, CaK visual observing is still a reasonably tough challenge for middle-aged eyes!
Jim 8)