Hallo Helmut,
nachstehend eine kompakte Beschreibung, die dir deine Fragen beantworten wird (habe sie momentan nur auf Englisch verfasst). Besitze ein PST der ersten Serie, das eine Bandbreite sicher unter 1A hat und den Tubus einfach abschrauben lies.
MbG Michael
Adapting a Coronado PST to a Refractor
Depending on the mechanical circumstances, adapting a PST could be an easy or a difficult thing.
First of all check if you can unscrew the golden tube from the body. The thread can be glued (probably because of security reasons) and it could be nearly impossible to take them apart.
It is an advantage when you can unscrew the distance tube in front of the blocking filter/eyepiece combination (maybe glued too). This helps a little bit to win additional focal length.
The indeed very sophisticated construction of the PST seems to be a system optimized for a focal ratio of 1:10. So a refractor of this focal ratio will give you the best contrast. For example, a 90/900mm, 100/1000mm or 120/1200mm will work out fine. A 1:8 system will work too, but the highest resolution of the objective might not be possible. Adapting the PST to a system of 1:6 and faster will result in a low contrast performance and is not recommended.
Since we observe in monochromatic light, an APO optic is not required. A bargain Chinese refractor which is well corrected in red light may be the first choice. The PST body needs a lot of focal length. It will be difficult to find a refractor with free focus of 18-20 cm. The only way is to cut the tube. Painful, but necessary! This must be done carefully and exactly, so that the optical axis is still in line.
Another mechanical part is the connection from the PST body to the telescope. Depending on the focuser, the construction is individual.
Now we have to simulate the heat/light protection of the original objective. Heat protection can be reached with an IR block filter, widely used by CCD photographers. To be on the safe side I combine two of these filters. Filters from ASTRONOMIK perform very well. Of course there are special heat- blocking glasses at the market. But they are made of thick glass and probably not good enough in optical quality. Our advantage: The filters are far from focus, so the heat intensity is well below the flammable focal point. Still there is heat enough that the glass could expand a little bit (depending on the diameter of the optics). Be sure that the filter glass can shake a little bit in their cells. This will prevent from breaking. For light reduction a red/deep red filter will do the job. The etalon is small, so a combination of 1.25” filters will fit fine when be screwed close to the etalon.
Voilà, that’s it. A friend, who is retired, has a lot more time to observe the sun. His eyes are still OK. I would never claim that this PST construction is completely safe. We have to be careful all the time. But it is a cheap way for high resolution observing an imaging.
