PST Filter Barrel adapter.

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Sunday: I finally got around to turning a supporting bush/adapter for the PST filter barrel. As mentioned before I have turned away the original PST eyepiece socket. I also had to turn a reduced shoulder so that the socket would fit inside the helical focuser. 

The adapter also needed a shoulder and a recess to allow the barrel so fit inside the 2" : T2 adapter.

Eventually I was able to screw the two halves of the PST filter barrel together while leaving just enough freedom to rotate. This was important to ensure the two halves were joined. The barrel adapter was also sunk into the 2":T2 adapter to bring it safely under flush. The helical focuser has been photographed twice to show its full range of motion. Forgive the oily fingerprints from working on the lathe.

 This series of images show the T2:T2 helical focuser and T2 : 1.25" eyepiece receptacle with compression band. The PST eyepiece socket was not remotely up to the job of supporting a binoviewer. The new fitting is superb at holding the binoviewer's weight.

The bright aluminium barrel adapter [far left] is sandwiched by the two halves of the PST filter barrel. The whole of which slides into the 2":T2 adapter. Though the front section of barrel and adapter go in first and only then can the rear section be screwed into place from the rear of the T2 adapter thread.

The brightly turned tail end of the PST barrel fits inside the helical focuser and could have been left full length provided it was turned down enough for generous clearance. The barrel is now held securely and squarely to the optical axis. It no longer prohibits the smooth action of the helical focuser.

Here an eyepiece has been fitted for scale. The telescope would not normally focus without the addition of a star diagonal or an erecting, terrestrial 45 elbow. After a quick look at the H-a sun with the single eyepiece I always prefer the much more relaxing view in the binoviewer.

Now the filter assembly is fitted into the AOK Swiss PST Etalon : 2" adapters. The PST etalon lies under the plastic knurled ring on the left. The heavy duty AOK fittings sandwich the etalon securely by the etalon's threads.

Turning the heavily knurled ring "tunes" the exact frequency of the H-alpha light passing through the etalon. There can be a difference in tuning between showing surface detail and prominences. Though in my case they seem to be close together in their tuning. PST [and those of other maker's] etalons are known to vary quite widely in quality.

It is a vital part of the modified PST's optical construction that the rear of the etalon ring is 200mm from the donor telescope's focus. The PST is designed to work at f/10 and will automatically stop down any variation on this theme. The etalon is sandwiched by a negative and a positive lens to ensure only parallel light passes through it.

Now the previous assembly is fitted with the binoviewer. Which has a standard 1.25" chromed nose-piece. GPCs and Barlow lenses can be screwed into the binoviewer nose-piece which has a standard 1.25" filter thread.

And, finally, the complete, modified PST H-alpha assembly. Which is fitted into a 60mm long x 2" adapter in the turned, tailpiece backplate of the 6" f/8 refractor. Which now works at 120mm f/10. A loss of 30m aperture but still 3x larger than the original PST objective. A 6" f/10 would give me back my full aperture but I consider the iStar example to be far too expensive for my tastes. They were once an affordable choice but their prices have inflated to almost double previous levels. 

A 90mm Ø Baader D-ERF rejection filter is fixed about half way down the inside of my 6" OTA. A vital part of the optical system to provide user safety. By reflecting away almost all of the sun's fierce heat from the much larger objective lens. The original PST has only a 40mm objective.

Click on any image for an enlargement.

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