Stage 2 PST Mod (dump the black box)

[Merlin66]

To complete the " Modifying a Ha PST" write-up I've just pulled the PST etalon off the black box! Usual issues with Loctite, but I eventually won!

I'm using the 90mm f8.8 Meade donor OTA effectively stopped down to 88mm by the D-ERF cell. The etalon was left in the same position used to focus with the Stage 1 black box arrangement, so I know it's close to the 200mm inside prime focus.

I used a custom made M50 to SCT thread adaptor, into a SCT Revelation 10:1 Crayford followed by a BF15 ( Ex my SM60 set-up)

Allowing 80mm for the BF diagonal the spacing ( to get a good focus on a 10.5mm plossl) was 145mm from the rear of the rubber knurled ring +80mm for the BF = 225mm Bearing in mind the "optimum" 200mm back focus should be measured from the rear lens of the etalon this looks very close to the design.

Surface detail and proms easily and clearly seen, some slight residual sweet spot ( this gets worse when you change the spacing... I moved the etalon 8mm further out of the Meade and the sweet spot/ ring was much more noticable)

So, another Stage 2 mod done successfully!!!

I'll try to get some images latter for comparison with the SM60 single and double stacked.

I'm sure the others (Andrew, Helen, Mike and Kev...et al) will have the same success.

Ken