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Andy Dodson | all galleries >> Solar Photography >> Evoloution of my Ca K filter > Straight through design, - no pentaprism.
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26-APR-2012

Straight through design, - no pentaprism.

Here's the modified Ca K PST stripped of it's pentaprism (which caused the astigmatism whether it was 'square' or not). In order to change the focal plane the gold tube was cut and rethreaded. On the back of the 'black box' is a 1¼" helical focuser from Hutech. The CaK filter is inside the black box and behind the focuser are two adapters and then the original 5mm Ca K BF.

Canon EOS 5D
1/80s f/6.3 at 50.0mm iso200 hide exif
Full EXIF Info
Date/Time26-Apr-2012 13:04:31
MakeCanon
ModelCanon EOS 5D
Flash UsedNo
Focal Length50 mm
Exposure Time1/77 sec
Aperturef/6.3
ISO Equivalent200
Exposure Bias
White Balance
Metering Mode
JPEG Quality
Exposure Program
Focus Distance

other sizes: small medium large original auto
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Alistair G. 28-Mar-2017 19:39
From what I read there is supposed to be an optimum distance that the PST Etalon has to be from the front objective lens in order to give the best contrast images, which is at 200mm (same as unmodified PST) from the objective, since the principle is that the objective of the PST has a 200mm focal length, then Etalon position is at 200mm from objective, then PST Etalon has 200mm focal length, giving 400mm total. By making the etalon closer to the objective you would suffer loss of contrast (so I read). BTW anyone reading this may want to examine their internal ERF in their Ha PST (lower down than the blocking filter in the bottom of the eyepiece holder assembly i.e. at the bottom of the lower of the 2 tubes) for "rust" that is common on these on PST's causing lack of contrast. PST pentaprism alignment is normally critical on getting the best contrast images, you seem to have been unlucky in that your prism itself seems to have been badly made. Best Regards.
Andy Dodson22-Jan-2013 07:22
This straight through design is no longer used for the Ca K photos as I always use a larger aperture : 100mm so far. It's not gone to waste though, as I already have a SM40 Ha etalon and so I reuse the PST 40mm objective on a 'sawn-off' gold tube with the black box (which is still useful with it's built-in sun finder & 1¼-20 threaded base) with a BF10 from the SM40 set as a Ha BF. It works well as a handy Ha unit
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