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EFR Combo trials
Finally got a bit of clear sky to check out the ERF combo/ PST etalon/BF10 on the ED80.
Firstly; IT WORKS!!!!!!!!!!!!
But,
1. The ERF must be slightly tilted, in my case I used a small piece of cork gasket about 2mm thick placed near the edge of the filter, this "moved" the annoying reflections away from the primary image, didn't seem to have much impact on focus or definition.
2. There must be a "sweet spot" for the etalon/BF focal ratio/ focal length. The best images were with a 10.4mm plossl at the prime focus of the ED80 ( 600mm fl, f7.5), the solar image covered about 80% of the field so it was easy to check around the edge for proms etc. Didn't try a higher magnification. However, when I inserted the TV x2 Barlow infront of the etalon ( It needed an extra spacer, 90mm long infront of the barlow) I found the vignetting from the system (BF filter?) terrible. Through a 21mm eyepiece you could see the edge/outline of the optical train as a "diaphram" about 50% of the field of view, inside which was an image of the sun, which appeared very highly magnified ( an optical illusion, due to the restricted field of view). Not very impressive!
So I have to assume that f15 is a bad number for the etalon/BF, whereas at f7.5 things looked very nice.
I must say, the #25 Wratten filter/UV-IR combo does its job and is a very effective ERF when used with the pST etalon.
I hope Karl ( and others) can verify the above and "home in" on the sweet spot.
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