Thursday, March 29, 2007

PST blocking filter and BF10

This morning, I did a very simple comparison of my BF10 (comes with Solarmax filter) and the blocking filter from the PST.

Some people said PST blocking filter is basically a BF5, yes, in terms of aperture, it really is, but the normal BF5/BF10 has two parts mounted in the nosepiece of the diagonal and the visual back side. My friend told me that the PST blocking filter has the same thread as the visual back part of a BF10, and I believe the similarity ends here. I mean the PST blocking filter has only part of a regular BF10/BF5.

I hold them side by side to my eyes on terrestial objects. I found a BF10 is basically opaque, I mean it cuts a lot of light so that regular terrestial objects are not bright enough to be shown after it. However, the PST blocking filter has quite high transmission, one can see a lot of things with it in front of your eyes. I feel it's more like a deep sky type narrow band H-alpha filter! It can filter out the harmonics left by the etalon, but it does not block a lot of light.

Then, I hold the PST etalon and PST blocking filter in front of my eye, to look at terrestial targets again, and I can still see basically everything clearly. That means this combination does not indeed cut a lot of light!

This simple experiment strengthens my idea that this combination is going to work behind a Herschel Wedge, what I mean is, Herschel Wedge should be able to act as a very scalable ERF for the PST etalon and blocking filter combination.

If that ever works, the problem will then be focus travel issue. My Borg 45ED II is going to work without problem, but my Ranger, or Sky90 is not going to work, since with the Herschel Wedge alone, all the back focus were used up with only 1 few millimeters left so that there would be no room for a PST etalon and blocking filter.

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