PST solarscope - What does the blocking filter block?

[Poita]

I'm looking at my PST, I understand the Energy rejection filter at the front, and the etalon in the middle, but what does the blocking filter at the eyepiece end actually block?
They are horrendously expensive, especially larger ones like the BF15, just wondering what wavelengths they actually filter that the ERF and Etalon don't block.

[sheeny]

My guess is it blocks everything except a relatively broad band around Ha. The ERF knocks the brightness of everything down a bit, then the etalon effectively blocks everything except those wavelengths that resonate between the two reflective surfaces inside the etalon. The frequencies that resonate of course build up till they are so bright they can be seen through the etalon. As there are many frequencies which can resonate inside the etalon, the blocking filter blocks them all except the one you want to look at. The blocking filter just can't let through as narrow a band of frequencies as the etalon, so you need both.

Al.

[Poita]

That makes sense. Has anyone done an analysis to see how narrowband the BF actually is?

[sheeny]

I'd be surprised if no one has.

(Waiting for merlin to jump in...;))

Al.

[Poita]

Me too! This thread is officially Merlin bait.

[Merlin66]

Bait taken!!

The etalon produces a series of very narrow bandwidth images (about 0.7A wide) spaced every 10A across the whole spectrum, from blue to far red. Like an optical "comb"
One of these images coincides with the wavelength of H alpha. The job of the blocking filter is to "block" all the other bandwidths and just allow the H alpha image to get through.
The D-ERF reduces the intensity of the UV-red region of the spectrum, but doesn't block the etalon "comb"

I have some spectroscope images of the various bandwidths for a couple of SM60 etalons and two samples of the Coronado blocking filters.
The bandwidth of the BF is around 5A centred on Ha.
I'll upload the images when I find them.
EDIT:
The images are shown in the first message of this thread:
http://www.iceinspace.com.au/forum/showthread.php?t=72540
Hope this helps,

[Poita]

They seem rather expensive then for a 5A filter, are there any alternatives?
I'm thinking purely for imaging, not visual use.

[Merlin66]

No.
There are no alternatives that I know off....
making a quality interference filter with such a small bandwidth (5A) is not easy......