A question about BF filters, and possible replacements?

[cathalferris]

I'm doing some research on the filters used in solar h-alpha telescopes at the moment (with a view to a version of the PST mod).

From what I can gather, the purpose of the BF is to allow only one of the "comb peaks" from the etalon through to the eyepiece, blocking any other comb peaks that made their way through the etalon and ERF filter.

I can't find information on the gap between these comb peaks, and it would be interesting as it may be possible to replace these very expensive BF filters with e.g. a stack of an IR/UV filter with an astronomical Ha filter of sufficiently narrow bandwidth.

Is there something that I'm missing in this? Technically what's missing?

It's a curiosity at the moment - I like to tinker but only when doing it right! I'm still enjoying the views through the PST, but I'll always be looking for ways to improve it.

[Peter Drew]

As far as I know, the only accepted way to improve a PST is to either double stack it or modify it to accept a larger donor telescope. The latter option has been exploited for some time now, if there was a cheaper alternative to the supplied BF I'm sure someone would have come up with it by now. It might not be too difficult to purchase a replacement BF5 second hand as many of the modders upgrade to BF10 or BF15. Merlin66 can give you "chapter & verse" re BF characteristics.

[cathalferris]

A bit more research and I may have answered my own question.

Apparently the bandpass on the night-time Ha filters is ~30-70A and the required bandpass for the BF is ~6A-9A - hence why the night-time Ha filters can't be effectively used as a portion of a BF stack. Decently low-bandpass Ha filters approach the BF cost anyway..

Still, good to gain an understanding of why.

[Merlin66]

The finesse of an etalon filter is the width of the "tooth" compared with the "gap" between them...

For a typical Ha etalon (Coronado/ Lunt) the bandpass ("tooth") is about 0.8-1.0A and the distance between each is 10A.

Therefore, as you say, if the blocking filter is to work on Ha it needs to have a bandwidth of around 6-8A centred on the Ha wavelength.

There are other BF type filters, other than Cornonado/ Lunt, available for industrial/ research applications - but these tend to be even more expensive!

[cathalferris]

Thanks for clearing that up.

After disassembly and cleaning of the new PST, I took a look through the etalon. Surprisingly it's a medium blue colour A bit surprising, but I do understand it