HELP - Solar Ha blocking filter problem

[michaelmorris]

Three years ago I got hold of a Coronado BF10 blocking filter and a Coronado SM40 Etalon which I have been using on a Revelation 66 APO with great success. It’s been about 6 months since I last used it. This morning I’ve trying to observe the Sun but I can’t get the image into focus.

I’ve inspected everything to see if I can isolate the problem. On inspecting the BF10 blocking filter I noticed that the first filter on the diagonal (the one pointing long the axis of the scope) appears to be fogged. It’s not condensation on either side of this filter (I opened up the base of the diagonal to check). The fogging consist of a disc covering about 90% of the diameter of the filter with a thin, much smaller diameter ring of relatively clear filter about 2/3 way to towards the centre. This ring is at a similar ratio of the diameter as the central obstruction on the Etalon.

Has anyone seen this problem before?

Any ideas what (if anything) can be done about it?

Thanks

[Ben Ritchie]

Sounds like the dreaded rusty BF10 problem? Mine had to go back for replacement

[michael.h.f.wilkinson]

Does sound like "rust". Ouch!

[Merlin66]

The front ITF (Interference Transmission Filter) unfortunately does deteriorate. This seems to be associated with thermal loading followed by moisture ingress.

The effect you describe is typical of the failure mode.

If the BF is still under warranty, it should be replaced.....

It is possible, I've been told, to purchase a replacement from Meade Europe, but around 250euro....

As I've mentioned before we are investigating various alternatives, but haven't found a definative solution as yet - the search continues.

(If you can email me a photo I can confirm the diagnosis)

[michaelmorris]

(If you can email me a photo I can confirm the diagnosis)

See attached

[Ben Ritchie]

Looks very much like the problem I had.

[Merlin66]

Hmmmm

Yes, it's failing, but I've seen much worst than that!!

[pmlogg]

Does the same sort of deterioration happen with the fixed blocking filter in the PST?

[Merlin66]

Pmlogg,

In the latest PST the "blocking filter assembly" - in the eyepiece tube, is very similar in design - an ITF filter followed by a narrow band blocking filter.

Yes. Unfortunately the ITF element is prone to failure....

[pmlogg]

Thanks for that. Is the key to delaying that deterioration to try to avoid moisture ingress - by dry storage and perhaps dessicant?

[Merlin66]

We have no direct evidence that it extends the life of the filters, but we do know that once the "seal" is gone, moisture does penetrate and accelerates the deterioration. Dry storage/ dessicants can only help....