Reflection ghosting from Blocking filter?

[Merlin66]

I use a Coronado BF15 blocking filter on my PST88 mod.
I noticed yesterday a definate ghosting of the FOV in a 21mm eyepiece.
Rotating the ERF, etalon tuning, blocking diagonal or eyepiece made no difference to the extent, intensity or position of the ghosting.
However, I noticed if I slightly loosened the blocking filter, the ghosting would move....
I'm therefore concluding that its caused by the reflection from the front "mini-ERF" element in the blocking diagonal bouncing back to the rear re-focusing lens on the etalon assembly and then reflected back again.
I'm looking at adding a series of baffles between the etalon and the diagonal. If this doesn't work the next stage would be to remove the "mini-ERF" - it's already redundant with the Baader D-ERF up front.
Any comments???

[swisswalter]

Hi Ken

yes, indeed, remove the little ERF, any glass not beeing necessary asks for trouble

[marktownley]

Yes, remove the mini ERf, no need for it...

[Merlin66]

Thanks everyone.
I tried a series of three baffles - made no real difference.
removed the "min-ERF" from the BF15 and the main reflections disappeared!
Just had the five minutes of clear sky (at -8 degrees)in Brussels....ready for the next time.

[Merlin66]

I have a various collection of T thread spacers between the etalon and the BF...I measured the position of each joint and cut a diaphragm diameter to suit the spacing - I had a total of four baffles....
Interesting comment..I have a BF10 from a SM60 scope - I should try that for comparison....

[marktownley]

Stephen has said before the BF15 isn't the best (I think...)

[Merlin66]

Can anyone expand on that comment??
When used with which system?
I checked both the BF10 and BF15 with the spectroscope..the bandwidth is almost identical but the BF10 had a slightly better transmission efficiency...that was about it.

[marktownley]

What sort of difference are we talking about % wise for the transmissions Ken?

[Merlin66]

I'd need to pull the data to be 100% sure, but of the order of 10-15% ie a 30% transmission going to 35-36% - something like that.

[Merlin66]

Neo,
it's not the blocking filter at the eyepiece end that's the issue...it's the "mini-ERF" inside the nosepiece (This is the same size for both the BF10 and BF15)-about 15mm diameter.
Which "tilt" do you see? In the BF diagonals, both the "Mini-ERF" and the rear blocking filter appear to be square to the body and the optical axis. Any chance of a photo?
Yes, like the tilt on the front D-ERF, a tilt on the BF assembly would make a difference.

[Merlin66]

Neo,
It's OK....I pulled the eyepiece holder off the blocking filter diagonal and yes, you can see the tilt of the final blocking filter from the inside clearly.
Thanks for the info, appreciated.

[marktownley]

Ken, do you have the bandpass/width / transmission data for this mini erf that you unscrewed?

[Photosbykev]

I add all this ERF glass and BF glass on my Lyra Optic Ha scope and almost before using it I need to remove some of it, like a mini ERF . Now where did I leave my hammer?

Well that was easy, I had a steel engineering right angle which was a perfect fit on the locking collar. A quick tweak to remove the big blob of silicon that Coronado had used to lock the collar in place and the filter is out.