Etalon reflection - Ghosts in the machine

[Merlin66]

Following on from other comments in other threads...
Mark may have something there with the polarising of the light - worth checking
I was re-reading "Practical Spectroscopy" by Harrison, Lord and Loofbourow. On pages 558 -573 where they discuss in detail (!!) the application of a Fabry-Perot etalon. The circular fringes (sweet spot)is well explained and on p561 there's details of the "secondary images" - the solution is to have each plate (front and rear) with a outer surface wedge of 3 -30 minute of arc. (5 min recommended)
I'm not sure our humble Lunt/ Coronado etalons have this feature.....

[swisswalter]

Hi Ken

interesting information. I have taken apart my LUNT50 etalon and it has indeed on both plates an untreated circulaire zone of roughly 10 mm

[Merlin66]

Walter,
Is that REALLY to two plates from an etalon!??
If so, How will you re-assemble?
The anti-reflection "wedge" just means that the plates are not of equal thickness, one point the edge being thinner than the other. I don't think you'd even see 5 min arc variation never mind measuring it...
BTW they also say the internal surfaces of the etalon should be within 1/30 wave(!) and the external surfaces better than 1/4 wave....

[swisswalter]

Hi Ken

yes these are the two plates from a LUNT50 etalon. I was able to recontact them, but not as good as LUNT can. So I sent them in to have the job properly done.

Here you see the stack recontacted again

[Merlin66]

Thank goodness!
The mirror surfaces enhance the transmission of the etalon.

[solarchat]

ya man, taking apart your etalon is a very costly experiment. Just dont drop it:

[Merlin66]

Ouch!

[marktownley]

[quote]Following on from other comments in other threads...
Mark may have something there with the polarising of the light - worth checking

Yup, it definitely shifts them, has to be a circular polariser though, but like I said in the other thread the negatives regards poor optical flatness and poor Ha transmission certainly of 'photographic' cp's outweigh the pros that are brought with using one. I will see if I can find a cheap Hoya HD on flea-bay and revisit the experiment...

[Merlin66]

Mark,
I have a few linear and circular polarisers...do you think standard eyepiece PL filters would help?

[marktownley]

Possibly, certainly worth a go. I think the key is determining where the reflections are occuring and then positioning the CP filter accordingly. For DS setups with external etalons the main reflection is caused between the 2 etalons - so, placing a CP between them is what is needed. When I did this the reflection you get in the fov with a low power eyepeice completely disappears, but with my <£10 2nd hand CP the image was soft and dim. (you get what you pay for!)

I also think there will also be a reflection from the 'mini ERF' on the coronado blocker that while not as significant as the etalon reflections will certainly impact on the image. My project for this week is to remove this and hopefully get to see what results are with it removed next weekend if the weather obliges.

Linear polarisers do not work at all btw, I tried these (I have the ones that came with my Lunt wedge)...

[marktownley]

I was re-reading "Practical Spectroscopy" by Harrison, Lord and Loofbourow. On pages 558 -573 where they discuss in detail (!!) the application of a Fabry-Perot etalon. The circular fringes (sweet spot)is well explained and on p561 there's details of the "secondary images" - the solution is to have each plate (front and rear) with a outer surface wedge of 3 -30 minute of arc. (5 min recommended)
I'm not sure our humble Lunt/ Coronado etalons have this feature.....

Ken, I have a question about the Fabry-Perot etalon that maybe the book might be able to answer... External etalons like our SMxx act like a comb filter - essentially generating an output of a series of sharp thin spikes with broad flat valleys in between with little or no transmission - in the case of the coronados I think I read the spikes were seperated by 10A.(?) My question is that i assume this happens when a broadband light source passes through it? Would the same thing happen if a narrowband light source passed through the etalon?

[Merlin66]

mark,
I'm seeing "ghosts" even with a single stack and no "mini-ERF"
re the etalon - yes it will give exactly the same bandpass and finnesse.
It's 0.8A spikes every 10A separation.

[marktownley]

mark,
I'm seeing "ghosts" even with a single stack and no "mini-ERF"

Very curious Ken! I would try a CP in place of the mini ERF, I think you will find this will remove the ghosts you're seeing. Do you have any pics that show these ghosts?

[Merlin66]

If the clouds ever clear from Belgium, I'd LOVE to give you all some images!!

[marktownley]

How 'close' are the ghosts together in terms of solar diameters? Are they overlapping? Do they go off in a straight line or is it curved?

[solarchat]

that sounds like a decontacted etalon if the ghosts are close together and in a straight line. If you fold the etalon up to a fluorescent ceiling light you should see several concentric rainbow colored rings from the center out If this pattern is not there, you have a decontacted etalon...

[swisswalter]

hi Theo

we are just having fun and learn a lot

[Merlin66]

No, nothing like that.
It's a large secondary reflection about 1/2 a solar diameter away from the sun's image and maybe twice the size.
I've only noticed it visually with the 21mm eyepiece.

[marktownley]

No, nothing like that.
It's a large secondary reflection about 1/2 a solar diameter away from the sun's image and maybe twice the size.
I've only noticed it visually with the 21mm eyepiece.

Phew! Atleast we're not looking at decontacted etalons... The increased size of the reflection is significant, what type / brand of ep is it?

[Merlin66]

I normally use the 21mm TV
The size and position of the reflection doesn't change with eyepiece rotation....

[marktownley]

Hmmm, not the ep then... Have you tried rotating the BF or etalons?

[Merlin66]

The ghosting is on a modded PST set-up...
I've changed the ERF tilt with no effect; I've rotated the BF10 diagonal - no effect...
I'll see if I can reposition (rotate) the PST etalon and note any differences....

No one else experiencing similar issues???

[peter drew]

I certainly am! I have a collection of 5 PST's in use with several different modded scopes, each seem to give different ghosting effects to varying degrees. There also seems to be a variation of throuhput via the BF's, the best reduction of ghosting in my units comes from retention of the mini ERF although the faintest prom details are sometimes lost due to the extra filtering. If you look down the front of the scope when it's pointing at the Sun there is a significant reflection of bright light from the front of the etalon, if this turns out to be a prime source of ghosting we may have to live with it. I'm not an imager (yet), does this ghosting cause problems for that purpose?

[Merlin66]

Peter, et al,
Mark may have come a long way to at least suppressing the problem!
Mark, I owe you a beer!!!
In the limited sun time I have, I've verified rotating the etalon doesn't change the ghosting. Closer inspection shows the most intrusive ghost overlaps the main image by about 1/4 the solar diameter; it's the same size as the solar disk. This doesn't alter with magnification or eyepiece - always the same relationship.
Well, I have a couple of polaroid 2" filters.....when a filter is placed between the eye and the eyepiece it is possible to find an angular position where the ghost for all intents and purposes fades away!!! Excellent outcome!!
I think the long term solution, at least for my set-up may be to include a 1.25" circular polariser filter either at the nosepiece of the diagonal or worst case on the eyepiec.
Well worth trying. It would be good to get independent verification......
Well done Mark!

EDIT: Ok my "window of opportunity" for today (and maybe this month!!) has passed.
I tried different set-ups and a "blind" test for the PL filter...there is a definate improvement with the filter in place. The filter orientation needs to be "tuned" to match the ghosting, but seems to be repeatable. Looks very promising.

[marktownley]

Excellent! Glad progress has been made with this. Will definitely invest in a Hoya HD CP later this week and explore this further myself

*Edit*
Just been looking on fleabay and you can get the Hoya HD CPs upto 82mm in diameter... Lots of applications

http://www.hoyafilter.com/products/hoya/HD-01.html

[peter drew]

This is interesting, Mark may have one hand on the "Nobel". It's a most annoying effect as the rest of the field is so good. Maybe we'll all end up wearing polaroid sunglasses, cool and quite appropriate actually.

[Merlin66]

Yeah, I tried adding baffles etc etc, but this seems to give much better results.
I hope you have the same success as I.

[marktownley]

Cheers guys Moral is where there's a reflection about get a polariser out

[Roel]

Thanks, Ken! I reduced most of the internal reflections by using baffles, but when I place the Sun's disk at certain positions in the FOV, some reflections still remain. A polarizer could be an easy remedy. A friend of mine has one, so I'll definitely try this one!

[Merlin66]

Well I had my five minutes of "solar access"!
Even through the thickening cloud and the tree branches today's host of proms was very nice.
I tried to fit the rotating 1/2 of a CPL filter to the nose of the BF diagonal but the depth of the Borg focuser wasn't enough!
Resorted to fitting it on the eyepiece....
The results...
Well the ghosting is much reduced, but the refraction/ interference from the branches make it very difficult. At this stage, until I can get good clear views, I'd say - well worth trying.

[swisswalter]

Hi Ken

thank you for these promising results

[marktownley]

Bargain!

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