Collimator for Omega Bobs Filters

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AndiesHandyHandies
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Collimator for Omega Bobs Filters

Post by AndiesHandyHandies »

Hi

Based on AstroRods experience with a set of these filters I have looked round and found that professional Fabray-Perot spectral telescopes use this configuration. Double collimators with collimated and tele-centric positions.

web.njit.edu/~cao/IBIS_Cavallini.pdf

Page 4 digram.

Somewhere it says the telecentric position is best for ghosts.
Peter Z uses the collimated position for whole Sun imaging.

Placing the filters close together gives mutiple reflections and they have to be set 200mm apart for them to drift out of the field. And the filters are not in an optimum postion as in Dave Groskys design where they are not at a pupil.

Could we put a filter at each pupil in the two collimators? At the moment we have 40mm binocular lenses in mind. At F3.5? thats 140mm FL and so the filters will be 280mm apart with a field stop in between. Peter Z uses double ES achromats so would we need to do the same?

Rod is suggesting a folded design with a 'newtonian' eyepiece position. We are aware of the mechanical issues so it is proposed to have two aluminium, carbon fibre if we are confident it will work, box sections on top of each other.

The collimator doubles the image size so we are limited to 10mm prime image. Or smaller to pick out a best part of the 'seconds' filters.

80mm F5 with TS 2" field flattener? 4mm image.
100mm F5 with TS 2". 5mm image.
100mm F10 maximum.
Whats a useable aperture for usual UK seeing?

Andrew.


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Re: Collimator for Omega Bobs Filters

Post by marktownley »

I'm sorry but I can't recommend the Omega Bob filters.


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Re: Collimator for Omega Bobs Filters

Post by Valery »

marktownley wrote: Thu Jan 24, 2019 2:26 pm I'm sorry but I can't recommend the Omega Bob filters.
That Bob took my honest $250 + shipping and sent me a piece of a dry shit.
Then he promised to send me the replacement, but then disappeared.

Bla bla guy.


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Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.
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Re: Collimator for Omega Bobs Filters

Post by AndiesHandyHandies »

Hi

The question is about optics which I am interested in a reply to!

Unfortunately I had previously purchased a night time sodium and mercury light filter from Omega which was excellent. And now have a 2" as well.
As a physicist I chanced to search Ebay on 'sodium notch filter' rather than LPR filter and they came up.
Bob did make them after a suggestion from the imaging community. And they are sold on ABS by a company in Europe.
It came with no holder as I did not realise I needed one. But it fitted in my Panasonic GX80, partly bought for astronomy body, and you could not tell it was there in daylight shots. And I get dark background star shots with a lens in a small town.

So I did not think to do 'due diligence' about the H-alpha filters.
A lad in my Astro club was needing to replace a PST filter so I thought to have a look for H-alpha filters and came upon Bobs 'Old New' ones.
They are seconds and someone said as I recall they may be for detectors rather than imaging.

I bought two sets, one for AstroRod as he does turning of bits for me. The first one had the ERF bonded to one H-Alpha and had many reflections.
After contacting Bob, prod through Omega site, we sent them back.
We only got two ERF ands and two H-alphas back, so I had to ask for the other two H-alphas which were sent.
These look better, may have had to send production ones?

Rod has used the first set and one does get reasonable images. Although with filter experiments and holders he could have bought a commercial scope,
so an expensive present in the end.

Also Peter Zetners images with Bobs filters look good. Properly mounted in kinematic holders. Rod reckons they are very much tilted compared to his.
Bob matches the two H-alphas to over-lap the response curve so that one can be tilted to be one band and the other tilted a bit more to match the band and for reflections to be off-set a bit.

Compared to a new commercial 80-110mm solar scope they are attractive for a project?

Andrew


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Re: Collimator for Omega Bobs Filters

Post by Merlin66 »

Notwithstanding all the grief being posted around Omega filters, I’m happy with the CaK series filters I got and use in the CaK filter stack.
Checking the transmission curves with the spectroscope verified the quoted curves.


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Re: Collimator for Omega Bobs Filters

Post by marktownley »

Andrew, what setup of Peters are you talking about? What are you trying to make here?

The scope in the link is using a solid etalon, not interference filters like Bobs. They may both be called 'etalons' but it's comparing apples to oranges. Using a double collimation system may work, but with double the glassware you will find the incidence for reflections goes up, of course polarisers suppress this but light throughput also goes down. When I made my double stack PST mod (2 pst etalons), I found I got best results running both etalons clocked in a single collimated light path, rather than each etalon independently in it's own collimated beam. The field of view was more evenly illuminated and there were less reflections.

I can use an 80-100mm scope effectively for solar imaging for probably 70% of the year, all depends when you are imaging.
AndiesHandyHandies wrote: Fri Jan 25, 2019 6:06 am Compared to a new commercial 80-110mm solar scope they are attractive for a project?
Personally, no. There are a few examples where Bobs CaK and CaH filters have produced decent results, but i've yet to see any images from Omega Ha filters that even come close to anything decent. For me the best route for a modded 80-100mm system would be to use a PST mod. I get daily 'coronado' updates from EBay and you can pick up some half decent PSTs for very reasonable prices. Solar minimum is a buyers market


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Re: Collimator for Omega Bobs Filters

Post by AndiesHandyHandies »

Hi Mark

Were you using a PST type layout with the lenses and etalons inside the focal point?

Peters images

www.pbase.com/p_zetner/omega_halpha

He used double achromats for the collimation / tele-centric lenses.

He used the filters between the lenses for whole Sun images and after them in the tele-centric beam for magnified images.

pbase.com/p_zetner/image/145615191

Andrew


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Re: Collimator for Omega Bobs Filters

Post by marktownley »

AndiesHandyHandies wrote: Fri Jan 25, 2019 9:39 am Hi Mark

Were you using a PST type layout with the lenses and etalons inside the focal point?
Hi Andy. Yes.


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