Double stacking ... please help

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MAURITS
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Double stacking ... please help

Post by MAURITS »

For this moment I use my 60mm scope with the red 60mm Lunt frontfilter.
When I double stack it, than I do this with my Lunt 35mm etalon.

You probably saw on my FD images that I have a little problem with the even illumination, so maybe better flats could possibly help.

The big question now

Can I solve the even illumination when I should use two 60mm Lunt frontfilters DS 60/60 instead of DS 60/35.

This is a picture from my 60mm scope with the Lunt 60/35 DS.

IMG_20200307_101800 klein.jpg
IMG_20200307_101800 klein.jpg (426.39 KiB) Viewed 297 times
Last edited by MAURITS on Thu Jul 22, 2021 3:30 pm, edited 2 times in total.


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Re: Double stacking ... please help

Post by Stardust5858 »

Tricky question. I've come to the conclusion that getting even illumination depends on how well both the single and double stacked echelons are matched. I've always used a 60mm with the 60mm Lunt. Even then I'm tweaking the double stack by clocking it with a 360° rotator. I've never taken flat's but use a combo of clocking, turning the blocking filter one way or another and tweaking the pressure tuner to get even illumination. Cak is so much easier, lol.


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Re: Double stacking ... please help

Post by MAURITS »

Thanks Ella for the feedback.


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Re: Double stacking ... please help

Post by pedro »

All etalons are different. I tried DS with two (pre-Meade) SM60 and the disk is not even, flats can help though

My SM90II/SM60 (pre-Meade) works like a charm


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Re: Double stacking ... please help

Post by Carbon60 »

Clocking and tweaking can certainly help, and as Pedro says, all etalons are different, so you might get lucky and eliminate the issue….or not. I triple stack my Lunt 60 (two DS modules on the front of my pressure tuned scope) and it takes quite some ‘fiddling’ to get close to an even view. It seems every time I set up the tweaking has to start again from scratch (clocking as well as tuning).

If I double stack, one module seems much easier to use than the other, so, same supplier, same design, different characteristics.

Stu.


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Re: Double stacking ... please help

Post by marktownley »

Like others have said there is a certain amount of luck in a getting a pair of etalons to match. I've been through numerous external etalons in double stack configuration with varying degrees of (un?)illumination.


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Re: Double stacking ... please help

Post by MAURITS »

Have I a benefit if I use DS 60/60 instead DS 60/35 with a 60 mm scope?

With other words, which show the overall best views?


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Re: Double stacking ... please help

Post by Bob Yoesle »

My gut would tell me it's better to use identically sized etalons. But as others have noted, there is a bit of random factor involved.

The key is finding etalons that require very little tilt to come on-band. External etalons are made with a central wavelength (CWL) above the H alpha 656.28 nm emission. This allows them to be tilt-tuned. However, if they are a bit too high in CWL, this requires more tilt to get on band, and creates some contrast non-uniformity (changing band-pass -- FWHM) across the etalon. This combined with the fact that double stacking reduces the overall FWHM only enhances any contrast non-uniformity the two-etalon system might have. Some of todays etalons are purposely made with higher CWL's to ensure enough tilt is used to get ghosts out of the field of view. Unfortunately this extra tilt also results in a greater contrast gradient.

My advice would be to somehow be able to select from various samples to find two etalons that require very little tilt to come on-band. The etalon that needs the least tilt should be your primary etalon and located at the front of the telescope objective. The other etalon becomes the secondary one, and uses the least tilt needed to remove the ghost solar image retro-reflection off the primary image. This will provide you the best contrast uniformity possible.

Two early 2000's era double stacked Coronado Tucson SM90 etalons; both requiring little tilt:


SM90 double stacking full disc .jpg
SM90 double stacking full disc .jpg (490.03 KiB) Viewed 218 times
Another factor for imaging is to place the ghost as close to the main image as possible without overlapping, which again usually minimizes the tilt of the secondary etalon:


160621 Ha 90 DS ED100 small.jpg
160621 Ha 90 DS ED100 small.jpg (453.12 KiB) Viewed 218 times


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Re: Double stacking ... please help

Post by MAURITS »

Thanks all for the good feedback.
My next purchase is the red Lunt 60mm front filter for double stack, DS 60/60.


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