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2 inch or 1.25 inch Baader 35nm H-alpha filter for solar imaging ?

Solar Filters Imaging
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#1 IR656nm

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Posted 16 October 2021 - 07:34 AM

Hi all !

 

I am considering upgrading my solar setup from a 1.25 inch UV/IR cut filter for h-alpha imaging with a Quark chromosphere.

I have been advised that a Baader UV-Red CCD filter or a Baader 35 nm H-alpha filter is better for h-alpha.

 

At present I put the 1.25 inch uv/ir filter on the end of the Quark (it screws on the end facing the telescope).

If I buy the 2 inch I will have to screw the filter on one of the extension tubes. If I buy a 1.25 inch H-alpha 35nm filter, then I could still screw it on the end of the Quark as before.

 

Is there a improvement using a 2 inch vs 1.25 inch h-alpha filter 35nm ?

And is Baader the only maker worth considering ?

 

I am thinking of buying a 1.25 inch Baader 35nm h-alpha filter as it only costs 65 pounds vs 118 pounds for the 2 inch version.

 

Am I making a serious mistake by only getting a 1.25 inch filter ??

 

Many thanks guys.

 

 

Magnus

 



#2 MalVeauX

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Posted 16 October 2021 - 07:43 AM

Hi,

 

It depends on a few things.

 

1) Aperture size of your imaging scopes (this determines the input energy).

 

2) Are you having heat issues?

 

3) Placement. Threaded to the end of the Quark is too close, you want your ERF to be as far into the light cone as you can without masking it so it should be the first thing inserting into your focuser.

 

4) Any dielectrically coated red imaging filter will do this fine. Personally I don't see a reason to get the HA 35nm over a standard Red imaging filter (35nm vs 100nm). But I would certainly suggest these over a basic UV/IR (which allows Blue & Green to pass).

 

5) 2" (50mm) is ideal for larger light cones, especially on faster base scopes. But it's not needed on smaller scopes with longer native focal-ratio. Again, depends on the scope and size of the light cone. You could ray trace to see the diameter of your light cone where you will place the filter, but it must be significantly into the light cone away from the plane of focus to work correctly.

 

Very best,


Edited by MalVeauX, 16 October 2021 - 07:45 AM.

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#3 SgrB2

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Posted 16 October 2021 - 07:45 AM

You want the filter to be as far forward in the optical

chain to protect the Quark and be far removed from

the focal point.  So use a 2" filter on the forward

most extension tube if your configuration is straight

thru (no diagonal).

 

Cheers,

SgrB2



#4 Tapio

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Posted 16 October 2021 - 07:45 AM

Where did you hear/read that these filters help with Sun ha imaging?
I'm little skeptical because the bandwidth already is so narrow - what would so wideband filter help?

Edited by Tapio, 16 October 2021 - 07:46 AM.

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#5 MalVeauX

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Posted 16 October 2021 - 07:57 AM

Where did you hear/read that these filters help with Sun ha imaging?
I'm little skeptical because the bandwidth already is so narrow - what would so wideband filter help?

They're for primary energy rejection as sub-aperture ERFs inside a refractor, prior to his imaging train containing the actual narrowband imaging filter(s).

 

Very best,



#6 TOMDEY

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Posted 16 October 2021 - 09:18 AM

Something to consider is that most (nearly all) filter substrates have terrible wavefront. This is not a problem if they are close to the image... where only a tiny footprint is involved in each field point. But if the filter is far from focus, the footprints ("cookies") get big to huge and can degrade the image. Nother consideration is that reflections twixt cascaded dielectric filters / etalons / windows create ghosts and have unusual spectral effects. A traditional Deep Sky Hα won't improve your system's bandwidth... it's a barn door compared with the eye of a needle. I wrote a paper on the subject of cascaded filters decades ago, because we were experiencing these kind of problems at B&L coatings that we were supplying to Xerox. They asked me to look into it and I discovered the issue... and fixed it.

 

Couple of illustrations from my ancient paper that closely relate and show the mechanism, which is a combination of both coherent and incoherent interference.

 

PS: We did interferometry on hundreds of OEM filter substrates... only a few select ones from Uncle Sam had great wavefronts. The rest of the commercial ones were terrible.

 

I'd say try what you describe and see what happens. Worst case... you just discard the filter after your experiments.     Tom

Attached Thumbnails

  • 10 incoherent interference.jpg
  • 11 toms filter doubling paper.jpg

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#7 SgrB2

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Posted 16 October 2021 - 11:57 AM

If you place the filter too close to focus, there will be too much thermal

load on the filter and the Quark which will affect the image and probably

damage the filter in the process.

 

Cheers,

SgrB2


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#8 davidpitre

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Posted 16 October 2021 - 04:04 PM

Tom. That’s some really good information about optical figure on most filters. It would be good to hear from those Who know the specifics on different brands.

#9 davidpitre

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Posted 16 October 2021 - 04:08 PM

Doesn’t Baader advertise 1/10 lambda for their DERFs?

#10 MalVeauX

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Posted 16 October 2021 - 05:06 PM

Something to consider is that most (nearly all) filter substrates have terrible wavefront.

Can you define terrible in this context and can you provide information that is current and relative from the past decade about these filters being spoken of with respect to their actual wavefront?

 

Very best,



#11 Great Attractor

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Posted 16 October 2021 - 05:29 PM

Doesn’t Baader advertise 1/10 lambda for their DERFs?

For D-ERFs, yes, but technically a 1.25" or 2" Hα 35 nm deep-sky filter isn't one (though as far as I remember the coatings here are exactly the same).

 

From their website:

 

 

 

Every single cell mounted filter delivered to our customers is cut as a round or square disc in the according size (1¼", 31mm, 36mm, 2", 50,4mm, 50x50mm, 65x65mm), and then is polished plane to a quarter wavelength on both sides on a computer numerically controlled polisher.

 

 

 

Many people use these small filters as internal ERFs and their images look OK, so I guess it's indeed λ/4 P-V or better.


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#12 MalVeauX

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Posted 16 October 2021 - 06:17 PM

 

Many people use these small filters as internal ERFs and their images look OK, so I guess it's indeed λ/4 P-V or better.

Yes, likely because the seeing conditions are worse than what the filter would be doing if seeing were non-existant and one could truly stress the optical performance of the filter(s) involved. Instead, all these great figures are pretty much not going to matter much when someone is imaging in greater than 1 arc-second seeing. When the wavefront is distorted in 1 to 2 arc-second seeing, it's not like something 1/10th lambda vs something 1/4th lambda is going to really show a palpable difference I don't think, for practical result purposes with imaging solar in poor seeing.

 

Otherwise, people with high end optics and high end filters would be able to perform magic feats through their poor seeing conditions. Neat thing is, they don't. They're limited by the seeing of course. lol.gif

 

Very best,


Edited by MalVeauX, 16 October 2021 - 06:22 PM.

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#13 briansalomon1

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Posted 16 October 2021 - 07:34 PM

Yes, likely because the seeing conditions are worse than what the filter would be doing if seeing were non-existant and one could truly stress the optical performance of the filter(s) involved. Instead, all these great figures are pretty much not going to matter much when someone is imaging in greater than 1 arc-second seeing. When the wavefront is distorted in 1 to 2 arc-second seeing, it's not like something 1/10th lambda vs something 1/4th lambda is going to really show a palpable difference I don't think, for practical result purposes with imaging solar in poor seeing.

 

Otherwise, people with high end optics and high end filters would be able to perform magic feats through their poor seeing conditions. Neat thing is, they don't. They're limited by the seeing of course. lol.gif

 

Very best,

I can second Marty's comment about seeing being the limiting factor in H-alpha.  The figure of a night time telescope matters quite a lot more than the figure on a daytime scope.

 

After observing in H-alpha for a few years I've seen for myself that it's only in times of very good seeing (and this is only at 60mm aperture) that I can see slightly better clarity and sharpness in a set of 25mm plossls compared to 24mm panoptics.

 

If you're using a single scope for both day/night then a high end objective makes sense.


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#14 TOMDEY

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Posted 16 October 2021 - 11:00 PM

Can you define terrible in this context and can you provide information that is current and relative from the past decade about these filters being spoken of with respect to their actual wavefront?

Very best,

I'll describe in a bit more detail what I did and saw >>>

 

We were qualifying laser-attack shields. They look just like clear glass windows, but are coated/tuned to reject 99+++% of offensive radiation. We astronomers think of a couple dozen layer coating as extravagant... but these are hundreds or more nearly magical concoctions. No surprise that beyond superlative coatings merit beyond superlative substrates... even for little few-inch witness-sample windows. We checked these in double-pass interferometry. And gosh --- etalon quality with even zero wedge. A relatively thin window that you can hand hold insert into a fluffed double-pass cavity and still see perfectly fluffed cavity (cept for the piston) is quite astonishing!

 

So I took advantage of our set-up and we guys started sticking every optical window we could find in there just to educate and amuse ourselves. This included my tubs of commercial astronomy filters from home, camera filters, camera windows, solar filters (with the coatings stripped off), plain old window glass, float glass, optical pellicles, thin Mylar sheet, Saran Wrap, unblown soap bubbles, old-fashioned polished plate glass, WWII optical windows, 6" and 10" thick fused silica optical windows from Edmund, etc., etc.

 

What I found was that (as expected) etalons are very good and our custom Edmund windows also very good (1/10-1/20 wave single-pass wavefront PV). Ditto the optical pellicles, some of the hand-crafted WWII parts, and old-fashioned polished plate... which is no longer produced. Virtually everything else (that we tested) was closer to window glass quality than ~optical quality~. But this is also not surprising, and why most filters cost ~$50 typ. Otherwise, the glass substrate alone would eat up the entire cost.

 

Terrible = few waves typ PV (random/irregular) SP over a couple inches diameter. And that will generally be good enough for filters residing reasonably close to focus. My recollection is that Premium Camera manufacturers offer fused silica optical quality integral camera windows as an option. FLI comes to mind.

 

If Baader filter substrate is 1/10λ --- That's Superior!      Tom

Attached Thumbnails

  • 16 commercial coated pellicle.jpg

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#15 IR656nm

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Posted 17 October 2021 - 01:49 AM

If you place the filter too close to focus, there will be too much thermal

load on the filter and the Quark which will affect the image and probably

damage the filter in the process.

 

Cheers,

SgrB2

Thanks SgrB2,

I will switch my uv-ir filter to a different location next time I image. Many thanks for this.

 

Magnus


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#16 IR656nm

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Posted 17 October 2021 - 01:52 AM

Hi,

 

It depends on a few things.

 

1) Aperture size of your imaging scopes (this determines the input energy).

 

2) Are you having heat issues?

 

3) Placement. Threaded to the end of the Quark is too close, you want your ERF to be as far into the light cone as you can without masking it so it should be the first thing inserting into your focuser.

 

4) Any dielectrically coated red imaging filter will do this fine. Personally I don't see a reason to get the HA 35nm over a standard Red imaging filter (35nm vs 100nm). But I would certainly suggest these over a basic UV/IR (which allows Blue & Green to pass).

 

5) 2" (50mm) is ideal for larger light cones, especially on faster base scopes. But it's not needed on smaller scopes with longer native focal-ratio. Again, depends on the scope and size of the light cone. You could ray trace to see the diameter of your light cone where you will place the filter, but it must be significantly into the light cone away from the plane of focus to work correctly.

 

Very best,

Dear Marty,

Thank you. I am not not sure about heat issues, I try to limit the time I image for, as well as not imaging when it is too hot.

I will certainly move my ERF filter to a better location as advised, I did not know I was doing it wrong.

 

Thank you so much.

Magnus


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#17 IR656nm

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Posted 18 October 2021 - 11:47 AM

Hi All,

 

I was just "reflecting" on the 1.25 inch uv/ir filter I have been using.

I was placing this on the end of my Diagonal before on a diligent basis.

When I tried imaging with the 60mm guidescope, I had to put the ERF (1.25 inch UV/IR filter) and I was swapping this out so regularly, I must have gotten into a habit of leaving the ERF on the Quark when I should have unscrewed it and put on the diagonal for imaging with the 70mm refractor.

 

So apologies if I made this mistake. It has only recently when I tried imaging with the 60mm guidescope and there is no alternative to placing the ERF, except on the end of the Quark's nose.

 

(unless I get a front mounted ERF) - I made enquiries to Baader-Planetarium, not sure what the answer will be.

I was hoping one filter+1 housing for the filter  could fit 60 to 90 mm telescopes.

 

 

Many thanks.

Magnus



#18 MalVeauX

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Posted 18 October 2021 - 11:53 AM

Hi All,

 

I was just "reflecting" on the 1.25 inch uv/ir filter I have been using.

I was placing this on the end of my Diagonal before on a diligent basis.

When I tried imaging with the 60mm guidescope, I had to put the ERF (1.25 inch UV/IR filter) and I was swapping this out so regularly, I must have gotten into a habit of leaving the ERF on the Quark when I should have unscrewed it and put on the diagonal for imaging with the 70mm refractor.

 

So apologies if I made this mistake. It has only recently when I tried imaging with the 60mm guidescope and there is no alternative to placing the ERF, except on the end of the Quark's nose.

 

(unless I get a front mounted ERF) - I made enquiries to Baader-Planetarium, not sure what the answer will be.

I was hoping one filter+1 housing for the filter  could fit 60 to 90 mm telescopes.

 

 

Many thanks.

Magnus

Hi,

 

I wouldn't bother with front mounted ERF for a 60mm to 90mm aperture. I wouldn't even really bother for 100mm or less. A simple sub-aperture 50mm (2") filter will do the job fine at on those apertures, like a UV/IR block or Red/HA imaging filter that is dielectrically coated.

 

Very best,


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#19 lorenzo italy

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Posted 18 October 2021 - 12:53 PM

Hi,
I normally use, as internal D-erf, a two inch Baader 35nm H-alpha, screwed on a 60mm extension mounted before the diagonal.
Just yesterday I checked, during the disassembly of the telescope, if there was any heat at any point.
The diagonal was at room temperature, so were the extension and the 35nm filter ring.
I have not touched the filter, but only so as not to dirty it ...
The telescope is 127mm in diameter and had been pointed at the Sun for over an hour.
I have not even noticed any deterioration in the image, after a long time that the telescope points to the Sun, perhaps due to internal turbulence generated by the heat.

 

Lorenzo


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#20 C0rs4ir_

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Posted 19 October 2021 - 02:19 AM

I always go with a front mounted D-ERF above 80mm of aperture, no experiments with the sun. As a sideeffect you get the best possible images and your precious Quark/blockfilter will work for many years to come.


Edited by C0rs4ir_, 19 October 2021 - 02:20 AM.

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