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Dielectric Diagonals

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#1 ButterFly

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Posted 13 May 2021 - 03:30 AM

I have wanted to give my 120mm f/7.5 more of a chance with afocal NV.  Diagonal goes along with that.  I have three 2" dielectric diagonals, and one 2" 45 degree prism with a smaller clear aperture of 36mm.  I'd prefer the full 46mm.  I was concerned the dielectrics would do poorly unfiltered.  It seems to be fine with my set of diagonals so far.

 

I compared the views with an without diagonal.  I didn't notice any difference whatsoever, except scattering from the dusty diagonal.  I was still getting the faintest stars and spillting the same doubles.  I just carried on with the diagonal.  Focusing without a diagonal is not always possible though.

 

To try and exclude dumb luck on targets, I just used half view through diagonal, comparing sky brightness.  Same brightness.  However, I was still looking at a different patch of sky through the diagonal and past the diagonal.

 

So I used the best broadband IR source I have - a turned OFF hot incandescent bulb's filament.  For added safety, I also used the 642.  I could certainly watch the filament dim as it cooled.  These peak above 1 micron for ordinary color temperature bulbs, so the spectrum flattens in IR as it cools.  The curly filament also has some structure.  No difference in brightness really or any effect on structure.

 

It seems promising so far.  Omega Centauri will be the final test I try before I'm sated.  For good measure, I'll do tests 2 and 3 with my dob's diagonal.  I have only been looking at brightness and resolution for visual use.  Is there anything else I should be looking out for visually?  What have been your experiences with dielectric diagonals?  I'm trying to avoid poisoning the well with brands, but how do your unknown named dielectric diagonals do with these fairly simple tests?

 

 



#2 a__l

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Posted 13 May 2021 - 03:57 AM

Your diagonals are accompanied by documents (spectral  graphs), what is this that was discussed here earlier?


Edited by a__l, 13 May 2021 - 05:20 AM.


#3 ButterFly

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Posted 13 May 2021 - 05:46 AM

No, but that would be helpful.



#4 chemisted

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Posted 13 May 2021 - 07:22 AM

Read the following thread completely:

 

https://www.cloudyni.../#entry11078493

 

As far as spectral response of dielectric mirror coatings Al Nagler confirmed many years ago that they fall precipitously as you go to higher wavelengths from 700nm.  See post #10 in the following:

 

https://www.cloudyni...ectro-i3-piece/


Edited by chemisted, 13 May 2021 - 07:24 AM.


#5 ButterFly

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Posted 13 May 2021 - 08:02 AM

I've seen those and that's what led to the concern.  Dielectric coatings can cover whatever range was desired.

 

With my diagonals and thoer random coatings, I see no difference in brightness or resolution.  Is there anything else I should look out for?  There are faint doubles all over the place in Omega Cen.  I'll certainly be looking at those.



#6 ButterFly

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Posted 13 May 2021 - 08:10 AM

Dark nebulas can be a good test.  Anyone know of any that don't become transparent until well into IR?  UKS 1 should be fun to try for, but it's a little hard.



#7 chemisted

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Posted 13 May 2021 - 08:47 AM

When they are above the horizon do a straight through test vs. any of your dielectric diagonals on NGC 6760 and NGC 6749.  The difference will be dramatic.  You are starving your NV device of precious long wavelength photons.


Edited by chemisted, 13 May 2021 - 09:10 AM.

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#8 Deadlake

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Posted 13 May 2021 - 09:37 AM

Does anyone know alternative to Baader 2" BBHS diagonal. Tried to buy one from last January and so far not possible. Baader deliver dates come and go...



#9 Jeff Morgan

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Posted 13 May 2021 - 12:37 PM

I've seen those and that's what led to the concern.  Dielectric coatings can cover whatever range was desired.

 

Yes, Clausing's web site shows a variety of coatings available, some of which reach into the NIR.

 

But the diagonal is a visual accessory. For example, from the Astro-Physics MaxBright description:

 

Advantages of the Astro-Physics Dielectric Coatings
Reflectivity - Reflectivity is above 99% over the entire 4000 to 7000 Å photo-visual range. Thin film coatings have extremely low surface scatter compared to aluminum or enhanced aluminum coatings. Dielectric coatings (if done right) will result in less scattered light. Examination with a laser source shows approximately a 5 fold reduction in surface scatter, a tremendous improvement over aluminum mirrors.

 

A couple of salient points. Firstly, dielectrics are an engineered coating, and there is no reason for them to do more in a visual accessory. And a big reason for them not to: $$$.

 

But if they did they would certainly trumpet that wouldn't they?

 

Secondly, the qualifier "if done right". I think we all know who A-P is talking about there wink.gif

 

Peak intensifier performance is about 770 angstroms with good response down to 900 angstroms. Means nothing for nebula of course, but for stars (as in "clusters") it could be significant. Don't know about dark nebula you mention, but that could be interesting.

 

The Baader BBHS claims performance down to 2000 angstroms:

 

https://www.baader-p...ive-properties/

 

I don't think Baader had a reason to target the extra spectrum either (unless they know of imagers using diagonals in their train). Rather their marketing is just talking up the intrinsic properties of silver. Certainly a happy "byproduct" for the NV astronomer!

 

But really, the ClickLock feature alone was enough for me to justify the extra money over buying a newer Astro-Physics (to update my old one). And it does have more "pop", but that could be partly due to the age of my A-P diagonal.


Edited by Jeff Morgan, 13 May 2021 - 12:40 PM.


#10 ButterFly

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Posted 13 May 2021 - 12:54 PM

Does anyone know alternative to Baader 2" BBHS diagonal. Tried to buy one from last January and so far not possible. Baader deliver dates come and go...

Don had some suggestions here: Largest Prism clear aperture.  Both prisms and mirrors.



#11 Deadlake

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Posted 13 May 2021 - 01:01 PM

But really, the ClickLock feature alone was enough for me to justify the extra money over buying a newer Astro-Physics (to update my old one). And it does have more "pop", but that could be partly due to the age of my A-P diagonal.

It’s the click lock which makes it worth waiting….



#12 Jeff Morgan

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Posted 13 May 2021 - 02:57 PM

Don had some suggestions here: Largest Prism clear aperture.  Both prisms and mirrors.

 

I used to have a 2"amici used in an 80mm apo.

 

The roofline is noticeable on brighter stars. How much this bothers is of course personal ...

 

BBHS is a safer bet for the money.



#13 chemisted

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Posted 13 May 2021 - 04:41 PM

 

 

Peak intensifier performance is about 770 angstroms with good response down to 900 angstroms. Means nothing for nebula of course, but for stars (as in "clusters") it could be significant. Don't know about dark nebula you mention, but that could be interesting.

 

 

I have been trying to stifle myself on making this post but then I pictured the mods having a double-secret probation grading system that follows this kind of stuff.  An angstrom is 1/10th of a nanometer so, for example,  H-alpha is centered at 656.3 nm and 6563 angstroms and peak performance of the tube is at 7700 angstroms.  I suspect it is just a typo but you never know just who is watching . . .



#14 GOLGO13

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Posted 13 May 2021 - 04:48 PM

The Baader prism diagonal I have lists the same range as the BBHS...so that's another option.



#15 ButterFly

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Posted 14 May 2021 - 07:23 AM

Very intersting.  6749 showed differences, but it was on the order of 6 stars vs 4.  The aperture is under 5", so the stuff that's different in my targets may be too dim to see.  The reflectivity in IR of my objective is a complete mystery.  If that light's not getting in in the first place, that's an easy explanation.  NASA says TV remotes are around 940nm.  If I can see the spot on a wall at 1x, and then it dims too much in the scope, done. 

 

The setup is still lovely on a video head aimlessly panning around.  If I'm missing some longer IR, oh well.  Most globs will get the dob anyway, so I don't think I'm missing too much staying put.  Omega Cen and the dark nebulae that low were very nice straight through anyway.  Standing back and looking at both was even better.  I'm visiting Tucson.  I tried to compare the milkier dark nebulae with and without, but no luck.



#16 Jeff Morgan

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Posted 14 May 2021 - 08:37 PM

I have been trying to stifle myself on making this post but then I pictured the mods having a double-secret probation grading system that follows this kind of stuff.  An angstrom is 1/10th of a nanometer so, for example,  H-alpha is centered at 656.3 nm and 6563 angstroms and peak performance of the tube is at 7700 angstroms.  I suspect it is just a typo but you never know just who is watching . . .

 

lol.gif

 

Thanks, been a pretty rushed week. Maybe need to slow down some!



#17 ButterFly

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Posted 16 May 2021 - 03:19 AM

Remote controls are very bright, excellent IR illuminators at the high end of the spectrum.  Point at the floor first.

 

I know exactly which diagonal does the best, and how it compares to straight through, and how it compares to the bathroom mirror.  Every single one of those diagonals has baffling issues - they are bare metal in IR.  That is a much more pressing concern to me right now.  I am fine with their reflectivity in IR, so hard pass on yet another diagonal.

 

Dielectric anti-reflective coatings can be as much of a concern as are dielectric reflective coatings.  If they are tuned for visual use only, the transmission can go way down outside of the band.  I checked my refractors and the transmission is still fine.  Check your coated optics.



#18 GOLGO13

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Posted 26 May 2021 - 02:27 PM

So what 2 inch diagonals are you guys using? It appears my only proper NV diagonal is my 1.25 inch Baader prism. But I would like to use my .7 reducer also. Alternative is use the afocal 40mm plossl. But I usually prefer prime focus.

#19 GOLGO13

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Posted 26 May 2021 - 02:30 PM

I should list my current diagonals. Televue enhanced aluminum. And a very old Televue unknown if it’s dielectric or not.

#20 chemisted

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Posted 26 May 2021 - 03:48 PM

Again, it is only a concern when viewing globular clusters for which a 2" is not necessary. I used a Televue 1.25" for many years and now prefer the Baader BBHS T-2.

#21 GOLGO13

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Posted 26 May 2021 - 04:31 PM

Ahh. I’ll just use the Baader prism then

#22 ButterFly

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Posted 26 May 2021 - 05:46 PM

Again, it is only a concern when viewing globular clusters for which a 2" is not necessary. I used a Televue 1.25" for many years and now prefer the Baader BBHS T-2.

It is a concern for any viewing outside the visual band - the band for which the dielectrics are optimized for visual use.  A dielectric anti-reflective coating can be just as damaging as a dielectric reflective coating, when used outside the band of its intended use.

 

Can you say with any degree of certainty that the coaters of your refractor's objective sprung for a VIS-IR anti-reflective coating?  If you pass an IR beam through your glass at a wavelength above 850nm, and the transmission is fine to you, is there anything left to complain about?



#23 alexvh

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Posted 30 May 2021 - 12:59 PM

Hey guys, is there a 2” diagonal that people are recommending for NV?

#24 Mazerski

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Posted 30 May 2021 - 01:58 PM

I use this one with NV:

 

https://www.highpoin...-adapter-amici2



#25 Tyson M

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Posted 30 May 2021 - 03:09 PM

One complaint I had with the BBHS diagonal is the coatings. On mine they were not that robust. I tried to clean a spot off and it was a coating flaw and by cleaning it I totally made it worse (ruined the diagonal mirror). A lot of this was my fault and lesson learned but still stings a bit, all because I didn't initially recognize that the coating failed




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