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High aperture CaK imaging with full aperture "ER"-like filter?

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#26 pbsastro

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Posted 13 October 2015 - 09:15 AM

Bon jour Christian :-)

Very interesting and strange that you typically get higher resolution at 396 than 393, since the wavelengths are so close. Moreover 393 being much narrower, should be less sensitive to seeing.
Do you have an explanation for this?

Could you please share the TOA 396nm spot diagrams? I only found for 435nm.

Yes, I agree a newton is the more cost-effective solution. It would have its own issues as obstruction and mirror scatter…
Another cost-effective solution could be to change spacing in an cheap achromat. I remember Valery doing this many years ago with a cheap 102 achro, to optimize for Ha. However I think, not sure, that in CaK case it would require spacing reduction, which may not be possible.

Yes, the D-ERFs are excellent. My 180 D-ERF was measured as transparent with the 96% Ha 175 :
http://r2.astro-fore...75-1400portugal

And to come back 100% on-topic, unfortunately a Baader CaK ERF will not happen. I e-mailed with Mr. Baader directly about this, and the development is too expensive, not possible for a such a small market as solar CaK.
It would have the big advantage of preventing heat from entering the tube, but is out of reality.

Pedro

Edited by pbsastro, 13 October 2015 - 09:16 AM.


#27 ValeryD

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Posted 14 October 2015 - 11:08 AM


And to come back 100% on-topic, unfortunately a Baader CaK ERF will not happen. I e-mailed with Mr. Baader directly about this, and the development is too expensive, not possible for a such a small market as solar CaK.
It would have the big advantage of preventing heat from entering the tube, but is out of reality.

Pedro

Hi Pedro and who may be interested,

 

Please,  contact me via PM if you need the info about CaK  ERF.

 

 

Valery



#28 DAVIDG

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Posted 14 October 2015 - 01:33 PM

 One needs to also consider when using refractive optics for CaK viewing/imaging is that many optical glasses absorb 20% or more below 400nm. So while you can have an  objective that is design to be  corrected well at CaK wavelengths, the optical glass used can absorb strongly in that region and greatly dim the image.

 So I would agree that a 150mm f/10 Newtonian offers both excellent optical performance and image brightness at 393nm.

 

    - Dave 



#29 ValeryD

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Posted 14 October 2015 - 03:41 PM

 One needs to also consider when using refractive optics for CaK viewing/imaging is that many optical glasses absorb 20% or more below 400nm. So while you can have an  objective that is design to be  corrected well at CaK wavelengths, the optical glass used can absorb strongly in that region and greatly dim the image.

 So I would agree that a 150mm f/10 Newtonian offers both excellent optical performance and image brightness at 393nm.

 

    - Dave 

Most of the modern glasses are MUCH more clear at the 393nm and absorb less than 3%.  But refractors have MUCH less light scatter and hence much higher contrast.   My own experiments confirm this for 100%.   Not say about the wave front smoothness and much greater stability of collimation in refractors. 

 

Valery.



#30 ch-viladrich

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Posted 15 October 2015 - 01:56 PM

Hello,

As said Valery, the problem is not so much on the transmission of the optical glass at 393 nm, which is quite good for most of the glass. The problem could be on the the anti-reflection coating which can cut a lot of light.

 

For example, given the tolerances of the coating, the transmission of TOA150 at 390 nm is between 86 to 91 % (data from Takahashi). This is not so bad.

 

Another alternative approach to "large ERF", is to have an Hershel diagonal with a special coating to reflect Ca K. This is something I have in mind. A friend has such an Hershel with Ca K and Ha coatings. There are pro and cons....

 

Christian



#31 ValeryD

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Posted 18 October 2015 - 01:06 AM

Hello,

As said Valery, the problem is not so much on the transmission of the optical glass at 393 nm, which is quite good for most of the glass. The problem could be on the the anti-reflection coating which can cut a lot of light.

 

For example, given the tolerances of the coating, the transmission of TOA150 at 390 nm is between 86 to 91 % (data from Takahashi). This is not so bad.

 

Another alternative approach to "large ERF", is to have an Hershel diagonal with a special coating to reflect Ca K. This is something I have in mind. A friend has such an Hershel with Ca K and Ha coatings. There are pro and cons....

 

Christian

Christian,

 

This is an obvious easy solution to make a special coating on the Herschel prism.  But several nuances lead me to a full aperture filter:

 

1.  Even smallest ( 3-4C) temperature gradients will produce micro scale turbulence inside the tube and image processing software will not deal with it.

 

2.  Such a coating will quickly degrade under a focused very dense sunlight.

 

3.  Every dust particle will scatter the sunlight much more than when these particles are on a front ERF.

 

4.  Coatings always scatter more light when light fall under a certain angle (about 45 in this gives case).

 

 

My tests in solar granulation imaging in a green light show the results,  that images were significantly crisper and less distorted through a full size ERF than with Baader Herschel prism.  I did this test with a 150mm refractor and a C11.

 

Since that tests my Baader prisms just gazers a dust.

 

 

Valery


Edited by ValeryD, 18 October 2015 - 03:03 AM.


#32 TH1

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Posted 25 October 2015 - 11:17 PM

How do you double stack CaK??

 

How rare is a prom in CaK? .. Must be incredibly dense.

 

LE152/CaK/Binotron/iPhone

 

TH

 

Attached Thumbnails

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#33 icecreamcat

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Posted 26 October 2015 - 05:23 PM

Could you guys not use a beam splitter?

 

I have one that seperates the uv from visual and infrared with 99% reflectance.

 

 

 

_DSC4002.jpg

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#34 Mike I. Jones

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Posted 27 October 2015 - 08:19 AM

Howdy all,

Bob Yoesle made me aware of this thread yesterday, and I'm pleased that a refracting objective solely dedicated to calcium K observing has generated interest here.  I'd be glad to design variants of the 150mm f/8 that I sent Bob.  I see mention of apertures of 100mm and 120mm, and possibly different focal ratios.  Those are all easy to do if the interest is there.

 

Bob's design is optimized over a temperature range of -10ºC to +50ºC, and not just at a single temperature.

 

ISTAR and Cumberland Optical come to mind as possible suppliers of coated, mounted CaK objectives.  ISTAR is well known for their high-quality, reasonably priced refracting objectives.  Cumberland makes the optics for Questar, and beautifully made the corrector optics I designed for Kevin Frederick's 17" wideband Chiefspiegler telescope over in the ATM forum.

 

What would CaK observers like to have?  Let's design up a few and get some quotes.

Mike



#35 DAVIDG

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Posted 27 October 2015 - 09:18 AM

Mike,

  Personally I think the scatter issue is over rated with reflective optics because besides the objective, the Etalon and all the optics in the system scatter light as well.   Lunt even had on the drawing board a duel CaK/ Halpha system  based on 200mm Newtonian.

   But if we want to go with custom design objective I would add one more spec to the design. I'm restoring a Hale spectrohelioscope that will soon be installed at Stellafane. In my research on these devices I came across one  design with an objective that I believe was made by GE and it was designed so both the CaK and H-alpha line comes to same focus. With such a lens you wouldn't have to refocus the SHS when changing wavelengths which would also be true if the telescope used both a Cak and H-alpha filter system with say a rotating mirror so one could quickly change from one view to the other.

 

                       - Dave 



#36 chonum

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Posted 27 October 2015 - 09:48 AM

I'll make a run of 203mm HaT in december.

Then I'm planning a run of large Ha ERF this winter (210/230mm, 6 units only...well 5 as I'll keep one for my CN212 !).

That is for sure.

 

Then if I have enough request, I may do the same thing with CaK ERF.

 

And no, it is not possible to have both >80% Ha and =20% CaK transmission on the same filter with those size.

The larger the ERF is, the more difficult it is to coat with a good homogeneity in the band.

In fact it is possible, but not in the amateur price range.

 

 

 

How do you double stack CaK??

 

How rare is a prom in CaK? .. Must be incredibly dense.

 

LE152/CaK/Binotron/iPhone

 

TH

 

Doing UV imaging with a large short doublet is a kind of suicide :)



#37 Mike I. Jones

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Posted 27 October 2015 - 09:59 AM

Mike,

  Personally I think the scatter issue is over rated with reflective optics because besides the objective, the Etalon and all the optics in the system scatter light as well.   Lunt even had on the drawing board a duel CaK/ Halpha system  based on 200mm Newtonian.

   But if we want to go with custom design objective I would add one more spec to the design. I'm restoring a Hale spectrohelioscope that will soon be installed at Stellafane. In my research on these devices I came across one  design with an objective that I believe was made by GE and it was designed so both the CaK and H-alpha line comes to same focus. With such a lens you wouldn't have to refocus the SHS when changing wavelengths which would also be true if the telescope used both a Cak and H-alpha filter system with say a rotating mirror so one could quickly change from one view to the other.

 

                       - Dave 

Dave,

Do you recall the aperture and f/# of that dual-band lens?  I can do a CaK/Hα objective, but even with any objective achromatized at CaK and Hα and minimum SA and coma at both wavelengths, the same SHS objective would still have to be refocused at all wavelengths other than those two, right?  Or is your SHS just going to operate at those two wavelengths?

Mike



#38 Mike I. Jones

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Posted 27 October 2015 - 11:01 AM

Here's a 150mm f/10 all-spherical objective that brings CaK and Hα together over a 0.56º FOV and achieves 1/20 wave RMS or better at both wavelengths, at the same focus, and over a -10ºC to +50ºC temperature range with less than 1/2 mm focus shift across temperature.  I was able to use preferred Ohara glasses in this design.  I know the move is toward CDGM glass due to its lower cost (at slightly reduced optical quality), and I could look at that if desired.

 

Dave - something like this one?  Different focal ratio?  OSLO file attached.

Mike

Attached Thumbnails

  • 150mm f10 CaK and Ha objective.png

Attached Files



#39 DAVIDG

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Posted 27 October 2015 - 11:17 AM

Mike,

   The aperture is 4" and the focal length of the for Stellafane SHS is 16'. It is the same design that is presented in ATM-1 and it was made commercially by  Sherburn and Howell. Presently it uses a singlet of the Cross design ie the back radii is 6x the front to reduce spherical aberration  and it's one a mount that allows about 8" of travel so it can  focus  from the H-alpha to the CaK positions. While there are interesting features at other wavelengths besides CaK and H-alpha those are  the two main ones that most would want to observe at.  So having the ablity to just turn the grating to either position without the need to adjust the rest of the optics would be a nice feature. Of course the others wavelengths  can be observed as well but one would   needs to refocus for the other wavelengths. 

  The GE lens was 6" f/15 I believe and that SHS was designed and built for the military and used to observe the Sun for flares that could cause radio transmission problems.

  So having a dual wavelength focus for a shorter focal length objective that will work with modern day narrow band filters might be a nice feature. 

 

                     - Dave 



#40 Mike I. Jones

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Posted 27 October 2015 - 11:55 AM

Dave,

This lens does what you want.  Good glasses, matching BFLs for CaK and Hα, over temperature.  The rear radius of 19 meters is long, but has to be there if the other three radii are equal.  If you try a plano rear it doesn't perform very well unless you let R3 be different than R1, or change glasses.  

Mike

Attached Thumbnails

  • 102mm f47_8 CaK Ha lens at 20C.png

Attached Files



#41 Mike I. Jones

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Posted 27 October 2015 - 12:08 PM

Or you could go with an equal radii solution with a plano rear surface by switching to Ohara S-LAL8 and S-TIH1 glasses.  The blanks cost about 3X BK7, but in this small size that shouldn't be a wallet buster.

Mike

 

Attached Thumbnails

  • 102mm f47_8 CaK Ha lens at 20C  S-LAL8  S-TIH1.png

Attached Files



#42 DAVIDG

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Posted 27 October 2015 - 12:12 PM

Mike,

  Excellent designs. Thank you ! What about good old BK-7 and F2 since I got a pile of that stuff ? 

 

                      - Dave 



#43 Mike I. Jones

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Posted 27 October 2015 - 12:35 PM

Like a diss?

 

R1 and R2 can be the same, but R3 must be a little flatter for R4 to be plano.  Too many fringes difference between R2 and R3 to count, but R3 is CC so you can just KE it directly.  One CC test plate does it for this lens.

 

Git er dun!

Mike

Attached Thumbnails

  • 102mm f47_8 CaK Ha lens at 20C  N-BK7 N-F2.png

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#44 BYoesle

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Posted 27 October 2015 - 12:57 PM

Wow -- awesome Mike!  Thanks especially for the OSLO files  :bow:  :waytogo:



#45 Mike I. Jones

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Posted 27 October 2015 - 01:55 PM

Dave's 102mm f/47.8 lens tests very nicely in double pass at the HeNe wavelength of 0.6328µm.  It would do equally well in DPAC at any near-monochromatic wavelength between CaK and Hα with the appropriate BFL shift.

Mike

 

Attached Thumbnails

  • 102mm f47_8 CaK Ha lens at 20C  N-BK7 N-F2 DPAC.png

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#46 DAVIDG

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Posted 27 October 2015 - 02:07 PM

Mike,

  Many thanks for the design especially the BK7/F2 one !  What I have done in the past for singlets designed for H-alpha work that have the radii chosen for zero coma and then require aspherizing a surface to remove spherical is setup them up in Double Pass using bright white light source  look through 1.5 nm bandpass H-alpha interference filter. When it nulls under those conditions I know it will work real well. So I can use the same procedure with your design and check it both 656nm and 393nm since I have interference filters for both of those wavelengths.

 

                         - Dave 



#47 Mike I. Jones

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Posted 27 October 2015 - 02:17 PM

Hα is easy to see, but will you have to use a CCD camera to see the CaK Ronchi bands?  



#48 ch-viladrich

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Posted 27 October 2015 - 02:38 PM

Hello Valery,

You raised some very interesting points :

(1) A "CaK Hershel" diagonal would reflect 100 nm of band pass centered on 400 nm, and probably some IR-light  If we consider the amount of energy received at the focus, there would be no difference  with the full size front ERF having the same transmission curve. The air inside the tube won't heat because air is basically transparent. Tube turbulence builds up when some mechanical or optical part is warmed by the incoming beam of light, and then warm the surrounding air by convection.
So, the heating of the air at the focus would depend on how much energy is reflected / absorbed by the filter. I made some tests with my Baader K-line filter. It does not seem to warm, but this is very difficult to measure.
BTW, we tested a 2" Herschel diagonal on the 435 mm refractor of my friend Serge Decounihout.

The temperature of the diagonal rose to 150 – 200°C (I do not remember the precise figure). This was because the mechanical parts of the diagonal was inside the light beam. Then he made the 4" Herschel diagonal you can see there :

https://coupolespuim...unetteserge.jpg

 

The 60 mm diameter light beam no longer touches any mechanical part and there no warming anymore.

(2) Soft coating would degrade but hard coating won't. This is the benefit of using hard coating.

 

(3) Scattering does not seem to be a big issue with my Baader Herschel prism. However, this is very difficult to evaluate quantitatively. But it is true there is potentially more scattering at the focus than at the front end of the telescope. This is the same for the Ha etalon or blocking filter.

(4) This is a big question. I hope to find a way to the answer testing the Ca K + Ha Herschel I have at hand. I am thinking of measuring the scattered the light at the solar limb and around a star (PSF). The biggest problem now is to have some good weather ;-)

 

A question for front ERF is the uniformity of the coating. The issue is not the uniformity of the FWHM, but rather the uniformity of the transmission in Ca K (and / or Ha) line.

 

In any case, what you've found in your tests is very interesting. At the end of the day, this is only only actual experiment which could say what is the good way to do it or not.



#49 ch-viladrich

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Posted 27 October 2015 - 02:45 PM

Thanks a lot Mike for all these OSLO simulations and designs :bow:



#50 Mike I. Jones

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Posted 27 October 2015 - 04:14 PM

Just to wrap the day up, Dave mentioned a 152.4mm aperture f/15 GE lens used in a military SHS to focus CaK and Hα together.  This is my all-spherical version of it, optimized over -10ºC to +50ºC.  Pretty nice lens, if the only two lines you want focused are CaK and Hα.  The Airy disk in the spots is for the 0.3933644µm CaK line.

Mike

Attached Thumbnails

  • 152_4mm  f15 CaK Ha lens 20C  N-BK7 N-F2.png

Attached Files




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