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

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#51 TH1

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Posted 28 October 2015 - 12:43 AM

152 stopped down to 102mm. The 152 is the telescope of the year and will blow away many.

 

I guess DS in CaK can't be done?

 

Rare CaK prom?



#52 chonum

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Posted 28 October 2015 - 01:35 PM

Sure it should blow some spherochromatism records.


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#53 ValeryD

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Posted 29 October 2015 - 06:38 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.

Hello Christian,

 

I have experimented with my C11 and Herschel prism.  Images of solar granulation are better with a full size ERF. 

 

In such a telescope with narrow baffle tube,  secondary mirror and it's cell,  prism's own cell etc,  too many sources 

of a heat.  With a full size ERF this problem is much lesser.  

 

When I spoke about scatter on the prism,  I did mean that if the prism will be coated with a coating for selectable reflection, this coating will scatter a light.  

 

Such oversized Herschel prism with such a coating won't be really cheaper than a full size ERF for refractor up to 8".

The only field where such a prism can work and have some cost saving is  large refractors 12" and up.

 

Valery.



#54 TH1

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Posted 09 December 2015 - 12:41 AM

Hello? 

 

CaK proms? Rare?

 

152 is stopped down of course to 99mm (don't trust 100mm)

 

SCT CaK? That can't be good can it? Can't imagine the battle there with plumes. A 

 

How can I bino in CaK?



#55 marktownley

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Posted 27 December 2015 - 02:16 PM

CaK proms? Rare?

 

Absolutely not, they're there every time you can see them in Ha.  It's just that 'off the shelf' CaK filters are primitive things that don't have much contrast or blocking out of band, plus, the scope you are using plays a big part.  I record them most times, check out my website http://brierleyhills...blogspot.co.uk/



#56 marktownley

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Posted 27 December 2015 - 02:56 PM

What an interesting topic to stumble upon here, i'm glad I did.

 

There's lots of things here I agree with.  Seeing at 393nm is a pain in the neck, I can do my full disks at 40mm pretty much 99% of the time, up it to 60mm and I reckon I can image at 80-90% of the time, going up to 80mm i'm down to maybe 40-50% of the time, whereas going to 100mm it's maybe 10-20% of the time; these estimates are based 'throughout the year', and, are obviously local to my seeing conditions, which is an urban environment - a 30 mile radius of concrete and conurbations, so, not ideal.  

 

I not really had any success above 100mm because of the seeing as described above and also I can't justify buying a TOA130 because i'm not sure how often i'd be able to realistically use it.  I have tried with 'lesser' scopes but the seeing just isn't in my favour, it comes back to what was said before about spherochromatism.  

 

Even at my small apertures the ERF is so important in getting really good results, I use a Baader blue (full aperture) at 40&60mm, a Beloptik Tri band full aperture at 80mm, and then the Beloptik tri band and Baader Blue (both) sub aperture with the 100mm.  Without the ERF it has a noticeably deteriorated effect on the image.

 

The other things that I find important in CaK imaging are a fast frame rate with the camera to help with the seeing, and also very high out of band blocking, this definitely improves contrast. 

 

I'd certainly be interested in a 'proper' large ERF for CaK, and would also be interested in a dedicated objective for CaK.  Count me in the project. :)



#57 marktownley

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Posted 01 January 2016 - 10:46 AM

I was having a peruse around the Andover site and found they sell BG25 in a 165mmx165mmx3mm piece for a mere $275 - 90% transmission at CaK.  Might be worth a punt?

 

http://www.andcorp.c...-bandpass-type/



#58 ValeryD

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Posted 05 January 2016 - 02:26 PM

I was having a peruse around the Andover site and found they sell BG25 in a 165mmx165mmx3mm piece for a mere $275 - 90% transmission at CaK.  Might be worth a punt?

 

http://www.andcorp.c...-bandpass-type/

Hi Mark,

 

It is difficult to make such a thin glass really flat without significant optical figure errors.  Especially for such a short wave length as 393nm.

A second problem is that blue glasses are very rarely tested for good homogeneity and stress birefrigency.

 

I don't think that a thickness of less than 10mm is suitable for high quality full aperture filter.

 

These Andover filters are made for more forgiving applications,  I believe.

 

 

Valery.



#59 marktownley

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Posted 06 January 2016 - 04:04 PM

Indeed, that is the problem Valery...




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