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Try Cak Altair Astro filter

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#1 lorenzo italy

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Posted 09 May 2021 - 10:52 AM

First image, decent, using the Altair Astro NUV 7nm 395nm filter.
It should be similar to the Baader CaK filter, and I only got it because it costs less ...
So far I have tried it with the 90mm refractor, but the results were poor.
I was beginning to think that the filter was also poor.
Then I remembered that there is a possibility that some refractors may be opaque to UV rays, so they may work poorly in that region of the spectrum.
I then mounted my old Astrosolar filter (I haven't used it for 10 years) on the 90mm Mak and put the filter on the nose of the ZWO ASI178mm.
I have finally achieved a decent result.
Of course not at the level of the Cak Lunt or Daystar modules, but still at a lower cost.

 

Baader Astrosolar ND5
mak 90mm Skywatcher
Altair Astro NUV 7nm 395nm
ZWO ASI178mm
Autostakkert3 and AstroSurface processing

 

07_48_52Z_lapl3_ap975 e.jpg

altair-nuv-7nm-1.25-filter-3-6910-p.jpg

 

 

 

 


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#2 MalVeauX

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Posted 09 May 2021 - 11:04 AM

Nice capture Lorenzo!

 

Your refractor, as long as it's long focal-ratio will do fine at 395nm most likely. The Mak (F3ish mirror) is actually poorer at 395nm than your refractor probably is. A fast refractor will more poor at 395nm unless its well figured for it, but that's unlikely. The longer focal-ratios (F8+) achromatic doublets will generally be fine for 395nm. Longer the better. If you have a bigger, longer frac, you can aperture mask it to 80~90mm and go F10+ (F15+ even) and have excellent results from an optics stand point.

 

The key difference however, regardless of all that, is seeing. Near UV like this is heavily influenced by atmospheric seeing. The day you tried your 90mm, it wasn't that the glass was poor for this or the filter was poor, it was poor seeing. Just like here, you were able to get a good image from a Mak, not because the Mak was better (its actually probably worse for this), but because the seeing was really good.

 

Very best,


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#3 lorenzo italy

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Posted 09 May 2021 - 11:29 AM

I have tried several times with the refractor, on several days.

Keep in mind that the refractor is an f5.5 ...
For the refractor I also built a 60mm diaphragm, the image was better but still with very little detail.

 

The turbulence was calmer today, but the mak image was "clearly" better.
I should have made a comparison between the two, but to be honest I didn't have the time to mount the refractor.
I will do more tests in the future, but I believe the culprit is the refractor.

 

Lorenzo


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#4 MalVeauX

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Posted 09 May 2021 - 06:24 PM

I have tried several times with the refractor, on several days.

Keep in mind that the refractor is an f5.5 ...
For the refractor I also built a 60mm diaphragm, the image was better but still with very little detail.

 

The turbulence was calmer today, but the mak image was "clearly" better.
I should have made a comparison between the two, but to be honest I didn't have the time to mount the refractor.
I will do more tests in the future, but I believe the culprit is the refractor.

 

Lorenzo

At F5.5, odds are yes, the refractor was likely not well figured for near UV wavelength. To compound this, did you use a barlow or extender? These are rarely good for near UV too, which can make it even worse. Couple those two things with poor seeing and you get mush.

 

The longer the focal-ratio, the better, no matter what you use. The ideal instrument is a really long focal-ratio newtonian mirror (greater than F6). But again you're always up against incredible seeing requirements using near UV wavelengths (393~395nm). A long focal-ratio refractor can do it though. Take a huge aperture achromatic doublet around F8 and mask it to 60mm~80mm and get a really long natural focal-ratio using the best figured part of the optics (the center) and it will do quite well in near-UV.

 

I have this same Altair NUV 395nm filter as well. It's a great filter, very well made, very good steep transmission profile. Great for high res photosphere imaging when seeing is good and great for Venus's clouds.

 

Very best,


Edited by MalVeauX, 09 May 2021 - 06:26 PM.

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#5 ch-viladrich

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Posted 10 May 2021 - 09:11 AM

Hello,

Please allow me to clarify something important : the near-UV Altair filter is not a Ca K filter. This is just a narrowband near-UV filter.

A Ca K (or H) filter has a bandpass smaller than about 0.4 nm, which is about 20 x smaller than the Altair (or Baader K-line) filter. So the difference of cost and result.

This being said, this is quite good to have an additional source for this kind of filters.


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#6 lorenzo italy

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Posted 10 May 2021 - 01:29 PM

At F5.5, odds are yes, the refractor was likely not well figured for near UV wavelength. To compound this, did you use a barlow or extender? These are rarely good for near UV too, which can make it even worse. Couple those two things with poor seeing and you get mush.

During the tests with the 90mm F5.5 I didn't use anything else in the optical path.
I think it is the telescope that is not suitable.

 

 

Hello,

Please allow me to clarify something important : the near-UV Altair filter is not a Ca K filter. This is just a narrowband near-UV filter.

A Ca K (or H) filter has a bandpass smaller than about 0.4 nm, which is about 20 x smaller than the Altair (or Baader K-line) filter. So the difference of cost and result.

This being said, this is quite good to have an additional source for this kind of filters.

Thank you for the clarification.
Obviously I don't expect the same results as the Lunt and Daystar modules, which are much more expensive and performing.
So how can I present my images?
Would "near ultraviolet solar photos" be okay?

 

Lorenzo



#7 ch-viladrich

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Posted 10 May 2021 - 03:17 PM

During the tests with the 90mm F5.5 I didn't use anything else in the optical path.
I think it is the telescope that is not suitable.

 

 

Thank you for the clarification.
Obviously I don't expect the same results as the Lunt and Daystar modules, which are much more expensive and performing.
So how can I present my images?
Would "near ultraviolet solar photos" be okay?

 

Lorenzo

Yes sure, "near UV" would be perfect.

 

BTW, here is a comparison between "near UV" and Ca K. Back then, we had some really big sunspots :-)

 

http://astrosurf.com...R1263-Kline.jpg

 

http://astrosurf.com...-AR1263-Cak.jpg

 

Clear skies

 

Christian


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#8 lorenzo italy

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Posted 10 May 2021 - 03:33 PM

Thanks Christian,
fantastic photographs.
The difference is obvious.
For now I will have to settle for the "near ultraviolet".

Lorenzo
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#9 astro rocketeer

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Posted 10 May 2021 - 03:37 PM

Yes sure, "near UV" would be perfect.

 

BTW, here is a comparison between "near UV" and Ca K. Back then, we had some really big sunspots :-)

 

http://astrosurf.com...R1263-Kline.jpg

 

http://astrosurf.com...-AR1263-Cak.jpg

 

Clear skies

 

Christian

Those are amazingly beautiful high magnification captures Christian!



#10 MalVeauX

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Posted 10 May 2021 - 05:02 PM

For now I will have to settle for the "near ultraviolet".

 

You could call it Solar Continuum UV-A I think?

 

Very best,


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#11 ch-viladrich

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Posted 11 May 2021 - 04:03 AM

You could call it Solar Continuum UV-A I think?

 

Very best,

Hello Marty,

Strictly speaking, this is not continuum since the bandpass includes two big absorption lines (Ca K and H).

 


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#12 lorenzo italy

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Posted 11 May 2021 - 01:05 PM

I have decided,
for my next photographs I will use this terminology:
"Photosphere in the near UV"
I should perhaps put in parentheses:
(it's not Cak) grin.gif

 

Lorenzo




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