C11 ERF-Concept

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C11 ERF-Concept

Post by Okay-1 » Fri Mar 09, 2018 8:40 pm

This modification of solar telescope has been moved from cloudynights.com to solarchatforum.com.

I'd like to use my C11 for Ha solar imaging... but the full aperture Energy Rejection Filters (ERFs) available as quite expensive - think US$ 2K+. I was wondering if I could accomplish the same thing with an array of 2" UV/IR cut filters and a film of red cellophane.
Fig 1
C11 ERF-2.jpg
C11 ERF-2.jpg (72.24 KiB) Viewed 742 times
The concept is to place red cellophane over the SCT aperture plate... to cut out visible light. The array of UV/IR filters, mounted in a blacked aluminum plate, serve to cut out the ultraviolet and infrared light. That allows only red light containing the critical H-Alpha at 6562.8 Angstroms to enter the SCT.
The 2" UV/IR cut filters are available from China for about US$20 each. (Yes, they work.) Allowing for the central obstruction, one would need about 16 of these... 16 x $20 = $320.

I'm certain there are objections to doing this... and I'd like to hear them.
Thanks in advance...

Person 1
It has been tried before. There's the obvious challenge of making it mechanically safe. Then you have diffraction effects that take away image quality. It's likely that a smaller clear aperture will give just as good, or even better, results.

Okay-1
Person 1, thanks for the PM. I've reproduced it here because I think it's relevant.
Here's the link to the discussion: https://solarchatfor...pic.php?t=16241
PST mod1 on a C8 without ERF
The cheaper filters might not have a good figure and introduce aberrations of their own.
Finally, you have to be super careful about safety. A loose filter can cause serious damage. Even with the filters in place I have no idea of how much light can the pst body take and remain safe.
I feel like most of the discussion about solar mods has died down in favor of quarks with ~100mm refractors. It gets you enough aperture, relatively safe and relatively cheap.

That solar chat happened in May 2015... almost 3-years ago; too bad they stopped experimenting after 3 filters... I would have liked to see the results of 5 filters. Thanks again for supplying that information.

Okay-1
I redid my conceptual image based on actual measurements of my C11 and UV-IR cut filter.
Fig 2
C11 ERF-5b-900.jpg
C11 ERF-5b-900.jpg (153.89 KiB) Viewed 742 times
Person 2
Why not add some more 1.25" filters close to the center and then make it?

Okay-1
Hi Person 2,
The sample 1.25" UV-IR Cut filter that I had cut too much into the red... cutting out all the prominences.
Maybe a good idea... if I could find some inexpensive working 1.25" filters.
Here's what it would look like.... like the business end of a Gatling Gun or Vulcan Cannon.
Fig 3
C11 ERF-5d-900.jpg
C11 ERF-5d-900.jpg (199.92 KiB) Viewed 742 times

12 of 2" filters and 12 of 1.25" filters.
I tried without success to find a free program to model the diffraction pattern of such arrays.

Person 3
If you want to see the effect on contrast of such a setup, you could cut a mask and then examine the Moon - with and without. Also, might be interesting to compare against masks with single apertures of interesting sizes. Of course, viewing the Moon is likely going to be under better seeing conditions than you would find with the Sun during the day.

Person 4
Does this thread not contravene the CN TOS regarding safety and Solar viewing/modifications ...

Okay-1
Great idea Person 3! When the weather co-operates I will try that. THANKS!
Hey Person 4... the thread is about imaging, not viewing... and no solar scope is being modified.

And that folks was when it was time to move the subject over to solarchat!
All comments and suggestions are Welcome.

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Re: C11 ERF-Concept

Post by marktownley » Sat Mar 10, 2018 8:31 am

Yes, these ideas keep doing the rounds...

You will get a fantastic starburst effect firstly with the pattern of sub apertures - try it on the moon and see.

Secondly my feeling is that it will let in too much energy with UV/IR and red cellophane. Red cellophane - seriously? Any idea of the optical properties of this in terms of transmission? Try some plastic wrapping film over the aperture of the C11 and look at the moon - see how how optically imperfect it is!

Bad idea personally.
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Re: C11 ERF-Concept

Post by christian viladrich » Sat Mar 10, 2018 8:46 am

Hello,

As told by Mark, these ideas come arround now and then.

The first easy test to do is to have a look at a star with the red celophane filter. I guess this will settle the case for the cellophane idea.

As for the obstruction pattern due to the subaperture UV filters in front of the C11, here are some examples I ran for a friend who has the same idea with sub-aperture Coronado etalons (1, 2, 4 ai-spaced etalons):

Image
Explanations are here :
http://astrosurf.com/viladrich/astro/in ... action.htm
Such calculation can be done for many more sub-apertures.

If you still want to push further that way, it would be better to use high quality red filters, rather that low quality UV filters + red cellophane filter. You need to find lambda/ 4 filters ...
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Re: C11 ERF-Concept

Post by christian viladrich » Sat Mar 10, 2018 10:06 am

Here is the simulation of the diffraction pattern. C11 on left, ring of 50 mm filters on right side. Same scale for both :
Image

The simulated apertures (C11 on left side, ring of 50 mm filters on right side) :
Image
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Re: C11 ERF-Concept

Post by christian viladrich » Sat Mar 10, 2018 10:31 am

BTW, 75 mm x 75 mm square filters are all available.
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Re: C11 ERF-Concept

Post by Okay-1 » Sat Mar 10, 2018 6:34 pm

Hi Mark,
Nice to hear from you again.

RE: Yes, these ideas keep doing the rounds...
Yes but they never seem to push through to a final image... unless I missed something.

RE: You will get a fantastic starburst effect firstly with the pattern of sub apertures - try it on the moon and see.
I would expect such an effect on a star but on an extended object like the moon??? - I will try it an see.

RE: my feeling is that it will let in too much energy with UV/IR and red cellophane.
I could scrap the cellophane; it's only purpose was to reduce the 'visible' light striking the secondary mirror. Maybe ??? I can get away with all that visible light as the UV and IR have been removed.

Hi Christian

Thank you very much for those simulations of the diffraction patterns from a native C11 and a C11 with a ring of 50 mm filters.
Actually, to my eye that diffraction pattern looks mild - importantly most of the energy is still concentrated in the Ariy disc.
I guess only an actual image of the moon with an without the ring-of-filters will tell the whole story.

If it's not too much work for you... any chance you could repeat the simulation for the mix of 2" and 1.25" filters shown in my Figure 3?

Thanks All!

V/R
Rick

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Re: C11 ERF-Concept

Post by marktownley » Sat Mar 10, 2018 7:18 pm

Hiya Rick,

I really think the cellophane is a bad idea, and using UV?IR I think you will have too much energy inside the tube. Looking at the simulation Christian posted of a ring of 2" filters adding in 1.25" filters too is just going to make things more dotty with the resultant image lacking in contrast.

I think the easiest and safest way to get the answers you are after is to make a filterless mask with the 2" and 1.25" holes and try it on the moon with a bit of imaging. Easy, cheap and should it not work you aren't left with a pile of filters with no use for them.

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Re: C11 ERF-Concept

Post by Bob Yoesle » Sun Mar 11, 2018 4:09 am

Agree Mark, the cellophane is a non-starter. If I were going to do this (and that's a big IF considering the diffraction effects), I'd use Baader 35 nm nighttime H alpha filters. Better than the UV/IR for the UV/IR and visible blocking, on an RG substrate (RG610 I believe) and optically polished to a much better flatness and overall quality. You should be able to get a quantity discount as well ;-)
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Re: C11 ERF-Concept

Post by Valery » Mon Mar 12, 2018 5:23 pm

As the "Person 2" I would to remind the proverb "There is no free lunch.".
And add more - normally, there are no cheap large size optics and large size interference filters.

You always get what you pay for.

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Re: C11 ERF-Concept

Post by marktownley » Mon Mar 12, 2018 5:37 pm

Valery wrote:
Mon Mar 12, 2018 5:23 pm

You always get what you pay for.

Valery
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Re: C11 ERF-Concept

Post by Okay-1 » Mon Mar 26, 2018 3:37 am

Well I made a mask for my C11.
First attempt - Original Mask.
Original Mask 950.jpg
Original Mask 950.jpg (150.46 KiB) Viewed 477 times
I suspected that the rough edges were contributing to image blur so I modified the mask using black tape to cover the edges of the holes.
Second Attempt - Modified Mask
Modified Mask 950.jpg
Modified Mask 950.jpg (173.04 KiB) Viewed 477 times
Here's the results on the moon tonight (March 25 2018) using the my C11 with and without the modified mask.

NO MASK - 10 best out 900 frames stacked in AS!3. No Image Processing.
20_11_21NM Crop JPG 950.jpg
20_11_21NM Crop JPG 950.jpg (71.38 KiB) Viewed 477 times
WITH MASK - 10 best out 900 frames stacked in AS!3. No Image Processing.
20_21_02M Crop JPG 950.jpg
20_21_02M Crop JPG 950.jpg (62.45 KiB) Viewed 477 times
The masked image is certainly not as clear as the unmasked image. Maybe sharpening the image might help?
Here's the sharpened masked image. (Sharpen: Lucy Richardson deconvolution, unsharpmask, high-pass filter)
WITH MASK - SHARPENED
20_21_02MPS Crop JPG 950.jpg
20_21_02MPS Crop JPG 950.jpg (239.74 KiB) Viewed 477 times
At this point, I felt that the sharpened masked image was better than the original no-mask image.

I then applied a similar sharpening routine to the original no-mask image.
NO MASK - SHARPENED
20_11_21NMPS Crop JPG 950.jpg
20_11_21NMPS Crop JPG 950.jpg (209.68 KiB) Viewed 477 times
After sharpening both the masked and no mask images, to me it is evident that the image made without a mask is superior. Just how much... hard to quantify this effect. Applying these moon results to a possible solar image, I think one could expect a similar performance.
i.e. Not as good as a full 11 inch aperture (but who expected that anyway) but it seems to work... how well is an open question.

What are your thoughts on these lunar results?

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Re: C11 ERF-Concept

Post by marktownley » Mon Mar 26, 2018 5:40 am

Excellent! Experiment is King! The results are pretty much what I would expect, the masked version has much lower contrast. On the sun, and certainly at the moment contrast is a lot lower than the moon so I would image if this was tried on the sun things would be very washed out.
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Re: C11 ERF-Concept

Post by Okay-1 » Mon Mar 26, 2018 5:36 pm

Thanks Mark,

Yes I agree... " Experiment is King!"... often too many talkers without proof.
Now in retrospect, the results makes sense; chopping up the aperture would cause diffraction effects that would lead to a loss of contrast.

The only thing I wonder about is resolution.
The masked C11 has the equivalent clear glass of a 152mm aperture scope. However, because the clear glass is distributed about the periphery of the C11, I would expect the masked C11 resolution to be higher than an unmasked 152mm refractor.
I wonder who wins? Would the loss of contrast have more effect than the increase in resolution or visa-versa?

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