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A big solar newton for ultra narrowband imaging, possible?

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A big solar newton for ultra narrowband imaging, possible?

Post by allhoest » Sat Aug 31, 2019 8:44 pm

The here subject cover the observing with aluminized mirror, thus having high flux suitable for Ha / Cak imaging.

Warning: Solar observation is dangerous, we know it.

There are solutions for reducing the danger: front filter, Herschell prism, de-aluminized mirror, etc. But solar observation still remains dangerous.

The here subject copes with the use of the full aperture of a telescope, partly without pre-filtering. We are clearly in a zone, where we should know what we are doing and where the dangers are. Dangers are direct (ie direct viewing), indirect (ie reflection), affecting equipment (ie reflection aside, heat-up / burning parts), etc.

If you don't know what you are doing, if you are not mastering your working process, strictly refrain for any solar scope action.
I can not take any responsibility for any action you would do in solar observation, especially with modification.
Certainly never put an eye in a moded scope
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Considering again the dangers, expected and unexpected, I'll take over word by word the comments posted along my previous post:
"anyone doing this should take all necessary precautions."
"I cannot stress enough the dangers "
"Use with caution and respect!"

--------------------------
In the here trial, a 300mm aperture scope, with coating, is used.
The amount of solar power collected by the mirror is estimated to 90w. As much as a quite powerful light bulb.
The scope is of newton type, thus a big part of the heat is concentrated on a circle of about 50mm in diameter at the location of the secondary mirror, then further concentrated to the focus and forth.

We all know that there big front ERF filters are difficult to make and also very expensive. A 300mm front ERF doesn't cost a few thousand of Eur, but several thousand of Eur.
So the idea of a smaller ERF set into the system.
Here the ERF would be as big as the secondary, thus about 50mm in diameter, located in front of the secondary.


I started with various trials, measuring temperatures, etc.
Image


You can figure what will come into these fingers...
Image


... A blue interference filter suffering the full flux
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The filter resists.
I tried also to partly target the filter, so to have one side "cold" and the other side "hot".
The filter still resists.
Image


The wished color (blue here) passes through the filter.
The remaining flux is reflected back.
This is one of the indirect danger of the application. In this picture the flux is reflected back toward the body of the telescope, leading to a local heating. I experienced also a returning flux against the carbon truss; they started then to generate smoke.
Image


The test filter was a 1.25" model.
Then I went for the final version, a 50mm filter.
The setup is quite artisanal. We are in the test phase.
Image


I tried with a blue "ERF" and a red "ERF" aiming for CaK and Ha
In my area, a small city suburb, surrounded by streets and houses, I could get some granulation.
Looks "interesting" at this stage. Here in blue.
Image


So, I went out of the city for further trials.
I could get better granulation pictures, in the red domain. In the blue, the pictures were awful.
Image


The next step was a trial in Ha.
Here also, I could get some results, but not easily. Only when the seeing was stable. And only with a focal reducer.
Image

Image




Very full of hope, I went to the St Veran Observatory site, expecting for further progress.
The first results were promising. Christian detected filigrees on my first image in KLine.
But the picture quality was poor.
Image


Finally, getting further progress was difficult.
We checked the temperature in the system with an IR camera.
Certainly, we don't know the emissivity of the filter surface, nor from the surrounding material, but it looks that hot parts to have are about 10°C higher than the surrounding.
Image courtesy Frederic Jabet.
Image


Even the focusser side is getting warmer, sign of energy passing through the filter and heating the setup the system behind
Image courtesy Frederic Jabet.
Image



In Halpha, the results were not better.
Now, I remember that I could get the former Halpha "reasonable" results only when clouds were passing. The clouds were leading to temporary cool down of the equipment.



An other fellow made a similar test in St Veran, with a different optic and he also had trouble getting high resolution imaging.
After one additional trial, I came to the conclusion of instrumental turbulence due to the local heat of some telescope parts, despite the system was fully open (made out of a truss system).
So, I went for removing the primary mirror coating.
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Here, the first pics in bad seeing and bad wind conditions, in KLine with no coating on the primary.
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I had later a chance to observe with the de-coated mirror in better seeing conditions. Here under 450nm
Image


The conclusion is clear.
Bye Bye cheap ultra narrow band imaging.


CS
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2019-Aug-31

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Re: A big solar newton for ultra narrowband imaging, possible?

Post by bart1805 » Sat Aug 31, 2019 10:19 pm

Very interesting Alex, thanks for posting. What kind of filter were you using as a blue Erf? I am experimenting with a 150mm aluminised Newton (f/8) and a Baader Blue 2 inch CCD. No succes yet...
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Re: A big solar newton for ultra narrowband imaging, possible?

Post by marktownley » Sun Sep 01, 2019 7:10 am

Very well documented post there Alex, which gives us all a lot to think about. Especially the thermals from equipment heating.
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Re: A big solar newton for ultra narrowband imaging, possible?

Post by LTHB » Sun Sep 01, 2019 8:44 am

Thanks Alex, very informative, in particular the IR images!

I've been experimenting with a Mewlon 210 with a front ERF for some weeks now. When I am taking a sequence of a dozen or so CaK videos, after processing the best image almost always proves to come from the first or second video. I suspect that the explanation for this is tube seeing that quickly builds up after the start of the session when some inner parts of the telescope are absorbing light (despite the front ERF) and getting warmer.
I'll have access to a thermal camera in a couple of weeks and then look into that. Removing the mirror coating is, however, not an option with the Mewlon...

Regards,

Frank


Edit (09/11/19): As I have been asked (by PM) I'd like to add a clarifying remark: I did not and do not want to state or to suggest that I am in any way dissatisfied with my DERF (or that I am looking at it as being the cause of the problem described above or anything like that), on the contrary: So far the DERF has made some very nice images possible (some of which I posted in other threads) – I just would like to get more of them, and this will be my aim for the next solar season!
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Re: A big solar newton for ultra narrowband imaging, possible?

Post by allhoest » Sun Sep 01, 2019 7:28 pm

Bart,
The Astronomik B, that I have, are passing from 385nm.
The Baader that I have form an LRGB set is almost passing at about 400nm, so it has a poor transmission at 390..395nm.


I would suggest that you get in touch with the supplier and ask for "selected" filters.
Astronomik should be responsive on the subject. I didn't check with Baader.


Frank, I understand that you are not willing to play with your Newlon. But anyway, we see that there room for bigger size, so a bigger newton?


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Re: A big solar newton for ultra narrowband imaging, possible?

Post by MalVeauX » Thu Sep 05, 2019 1:10 pm

This is very interesting Alex.

The idea of keeping a fully coated mirror in a newtonian reflector and simply finding a way to install an ERF in front of the secondary mirror and it handling the thermal load is very interesting. That keeps transmission really high for high resolution imaging with dim filters, like etalons. It is interesting for HA. Really interesting of course for short wavelength, like 430nm or similar, in white light.

After dealuminizing the primary mirror, is the transmission high enough to still image with dim filters like HA etalons? Or CalciumK with an appropriate 2.4A filter? Or does it become a white light only filter?

Very best,

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Re: A big solar newton for ultra narrowband imaging, possible?

Post by christian viladrich » Thu Sep 05, 2019 6:48 pm

Unfortunately, these tests proved that this idea was a dead end.

For Ca K imaging with my TOA150, the 50 mm internal ERF is 33% of the diameter of the objective. I have no internal turbulence.

With the 300 m Newtonian of Alex, the ratio is 16%. Now, we can say this is too low...
Maybe, another factor is that with the TOA, the distance between the internal ERF and the focus is about 250 mm, while in Alex setup the 50 mm ERF is next to the diagonal mirror.

As said by Alex, other tests were made with a 200 mm F4-12 Cassegrain with the same bad results due to strong internal turbulence...

So, we are left with full aperture ERF ...

Frank : interesting feeback. Still a little bit unexpected. I have no internal turbulence with my 200 mm ERF and C8 (in Ha).

Marty : there is not enough light for Ha or Ca K imaging with a not coated mirror. This is only for white light imaging (say 1 to 2 nm FWHM).

As a side note, these FLIR cameras are very interesting to check was is going on inside the optical tube in terms of thermal balance.
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Re: A big solar newton for ultra narrowband imaging, possible?

Post by Merlin66 » Fri Sep 06, 2019 4:59 am

Interesting stuff.....
I assume the ambient temperatures were around 18 deg? that would represent a temperature increase of around 10 deg for the "ERF"
That doesn't sound too bad to me....
The ambient temperatures down here in Oz in Summer can be up to 40 deg in the shade!
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Re: A big solar newton for ultra narrowband imaging, possible?

Post by christian viladrich » Fri Sep 06, 2019 7:14 am

We were on top of a mountain, 2930 m altitude. Ambient temperature was from 2°C to about 15°C.
So an increase of 10°C is huge. Basically, a delta of more than 2°C between ambient air and an optical part is detrimental to the seeing.
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Re: A big solar newton for ultra narrowband imaging, possible?

Post by Merlin66 » Fri Sep 06, 2019 7:37 am

Christian,
I understand....
But I would think many internal ERF applications might give significant heat loads on filters in the optical train.
The point is the actual temperature of the element may not be as critical as assumed.
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Re: A big solar newton for ultra narrowband imaging, possible?

Post by allhoest » Fri Sep 06, 2019 6:43 pm

I was off a few days, sorry for not being responsive.

After stripping the coating, I checked the mirror temperature vs surrounding.

The mirror T°, taken from the side
Image



The bottom cell supporting ring (Wood paint in white)
Image



The center cage (Wood paint in white)
Image


The IR measurements are taken with a small portable camera. Courtesy Frederic Jabet.
We don't know the measured surface emissivty; the camera is giving numbers you have to trust or not.
The difference in T° is about 3°C.



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Re: A big solar newton for ultra narrowband imaging, possible?

Post by Merlin66 » Sun Sep 08, 2019 3:29 am

Christian,
Could you use the IR camera to measure the temperature of a blocking filter diagonal against an ambient temperature.
Ideally on different aperture scopes.
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Re: A big solar newton for ultra narrowband imaging, possible?

Post by langleif2 » Sun Sep 08, 2019 6:12 am

The DOT is an open telescope, which means that the structure is physically open, and the wind can blow through. Because the wind blows along the mirror the air has a more or less constant temperature.

It focuses the incoming beam onto a water-cooled 1.6 mm field stop that reflects most of the image out of the telescope and transmits only a 2.5 by 2.5 arcmin subfield to the re-imaging optics and cameras.

Based on this information your project with some effort actually can be done. :band

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Re: A big solar newton for ultra narrowband imaging, possible?

Post by bart1805 » Sun Sep 08, 2019 8:20 am

langleif2 wrote:
Sun Sep 08, 2019 6:12 am
It focuses the incoming beam onto a water-cooled 1.6 mm field stop that reflects most of the image out of the telescope and transmits only a 2.5 by 2.5 arcmin subfield to the re-imaging optics and cameras.
Can you compare that effect with using a very small secondary mirror in a Solar Newton? A lot of vignetting, but maybe interesting when you just want to use it for highres imaging. CS! Bart.

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Re: A big solar newton for ultra narrowband imaging, possible?

Post by langleif2 » Sun Sep 08, 2019 12:47 pm

Just saying and yes, this is all about high resolution imaging. I know (from a German forum) of 3 H-alpha solar observation rebuilt Newton telescopes with 7, 10 and 12 inches opening. The 7-inch model has a 180 D-ERF in the front, the 10 and 12-inch model has a dielectric coated primary mirror which only reflects H-alpha with approximately 70 nm FWHM. All three "work" with a coma correcting telecentric for collimation and with PST etalon (without lenses) plus 20mm block filter as a Ha system. Now when your mirror is uncoated, this might be an option. Well, a costly one!

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Re: A big solar newton for ultra narrowband imaging, possible?

Post by LauraMS » Sun Sep 08, 2019 1:22 pm

@Bart: A small secondary only to some (minor) extent could replace the 1.6mm DOT pinhole - the pinhole is only 1.6mm. A small secondary would be something like 20mm minor axis. Moreover, the DOT pinhole is mounted in a quite complex structure made of copper, with water cooling channels, silvering and pressurized air.

Moreover, a small secondary limits - among others - some of the rays of the primaries' near edge rays. I.e. those, which make the high angular resolution possible.

So, a small secondary may not be the way to get thermal power managed.

Best wishes,
Laura

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Re: A big solar newton for ultra narrowband imaging, possible?

Post by christian viladrich » Sun Sep 08, 2019 7:30 pm

The DOT is a Gregorian telescope, not a Netwonian. This is a huge difference and explains why a cooled diaphragm can be installed at the primary focus. Remember that in a Gregorian telescope the focus of the primary mirror is in front of the secondary mirror, not behind. This makes thing much easier.

Still a Gregorian is very cumbersome, and a liquid cooling system is a bit complex to handle. Not a solution for an amateur portable telescope.

Langleif2 : can you be more specific about the coated 12-inch mirror ? What company did the coating ?
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Re: A big solar newton for ultra narrowband imaging, possible?

Post by bart1805 » Sun Sep 08, 2019 8:51 pm

Thanks Laura and Langleif!
I understand the difference between the DOT and a Newton with a small secondary is huge. This winter I am going to grind my first mirror, 8 inch f/8. Will try different secondary sizes to see what the results are going to be.

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Re: A big solar newton for ultra narrowband imaging, possible?

Post by LauraMS » Mon Sep 09, 2019 7:23 am

Good luck with grinding your own mirror, Bart. Have done this loooong time ago. Fascinating experience to be able to obtain such high accuracy with so little technical means. With your own mirror for a solar Newton, observing will be even more satisfying!

CS, Laura

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Re: A big solar newton for ultra narrowband imaging, possible?

Post by langleif2 » Tue Sep 10, 2019 5:37 am

christian viladrich wrote:
Sun Sep 08, 2019 7:30 pm
The DOT is a Gregorian telescope, not a Netwonian. This is a huge difference and explains why a cooled diaphragm can be installed at the primary focus. Remember that in a Gregorian telescope the focus of the primary mirror is in front of the secondary mirror, not behind. This makes thing much easier.

Still a Gregorian is very cumbersome, and a liquid cooling system is a bit complex to handle. Not a solution for an amateur portable telescope.

Langleif2 : can you be more specific about the coated 12-inch mirror ? What company did the coating ?
Hello Christian, thanks for the description. I asked Airylab about the company and they told, that at least one of them was done by MCM ITP in France. It's stands for Materials and Components for Missiles, Innovation and Technology Partnership. You can go for further research from there, please tell, if you do. It sounds rather expensive, though.

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Re: A big solar newton for ultra narrowband imaging, possible?

Post by christian viladrich » Tue Sep 10, 2019 7:11 am

Hello lanleif2, thanks for this clarification.

I am afraid you were a bit mislead. I know rather well AiryLab and MCM. Alex do so since he is the one you got his 250 mm coated by MCM. So far, this is the larger Newtonian telescope with such a coating on the mirror. This telescope and MCM coating is presented in our book "Astronomie Solaire" (unfortunately in French language). You can also do a search in Solar Chat archive.

There are only three other mirrors coated (ERF) by MCM (all in 200 mm). None of them are operational.

MCM is not able to coat (ERF) mirrors larger than 250 mm (except mirrors with a central hole like primary of Cassegrain). This is why there is a 280 mm Hat telescope with an ERF coated Schmidt plate.

BTW, MCM has nothing to do with Missiles ;-)

In a nutshell, after having tested (with Alex and Fred) a number of options since the last 4-5 years, we are unfortunately left with the use of a large ERF for large Ha solar Newtonian.
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Re: A big solar newton for ultra narrowband imaging, possible?

Post by langleif2 » Tue Sep 10, 2019 1:22 pm

Hello Christian
Okay then, there's a member in forum.astronomie.de in the heading "Sonne" with the alias "Fensterrollo" telling, that he actually looked through a 12" and there was also a 10". As he say: "Impressive at quiet seeing in France to the star party". If you contact him, you will eventual get some clarification, I would think. But if you guys are the only folks behind the mirrors, then it must be a special built Newton and not a common one. The thread in the German forum doesn't tell.

Yes, I must admit that I thought it is has to do with missiles :seesaw
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Re: A big solar newton for ultra narrowband imaging, possible?

Post by allhoest » Tue Sep 10, 2019 8:38 pm

In a nutshell, after having tested (with Alex and Fred) a number of options since the last 4-5 years, we are unfortunately left with the use of a large ERF for large Ha solar Newtonian.
Or a newton with a central hole and treat it like a SC corrector plate, ie by MCM.
But like in every treatment, there is a risk.

Second issue: there is no more central part reflecting the laser for collimation.
You need then a very sturdy setup suitable to keep the collimation stable (star collimation).


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Re: A big solar newton for ultra narrowband imaging, possible?

Post by bart1805 » Tue Sep 10, 2019 9:12 pm

allhoest wrote:
Tue Sep 10, 2019 8:38 pm
Second issue: there is no more central part reflecting the laser for collimation.
You need then a very sturdy setup suitable to keep the collimation stable (star collimation).
Maybe that can be solved when using something like this:
https://www.teleskop-express.de/shop/pr ... copes.html

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Re: A big solar newton for ultra narrowband imaging, possible?

Post by allhoest » Wed Sep 11, 2019 6:11 pm

I use the Concentrer to prepare the collimation, for centering the secondary.
But for finishing the collimation, the Concentrer requires the central spot and dark conditions:
http://www.spheretec.de/pdf/Collimating ... epiece.pdf

When there is not central spot, then star test is the natural option to go.
As alternative, maybe the goldfocus solution may help; I didn't evaluate it however.
http://www.goldastro.com/

CS
Alex

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