Filter holder for a secondary mirror (Newton).

Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
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Filter holder for a secondary mirror (Newton).

Post by allhoest »

Hi,

Some time ago, I started experimenting with a filter located before the secondary mirror (Newton), as prefilter.

Here, the original version (2019):
Image



Here a more professional version (2021), made by a machinist, mounted a modified secondary holder.
Image




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Re: Filter holder for a secondary mirror (Newton).

Post by minhlead »

Interesting Alex. How did you treat your pri. Mirror to reflect the reds only? I mean did you DIY it or there are a service that handle this kind of coating?


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Re: Filter holder for a secondary mirror (Newton).

Post by marktownley »

Good engineering there Alex!


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Re: Filter holder for a secondary mirror (Newton).

Post by RodAstro »

Hi Alex
That looks like a nice neat solution.
Just wondering what filters you are using and how much of a drop in temperature they manage?
Also what size primary are you using and is it coated?

In the past I used my 250mm newt full aperture and fully coated for solar projection for several years.
There was no noticeable heating of the secondary, or a prism used after the eyepiece to divert the beam down to an observing box at the rear of the scope.
I did make a symmetrical eyepiece out of two 35mm air spaced objectives so no glue was involved between the elements to melt and that worked very well.
I could leave it tracking the sun for several hours at a time and as I say everything was not far above ambient temperature and easy to touch.
The back of the primary was actually cool as if it were in shadow.
I came to the conclusion that good clear glass does not get that hot with a beam passing straight through it and the coating of the secondary is very important.
If the coating is in very good condition and does not leak any light through to the glass and the secondary holder, then the beam/heat is just diverted.
My secondary holder was also made hollow only contacting the secondary around the edge away from the suns image and quite long so no heating from any leakage.

What we have to remember is that there is no heat in the light beam until it is inhabited, it is then the light energy is converted to heat energy.
I have just re-clad my observatory run off roof in 1.2mm aluminium from roofing felt and it is now 47degrees C cooler in direct sun light and only 2 degrees C warmer than the surrounding woodwork, it just shows how little heating there is when the energy is reflected away.

These are just my personal experiences and wont be the same for every system.

Cheers Rod


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Re: Filter holder for a secondary mirror (Newton).

Post by allhoest »

I used Astronomik interferential filters (The one which are marked "Not for solar use": type 2c, Ha 12nm.
The above pic shows also an Edmund Optics extended hot mirror, which is fused silica based, so far I remember.
To my experience, they all resists the full flux reflected from a 300 f6 aluminized mirror.
At your own risk...

I didn't check their temperature, but I can take them by their side, to remove them. They don't heat up, at least not significantly.

I have also the feeling that the secondary heats up, especially when not filter is used. This is my next investigation step.

I have also no bad experiences with optics (barlow, etc.) located after the secondary. They don't break.
My feeling is that the heat that they take depends on their absorption, which depend on the wavelength. Combining this absorption curve and the sun energy vs the wavelength gives an estimation of the absorbed energy by the glass.

CS
Alex


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Re: Filter holder for a secondary mirror (Newton).

Post by Rusted »

RodAstro wrote: Tue Aug 17, 2021 11:33 am Hi Alex
That looks like a nice neat solution.


What we have to remember is that there is no heat in the light beam until it is inhabited, it is then the light energy is converted to heat energy.
I have just re-clad my observatory run off roof in 1.2mm aluminium from roofing felt and it is now 47degrees C cooler in direct sun light and only 2 degrees C warmer than the surrounding woodwork, it just shows how little heating there is when the energy is reflected away.

These are just my personal experiences and wont be the same for every system.

Cheers Rod
I can add my own experience. I once had a block built, optical workshop with a massive, cast concrete roof between 4" and 6" thick It was coated in black, asphalt type, weatherproofing paint and it absorbed and stored a lot of the sun's heat in summer! So I covered the roof in aluminium sheet. Instead of my pitch laps literally running off their tools overnight, it became very pleasant to be in there. Probably about 25-30F cooler.

Conversely, shiny stainless steel sheet gets very hot in sunshine. So this material is best avoided for solar instruments.


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Re: Filter holder for a secondary mirror (Newton).

Post by Bob Yoesle »

Regarding thermal control, you ideally want to use materials with both high reflectivity, and high IR emissivity.

This generally won't be bare metal - including aluminum. From my own experience, I found flat white "cool roof" paint with very high emissivity to be the best choice for my observatory, and this also would appear to be the best choice for a solar telescope's structure.

I used Henry 687 Enviro-White, which can be thinned with water for traditional multiple-coats application to surfaces.


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Re: Filter holder for a secondary mirror (Newton).

Post by Rusted »

Thanks Bob for sharing your useful research into solar heating with familiar materials and finishes. :bow

Even something as simple as the objective cell, or a D-ERF cell, has the potential to heat up. I have noticed that the, all too familiar, black anodizing on aluminium quickly develops a tan when regularly exposed to our favourite, object of study.

I should have tried my laser-guided thermometer on the black objective cell and bright aluminium, D-ERF cells by now but seem to have completely overlooked it. Overcast and raining today. So obtaining this "utterly vital" information will have to wait for another day. :lol:

My green dome is a real heat absorber but I have plans for white, roller blinds for concealing the shutters. Which, by a delightful coincidence, conceal quite a large percentage of the solar-exposed, dome exterior, when open. The shutters are always closest to the optical axis. So have potentially, maximum, negative, thermal effect on seeing. Painting the dome all white would be instant grounds for divorce. I just wish I knew whether dense foliage was as cool as we all hope. Painting the trees would be a quite a chore. ;)


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Re: Filter holder for a secondary mirror (Newton).

Post by allhoest »

Conversely, shiny stainless steel sheet gets very hot in sunshine. So this material is best avoided for solar instruments.
I guess this belong to my spider.
I wanted the material of the spider to be aluminum, because it has a very high reflectivity.
The machinist proposed me high reflective stainless steel. There are many kinds of stainless steels, including "bright" stainless steel, so I accepted.
At the end, there aren't as "bright" as I expected.
Well, putting things in perspective, only the 1.5mm thickness of the material face the sun.
CS
Alex.


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