Newtonian vs SCT thermal handling with full aperture D-ERF

[MalVeauX]

Hey all,

So I've tried to use both my 200mm F6 Quartz newtonian (quartz primary & secondary) with my full aperture D-ERF as well as my 200mm F10 SCT with the same D-ERF. I got the newtonian because it's F6 quartz mirror would be better for short wavelength imaging (gband, calcium) compared to the SCT mirror (F2~F2.2) which has significant issues at short wavelength at full aperture. I ran into something interesting though. The D-ERF on my SCT handles heat fine. The image is steady. No signals saying there's something getting too hot in the optical train or in the tube itself. And to my "hand test" there's no heat coming out when I combine the full aperture D-ERF and then a Baader Red CCD-IR block (50mm) filter right before the imaging train as the 2nd D-ERF. Handles heat great in the SCT. But this is not the case with the 200mm F6 Newtonian! The heat is not handled right, something is getting too hot I think. With the full aperture D-ERF over the 200mm aperture newtonian, there's a weird nervous bend or spasm effect noted in the image (and its not the seeing), no matter what and it makes it impossible to get good focus and image. I wonder if it's the secondary mirror getting hot being so close to the aperture of the scope and getting light from both sides? The heat coming out of the focuser is also significantly higher than my SCT, even with a 2nd DERF there, it's hotter on my hand and melts things at the point of focus. Any ideas why the newtonian with the same DERF setup cannot handle the heat, but the SCT can? I would think the newt would be better for heat being an open tube unlike the sealed nature of the SCT?

Not sure how to fully thermally regulate the newtonian (with fully aluminized mirrors) with D-ERF's. I woudn't think the issue is the primary mirror. I would think maybe it has something to do with the secondary mirror? But then I think of SCT and how does it handle it on its secondary mirror?

Thoughts?





Example of the nervous twitch with the newtonian. The seeing was good, so it wasn't that. If seeing was poor the features would be out of focus and blurry. Instead, they all have a movement to them, a nervous twitch, that happens faster than seeing changes and isn't frozen via camera at short duration exposure. It's almost like a guasian motion blur applied. Because of this I stopped using this scope for solar. This was with both D-ERFs in place (full aperture front, and internal before the imaging train).



Very best,

[GreatAttractor]

One could think that perhaps SCT reflects/absorbs much more IR than the Newtonian, but the only difference would be the thin glass corrector... seems strange.

This, however:

The heat coming out of the focuser is also significantly higher than my SCT, even with a 2nd DERF there, it's hotter on my hand and melts things at the point of focus.

could be simply due to the different f-ratio. At the focal plane, the energy density (within Sun's image) will be (2000/1200)² ≈ 2.77 times higher for the Newt.

Does this Newtonian work fine with Baader solar film? If you haven't tried, maybe the black tube simply heats up (even at this small, grazing angle to the Sun) - try wrapping it in something reflective.

[bart1805]

I agree. You can't compare these telescopes because of the difference in focal ratio. If you had a SCT that is f/6 and could compare it with the C8 the temperature would be a whole lot higher measured in the f/6 telescope.
Besides that: secondary mirror is closer to focus than the secondary of the SCT, with a Newton you would need an open structure and definitely not a black tube. And besides that: the second ERF used with the Newton is really close to the secondary, that is not the case when you use it with the SCT.
Solutions:
1) Go slower than f/6
2) Let MCM coat your secondary so it only reflects H-alpha
3) Remove the aluminium of your primary: great instrument to use with G-Band. But you can't use it anymore for normal imaging (besides the moon) or for H-alpha.

[marktownley]

Interesting findings Marty, not sure straight away why - let me give it some thought...

[george9]

Like the others, I suspect the hand test is due to the f-ratio.

The two secondaries about about the same size, so the power being delivered to them is not too different--8" disc of sunlight compressed to a 2-3" disc--but I am sure the Newtownian is a little smaller.

The Baader red CCD will drop some of the blue light that the Aries ERF lets through, but the Aries already blocks most of that. And otherwise I don't think the Baader does much more beyond what the Aries does (the Baader is 100% where the Aries lets red light through, and otherwise the Aries is already blocking where the Baader does).

Maybe it is simply differences in local heating at the Quark due to the f-ratio.

George

[Rusted]

Mention of the tube heating at grazing angles does seem rather unlikely.
No direct light reaches the tube if it is continuously aligned on the sun.
Particularly with a larger, filter holder rim further blocking sunlight from the tube exterior.
The inside of the tube is slightly more exposed but only to D-ERF filtered light.

This calls out for a quick test to stir the air inside the tube.
Use a hair drier, blowing cold air past the primary gap without the heat on.
That will quickly confirm if there is an air heating or stratification problem.

The Newtonian is effective closed off by the D_ERF filter.
Normally the tube would be wide open. No cooling air for the secondary?

Try a simple standoff for the D-ERF filter to allow some air movement around the rim?
You should get a nice chimney effect up the tube if the primary cell is open enough.

[christian viladrich]

Hello Marty,
This is kind of strange ...
Regarding the secondary mirror :
- On the SC, the f-ratio of the primary is F/1.5, so I would say the secondary of the SC gets a more focused beam. Still, the problem is with the Newtonian, not the SC. So, this is not an explanation.
- Coult it be that the source of turbulence is the Baader red filter ? This filter "sees" a f/10 beam with the SC and a F/6 beam with the Newtonian. Still, I am not expecting the red filter to heat ?

Could you find a way to measure the temperature of the red filter ? On my side, I've bought a FLIR camera that can be attached to a smartphone for future tests.
Or find a way to measure the temperature inside the Newtonian OTA. Near the primary and secondary mirror ?

[Merlin66]

George,
Marty's talking about the same Baader D-ERF used on both scopes - not Aries.....

[bart1805]

George,
Marty's talking about the same Baader D-ERF used on both scopes - not Aries.....

Marty uses an Aries Triband as a front Erf.
Did you try other filters as a secondary Erf? For example something like this:
https://beloptik.de/en/uv-ircut-on-kg3-filter/

[MalVeauX]

Sorry if there's confusion.

Front D-ERF is Aries Triband, 214mm aperture, fits over both scopes in its holding cell.

Second D-ERF is Baader 50mm Red CCD-IR Block filter and secondary before imaging train.

I use both filters in same configuration on both scopes. The only difference are the scopes and mirrors. So, I've just been trying to figure out why the SCT handles the heat way better than the newtonian.

It's bad weather, rain, but as soon as I have a sunny day I will try to get some temp readings with instant-temp gauge. I don't have heat camera. Would love one.

Very best,

[george9]

Regarding the secondary mirror :
- On the SC, the f-ratio of the primary is F/1.5, so I would say the secondary of the SC gets a more focused beam. Still, the problem is with the Newtonian, not the SC. So, this is not an explanation.

Christian, even thought the SCT primary f-ratio is shorter, wouldn't the SCT put the secondary closer to make up for it? It seems to me the heat delivered depends only on the size of the secondary, assuming you place it so that it is just fully illuminated by the primary in both cases. A certain number of watts come in the primary, and they are compressed to an area dependent on the size of the secondary. Near the focal point, the shorter focal ratio produces a smaller Sun, and so the heat is compressed to a smaller area. But back near the secondary (where the width of the cone of light is much wider), the size of the solar image doesn't seem it would have a significant effect. Do I have that wrong?

And the Newtonian does probably have a smaller secondary, so that could be a minor effect. But I suspect the main effect is f-ratio nearer the focal point.

George

[Merlin66]

OK.
The terminology got me....
A D-ERF is a Baader product and design...
A Tri-band Aries is a completely different concept.

[MalVeauX]

OK.
The terminology got me....
A D-ERF is a Baader product and design...
A Tri-band Aries is a completely different concept.

Sorry Ken, I might have called it something it's not. Not sure what to call it then, other than specifically the Aries Triband (I think gen2?).

Very best,

[bart1805]

A D-Erf is a Dielectric Energy Reflection Filter. In my opinion both the Baader and the Aries Erf's are D-Erf's.

[Merlin66]

Yeah but....not the same.
Let's stick to Baader D-ERF and Aries TriBand ERF.

[marktownley]

OK, my thoughts on this. I think what George says that is related to the f-ratio is correct, however as a function it will be related to the 'square' of the focal ratio - 36 v 100 - 3 times the difference effectively.

I wonder if the effect you're getting with the images is due to wavefront distortion on the Baader red filter, as only the central section of it is going to be in the light cone. I've had similar results with CaK when using Baader sub aperture ERFs. Maybe try without the Baader red and see if you get the same?

[MalVeauX]

Ok, I got a chance to try to measure some today.

Setups were with same two filters, Aries TriBand D-ERF (393, 430, 656) and a Baader 50mm Red CCD-IR Block Filter placed in the focuser. I used an instant read thermometer. I tested it in the air and it was correct with air temp, and placed it in the unfocused beam and it registered the heat difference there too. Not sure how accurate, but it showed enough information to confirm a few things. The other measurement was the "hand test." If it was hot on my hand or not, when out of focus.

200mm F10 SCT (Edge version) with both filters. Hand test: no warmth. Thermometer: 68F. Ambient is 68F. So this is why its stable. It handles the heat load no problem and is thermally acclimated with ambient.

200mm F6 Newt (Quartz both mirrors) with both filters. Hand test: warm to hot. Thermometer: 109F. Ambient 68F. So that explains why its warmer on my hand, seeing as its so much hotter with more energy coming through. That means at the focus point, it's blistering hot, burning level hot. So this is why my cameras never stay focused and things are twitchy, as they're likely unstable due to near-burning-melting level temperatures. I'm fortunate I didn't melt my sensor I guess in the past when I was trying to image with this. And of course, it means the cameras and sensor covers can handle quite a lot of heat. Also means, seriously not good for eyeballs in this configuration.

So that's that. I'm writing off the newt as a narrowband solar instrument. It will go back to being a night scope. Maybe it would be different if it were also F10. But I'm not fooling with a 8" F10 newt, that's just too big. I'd rather use a 8" F10 refractor if I had to go full size like that. My mount cannot handle it anyways, so off it goes.

I will be sticking to 6" achromatic fracs and the 8" SCT and that's as big as I can go for my seeing and those two designs, slow ones, handle heat better.

Very best,

[christian viladrich]

Interesting Marty.
I was wondering what is your optical setup at the focus after the Baader red filter when you are imaging.

[MalVeauX]

Interesting Marty.
I was wondering what is your optical setup at the focus after the Baader red filter when you are imaging.

Hi Christian,

After the Baader Red filter (DERF #2), sometime I used a Quark followed by camera. Also now, PST etalon imaging train. The Quark has its additional blocking filter, but still got hot on the secondary mirror I think, too hot. With the SCT both work fine and never get hot.

Very best,

[bart1805]

How far before the Quark was the Baader Red CCD mounted when used in the SCT or the Newton?

[MalVeauX]

How far before the Quark was the Baader Red CCD mounted when used in the SCT or the Newton?

About 50mm or so, give or take, sometimes 35mm, I use an extension to ensure there's space and put it as deep into the light cone as I can.

Very best,

[george9]

Marty, sorry it hasn't worked out (at least yet) but thanks for posting as we learn how to use larger scopes.

A Barlow high enough up the light cone might address it by increasing the focal ratio, but then might as well use the SCT. And you would have to make sure it works at 393nm for that work. A classic Cassegrain with its higher focal ratio should better handle the heat and still avoid lenses.

I am interested because of my work with my C11 and double stacking it. What heat load gets where.

I am sure there are better formulas, but here is what I came up with for judging the load at any point in the optical train. It gives a ratio of how intense the light is compared to unfiltered sunlight. As an approximation, just add the diameter of the light cone to the diameter of the solar image at that point. The ratio of that circle to the objective aperture squared is the relative intensity.

intensity = { d / [ (f-x)/108 + xd/f ] } ^2

d=diameter, f=focal length, x=distance from focal point (toward objective)

(f-x)/108 is the size of the solar image at that point
xd/f is the size of the light cone

It says that light at f/6 is almost 3 times as intense as at f/10 at the focal point. And that a Quark should see about twice the intensity of light at f/6 versus f/10 for a 200mm scope (less than 3 times because it is further up the light cone). And that two secondaries both of 40% diameter get the same intensity at f/6 or /10.

George

[Valery]

Marty, for 393nm and G-band just buy a UV corrected barlow made by someone in the Netherlands.

[MalVeauX]

Marty, for 393nm and G-band just buy a UV corrected barlow made by someone in the Netherlands.

For the SCT? Or do you mean for the Newtonian?

I picked up a barlow a while back from thread relative to this, where the spacing was changed to undercorrect and improve 393~430nm performance on a SCT. But never got around to modding it with correct distances as they were unknowns.

If you have a contact or a link to the appropriate barlow, I'd be happy to get it!

Very best,

[bart1805]

Was looking at your picture again, it seems to good to be really affected by tube-seeing. Maybe it has nothing to do with the heat itself. Pure mirror systems are the best for UV and IR imaging. SCT systems are not because of the corrector. The Baader Erf opens up above 1500nm, maybe that is also true for the Baader Red CCD. Don't know the exact specs of the Aries Triband.
It could be IR leakage caused by the pure mirrors and the opening of the ERF's. That also could be the reason why Paul with his Mewlon is less successful than I would have expected with that good an optical system. The distance of the secondary of the Mewlon to the camera can be compared to that of a SCT, so it could be related to the missing corrector.
The solution could be (when H-alpha imaging) to use a UV/IR block, the Beloptik filter I mentioned earlier or a Baader 35nm. The 290MM you are using has just an AR window, so nothing is blocked there.

The barlow Valery mentions is this one:
https://www.astrosystems.nl/uv-barlow-
I use a ADC from ASH: great piece of equipment!