Solar unplated Newton N250/F5.6 , first light 9.6.

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Solar unplated Newton N250/F5.6 , first light 9.6.

Post by pupak »

I spawned N250 in three days at a deathly pace. The sight of today's test drove me forward until I finally assembled it, quickly collimated it and threw it out. As expected, the sun was clouded with haze and cloud followed by cloud. I did a few sequences, but it wasn't worth much.
But it brought several insights:
1/250mm will provide usable H-a intensity for Quark (13ms / G-220 ) on IMX432.
2/ The contrast is lousy. There is evidently a problem with parasitic light.
3/ The mirror is made of who knows what. The glass is slightly bluish and heats up to 40-45 degrees in an hour. Celsius. It would probably need clear glass. Since it was noon, the seeing was terrible and it was difficult to focus.
4/ The increase in resolution is very debatable when the conditions are not good.
It's not an outright disappointment, but the champagne stays in the fridge. :D

Just one example of the sun and a photo of telescope.
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TZ4x - F22.5  IMX432
TZ4x - F22.5 IMX432
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TZ4x - F22.5  IMX432
TZ4x - F22.5 IMX432
s1c.jpg (889.01 KiB) Viewed 782 times
n2.jpg
n2.jpg (801.22 KiB) Viewed 782 times
n1.jpg
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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by pupak »

So I found some sequences with the IMX429 and one is pretty cool. The N250 does a little bit to repair the reputation.
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IMX429
IMX429
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IMX429
IMX429
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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by DavidP »

I love following your solar scope journey.
So what’s the story on the mirror?
Did you have this mirror made? Do you know the quality? Is the back polished?
I’m looking forward to seeing what this set-up can produce with some time experience and patience.
I’m also interested in your feeling that you have stray light damaging contrast. Do have ideas on baffling?


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by pupak »

DavidP wrote: Fri Jun 09, 2023 5:14 pm I love following your solar scope journey.
So what’s the story on the mirror?
Did you have this mirror made? Do you know the quality? Is the back polished?
I’m looking forward to seeing what this set-up can produce with some time experience and patience.
I’m also interested in your feeling that you have stray light damaging contrast. Do have ideas on baffling?
I bought the cheapest mirror from APM for €200. I don't know anything about him. I don't know the type of glass or the quality. I didn't want to destroy a €1500 mirror and find out it didn't work. Now I will experiment and try to improve. Then maybe I'll try another better mirror.


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by arnedanielsen »

A very promising new light with your new creation! Looking forward to follow the evolution of this scope!

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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by MAURITS »

Great setup Petr.


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by pupak »

10.06.
The conditions weren't good today, but it peeked out for a while, so I tried a shot at F16.8 (TZ3x) and surprisingly it didn't turn out too bad.
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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by Dennis »

Very interesting new setup and great shots.
The contrast issue must be related to stray light.. maybe the back of the main mirror has to be treated in some different way.


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by pupak »

Two factors will probably have a decisive influence on the level of contrast.
1. low H-a intensity
2. parasitic light from the back
Both things will be quite difficult to resolve satisfactorily.
At the moment, the only viable solution I can see is silvering the primary mirror with an extremely thin layer that will increase the H-a level while blocking the rear light. But finding the right amount of plating won't be easy, and I'm not sure if I even want to go for it.


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by rigel123 »

Your work with metal is amazing!


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by SteveZ »

Fascinating rig Petr. Hope you get your mirror issue worked out.


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by Averton »

Another interesting instrument you've created Petr. We wonder whether a simple test of something on the back of the mirror might be worth a try.


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by pupak »

When you consider that the filters cut out all of the non H-a band, then the reflected back light is a fraction of the H-a intensity and should not have a major effect on contrast. This is also proven by the use of an uncoated mirror for white light, where there are definitely no problems with contrast and the susceptibility is an order of magnitude higher.
Maybe someone with real experience will chime in and give advice. So far, I'm pretty confused. I'll take a picture of it, but it's definitely not usable for visuals. Maybe the mirror is bad, but I don't have the funds to buy another right now. I will have to sell some telescope to buy new toys. Interested parties are welcome. :)


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by DeepSolar64 »

Great job, Petr!!


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by marktownley »

Always nice to experiment Petr, it's how we learn!


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by DeepSolar64 »

True that!


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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by thesmiths »

pupak wrote: Sun Jun 11, 2023 4:41 am When you consider that the filters cut out all of the non H-a band, then the reflected back light is a fraction of the H-a intensity and should not have a major effect on contrast. This is also proven by the use of an uncoated mirror for white light, where there are definitely no problems with contrast and the susceptibility is an order of magnitude higher.
I've worked a bit with unsilvered mirrors. It's not something I would have used for H-alpha (I used only for white light and CaK). But it should be ok. I found the issue with stray light can be solved if you put enough shielding behind the secondary. The problem with the current configuration is the camera often points directly at the sky, where there is a lot of light (relative to the reduced intensity from the primary). I bet you would see a difference if you did a meridian flip (so the camera was pointed at the ground instead of the sky). Or you could configure things so that the camera is always pointed down -- that will help a lot.

I'm assuming the secondary is mirrored (in some configurations for white light visual, both are unsilvered, but I would not recommend this).

APM always sell good stuff; I would not worry about that. The fact that the glass warms up should not really be too much of an issue -- at least there will be no tube currents. I'm assuming the glass is a special composition with low thermal expansion, which is probably why it has a slight colour to it.

If my suggestion with the secondary shielding does not improve things enough, you could also put a piece of 250mm black cardboard behind the primary. It would not normally be necessary if the primary was held only by the edges, but you have a somewhat reflective metal frame right behind the primary (I assume because the mirror is quite heavy) and this might cause some stray reflections. Only a few percent of the light reflects off the front surface of the primary back to the secondary but essentially all the light from the reflective frame goes back towards the secondary. If you want to get really fussy, you should also blacken the clips on the front of the primary. Maybe the easiest is just to paint or black anodise the whole of the primary mirror holder.

There should typically be a shield behind the secondary. The camera should generally be pointed down, not up.
There should typically be a shield behind the secondary. The camera should generally be pointed down, not up.
1751.jpg (225.76 KiB) Viewed 451 times
Normally the mirror would be held like this, by the edges.
Normally the mirror would be held like this, by the edges.
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Re: Solar unplated Newton N250/F5.6 , first light 9.6.

Post by pupak »

Thank you for the stimulating comments. With H-a, secondary shading does not play a role, as the filters block out all ambient light. They are really necessary for white light, and the aperture at the entrance of the focuser is also good. Mirror reflections will be more of a problem. A clear glass mirror with an etched back for matting and reducing reflection would probably be ideal. The mirror I have probably won't be ideal for these purposes. It would also be good to completely redo the support system. It was the first attempt, so it would be a miracle if it worked 100%.


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