Solar newtonian for white light?

[vlaiv]

Here is an idea, not sure if it is feasible or not, but sounds interesting "on paper":

I was thinking about process of making a primary mirror and all that is involved in doing it and one thing that I've noticed is that star test can be done on uncoated primary.

Might not make sense at first, but when one gets to analyze what is going on, it works really well. Uncoated glass has something about 4% of reflectance rate if I'm not mistaken, and that is less than two magnitudes, so star testing on star that is first-second magnitude will yield bright enough image (same as using coated mirror on 3-4 mag star). But this thought process led me to interesting idea: since Herschel prism has reflectivity around 4.5%, uncoated mirror would reflect the same amount of white light, or even less if coated properly. In place of secondary mirror one might use regular prism for scopes of high f/ratio (like 6" f/8 - which would due to small central obstruction be probably suitable for solar observing) to avoid secondary heating (not sure if it would work).

After that one would normally use ND filter as with Herschel wedge to bring down light intensity even further to safe levels.

There are bunch of technical issues to solve - like what to do with excess light that is not reflected of mirror and goes thru (scatter inside glass blank) - how to avoid it bouncing around and lowering contrast and heating issues (how to safely direct it away from ota) - will mirror be thermally stable enough in this situation to provide good image?

What do you think about this idea?

[michael.h.f.wilkinson]

Such solar Newtonians have been made (in fact, the Kit Peak Solar scope has an uncoated mirror).

[vlaiv]

So idea works, glad to hear it. No one makes such scopes for amateurs I presume? It can certainly be done ATM.

[Alien 13]

Its an idea I have thought about having a go at, in my case I would use it for imaging only so cant see the need for additional filters if both primary and secondary where uncoated.

Alan

[Alien 13]

7 minutes ago, vlaiv said:

So idea works, glad to hear it. No one makes such scopes for amateurs I presume? It can certainly be done ATM.

I am sure there are examples of small Newtonions built like this on the net.

http://www.stargazing.net/Astroman/Solarconst.html

Alan

[vlaiv]

Just now, Alien 13 said:

Its an idea I have thought about having a go at, in my case I would use it for imaging only so cant see the need for additional filters if both primary and secondary where uncoated.

Alan

I thought about uncoated secondary, but I was not sure that it would work - too close to focus point, and one either has to color back surface of secondary with black paint to stop second reflection (like in prism) that would send out of focus light in focuser and that might be a problem in itself - too much heat accumulating at secondary base. In other case one needs to remove excess light somehow from secondary, but I have no idea how it should be done and where that light might end up.

[xtreemchaos]

its a grand idea and would work, I remember having a look through a 9.25 in the 80s, but its a lot of hard work and cash when a sheet of baader solar film would do the same job, saying that it would be fun making one. charl.

[Uplooker]

It does work. I have seen examples on Solarchat where people have used a normal Newt - 6 inch, and removed the mirror coating from the primary. Have a search on Solarchat forum.

 

[Alien 13]

4 minutes ago, vlaiv said:

I thought about uncoated secondary, but I was not sure that it would work - too close to focus point, and one either has to color back surface of secondary with black paint to stop second reflection (like in prism) that would send out of focus light in focuser and that might be a problem in itself - too much heat accumulating at secondary base. In other case one needs to remove excess light somehow from secondary, but I have no idea how it should be done and where that light might end up.

The guy in the link I posted did indeed paint the back of the secondary black..

Alan

[JohnSadlerAstro]

Hi,

Surely this would be a good use of a Newt with a damaged/degraded primary coating? Unless the instrument is a particularly expensive one it seems more sensible to remove the coating and use it for solar imaging than to have it professionally recoated. Though, you would have to wait for many years before a well-kept scope was in a bad enough state.

John

[Alien 13]

I expect that any design of this type needs to take into consideration what happens when the Sun is off axis and the focus point is on the wall of the tube or eyepiece hole, perhaps careful placement of baffle tubes might help.

Alan

[vlaiv]

7 minutes ago, Alien 13 said:

The guy in the link I posted did indeed paint the back of the secondary black..

Alan

Interesting read, not sure if modern coated / protected mirrors would have their reflective surface removed that easily - but one can always scan classified for old aluminum mirrored scope

One interesting point that I did not think about - Use of IR/UV cut filter when observing for eye protection is very important point.

[vlaiv]

1 minute ago, Alien 13 said:

I expect that any design of this type needs to take into consideration what happens when the Sun is off axis and the focus point is on the wall of the tube or eyepiece hole, perhaps careful placement of baffle tubes might help.

Alan

Don't think that you need to worry about that if you select secondary of proper size - if sun is at an angle that would project image to outside of focuser tube then it will miss the secondary (partially - thus lowering levels of radiation, or completely).

Smaller secondary has a benefit of smaller central obstruction - better scope performance.

[michael.h.f.wilkinson]

Could the secondary not simply have the same design as a Herschel wedge? Even then assuming 4% reflectivity of both primary and secondary, you attenuate the light only by a factor of 0.0016 (1.6 x 10^-3, or ND 2.8, which is already stretching it for planetary cameras (ND 3.8 recommended). To reach  ND5 for visual, you still need an extra ND 2.2. I would personally go for a coated secondary, and use the same filters as have to be inserted after a Herschel wedge. The open construction of the solar Newtonians I have seen on SolarChat prevent any tube currents which are the bane of using my C8 for WL