7 replies to this topic

[Biff]

I've been doing some reading and this old thread seemed to be the most on-point for what I'm looking into. There are some points I'd like some clarification on... 

 

The first is the required reduction in light, which seems there's two answers for. One is reduction for comfortable viewing, the other is for safety. I imagine if you were using a standard refractor with a full aperture solar filter and that filter fell off unexpectedly, I would assume you'd cook your retina before you had a chance to react. What is a ballpark reduction that the image would be even painfully bright but give you ample time to react without damage. I'll come back to this point.

 

I have a 105mm/1200mm uncoated primary and uncoated secondary. I may also have a prism that I could use inplace of the uncoated secondary if ghosting of the back surface proves to be a problem. I've seen DavidG's design, and I like the elegance of it, but I don't want a right angle after the focuser - just seems there'd be a lot of uncomfortable viewing angles. I was wondering if I could use an ND filter at the EP (post #9 in that thread suggests an ND 1.8 would be sufficient), stacked with half of a polarizing filter since the light reflecting off the secondary will be partially polarized I could spin the EP to tune brightness. Any reason this won't work?

 

One concern is safety as mentioned above. What if the filters fell off? The light at the EP after two reflections and no filtering will be ~0.16%. I'm sure you'd be seeing spots but is that attenuated enough to be able to react without pemanent damage?

Edited by Biff, 27 January 2023 - 10:58 PM.

[BYoesle]

Hi Ryan,

 

Sounds like it would work and be safe.

 

Most designs would use an uncoated primary and an aluminized secondary, which would effectively give you the same brightness as a Herschel wedge. You'd then similarly use a ND3.0, UV/IR block, and a couple of ancillary ND filters like a 0.9 and/or a Baader Continuum, polarizer filter, etc.

 

You also might want to check out the details of Christian Viladrich's 12 inch solar Newtonian.

 

But at ~ 100 mm of aperture, any good quality refractor and Herschel wedge would be more preferable (no secondary obstruction or spider diffraction, etc.)

[Helen P]

Would a 200 mm version be worth making.

We have an old 200 mm F8 OTA lying around at the astro club which should be re aluminized before it can be used again. ON top of that everyone now uses either refractors or larger reflectors for their observations or photography.

That is why I tought of turning this into an uncaoated miror solar OTA in stead of having it end up as scrap. But indeed the the question remains: is a 200 mm worth the effort (apart from the pleasure of ATM'ing something unusual)

Edited by Helen P, 28 January 2023 - 09:29 AM.

[ch-viladrich]

Would a 200 mm version be worth making.

We have an old 200 mm F8 OTA lying around at the astro club which should be re aluminized before it can be used again. ON top of that everyone now uses either refractors or larger reflectors for their observations or photography.

That is why I tought of turning this into an uncaoated miror solar OTA in stead of having it end up as scrap. But indeed the the question remains: is a 200 mm worth the effort (apart from the pleasure of ATM'ing something unusual)

Yes, this is indeed a good idea. Co-author François Rouvière made a 210 mm f/8 solar telescope starting from a mirror he made more than 40 years ago. He is getting very good solar pictures with it.

 

Check some pictures there (see "Gallery") :

https://solar-astron...ex.php/preface/

[PatrickVt]

Would a 200 mm version be worth making.

We have an old 200 mm F8 OTA lying around at the astro club which should be re aluminized before it can be used again. ON top of that everyone now uses either refractors or larger reflectors for their observations or photography.

That is why I tought of turning this into an uncaoated miror solar OTA in stead of having it end up as scrap. But indeed the the question remains: is a 200 mm worth the effort (apart from the pleasure of ATM'ing something unusual)

 

I agree with this.  I, too, have thought about converting a 250-300mm reflector solely for solar astronomy.  It is quite a bit of work and I highly doubt that my location would even support that aperture during the day.  Honestly, I suspect few populated locations would.  The only reason I could use to justify this project would be just to build something on my own...  so, "just because".  Then I would question whether I could really put it to the test without the necessary seeing conditions.  Then it would just be taking up valuable storage space with little purpose other than being a conversation piece.

 

Patrick

[MalVeauX]

Hi Biff,

 

Like Bob pointed out, to mimic the transmission throughput of a wedge on a frac, the newtonian mirrors would have an uncoated primary and a coated secondary. You can go with two uncoated, but the image will dim significantly. As Bob pointed out, you still need the same filters in line, notable, the ND3.0 filter and then probably a polarizer and then a selective filter like a red or green filter depending on seeing conditions. Personally I would not go this far on a newtonian for only a 100mm aperture since quite a lot of that aperture is including the secondary obstruction. At smaller apertures like this, I would much prefer a refractor instead. I would mostly reserve the idea of solar newtonians for 150mm or larger systems personally.

 

Would a 200 mm version be worth making.

We have an old 200 mm F8 OTA lying around at the astro club which should be re aluminized before it can be used again. ON top of that everyone now uses either refractors or larger reflectors for their observations or photography.

That is why I tought of turning this into an uncaoated miror solar OTA in stead of having it end up as scrap. But indeed the the question remains: is a 200 mm worth the effort (apart from the pleasure of ATM'ing something unusual)

Hi Helen,

 

This entire depends on your seeing conditions. A 200mm would need around 0.4~0.6 arc-second seeing moments to have higher resolution than a 150mm at 0.7~0.9 arc-seconds (otherwise the difference is just image scale really, seeing is the great limit to resolution). Either way, you need sub-arc-second seeing to benefit this apertures in longer wavelengths, even better seeing needed for shorter wavelengths like continuum. The other issue that is rarely brought up is that these instruments (newtonians, etc) need collimation to be very good if attempting to image at critical sampling, just like one would need for any solar system imaging. And the same issues with thermal acclimation applies with these thicker mirrors, they will take more time to acclimate or focus will shift as the temperature changes through a session so it takes a lot more preparation to use this at a high level than other options. Something to consider, you can always mask the aperture down as needed to image to the actual seeing each session. So it's not like you're stuck at 200mm if you did this.

 

The examples of people using 200mm apertures (or more) in solar anything all have one thing in common: sub arc-second seeing conditions.

 

If your passion is not high resolution solar and you don't have sub-arc-second seeing often enough to even attempt it really then I would advise that you stick with refractors (up to 150mm aperture) and a herschel wedge instead. No collimation woes. Thermal acclimation is way less of an issue and much more rapid. Much better potential for the focuser and imaging train with less limits compared to that found on a newtonian. Even then, a 150mm aperture refractor produces high resolution solar images under excellent seeing (0.7~0.9 arc-seconds needed).

 

If you just like to experiment, tinker and build things to have proof of concept for fun, by all means, do it all!

 

Very best,

Edited by MalVeauX, 28 January 2023 - 10:41 AM.

[Biff]

Hi Ryan,

 

Sounds like it would work and be safe.

 

Most designs would use an uncoated primary and an aluminized secondary, which would effectively give you the same brightness as a Herschel wedge. You'd then similarly use a ND3.0, UV/IR block, and a couple of ancillary ND filters like a 0.9 and/or a Baader Continuum, polarizer filter, etc.

 

You also might want to check out the details of Christian Viladrich's 12 inch solar Newtonian.

 

But at ~ 100 mm of aperture, any good quality refractor and Herschel wedge would be more preferable (no secondary obstruction or spider diffraction, etc.)

Thanks for the reply! I'll have to read that link in more detail but there's some fantastic info in there. I didn't think about the thermal properties of the glass WRT wavelength. I have a few 200mm quartz blanks here that were orginally planned for making a reference flat. That was before reading about Ed J's breakdown of the Releigh water test, so I might be able to part with one for a solar scope. He has his Chief designs for corrected off axis scopes, maybe I'll have to hit him up to do a design. Still gives me 2 uncoated reflections to lose light. 

[brent1123]

Perhaps a naïve question for this topic, but if an uncoated Newtonian is only good for White Light (and perhaps CaK), why bother at all since full aperture White Light filters are so cheap? If the end result is more or less the same - safe reduction of light intensity, IR/UV cut off, then what is the practical difference?