f10 mirrors?

[Poita (Peter)]

I'm looking to make a white-light solar scope, like this one here:
http://www.considine.net/dgroski/wlnewt/
and need to find an f10 parabolic mirror.

Can anyone point me to a supplier in Australia?
It doesn't need the coatings, so even second hand ones with old mirror coatings would be fine as they would be being stripped off anyway.

Any help appreciated.

[ZeroID (Brent)]

f10... Closest you'd find commercially would probably be a 4'5 (112mm) f8 I'd think. Standard size out of China factories. Easy to strip the silver (alum) off. Wee bit of Caustic Soda diluted ( Drain Cleaner ).
I've setup an old 4" without coatings for solar, quite effective.
Baader N5 Solar film is quicker and cheaper though.

[Poita (Peter)]

Thanks for the info., I was wondering how to remove the coatings safely.

True, the solar film is quicker, but this should result in better optics.
It is only cheaper if you already have a scope though.

The main attraction is though, that there are no filters to be damaged, punctured, accidentally removed or fall off for outreach work.

This will be designed for outreach and maximum safety, and also as an example that DIY is possible for schools etc.

[anj026]

A 6" f8 or 8" f6 stopped down to 120mm would be f10.

[Wavytone]

I made something like this in the 70's, but I used a 4" f/10 mirror tilted off-axis to reflect the image off two glass wedges in an arrangement similar to a 3 mirror schiefspielger (though in my case the two wedges were flat mirrors, not curved). The two wedges were originally prisms extracted from some surplus optical junk I scored in a disposal store.

FWIW a f/10 mirror is effectively spherical at apertures up to and including 25cm, you won't be able to tell the difference.

It takes preferably 4 reflections off plain (uncoated) glass to reduce the brightness. The light from 3 uncoated reflections will still be painfully bright and probably damaging to your eyes.

Regarding the mirrors, to get maximum contrast the glass each mirror is made of needs to be fairly clear and the back surface should be polished clear- put a black matte surface behind it to absorb the light.

The reason for mentioning this is that many cheap 4" mirrors are made from glass which is less than clear, or have a fairly rough ground back that will create a fairly bright background scatter, so your view of the sun will be against a rather bright background of scattered light. An alternative is to grind and polish your own from a thick piece of plate glass and a mirror grinding kit - a 4" f/10 can be done in under 20 hours hard labour.

Then there is the problem of the star diagonal - most are cut from a plane-parallel piece of glass., though the backside won't be optically flat. If you strip the aluminium off the front surface you will get an annoying second reflection from the back surface, which will degrade your image quite badly.

A better solution for the diagonal mirror would be to use the hypotenuse (the long side) of a 45-45-90 degree prism, this way the unwanted 90% will go through the back of the prism or be bounced around through odd angles - you won't get a second image at the focal plane. For a prism, you could cannibalise an old star diagonal or a small pair of binoculars. Some IIS members may even have a spare old one lying around. Another prism would work nicely in place of the Herschel wedge too.


Have fun, but be careful.

[Poita (Peter)]

Thanks for all that great info.
That is why the herschel wedge at the eyepiece end, to dump off the remaining energy and give safe views.

I'm always extra careful with solar projects, everything will be checked multiple times.

[Astro_Bot]

Originally Posted by Poita

The main attraction is though, that there are no filters to be damaged, punctured, accidentally removed or fall off for outreach work.

That's always my biggest fear with solar - if someone flips the ERF off before I can stop them.

[Wavytone]

Originally Posted by Poita

Thanks for all that great info.
That is why the herschel wedge

Won't be enough- it will still be painfully too bright and possibly unsafe.

[Poita (Peter)]

I've seen a few scopes based on this design no-one has reported it being too bright, but I'll be doing more homework before anyone goes near the eyepiece. One can never be too careful when it comes to the Sun.

From the design sheet:

The solution was still to use the Newtonian configuration but to have all the optical surfaces uncoated. For safe viewing, the image of the Sun needs to be reduced by 100,000 to 1 in brightness across the full spectrum. This is an Optical Density of 5 ( LOG(100000) = 5). Bare glass reflects about 5% of the light striking it. This translates into an OD of about 2. The two reflections in a Newtonian would results in an OD of 3.5. Still too bright for comfortable viewing so a third reflection is needed. I used a homemade Herschel wedge to achieve this. I also took advantage of the fact that when light is reflected off of bare glass at an angle it becomes polarized. The closer the angle of reflections gets to the Brewster angle of 57 degrees, the more complete the polarization. With both the diagonal mirror and Herschel Wedge placed at 45 degrees the light becomes crossed polarised and the image brightness can be farther reduced.

[ZeroID (Brent)]

I've got a few binoc prisms downstairs doing nothing if you need. Out of the old 10x50 that I cannibalised to rebuild another pair. Cheap enough to post I'd imagine, just let me know if you want to DIY some bits.

As an offer to anyone playing DIY I get front silvered mirrors from laser ROS units plus various curved glass strip lenses, scanner parts, small lenses, small stepper motors, quite few electronic components and stuff. Let me know (PM or post a thread ) and if I can supply then we can figure something out.
Love DIY, I'm always pulling things apart to see if they have anything usefull inside. .
I have quite a collection of very small ball bearing races which could be useful for crayfords builds or ???

[GrahamL]

I have an 8 " (f8 I think ) ) somewhere you can have for postage If you like Pete, the coatings are almost completely gone so its no good to me for the moment . drop me a pm or two And i'll try and locate it if your innterested.