9 replies to this topic

[AnthonyQ]

I have an old Jason 60/900 EQ refractor that came with the solar projection screen that is a bit scratched and scuffed.

 

Is there a recommended or preferred flat white paint that folks use to refinish the screen surface, or any old flat white Krylon oughta do it?

 

Thanks for any assistance!

 

-Anthony

[photomagica]

Any flat white will work really well. Krylon is definitely a superior spray coating compared to others I have used, so is a good choice.

Bill

[Couder]

I have used the flat white, gloss white, roll up projector screen, and the side of my barn. Quality varies, but I'm guessing mainly what you're after is to see what the Sun looks like at that time. Attached are some pictures with explanations:

MVC-002 is on the white screen like you have with gloss white

HPIM3231 is on projector (35MM slide) screen

20170821_133452 s is on white poster board, you can see where I pinned to the woodwork

 

Right now I can't find other pictures, but you can see - even with being slightly out of focus on some - the Sun shows up good on any of the surfaces.

 

 

Attached Thumbnails

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[TOMDEY]

Theoretically, Lambertian White would be preferred. The best in that category is Eastman White Reflectance Coating, which is used to coat the inside of Integrating Spheres. In actual practice, any decent white matte paint will do. A good ceiling paint will work just fine. Even a piece of premium matte print paper is right up there in performance. I actually measured all sorts of flat white and flat black paints decades ago for reflectivity and BRDF (Bidirectional Reflectance Distribution Function). Then I wrote a "White Paper" (Heh!) on the results, and presented that at astronomy club meetings around the Great Lakes Region.

 

If you really want to go ultra-deluxe there is this phenomenal material called "Single-Surface Fire-Flashed Opal Glass". It's near impossible to find, but it is essentially window glass that has been treated on only one side to serve as a premium white rear projection screen. The advantage to this is that you are behind looking straight at it, rather than off to the side looking down at it. Problem is finding it. If you search "Opal Glass" the only things that come up are cheap imitation white plastic that is milky throughout and entirely useless as a projection surface. I discovered the real stuff at the Bausch & Lomb Optical Labs, back in the 1960s. It has to be single surface treated or it is worthless.   Tom

Edited by TOMDEY, 04 April 2021 - 03:24 PM.

[Michael Covington]

Yes, whiteness is not critical (the sun is bright!) but you want a good matte surface.

[PirateMike]

Single-Surface Fire-Flashed Opal Glass

Tom

Is this the right stuff?

 

https://www.uqgoptic...windows-plates/

 

 

Miguel   8-)

 

.

Edited by PirateMike, 04 April 2021 - 03:44 PM.

[TOMDEY]

Is this the right stuff?

https://www.uqgoptic...windows-plates/

Miguel   8-)

WOW! Thanx for finding that. Their description sure sounds like the stuff that I (think I still) have. Their tech specs seem peculiar though. That the treated layer is >>>

 

Flashed Opal Layer Thickness:
0.45mm (+0.35mm /-0.20mm)

 

Which seems crazy thick and crazy variable to me. I sure recall it being much thinner, but could be mistaken. Hopefully, I can find my stash out in the barn and measure it somehow optically. I thought the thickness was like a handful of microns... but maybe I'm just a dope. Or maybe B&L ordered ~select thin~ or even made their own?! I'll research that some more and then also call them tomorrow. Well, their description that the bulk sheet simply is not produced anymore is certainly consistent with my recollection.

 

This is just my personality --- I'm suddenly panicked that I may not be able to find my stash of Single Surface Fire-Flashed Opal Glass, or that I won't ever be able to get more "Just In Case". In case what, I haven't the slightest idea, but I won't be able to sleep until I know we have that material in a box under the bed.

 

No wonder my employers treated me well --- I was probably the only optical scientist who obsessed over optics to the point of being a mental case... quite useful in the Research Labs. Igor was my best friend.        Tom

[photomagica]

About solar rear projection - after working with solar projection in both front projection (painted surface) and rear projection (ground glass and special rear projection screen material) I would recommend front projection. Regardless of the quality of the rear projection material it gives an image with lower apparent resolution and lower illumination. In one case I replaced the small, roughly 10" diameter rear projected solar image in a museum - an image that appeared dim and a bit fuzzy - with a front projected image from the same telescope that was 6 feet across. The same coelostat-fed 6" refracting telescope fed the image by eyepiece projection in each case. The difference was so dramatic people could not believe it was the same telescope. The large 6' image was incredibly detailed in good seeing and one could just tack a piece of paper to the screen and sketch a sunspot by tracing.

 

In the museum, the front projection screen was painted with matte ceiling-while latex. We washed it occasionally and repainted every year or two. On a smaller screen I'd recommend a matte white spray paint to best preserve fine detail. I have used smooth, matte white board from an art supply store very successfully.

 

In rear projection a very fine Fresnel lens may be combined with the screen in order to enhance the brightness. This was done in rear projection televisions and is also used to enhance screen brightness in view cameras. In general, it also degrades fine detail unless the lens is made with incredible precision, as are the small Fresnel lenses for SLR viewing systems.

 

Griffith Observatory has a rear projected solar image that is perhaps a foot or so across and I've always considered it highly disappointing in the level of detail. I would not recommend solar rear projection.

Bill

[Terra Nova]

I agree with both Tom and Bill. Having done rear projection onto a ground glass focusing screen from an old large format plate film camera at various times over the past fifty-odd years of both the sun and the moon, I have always found it to be a fun and interesting exercise. Not one that produces superlative results in terms of resolution and detail, but still, one that produces what I have always found to be a most interesting alternative view. There is something cool and different about it, and it’s particularly fun to use to show other people.

[AnthonyQ]

Theoretically, Lambertian White would be preferred. The best in that category is Eastman White Reflectance Coating, which is used to coat the inside of Integrating Spheres. In actual practice, any decent white matte paint will do. A good ceiling paint will work just fine. Even a piece of premium matte print paper is right up there in performance. I actually measured all sorts of flat white and flat black paints decades ago for reflectivity and BRDF (Bidirectional Reflectance Distribution Function). Then I wrote a "White Paper" (Heh!) on the results, and presented that at astronomy club meetings around the Great Lakes Region.

 

If you really want to go ultra-deluxe there is this phenomenal material called "Single-Surface Fire-Flashed Opal Glass". It's near impossible to find, but it is essentially window glass that has been treated on only one side to serve as a premium white rear projection screen. The advantage to this is that you are behind looking straight at it, rather than off to the side looking down at it. Problem is finding it. If you search "Opal Glass" the only things that come up are cheap imitation white plastic that is milky throughout and entirely useless as a projection surface. I discovered the real stuff at the Bausch & Lomb Optical Labs, back in the 1960s. It has to be single surface treated or it is worthless.   Tom

“White paper”.

 

Nice!