Any telescope makers out there?

[Roger Feeley]

In my new house, the rising winter sun projects from an upstairs window on to a big blank wall. The distance is about 48'. I think it would be cool to build a little obscura lens and project a detailed image of the sun. I would like it to be about 3' in diameter which would be much bigger than a simple pinhole.

Can anyone tell me what sort of lens I would want and where I might find one for cheap. My idea is to just take a sheet of plywood and make a primitive lensboard that would sit inside the window. They we can sit and admire our heliostat.

[Lee Schierer]

I think you might have problems concentrating the sunlight into anything but a brighter spot on the wall without blocking off the rest of the window.

[Dave Garrett]

reminds me of the time i took apart a projection tv and leaned the fresnell lens up against the window without thinking and 30 minutes later smelled smoke and nearly had a house-fire! that's one way to appreciate our all-powerful sun!
I agree with Lee, you'll have to block out everything else to have a focused image.

[Mark Bolton]

Additionally focal length is going to change throuout the day (distance between optic and focal plane (wall) )

[Wade Lippman]

Additionally focal length is going to change throuout the day (distance between optic and focal plane (wall) )

Unless it is a very big lens, the distance isn't going to change enough to matter. A 5" lens with 48' focal length is a f120.

Roger, you need a lens that has a focal length equal to the distance from the lens to the wall; 48'. Good luck finding anything like that. I don't know what the diameter would be, but I expect it will much more than 3'.
The bigger the lens the brighter the image. It won't be terribly bright because it will be so big.

[John K Jordan]

In my new house, the rising winter sun projects from an upstairs window on to a big blank wall. The distance is about 48'. I think it would be cool to build a little obscura lens and project a detailed image of the sun. I would like it to be about 3' in diameter which would be much bigger than a simple pinhole.

Can anyone tell me what sort of lens I would want and where I might find one for cheap. My idea is to just take a sheet of plywood and make a primitive lensboard that would sit inside the window. They we can sit and admire our heliostat.

Roger,

Caveat: I am but an amateur astronomer and not an optics designer. However, I have projected sun images many times to show eclipses of the sun to school children. Sorry, I haven't used a simple lens - all my experience is with by projecting with telescopes. Although I have a good solar filter for my 8" telescope, it is easier (and safer) to project an image of the sun into a cardboard box with a piece of white paper taped inside. That way, more than one kid could see it at a time. Just aim a telescope or binoculars at the sun and project the image onto some white surface. I'm not sure how to do the calculations, but it would be easy enough to try some optics and see how large an image they project at a given magnification and for your distance. For the cardboard box, I use a small telescope with a right angle viewfinder (don't remember the magnification) and generally get a sun image of perhaps 6" diameter at a distance of a foot or two. (If I'm remembering this correctly)

Some issues:

The earth is moving very fast so any fixed compound projection lens will only display the sun image for a very short time at the size you mentioned. A simple lens such as you mentioned would need only crude alignment but I don't know if it would provide a suitable image. I use a motor drive on my telescope and without it I have to constantly move the scope manually to keep the image of the sun or moon (or anything) in the field of view of the telescope. If the optics are not aligned properly to point exactly at the sun, the image will be vignetted and quickly lost. The ideal system would have a computer driven alignment method. (When I worked at the Lab we actually designed such a system to keep large parabolic solar collection optics pointed exactly at the sun throughout the entire year. There are several ways to go about this - ours was an open loop system that relied on a precision time source and careful calibration. Our prototype mirrored collectors were about 4' in diameter and imaged an incredible and dangerous amount of light energy onto a small area. I can tell you more about the optics and alignment of such systems than you could possibly want to know!)

The brightness will depend largely on the objective lens size. A bright image needs a big lens. I did get an acceptable image, easily viewed in the shade of the cardboard box in the daylight, but I wouldn't call it bright. Results would be much better if you darkened the room and window except for a small opening for your optics, as you planned with your plywood.

I have never tried to project with a simple lens (a single lens instead of optics as in binoculars or telescopes) but it seems it would work but perhaps with serious image degradation (chromatic aberration, etc). Such a simple lens would be easy enough to test - I keep a box of lenses to play with. If a test with a simple lens works OK, then a Fresnel lens would be far cheaper than a solid lens - I've had them 12" in diameter and I remember seeing them advertised much larger. Seems to me the trick would be getting one with the right focal length to get a useful inverted image at the 48" you mentioned. Also, the image quality from a cheap Fresnel lens might be pretty dismal.

And important: DANGER DANGER DANGER! If you have a solar projection system (such as the binocs/telescopes I used) mounted in the room and aimed at the sun and some poor fool puts his eye up to the eyepiece he will be instantly and permanently blinded in that eye. That's exactly what a kid would do. Safer would be mounting the optics very high in the room where no one could get close, or provide a set of mirrors to reflected the sunlight up to the ceiling and then project the image down through the optics to the floor. Or put some sort of protective cone on the ocular side. Even with precautions some enterprising kid might figure out how to get up there and put his eye up to the projecting lens.

Another danger - collecting enough light to project a large image might come with enough heat to start a fire if something flammable somehow got right at the focal point. (In the 60s I made an imaging collector from cardboard and aluminum foil that could boil water.) In your case the early morning sun near the horizon might be dimmed enough by atmospheric filtering to reduce the danger.

A pinhole "lens" is quite safe although as you mentioned will not make a large image nor will the image be very bright.

An interesting aside: the next time there is a solar eclipse in any season but winter and when the sun is well overhead, walk under a fairly small tree with a smooth surface such as a sidewalk beneath. You will see many little solar eclipse projections on the sidewalk! These come from pinholes formed between overlapped leaves in the trees. I've also handheld small binoculars and telescopes during such eclipses and projected the images onto the sidewalk. As mentioned above, for a useful projection the alignment of the optics is critical and not all that easy to do by hand.

Here's an idea: put a small computer-guided telescope in that room, capture the real-time image on a CCD sensor, and pipe the picture to a large screen TV in the den! I know, not the same thing... :)

JKJ

[Wade Lippman]

Seems to me the trick would be getting one with the right focal length to get a useful inverted image at the 48" you mentioned.

JKJ

You think he has a big blank wall 48" from a window? Probably changes your advice a bit.

[John K Jordan]

You think he has a big blank wall 48" from a window? Probably changes your advice a bit.

Yes, thanks Wade,

I did think that - I misread 48' for 48"! That changes the optics but probably makes the need for a very large collecting lens and a dark space even more important. Interesting challenge.

BTW, I know a guy near Albuquerque who pipes sunlight into his house nearly the entire day. He mounted a large mirror on the side of the house directing sunlight to another mirror which directed the light into the house, for dispersion in his case. Move a large mirror would be simpler and cheaper than moving a large lens. It would be fun to try the simple lens idea. If I get time I might mount some lenses on the door to the deck and see what happens. It nicely catches the morning sun rising off the horizon and puts it on the wall maybe 20' from the door. Also shines it right in my face when I'm sitting in my easy chair with coffee and a book!

JKJ

[Wade Lippman]

Yes, thanks Wade,

I did think that - I misread 48' for 48"!

It happens. Just yesterday I read a magazine article that says that Lewis County NY gets 200 to 300 feet of snow a year. Maybe a century...