Solar pinhole camera

Hi folks. I am intrigued with the possibility of building a pinhole camera for taking photos of the sun. Has anyone done this, and if so can you comment?

Here are a few parameters I am thinking about. First, I think the "focal length" should be very long in order to provide a large image on, let us say, a medium format film, or maybe even large format film. Regarding the focal length, we are probably talking about several feet long.

For optimal resolution the diameter of the pinhole should be proportional to the square root of the focal length. This means that there will be a loss of image brightness on the film plane, i.e. the brightness will be inversely proportional to the focal length. I don't think the loss of image brightness is going to be much of a negative issue because the sun is so enormously bright to begin with.

Aiming of the device might be a bit of an issue. I am thinking that one should stack the pinhole camera and a pinhole viewer.

Mounting the camera might be a problem too.

What do you think?

Have not done it with pinhole, but I would guess it is possible; using a pinhole to project the image on a piece of paper is one approach to eclipse viewing. If you're trying to get a round, stationary image of the sun, as opposed to a track of its passing, there could be some mechanical stability problems at high magnification. I shot the total eclipse last August and even a 300mm plus 2x telextender was well short of filling the frame of the 35 mm camera I was using, yet 600mm is essentially a 12x telephoto. In the process of doing that I had to constantly make adjustments as the sun seemed to be really cruising across the sky. There are, of course, clock drive telescope mounts, and these days, all sorts of computer driven technology (but it doesn't come cheap!) My gut sense is with the trade-off between f-stop and diffraction, given an "optimum" pinhole you might need some ND filtration to rein in exposure -- but I could be wrong. My full sun pre-eclipse shots used a special metallized Mylar solar filter through which one can barely see the energized filament of a clear 60 watt bulb. I doubt you will find too little light to be a problem.

If I were trying it, I would likely start with using print paper rather than film for a lower sensitivity, try a shot or two and see how it does. Not sure how big you want to go, or how much money you want to spend, but some of the big box stores carry round fiber tubes of substantial diameter, used for pouring concrete foundation pads. They are not actually all that pricey.

I've tried this for both solar and lunar photographs. First I used about 10 feet of black plastic pipe, and the pinhole was about 0.07 inches. The rear of the camera rested on the ground and the front was positioned so its shadow was centered on the rear. Image quality was poor Pictures of and from this camera are on page 125 of the first (1995) edition of Pinhole Photography: Rediscovering a Historic Technique by Eric Renner. A somewhat shorter camera was fabricated for lunar photographs. Exposure with that was maybe 2 seconds with 35mm ISO 3200 film. A 32 foot long pinhole camera for photographing a solar eclipse on 5x7 film was too cumbersome, so the camera was shortened to 24 feet. The front of the camera was suspended from a telephone pole, and the camera was positioned by moving the rear of the camera until the shadow of the front was centered on the back. Image quality was somewhat better than the little 10 foot long setup. An ordinary leaf shutter provided the short exposure times.

I tried to build a pinhole projector to count sunspots and gave up when I realized there is a fundamental problem. When the projected image of the sun is small, the edges on it look crisp, and you can even see darker areas where groups of sunspots are. But as you increase the "FL", the resolution decreases, so the image of the sun gets bigger and bigger, but the details get fuzzier and fuzzier. I'd advise building a simple projector out of cardboard so that you can see what it looks like and what you'll get, before you go to the trouble of making something lightproof that can be used as a camera.

At the aforementioned eclipse exercise, there was a guy with a "Sunspotter" that appeared to be either homemade by a highly skilled craftsman, or, as it turned out, a commercial product. It projects a 3 inch image on paper with a "folded Keplerian telescope" using a 62mm objective (indeed a lens) and mirrors.
Two iPhone shots thereof, one attempting to show the results. (Standing there I could see several sunspots.)

That appears to be an earlier version than what now appears on their website -- some serious plywood there!

As I speculated earlier, I think there is a bit of conflict with diffraction limits for doing it with pinhole, I seem to recall that is true in general of "telephoto" pinhole cams, the wide angle stuff is much more impressive in terms of sharpness and (apparent) resolution.

Yep, even a very simple lens or the cheapest thrift store binoculars will make a very good projected image of the sun. That one uses a very clever design with mirrors to contain a long focal length into a reasonably small space. That's a neat contraption!

For image size the scale is; for every 100mm of focal length the diameter of the sun (or moon) is .9mm (point nine or 9/10 of a mm) at the focal plane. So this makes it easy to calculate how much focal length to use for a given image size. It works out to a very long distance. The contraption pictured must be using eyepiece projection to get a that size. I have a ancient 60mm 2 element air spaced refractor telescope with a focal length of 700mm and I used a simple 3 element 15mm eyepiece to project a 3 inch image for the August 2017 eclipse. This worked out well and friends and neighbors could view the sun safely. Sunspots were clearly visible.