1st April, 2014.
Today was the first really nice day we've had since November. The temperatures weren't terribly high still, just 49° F (9° C), but in the sunlight, it feels so much better than the winter we've had. It was also remarkably clear today, with that wonderful Sun shining like mad up there in that sapphire sky.
In other words, it was a perfect day for solar astronomy and a little experimentation.
Back in early fall of 2013, I took a small chunk of broken first surface mirror I had and attached it to a piece of cardboard, and played around with solar pinhole mirror projection. This is the opposite of regular pinhole projection, since there is no hole, instead just a very small reflective source. My test last year weren't perfect, but they showed that yes, a solar disk could be projected. The problem was that I was projecting the image onto a screen outside, where it was too bright.
For today's experiment, I chose to cast the reflection into the condominium, and with much better results.
The mirror and its base were taped onto the head of one of my tripods, in a manner that allowed it to be easily aimed (much like a camera, in fact).
The tripod was then carried outside onto the back porch. It was shortly before noon, and the Sun was fast approaching the meridian. It took a little work to get it aimed. I chose a spot on the dining room wall, next to the large decorative clock that hangs there. This allowed for an image that was a little over 5" (125 mm) across). This was perhaps 25 feet (8 meters) from the mirror.
It worked, though the resulting image was not as sharp as I would have liked.
This is the trade off. You can choose sharpness over contrast, making the pinhole (or in this case the reflective surface) smaller. The resulting image would be darker, however. For today's experiment, I chose contrast over detail.
Amazingly, after the images were pulled and processed, I was surprised to discover that the largest patch of sunspots on the main disk, group 2021, was faintly visible in the image. A little sharpening, and you can sort of pick them out.
|In this projection of the Sun, north is to the left.|
|The Sun, 1st April 2014, courtesy Spaceweather.com|
Certainly, the method can be improved, but it is severely limited. You will never have the acuity that a lens system will provide. As a safer method for observing things like eclipses, transits and larger sunspots, this method should be sufficient.
I do wonder to what extreme it could be taken. Maybe that will be a "project" for the future.