Questions about a pinhole projectors

[AutoBoof]

Two questions.

1. What is the best pinhole projector method...I remember hearing phil talking about a makeup mirror design?

2. Can you use a pinhole projector on the moon?

[Jeff Root]

A video that Phil linked to showed a guy using a standard
(diameter 4 inches or so) makeup mirror to reflect an image
onto the side of his house.

I only have a dial-up connection, so downloading the whole
video would have taken more than an hour. I quit after it
was half done. I never learned whether he used the flat
side or the magnifying side of the mirror.

He covered most of the surface of the mirror with aluminum
foil. I didn't watch long enough to get a good look at the
opening, but I expect that it was circular. I have to presume
that the reason for reducing the aperture was to increase
the sharpness of the image.

But the whole thing makes no sense to me.

If he was using the flat side of the mirror, the opening in
the foil (the exposed area of the mirror) was way larger
than a pinhole. It would not make a sharp image. If he
had used a much smaller opening, it would be *very* dim
at the distance he was projecting (20 to 30 feet). With
a large opening, the reflected image would be the shape
of the opening, not the shape of the Sun. The image he
got on the side of his house would be round in this case
simply because the shape of the hole in the aluminum
was round.

If he was using the magnifying side, it isn't obvious to
me why he was covering part of the mirror. The whole
surface should be useable. Most likely, the low quality
of the cheap mirror means that reducing the area used
can improve the image quality significantly. The old
makeup mirror I have does not project a good image
of the Sun when using the whole surface. I tried it
yesterday while downloading the video, but I didn't
try covering part of the mirror. I'm sure that would
have improved the image.

In any case, the magnifying mirror has a specific focal
length. The "real image" (that's a technical term in
optics that you can look up) of the Sun is in focus at a
specific distance from the mirror. For my magnifying
makeup mirror, that distance is about 2 feet. The guy
in the video was projecting the image 20-30 feet. No
way was that an ordinary magnifying makeup mirror.
He must have been using the flat side, in which case
it seems unlikely that the bright circle he was getting
was an image of the Sun rather than the shape of
the reflective area.

Also, aluminum foil is easy to attach to the mirror --
just wrap it around -- but it reflects light, too. The
light reflected by the foil reduces the contrast in the
image. (Though at the distance in the video, that is
probably negligible compared to all the light from the
environment.) I'd recommend a piece of paper or
cardboard instead.

If doing a pinhole projection, there is an optimal size
for the pinhole, and it is pretty small. You should be
able to find instructions for making a decent pinhole
on the Internet. "Optimal" means "sharpest image".
A hole larger than optimal will give a brighter but
fuzzier image. The farther your screen is from the
pinhole, the larger but dimmer the image. Get your
eyes closer to the screen than the pinhole for the
best view. There is no focal distance in pinhole
projection, although intermediate distances work
much better than very close or very far.

I can use a pinhole projector on the Moon, but the
cost of transporting pinholes to the Moon has... oh...
you mean... Yes, I can project an image of the Moon.
I did it just now. Full Moon. I probably used too small
a pinhole, and I couldn't get away from the streetlights,
and didn't try to make an enclosure to block their light,
so all I could see was a faint, very, very, very tiny dot,
not worth much. The streetlights made better images.

-- Jeff, in Minneapolis

[captain salty]

There is a mirror projection technique here http://www2.eng.cam.ac.uk/~hemh/transit.htm . Uses a 5mm x 5mm section of a mirror, projected 30m into a dark room to get a 30cm diameter image of the sun.

[Jeff Root]

That 5 mm square opening for the mirror "pinhole" is
larger than I thought would work and smaller than the
opening I thought was on the mirror in the video.

I thought the opening had to be larger in part because
the mirror was at a large angle to the Sun, so the foil
used to mask it would make quite a shadow on a rear-
surface mirror, since the foil would stand off from the
mirrored surface by at least the thickness of the glass.

Farther down the page, it appears that the Astronomer
Royal Sir Martin Rees has an unusual built-in collapsing
height adjustment mechanism for reaching the eyepiece.

-- Jeff, in Minneapolis

[RickJ]

For a solar eclipse I rigged a 3/4" secondary on top of the building I was in to cast a reflection onto the side of the shaded side of a white building 250 feet away. It made a huge image of the sun hundreds could see at one time. I put it high enough on the wall no one blocked the view either by their shadow or just being in front of others as all had to look up to about the second story level to see it. Not stopped down other than by the angle needed to hit the building. Your resolution is limited to about the size of the pinhole or reflecting surface. Smaller increases resolution though too small adds diffraction that lowers it. If the "screen" is highly reflective and shielded from ambient light so the image is large like it was on the side of the building then even large sunspots were seen with that 3/4" first surface mirror. Due to the angle needed the sun saw about a 1/2" mirror however.

First surface mirrots will be better than ordinary second surface ones as the glass will also cast a reflection lowering contrast some. Don't use a magnifying mirror. Only a flat one!

Venus requires better resolution than an eclipse. If the screen is reflective and well shielded some combination of distance and hole diameter should give usable results. Might take some experimentation on the fly to find the right combination. The shorter the distance from mirror to screen the smaller the hole. Also the closer the viewer to the screen the smaller the hole needs to be as well. Those 75' from the wall, where the walkway ran, had the best, apparently in focus, view. Getting too close is like enlarging a picture too much. Though it wasn't bad from my roof top perch 250' feet away.

Rick