Building a Solar Projector

All of the excitement surrounding 2017 solar eclipse was the catalyst I needed to motivate me to build my solar projector. I had always thought that it would make a neat project, but the up-coming eclipse gave me a reason to continue. However, after the eclipse was over, I discovered how much fun it was to view sunspots, so my enthusiasm for the project continued.

The basis for the design was an old 20x to 30x adjustable power spotting scope that had been languishing on a shelf in the basement for well over 20 years. My first step in the design was to run a quick check to see if the telescope was even a suitable candidate for the project.

I mounted the little scope on my tripod, pointed it at the Sun and checked to see if the light would be concentrated enough to burn anything. I very gingerly placed my hand in front of the eyepiece to check for heat. Apparently, the 50mm objective lens was small enough that even in the bright July sunlight the image focused on my hand wasn’t even warm. I held a tablet of paper up and found that I could produce a very nice 1.5” diameter image of the sun, at about 18 inches. It sounded like the idea was going to work…so off to the shop!

I wanted to come up with a structure that was easy to build and be modified easily as I refined the design. I chose to use ¼”plywood for the front and rear panels and use ¼”-20 “All-Thread” to connect them. It made a cheap, lightweight structure. What I did not account for was the incredible amount of instability that this kind of construction would have. Even the slightest breeze could cause the rear panel to wiggle and twist making viewing difficult…so, back-to-the-shop!

I added a “lower shelf” (visible in these photos) to support the rods about half way back and provide a more solid, tripod mount for the projector. Doing so made the rear panel much less likely to “twist. I realized that adding cross-bracing on the rear panel held it flat and added some additional rigidity.

I added a rotating lens cover, partially to keep the lens clean when the projector was not in use but mostly because I didn’t know if heating “might” eventually occur if the scope remained pointed at the Sun for the entire 4+ hours of the eclipse event. During the eclipse, I frequently touched the body and eyepiece of the scope to see if I could detect any heating. There was effectively none.

You can see in this photo how large (about 1.25" in diameter) and sharp the image was. Notice the contrast of the image against the paper "screen".

Here is one of the pictures that I did take during the eclipse. It demonstrates how good the image was on that day. There was, I believe, a total of 5 visible sunspots that day. This photo shows just three of them reemerging as the Moon’s shadow began to recede. I was “sort of pleased” with my design, but I could see that it could still be better…so, back to the shop.

Long after the eclipse was over, I continued using my solar projector to look at sunspots. We seem to have had a bumper crop this year. I decided to try increasing the image size by increasing the projection distance. By extending the overall length of the device to 24”, I was able to increase the image to just a bit over 4”.

One problem that the first design had was that it was very “rear-end-heavy”. If, once you centered the unit on the Sun, you didn’t tighten the grip on the tripod tightly enough; the unit would slowly sag to the rear, causing the sun to slowly “slide off” the top of the screen. I addressed the problem of the unit being “rear-heavy” on the new unit by adding two 1x2 rails running from front to back. I experimentally found a point where I could place the tripod mount to pretty much eliminate that problem.

I should have realized this earlier, but by spreading the incoming light over an area about 3 times bigger in diameter made the image about 9 times dimmer! The poor contrast due to the light coming in from the sides made it more difficult to pick out the sunspots. (If you compare the photos of the two versions, you can actually see how much less contrast there was with the second version). I purchased some black felt to drape over the unit to act as a sun shield. That helped the contrast issue, but its weight through off the “balance point” causing the annoying “sag” to return. And it that wasn’t bad enough, the wind would flip the draped cloth around making it sort of hassle to get it positioned for the desired shade.

And, with the eyepiece positioned along the unit’s center-line, I could not get a good angle for taking photos. Clearly, this still wasn’t the best solution...so, back to the shop!

I found a number of solar projector designs that projected the image onto a “screen” so that it could be viewed from the rear. If I could make that work, I could, perhaps, finally take pictures without having to deal with interference from that pesky eyepiece. I found the cost of “professionally-produced” ground glass screen to be completely out of the question, but I did find a video about making your own ground-glass slides[1]. It “seemed” like it would be easy, but after a number of hours spent hand-grinding a piece of window glass, all that I had gained was an appreciation of why the professionally ground glass was so expensive!

Then I tried a different tack. I asked a friend with a sand-blasting machine to see if he could create something “like ground-glass”. The resulting finish on the glass was much too coarse. The image was too fuzzy and “sparkly” but that the idea may not have been as half-baked as it may seem. It “might” have worked if he had used a finer grit sand and had a more controlled method for spraying.

Then I ran across an article where the writer used rear-projection screen material[2]. The article calls out a good source for the rear-projection screen material: Carl’s Rear ProjecKon Film”[3]. If you know what you want, they can supply a piece of any of their material cut to any size you want…at a reasonable price.

I had no way of knowing which material would be the best for my application but I knew I didn’t need a very big piece (~8” square). As it turns out, at that site, you can also buy a “sample pack” of about a half dozen 8½ x 11 inch sheets of different types of films for about $4 plus shipping. I chose the most transparent one in the sample kit (the others seemed way too dense). The material is mounted so that the image is projected on the “dull finish” side and is viewed from the “shiny” side.

I stretched my rear projection material on a 7” embroidery hoop. It is very simple to do and the clamp holds it nice and tight. I then just trimmed off the excess.

[1] See: http://roaring20sevent.com/wpe/downloads/ground%20glass%20v1.pdf

[2] see: https://www.astrosociety.org/tov/Build_a_Sun_Funnel2.pdf

[3] See: http://amzn.to/1SsLTRN.)

Now that I had a suitable screen to project the image on, I decided to design and build a new solar projector around it with enclosed body (to help with the glare situation) and one that used a mirror to bend the light path to shorten the projector’s overall length and to make observation a bit more comfortable. Schematically, this was what I had in mind:

I build the main enclosure out of ¼” plywood. The material is pretty cheap in 2’x2’ or 2’x4’ pieces at the big box stores. If you pick and chose from what you find there, you can score some really nice, clear material and is also flat (that greatly improves the finished product). But, DO look over what is there, because you’ll probably want your solar projector to look as nice as possible and some of it can be pretty warped and “rough”.

I used pieces of ½” square dowels to join the seams together. That worked pretty well, but if I had that to do to over I would probably use ¾” square dowels instead. They are a little heavier, but they would be a bit more forgiving from the standpoint of getting things lined up and getting holes drilled. I used 1” drywall screws (I already had a big box of them) to hold things together. Unfortunately, these screws were “just” long enough to have the tips pop through the dowels on the inside. From a construction standpoint it really doesn’t matter, they still hold well, but every single time I grabbed the enclosure by and edge to move it my fingers managed to find one of those nasty little points…and trust me, they are really SHARP! Note to the wise: use the thicker dowels or shorter screws!

Here’s a shot of the right side of the unit. Notice, that I left an access hole for scope adjustment. I don’t know yet if leaving this opening for light to get in will be a problem or not. If it does become a problem, there is enough clearance in the mount to clear a thin piece of “something” to cover opening. I discovered (after the fact) that the opening turned out to be “OK” but not “quite” big enough to get my hand though in certain positions where the cradle covers part of the opening…not sure just now how to correct this after the fact without a major effort.

The mount pivots had not yet been drilled for or installed at this point when this picture was taken. The left side is just the mirror image without the hole.

This is the telescope mounting bracket and the front face of the projector. Notice the scope cradle made of heavier, ½” plywood to provide the highest possible stability. The rib and the dowel pin to help align and hold the scope into the correct position. I use a 1/4" x 20 bolt attached to a wooden knob from beneath to lock down the scope.

You will note on the schematic diagram the approximate position of the mounting pivot. It is located approximately at the center of gravity (CG) to minimize the tendency for the projector to sag once set in a particular orientation...something I learned was VERY important!

To locate the CG, I assembled the unit, including the mirror and telescope and sat it (in the same orientation as shown above) on a short length of dowel rod. I then rolled the box back and forth until it “balanced”. I marked that point. I then flipped the enclosure on its back (scope pointing up) and repeated the process. Where “vertical lines” from each of the two points intersect is the approximate CG. I transferred the point to the other side and drill a ¼” diameter hole at that point on both side, (see below).

I made the two pivot/mounting screws by sandwiching a 2” carriage bolt between 2 circular pieces of ¼” plywood. The finished screw assembly is glued on the inside of the enclosure and an additional circle on the outside of the enclosure to provide clearance between the projector and the cradle. I made two “Hand knobs”, using a similar technique only this time I sandwiched a ¼”x20 nut between the two plywood disks. I took the time to carefully sand these and given them a couple of coats of varnish so they were nice and smooth.

Here is a view looking down into the viewing port without the Projection Screen in place. You can see the 5” mirror sitting at a 45° angle at the bottom. I’m sure that if I was really into perfection, I would have used an expensive “front surface mirror”. But the light levels are really high here, so I’m not sure that is necessary. But the real determining factor for me was its $0.99 price tag at the craft store! I did pick through what was there and picked one the best one.

The flat plate with the central circular hole is where the projection screen will be mounted.

Looking ahead and trying to foresee any alignment issues, I did make the provision for making adjustments to the mirror orientation. The mirror is attached to a sub-plate using some small nylon clips (see photo). In turn, the sub-plate has three ¼” holes that line up with 3 similarly positioned holes in the enclosure. In the event that the mirror’s position/orientation is determined to be off, I can replace the three mounting bolts with the typical 3 bolt/3 spring arrangement that will permit me to adjust the mirror in place. So far, it appears that the solid mount is “close enough”.

I made four little notched blocks to hold the projection screen in place. The notches on the end were to lock into the dowels in the corners. I later cut them off to permit them to rotate. Not having to remove them completely greatly facilitates the removal of the screen for cleaning.

You can just make them out (they re painted flat black like the rest of the interior) in this photo I took during “first light”. You can judge just how large the image is in this 7" diameter screen. Regrettably, the Sun decided not to provide any sunspots for this inaugural photo.

The cradle for the solar projector is made primarily from ¼” plywood, but has a ½” plywood base and a couple of pieces of 2x2 fashioned to add stiffness to the vertical members. I also inset two ¼” plywood stiffeners into the vertical members and the 2x2’s. You can see the inset seam on the far side.

For now, I anticipate that I will just leave the mount as an El-Azimuth mount. But when I do get around to building a stand for the projector, I plan to eventually include a “~40˚ wedge” that will convert it into a Polar mount. Then, in 2024 when the next eclipse occurs (this time just a few hundred miles away from here, going right through the middle of Ohio), tracking the sun will be much easier because movement will only be required in 1 axis.

Here is the photo of the final design.

The lens cover is based on an iris mechanism by Carl Bass on Instructables, https://www.instructables.com/id/mechanical-iris-1

Should you decide to build the iris mechanism for either a solar projector like this or just because you think it is neat (that's really why I built it!) don't feel that you have to have the pieces cutout with a CNC machine. There is nothing wrong with doing that, but I just printed out the design on paper, glued it to some "good" 1/4" plywood and cut it out with a scroll saw. CNC is obviously easier, but the scroll saw did a good job...it just took longer.