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How to Make a Flat Frame Panel for Dark Sites (Cheap)
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How to Make a Flat Frame Panel for Dark Sites (Cheap)
by Dean Tullsen
Having recently dived into astrophotography, I have found that most problems I face have been faced before, and have a plethora of solutions. Typically, those fall into a couple of categories – expensive, and cheap do-it-yourself. But backyard astronomers being the type of people they are, they tend to share one endearing characteristic – heavily over-engineered for the problem at hand.
But recently, I took my first trip to a shared dark site, owned by the local (San Diego) Astronomy Association. I was quite nervous about doing something terribly inappropriate on my first visit. As an astrophotographer, I had already worked through the curve of taking light frames, then realizing I needed dark frames, then flat frames, etc. So one of my questions before my first shared dark site visit was “how do you take flat frames at a dark site?” My current flat frame solution uses the laptop I’m already using, loading up a powerpoint presentation with one solid color slide, going back to APT to start the flat frame sequence with a built in 20-second delay, back to powerpoint, carefully flip the laptop over (trying not to touch the screen, or flip in the wrong direction so that the presentation changes orientation, or lose any of the cables attached because the laptop is still controlling the camera, all in the dark), carefully balance the screen on top of the scope pointed straight up, and hold it there while the flat frames are taken. This technique has a number of issues, but here are two. If there is any moisture at all, the laptop tends to slide all over, even with one hand holding it on – it’s inevitable I’m going to drop it someday. Second, it flashes the bright powerpoint screen all around for everyone else to see.
Solutions for light frames abound, but they tend to solve the first problem (and that only partially, I’d still worry about dropping some expensive equipment that is balanced on the scope) and not the second – they are not light-protected. Searching more carefully for my specific problem, the advice I find is this: (1) wait until morning and use the morning sky, or (2) keep your camera attached to the scope until you get home so you can take a properly aligned flat frame. Both solutions fail to satisfy for a few reasons, one of which is this – they do not feature my favorite characteristic of astronomy equipment – heavy over-engineering.
So my goal, before my second trip to a shared astronomy site, is to create a flat frame panel with the following characteristics.
-it should be light and compact
-it should leak no light when turned on
-it should fit tightly to my telescope, not requiring me to hold it on nor requiring the telescope to be pointed vertical.
-it should be very cheap (because I find cheap solutions so much more gratifying).
The scope I am targeting is a Skywatcher 80ED. It has a dew shield with an outer diameter that is a surprisingly wide 115 mm. Obviously, designs for other scopes will need to be adjusted accordingly. Most will require small changes, until you get to large apertures for which you cannot find a large enough electroluminescent panel. The materials for my design are the following.
-an electroluminescent (EL) panel with battery pack (just over $20 from Chinese bargain outlets)
-2 9x12 EVA foam sheets (about $5 each from Walmart)
-1/4 inch plywood, less than 2 square feet (free, from the scrap wood heap in my garage, or under $10 from Home Depot)
A word about each of these.
Electroluminescent panels come in many shapes and sizes, but mostly come in stock paper sizes (e.g., A5, A6 , …). I did see some 4in x 4in but that wasn’t big enough, nor was the A6 (105 mm x 148 mm), so I had to get the A5 (148 x 210). The descriptions all claim that these can be cut to size if you know what you’re doing, but because I went ultra cheap and waited 2 weeks to get mine from China, I chose not to risk it and adapted my design to the full size. You could certainly make one more compact than I did if you’re willing to cut – but notice that the power elements do place a limit on the size reduction. Make sure you order something that comes with a battery pack. You don’t want to have to deal with the non-standard power connector these come with, and you certainly don’t want to have to plug this in – if you’re doing astrophotography, you’ve already got a dozen cables running everywhere, and you don’t need another one.
For the foam, you want something pliable but firm and strong, and something that will not continue to shed foam particles after it is cut. EVA foam is a form of closed cell foam, which is probably what you want. The ones I bought were supposed to be 6mm thick, but I don’t think they were. You could go thicker, and have a longer nose on the design, but mine works perfectly as it is.
I just used the plywood I had available. Any wood will do. I would have been tempted to try and go thinner if I was buying new wood for this, to minimize weight, but in retrospect I’m glad I didn’t – that is because thinner wood would have been harder to work with, and in the end I find that most of the weight is from the battery pack, not the wood. If you do go out and get new wood, you can get a small piece of Baltic Birch plywood, even from Amazon. That is a high quality piece of plywood. Stronger than other plywoods and lighter and stronger than other natural woods.
Step 1 – Cut the main wood piece with a .5 to .75 inch margin (over the size of the EL panel) on all 4 sides. The margin on 3 sides is to have room to glue. For the fourth side, the extra space has two functions. First, it allows you to tuck the delicate connection between the EL panel and the power cable under the foam for stability. Second, it allows you to push the EL panel in further to prevent light leakage. But you will not glue the fourth side, because you’d like to be able to replace the EL panel if you need to, or tuck in an extra piece of paper to reduce the intensity, etc.
Step 2 – Cut the first foam sheet to the exact dimensions of the wood piece. It turns out you can use a table saw to cut the foam sheets.
Step 3 – Cut the other foam sheet in half across the middle so as to make 2 identical 9 x 6 inch foam sheets. These will be stacked with the first foam sheet to create a 3-deep foam nose that attaches tightly to the telescope dew shield. If your telescope diameter is larger than 6 inches, meaning you can’t cut this piece in half, then you probably want to buy 3 foam sheets, so you’re still 3 deep on the foam sheets (or get thicker foam sheets).
Aside – I had no idea what kind of adhesive to use to attach foam to wood or foam to foam. A few quick experiments with scraps from the wood/foam cutting steps compared the two options available in my garage – gorilla glue (super glue) and wood glue. The gorilla glue worked great. The wood glue did not. Another big advantage of the super glue is that you can apply a very thin bead, while the wood glue soaks into the foam and spreads way out, which would be a big problem in the last step.
Step 4 – Line up the two matching foam pieces, center them on the larger foam piece, then glue them all together. Just a bead around the outlines of the smaller pieces is sufficient. You’re going to cut out the center, so no need for glue there, anyway.
Step 5 – Now the most difficult part, cutting a circle. Lacking any helpful special equipment (a hole saw the right size, or even a band saw), my weapon of choice was an Xacto knife. I used the dust cover from my telescope as the guide. This only gave me the inner dimension of the dew shield, not the outer dimension, but calipers indicated that when I traced it with a pencil, the outside edge of the pencil marks were about 115 mm. So I went with that. I then cut the circle carefully, making sure I had something between the foam and my knees – it turns out my glue experiments yielded a piece of wood with foam attached – the perfect backstop for this step.
I started with small cuts and went round and round many times, removing layers as they came free. I found if I tried to cut deeply initially, given the triangular shape of my xacto knife, then I was making a circle from a series of long lines, while shallow cuts gave me a circle made with short lines – much better. When I was done, I was not particularly happy – it was uneven, and evening it out or widening (a step I would also need) was too fine an operation for my knife (maybe needs sharpening?).
I figured if I can use a table saw to cut foam, maybe I can use sandpaper on it, as well? So I took 80 grit sandpaper and started to work the edges. This removed a lot of the loose and uneven parts, and made it start to look more like a real circle. I tried to fit it onto the scope but it was tight. It was possible to get it on, but with too much effort. More work with the sandpaper, and use of calipers to identify where the diameter was shortest, eventually gave me the circle I wanted with a nice tight fit but not requiring a fight to put on.
Note that I was very nervous about building something that would drop debris onto my lens. Sawdust, yes, but especially foam. If the foam just constantly erodes, then that is a nonstarter. So after sanding, I was just constantly working the foam, playing with it, rubbing it, getting any excess residue off. After the initial residue was removed, I never was able to work any new residue off, so I was comfortable with moving forward.
Step 6. The EL panel came with an actuator (with an on-off switch) and a battery pack. I wanted them semi-permanently attached to the back of the panel, both to avoid any more messing with wires than I already do at night with my equipment, but also to give me some natural handles for the flat frame panel. So I cut out two little boxes from the plywood to house the battery pack and the actuator, with extra room in the actuator box to stuff all the unneeded wire lengths. Make sure you orient the actuator so you can reach the on-off button!
Step 7 – Glue the boxes to the top (better looking side) of the main piece of wood. Here, of course, you’re going to use the wood glue we have been avoiding so far.
Step 8 – Flip it over and glue the foam pieces to the other side of the wood, with the largest foam piece matching up with the wood piece. Start by placing the EL panel on the board, with the wire connector on the side closest to the actuator wire coming from the underside. Then place glue along the outside edge of the wood around the other three sides of the panel, leaving the connector side open.
Leaving this last slide unglued allows you to slide the EL panel in and out, if necessary.
Now I connect the actuator wire to the EL panel and it’s done. If the batteries or the actuator slide out too easily, you can put in some cardboard or tape or some other filler to make them a little tighter. My flat frame panel now fits snugly onto the end of my scope and can be used at any angle, without leaking light to my light-sensitive neighbors.
It is not aesthetically pleasing with the unfinished plywood, because I was too impatient to wait for stain or paint to dry before I connected it all together. But I’m going to go back and protect the exposed wood with Tung Oil and it will look much nicer.
- okiestarman56, bcloud, ram812 and 6 others like this