Thermoforming a Hartmann mask

I decided I needed a Hartmann mask to assist in focusing my Vixen ED80Sf for photography.

What's a Hartmann mask? Lots of info on the web:

http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM or

http://www.weasner.com/lxd/techtips/hartmann-mask.html or many others.

Now, making a Hartmann mask bears almost no similarity to brain surgery. Just about anything will do. In an emergency, in the field, you can even use a paper bag with a couple of finger holes poked in it. For a while, I had been using a piece of 1/4" foamcore with two holes in it. Taped to the side of the dewshield, it could be flipped into positiion anytime it was needed.

But I believe that nothing should be left simple if a way can be found to make it complex and wonderful. Now, there are any number of ways to do this. I thought of the following: papier mache or fiberglass layup over a core; hammerforming sheet aluminum over a former; spinning an aluminum disk over a "chuck" (as it's called in the spinning biz); gluing up a cardboard disk and cardboard ring. All will work, but lack the required degree of overkill.

So I hit upon thermoforming a lightweight plastic cap to act as a mask. For material, I found the lids of some plastic tubs used as packaging for cookies. The tubs are marked with the recycling symbol and PETE, for polyethylene terephthalate. So that's what I have.

First, I made a plug of the exact size and shape as the desired inside dimensions of the finished mask, out of 3/4" plywood on my lathe. The diameter is a little larger than that of the dewshield, to allow room for some felt pads on the inside of the finished mask. I made mine 4.60 inches in diameter. I put about a 1/8" radius on the corner. There is a 1/4" central hole which is useful for placing a steel dowel to later hold the plug in the drill press for thermoforming, and for using a 1/4" transfer punch to mark the center of the finished part . Also I found that I needed a ring (attempts to form over the plug alone didn't work). The ring is a piece of scrap particle board, 4.64 inches in inside diameter. The inside corner also has a 1/8" radius.

To form, I placed the ring on some spacers, mounted the plug in the drill press chuck via the 1/4" dowel (the drill press is not running for this; it's just being used as a light-duty arbor press), placed the plastic stock between the two and lightly brought the plug down to contact. Then I heated the stock, as evenly as I could, with an electric heat gun, as I slowly pushed the plug through the ring. The edge of the material will pucker up, but the section drawn into the ring should be smooth. When the plug has been pushed through to the point where its back is flush with the top of the ring, it's finished.

One finishing touch is to leave the plug and stock in the ring, but turn them over and pass the heat gun over the face of the plug, to smooth out the plastic.

When cool, push the plug and formed plastic backwards, out of the ring.

(In a later variation of the tooling, I turned the ring edge down so that the plastic lids would snap onto this ring, but it turns out that makes no difference, as once they begin heating, the internal stresses are released and the edge wants to flip up).

To trim, I could have used the lathe, but found it easier to just use a 3/4" block of steel and a sharp razor blade, to score the plastic just below where the puckering starts.

Peel the plastic off the plug and check for fit.

Next, drill or punch some holes. Use a compass to mark the diameter of the telescope lens (in this case, 80mm) and place holes with their edges just touching that. I made 3/4" holes using a spade bit. One can also use a stepped bit, or a large punch (such as Greenlee chassis punches). There are various designs for Hartmann mask hole configurations: two holes, three holes, round or triangular. Take your pick.

To finish, I sprayed the outside of the mask with some "vinyl dye" from the auto parts store, used to re-color automotive vinyl upholstery. And glued in three felt pads.

Finally! My misspent youth of playing with vacu-formed slot car bodies pays off!

Then, I got to thinking. That size seemed familiar. I wonder if...

Sure enough, it turns out that the lid from an ordinary supermarket margarine tub will fit. Tightly, but it fits.

But I like mine better.

Pete Albrecht