A simple wooden Crayford-style focuser

My son wanted to build a telescope out of a mailing tube we had, so we bought a 70mm lens (300mm FL).  The part of telescope making that I like the least is focuser-making.  But I think I now have a Crayford-style design that is very easy to make.  It is easier to make than my previous wooden Crayford and helical Crayford, and has about the complexity of my push-pull Crayford but works an order of magnitude better.

It's a block of hardwood (cherry, from an old crib), 2.75"x2.75"x1" in dimensions.  I used my drill press to drill out a 1.5" hole (A) in the middle for the 1.5" outer diameter aluminum focuser tube.  I then drilled a second hole, 3/8" in diameter, all the way through from one side to the other (B), so that hole A and hole B would meet and overlap by 1/16".  This second hole is for the focusing rod.

Finally, I drilled two more holes from one side to meet up with hole B (C and D) for adjustment screws that press on the focusing rod through PTFE pads, and tapped them to fit the adjustment screws simply by forcing a screw into the wood.  These PTFE pads (E) are small strips of 1/16" PTFE, bent into a C-shape, with the back of the C facing the adjustment screws.

I purchased an 8" x 1" x 0.0045" strip of self-adhesive PTFE tape (F) on ebay from mousetape.com for about $3 shipped.  I stuck a strip along one side of the focusing tube.

I then enlarged the 1.5" hole in the middle slightly.  My method of doing this was to use a sanding drum mandrel in my drill press with a somewhat oversized sanding drum (which would stick out on both sides past the work piece), with the work piece resting on another piece of wood with a hole in it, which was on the drill press table.  As a result, I could keep the work piece aligned at right angles to the sanding drum while moving it about so as to enlarge the hole.  I enlarged the hole until it was about 1.56" in diameter.  I then finished the exposed raw wood with Titebond II diluted with water and sanded.  I stuck another strip of PTFE tape (G) inside the focuser hole, opposite where the focusing rod will go.  Then I put it all together, and it worked just fine.

Update 1: I added knobs.  I had some oak circles from using a hole saw on some piece of oak some time ago, and after sanding them (by putting on a bolt and spinning with a drill against a sanding block), I press-fitted them on the focusing rod.  If it starts slipping, I'll drill a hole through the rod and knob and put in a screw.

Thanks: I am grateful to John Wall for the idea of putting PTFE on the focuser tube.

Update 2: I just had a bit of trouble with the wooden thread for one of the adjustment screws getting stripped. Treating the hole with CA glue helped, though.

And I added a screw on one end of the focuser tube as a stop, and I added a screw to hold the eyepiece. Because the focuser tube is only 2" long, so as not to compromise the length of movement, we drilled little depressions in the main block for the screws.

Update 3: And here is a not-to-scale diagram of what the main holes in the block of wood look like.

Various things

It's been cloudy and rainy for a couple of days.

I'll be building a 70mm F/4.3 refractor with my five-year-old son.  Lens and diagonal have arrived ($33, from Sheldon Faworski).  We have a mailing tube of just about the right size.  I ordered some screws and nuts from Amazon to attach the lens to the tube.  I am working on a simplified PTFE-on-PTFE Crayford variant for the focuser, made out of one piece of wood, plus some hardware.  I'll post details once it's done.

I've spent some of the weekend playing with using a Wii remote with an IR pen.  This might be nice for operating Stellarium at public star parties.

Advanced rubylith uses

Rubylith is a deep red plastic sheet, originally used in the printing industry.  Amateur astronomers now use it for such things as covering laptop or phone screens to make them red and help preserve dark adaptation.  It's a nice product for that purpose, but it has other redding-out uses.  In an earlier post, I described a method for using Rubylith to red-out the backlight on my phone.  That method had two problems.  First, the cyanoacrylate glue melted the Rubylith and gummed up keys, requiring extensive cleaning.  Second, light sneaked out around the edges of the Rubylith bits.

But there is a simpler method that I'll explain after a bit of background.  I bought some Rubylith from Jay, an amazingly wonderful ebay seller.  The one sheet I ordered unfortunately got damaged in transit, but Jay immediately sent me two (!) fresh sheets, superbly packed, at no charge.  Along with these sheets, Jay emailed me with a lot of fascinating information on the material.  The directly relevant part of that information is that Rubylith consists of a thicker clear sheet bound by static to a very thin red sheet, and if one cuts very carefully, one can cut through the red sheet layer while leaving the clear sheet intact, and then peel off the red sheet.  I tried to do this, and failed, and emailed Jay that I failed.

Imagine my surprise when today in the mail there came another tube mailer from Jay.  Inside was another free sheet of Rubylith, with squares of the red portion cut out to demonstrate how to separate the red and clear sheets.  (What a great guy!)  It looks like my initial mistake was that I was trying to peel from the edge.  But the trick is you cut out the red portion you want, being careful not to cut through the clear portion, not near the edge, and then peel.  Jay used a #10 X-Acto knife, while I just used box cutters.  Also, you have to know which side has the red sheet--it's less slick side.

Anyway, the red sheet is very thin and very clingy (my calipers say it's about 0.04mm, versus the clear acetate layer which is about 0.10mm).  And I had a use for it.  My phone had a thin latexy membrane between the dome switches and the keys themselves, probably to keep dust out.  I removed that membrane, and cut a clone out of the red layer on the Rubylith.  I took the occasion to clean out some dome switches, and then replaced the original membrane with the Rubylith membrane.

Result: a very nice and deep red on those keys covered by the membrane.  (Some of the redness is from my previous treatment.)

In case anybody is curious, here is Jay's whole email about the history of Rubylith and gel filters.

Changing the subject to night vision, and your search for a red safelight. I'm thinking I should give you some quick background on Rubylith. Even further back in the day (mid-1960's), I used Rubylith for stripping. It's original use was in the printing industry. It is a product that is actually two layers. The red part is incredibly thin, and is bonded (by static electricity, I think) to a much thicker clear plastic.  They are meant to be separated -- stripped -- apart.  Whenever a magazine or newspaper had a photograph they wanted to insert in an article, someone had to manually cut an opening where the photograph would show.  Using a VERY sharp X-Acto knife, and a VERY light touch, a stripper would cut through the Ruby, but not through the plastic. Usually, the stripper would cut a box or rectangular shape. Then they strip away the Ruby.  Actually peel it up off the plastic, leaving a clear box for the photograph to show through. When you get the two new sheets of Rubylith I sent you, you can check this out by experiment.

Since I cannot show you the two different sides of the Rubylith, try this: Take a piece of tape, and stick it to one side of the Rubylith at a corner. Pull up. Either the Ruby will peel away, leaving clear plastic; or you've stuck the tape to the wrong side. Once you get it apart, now is time to experiment. 

I'm thinking the SuperGlue you used, will dissolve the Ruby, but not the clear plastic. And that is why some of the Rubylith seemed to melt, but some of it not. Depending on which side you put the glue. I would not recommend SuperGlue. The workplace where Rubylith is designed to work, uses Rubber Cement. Very, very rarely, would you use Rubber Cement; and then only on the plastic side. Actually generally speaking, glues of any sort were not used with Rubylith.

It is beyond the scope of this letter to describe how the old-style printing processes worked. But Rubylith was an overlay on top of the photograph. It was held in place by a vacuum frame. Then a composite photograph was made; which, using several steps; put the photograph together with the type; and made the final article.

As to Roscolux. Yep, as you might guess, back in the day (late 60's -- early 70's) I worked with that stuff too. I just wanted you to know that the word "gel" is short for "gelatin". As in JELL-O. Roscolux used to be water soluble. You could eat it. The various colors of gel tasted differently, but all would dissolve in water. I'm thinking now-a-day, Roscolux has changed to some kind of plastic-based formula. [Yes -ARP]  But; just; test the stuff with an eyedropper of water first. Or, maybe suck on it. It will either dissolve, or it won't. [It doesn't. -ARP] As to the obvious question, "why gel?", the answer is heat. Roscolux is a product for Theater.  No, not the local movies where you see "Tron." I'm talking legitimate Theater, where the audience sees -- not celluloid images -- but they see real people, standing in front of a real audience, and giving a real performance, in real time. Those people on the stage, must be lighted. And depending on the color of the costumes; and the color of the background; the lights must have color also. Since a typical Theater stage is about half the size of a football field, there must be a corresponding amount of light (think of a night football game). If all the light is just white light, then everything down there on stage will be washed out, faded, by the light. So we color the lights with gels. They must stand up to a whole lot of heat. At a minimum, we're talking a 500 watt bulb. At a maximum, a carbon arc follow spot, which draws 23 amps (about 3,000 watts). Only gel can stand up to this kind of heat. It does not melt. And with this incredible amount of heat, no one worried about humidity. Just check. I'm thinking Roscolux has updated their gels in the last (*ahem*) four decades. Probably they are now plastic, not gel. Just, check first.

Anyway, as I said, Jay is a great guy, so if you're buying Rubylith, I don't hesitate to recommend him. And if you have questions about the product, he's great at answering them. Thanks, Jay!

(By the way, my son wants me to make a telescope for him and his sister out of the heavy-duty mailing tube Jay sent me the Rubylith in.  I am thinking a nice 70mm lens would fit well.)