6" F/5 Dobsonian: Secondary mount and first light!

I glued the secondary mirror to its support the standard way, with silicone sealant and some toothpicks to ensure spacing, which toothpicks I removed after a couple of hours of drying.  It's a nice-looking 1.5" minor axis mirror.

I had a hard time deciding on the secondary mirror mount.  I wanted two-axis collimation done in Coulter-style, by having the support rotate along one axis and the secondary mirror along the other.  But I was having a hard time figuring out what the support should be.  I considered a two-vane support going across the aperture, using maybe a hacksaw blade (I went to the length of grinding teeth off it) or a steel ruler or even a hacksaw blade epoxied to a steel ruler.

But I also had some 1/8" thick 6" long 1" wide steel strips.  I bent one near an end--sandwiched it between two others, and hit it with a mallet, and then did the final fine adjustments by grabbing the end with an adjustable wrench and just bending by hand.  It was hard finding the exact right place for it and the exactly right bend.

In the evening my friend came.  We put in the primary.  And then tried to mount the secondary.  I was very nervous about dropping something on the primary.  My initial solution was to tie a safety line to my secondary support strip, and tie the other end to the focuser so if I dropped it, it wouldn't hit the primary.  I never dropped it, fortunately.  Then I had an idea for a better safety measure--I gently put a scrunched up T-shirt on top of the primary.  Good idea!  I dropped a fender washers two or three times on it in the course of the evening.

With my friend there, we measured off half of the diameter of the tube and cut a piece of wood to help center the strip.  Moreover, we cut a small piece of hardwood to act as a small carpenter's square, for squaring the strip against the focuser tube (which I racked in all the way).  Also, to get the vertical positioning right, I put a laser collimator in the focuser and used Inkscape to draw a little centering target which my friend cut out and we taped over the front of the secondary.  (My laser collimator emits a cross, so I aligned it with one of the crosses on the target.)

Eventually, I gave up trying to get it completely perfect, and mounted it.  Then I drilled a hole for the secondary mount on the support strip after aligning the target on the secondary with the laser (sorry, no offset).  And mounted the secondary to it.  (By the way, that big screw will have to be trimmed slightly.  And I will glue its head down so as to make the collimation entirely tool-free.

The secondary support is mounted to the tube with a carriage bolt.  One end of the carriage bolt is capped with a plastic wingnut, and the other goes into a plywood disk on the outside of the tube.

I collimated with my laser, and it was time for first-light.  The sky was clear.  I plunked my "30mm" Rini (probably 26mm or so) in the eyepiece for 28X with an approximately two degree field.  And, hurrah, stars came into focus, while handholding the tube.  With another friend's help (he saw us over the fence working away, and dropped in to kibbitz), and the scope leaning on the earlier mentioned T-shirt on the trunk of my car in the driveway, we saw a lovely but tiny sharp image of Saturn.  We then looked at Mizar and Alcor, and split Mizar.  Didn't look at any deep sky objects.  At one point it looked like I saw a dark nebula, but it was just leaves on a tree.

The diffraction spike from the single vane support didn't bother me and more importantly didn't bother my friend--it's his scope, after all.

So, the tube is done is done.  Now it will be time for the mount.  We'll make a hexagonal ring, about 7" wide, around the middle of the tube (the center of gravity with my 13mm Hyperion plus a Barlow is in the middle;  it'll be even further front once my friend attaches a Telrad), attach the altitude bearings to that, and make a standard Dobsonian box, probably on a circular base.

6" F/5 Dobsonian: Mirror cells

I used a router and a hand saw to make a simple secondary cell out of 1" square poplar rod.  It will eventually attach it to a simple two-vane hacksaw blade spider (I used a cheap Harbor Freight rotary tool with a cut-off disk to grind the teeth off the blade), and it will adjust about two axes by direct rotation.

The mirror came with a plastic primary cell.  Unfortunately, that would have required quite a bit shimming to fit in the 8" inner diameter tube, as the cell was rather small in diameter.  So I made one.  As I often do, I laid it out in Inkscape.  (The annotations are for the benefit of the blog--they weren't there in my original file.)  I am very bad at measuring (but getting some cheap digital calipers for $8 from a Hong Kong seller on ebay helped a lot--and then I got $4 back as one of the jaws was chipped and a set screw was crooked, so it was a great deal) so my best way for precision is to print out a cutting template, attach it to the piece and drill and cut right through it, or at least make marks with a pencil or drill bit through it.

The plan was to use scrap material.  The cell itself was to be a triangle with corners snipped off made of two layers of 1/2" Baltic Birch plywood, cut from the circular inner waste generated by routing the tube reinforcement rings.  I cut the cell with a jig saw, not very precisely, and cut a ventilation hole with a hole saw.  I then drilled and countersunk holes for collimation screws in only one of the two triangles.  I JB Weld'ed 2" long machine screws into their countersunk holes, and then I glued the other piece of plywood on top of this one (with Titebond II), covering up the screw heads, and clamping hard.  As a result, the heads of the screws are trapped within what is basically a solid piece of plywood (two pieces of plywood glued together are basically one thicker piece of plywood!)

There would be a rectangular baseplate made from a piece of scrap red oak that I had.   The baseplate would be mounted in the tube, near its bottom (not quite at bottom, as the plywood reinforcement ring on the tube would   I rounded the corners of this scrap piece with the router using approximately the right diameter to fit in the tube, bearing in mind I could always sand it smaller (as I indeed had to) but couldn't make it larger.  (By the way, here is a hint for how to measure things off from the router.  I cut things with a 1/4" up-cut spiral bit.  For measuring, I replace the spiral bit with a small length of 1/4" stainless steel rod, and measure from that with the calipers.)  Notice from the diagram that the rectangle would be somewhat off-center in the tube--this was on purpose, to fit better with the collimation screws.  I drilled holes for the collimation screws (actually, I did that while drilling them in the triangular cell, because they needed to line up exactly).

I also made a bit of a countersink around the holes, so that the collimation springs that would go on the screws (I had some springs that someone once sent me) would be countersunk, which would make the assembly have a lower profile.  I didn't have a paddle bit of the right size for these countersinks, and I didn't think I could control my router for such small work, so I did something wacky.  I chucked my 1/4" spiral cut bit into my cheap Harbor Freight drill guide, locked its height, and used a drill to route out the countersinks.  It wasn't very neat, but it was good enough, and the lower speed was less scary.  May not have been good for the bit, I already somewhat damaged the bit in an earlier episode when it was accidentally cutting against cement.

I also cut three ventilation holes in the baseplate with a hole saw.  The result coincidentally (honest!) looks like a certain rodent from a company that protects its intellectual property with a zeal that one may think violates the Greek maxim meden agan.  In defense of this design, I will say that the placement was entirely functional.  I made the inner hole as large as I could while keeping strength around the collimation screw holes, and the small holes are also located in such a way as to be fairly symmetric, and not too close to any of the alignment holes or the inner hole.  Four wood screws hold the base plate to the tube.

The mirror is glued to the cell with three 3/4" blobs of silicone sealant, with the positions optimized with Plop. I used some removable 1/4" particle board spacers to make sure the blobs wouldn't flatten out.  One wants the mirror to float on the blobs, and not have stresses put into it by the differences between thermal expansion of wood and glass.  By the way, when I removed the mirror from the original cell, I was horrified to see it was attached with two blobs of 1/16" thick rubbery stuff.

My 8" F/4 travel scope: Secondary mirror

I get confused by three-screw collimation arrangements, and I couldn't figure out how to make one. So the secondary mirror collimation is done like in my Coulter scopes, by rotating along two axes (and one can adjust the position up-down because the support stalk fits in an elongated hole in the focuser board). Basically, the cell is two pieces of softwood that swivel around a screw. The gray square knob at was made by putting a nut in a square made out of painter's tape, and pouring JB Weld into this cast.


The stalk started as a piece of threaded rod. Unfortunately, the support wasn't solid enough--collimation shifted too much with elevation. So I JB Weld'ed (there is a theme here, isn't there) a small square steel rod in parallel with it to reinforce, which helped.


And here is a picture of the cell before it was painted and the mirror was glued to it (with silicone glue). I later cut and sanded it smaller.
I did have one problem with the scope--when I viewed Jupiter, I would get a long smear in one direction. It turned out that the secondary mirror had a distorted edge on one end. I tested by looking through the telescope at an LED flashlight with aluminum foil and a pinhole, if memory serves me. I blacked out the bad part, and the smear disappeared. It might be a good idea reorient the primary so that the blacked out part of the primary aligns with the blacked out part of the secondary, but currently the blacked out part of the primary aligns with most of the secondary stalk, which is also good.