Saturday, May 15, 2010

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.


  1. great job, do you think i can find such mirror with such price to be shipped to the middle east, jordan.
    i've been looking for a mirror for a long time

  2. Sign up with CloudyNights and send a PM to user MASILMW. He's got lots of mirrors. Tell him I sent you.