Sunday, June 9, 2013

Lightbridge Part 4 - Mirror Mirror

Are we done yet?
Is it purple?  Is it blue?  It depends!


That's not a scratch or blemish in the finish,
It's the flash lighting up the back of the dining
room chair.

Last year I was working on painting and cosmetic changes.  I finally got the tube clear coated sometime last fall, I can't remember when it was exactly but I am very happy with the results.

I had the scope out once or twice over the winter, but for the most part the scope sat unused for the duration.  Now that the weather is starting to get better and Table Mountain is just around the corner, it was time to get back into the project.

The mirror has always bugged me since it was damaged and I know I can see the problems with it.  I spent a lot of time thinking about options and exploring the pros an cons of each.  Since I can't afford a Zambuto (and would that be sacrilege to put a Zambuto mirror in a lightbridge?) I looked at sending the damaged mirror in to someone like Swayze, but at close to 600 bucks and the fact that the mirror has a chip on the edge, It didn't feel like a good investment.  That's when I found the GSO mirror set that Jim's Mobile was selling.  GSO is the OEM mirror supplier for Meade.  The new mirror was under 600 bucks, if I want to I can still send in my old busted mirror to see what, if anything can be done.

Ooo  Unboxing
 Ordered on Monday, Arrived on Thursday.


White trashcan lid.  Looks familiar.
Looks like it comes from the OEM
and not Meade.

The mirror arrived safe and sound.  I was actually surprised how it was packaged, but I don't know what I was expecting exactly.

Here you can see some of the oxidation .

When I got into the scope, I noticed that the coating on the primary was starting to deteriorate.  This is just what happens when the coating gets damaged and stored in a high humidity environment like....Seattle.
Glad I opted to replace the mirror.  This is a slow death for an aluminized mirror.

Pretty standard setup, wasn't anything I wasn't expecting
Now that I had a new mirror it was time to pull the old one out and prepare the mirror cell for the new mirror.  I started by asking questions in the Lightbridge Yahoo group as well as searching around on Cloudy Nights.  Some helpful folks in the group pointed me to a really comprehensive post about some pretty intensive mods to a 16" lightbridge and another one about a 12" light bridge.


I got this stuff from Amazon
Razorblade made quick work of the foam adhesive pads.

 I opted to replace them with UHMW Polyethylene pads like the example in the one cloudy nights thread.

Then I added eyelets to the mirror supports so that I could connect them together and keep them oriented properly.

I tried drilling holes, but opted for built
up plastic and crazy glue

 I used some rubber/elastic thread stuff that we had laying around for beads.

With UHMW and without
Next was the mirror clips.  The stock clips are rubber and exert significant force on the mirror that affects optical performance.  The clips only function is to prevent the mirror from falling out when the scope is horizontal.  First I removed the rubber bumpers and put more UHMW Polyethylene on the underside.

Flattened and Original
Since the mirror is about 1.5 inches thick and sits about a half inch into the cell, it turns out that 1.25 inch nylon spacers are almost perfect for replacing the rubber bumpers and putting the clips at the perfect distance from the mirror.  Only problem is that being round, they touch the edge of the mirror which can also cause pinching.

Laying them out on the table keeps
the flats all pointed the same way.
My solution, put them into the vise and use the orbital sander to flatten one side.

This also posed a problem with the spacers twisting when I tightened the screws.  Nylon is a difficult plastic to glue and most stores won't carry the products necessary to glue them together.  I didn't need anything crazy since they are going to be under compression so there isn't a lot of shear strength involved, I just need something to keep them oriented properly.
All Glued up
I did a little digging around and ran across a posting suggesting using hot glue.  I had some construction sticks so I used them.  Its not pretty but it works really well.
Laid out, with the test holes on the end.

Next step was the edge support.
I picked up some 1 inch by quarter inch aluminum bar stock.
I figured this would be stiff enough to support the mirror edge.

Using the calculator here, I was able to determine that the optimal placement for the mirror edge supports was .71 inches from the back of the mirror or just shy of 3 quarters of an inch.  I ended up doing something similar to the methods in the cloudy nights thread as well as some of the ideas in this post (incidentally, this was also the post that led me to think about replacing the rubber completely.

I picked up a tap and die set from Harbor Freight, made a couple test holes and tapped them and then laid out the 90 degree edge supports.
Tapping the hole for the
nylon screw

 First I drilled two 1/4 holes, the original intention was to have two screws securing them to the cell, turns out I didn't need both.

Holes drilled and tapped, ends trimmed, ready for the final cut.

I took my time with the hacksaw, put a new blade in and used plenty of oil.

I was very pleased with the results.

Next step was to drill and tap holes into the cell itself
Tapping the hole
Tapping the cell was a lot easier than the bar stock.  It went much quicker than the other holes I tapped.
Overview of the whole cell post modifications.

Finally the mirror cell was all back together.
The rubber stabilization cord is in place, the mirror edge supports are also in place.

Now all I had to do left is to put the new mirror into the "new" cell.

I had placed some more of the UHMW strips around the edge of the cell, but the new mirror did not have the clearance so I removed most of them and only put one back in opposite of the edge supports.
I also placed a small patch of the slick pad plastic
under each nylon screw to help reduce friction.

This shot shows the clearance of the new posts and clips.

Another view of the mirror clips

Closeup of one of the edge supports.  I rounded the end
of the nylon screw to reduce friction.

Of course, it was cloudy tonight so it will have to wait for testing.

I also installed the 2 speed focuser upgrade from agena astro products.  Can't wait to get it out under the stars, I will do a full write up once I have a chance to use it.  I still have to get a shroud made, the upper cage  painted, and some mods for the secondary as well as new springs and knobs for the mirror cell (if I had planned a little better, I would have ordered them already.


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