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This page is documenting some of the fiddling I have been doing with my 18" Obsession telescope.  Some of these ideas have been distilled from the Obsessions Users forum or from the Big Dobs Users group on Yahoo:

***This whole page is IN WORK as is (for fun) the telescope***

Contrast Improvements

Upper Tube Assembly (UTA)

These two easy additions have really helped my contrast:

bulletI have installed a filter slide to hold my two inch filters, but it also serves a a baffle for the focuser.  I also find that I use my O-III and UHC much more often now because they are so convenient.
bulletI also made a light shield to go into place opposite the focuser, mounted to the inside of the UTA.

Lower Mirror Box

From: "Banich, Howard" <howard.banich@nike.com>
Subject: RE: Re: Mirror box bottom light baffle / Sawdust + 3M undercoating.

OK, here are the steps I went through:

1.) Created and saved lots of clean sawdust while working on an assortment of projects.

2.) Bought two cans of 3M automotive undercoat spray and read instructions on the can. 

3.) Practiced on scrap wood until I was comfortable with the process.  I needed the practice to get an even spread of the sawdust and to control the spray - it comes out fast and thick.

4.) Bought a grated cheese dispenser - this made spreading the sawdust evenly much easier.

5.) Practiced on scrap wood again. Much easier, and it was his point I knew this technique would turn out well (let the practice pieces dry overnight so you can see the finished texture).

6.) Removed the primary mirror from the mirror box. Stored it in its shipping box and covered the box with a shop towel to protect from over-spray. 

7.) Removed mirror cell and altitude bearings, and covered them with shop towels to protect from over-spray.

8.) Rough sanded all the areas I intended to spray. I used 80 grit sandpaper and did the sanding by hand - nothing major, just enough to roughen the surface.

9.) Masked off all the areas I did not want to spray. I used painters masking tape and covered the outside of the scope with newspaper and made sure to plug all the bolt holes so spray wouldn't sneak though to the outside wood finish.

10.)  Read spray instructions on the can again.

11.)  Sprayed on first layer of 3M undercoat.

12.)  Immediately sprinkled sawdust on the still wet first layer. Made sure the sawdust was sprinkled evenly.

13.)  Sprayed on the top layer of 3M undercoat, making sure I completely covered the sawdust from all angles. The first layer was still wet at this point.

14.)  Let dry for about a week, then reassembled the scope.

From step 6 it took me an afternoon to complete. By itself the 3M spray has a pebbled, flat black finish when dry but with the additional roughness of the sawdust it comes close to eliminating light reflection, and I'm very pleased with the result. Hope this is what you were looking for, and good luck if you decide to give this a go.

It was pretty easy to do. I sprayed a layer of the 3M undercoat and then immediately sprinkled on the sawdust while the undercoat was still wet. Then I sprayed on the top layer, making sure I covered the sawdust from every angle.

Clean sawdust is important as it's easier to spread than dirty sawdust.  Also, I used a shaker container made for sprinkling grated cheese on food to sprinkle the sawdust, which turned out to be important to get it spread evenly. Because this was the first time I'd tried this method I first practiced on scrap wood to make sure I was comfortable with the technique.

As you probably know, spray painting anything involves some pre-work. I took out the mirror, mirror cell and removed the altitude bearings so all I was working with was the wood mirror box. I rough sanded every surface I was going to spray and then masked off everything else. This took longer than the actual spraying but helped the procedure turn out nicely.

Aside from suppressing stray light, I like the rough and very black finish contrasted to the beautiful wood finish of the outside of the scope. Looks cool! I'll use this technique again on my next project.

20" Obsession #038
West Linn, Oregon


Light shield for the bottom of the mirror box

Another idea I got from Howard Banich; a piece of canvas, coated in flat black paint (or better yet, the coating he described for the inside of the lower mirror box).  I have attached this block to the bottom of my scope using snaps instead of velcro.  This has a hole in the center to allow the muffin fan to draw air through, and I can unsnap it most of the way down when I'm cooling down the mirror.  The hole can be completely covered over to keep dust out during storage.


Yet to Come...


    Heated Secondary - not wired up yet.


Boundary Layer

Muffin Fans

I just bought a squirrel cage fan that I am going to install on the side of the mirror box to break up the boundary layer on the mirror.  This will be done in conjunction with the installation of a dual thermometer that I will  use to tell me the ambient and actual mirror temperature simultaneously.