Star Diagonal Mod

February 22, 2010

I did some reading on diagonals, and it appears that prism diagonals are not the best choice for short focal length achromatic refractors as the chromatic aberration is amplified by diffusion in the prism.  It can work to your advantage if you have an under-corrected achromat, but in my case my 80mm home built is just a typical achromat.  The fact that the 1.25” Meade prism diagonal focused successfully on my 80mm refractor made me confident that a similarly sized mirror diagonal would also work.  So I took a gamble, and ordered myself a 1.25” mirror diagonal from Island Eyepiece.  I originally was looking for a Williams Optics dielectric diagonal, but the guy at Island Eyepiece said that the Orion brand one was cheaper, easier to get, and was essentially the same as the W-O since they are both made in the same Taiwanese factory.  Well I got the mirror diagonal in and pooh, I can’t get focus with it!  The documentation that came with the diagonal says that it adds 90mm to the focus length…I only have 80mm of focuser travel, and all the eyepieces I have focus right around 80mm in straight-through configuration.  The 1.25” to 2” adapter also adds another 10mm.  Now what?

I was not going down without a fight.  I disassembled the diagonal and measured all the pieces so that I could generate a to-scale computer drawing of it (see sketch #1).  I then looked at all the areas where I could save focus length. 

I figured out the following areas:

• The shoulder on the 1.25” to 2” adapter is the standard 10mm high.  If I machine this off, and replace the thumbscrew with a set screw, I can save all 10mm.

• There is a 1mm high shoulder on the male end of the diagonal that I can machine off.

• The end of the eyepiece, when inserted into the diagonal, is far from the mirror surface.  Some careful machining would allow me to reposition the female end of the diagonal further in, recovering some of that distance (see sketch #2).

Incorporating the three modifications above I estimated that I could recover ~23mm of focus distance.  Take this off of the starting point of 100mm (90 + 10), and I should just squeek into focus.  Gulp, here I go!

I brought my son into work with me so he could watch me do the machining I needed to do to the diagonal and adapter pieces.  He really liked the long curly cuttings that came off.  They were soft like tinsel so I let him have them (if I had been machining steel the cuttings would be more like tiny razor blades!).  The machining went well, except for an instance where the diagonal piece came loose in the chuck, resulting in some marks on the outside that I later touched up with paint.

I painted the parts that would be on the interior of the diagonal flat black, and all the other parts I just touched up the glossy black paint.  Looks pretty good so far.

I did the final re-assembly tonight.  Everything fits together nicely.  It was forecast to be raining all weekend, so I tried the diagonal in my 80mm just looking out the window at a distant object…yeah it works!  My 26mm Plossl eyepiece has about 4mm of focus-in left, the 10mm Radian has about 11mm.  Phew!

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