Tribahtinov mask for Celestron CPC 925 Edge HD & MyC925-Knobs

A symmetric and rounded Tribahtinov mask for the Celestron 925 Edge HD. 
Based on the Satakagi Tribahtinov web calculator. Also included M3 knobs as a replacement for the original screws.

• https://satakagi.github.io/tribahtinovWebApps/Tri-Bahtinov_symmetric.html
   

Tribahtinov mask

Design

This is a Fusion 360 design. I added the design files.

There is three parts design to fit on the Prusa MK3x plate. 
I also added a one part design (diameter 265 mm), if you have a large printer. 

The part is 1.8 mm thick for sake of stability. 

I do not like to glue parts together, therefore the screws. All screw holes are threaded. 

• 6 x M3 / 10 mm  (2 per parts)
• 3 x M2 / 10 mm (1 per part)

You can also glue the parts together. Super glue is perfect for this. But screwing is more safe in many aspects, that's my opionion. And you can fix a part, if broken (or melted in the sun).

!!! Super glue WARNING: 

Be absolutely sure, that the glue is completetly dry, when you put the part onto the Schmidt plate. Super glue emits very fine spurious gases even after hours, which will stick onto the glass on a molecular level. You will NEVER be able to remove this fine film. So, if you glue with super glue, let the part dry and evaporate out on a warm place for at least a couple of hours! 

Be warned.

Test inside obstruction ring (secondary mirror)

The inside obstruction is 91 mm. There is an STL you can use to test this, if you are unsure.

"Mask" for the mask

To be able to collimate each screw, I added a simple mask for the mask :-) You then can collimate number by number each screw. 

For how to do this, see the attached picture. Lose or tighten the RED screw and counter screw the BLUE one. Do this for all three directions.

Print instructions

PLA Max, 6 mm nozzle, 0.3 mm layer height and 25% infill. 

Modification ideas:

• You can try to alter the height of the plate to 1.2 mm to safe material (use “q” in fusion to reduce the layer height), but I would not recommend this. I also would not recommend reducing the overall height for stability. 
• You also can use threaded inserts M3, if you like. They should simply fit into the bore. 
• Or probably you like to integrate a hex nut into the model design. Feel free.
• This might probably fit as well to a SCT 925. E.g., with an adapter ring in the middle (or put some other material between). But probably better take another design for the SCT.

Nobs

Mount the knobs

First of all and a very important note!

Demount and remount ONE screw after the other. 
Otherwise your secondary mirror will FALL INTO THE TUBE AND CAN BE DAMAGED. As well as yout inner  tube and your main mirror. '

Be warned! I am not responsible for any damage you might produce beeing not careful and not knowing what you do.

Design

This is a Fusion 360 design. I added the design files.

Like the well-known “B…Knobs”, I decided to do this by myself. 

• Three M3 / 30 mm long A4 screws (stainless steel)
• Three washers M3 or better M4

The original screw is M4 and 9.2 mm long. I designed a simple knob. At the bottom, the plastic is 6.5 mm in diameter. The washer is a little smaller (see picture). It is no problem to put the sheet onto it.  The screws are 30 mm long. So there is enough space.

Print instructions for nobs

PLA Max, 0.6 mm nozzle, 0.3 mm layer height, 100% infill.
And yes: this is exact enough with a 0.6 mm nozzle :-)

Usage

The usage of the Tribahtinov mask for collimation is not often shown. I designed a “mask” for the mask, so that one can collimate each screw after the other. 

First step:

Align  the little protrusion  with the numbers. Or if you built the three part one,  align it where the three parts come together. 
 

I call the lead screw the most important one - it is the one opposite to the little protrusion (and number).  There is a little notch in the inner ring, that indicates for this screw, which is the main one for the direction to collimate. 

Look at the pattern of the star in a live view (this should be done with a camera).

There are two lines in an 30° angle, and additionally one, that is between or parallel to the middle line of this two. This one is the collimation line. You have to put it into the middle of the two others. When you loosen the lead screw, you can detect the direction.

Start with LOOSEN (!) this lead screw for 1/6th (not more). Then immediately after, TIGHTEN the other two screws about 1/12th. 

You should see, if the middle line goes into the right direction. If not, LOOSEN BOTH opposite screes to the lead screw 1/12th and TIGHTEN the lead screws 1/6th. 

This is only for the first time to determine the direction to collimate. 

Then go further only by loosen or tighten by a 1/10th or 1/20 respectively.
Do this as long, as you ROUGHLY are in the middle.

Then go to the next screw, and the next, and start over again. After several passes, you should have every single screw aligned.

• Use several passes
• Do not overtighten the screws
• Take your time
• Use a camera and a tablet/handy while collimating

Cheers, Axel