A 13" Mirror cell for a telescope

[pdentrem]

I am going to build a new mirror cell for my telescope. It is a Newtonian reflector with a 13" mirror. A mirror cell is the support structure for the mirror. Without a proper support system a mirror would be distorted to uselessness. In using a program called PLop, one inputs the mirror data and the program will then plot out the positions of the supports. You can run the program several times and change the number of support points to find the system that works best for the mirror.

The distortion is very minute, like .0000125mm, which is 1/40 of the wavelenght of 500nm Green light. I started a design in Autocad using the information for a 6 point support system. The red triangle is 1" square tube and the small yellow bars along side are the supports for the mirror. One also has to align the primary mirror with the secondary mirror which is in this case 50" above the primary. The red triangle structure will have a fixed pivot, which is at the top and two adjustable ones on either side to collimate the mirror system to get good views through the eyepiece.

I include 3 files, a gif of the 3d drawing of the cell system and 2 plots for the results for the 6 point and 18 point support system. The 18 point is better but overkill except for one who is OCD. Note the small size of the numbers at the right side of the plots. TINY!

More will come along over the summer as I build this up.
Pierre

 

[Tony Wells]

I don't know any machinists who have OCD :biggrin:

 

[12bolts]

Nice! My GF wants me to eventually build a telescope on the lathe. I will be watching your progress.

Oh boy, that quote could be turned around into so many connotations:lmao:

Cheers Phil

 

[pdentrem]

Small update

I started during the last week with modifing the current wooden mirror cell, which started life as a 9 point support system. Originally made back in the pre Plop days. This is a proof of concept as the old will be replaced by the new steel cell, but I still want to use the scope of the summer and fall. This will improve what I have and be a prototype to work off of.

What I have done is remove the old support structure and built new rockers (3 rocker with 2 points on each) and placed them in the correct places. I then made 3 new outer retainers that hold the mirror in the cell but do not touch the mirror. The actual support for the mirror is a cable that wraps 180 degrees of the mirror.
The 2 pictures show the 3 supports, 3 retainers, which one, lower right, is open to allow the mirror to be removed/installed, and 1 of the 2 blocks, top left, that the cable will attach to. I hope to have the cable system made up by the weekend.
Pierre

 

[pdentrem]

I now have installed the cable sling and reinstalled the mirror back in the lower assembly. I hope to test Friday night. I will paint the last piece of wood later. Have airshow to go to on Saturday.

First pic shows the adjuster end of the cable, with the cable being held up by a little dab of caulking. The second pic is showing the cable wraping around the mirror at it COG. The last is showing the mirror back in the lower assembly ready for testing tomorrow night. Weather permitting!

 

[Tony Wells]

Very cool, Pierre.

 

[pdentrem]

Getting closer to finish line. Tonight I worked on finishing the cable sling hardware. The adjustable side needed to have metal removed to lower it. The cable sling has to be on the COG line of the mirror. I started them tall to allow final fit and this was a removal of about .250" on the stud head and half of that of the bottom of the cable stay. The cable stay is held by 2 set screws and the threaded stud is where the adjustment is made. The cable stay part simply is rotated and locked in position when the cable sling is in the right orientation.
Pierre

 

[savarin]

Nice work, cant wait for the finished product.

 

[pdentrem]

Here is a not very good shot of the scope without the shroud in place.
Pierre

 

[savarin]

Nice, just the two truss tubes?
Does it pack down to something like a travel scope?