200mm f/4 mirror cell upgrade

[RyanJones]

Hi All,

I noticed in my recent images that I was getting some astigmatism. I had carried out some modifications to my focuser to stop the tube tilting in the body and consequently holding collimation better. I had also turned the focuser to face downwards to create better balance and a lower center of gravity. Obviously with the astigmatism, my attention turned to my primary mirror. After a bit of thinking and looking I realized that when I had turned my tube it had also turned my mirror cell such that the mirror was resting on a single side support in the basic 3 point cell. That got me researching.....

I learned an incredible amount about the cell. How it is supposed to JUST support the mirror and the benefits of multiple supports to distribute the load ( short of a sling setup ) Also the benefits of a floating support system under the primary. Probably not all entirely nessasary on my 200mm mirror but while I was there .....

What I came up with was a system that consisted of a 9 point floating cell with 120 degree waffletree side support. The idea was to use the exsisting mounting points as much as possible so that should it not work, it will be reverseable.

The original cell appears in the first picture. First I made some aluminum triangles with hard plastic inserts sized and mounted such that the outer support is at about 75% from center and the inner support is positioned just outside the shadow of my secondary. Shown in the second image.

Then I set about making the 120 degree side supports. I didn’t have enough room to use bearings as per some other designs I’d seen. Instead I used nylon screws. The radius bars on the side are aluminum C channel that I manually bent into the correct radius. They are mounted in the center using the original screws from the 3 point cell. Shown in image 3.

I painted the radius bars and then fitted everything back together. The original screws have nuts fitted so that I can adjust the height such that the nylon screws contact the mirror at the center of gravity of the primary shown in the 4th image.

Fully adjusted I can still turn the mirror in its cell showing how gently it is holding the mirror. I fitted it back in the tube and did a quick collimation with the laser. Preliminary testing shows that it doesn’t change collimation from the horizon all the way through to the zenith. Now I’m just waiting for the clouds to clear so I can do a star test..... fingers crossed.

Thanks for looking, comments always welcome.

Cheers

Ryan

[VPAstro]

Hi Ryan, I am interested to see how this goes, as I am having a similar issue with the primary mirror. Looks like a great solution, Andrew...

[peter_4059]

Hi Ryan,

Looks like a good improvement to the basic cell. How did you bend the radius bars without distorting the sides? Are the three triangles for the 9 point supports free to move? It looks like a bolt attaching them to the original cell.

Peter

[multiweb]

Pretty cool project Ryan. Also interested on how you've bent that alu C channel.

[Sunfish]

Thanks Ryan. Great post. I did something similar with nylon screws and three triangular supports for my home made mirror but have not tested. Not neat as that with your curved retainer support. I wonder how the skywatcher mirror support compares with the GSO variety and if it suffers some of the same problems.

[RyanJones]

Thank you Andrew, Peter, Marc and Ray.

So far it seems to have been a successful project. My stars seem better and the mirror cell appears with a laser to hold collimation much better. I tested it with a meridian flip and in various areas of the night sky and the stars at least visually seem to maintain a even shape. I’m looking at getting an auto collimating eyepiece soon which will really test the theory but at the end of the day it’s the overall outcome that matters.

One of the challenges I faced upon completion was that the primary obviously moved slightly up the tube. This meant that initially I couldn’t get enough out focus. I solved that with a 2” nose piece on the front of my MPCC. This has also had the added bonus of making my focal point more central in my focuser tube’s travel. I had already modified my focuser to hold the tube more concentric so now having it half way through its travel it has even less flexure. 2 birds one stone.

With regards to the triangles and their fixings. As Peter quite correctly noticed they are bolted to the original cell. On both the bolt head side and inbetween the cell and triangle I have rubber washers. The bolts are tightened just enough to take up the clearance allowing the triangles to flex as required to find even surface support.

As far a bending the aluminium C channel goes. That was done by hand. I bought a 1m length. With a block on the ground to wedge one side up, I gently pushed down in the center to begin the bend. Working backwards and forwards over the length to create an even curve. Once the radius tightened significantly I then moved to bending it over my knee. Once I had created enough of a radius in the center of the length I proceeded to cut out the section lengths that I required. A very crude way of going about it I know but I didn’t have a metal bender. Having seen what I could achieve with my hands though, I think I may invest in a small vice mounted one for some other projects I have in mind.

Thank you for your interest guys :)

Ryan