Sling or Cable?

I'm reposting my question here, as it was getting limited response in the Dob forum:

For all you truss-tube dob owners, who uses a sling and who uses a cable to support their primary? For those using a sling, have you had any problems with sling induced astigmatism? What size is your primary? Is a sling adequate for mirrors below a certain size? Any other comments on this subject?

I use a sling on a 10" f/6 mirror 1.6" thick and have no problems with it. Mine is a Nylon sling about 1.5" thick. Kevlar is also used and will not stretch.
A metal sling can also be used, such as strapping tape that is used to tie lumber and boxes. Also aluminum has been used as a permanent sling, with no adjustment.

I use a nylon sling about 1.5" wide - kinda looks like something a seatbelt would be made out of. I have not really noticed any problems from it. It is on a 20" f/4.0 1-7/8" thick mirror. The sling has served well since 1991 when the scope was built. If I were to observe much below the 15-20 degree angle, I would probably notice some distortion, though.

I have a Webster mirror cell that uses a stainless steel cable for the sling. I've only had it for the past few months and so far it works very well. I was able to see Omega Centauri 10º above the southern horizon with no problems whatsoever with distortion or movement of the mirror. It was expensive but IMHO worth it.

I use point supports in pairs, similar to the ones Jim Nadeau (nightsky scopes) uses (see pic)--my mirrors are a 16" that is 1.5" thick, and a 10" that is 1.25" thick--I do not detect any astigmatism at low elevations. I find this sort of system much easier to build. Now Jim's edge supports move with the collimation adjustments, but you can build a simpler cell where the supports are stationary--the mirror will just slide on the supports as you adjust collimation if you face the supports with an appropriate material--felt pads on my 16", and teflon on my 10" (which being a much lighter mirror, got hung up on the edge supports when lowering the collimation bolts--until I went to teflon).

Another thing I like about this system is that the mirror really stays put. I tried (on another project) just gluing a thin 13" mirror to the cell with no edge support and noticed a large shift in position at low altitudes.

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Summing up what I've read so far, it appears there are two schools of thought. I originally read about cables on the Yahoo bigdob group, with Nils Olaf being one of the chief proponents. Dave sums up that school of thought nicely:

What you want is something that has as little friction against the glass as possible, does not stretch even a millimeter with temp or tension, and something that does not go near the face edge of the glass...

But Floyd and others, including myself I think, believe:

It seems obvious that the cable being narrow would not distribute the weight over as large an area as a strap or sling.... You want to place the weight over the largest area to minimize distortion...

So is either of these a proven fact, or is it merely a matter of preference? Also, once again, I wonder whether the cable method, if valid, only applies to mirrors over a certain size?

I've used metal slings on both my 16" and 24" Dobs, and my solution to avoiding stiction has been to wrap adhesive-backed teflon tape on both the sides of the mirror and the face of the sling. The coefficient of friction for teflon on teflon is ~0.04 -- about as low as you'll find for *any* materials. (For reference the coefficient for steel on glass is between 0.5 and 0.7)

I've successfully used my 24" Dob to point at Omega Centauri from mid-Atlantic latitudes (Virginia), so this Teflon/sling combination performs well down to what are essentially horizontal viewing angles. FWIW, this sling replaced a conventional nylon one on my 24" scope; part of my motivation to change was watching the primary move over 1/2" in the scope between night and day at WSP two years ago in response to changing temperatures and humidity. The new steel sling is basically insensitive to these environmental factors.

I have used both thin tape and wider 'bumpy' textured teflon tape, and make my slings out of different thicknesses and widths of steel lumber strapping tape. (I wanted some 'give' in the sling supporting the relatively light 1/20th wavefront 1 3/8" thick 16" mirror, whereas on the 75 pound 24" I was looking for something stiffer.)

You can see examples of both slings and types of tape on my web site, http://www.jimrichberg.com/astronomy/
Insert "sling" into the FreeFind search tool in the upper left corner hand side of my home page and you'll find a half-dozen photos or references to the slings' performance.

Clear skies,

Jim