"Thanks for your interest in my mirror edge support sling. First, let me confess that I designed this sling for selfish reasons. My wife bought me a used 18" Obsession in perfect condition as a birthday present. The primary mirror was from Pegasus (John Hall) and it came with a excellent inspection report of P-V wavefront error 1/14 wave, 0.975 Strehl. The first night I had it out the seeing was excellent. With a star image in the center of the field I racked the focuser and saw the Fresnel rings go eliptical, switching major axis direction 90 degrees from in front of to behind focus - the hallmark of astigmatism. Needless to say, I was perplexed. An inspection of the mirror box revealed that the nylon web sling belt did not hang straight down to where it contacted the mirror edge. The mirror was adjusted forward with the collimation screws, and the sling angled forward going down to the mirror. It was evident that the sling was not only supporting the mirror upwards in its cell, but was also pulling backward on the edge, bending the mirror to a potato-chip shape. Seeing this was the catalyst for the production of the prototype Glatter Sling. After I installed the new sling the astigmatism disappeared, and I realized that I potentially had a new product.
I will copy below the text of my upcoming webpage information on the sling, together with some photos of the sling installed on an 18" UC Obsession:
Astigmatism free, perfectly collimated views, no matter where you point your telescope, and no matter where you position the focal plane with the primary! "
The Glatter Sling ™ for Obsession and other Dobs is a new development in primary mirror edge support systems that eliminates the problems of astigmatism and miscollimation present with many commercial and ATM slings. It is produced as a bolt-in replacement sling kit for Obsession telescopes, as well as a component for amateur telescope makers.
What's wrong with some slings ?
In alt-az mounted Newtonian telescopes such as Dobsonians, U-shaped slings provide the best edge support for the primary mirror because they produce the least optical surface distortion. However, slings have a few potential problems. They are:
1. Up and down motion of the mirror in the cell, which decollimates the optics. All plastic webbing slings (Nylon, Kevlar, etc.) stretch significantly under the mirror's weight when the scope is pointed towards the horizon, and then relax and pull the mirror back up when pointed towards the zenith. The mirror moves up and down in the cell, de-collimating the scope. This can be easily seen with a laser collimator. Slower excursions can occur when plastic slings absorb water from the air and elongate.
2. Side-to-side pendulum motion which decolimates the optics.
When the scope is pointed towards the horizon there is very little frictional force between the mirror and the back support pads. If the mirror is not restrained laterally it can easily swing sideways when the scope is slewed rapidly in azimuth, if the rocker box has not been carefully leveled, or if the scope is used on an equatorial platform. The mirror may "stick" on the back pads and not return to its lowest equilibrium position, de-collimating the mirror.
3. Out-of-plane forces which astigmatize the mirror.
If the points that the sling is supported from are in front of or behind the mirror, the sling will angle backwards or forwards towards the mirror edge, and exert an out-of-plane force at the points where the sling joins the sides of the mirror, pulling on and "potato-chipping" the mirror. Since the mirror is provided with collimation screws that can adjust it forward or backward with an inch or so of travel, sling parallelism to the mirror can not be maintained when the sling is suspended from fixed support points.
Also, to avoid distorting the mirror's surface the sling must support the mirror with a force exerted effectively at the mirror's center of mass, and acting parallel to the plane of the mirror. Because of the concavity of the mirror the position of the mirror's center of mass is not half-way through the thickness of the mirror but a little more toward the back, and therefore the sling should be positioned on the edge of the mirror this same distance from the back. With a web sling or a wide metal band it is difficult to insure that the sling support is effectively under the center of mass.
The Glatter Sling:
My sling implementation avoids all the above mentioned problems.
1. As have others, I use wire rope in order to eliminate sling stretch. Stainless steel wire rope, preferably plastic-jacketed to provide a cushion between metal and glass, provides an almost ideal sling material. With adequately sized wire rope, stretch under the mirror's weight is negligible. With the narrow cable it is easy to ensure that the sling acts at the plane of the mirrors center of mass. Small self-adhesive velcro guide strips, forming a notch for the cable, are placed on the mirror edge at the bottom and sides keeping the sling in place front-to-back.
2. I replace the split bolts with hardened stainless steel shafts and hang the sling from low friction linear ball bearings that ride on the shafts. The sling support points move with the mirror when it is adjusted forward or backward, and the sling always hangs straight down onto the mirror, parallel to the mirror plane, eliminating out-of-plane forces and the resulting "aslingmatism".
3. I replace the mirror edge bumpers with ones that have an off-center axial hole, so that they can be eccentrically adjusted by rotation to just miss touching the sling at each side. This restrains the mirror to its lowest equilibrium position, eliminating sideways miscollimation.
Obsession has made the sling supporting split bolts in two diameters, 3/8" and 1/2", so the replacement sling kits come in both sizes. Check and specify your split bolt diameter when ordering.
The Glatter Sling kit includes:
(2) hardened stainless steel threaded shafts with stainless steel nuts, lockwashers, and e-clips
(2) stainless steel linear bearings with cable retention rings
(1) stainless steel wire rope sling, cut and stripped to length and terminated with epoxy
(2) stainless steel wire rope clips
(6) self-adhesive velcro hook strips
(3) velcro loop strips
(1) 5.5mm nut driver sling adjustment tool
(2) Delrin eccentric bumpers
Installation Instructions
