Quote:
Originally Posted by ralph1
I was pondering this last night while observing. I would like to stay with F/13 if at all possible but the cantilever of the mount is a problem. Unfortunately, the GEM uses cantilevers at every possible oppurtunity while a yoke mount doesn't use them at all. With a long OTA critical balancing would become much more important but I don't mind spending an extra 10 minutes getting as close to perfect balance as possible. When you say serurrier equivalent what do you mean? I thought the point of a serurrier truss that it flexes equally on both sides. I think a horseshoe would be better than a yoke because of the bigger bearing surface and the ability to observe faint galaxies in octans  . On a different note, the original problem solved itself( the decreasing sagitta) so now I can sleap knowing my mirror is behaving. |
All rigid structures flex and bend and with telescope tubes the amount of flexing and bending will change depending on the change in position of the tube relative to horizontal. The serurrier truss was very cleverly designed to ensure the amount on flexure on either end of the tube was always the same as the othe end so that the optical centres of the primary and secondary mirror always remained aligned and there was no loss of collimation. In my experience most amateurs don't do the differential calculations to ensure this. We just make a truss and hope the triangular rigidity is sufficient to minimise any tube flexure.
As to your mount. With such a long tube and small mirror your centre of mass is likely to be close to the centre of the length of the tube. In other words your centre bearings will be around 1 metre up the tube. This will present problems with horse shoe and split ring mounts as the diameter of the ring for the right ascension bearing will have to be almost 2 metres unless you either weight the mirror end of the tube to change the balance point or offset the declination bearing attachment points from centre on the RA ring.
If portability is an important consideration I would probably recommend a dobsonian mount with larger altitude bearings for better stability. You can always put it on an Equatorial platform if you want to do photography. You can still make a suitably stable EQ mount but I'd make large thrust bearing plates as part of each axis to maximise your bearing surfaces and reduce vibration/increase stability. Richard Berry's book How to Build Your Own Telescope has a design inside for an EQ mount with a thrust bearing RA axis which you could adopt. I think his used Teflon and Formica like dob bearings as the bearing surfaces but you could easily incorporate roller bearings (or even a lazy susan) for better accuracy if you were inclined.