confusing mirror grinding

[sharpiel]

Originally Posted by ralph1

Yes, the mount is a GEM. I did consider a horseshoe or yoke mount but I decided it wouldn't be portable enough, if a 2 metre long telescope can ever be considered portable. Another idea I had in mind was a dobsonian with the azimuth axis pointed at the SCP but when this popped up I thought I would use it instead. Sorry, no pictures at the moment but I should be able to take some tomorrow. When I got it there was a box full of goodies, one of them being setting circles from astro-optical supplies so the mount could have originally come from there too. I've had a play around with it but I can't see how to put them on, or even anywhere to disassemble it so the setting circles could be unrelated. Possibly a surrurier truss would be good, it would have a mounting point in the middle for the GEM, ultimately the tubes purpose is to hold the optics in alignment. Another possibility stolen from garyseronik.com is a simple metal bar with the mirror and focuser at each end.
Ralph

I doubt Gary Seroniks single strut will work for you at 2 meters. You're going to experience a lot of Collimation issues due to flexure of the support strut over that distance. I'd suggest a reasonably light serurrier equivalent for best rigidity, collimation/optical alignment and vibration damping.

If I'm not mistaken the pics of the mount are one of the Samson (?) mounts Astro Optical sold. Mark or others could confirm how much leverage load they can handle but I wouldn't think that would be suitable either for your planned OTA. It's not the weight which will be your main issue but rather the length of your OTA will act like a long lever magnifying vibrational effects at the extremities even for a very light OTA. You may find the OTA will develop a resonating effect which may be difficult to damp out even in light breezes. You could hang a light chain from the OTA to absorb vibrations.

As you have this mount already no doubt you'll do your best to make it work but an altazimuth or other mount where the centre of gravity is between bearing points (English yoke, horse shoe, split ring) all of which can easily incorporate large bearing serfaces that damp out vibration, will be much more stable for your long OTA than a cantilevered design like a GEM.

[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.

[Wavytone]

Yes that's an old AO Samson mount - I had my 8" f7 Newtonian on the same back in the 1970's and IMHO it was quite poor - despite the beefy shafts there is a surprising amount of flexure in the equatorial headband as well as in the tripod (it can and will oscillate in azimuth).

It isn't hard to make a dob mounting that is stiffer and smoother.

Speaking of which I recall a 6" f/15 Newtonian at wiruna once, it had a square OTA made of 2mm 3-ply which was quite light despite the length and perfectly stiff enough. This was mounted on a Dobsonian base, the radii of the bearing surfaces had been increased in proportion to the length of the OTA, the result was quite good free of vibration and easy to use.

[Satchmo]

Nick- the 6" 15 you are remebering was probably Wayne Stuarts 6" F11 - it had 3mm ply square tube and aperture stops all the way down to strengthen the tube . It was actually parabolised ( I made the mirror using a collimated light source ) . Unfortunately ( at least in Summer when I used it ) it was next to useless on the planets . Heat always emanating from all the wood and tube currents jumping over the aperture stops into the light path and ociurse trapped in the corners between the formers. I learnt from that that you shouild always leave a Newt tube as smooth and obstruction free as possible to allow the warm air to escape from the tube as uninterupted as possible .

Ralph- again I'd respectfully suggest leaving your tube around 60 max 70 " . That was the longest tube you could put on the Samson mount . By all means figure your mirror on star at focus with a ronchi grating until the bands are straight . Whether you parabolise a little or not is moot as you would be chasing the needed sphere a little just as you might chase a slight parabola. If you null test on a very distant light or star its all the same . Anyway if it your first experience just treat it as learning curve to a perhaps bigger part of the hobby . You can buy a chinese 6" F8 mirror for half the cost you can get one aluminised over here anyway .

[ralph1]

That 6 inch F/15 sounds interesting; Mine's only 1 inch longer focal length and 1 inch larger aperture so the builder could have some relevant tips on long focus newtonians. Do you know whose it was?
What was it like hand-tracking with a long focus scope on an alt-azimuth mount, obviously assuming it had no drive system?
Can you think of any modifications that could be done to the mount to improve its rigidity? When I got it it had an 8 inch F/7 newonian with it as well, which had a damaged mirror, but I gave it a quick test terrestrially and it seemed to be fairly stable, the magnification would have been about 120X. The focuser installed on it was 0.965 inch so I was using the eyepieces I got with it. I since discovered that it was screw-in and could be replaced with the 1.25 inch focuser so I could reassemble it and try it out at up to 466X, which is more than I reasonably expect a 7 inch to use.

[ralph1]

I was having a look for it in some old threads; I think I may have found it in the 'Mt Kaputar 86&87? Are you in the pic?' thread from 2008. I'm assuming it is the one on the far right, not the EQ mounted one in the middle.
It's a learning experience not just to a bigger part of the hobby but to a bigger aperture

[sharpiel]

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.

[Wavytone]

Hi Mark ...

yes it probably was Wayne's scope. I wasn't aware it was "only" f/11. Although I never got a look through it I'm not surprised re the thermal problems - my 12" started life with a closed box tube which was likewise dreadful, which lead to chopping it up to make an open truss using glass fibre composite construction to lighten it and let air flow freely.

It's the scope at far left http://www.iceinspace.com.au/forum/a...se.php?a=51343

[ralph1]

Originally Posted by sharpiel

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.

It would also mean that a dobsonian would need a rocker box 1 metre tall; A GEM would be suited to a telescope mounted in the middle.
I had another look at the mount and the vibrations could be improved by filling the pier section which is hollow with sand or some other heavy thing, lowering the centre of gravity of the mount. It wouldn't help vibrations originating in the equatorial head but any vibration in the base would be minimised.
A truss design of any kind would serve to increase the weight concentration at each end even though it's lighter overall so my basic understanding of mechanics tells me the vibrations would have lower frequency and would take longer to dampen out. Is it plausible to put weights in the middle of the tube at the balance point to increase the frequency and decrease the damping time? Such weights could be attached to the centre section of a serurrier truss.
Ralph

[sharpiel]

What do you intend to use your telescope for? Is it for visual or photographic use?

[ralph1]

Visual exclusively. I haven't dabbled with anything more complex than camera-against-the-eyepeice images and I don't think a long focal length undriven telescope is the one to graduate to. The next project might be some sort of photography telescope; I like the idea of a schmidt camera but that won't be for at least a year as I don't expect to finish the current scope within six months.

[sharpiel]

Ahh gotcha. Well make your OTA nice and light and open so as not to catch the wind as much as possible and do the best with your current mount. Installing a motor focusser will help eliminate vibrations when focussing.

If you find vibrations become an issue you can always switch to a dobsonian in the future. Best of luck!