How do I reduce mount (CG5) / OTA (C9.25 CF) wobble?

[g__day]

Guys and gals,

I'm interested in your suggestions as to how to reduce my set-up's wobble during focus adjustment / general viewing factor. I definitely feel the mount to dovetail attachment is fine, but the dovetail to OTA attachment is a bit under capacity and risky. I ponder maybe a better (more rigid) dovetail bar with the appropriate rings would work better; especially seeing I have a 80mm Meg piggy backed. See attached piccys:

The basic set-up is okay, but I first noticed when you tried to adjust focus using the Celestron's primary mirror focusing knob there was "alot" of jiggling in the image - enough to make achieving best focus hard. I eventually side-stepped this by moving to a Meade 1209 motorised focuser.

But I still feel the scope to mount's dovetail attachment design is allowing the wobble. Although the CG5 attachment bar is rigidly held by the mount, push even gently on the top of the scope and it will move and vibrate freely a millimetre or two. Also the CG5 dovetail bar attaches to the OTA by only three little bolts, carrying alot of weight, so it would probably be a much better and safer idea if the bar was firmly holding two (or three) appropriately sized O rings - around 10", and these held the OTA to the mounting bar, rather than three little screws from the mounting bar into the OTA itself.

What are people's observations and suggestions please? Should I just buy two or three rings and mount the OTA to these and firmly attach these to the CG5 dovetail bar for instance?

Many thanks!

[Dennis]

You may not want to hear this, but I have the same OTA’s as you; a C9.25 XLT CF and a WO 80mm Apo triplet and they are mounted on a Tak EM200 with the orange Celestron CGE dovetail rail (a Losmandy fit).

The (wider) CGE dovetail, like your (narrower) Vixen style dovetail, fits to the OTA with the same 3 screws. I don’t experience the same wobbles you describe, although our interpretations and grading of these wobbles could obviously be quite different.

Suffice to say, the OTA is quite rigid on the mount and when adjusting the focus via the C9.25 focus knob, I don’t experience any unacceptable wobble, the main component being the ubiquitous slight image shift.

So, I’m not sure you would gain the improvement you are looking for, if the mount itself is an equal contributor to the wobbles?

Cheers

Dennis

[g__day]

Dennis, thanks

I have the CGE dovetail too - its the one at the top of my mount attaching the Meg via O rings to my C9.25. The Meg to the C9.25 seems pretty stable so I can summise the CGE is a much more rigid dovetail for two reasons:

1. Although it still attaches by 3 screws 1 forward and 2 at the rear, the two screws at the rear of the dovetail are 40mm apart on the CGE vs 20mm apart for the CG5 dovetail. This provides a much more stable and rigid coupling.

2. The CGE bar being twice the width of the CG5, with a curved base and flat top, has more metal in it and that makes it less prone to flexure in and of itself.

So these two factors combine to increase rigidity, although I intuit it is point 1. above that is predominantly determining rigidity.

[sheeny]

G'Day g_day,

I would agree with your assessment of the CG-5 dovetail. I had similar problems and like you I went to a motorised focusser, but I still notice unacceptable movement in the wind. I expect the CGE dovetail is probably significantly stiffer.

I don't think the size or number of the screws attaching the dovetail to the OTA is a concern. They should be plenty strong enough.

Apart from the alternate dovetail and rings solution, and alternative you might like to try is to make a new solid dovetail bar to replace the CG-5 bar.:shrug: I suspect the grooves in the bottom of the CG-5 dovetail bar seriously diminish the bar's polar moment of inertia and allow it to twist easily between the mount and each end. I haven't got my act together to try that step yet:P .

Al.

[Dennis]

Ahh, yes, I see the (larger) CGE top dovetail now.

Just for some background information, I have a Vixen GP and a Vixen GPDX mount, which I think are similar to the CG5? When I only had the GP mount, I was new to astronomy and it felt rock solid for my 4” f9 refractor and VC200L 200mm f9 Cassegrain. I then acquired the GPDX and the OTA’s felt even more solid on the GPDX, making the GP mount (almost) feel more like a toy in comparison.

I once fitted my C9.25 to the GPDX using the (Vixen sized) Celestron dovetail rail and the whole thing felt quite wobbly. Part of this could be attributed to how the GPDX head clamps the rail, with a single main, clamping screw and a smaller, secondary locking screw. There was nothing weak or loose, but the overall system did flex and was very, very vulnerable to shakes from the slightest breeze or touch.

I then fitted the C9.25 with (Vixen sized) Celestron dovetail rail to my EM200 with a Vixen saddle plate and the OTA felt very solid. The Vixen saddle plate on the EM200 head works like the Losmandy system; it forces a bar against the dovetail rail to lock the assembly in place.

When my WO saddle arrived and I re-fitted the CGE rail to the C9.25 and slid the OTA onto the EM200, once again the rigidity seemed to increase compared to the Vixen rail.

The GPDX is indeed an excellent mount and very rigid for its class and size, but it now feels quite wobbly compared to the EM200, as no doubt a Paramount ME would make the EM200 feel wobbly by comparison.

So, what I concluded was:


The smaller Vixen mount and components were overtaxed by the C9.25 whereas the EM200 was designed to cope with that class of weight and torque.
The Losmandy style clamping method of a bar, versus the Vixen single screw provides superior stiffness in mounting the OTA to the head.
The CGE (4”) dovetail rail is more stable than the Vixen GP style (1 7/8”) dovetail rail.
The greatest reduction in wobble was attributed to upgrading to the larger, heavier EM200 which I presume means beefier components and more mass, as well as the tighter tolerances.

So, I strongly suspect that by you improving the rigidity of the OTA component by using the superior method of tube rings on a rail, you would still be left with the inherent “weaknesses” of the CG5 clamping method along with the smaller, lighter components of the CG5.

Cheers

Dennis

[g__day]

Dennis and Al - thanks!

I ponder how had it is to machine a steel bar rather than aluminium, and attach tube rings to that.

I looked at a EMM200 closely at Magellan Observatory last week, and they look rock solid in comparision to the CG5 mount - definitely the class of mount I'd like to have in the future!

[Dennis]

The Vixen mounts at $750 (GP), and $1400 (GPDX) are excellent mounts for astro photography when used around the mid-range of their weight carrying capability, but the step up to the Tak EM200 does come at a big cost, between $5300 and $7600 for the head only, tripod extra!

It seems that in astronomy, very little is free. :(

Cheers

Dennis

PS - the above Tak prices are from the AEC website (http://www.astronomy-electronics-centre.com.au/)at a time when the Yen is ¥94 to the $. When I purchased mine, the range was from $7300 to $9500 (IIRC) based on a much less favourable exchange rate, so now is a good time to purchase Tak gear if anyone is interested.

[gbeal]

While it is mere gut feeling, I have never liked the three screws into the front and rear cell method of connecting the rail to the OTA, just doesn't seem right. Rings would be my preference.
Maybe you could also try by-passing the dovetail and saddle idea altogether. It seems you are semi-permenant, and perhaps you could bolt the lower rail directly to the top of the mount head (where the current saddle attaches). The EM200 allows this, with the Tak clamshell fitting directly, with two allen bolts.
This may remove any wobble from where the rail fits within the saddle, and if nothing else will get your weight closer to the pivot, albeit only slightly.
I don't think steel as a rail is any better to be honest.
Dennis (as usual) is right, you normally get what you pay for with a mount, and this ultimately could be the next remedy, upgrade. Try to work the current system first though.

[sheeny]

I see little advantage in going to a steel bar unless you just happen to have the steel lying around at the right price. If/when I have a go at it I plan to use solid Aluminium bar. It may be a little more expensive but it will also be significantly lighter than steel. I'll just have to see which one of the local engineering firms owes me a favour at the time to get it machined;) .

Al.