Scope sliding off ADM saddle !

[eyepod]

Has anyone with a ADM Dual saddle had problems with the scope sliding off unexpectedly? I have had this happen twice now and can only assume its the cold weather. I purchased the EQ5 CG5 dual saddle upgrade for my EQ5 Pro and have a 130pds with a long skywatcher dovetail attached. Wednesday night i put it on the mount and tightened it nice and firm, about 20 minutes later after i had polar aligned etc i returned the scope to the home position and bang! ... lucky for me the bolt at the end of the dovetail stopped it crashing to the floor. I checked the dovetail and it clearly needs tightening again. The temp outside was about -1. The same thing happen last night after i had double checked it was firm, again the bolt just stopped it crashing to the floor. (the astro gods must be smiling on me!) The temp outside last night was again -1 to -2 so i can only assume the saddle contracts after you think its tight. I am now going to wrap a velco strap round the dovetail saddle etc before i think of putting the camera and other expensive bits on the scope, Anyways just thought i would post a warning If your upgrading from the old style saddle. : ) 

 

regards

 

Jason

[brantuk]

Yup - caught me out once too - no damage fortunately cos I caught it all in time. Pop a bolt in the top end of the dovetail bar to stop it sliding - as near the saddle as poss and hopefully where the scope balances. Then it will only slide a short (if any) distance before stopping. It happens when the dovetail gets cooler and shrinks quicker than the saddle and it's clamps. I put a bolt through all my dovetails now - even if it means drilling a hole. Hth

[Ibbo!]

I have drilled an extra hole in the plate and put an stainless nut and bolt in so it cannot slip through.

Mine that did it had a C11 on it.

[eyepod]

its not just me then...! perhaps these saddles are due a little redesign, after all they get rave reviews which makes you have confidence in its grip. I'm not saying there not good.. its a quality item, but a least a little warning in the box to be extra careful in cold weather.?

 

Jason

[Davey-T]

Mine came with the screw so they must be aware of it.

Dave

[steppenwolf]

I use ADM saddles and dovetail bars for my dual rig system and have never had a hint of slippage but I still have a safety détente bolt on all of my dovetail bars to be certain. Something that I am very conscious of when tightening mine is ensuring that the fixed and moving sections of the saddle are definitely parallel with one another - i.e. gripping the bar evenly along its whole contact edge on both sides of the bar.

[RobertI]

I find that when I attach the scope to the ADM saddle the bolts appear to be fully tightened, but if I hold the tube and 'wiggle' it up and down, I am able to tighten the bolts a bit more, indicating that it wasn't fully secure in the first place. I now 'wiggle' by default and have not had a problem (yet). I still think you cannot beat an angled safety screw in the saddle but they do mar the dovetail. 

[brantuk]

1 hour ago, eyepod said:

its not just me then...! perhaps these saddles are due a little redesign, after all they get rave reviews which makes you have confidence in its grip. I'm not saying there not good.. its a quality item, but a least a little warning in the box to be extra careful in cold weather.?

 

Jason

The thing is Jason - over the lifetime of a mount a lot of folks change the saddles and use different manufacturers dovetails with them on different scopes. So invariably you end up with different metals of all sorts coming into contact with each other. And who checks the coefficient of expansion and contraction for every item they buy - much less match them all up? I know I don't lol.

Screwing correctors and visual backs onto a scope is another typical area where this happens - some shrink up solid and won't budge when you want to take them off - until it warms up a bit or you apply some sort of heat. It's good to be aware of it though and take appropriate action to prevent damage occuring.

[Merlin66]

I use a C11 on an ADM saddle and fitted a safety clamp on the top side of the dovetail. This gives added comfort when positioning the scope on the mount and repeatable positioning (for balancing)

http://www.ebay.com.au/itm/Astrodevelopments-Losmandy-style-dovetail-safety-clamp-for-scope-mounting-plate-/270977887187

Highly recommended.

[triton1]

Even though mine has the angled screw I also added a cap head under the dovetail and balanced the scope around it the bonus aswell is the scope always ends up in exactly the same position.

[ollypenrice]

Yes, I've had slippage with ADM as well. The stop-bolts did their job so no harm done. It was hot-cold-hot-cold that did it, I think. The item in question was a Vixen to Losmandy adapter.

Olly

[steppenwolf]

Is there a pattern here? I exclusively use Losmandy style dovetail bars rather than Vixen even though I have dual size ADM saddles. Is the problem confined to the Vixen type? I only ask as Olly's comment mentioned a V to L adaptor but he doesn't say which way!!

[Grotemobile]

Thanks to all of U for the warning.

Just put a bolt in my dovetail. Never

have give this a thought, up to now.

 

Steve.

 

[Stub Mandrel]

Are we talking aluminium dovetails fitted inside cast iron clamps or fitted with steel clamp screws?

The aluminium will contract less than the CI/steel if the temperature drops by, say 20 degrees. Figures in um per meter per degree are:

Aluminium 22

Stainless steel ~15

Carbon steel 12

Cast iron 11

Over a 5cm dovetail width and a 20-degree temperature drop the change in dimensions would equal the figures above in um, so the difference between  cast iron and aluminium would only be 0.011mm or less than half a thou.

That doesn't seem enough to cause disaster to me, so I suspect something more subtle is happening, such as temperature cycling loosening fixings (as Olly describes).

The dovetail on my cheap as chips Bresser has a broad notch for the fixing screw -  a useful idea that gives extra insurance with the stop screws.

Neil

[eyepod]

Mine is a skywatcher 33.5cm dovetail on a dual saddle which i think are 6016 aluminium. Both times it happened to me the mount came from a warm room at 20c going outside into -1 to -2c. It slipped after about 30 mins, both times the mount was tight i always check with a torch along both sides of the dovetail to check it is seated correctly. I can't think what else would have caused it to slip other than a change of temp?

 

Jason

[steppenwolf]

It certainly sounds like you are doing everything right, Jason so from your full description (warm room to cold) temperature change would seem to be the issue - my own situation is different, of course, as my two elements are out in the cold all the time!

[eyepod]

2 hours ago, Grotemobile said:

Thanks to all of U for the warning.

Just put a bolt in my dovetail. Never

have give this a thought, up to now.

 

Steve.

 

Glad we have saved someone from an expensive disaster!

Jason : )

[CraigT82]

I would have imagined the problem would be worse with the losmandy bar rather than vixen, as the bar is larger so it contacts more?  (still contracts by same % but larger bar equals larger overall contraction?).  I'm probably wrong!

 

I always put mount and scope out to cool separately, only mount it up once everything is cooled to ambient or thereabouts.

[ollypenrice]

On 14/2/2016 at 11:22, steppenwolf said:

Is there a pattern here? I exclusively use Losmandy style dovetail bars rather than Vixen even though I have dual size ADM saddles. Is the problem confined to the Vixen type? I only ask as Olly's comment mentioned a V to L adaptor but he doesn't say which way!!

Erm, he doesn't remember! (Come on, you know what he's like...)

I have adapters both ways and can't remember which way did the slipping. I think it was the Losmandy. A while back there was a considerable correspondence on the subject of Losmandy dovetails and it seemed that there was no clear industry standard, with poor grip, slippage and even total incompatibility being reported when brands were mixed. It's a nervous subject. To let Yves sleep at night I welded up a gadget to ensure that his 14 inch couldn't fall off the Mesu if some slightly questionable saddle plate locks failed. I vote for belt and braces.

Olly

[steppenwolf]

Yup, I agree, belt and braces is a good policy - welding you say?

[ollypenrice]

10 hours ago, steppenwolf said:

Yup, I agree, belt and braces is a good policy - welding you say?

I didn't weld the dovetail and saddle together! Eek. Trying to stop Yves from having his scope back didn't go that far...   No, I made a bracket that connected the dovetail to the saddle in the event of a saddle clamp failure. It was an odd and discontinued kind of Baader saddle which made us jumpy.

Olly

[steppenwolf]

Quote

I didn't weld the dovetail and saddle together! Eek.