Vixen GP tube mount problem

[Dave_D]

So, i'm going to be stuck with this Vixen GP for quite a while until funds allow me an eq6 or similar. The problem i'm having is the fixing between the mount and the dovetail. As you can see from the pics below, the fixing is (to seriously understate) less than ideal in my opinion and allows the tube to move in relation to the mount and allows no adjustment for cone error. Pressure on the bottom of the tube shows that the eyepiece end is moving a good 5mm regardless of how tight the dovetail bolt is screwed in which can only make be think that flexure must be attrocious.

A couple of hours searching the net and i'm no closer to any kind of solution. only thing i could think of is some kind of angled bracket that would sit between the GP mount and the dovetail to 'take up the slack'. Anyone got any ideas what i can do? or has anyone come across some sort of plate that would bolt onto the top of the mount?

[Nakedgun]

Looks like you could grind away the sides for the plate which are too thick and are not allowing the dovetail plate to bottom out in the saddle. Or, re-attach the rings to one of Anthony's dovetails:

http://admaccessories.com/Productgallery3.htm

[michael.h.f.wilkinson]

I must say I have not encountered this problem on my GP mount. When I mount my dovetail, I do not get the gap you have between base of the dovetail and mounting puck (neither with the C8, nor LS35, nor APM 80mm). This gap is probably the source of your problems. Grinding down the dovetail or replacing it as suggested is probably the best solution, as the dovetail bar seems to be at fault, rather than the mount itself. 

[Dave_D]

I must say I have not encountered this problem on my GP mount. When I mount my dovetail, I do not get the gap you have between base of the dovetail and mounting puck (neither with the C8, nor LS35, nor APM 80mm). This gap is probably the source of your problems. Grinding down the dovetail or replacing it as suggested is probably the best solution, as the dovetail bar seems to be at fault, rather than the mount itself. 

Yeah, i figured the dovetail is the actual problem. Being an ex-draughtsman and a stickler for correct measurements, i just got my digital callipers out on the dovetail. I was absolutely gobsmacked on how inaccurately machined this, for want of a better description, 'lump' of metal is

[John]

Lets point the finger in the right direction here - the dovetail bar is Orion Optics. All the OO scopes I've had have this cast type bar and it's somewhat crudely done to say the least. What you could do is to replace the OO dovetail bar with a flat plate that has radius blocks that fit the tube rings (Orion Optics should be able to supply one of these) and then get a nice CNC dovetail bar and bolt it to that plate. Thats the arrangement that was fitted to the rings of my OO 12" F/5.3 optical tube before I converted it into a dobsonian. Seems a pity to change a fine mount on account of a badly made dovetail bar.

[Dave_D]

Lets point the finger in the right direction here - the dovetail bar is Orion Optics. All the OO scopes I've had have this cast type bar and it's somewhat crudely done to say the least. What you could do is to replace the OO dovetail bar with a flat plate that has radius blocks that fit the tube rings (Orion Optics should be able to supply one of these) and then get a nice CNC dovetail bar and bolt it to that plate. Thats the arrangement that was fitted to the rings of my OO 12" F/5.3 optical tube before I converted it into a dobsonian. Seems a pity to change a fine mount on account of a badly made dovetail bar.

Don't get me wrong, it's a good mount. The problem with it (well, not really a problem) is that it's rated at 7kg loading and the scope plus 'stuff' is 11kg  that's the only reason i'll be eventually getting a better mount. I just stumbled across this: http://astro.neutral.org/lxd55_diy_dovetail.html which is exactly what i'm looking for, especially as it could be made longer that the present dovetail. I'd like to get at least 12 inches between the tube rings as opposed to the 7.25 inches of this dovetail. 

[michael.h.f.wilkinson]

Actually, the rating I got on my mount is: 4" Frac, 6" Newtonian, or 8" SCT. Vixen factors (factored) weight and length of scope into the equation. Besides, the EQ5 is rated at more than the GP, but I hear the GP is more robust in reality. Official weight ratings have a lot to do with the degree of optimism of the manufacturer/salesperson.

[Dave_D]

Actually, the rating I got on my mount is: 4" Frac, 6" Newtonian, or 8" SCT. Vixen factors (factored) weight and length of scope into the equation. Besides, the EQ5 is rated at more than the GP, but I hear the GP is more robust in reality. Official weight ratings have a lot to do with the degree of optimism of the manufacturer/salesperson.

I can agree with that as i've got some reasonably good pics so far, the mount seems to track reasonably well unguided. What's bothering me though is that it's a 10" scope, admittedly, the OO GX250 is a lightweight by all accounts but i've yet to add my autoguider scope which should be arriving soon and any wind stronger than a gnats fart and i may as well not bother setting up. hopefully, now the eq8 is out there'll be a flood of cheap(ish) eq6's hitting the second hand market  (crosses fingers, toes, eyes and just about everything else)

[John]

A 10" Orion Optics OTA will weigh about as much as an 8" Skywatcher does of a similar focal length.

[Dave_D]

Tube rings and dovetail (which, to be honest were garbage) replaced:

hopefully i'll get a bit better stability of the OTA in relation to the mount now. The vixen puck is still a major point of issue though, and unfortunately i can't do much about it as the puck and motor assembly are a single unit... unless anyone knows different?

[cotterless45]

change the puck, mounting bar and saddle to Vixen,

http://admaccessories.com/V_Series_Saddle.htm

[Dave_D]

change the puck, mounting bar and saddle to Vixen,

http://admaccessories.com/V_Series_Saddle.htm

you sir, are a hero   and FLO stock them too... double   

[Dave_D]

and the parade is rained on... it's days like this i really regret getting a scope on a vixen mount.... 

[Dave_D]

found this thread: http://stargazerslounge.com/topic/122648-adm-dual-saddle-upgrade-for-eq5-and-cg5-series-mounts/

sad that it doesn't fit as this would be exactly what i need 

[michael.h.f.wilkinson]

Odd that you do not get along with the GP mount. It has been a total stalwart for me, for over 18 years now). I have had no issues with the Vixen puck at all (even if Losmandy wedges do give better stability).

[Dave_D]

Odd that you do not get along with the GP mount. It has been a total stalwart for me, for over 18 years now). I have had no issues with the Vixen puck at all (even if Losmandy wedges do give better stability).

It's a good mount, unfortunately it simply doesn't have the capacity to do what i want, thats all. I got my autoguider the other day and with the ota+autoguider it was just too much for the mount. Had to pull the plug on the power REAL quick when i tested it all because of the rather worrying loud grating noise the mount made as it tried to slew everything from a horizontal position. So when i eventually get a mount than can handle the load, this GP will be the home of something like a SW 130PDS or vixen R130SF which it will handle magnificently.

[michael.h.f.wilkinson]

It's a good mount, unfortunately it simply doesn't have the capacity to do what i want, thats all. I got my autoguider the other day and with the ota+autoguider it was just too much for the mount. Had to pull the plug on the power REAL quick when i tested it all because of the rather worrying loud grating noise the mount made as it tried to slew everything from a horizontal position. So when i eventually get a mount than can handle the load, this GP will be the home of something like a SW 130PDS or vixen R130SF which it will handle magnificently.

Or a C8

[E621Keith]

Dave, I just saw your PM and then saw this thread.

I don't think there is a problem with your mount if the movement is only 5mm at the eyepiece end of your 250mm Newt. I doubt the ADM saddle upgrade will solve the problem.

First of all, as John mentioned, your dovetail is an Orion dovetail. It works differently to the standard Vixen dovetail. Normally, the Vixen saddle's locking bolt pushes the Vixen dovetail into the base of the saddle and the opposite flange. Unfortunately, Vixen GP's saddle had raised 'Made in Japan' markings on the saddle base which could make flat dovetails unstable. Vixen own brand dovetails have a hollowed section in the base which provided clearance for these letters, but other brands' dovetail may not. Orion's dovetail was designed to engage the top of the Vixen saddle. The locking bolt forces the top of the saddle flange into the dovetail. This design also lifts the dovetail off the saddle base, so the 'Made in Japan' letters will not interfere with the fit. From my own experience, I felt the Orion style Vixen dovetail plate was more secure in a regular Vixen saddle than a normal vixen dovetail. 

Your Orion dovetail plate has integrated stoppers at each ends of the dovetail to prevent the scope from accidentally slipping off. However, these stoppers will make the dovetail plate incompatible with ADM's or other long pressure plate type saddle. If you buy the ADM saddle upgrade, you must also replace the dovetail.

Anyway, I don't think the saddle plate or the Vixen saddle is the source of your problem. 

1. Can you describe the 5mm movement you felt at the eyepiece? Is it a loose 5mm movement or a damped 5mm movement? If it is the former and the scope is rattling, then there is something wrong. If the movement is lightly damped, can you look at the joint between the saddle and the Dec axis (the line between your motor and the clutch handle). Can you seen any movement between the two when you push the scope while the dec clutch is locked. If you can see movement, then it's caused by backlash error on the dec worm wheel. A saddle upgrade will not solve this problem.

2. Next, take a step back and look at the whole mount when you push the scope with all clutches locked. Is the flex restricted to the dec axis only, or is your whole mount flexing? 

I can see both types of movements when I mount my 14kg 125/6.4 refractor on my GPDX. I also get the first type when I mount that scope on my IEQ45, but thanks it IEQ45's unique anti backlash mechanism, I get more movement than the GPDX

This video showed the amount of movement I get when I pushed my scope at the eyepiece end. Skip to 1:07 for the GPDX's flex. 

http://www.youtube.com/watch?v=IcDI46JBzWs#t=0

If the movement on your mount is similar to those in the video, then it's caused backlash error and overloading the mount. The saddle is not the cause of the problem and upgrading your saddle will not help. The only way to reduce this movement is upgrading to a bigger mount.