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How much do wider bars matter? (Vixen v. Losmandy)

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#1 stevetaylor199

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Posted 03 January 2013 - 02:08 PM

I own a recent Atlas mount and I was fortunate enough that it included a dual-width saddle out of the box. I have a number of scopes to mount on it, and I'm wondering when it's beneficial to use a wider, Losmandy-style bar instead of a narrower, Vixen-style bar.

I've been using a C6R refractor as of late, and its fairly long tube does create a big polar moment that I would like to tighten up as much as possible. I currently have a 13" Vixen-style bar attached to the bottom and the top of the standard rings. (The top one makes a very convenient handle, and I'd also like to get ADM dovetail adapters so I can attach a Telrad or ST80 to the top bar, for finder purposes.) Would it make a difference in stability if I upgraded either, or both, bars to the wider Losmandy units? The stock rings only have one point of attachment to the bars, with a single bolt.

Going larger, I have mounted an 8" F6 reflector on it, and the rings for that tube have three mounting holes available with a wider, flat area that would be in better contact with a wider bar. Can I really see any gains in overall stiffness and stability by going wider on the bar? Does it matter if I mix a Vixen bar up top with a Losmandy on the bottom?

Finally, I have an ST80 (3" F5 cheapie refractor) that I would like to use as a guidescope underneath a Canon DSLR with a telephoto lens. Rather than just using the single 1/4" screw on one mounting ring to support the camera, I thought it would be better to use a bar on top of the scope that is attached to both scope rings. Then, I'd use a dovetail adapter to attach the camera to that upper bar. (Are the camera mounts from ADM any good for this? They do have some ballheads that are getting too small for a 40D plus a 70-200 F2.8, and they have a very simple mount with a 1/4" screw only. I was thinking of just getting their universal adapter and attaching my own ballhead to that.)

#2 Lane

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Posted 03 January 2013 - 06:33 PM

I have not found any difference in using V or D style dovetails in regard to how stable they are. I use a V sytle dovetail on all my scopes except the C11. And the only reason I use a wider dovetail on that scope is for ease of mounting. The wide dovetail helps me keep it steady it in the saddle since I can only hold it with one hand while tightening the saddle bolts.

#3 vct123

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Posted 03 January 2013 - 08:12 PM

I have a Losmandy "D" type dovetail on my C6R and my c9.25.
Both came with the Vixen dovetail and it makes a difference in the stability of the scope, How much difference ?, well I think it depends on the scope. I don't think a C8 on a CG5 need a wider dovetail, however using the same CG5 and a C9.25 will benefit from the wider dovetail. A C6R just because of the length, I think also benefits from the wider dovetail.

#4 vct123

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Posted 03 January 2013 - 08:16 PM

I also find the ADM d dovetail with the bridge on the front and rear of the dovetail give you a nice handle to hold the ota as well as more stability. It give you a place to get your hand in between the scope and dovetail.

#5 Lane

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Posted 04 January 2013 - 04:10 PM

It must have something to do with the type of dovetail. The ADM type is solid aluminum, while many others including the stock Celestron are hollowed out.

Using a standard Celestron D series on my C-9.25
-vs-
Using an ADM V series on my C-9.25
resulted in no difference in stability at all.

The weight is about the same for these as well.


I also use a solid V series dovetail on a 6" refractor with no stability issues. Might be a different story with one of the hollowed out dovetails.

#6 Pat at home

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Posted 04 January 2013 - 05:54 PM

Only today I received ADM replacement "DV" puck and saddle for my EQ6 which only had the vixen sized saddle with two clamp bolts that chewed into the dovetails. I also received a wide dovetail for my scope. Maybe only in my mind, but there definitely seems to be a lot more stability at the mounting point with this setup as opposed to the original cheap dovetail and old 2-bolt saddle.

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#7 stevetaylor199

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Posted 04 January 2013 - 09:11 PM

As Lane mentions, I realize there is another variable, in that the Orion bars I have are I think of the extruded type, and are not solid like the ADM units. These examples here from Orion are what I am currently using. They aren't flat across the face, but they do have the set of two smaller screws for correcting cone error. (Has anybody ever bothered to address cone error on their scopes? Can it be done with the ADM bars too?)

I'm not sure if there's any more flex in the Orion type vs the solid ADM type. I was wondering if there were advantages in the extra width being held by a Losmandy-type bar in the EQ-6 saddle; vct123 suggests there are, which is good enough for me to give it a try on my larger, longer OTAs.

Pat at home, thanks for the pic of your nice-looking rig. I am sure you enjoy the ADM saddle. I have no idea how the scope makers ever got the idea that was okay to simply use one or two bolts to directly clamp a dovetail in place. (Because there's nothing more natural than burying a steel screw into soft aluminum, every time a scope is set up, right? :p ) Thankfully the newer Atlas comes with that feature; heck, it helped justify the cost for me. A few other details on Pat's scope I appreciate are that little stand (and handle?) on the focuser flange, and the finderscopes. Having an ST80 on top of the C6R would be nice.

#8 oo_void

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Posted 04 January 2013 - 10:39 PM

If you have a modern, Goto mount, the alignment process will usually model out cone errors.

Just guessing here but I'd think that the wider platform of the Losmandy style rail would minimize flex as compared to a Vixen plate as the scope moves to E / W extremes.

#9 Jared

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Posted 05 January 2013 - 12:24 PM

It matters a lot more for imaging than for visual use. For imaging, controlling flexure can be very important. Fewer guide corrections required = tighter stars.






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