Ring spacing/dovetail length

[philhilo]

What is the optimum mounting ring spacing for an OTA? 

Yes this is a bit of a how long is a piece of string question, however specifically I have a SW 200PDS - big thin metal tube with heavy mirror at one end and a heavy imaging train hanging off the side at the other - and I am changing from the supplied vixen dovetail to a losmandy as allegedly the latter is better, and I can put the former on the top to mount the power/data box/provide a handle/improve stiffness. However the dovetail I was recommended has shorter hole spacing - by 50mm or more. That got me thinking what is the optimal spacing? I did some research but no joy (looking at the mechanics the tube rings mounting points are probably the weak point in terms of stiffness).

Cheers,

Phil

[RT65CB-SWL]

Not sure myself, but I am sure if you ask a local engineering workshop to fabricate one to your specifications they would assist/oblige.

Edited September 4, 2022 by Philip R

[CraigT82]

As a user of large newts I often wonder about this.

I imagine holding a long stick in my hands, holding with hands wide apart would allow you to resist externally applied forces (e.g. like wind gusts on the OTA etc.) better than holding with narrow spaced hands, hence I’ve always gone with widely spaced rings. Also I think as the weight of a newt is concentrated at the ends then wider spaced rings would resist flex in the tube itself better too. 

Saying that I’m not an engineer and am just guessing really.  I’ve seen a few pics online of large/long scopes with closely spaced rings and I’m sure I’ve read something somewhere saying that closer spaced rings were better but can’t find it again. 

I wonder if there are any mechanical engineers on here who could share their thoughts? Following with interest! 

[markse68]

Tubes are really strong if they hold their shape but a long relatively thin dovetail plate could flex. I like the ADM vixen dovetail plates with the large backing plate as they have a decent thickness to them which is less likely to flex.

Mark

[CraigT82]

Yeah without a doubt a bigger beefier dovetail is needed with wide ring spacings or with big tubes, I’ve only ever used losmandy style plates with my Fullerscope or my 300p. 

[philhilo]

2 minutes ago, CraigT82 said:

Yeah without a doubt a bigger beefier dovetail is needed with wide ring spacings or with big tubes, I’ve only ever used losmandy style plates with my Fullerscope or my 300p. 

Logically a longer dovetail would need to be stiffer, but I would imagine there is not going to be much flex in the part held in the saddle. The parts outside of the saddle need to be super stiff. The 200PDS is nowhere near the size of your 300P, however its still coming in, fully kitted at over 13Kg. I have seen people adding stiffening (dual ring structure) around the imaging train as there must be some droop going on but that is a different issue. 

Logic suggests a super stiff long dovetail should prevent twisting in the OTA and be stiff enough to avoid twisting itself, however this all adds mass to the rig which may cause other problems. But there again adding a top bar may stop twisting.......

[philhilo]

27 minutes ago, CraigT82 said:

As a user of large newts I often wonder about this.

I imagine holding a long stick in my hands, holding with hands wide apart would allow you to resist externally applied forces (e.g. like wind gusts on the OTA etc.) better than holding with narrow spaced hands, hence I’ve always gone with widely spaced rings. Also I think as the weight of a newt is concentrated at the ends then wider spaced rings would resist flex in the tube itself better too. 

Saying that I’m not an engineer and am just guessing really.  I’ve seen a few pics online of large/long scopes with closely spaced rings and I’m sure I’ve read something somewhere saying that closer spaced rings were better but can’t find it again. 

I wonder if there are any mechanical engineers on here who could share their thoughts? Following with interest! 

Basically my thoughts!