Mounting of Scope to mount.

[LDUNN1]

Is there a 'magic' formula or recommended advice when deciding how far to space tube rings on a scope & what length clamp to use for a given dovetail/ring spacing length?

......or is it just make the tube spacing as long as the scope will permit, & then make the clamp as long as the dovetail will permit?

I am looking to upgrade the standard Losmandy clamp on my nEQ6, which only has a clamping length around 4" long.

From memory, my tube rings are approx 280mm apart on 300mm dovetails. My long-ish refractor would permit a significant increase in the tube ring spacing, but without a corresponding increase in clamp length, I can not see much of a benefit in just doing this.

Any thoughts or advice, or even peoples own experiences or 'best practices' would be good to hear.

Many thanks in advance & in anticipation of a response.....

[daz]

I replaced the puck on my EQ6 with the one from ADM, so I could bolt the ADM universal saddle plate to the head. My rings then bolt to the saddle plate.

I use this for both my 12" newt and ED80!

[LDUNN1]

Thanks for your response Daz.

I am not sure I totally understand your set up as described above, but lets have a go:-

You have replaced the SW puck & saddle with one of these:-

D Series Saddles

I presume you have bolted to either a Losmandy or Vixen plate like these:-

D Series Dovetails

& that your OTA mounted on that plate then sits in the saddle clamp above.

......this is similar to what I have now, BUT I am currently using the SW supplied Losmandy clamp & an AltairAstro Losmandy plate (the pretty blue ones!).

My question however is one of tube ring spacing & saddle clamp length, relative to the length of the scope.

I currently have my tube rings mounted approx 285mm apart on a Losmandy dovetail plate 300mm long, which is secured in the SW Losmandy clamp which is only 4 inches long.

I am interested if there is an accepted recommendation of tube ring distance apart for a given length of scope. ie would I get a stability improvement if I went to (found!) a Losmandy dovetail plate say twice as long 600mm & mounted the tube rings around 585mm apart.

.....if I did this & kept the SW 4" long clamp, that would seem to then be the 'weak' link. In which case, there maybe no advantage going to the longer dovetail plate without a coresponding saddle plate length increase.

......& if so, if you had a Losmandy saddle plate 600mm long, is 7" saddle clamp long enough to get an improvement over the SW 4" long one? Or do you have to go longer? What if you could get a saddle clamp that was 550mm long to hold your 600mm long dovetail plate - then I see that the 'weak link' might be that this long saddle plate is only bolted with a couple of bolts onto the mount head/puck which are only 50mm apart......is there any point in going to an overly long saddle clamp?

There must be some 'rules', formulae or recommendation somewhere between the relative distance of tube rings to scope length to saddle clamp length.......

[m.tweedy]

The longer dovetail bar enables you to slide the tube backwards or forwards along the saddles length to achieve balance.

[daz]

Thanks for your response Daz.

I am not sure I totally understand your set up as described above, but lets have a go:-

You have replaced the SW puck & saddle with one of these:-

D Series Saddles

I presume you have bolted to either a Losmandy or Vixen plate like these:-

D Series Dovetails

& that your OTA mounted on that plate then sits in the saddle clamp above.

Close. I changed the puck to the EQG-A Adapters

and then put a D Series Universal plate on that D Series Universal Bars

I guess what I didn't say is that my 12" rings then bolt to the extreme ends of this plate to give me maximum stability. It made sense to me that if the rings are too close together then there is instability, however, I don't know if there is a maximum distance that should not be exceeded.

[LDUNN1]

Yeap Michael, I've been using the positioning of the dovetail to balance the scope, that bit is fine & not in question (other than my setup being quite sensitive to achieving correct balance!).

Daz, thx for those links - that helps me understand your setup - so you do not have a saddle clamp at all, your dovetail is permanently bolted to your ADM puck......I guess you have a permanent setup in an observatory. I don't know if I would want to bolt up a dovetail to the puck everytime I went out observing to a dark site, but I can see how that can eliminate one potential source of instability.

Like you it does not make sense to me to have the tube rings too close together, but it is not so much a 'maximum distance that should not be exceeded' that I am seeking, more if there is an optimum distance beyond which you don't really gain an advantage from - all you would be doing is adding weight (longer plate).

To my mind, if there are recommended guidlines on tube ring distance apart & clamp length, then it must take into account the length of the scope & the clamp length must be a percentage of the plate length - taking into account Michael's correct observation about needing to maintain the ability to shift the plate in the clamp to balance the scope.

Like it made sense to you to space the tube rinsg to the end of the plate, I am looking at the 'sense' in having as long a plate as the scope will permit, if it is then only clamped by 4" in the center by the SW losmandy clamp that came with the nEQ6.

......actually Michael, I've just thought some more about this as I was writing - I don't slide the plate in the clamp - I have tube rings that open up to accept my scope & I slide the OTA up-down within the tube rings & then tighten tube rings to clamp the OTA in place. My tube rings are permanently bolt to the ends of the dovetail plate, with a guide scope permanetly bolted to the top (also in tube rings & not guide rings) via a vixen dovetail (the vixen dovetail does not sit in a clamp it is bolted to the tops of the main scope tube rings & teh guide scope is bolted to the vixen dovetail on top) - so I don't need to slide the main scope's Losmandy dovetail in the nEQ6 clamp at all.

.....I feel that I am not explaining my question very well!