Thoughts on tandem mounting. Part II: Piggyback madness?

[NigeB]

Hello All,

 

OK: so I've tried some side-by-side approaches (as per my last thread, Thoughts on tandem mounting? ), to get my TOA150 and C11 together on the Mesu, and while they worked, there was a lot of flex in the system. I'm not bothered about differential flexure since I use an OAG, but it led to unacceptable damping times.

I know people have managed to get tandem mounting to work, but most of the setups I've seen are carrying lower mass OTAs (lower, at least, than the refractor), and I'm wondering if I'm asking too much to tandem these 'scopes, hanging them cross-wise off the central mounting plate as that requires, and still have a short (<4 sec) damping time (well, that's short enough for me - I'm sure others have much stiffer setups!) With the tandem configuration, I was seeing 6-8 second damping, with "beats" as the different natural frequencies of the two ends of the assembly interfered - yuk -  though the imaging performance seemed to be little affected - the image below was a single 300 sec integration through the refractor in this configuration. No wind, though!

So I've been thinking about piggybacking, putting the lighter C11 on top of the Tak. The advantage is that both telescopes are clamped and supported along most of the length (~35 cm) of their dovetails; the saddle plate on the Mesu is 30 cm long, so there's also excellent support at the interface with the mount. All of which might provide a stiffer assembly.

The disadvantage is that raising the C11 above the refractor increases the moment arm. But I think I can partially compensate by removing the 4.8 kg counterweights at the focuser end of the Tak. These balance the objective cell, and usually when people remove them, they have to slide the OTA back a long way (i.e. pushing the focuser end away from the mount) to regain balance, causing mount clashes and eyepiece accessibility issues. But I'm putting ~12 kg of C11 on top - and the heaviest bit of the C11 is at the opposite end to the heaviest bit of the refractor. So I think I should be able to push the C11 out towards the rear to achieve balance with little or no shift in the refractor position. The total weight of kit (OTAs, rings and plates) will be about 35 kg, well within the mount capacity - but my worry is rigidity, not weight.

Bern's Modern Astronomy page includes a link to Terry Hancock's site, where he has an AT10RC and a TMB130 + guidescope, piggybacked. Given the quality of Terry's imagery, I figure there's nothing intrinsically wrong with this basic approach. But I'd be interested to hear views in support or against, before I start making and buying more bits!

If this doesn't work, there might be a mint condition, barely used C11 on Astro Buy Sell shortly .

Thanks

Nigel

 

[ollypenrice]

Yves and I experimented with a monster side by side outfit once, on a Mesu. On one side we had a 14 inch ODK and on the other a 10" Russian Photomak of considerable bulk and cold war musculature. We never got it to work. We never thought it was the mount. That could deliver the goods in the 14 inch (which had the OAG) even when the Photomak was on board. But the photomak trailed. We don't know why this was so but who knows whether the mirror of the ODK was moving, and being corrected by the OAG, or whether the mirror was moving in the Photomak, or whether they both were, or whether the dual bar was flexing?  (Unlikely. It was a big so-and-so.)  I think you have to guide a reflector with its own autoguider. I don't know this to be so, I just strongly suspect it. I suppose there might be the option to let the 'other' scope run in an active optics unit but rather you than me. The idea is to save time and that route might well do just the opposite. I think you can 'tandem' refractors (I know you can, because we do) but can you 'tandem' reflectors? Maybe not.

Olly

[Jessun]

Hi Nigel,

I use three scopes in tandem on the same plate - a big ol' plate - if that at all is still one the cards for you. It never gave me any problems apart from the little issue of alignment. I basically bolted them down and accepted a bit of loss at the combined edge.

I am installing the new FLO adjustable saddles now to square things up a bit. I don't know how solid they will be.

/Jesper

EDIT:  They are short focal length fracs though so not quite relevant actually. Just realized what you were putting on there.

[perfrej]

If reflectors are involved in such a venture, OAG simpy cannot be the way to go. It would compensate for one of the scopes' flexure and expose the other one to it. Alas, I would try a separate guide scope, most likely only to find that flexure in one or both of the reflectors is troublesome...

Jesper is on refractors, and that is so much easier. For the proposed setup, I would think it would be doable if used side-by-side and laterally balanced, with OAG on the refractor or a separate guide scope. The C11 would have to have mirror lock...

 

Twopence...

 

/p