Mounting & OAG advice for dual imaging rig needed

[reddoss]

This is my new Altaitr Astro 115 mm triplet APO that I have bought to build a dual imaging rig. I tested it out in the Brecon Beacons last night, Olly Penrice's 2011 Astronomy Now review is right, it's superb, a poor mans Tak I would say.

My plan is to piggy back it on to the MN190 and put them both on the EQ8 I have just ordered. At the moment, to mount the guide scope on the MN190, I use a steel bar which attaches to the quarter Whitworth bolts in each mounting ring.  I will be attaching the 115 APO to a Losmandy dovetail bar (this will be the same length as the Losmandy dovetail bar for the MN190). I guess the existing Whitworth bolts will not be strong enough to take that load. Does anyone have any advice or practical  solutions? This brings me on to my second question.  Should I get an OAG  or should I simply mount  the guide scope on top of the APO as in the photo?

If I do get an OAG from FLO, will it work (eg enough room to focus etc) with my existing Mead DSI guide camera?

Any advice will be gratefully received.

Cheers,

           Justyn 

[ollypenrice]

I'd be a bit nervous about the piggy back style.  I like it and use if for guidescopes but you are adding an almighty moment by putting a susbstantial imaging refractor so far from the mount's saddle plate. You are also asking a lot of the 190 tube rings. Our dual tandem Tak uses a side by side arrangement. Because our scopes and chip sizes on both sides are identical we need to be sure the scopes are perfectly aligned and for this we sourced a used Cassady T GAD, no longer in production. However, in your case I assume that the refractor will give a slightly larger FOV than the Mak Newt and an imperfect alignment may still give you a full overlap over the 190 image. (I recommend Registar for combining images.) I'd go for a good side by side bar, holding the tubes as close together as possible.

We just use a guidescope on our tandem. (I say 'just' but it is, in truth, Tom's Takahashi FS60! Posh or what??  ) I intended to put the guidescope and T GAD on the other side so that the offset on the side by side would move the OTAs to the east when imaging south. This would mean longer before a fllip-inducing collision. However, in a dyslexic moment I ended up making the hardware to put it on the wrong side! A case of  'Do as I say, not as I do...' 

Olly

PS, I'm glad you like the 115. I did think it was good, very sincerely. If you pay four times as much for an FSQ you get a much bigger flat field, a significantly faster F ratio and an R and P,  but no radically different quality within the field it can generate.

[reddoss]

I'd be a bit nervous about the piggy back style.  I like it and use if for guidescopes but you are adding an almighty moment by putting a susbstantial imaging refractor so far from the mount's saddle plate. You are also asking a lot of the 190 tube rings. Our dual tandem Tak uses a side by side arrangement. Because our scopes and chip sizes on both sides are identical we need to be sure the scopes are perfectly aligned and for this we sourced a used Cassady T GAD, no longer in production. However, in your case I assume that the refractor will give a slightly larger FOV than the Mak Newt and an imperfect alignment may still give you a full overlap over the 190 image. (I recommend Registar for combining images.) I'd go for a good side by side bar, holding the tubes as close together as possible.

We just use a guidescope on our tandem. (I say 'just' but it is, in truth, Tom's Takahashi FS60! Posh or what??  ) I intended to put the guidescope and T GAD on the other side so that the offset on the side by side would move the OTAs to the east when imaging south. This would mean longer before a fllip-inducing collision. However, in a dyslexic moment I ended up making the hardware to put it on the wrong side! A case of  'Do as I say, not as I do...' 

Olly

PS, I'm glad you like the 115. I did think it was good, very sincerely. If you pay four times as much for an FSQ you get a much bigger flat field, a significantly faster F ratio and an R and P,  but no radically different quality within the field it can generate.

Dear Olly,

               Many thanks for the advice.I will give the side by side option some serious consideration. 

The new batch of 115's have been fitted with R and P focusers..

[tony210]

Hi - I am using an EQ8 with OAG and I am amazed by the improvement with star quality since moving over from a guide scope - setting up with a Skywatcher OAG was tricky but once you have got the focal lengths allied between the main camera an the OAG guide camera set up (best done in daylight or with the moon! ) the results are great!- Best wishes with your set up -Tony.  PS side by side was always better than piggy back  when I was using a guide scope.

[steppenwolf]

The advantage of a side by side arrangement is that both telescopes are mounted close to the mount's centre of gravity so there is a smaller moment of inertia which helps with guiding and general mount control.

Differential flexure is your main enemy with any dual telescope system and although an OAG helps hugely with this problem, my own tests confirm that it doesn't eradicate it. However, for a long time, I fought the change to OAG from a guide 'scope but have to concede that for me, the change has been worthwhile although getting both the imaging and guide cameras in focus at he correct distance from a focal reducer or field flattener can be a challenge!

Sent from my iPhone from somewhere dark .....