How to fit (and guide?) a Canon 70 - 200 on an equatorial mount?

[Dezerker]

Hi folks,

I have a great opportunity coming up to test a Canon 70 - 200mm IS II 2.8 on an HEQ5 mount.  As far as I can see the lens 'ring' (which isn't the same as the tube rings folks use for astrophotography) has a slot to fix it to an arca swiss plate, but I'm trying to find a way of attaching it to the Vixen Style dovetail bars.  Can anyone recommend a way to do this?

I'd also love to try guiding with this setup and have a ZWO 30mm guidescope I could use - but how to attach it to the above?  Using the camera hotshoe is apparently not recommended because of potential flexure, and some recommendations seem to point to an L-bracket - arca swiss plate combos to hold the guider on one side of the dslr - which to me would surely offset the balance of the rig...

The lens is quite a big old beast, so in an ideal world I'd have it 'ringed' like a telescope and attach the guider to the top of the rings....but I've not seen anyone who makes rings for this lens - though PLEASE correct me if I'm wrong!

Any thoughts?  Thanks in advance!

[michael8554]

Bolt the Canon Tripod Ring Mount to the Vixen Style dovetail bar with a long Tripod Bolt.

3/8-16 or 1/4-20 are the common sizes.

A single fixing can easily rotate, so I would drill an extra hole in the ring mount, to take a smaller diameter bolt and nut.

Michael

 

[Dezerker]

Thanks Michael.  I've had a tinker with an old dovetail today - it'll work (if I get some slightly shorter bolts than I currently have), though it's a little scary having all that weight going through a single bolt (unfortunately the lens is a loan for now, so can't drill additional holes!), and I fear you're right about the possible rotation.  The shape of the tripod ring foot isn't a great fit with the raised edges of the dovetail either - I might have to look for a flatter style of dovetail if there is one.

I was pondering whether using a long dovetail might enable me to fit and balance the guider underneath the dovetail at the very front, though I'd probably need a 14 inch dovetail to allow enough room between the mini guidescope, camera and cables and the HEQ5 head...

Further digging has thrown up this Vixen - Arca adapter which looks like it'd be sturdier for holding the lens - but it's short, so would rule out the 'bolted underneath the dovetail' style of guiding....

[Dezerker]

More googling...looks like FLO do a flatter-surfaced (and cheaper) dovetail, but it's too short to attach a guider underneath at the front.  Frustratingly the longer one that might enable me to attach a guider isn't flat-surfaced.

I guess I could screw the lens collar foot into the backside of a vixen dovetail (the 'wider' side) for more stability, then bolt that dovetail to a longer one that's the other way around to fit the HEQ5 head, but (without drilling holes) It's reliant on a lucky alignment of the holes to screw through to join the two together...

Feel like I must be overthinking this and missing something really obvious here - how are people guiding their bigger DSLR-lens set ups on equatorial mounts?

[900SL]

I use a dslr L bracket and side mount the guide scope

 

[Dezerker]

21 minutes ago, 900SL said:

I use a dslr L bracket and side mount the guide scope

 

Thanks - it looks like a popular way of doing it - how much does it affect the balance of your setup?

I'd have thought that approach would make guiding accurately slightly more challenging with the off-centre balance - does it not keep pulling one way?  I'd also be worried about stressing the gears on the mount, though I guess it's not a huge weight off the side.