Strange Flexture Observation on a dual rig

[tomato]

The recent welcome run of clear nights has allowed me to note a strange phenomenon on my dual Esprit 150 rig. I have been imaging M51 which is pretty much overhead from my location and on three occasions around midnight I have observed trailing on the non guided scope for about 3x3 min subs which then disappears. I should note the guiding has been steady and constant on all 3 nights at 0.4” total RMS. Also it is definitely a trailing issue rather than a change in focus, which would perhaps make more sense (except the motorised focusers are backlash free).

Flexing between the scopes can and does occur on the rig but usually when they are near to a horizontal orientation, why would it occur when they are near vertical? Maybe slippage in the tube rings if not perfectly vertical would manifest itself as trailed stars?

[michael8554]

If guiding errors were definitely the same during the trailing episodes (check the guidelog at the times of the exposures), then it's differential flexture from who knows where.

If the guiding errors increase (were the trails in Dec or RA ?) then that's your culprit.

Michael

[tomato]

Thanks for the reply. The subs from the OAG scope were fine, no trailing, so it must be differential flexture. I’ll tighten everything up before the next outing, see if that fixes it.

[ollypenrice]

My somewhat fatalistic opinion: the only way to make high res dual rigs work is to follow Peter Goodhew and use an active optics unit on the slave scope. There is now a wide body of opinion 'out there' that you can't beat diff flex on high res dual setups by any other means and I'm persuaded by this.  I was an early dual rig adopter with twin FSQ106/Atik 11000 systems on a Mesu and with a Cassady T-Gad alignment device. It just worked - perfectly - from day one. I confidently assumed, therefore, that the transition from that system at 3.5"PP to a TEC 140 dual rig at about 1"PP would be easy. It wasn't and still isn't. Sometimes the 'slave' trails, sometimes it doesn't. I've had plenty of conversations with others similarly afflicted and the only winner I'm aware of is Peter with his AO unit. He says his FWHM on the AO-corrected slave is actually better than than that of the guided tube.) I'm not up for fighting with one of these notoriously difficult devices so I'm at the point of using two mounts, one per TEC 140, as being the least painful way forward. (I do already have two mounts.)

One thought: since dual rigs are popular I wonder if a manufacturer might be able to make a factory-assembled one-piece dual rig with a monobloc chassis for two optical paths. Or would diff flex in the focusers still get us? Who knows?

Olly

[tomato]

1 hour ago, ollypenrice said:

My somewhat fatalistic opinion: the only way to make high res dual rigs work is to follow Peter Goodhew and use an active optics unit on the slave scope. There is now a wide body of opinion 'out there' that you can't beat diff flex on high res dual setups by any other means and I'm persuaded by this.  I was an early dual rig adopter with twin FSQ106/Atik 11000 systems on a Mesu and with a Cassady T-Gad alignment device. It just worked - perfectly - from day one. I confidently assumed, therefore, that the transition from that system at 3.5"PP to a TEC 140 dual rig at about 1"PP would be easy. It wasn't and still isn't. Sometimes the 'slave' trails, sometimes it doesn't. I've had plenty of conversations with others similarly afflicted and the only winner I'm aware of is Peter with his AO unit. He says his FWHM on the AO-corrected slave is actually better than than that of the guided tube.) I'm not up for fighting with one of these notoriously difficult devices so I'm at the point of using two mounts, one per TEC 140, as being the least painful way forward. (I do already have two mounts.)

One thought: since dual rigs are popular I wonder if a manufacturer might be able to make a factory-assembled one-piece dual rig with a monobloc chassis for two optical paths. Or would diff flex in the focusers still get us? Who knows?

Olly

A very thought provoking post Olly. I accepted that a dual 6” refractor rig imaging at 1” per pixel was always going to be challenging on several fronts, and    so it has proved to be. To be fair since @Tomatobro’s designed and fabricated  top bracing plate, differential flexture has been managed to (for me) an acceptable level, but it does show up from time to time. To be fair, after this recent observation I checked and found both the bracing plate retaining bolts and the tube ring clamps looser than they should be, I will repeat the same imaging orientation and see if it improves after tightening.

The AO solution sounds like an interesting and challenging project, I presume the SX unit would be the one to go for?