Dual Rig Problem

[Xsubmariner]

I have recently set up a dual imaging rig consisting of 2x 130mm APOs, one with colour Atik 383L camera and the second with Atik383L mono + Lu filter. Now that I have SGPro configured to run 2 concurrent sequences on one PC. The dual SGP setup appears to be working ok, given the Atik drivers allow camera selection by serial number I have been able to configure each SGP sequence to the individual scope setup.

The Colour camera rig (primary) includes OAG and ZWO120mm guider, the mono rig is slave and just runs capture using the Camera, Atik EFW and Pegasus focuser.

The system appears to work well with the primary system managing guiding and mount control including plate-solving. Due to SGP inability to manage 2 imaging cameras, I am limited to this control setup and am unable to dither which I’ve switched off (plan to use bad pixel map). 

Last night was the first time I ran the setup with an OAG and 20min exposures. Previously I ran the system with a separate guide camera and 5min exposures. 

Please can someone advise why my mono image captures are elongated, see below. I watched several frames being captured and guiding was steady at 0.4 arc/sec. Without dithers there appeared to be no mount movement beyond guiding and yet the mono image is elongated on every frame.

The images were aligned between the setups using a heavy duty ADM adjustable saddle to position the TS130 with the AA130 and platesolves were used to align image rotation. 

I am confused why the colour frames are ok yet the mono frames are elongated. Even when I ran the slave capture time at half the primary the image was elongated ( see second image).

thanks Martin

 

 

 

[Laurin Dave]

Looks like its differential flexure between the rigs.. that the shorter exposure have less elongation somewhat confirms this.  How beefy is the dual saddle/alignment ? (just re-read your thread and its quite beefy - I'd check its all as tight as it can be)  @tomato had a thread on this issue with his dual Esprit150 not that long ago..  worth a search for 

Dave

Edited March 23, 2020 by Laurin Dave

[tooth_dr]

Was going to say differential flexure too. Some play between the two scopes. I’ve seen it in my own dual rig of 80mm scopes when the tube rings weren’t really tight on one scope. 

[tomato]

I left the top  bracing plate loose on my dual Esprit 150 rig after aligning the cameras and got this much sag over a 3 hour period. I was only taking 2 min exposures but even so the star elongation was still visible on each sub.

 

[Tomatobro]

Tomatoes top brace under construction

[tomato]

And here it is in position, (with the  nuts tightened)

[Xsubmariner]

Hi Laurin, tooth-dr, tomato and tomato bro,

Thanks for the feedback, I haven’t fitted a top brace to the telescopes and perhaps that is worth trying. I will have to align images using the ADM adjustable saddle then then fit some form of top brace. One thought that comes to mind is the loss of adjustment points for balancing the rig for the Mesu. 

Here is a picture of the ADM saddle, perhaps my error was in believing this was adequate for the arrangement.

 

Thank again 

Martin

[Laurin Dave]

I'd try tightening everything up to start with and see if you can move anything however slightly by hand.. also check both focusers as these are also a source of flexure and adjust if necessary 

Dave

[Xsubmariner]

Thanks Laurin Dave your advice was correct.  I found a lock nut on the ADM saddle elevation adjustment was loose and this appeared to cause the flex which ruined a nights imaging. 

Managed 6x 20 min frames on each scope tonight and no star elongation, great.

Thank you everyone who responded.  

[Laurin Dave]

Pleased you got it fixed 🙂

[tomato]

I use the same ADM adjustable saddle, it is right on the limit of what it can handle weight wise with a 150 mm refractor on there. I struggle to adjust the lateral bolts on it, these will get upgraded when the lockdown gets lifted.
 

@Tomatobro drilled and tapped the fixed saddle at both ends to take a small counterweight bar so Dec balance can be achieved without moving the scopes.

[Laurin Dave]

12 minutes ago, tomato said:

 

@Tomatobro drilled and tapped the fixed saddle at both ends to take a small counterweight bar so Dec balance can be achieved without moving the scopes.

Clever... I could do with a Tomatobro