Twin GSO RC 8

[Panagiotis Papadakos]

Hello,
 
I have been thinking about getting one more RC so that I can double the data per night.

However, I have not seen such a setup. Only RASA-based ones.

I know I will not have a problem with my mount (a mesu-200). But I am not sure about flexture issues since fl is about 1620

Any thoughts on this? 

CS
Panagiotis

Edited October 14, 2023 by Panagiotis Papadakos

[Elp]

I haven't attempted such at that focal length but when trying it with camera lenses if your aim is to image the same exact target it's not simply a case of making sure they're parallel, if you do mount them parallel your main scope will centre the target and the second will be either side of centre and usually the target not even in view. You'd have to devise or source a mount which allows for pan, roll and maybe tilt adjustment.

The next step I tried was pan adjustment on the second camera and it still didn't centre the target. My next step is roll adjustment which is haven't tested yet. I'd doubt it needs the tilt adjustment from my tests.

[Nik271]

5 hours ago, Elp said:

If you do mount them parallel your main scope will centre the target and the second will be either side of centre and usually the target not even in view.

The optical axes of the two scopes will need to differ by the parallax of a galaxy measured from two points about 1 meter distant. This is so small that for all practical purposes the axes of the two scopes can be considered parallel. Getting this is not going to be easy at this focal length. Use largest sensor size available,  this should give some slack in case of small misalignment.

[tomato]

Based on my experience of mounting two 6” 1050 mm FL refractors side by side you will definitely need one of them on an adjustable saddle to achieve getting both scopes centered on the same target. These saddles are commercially available but are usually designed for 80-100mm refractors. This saddle will need to be of substantial design to take the weight of the OTA and not introduce flex into the system. I have to use a bracing plate across the top of the tube rings to prevent flexture occurring over the course of an extended imaging session. This has to stay loose while the scopes are aligned and then tightened down very carefully to prevent distortion and re-introducing misalignment. You might get away without the bracing plate on the shorter, lighter scopes you propose to use.

Good luck with your project!

 

[ollypenrice]

The only adjustable saddle I know of with any hope of holding your OTAs parallel is the Cassady T-Gad, now out of production. I have just sold my own example. There is no problem inherent in the separation between the instruments. It took more than a century of positional astronomy to detect the parallax of the Earth's 200 million mile shift in position over six months of its orbit. Nor is it the end of the world if you don't have perfect alignment: you just have to edge crop. However, this assumes a good alignment device.

You have the additonal problem of mirror flop. My honest opinion is that it would be nigh-on impossible to make a long FL dual reflector system work.

Peter Goodhew uses twin refractors at high resolution, aligned to each other, one carrying the guider and the other using an active optics unit. I seem to remember his saying on here that it worked and that the slave scope with AO unit actually got the better FWHM.

It really might be more productive just to use two mounts.

Olly

[licho52]

The more straightforward approach is to upgrade to 10" or 12" RC with FF sensor if you're worried about narrower FoV.  It'd have almost 2x(10) or 3x(12) the light gathering power plus extra resolution.

Running 2 x 8" is more trouble than it's worth and you'd need 2 cameras.

[Panagiotis Papadakos]

Thank you all for your great responses. It seems like this project does not worth it!

[tomato]

It would have been a lot of work that’s for sure, with no guarantee of success.

But, to paraphrase JFK, “we choose to do these things, not because they are easy, but because they are hard.”