Side Saddle or Piggy back? Does the image drift the same?

[Star101]

I have on my Mesu 200, a C11 with a TS65Q on top. If I guide with one and image with the other, the target image will drift.

My question is, if I had them side by side instead of piggy back. Would the target drift the same?

And yes, eventually, dual imaging rig is my thinking here.

[baggywrinkle]

Yes, they will drift at the same rate if on the same saddle plate...

[ollypenrice]

I think you need to clarify your use of 'drift.' What exactly is the effect you are seeing? The candidates are

- differential flexure, meaning the guide scope and the main scope moving relative to each other so that the guide scope tracks properly but the imaging scope moves relative to that correct guiding to produce trailed stars.

- field rotation due to polar misalignment. In this scenario the main image will slowly rotate around the guide star. If the guide star is in the middle of the image that will be the centre of rotation but if the guide star is off the imaging scope's axis then that will be the centre.

It is always best to guide a reflector with an off axis guider because the guider uses the same light cone as the imaging scope. Mirrors are always susceptible to 'flop' which is an overstatement of what is usually a very small movement in the primary mirror. A separate guidescope cannot 'see' this movement so cannot correct for it. An off axis guider can see it and will correct it. There is also an enormous difference in resolution between the little Quad and the big SCT. Even without flexure I would expect your resolution to be below what is needed to guide the SCT.

The SCT is a big scope with large diameter tube rings. I would always worry that these would flex so I'd favour a side by side arrangement on this one. But I would go for an OAG for the C11.

Olly

[Star101]

differential flexure.

Thanks Olly, I know you have the two scopes side by side on the saddle and appear to have no issues at all. 

I do use OAG. If I use a separate guide scope which stays on the target star, the object in FOV on the C11 drifts across the FOV. I was wondering if this was because my two scopes where parallel and not divergent, pointing to the same object. Having read your comments, I see this setup is not ideal and that saddle is best.

My current optional setup would be C11 with Hyperstar (560mm) and TS65Q.(420mm) I already have two cameras, Atik 4120EX OSC and ZWO ASI183mm Pro. So, all I need is the mechanics to connect them side by side. I would love to have two identical scopes side by side but hey, maybe one day

Something like these.

https://www.widescreen-centre.co.uk/dual-mounting-bars.html

https://www.firstlightoptics.com/misc/jtd-dual-rig-telescope-alignment-saddle.html

 

Dave.

[ollypenrice]

I can't say whether or not dual imaging with a reflector would work. It didn't for us but we were trying it at long focal length/high resolution. Mirror movement in the reflector would be the killer if it happened and I think it might.

Olly

[JamesF]

Out of interest, Olly, by "reflector" in this instance are you really just talking about SCTs and Mak-Cass designs, or is your perception that mirror movement is also a problem with designs where the mirror is not actually intended to move during normal use (newts, Mak-Newts and RCs springing immediately to mind, I guess)?

I understand why the former can have problems with the mirror moving as the orientation of the OTA changes because there has to be some freedom of movement between the mirror carriage and the baffle tube, but do fixed-mirror systems also have similar problems that are inherent in the design?

James

[Star101]

7 hours ago, ollypenrice said:

I can't say whether or not dual imaging with a reflector would work. It didn't for us but we were trying it at long focal length/high resolution. Mirror movement in the reflector would be the killer if it happened and I think it might.

Olly

Thanks Olly.

Yes, I didn't think about mirror flop.  Having taken the time and trouble to make sure bother scopes have exact same center of field. Move to a different part of the sky, the mirror moves a fraction, and its out of alignment again. Its not going to work, is it!! I will be giving myself a headache with the C11. If I am going to set up a dual rig, it would be better using two refractors side by side. 

[ollypenrice]

1 hour ago, Star101 said:

Thanks Olly.

 If I am going to set up a dual rig, it would be better using two refractors side by side. 

I know this works at low resolution. We are girding our loins to try a dual TEC140 rig at high resolution. The bits are on their way. Will it work? It would be nice if it did!

Olly

[Star101]

1 hour ago, ollypenrice said:

I know this works at low resolution. We are girding our loins to try a dual TEC140 rig at high resolution. The bits are on their way. Will it work? It would be nice if it did!

Olly

Congratulations Olly, Dual TEC140, WOW!! ( droool ) I'm so envious. I hope it works flawlessly and I look forward to first light

[ollypenrice]

15 minutes ago, Star101 said:

Congratulations Olly, Dual TEC140, WOW!! ( droool ) I'm so envious. I hope it works flawlessly and I look forward to first light

It hasn't happened yet... It's a three way adventure with two friends.

Olly