Dual setup Rasa and Meade

[dan_adi]

Hey guys,

Do you think it’s posible to dual mount a 12 inch Meade ACF with an 11 inch RASA on a Mesu mount?

I’m thinking beacause of f 2.2 ,the exposures on RASA will be much shorter then the f8 Meade so I just might be able to get away with it, if optec will respond to my request of a Libra adjustable plate for the RASA :)).

The guiding And real time focusing will be through an ONAG XM mounted on the Meade.

 

[ollypenrice]

I think you'll be able to mount them both. My Mesu once carried, experimentally, a 14 inch ODK and a heavy 10 inch Russian Photomak. It coped with the weight but it would not work as a dual imaging rig, which was the experiment in question.

I think any reflector is best guided by OAG (or ONAG) because mirrors can move. Guiding the short FL RASA by parallel scope might work but I strongly suspect that the Meade might introduce mirror movement. 

The focal lengths are so different that I doubt you'd be wanting to do dual scope imaging at the same time on the same target? Even if this were your intention I would say it wouldn't work because, with two mirrors potentially moving, you will never eliminate differential flexure. We certainly never did so in our experiment and we came across others who had also failed. However, the Mesu handled two refractors for dual imaging with no difficulty whatever. It isn't the weight, it's the rigidity of the optics.

Be aware that your two scopes would be a long way apart side to side. This means that you would encounter a pier collision very early and the lower scope will find itself pointing at the observatory wall a long time ahead of the other one.

What would be the purpose of this dual instalation? It would need to be a good one to be worth the enormous complexities involved in setting it up.

Olly

[Davey-T]

Hi Dan and welcome to SGL.

As Olly asked what are your expectations of this setup ? an ONAG introduces challenges of its own.

Dave

[dan_adi]

Thanks for the quick reply. I had the crazy idea that I could image the same target at the same time with the 2 big scopes  , so I had to ask someone with more experience. So for dual imaging of the same target I'd better ad a refractor with the same focal ratio as the Meade SCT or at least close to it in order to reduce the differential flexure ? 

 

 

  

[dan_adi]

Also this is a fine article on dual setup imaging I found a couple of days ago. 

https://astronomytechnologytoday.com/2017/05/29/a-practical-automated-multi-camera-astro-imaging-system-design-one-mount-one-computer-two-imaging-cameras/

[ampleamp]

Peter Rejto describes very well some of the challenges of a really good dual system and will undoubtedly have given you some great ideas. Olly has given some sound advice on the practical aspects of long and short focal lengths, mirrors, and don't under-estimate how soon the lower scope starts imaging the wall.

Things to also consider no matter what system you may put together include - 

do the scopes point at the same place in the sky; depending on chip size, even at medium focal lengths on a standard dual plate they probably won't so you will need to use shims (tedious) or an alt.az adjuster (weight capacity) to achieve this

differential flexure will not be your friend and you may well need a good mechanical solution and even then...

cable management - there will be a lot of them!

timing of subs if you use dithering

balance

cost, £ and time

a doubling of systems imo equals at least a quadrupling of all issues, do you intend to be present for all these?

I am putting  a dual 1200mm system together and am working through all of the above, and more, but I have some very personal reasons for embarking on this and i will make it work ;-).

Good luck!

 

 

[ollypenrice]

The point of a dual rig is to take the same picture on two parallel scopes. Focal ratio has nothing to do with it. The most important thing is that the object being imaged should be imaged, first of all, at a roughly comparable pixel scale. Ignore focal length and pixel size on their own and concentrate on pixel scale in arcseconds per pixel. And make sure that the field of view is compatible or you'll get all of the object on one scope and only part of it on the other.

The most logical dual rig is one which has two identical scope-camera setups in parallel. Here's one we've used successfully:

If you go down this route you will need to get both scopes pointing in the same direction. Just bolting them onto a dual bar will not get them anywhere near parallel. You can see that the scope on the right above is mounted on a Cassady T-GAD tilt-pan adjuster which allows us to be aligned almost pixel perfectly so we lose almost nothing round the edges. These adjusters are expensive so you do have another option: use one system which gives a larger field of view than the other and hope they will overlap without the tilt-pan device. The smaller FOV will then define the framing of your image.

Now, how much difference in pixel scale will work? I would suggest that you could shoot RGB at half the resolution of luminance so combining Lum at 1.5 arcsecs per pixel with RGB shot at 3 arcsecs per pixel would be OK. Personally I'd prefer a smaller difference than that. A bigger difference than that would be an enormous processing challenge and I would not expect to ba able to pull it off. I've tried.

You can find your pixel scales here: http://www.12dstring.me.uk/fovcalc.php

Olly