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Use cases for combining 115mm with 81mm on same mount?

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#1 psychwolf


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Posted 30 June 2020 - 11:36 AM

My mount has compatibility to support 10lbs of a lightweight doublet, say something like a Vixen SD115S. If I already have a Vixen ED81S WT for imaging, will there be a decent enough payoff from imaging the same scene simultaneously with a larger aperture on the same mount?  As it is now, as pictured - that 10lb counterweight doesn't need to go down from highest position to balance the 81s once the DSLR and guide scope is attached, so I've got room to add to around near half of the 33lb weight limit claim by Meade.


My hope is that I could get greater clarity in smaller sections, while the 81mm works on the wide angle to help save imaging time. But is that an actual use case any of you do?

Or would you suggest putting some other type of scope on the mount instead say a lightweight reflector or a Rasa 8? I'm considering if I go all refractor this is turning into a great solar imaging rig with some decent filters, but if mm difference is minimal I'm okay just taking a scope off and using the $ on a scope that opens up more potential from where I am.


I will have a ZWO ASI071 soon, which may be a great candidate to throw on the new scope upgrade next year, whatever scope I go with. That should free up my Rokinon 135 + DSLR for a star tracker to do early imaging on other targets as I do detailed work on others.


Thanks for any pics of your rigs or use cases that may help me decide on future direction next year, which is always on my mind. To me, an upgrade from 81mm doublet to the 115mm doublet range seems like the natural next step, but I'd like to hear from others who have gone there before. I'd love to keep it steady with the Vixen to maximize compatibility with accessories for the 81s - so *cough*... also let me know if you're considering selling a Vixen in that focal range in the near future! That 81mm has been a beautiful scope to use. 

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Edited by psychwolf, 30 June 2020 - 01:33 PM.

#2 OhmEye


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Posted 30 June 2020 - 11:56 AM

I went from a 60mm doublet to a 130mm APO, I wanted the FoV of a 100mm with more speed and the 130x0.79 reducer puts it in what to me is a sweet spot of 735mm. The use case for mounting both is rather mundane, I use the doublet for the guide scope.


I haven't thought of any "side by side" use case, I don't see a lot of benefit for my interests to collect much wider FoV with a different pixel scale and any extra complexity to combine and process it. There is more disparity between 60mm and 130mm than 80mm and 115mm though, so I'm curious what others come up with.

Edited by OhmEye, 30 June 2020 - 12:01 PM.

#3 OldManSky



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Posted 30 June 2020 - 12:00 PM

I have a 103mm and 71mm setup on the same mount.  It's largely for convenience -- everything is setup once (I'm on a permanent pier in an observatory), balanced, and configured.  Which means every night I just turn the power on, unpark the mount, and go.  Nothing else to do.


I'm not currently imaging through both scopes at the same time, although I'm considering it -- saving up for a 6200 camera now, so when I get that and the 183mm, I can image through both at the same time.  And all it would take would be one new power cable from the mount pass-through power.  Right now I just move the flattener and camera/EFW from one scope to the other.  It takes a DEC re-balance, but not RA.  Makes things simple.


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#4 Couder


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Posted 30 June 2020 - 12:22 PM

If your mount will carry it, I say go for it. You don't have to use both scopes every time you setup. But it is nice to know you can if you want to. I use a 6" F15 and a 4" F15 refractor on the same mount. I built the custom dovetail system myself. There are 2 uses: I take the setup to star parties (star and solar) and I can watch through the 4" to make sure when someone simply has to put their hands on the eyepiece that it is still on target. If there is a big enough crowd, I lock down the AP 1200 mount and let 2 lines of people look through both scopes at the same time. The other use is I keep a camera on the one scope, when an opportune time arises, I take pictures, using the other scope for a guide scope.

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#5 psychwolf


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Posted 30 June 2020 - 01:49 PM

Wow I've never seen a scope like that in person, so some other year I'd be lined up to see through it as well!


Okay, I'm being lazy in my post - here are some numbers to work with:

My ED81S with a reducer takes it from 625mm to 419mm

The ED115S starts at 890mm which would go to about 600mm.


To me on paper that seems like a big enough difference, but it's all being imaged at about 5.5 with reducers. There's also a compatibility issue I'm not sure about with reducers and filters on these scopes, so I might be at 7.7 at 890mm for the 115, which is decent enough when I do want to use the dual narrowband 2" on there.


In the back of my mind I start thinking what if I just took the 81S off or in visual mode some nights and went with the Rasa 8 giving me f/2, short exposures at 450mm with compatibility with a ZWO I'll be getting... now my mind wanders at the combinations, although since I have to drive to my site and set up at dusk, shorter imaging time is underlying any decision while also saving up so I can have a backyard of my own some day.

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