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Mount and flex issue info..

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#1 Panza


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Posted 25 June 2010 - 06:25 AM

My first real mount (apart from photo-tripods) was a non go-to G11, which served me well. After a while I regretted I didn't buy it with go-to though.
So for the first-time mount-buyer my advice is to get a cheap(er) mount with go-to, to check out if you use the go-to functions. That will save you the grief of buying an expensive mount without goto only to have to replace it later, or if you figure out you don't use the go-to, it will save you the difference between a go-to and a non-goto.
The issue with that is off course that most high-end mounts have go-to as standard.

Anyway: I started doing some astrophoto with a 400mm canon lens and camera piggybacked on the 10" LX200 on the G-11. It worked, but after a while the limitations showed themselves. The mount itself seemed fine but the piggy-back setup was not. The 400 f2.8 and 1D camera is quite heavy and I doubt the LX200 is really meant to support that kind of load. To add to the flexiness I had the 400mm mounted in 3-point rings. Nice enough if the lens was made of some stiff metal, but to save weight some of it is made of plastic (or very thin sheetmetal) which does not support the points of the rings very well.
When my time in the queue came up I ordered the AP 900GTO and sold the G-11. That made me very satisfied mount-wise.
The piggy-back issue was still the same though, so I bought a Stellarvue SV105 along with Orion Starshoot cameras for imaging and guding. Even though the DSLR imaging worked reasonably well I couldn't take the filters out of my 1Dmk4 as I use it for other photos just as much. The SLR lenses also have their limits.
To make the mounting even more stable I have decided to get a Robin Casady tandem setup. LX200 on one side and the SV105 on the other side. I thought I'd mount one of the scopes on a TGAD. I hope that will make for rock-solid mounting of both scopes. I assume I will have to do all mount-alignments with the scope that is orthogonal to the mount axis, or everything will go pear-shaped ?
I will come back with pictures and a report of how things work out once I have everything in house.



    True Blue

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Posted 25 June 2010 - 07:12 AM

The best way to solve flexure issues is with an OAG. Especially with an SCT where mirror flop can occur. A tandem set up may actually be worse.

#3 mariobmd


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Posted 25 June 2010 - 08:05 AM

I agree with LLEGEE. I have used tandem set up for guiding and the flexure is much worst in my experience, even with casady saddles and accesories. This is with using refractors! Piggyback system works well for me but the best is OAG.


#4 Panza


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Posted 25 June 2010 - 10:22 AM

Very interesting and enlightening...
I will reconsider. No rush really as the nights are still almost like day here up north..

What are the drawbacks of the off axis guiders ?

#5 Peter in Reno

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Posted 25 June 2010 - 12:12 PM

I think there are more advantages than disadvantages using OAG over guide scope. When I switched to OAG with my CPC0800 and Mitty wedge, I was worried about not being able to find a guide star due to limited FOV but so far guide star is always there the first time without hunting for it. The drawback I have found so far is my SXVR-M25C has a physical large CCD sensor and I had to adjust the OAG pickup prism by raising the prism until the prism no longer block the CCD sensor. It's even worse with focal reducer and I had to raise the prism even further up. But it was no big deal and works fine.

OAG is usually tricky to beginners but once you get the hang of it, it's just as easy as using guide scope. OAG made a huge difference over guide scope. With guide scope guiding a SCT with moving primary mirror, I always get image shifts in between subs, but with OAG, there is no image shifts. The only image shift I find is field rotation due to slightly mis-polar alignment and that's not OAG fault. The field rotation is easily fixed by stacking. OAG helps reduce weight by eliminating guide scope.

I find the best way to parfocalize both guide and image cameras is test it during daytime. Once both cameras are focused, then you are ready to use it at night. I use a street light about half a mile away and works well.

Get a high quality OAG. I read quality of Orion's OAG is sub-par. I don't think Celestron or Meade OAG are good for astro CCD cameras because they are designed for SLR cameras with deep back focus. Try to find OAG with the shortest guide port. The longer the guide port the further away an image camera must be from OAG. Look for OAG with adjustable (lateral and vertical) prism. I find my Hutech OAG very well designed and built. Hutech offers many adapters to work with different cameras and scopes. I am able to put a filter in between OAG and camera so the guide camera won't be filtered (filter dims guide stars).

One thing you can't do with OAG that you can do with guide scope is guide and image a comet at the same time. Comets move a lot faster than stars so if you guide on a star, the comet would not look good. If you guide on a comet, you can't do that with OAG. If you are a comet hunter, guide scope is better.

If you get an OAG, I highly recommend Lodestar for autoguiding. Lodestar has a high sensitivity CCD sensor and helps find faint guide stars.

Research and Google for OAG and there are plenty of useful information about OAG.


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