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Old 13-06-2014, 07:53 PM
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graham.hobart (Graham stevens)
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RC 10 and ao 8?+ff?

Hi all, assembling my super duper collection of goodies now and considering an ao unit, namely the new ao 8 that fits neatly with my stt 8300 SBIG.
My query is about back focus and if anyone has tried this before? For tips etc
My imaging train is GSO RC 10. Feather touch 3 inch focuser, astrotech flattener, spacers? then STTFWG8 then STT 8300'.
I know that the integral guiding sensor in the filter wheel has a reducer, and I assume this is taken into consideration with the spacing of the AO as it is manufactured specifically to modular fit together.
If my SBIG back focus is 17.5mm, and the spacing of the FF is 57mm -/+ 4mm then if I can fit the filter wheel and ao8 in that 57mm would it work ? (-17.5 '=29.5mm -actually thinking out loud there is no way that the imaging plane to ff distance is going to bye less than 29.5mm with the Ao 8t and three filter wheel so forget the Ff!
So anyone just have the camera filter wheel and ao8t on the GSO 10 and can give me an idea if it all works ok? I believe imaging at f2000mm with that weight on my pmx it would be advisable to get an ao 8 unit?
Mucho Gracias for advice

Graham

Last edited by graham.hobart; 13-06-2014 at 07:55 PM. Reason: Spelling
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Old 14-06-2014, 07:53 PM
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Graham I am going through this myself. What I did was worked on the premise that whatever the metal back distance is from the FF can take up camera filter wheel and AO if there is room. In my case I have 106mm to work with. 61.84mm for the camera and filter wheel, 30mm for the AO and then a small adapter to the FF.

Just remember though all that takes up usable back focus and that needs to be subtracted from the whole back focus of the scope.

So with your camera is the back focus including filter wheel and guide camera prism 17mm? Or is it more with filter wheel and guide pick off prism?

How much room does the focusor take up?

What is the overall back focus of the scope?
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Old 14-06-2014, 10:55 PM
Star Catcher (Ted Dobosz)
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Hi Graham
I don't think you would come anywhere near the PMX payload capacity. The metal version RC is about 17kg, feather touch FOCUSER about 2kg, camera about 1.5kg, AO unit about 1kg. Any other bits and pieces would add minimal weight. So all up you have less than 22 kgs of equipment.

I think the payload capacity is 41kg for the PMX? So if the mount has a good PE, it is likely to continue having a good PE with your gear. That said, an AO unit will generally give tighter stars and as a consequence make it more apparent when the RC is not collimated i.e. the blur circle no longer hides star shapes.

Whatever ends up being the optical train, you will definitely need a sliding counter weight somewhere at the front of the RC tube to balance a very back heavy instrument.

Ted
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Old 17-06-2014, 09:25 AM
SpaceNoob (Chris)
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I doubt you'll have problems with that setup. I run a setup slightly heavier:

CDK 12.5 -> Pyxis 3" rotator -> AO8T -> FW8G -> STT8300M.

There is also an additional counterweight on the dovetail of the OTA to balance the image train.

I was considering dual OTA (side-saddle) setup with my FSQ106 but that could be pushing it a bit.
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Old 17-06-2014, 12:33 PM
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I have my GSO RC12 with my huge imaging train on the back and it guides near perfectly. No problem with the weight.
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Old 17-06-2014, 12:39 PM
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graham.hobart (Graham stevens)
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RC10 and AO8 and SBIG STT etc

Excellent- but I am still having trouble locating a back focus for the RC 10- it is mentioned as "generous" but what does this mean?

Thanks guys for chipping in anyhow.
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Old 17-06-2014, 12:53 PM
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Contact Jim Sheng direct Graham. He has ray trace diagrams that will get you nearly there.
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Old 17-06-2014, 08:17 PM
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graham.hobart (Graham stevens)
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Ff etc

So now I am really confused.
SBIG stt 8300 back focus is 17.5 mm
STTFW8G "mechanical back focus is 54.06mm
Does this include the camera or add on?

My Astro tech flattener has a back focus of 57mm so am i going to be screwed if it all adds ups.?
What does it mean mechanical back focus ? Is that distinct from optical back focus?
Arrrghhhhh!!!
Graz
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Old 17-06-2014, 08:41 PM
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Told ya.

According to the manual and the appendix drawings from the front of the integrated filter wheel to the sensor is 54.06mm.

Your next question is whether you can get an adapter made that is 3mm from the camera to the flattener. Based on my experience the filter wheel opening is 3" but it might be different for this camera. In my case I needed to make an adapter that can bolt directly onto the camera. The shortest one was 12mm.
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Old 17-06-2014, 08:55 PM
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graham.hobart (Graham stevens)
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Ff etc

Thanks Paul - yes been knocking my brains out this one,!
According to the PDF the screw thread on the fw is an sTL type 2.156"
So the stt is a standard 2" ? And the filter wheel as above.
What's really scaring me now is I bought the Feathertouch 3" with the standard compression rings. Having played a little with this camera on my refractors I realised just how heavy this varmint is and Now I wonder whether I should have gone with a screw fastening between camera and oh **** everything else !!!!!!!
It's not an easy camera to play with that's for sure, bloody weighs a ton!!
Cheers
Graz
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Old 17-06-2014, 09:11 PM
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Careful of compression rings and cameras. I had some gear fall out when temps went down and it lost grip. I have never had that with AP compression rings. Screwed adapters are always preferable for surety but if you do use a compression ring consider a safety lanyard attached to the camera and the focuser.

Greg.
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Old 17-06-2014, 09:15 PM
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graham.hobart (Graham stevens)
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Ff

Paul, precise parts making me a 3mm SBIG fw8g to Astro tech ff - result !!!!
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Old 17-06-2014, 09:56 PM
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graham.hobart (Graham stevens)
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Ff etc

Sure is a good point Greg. Learnt that with the cooler modded 60da. Didn't bounce well and only now getting it repaired at cost at centralds.

Velcro straps around everything now.
Graz
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Old 22-06-2014, 10:59 AM
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Yep either dove tail or screw connections are the way to go. Both the Pyxis rotator and the Atlas are dove tail and work really well. Never use compression anymore simply because they move a little with heating and cooling.
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