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yock1960
professor emeritus
Reged: 06/22/08
Posts: 978
Loc: SW Ohio, USA
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OK. I know that I need to use a barlow in order to be able to reach focus using the 'normal' port on my ETX-80. This limits the area of the sky I can reach with the webcam in the rear port. My solution 'was' to use a wedge & polar mode, this hasn't worked out too well due to drive slippage. I may be able to overcome this in time, but I was thinking about the possibilty of using a focal reducer, but my grasp of optical theory is not sufficent to know if this would work for not enough 'in' focus travel.
From a simplistic view, it would seem that if doubling the focal length makes focus possible, then halving it say, would make the focus issue worse. Is that correct?
Or...and this is a stretch, combine the barlow and reducer. What this would look like through the eyepiece or in images I don't know. 
Until my ship comes in......and I don't have a ship.....gonna have to get frugally creative. 
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LXD75 GEM
Orion Starblast 4.5" Imaging Reflector
William Optics Zenithstar 66 SD APO
Meade DSI II OSC
Meade DSI III OSC
Orion Starshoot Autoguider
Televue Powermate 2.5x
Discovery 6" Dobsonian
Nikon Action Extreme 10x50's
Gallery
Edited by yock1960 (10/03/08 11:32 AM)
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1_old_dog
sage
Reged: 09/07/07
Posts: 435
Loc: Smoky Mnts
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Hi
Interesting? We don't seem to have the issues you mention about focus. My son uses a modded quickcam setup with his ETX60 and he doesn't use a barlow anywhere in the equation. The scope focuses just fine, maybe I'm missing something here.
He didn't bother to put anything away last night, so I took this pic of his scope and cam are setup.
OD
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old stuff but good stuff
http://chu65nang67.us/nam/vietnam.html
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yock1960
professor emeritus
Reged: 06/22/08
Posts: 978
Loc: SW Ohio, USA
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Yeah, it works fine when mounted in the rear port, but when I try using the right angle port.....no joy....have to use a barlow. I can't remember if using the builtin one worked or not. I'm just trying to find a way to do some hi-altitude imaging and about 50 degrees is about the max., when using the rear port. As I said, wedge/polar mounting has not been a success so far, slewing in RA wants to slip and even if I help it a little, centering objects is very jittery. I haven't had much feedback from other users, but there does seem to be some variability in how these scopes/mounts handle polar mounting.
Steve
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LXD75 GEM
Orion Starblast 4.5" Imaging Reflector
William Optics Zenithstar 66 SD APO
Meade DSI II OSC
Meade DSI III OSC
Orion Starshoot Autoguider
Televue Powermate 2.5x
Discovery 6" Dobsonian
Nikon Action Extreme 10x50's
Gallery
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1_old_dog
sage
Reged: 09/07/07
Posts: 435
Loc: Smoky Mnts
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Hi Steve
Now I get what your saying, I'm a little slow today.
If your built-in barlow is like the one in our scope forget about it, in my opinion it's a piece of junk.
OD
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old stuff but good stuff
http://chu65nang67.us/nam/vietnam.html
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nytecam
Postmaster
Reged: 08/20/05
Posts: 5750
Loc: London UK
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I've tried a Barlow on my ETX-70 to increase the focal length and image scale but except for the moon [using just red or green channel in mono] images were soft due to exagerated chromatic aberration 
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Nytecam 51N 0.1W
Meade 30cm LX200+ETX-70+e-finder+C8+Ha+CaK PSTs SBIG SGS+homebuilt spectrographs
Starlight SXVF_M9+Lodestar CCDs/Canon 300D DSLR/Fuji E550
My observatory build-ETX-70 imaging-my videos
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