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Old 17-04-2008, 10:52 AM
Craig_L
Craig

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Canon ExtenderX2 as barlow

Just wondering whether anyone has used the Canon 2XExtender as a 2X barlow for astrophotography. It's difficult for me to use a field flattener/reducer and then add a barlow (or do it vice versa) if I want to extend the focal length, so I thought of coupling the flattener/reducer direct to the Canon Extender. Don't know how far this would degrade the image and if it would still be flat to the edges, as it is now. Any thoughts?

Craig
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Old 17-04-2008, 11:42 AM
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dugnsuz (Doug)
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The 2x has a rep for introducing a bit more CA than the 1.4x in Canon L lenses which one would think would be the optimized combination.
So, I would assume you might have an optical bad time by attaching it to your scope - also going to double the effective focal length of the system.
Poor results, and longer to image them!!!!
I'm guessing though as I've never tried this combo out - hopefully there's someone who has.
Cheers
Doug
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Old 17-04-2008, 12:32 PM
Craig_L
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Thanks Doug.
The Sky 90 with the field flattener/reducer is fast at 4.5 but only 400mm. I sometimes want to get in closer, hence the 2X Canon extender idea to bring it up to 800mm. Guess it would be F9, still doable, but the sharpness of stars worries me. If I was only imaging the moon or planets, I could use the Tak extender 1.6.
Perhaps I need a new scope or try my 10" LXD 55 at 1000mm.
Craig
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Old 17-04-2008, 12:52 PM
Craig_L
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On second thoughts, don't think this is a good idea. There's an interesting link to a site where you can compare the clarity of different lenses and with the extender 1.4X or 2X by using the mouse to scroll over the test image.

ISO 12233 resolution chart sample crop comparison tool
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Old 17-04-2008, 01:19 PM
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I use to use (until someone nicked it) an Olympus 2x telconverter with a 400 mm lens and my Orion ED80, and it worked fine.

If the price of Canon lenses come down (as has been suggested in another thread), then I might consider getting the Canon 1.4x.
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Old 17-04-2008, 01:39 PM
Craig_L
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Thanks Susan.
The Canon 2X isn't so expensive in the US - about AUS$ 300 but it certainly is here - around $520. STill don't know what to think given the test in the link I posted above.
Craig
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Old 17-04-2008, 04:52 PM
Dennis
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Hi Craig

Here is a photo of our Pentax *istDS on a WO 80mm EDII with an x2 photo Teleconverter and a couple of (hollow) extension tubes to allow the combo to reach focus.

Cheers

Dennis
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