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  #1  
Old 24-05-2011, 02:26 PM
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koputai (Jason)
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Barlow, Powermate, or Tele-Extender for DSLR?

Hi All,

I'm after advice on which method is best for multiplying your focal
length using a DSLR on an ED80.

Options I see at the moment are:

Televue 2" 2x Big Barlow
Televue 2" 2x Powermate
Canon 2x Tele-Extender

Which would be the best for optical quality and usability?

Of course the Barlow/Powermate could also be used with other
cameras on any scope, so might be more utilised.

Cheers,
Jason.
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  #2  
Old 24-05-2011, 06:53 PM
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dugnsuz (Doug)
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Hope I can help you here Jason re the extender + scope question.
Here's a relevant link (sorry High Res image links are void now)

http://www.iceinspace.com.au/forum/s...t=tak+extender

Extender in question is the 1.4x
The 2x Ext I wouldn't go with as too many aberrations are introduced (I've heard!! - no 1st hand experience, have only owned the 1.4x)

Cheers
Doug
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Old 24-05-2011, 07:01 PM
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dugnsuz (Doug)
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Found a thread re the 2x Extender...
http://www.iceinspace.com.au/forum/s...t=tak+extender
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  #4  
Old 24-05-2011, 07:17 PM
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koputai (Jason)
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Thanks Doug. Jeez, you can't complain about the results with the 1.4x can you!

Cheers,
Jason.
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  #5  
Old 24-05-2011, 07:27 PM
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Quote:
Originally Posted by koputai View Post
Thanks Doug. Jeez, you can't complain about the results with the 1.4x can you!

Cheers,
Jason.
No Jason, but there was a Tak involved!

Craig's post #4 in that thread really shows how good the 1.4x Ext can be
Doug
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Old 24-05-2011, 07:51 PM
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telemarker (Keith)
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There's a ton of information here. It'll keep you busy for a few hours. Be warned though, you might be tempted to pull a few lenses apart.

Keith
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Old 24-05-2011, 08:00 PM
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There's a ton of information here. It'll keep you busy for a few hours. Be warned though, you might be tempted to pull a few lenses apart.

Keith
Those birding guys love the ED80 as much as us!
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Old 24-05-2011, 09:27 PM
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Quote:
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Those birding guys love the ED80 as much as us!
Too true. There are some amazing photos taken with it too!
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Old 27-05-2011, 03:02 PM
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None. Use tiny pixels.

You did not say what kind of imaging you are after. If it is deep sky or birding, using Barlows or tele-extenders have had their day. Technology has moved on. If you are after planetary imaging you ought to go with a Powermate that takes you to f25 to f35, or thereabouts (depends on the pixel pitch of your camera). Let me explain.

For DSOs and for birding we need the captured images to be sharp (i.e. not waaaay over-sampled). Most refractors, even mucho $ premium APOs deliver stars that are no smaller than, say, 8 to 15 microns in diameter, worse off-axis. So if you have a camera sensor that has pixels in the 4 to 5 micron range, you are already at Nyquist, for that OTA. So, to eke the most resolution out of your lens/scope for a few to several hundred $, it is far more effective to upgrade your camera to one of the latest, small pixel DSLRs, eg Canon 600D, 60D, etc. A side bonus is that you do not lose any f-stop in doing so. For older DSLRs with 6 to 7 micron pixels it was still attractive to use 1.4x extenders, but not 2x. It's only with the earliest DSLRs with 9 micron sensors (and film) that 2x tele-extenders were useful. Ditto for 2x Barlows.

Now, for planetary imaging the point to start is with video capture, not stills. So one needs a webcam or a DSLR with LiveView. Next we use only the on-axis performance of the OTA or camera lens. On-axis performance can be MUCH better than even slightly off-axis. The cheaper semiAPOs have too much field curvature and if you use both a field flattener and a Barlow you are entering uncharted waters, presumably quite murky. Hence for planetary we are chasing the diffraction-limited theoretical resolution of the OTA on-axis and we do not mind if the captured videos are all bloated and fuzzy. The sharpening comes in post processing a stack of several hundred frames. So for planetary capture with 5 micron pixels we would need to be at f20 or higher magnification (basically > 4x the pixel pitch in microns). Hence 2x Barlows are not sufficient. We would need 3x to 5x Power Mates for such use. More on all this and other stuff on my website:
http://samirkharusi.net/
Direct links to testing tele-extenders:
http://www.pbase.com/samirkharusi/te...tele_extenders
Magnification required for planetary webcamming:
http://samirkharusi.net/sampling_saturn.html
Deploy that itching credit card on a camera upgrade, modded of course, rather than on a 2x. Besides, you'll get reinvigorated to take more pics of the family while awaiting nightfall
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Old 27-05-2011, 03:21 PM
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koputai (Jason)
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Whoa! Thanks Samir, greatly appreciated.

Cheers,
Jason.
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  #11  
Old 28-05-2011, 11:36 PM
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I just bought the tele vue 2" power mate for my 5d MKII. The seeing is terrible where I am tonight but so far so good. Once I get a decent nights work out fo it I'll post some pics.
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  #12  
Old 07-06-2011, 06:35 PM
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Hey guys.

As promised, here is an image of Omega Cent, taken with the 5D MKII and the Tele Vue 2" 2x power mate. this is cropped as the vingnetting is quite bad. I think with a bit of work it may just be a keeper. If you have some really nice glass then I think this will work out really well.

http://www.iceinspace.com.au/forum/s...ad.php?t=76658
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