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Old 11-11-2008, 09:45 PM
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A question on Vignetting...

I get slight vignetting when using my Megrez 102 for imaging.. I am about to get a moonlite motorized unit for it, however I contacted moonlite and they said that for $50 more, they can make a custom flange for the M102 to take their 2.5" focuser, which is a shade more expensive than the 2" model, but if it will lessen the vignetting, I'll go for it..

Would a 2.5" focuser reduce vignetting in the system??

I would have thought so due to a wider opening right the way down to where the 2.5" -> 2" adapter, then straight onto the chip... So maybe 3~4cm of 2" tube in the system rather than 10~15cm with the standard 2" focuser...

Any views on this?
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Old 11-11-2008, 10:54 PM
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Octane (Humayun)
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Alex,

Save your dosh and take flat fields, instead.

Regards,
Humayun
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Old 11-11-2008, 11:32 PM
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Tandum (Robin)
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Do the canon images show the same? I've recently found that the qhy8 sensor is 2mm larger across the diagonal than the 40D sensor. Not sure which canons you have

[edit] Found that the 350D sensor is the same size as the 40D but the 300D is bigger. The qhy8 is 28.4mm across the diagonal while the 300D is 27.26mm and the 40D and 350D are 26.68mm.

There is a 13mm difference between 2" and 2.5" but a $30 light box is really what you need.
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Old 12-11-2008, 05:26 AM
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Im putting a moonlite focuser on it anyway. that way its computer controlled focusing... The less I have to touch the rig after setup the happier I am... So for the extra $71 a 2.5" is probably worth it anyhow.

I do also need a light box for flats...

Robin, the 350D gets a hint of vignetting also.
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Old 12-11-2008, 06:33 AM
Dennis
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Hi Alex

I think that all long lenses (telescopes) “suffer” from vignetting; it seems a difficult to avoid property of their design?

I think that the Canon 50D has a software setting that can “compensate” for vignetting on Canon lenses?

Cheers

Dennis
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Old 12-11-2008, 12:59 PM
Hagar (Doug)
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Hi Alex. Vignetting is a curious thing which is very rarely resolved totally with increasing the focuser diameter. One of the main causes is the fact that the light is focused to a spot on the CCD and as such there is nearly always an illumination gradient or vignetting to the corners of the larger CCD's. The only real fix is flat framed images to correct the gradient. My Fs 102 has a 2.7" focuser but still suffers some vignetting which is easy to fix with Flat frames.

I have found an interesting option to use for flats and a light box and will be investigating it further. I have some reports that it works terrific. Time will tell.

http://www.elec2go.com.au/webcontent8.htm
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Old 12-11-2008, 01:14 PM
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Doug, That website has some great brainstorm ignitors!! The A3 EL sheet is a flat box, that rolls up to fit in a tube!! Fantastic...

Considering ordering!
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