#21
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"John Deer" wrote in message ... I found a huge improvement in quality between the C8 and C11. Perhaps there is less variation in the C11. If you want to see more deep sky objects a 10" or 12" dob makes more sense. To you maybe. |
#22
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Actually, I don't think it makes any difference at all to the brightness of
the moon. This is true in a telescope of any size. Surface brightness cannot be increased by a passive optical system as it would be a violation of thermodynamics. The Moon, or ANY luminous object will look brighter in a larger telescope than a smaller one for a given magnification, or the only gain would be resolvimg power. Actual surface brightness does NOT need to be increased for an object to appear brighter; the distribution of availible light energy increases the brilliance of an object for a given magnification. The larger the aperture, the more light energy is gathered and availible to spread over a given expanse of image plane. This is not the same as claiming that any telescope will show the Moon brighter per unit area than it actually is, or looks to the naked eye ( which it can't). Nor did I imply that. DC |
#23
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BTW, I gave the Newt
a tune-up today so I can get back to using it for DSO's. It is currently my biggest aperture scope. I need to get a new focuser for it. Bill, I guess I missesd it, but hat ever happened to that 18" er? Dan |
#24
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If you want to see more deep sky objects a 10" or 12" dob makes more
sense. To you maybe Actually a 10 or 12 inch DOB will have a much shorter focal length than the C11 and use 2 inch eyepieces effectively at that focal length thus allowing for a significantly wider possible field of view. This will allow one to see some larger DSOs that would otherwise not fit in the FOV of a C11. In the other sense, the Newtonian could fit more DSOs in the eyepiece at any given moment due to that larger FOV. jon |
#25
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#26
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matt wrote:
don't forget that commercial SCT's have a limited field of view due to baffle tube vignetting . None exceeds 1.25 degrees , so there's no such thing as a wide field SCT. Haven't done the math, but the vignetting at about 2 degrees in a C5+ with a reducer, although visible, is not visually offputting. I wouldn't image through it, though. -- Brian Tung The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt |
#27
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Haven't done the math, but the vignetting at about 2 degrees in a C5+
with a reducer, although visible, is not visually offputting. I wouldn't image through it, though. --------------- I have a C5. With a F6.3 reducer/corrector, the focal length ends up at about 800mm and a field of view with a 1.25 inch eyepiece of around 2 degrees, though the hole in the back of the scope is quite a bit smaller than 27mm so vignetting is going to be a factor. jon |
#28
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Jon Isaacs wrote:
I have a C5. With a F6.3 reducer/corrector, the focal length ends up at about 800mm and a field of view with a 1.25 inch eyepiece of around 2 degrees, though the hole in the back of the scope is quite a bit smaller than 27mm so vignetting is going to be a factor. Yes, I know that vignetting is a factor. When I said I hadn't done the math, I meant that I hadn't computed the extent of the vignetting at the edge of the (maximum) field. The question raised earlier was whether the baffle tube introduces extra vignetting above and beyond what the rear port does. Of course, the C5+ rear port introduces vignetting that it doesn't in the larger SCTs. -- Brian Tung The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt |
#29
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"matt" wrote in message ... wrote in message .com... If you want to see more deep sky objects a 10" or 12" dob makes more sense. To you maybe Actually a 10 or 12 inch DOB will have a much shorter focal length than the C11 and use 2 inch eyepieces effectively at that focal length thus allowing for a significantly wider possible field of view. This will allow one to see some larger DSOs that would otherwise not fit in the FOV of a C11. In the other sense, the Newtonian could fit more DSOs in the eyepiece at any given moment due to that larger FOV. jon don't forget that commercial SCT's have a limited field of view due to baffle tube vignetting . None exceeds 1.25 degrees , so there's no such thing as a wide field SCT. Then again, 1 degree of field can be plenty for those objects for which one wants large aperture. Just ask someone with a 20" F5 Dobsonian. |
#30
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"Brian Tung" wrote in message ... The question raised earlier was whether the baffle tube introduces extra vignetting above and beyond what the rear port does. Of course, the C5+ rear port introduces vignetting that it doesn't in the larger SCTs. Here are the measurements for the inside of all of the [Celestron Primary] baffle tubes: 5" 25mm 8" 38mm 9.25" 48mm 11" 54mm 14" 54mm (Reposted from June 2003) |
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