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#31
September 28th 05, 10:49 PM
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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 --------- A 20 inch F5 has the same possible FOV as a 10 inch F10 scope but 4 times the mirror area,, assuming both can fully utilize an 2 inch eyepiece. It also gathers 4 times the light, one is getting something back for the loss of field of view. Personally I like the large 2 degree field of view possible with a 10 inch F5 Newtonian, essentially it is a design capable of "rich field" performance. That potential wider field of view of a faster scope is an advantage, something that has to be considered when making a choice. jon 
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#33
September 29th 05, 12:25 AM
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Brian Tung wrote:
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, The C-11 has the larger rear port available which removes the inherant vignetting caused by the smaller visual back. Regards Bill -- William R. Mattil : http://www.celestial-images.com
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#34
September 29th 05, 12:27 AM
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"decaf" wrote in
ups.com: 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. Ok understood. Klazmon. DC
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#35
September 29th 05, 01:45 AM
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I still have both of mine and I agree. Sometimes I just don't want to lug
out the 11" one. I keep contemplating selling the NS8GPS, but..... "rat ~( );" wrote in message ups.com... Another Oink from Doink..... oink oink! Yeah, I owned 8" and 11" SCT's at the same time and the 11" blew the 8" clear out of the water. Even against my well-optimized 10" Zambuto Newt, the one extra inch of the 11" still gave it the edge in terms of light grasp (fainter stars visible at the same x). 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. rat ~( );
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#36
September 29th 05, 02:54 AM
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I used to own a C11 after selling the C8.
The C11 needed a lot of work to set up for one person. A semi/permanent housing is a solution but adds quite a bit to the cost and requires extra space. Then again your limiting yourself to your own backyard skies. Motorized drives are great when they work reliable, But dobs can have electronic setting circles that are just as useful for locating objects quickly. More important to me is aperture and quality. After selling the C11 I got a 16" dob. Given the choice of only the C11 or C8 I would take the C11 for optical reasons, but I sure would hate setting it up and taking it down alone. JD "Stephen Paul" wrote in : If you want to see more deep sky objects a 10" or 12" dob makes more sense. To you maybe.
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#37
September 29th 05, 04:49 AM
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wrote in message oups.com... Personally I like the large 2 degree field of view possible with a 10 inch F5 Newtonian, essentially it is a design capable of "rich field" performance. When I had my 12.5" F4.8 Dobsonian, I never could tolerate the views in a larger field eyepiece than the 24mm Panoptic (1 degree field at 67x). I just don't like overly large exit pupils. In my eyes, the stars are all distorted. Even the 5mm exit pupil of the 24mm Panoptic was loaded with astigmatism, and I would only use that as a finder eyepiece before jumping to the 2.7mm exit pupil of the 13mm Nagler T6 (0.7 degree at 117x). On the other hand, in the 80mm F7.5 refractor, the 24mm Panoptic has a more tolerable 3.2mm exit pupil, and a whopping 2.5 degree field of view at 25x. I don't use 2" eyepieces in that scope either. In the C9.25 SCT, the 35mm Panoptic has a tolerable 3.5mm exit pupil, and a 1 degree field at 67x. Same field as the 12.5" Dob, but with an exit pupil that doesn't blow out my eye. Even so, I don't use the 35mm Pan in the C9.25 all that much either. My starting eyepiece is always the 24mm Panoptic. Regardless of which scope I'm using. If I want a larger field, I go down in aperture as well as focal length, to keep the exit pupil smaller than 4mm. That potential wider field of view of a faster scope is an advantage, something that has to be considered when making a choice. Between an 8" SCT and an 11" SCT?? :-)
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#38
September 29th 05, 02:44 PM
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Bill:
Nice to see you here.. 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. u have a citable source for this information of course ? Though it needs to be noted that your statement regarding the focal length of the Newtonian is superfluous information given the subject line. You did read that didn't you ? ------- I guess if you need me to point out that the vast majority of Newtonians are F6 and below.... My own 12.5 inch is F4.06 which with a 2 inch eyepiece is capable of a tad bit over 2 degree FOV. As far as reading the subject line... Possible you have noticed that myself and others have suggested to the original poster that other options are available to him and that he ought to consider them before making a purchase. My first scope too was an 8 inch SCT, I am certainly glad I found other designs. Yet you choose to ignore the opposite of this. And that is that smaller DSO's will have a greater image scale than with a widefield instrument. ---- It is very difficult to increase the maximum possible field of view, that is inharent in the scope. It is very easy to increase the effective focal lengh or "image scale." That can be done for a few dollars with a barlow. In the other sense, the Newtonian could fit more DSOs in the eyepiece at any given moment due to that larger FOV. So can my naked eye .... but that's not the subject here. Your blatent bias towards Newtonians (actually Dobsonians) has skewed your reasoning here. -------- How many galaxies can you see in the Virgo Cluster with your naked eye..... I have no problem admitting that I enjoy DOBs. I also own two SCTs. However the point here is not to play favorites, rather it is to help the original poster make a choice that he can be happy with. By letting him know what the advantages are of other designs, it just may be that he will spend a bit of time investigating those choices. I have no doubt that for many folks, the C-11 or the Nexstar 11 would be a good choice and a scope they would be happy with. I also know that this past summer, I shared the eyepiece of my 10 inch DOB more than one owner of an 8 inch SCT that was very impressed when viewing through a DOB, not only the optical performance but the ease of use and the ease of setup. Yes I like Newtonians and I like DOBs, but when someone is trying to decide between an 8 inch DOB and a 12 inch DOB, I have no problem with someone else suggesting they consider an SCT. Let us all keep an open mind all the possibilities. Best wishes and clear skies... jon
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#39
September 29th 05, 02:47 PM
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Jon Isaacs said: This will allow one to see some larger DSOs that would otherwise not fit in the FOV of a C11. and William R. Mattil responded: Yet you choose to ignore the opposite of this. And that is that smaller DSO's will have a greater image scale than with a widefield instrument. Yes, Jon ignored the opposite -- and with good reason. The situation is not (alas) symmetric. In some sense, a telescope's focal ration is irrelevant. It can be decreased with a focal reducer or increased with a Barlow lens. In fact, that's the heart and essence of the Cassegrain design. An SCT's primary mirror has a focal ratio of f/2 or thereabouts, but the SCT uses the hyperbolic secondary mirror as a focal extender -- very much like using a Barlow on a Newtonian. Unfortunately, whereas the ability to jack up the focal ratio with a Barlow is essentially unlimited, the reverse is not true. An instrument's maximum true field of view -- the whole reason for reducing the focal ratio for visual work -- is limited by several other factors, including physical obstructions in the light path and off-axis aberrations in the objective. So it is an absolute physical fact that the maximum practical true field of view of an 11-inch f/5 Newtonian is *inherently* wider than the maximum practical field of view of a standard commercial 11-inch f/10 SCT. Whether you actually care about the wide field of view is a whole 'nother matter; that's a subjective judgment, and I can't argue about it. But it is an objective fact that assuming equally good optical construction, everything that you can see in an 11-inch f/10 SCT will show equally well or better in an 11-inch f/5 Newtonian, while the reverse is clearly not true. Mind you, SCTs have other advantages. Most obviously, an 11-inch SCT tube is much more compact than an 11-inch Newtonian tube. Which means, in turn, that a high-quality tracking drive for an 11-inch SCT is cheaper, smaller, and lighter than a high- quality tracking drive for an 11-inch Newtonian. - Tony Flanders
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#40
September 29th 05, 03:03 PM
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That potential wider field of view of a faster scope is an advantage,
something that has to be considered when making a choice. Between an 8" SCT and an 11" SCT?? :-) Fortunately none of us, you, me, the original poster, are limited to making a choice between an 8 inch SCT and an 11 inch SCT. That is why myself and others have brought up the possibility of choosing another design. Sometimes people get stuck with a specific type scope because it was the first thing they had and have not had experience with other designs. While it seems unfortunate that your eyes don't accomodate larger exit pupils, I find exit pupils of 5mm quite acceptable as do many others. I like those big fields of view, particularly when viewing targets like the Veil. A current favorite is IC4756. While this OC is only about 1 degree in diameter, it looks best to my eye when framed by a 1.4 degree FOV. I might point out too that you are comparing a 9.25 inch scope with a 12.5 inch scope. It is generally easy to get those wider fields of view with a smaller scope. A more realistic comparison would be a 12 inch F10 SCT. Jon Isaacs
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