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#11
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"Mark D" wrote in message ... SCT's do have one certain advantage versus Newts, they are more portable, due to the folded design. Mark The idea of an SCT being more portable that a Newtonian, must have been with respect to both being on a GEM. Given the same apertures, in practice I don't see this being all that significant, or even true when compared to a Dob. A tripod is often nearly as long a Dobsonian OTA, and a Dobsonian base is often no larger than an SCT in a hard carry case. The Dobsonian base was the great equalizer of Newtonian portability. |
#12
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Stephen Paul wrote:
The Dobsonian base was the great equalizer of Newtonian portability. I'd throw in truss tubes and equatorial platforms in there with the Dobsonian base. A solid-tube 14-inch Newtonian would be a beast, even on a Dobsonian base. And it still can't track or do long-exposure work without the equatorial platform. -- 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 |
#13
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Although I have long since sold off all of the SCT that I have owned, I
have gotten the best planetary views ever through an 8" SCT. Jupiter looked like the Voyager pictures on nights of exceptional viewing. However, for deep sky, there is nothing like a Newt. Even a 4" scope like an Astroscan will show the green color of bright nebulae. All-around scope? Toss-up. When you factor in price, the Newtonians come up as a better deal. That is why my primary scope is a 10" Newt on a german equatorial. |
#14
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"Gil" wrote in
oups.com: Although I have long since sold off all of the SCT that I have owned, I have gotten the best planetary views ever through an 8" SCT. Jupiter looked like the Voyager pictures on nights of exceptional viewing. While I don't have anything against SCTs, it is hard to take that statement at face value http://www.solarviews.com/raw/jup/redspot.jpg -- Pierre Vandevenne - DataRescue sa/nv - www.datarescue.com The IDA Pro Disassembler & Debugger - world leader in hostile code analysis PhotoRescue - advanced data recovery for digital photographic media latest review: http://www.pcmag.com/article2/0,1759,1590497,00.asp |
#15
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Brian Tung wrote:
Stephen Paul wrote: The Dobsonian base was the great equalizer of Newtonian portability. I'd throw in truss tubes and equatorial platforms in there with the Dobsonian base. A solid-tube 14-inch Newtonian would be a beast, even on a Dobsonian base. And it still can't track or do long-exposure work without the equatorial platform. I saw this very size solid-tube Dob get shoved into the back of a Scion xB last Friday. FTM it had some form of alt/az drive, digital setting circles and an integrating video camera. Sweet setup. Glad I didn't have to pay for it though. Shawn |
#16
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The idea of an SCT being more portable that a Newtonian, must have been
with respect to both being on a GEM. Given the same apertures, in practice I don't see this being all that significant, or even true when compared to a Dob. A tripod is often nearly as long a Dobsonian OTA, and a Dobsonian base is often no larger than an SCT in a hard carry case. The Dobsonian base was the great equalizer of Newtonian portability. ====================================== Agreed Steven, I should've perhaps elaborated in this regard as you say. There's no doubt that a 20" Obsession Dob can be up and running faster than say a C-14 on a AP1200GTO Mount. Still, talking about OTA's alone, the SCT does have some size advantages particularly as far as transporting goes. Mark |
#17
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While I don't have anything against SCTs, it is hard to take that
statement at face value ====================================== Hi Pierre, I can understand where you are coming from, and perhaps the poster you responded to, was in fact telling the truth in that this SCT he viewed through showed the best images he has ever "personally" seen. If someone were to go through the trouble of building a top quality no holds barred SCT, lets say with a 1/10 wave or better final wavefront, Quartz Mirrors, the best coatings, the best Mechanics, specifically addressing a top quality no shift Focuser, a Rear Cell cooling system which aided cooldown, I see no reason why an SCT such as this would not be a desireable scope to some. (If the price was right) Doubtful though that many would pay the asking price of such a scope, when folks like Meade, and Celestron are on the scene. Perhaps most would say "Why should I buy a 8" Schmidt OTA for $5,000, when I can buy a pretty good one for a 1/5 of that cost"? Also too, SCT's do have shortcomings versus other compound designs, so amateurs would probably say why make a high quality SCT, make a good Mak instead, or an RC? There are some better variations of the Schmidt design, but I've yet to see really anyone implement them commercially, unless you wish to say the Meade RCX-400 as being one, which I guess you could say it is. Mark |
#18
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Chris L Peterson wrote: On Tue, 02 Aug 2005 01:11:24 -0700, szaki wrote: Newt has a smaller COB 20-25%, SCT 35% or more, would it show visually? I own a C-11 and a C-8 right now, but splitting tight doubles is very ugly with a SCT. So far I haven't got any great views on planets any of my SCT's, 2 bands in Jpiter. Finaly I all ways grab my 4" APO, all though it's dimer, but stars looks like disks/spheres, not seagulls. JS You obviously have a problems with the SCTs. They should show perfectly pinpoint stars centrally. A CO may or may not show its effects visually. I've never really convinced myself that I could see the difference between planetary images in obstructed and unobstructed scopes. As far as tight doubles go, obstructed scopes frequently outperform unobstructed ones because of the way the Airy disks superimpose. People who spend a lot of time observing doubles often modify their apertures with rotatable linear obstructions for that reason. Theoretically, an SCT is generally better than a Newt because it is more corrected. In practice, so much depends on the individual quality of the mirror and other components that it is difficult to generalize. No its not. Count the optical surfaces. The probability of error at any (or through any) surface multiplied by the number of surfaces in each system, gives a rough idea of the probability of success with one system vs the other. Not to mention the fact they are basically different systems to begin with, unless of course you scramble eggs in a hub cap and hit golf balls with the frying pan .... which many people do in an over populated highly stressed out world! Jerry _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
#19
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...Getting really good collimation, is the key to the SCT. Many
people stop their collimation at the point where it is only really 'barely acceptable', rather than really good... I heartily second that observation. From personal experience, I remember when I got my 10" LX200 SCT I was disappointed with the performance out of the box (compared to a little Maskutov I'd been using prior to that). A browse of the instructions revealed it might be collimation so I adjusted it. Aha, an acceptable image (better than the Maskutov as you would expect for a scope of 3-4x the aperture)! Sometime later, when using higher mag, I noticed the concentric rings of defocussed stars were not quite concentric, so I adjusted collimation some more. Wow - even better images (airy disks clearly visible on all stars now). Finally - after reading Thierry Legaults info on critical collimation, and getting Suters book on star testing. I did a TINY adjustment on a night of exceptionally steady seeing (up to 846x magnification using a Televue zoom EP at 3mm).That noticeably improved things yet again; but MOST noticeably on planetary detail for that last tweak. Now I'm utterly content with the performance of my SCT - to the extent that I believe, within the variation of mass produced instruments, I've got one of the "honey's". The point though, even though I KNEW collimation was important, it was several months before I learned just HOW important those last miniscule tweaks really are. All part of the learning experience. Cheers Beats |
#20
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On Wed, 03 Aug 2005 00:25:07 -0500, jerry warner
wrote: Theoretically, an SCT is generally better than a Newt because it is more corrected. In practice, so much depends on the individual quality of the mirror and other components that it is difficult to generalize. No its not. Count the optical surfaces... I'm simply discussing the design, not the implementation. If you look at the spot diagrams of a well corrected SCT, it performs better (on the whole) optically than a Newt. To get good optical performance from a Newt any significant distance from the optical axis requires additional correction. However, even in practice, the additional surfaces are generally insignificant. Optical manufacturing techniques are quite good these days. Plenty of high end refractors have even more surfaces, and yield very high quality images. The value of additional surfaces in improving correction exceeds the harm (from light loss or scatter). But I do agree with you that you can't just do an A/B compare of two different optical designs. There will always be cases where one or the other is a better choice, regardless of arbitrary indicators of performance. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
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