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C8 and other 8"SCT's and DeepSky observation

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#1 vpatil

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Posted 23 September 2013 - 12:29 PM

I was wondering if C8(and other 8" SCT's) is really a good telescope for visual deepsky observation?

According to telescope optical equations, Minimum magnification of C8 is200/7 = 29

And Eyepiece needed to get that brightness is 7*10 = 70mm.

Ok I can understand from the above it isnt possible to operate at 100% surface brightness.

Then if I operate at optimal surface brightness, then optimal magnitude will be 200/2 = 100. Which might not be such good magnification for deep sky given 8% surface brightness.

So most 8" SCTs come with f/10. So are they really good for visual deep sky?

#2 Larry F

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Posted 23 September 2013 - 12:53 PM

The answer is, it depends.

If most of your interest in deep-sky is observing galaxies, planetary nebulae and globular clusters, then the narrower SCT field isn't really a problem in terms of image scale. Not so great for open clusters like the Double Cluster, or wide-field Milky Way sweeps, but even most nebulae will fit in the field of a 32mm eyepiece (gives the greatest true field, about 0.82 degrees if the eyepiece FOV is 52 degrees).

If you live in a light-polluted area, narrower field and higher magnification improve contrast of low-surface brightness objects (read Rod Mollise's excellent book "The Urban Astronomer's Guide" for a discussion of this). A wide-field, low-power scope like a fast Newtonian will wash out the detail.

If you observe from a truly dark-sky site, then by all means large and fast is the way to go. Aperture rules.

f/10 SCTs are a good compromise, though, when compared to fast dobsonians or refractors. They aren't so large as to be unwieldy, nor too small to disappoint, nor too expensive to afford. A computerized fork-mounted SCT is easy to work with and finds objects accurately. SCTs handle binoviewers better than other types of scopes, which is really great for planetary and lunar viewing. They are not the best for any single object, but they're probably the best for all objects.

#3 HeyJP

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Posted 23 September 2013 - 01:14 PM

An 8" SCT is one of THE most popular sizes of good telescopes. Great bang for the buck. With reasonable clear, dark skies, you can fairly easily find all of Messier and Herschel deepsky objects. The Orion Nebula M42, Ring Nebula M57, Dumbbell Nebular M27, many globular clusters (M13, M19, M4, etc) are all SPECTACULAR in an 8" SCT. And of course the "closer things" Moon, Venus, Mercury, Saturn, Jupiter, Uranus, and Neptune... are great subjects with an 8". Then, throw a camera on one, and you can do some GREAT imaging.

An 8" is a terrific scope for high portability, high capability, and low cost.

Jim

#4 vpatil

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Posted 23 September 2013 - 01:21 PM

Yeah I agree, but with 8" newtonian and f-ratio's around 5-6 which are available for almost similar price. Smaller f-ration is another big factor for deep sky?

Lower f-ration can allow me greater chance to opt for variety of lenses too.

Then what is advantage of SCT's?

Newtonians can also work with higher FOV eyepiece.

#5 Mr Greybush

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Posted 23 September 2013 - 01:45 PM

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#6 Sorny

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Posted 23 September 2013 - 01:56 PM

f/5 or f/6 scopes will require more expensive eyepieces. An f/10 scope is much more forgiving of eyepieces. Said f/5 or f/6 newt is MUCH larger than an equivalent aperture SCT. The prime benefit of an SCT is extremely compact package per unit of aperture.

High FOV eyepieces work in all telescopes. My biggest FOV eyepieces are 70 degrees, not because I can't use larger, but because I happen to like the FOV. Hell, the most often used pair of eyepieces I have are a whopping 65 degrees and used in my binoviewer.

#7 rmollise

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Posted 24 September 2013 - 06:41 AM

I was wondering if C8(and other 8" SCT's) is really a good telescope for visual deepsky observation?

According to telescope optical equations, Minimum magnification of C8 is200/7 = 29

And Eyepiece needed to get that brightness is 7*10 = 70mm.

Ok I can understand from the above it isnt possible to operate at 100% surface brightness.

Then if I operate at optimal surface brightness, then optimal magnitude will be 200/2 = 100. Which might not be such good magnification for deep sky given 8% surface brightness.

So most 8" SCTs come with f/10. So are they really good for visual deep sky?


Yes, a C8 is excellent for deep sky use. Forget your theories about "surface brightness," get to a star party and try one. ;)

Very low power is not very useful for most deep sky objects. Most are small, requiring a minimum magnification of 100x - 150x. Frankly, most deep sky observers misguidedly use too little rather than too much magnification. And very low power is next to useless if your sky is affected by light pollution.

#8 NorthernSoul man

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Posted 30 September 2013 - 01:53 PM

Another vote for the SCT - for visual I think you can't go wrong - short tube, solid alt az mount, GOTO, no tube rotation needed as with a big newt on an eq mount, great eyepiece position, hooked up to a webcam - amazing - Go For It. Paul.

#9 Jon_Doh

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Posted 30 September 2013 - 03:47 PM

For an sct and deep sky viewing add a focal reducer to broaden you field of view. I use a 6.3 reducer with my f10 8" sct and it makes open clusters real purdy. Also helps see more of big nebulae like the Great Orion and Veil. When I want to look at planets or tiny globs I just take it off, although I have left it on an just cranked up the power. Works fine that way too, though most will recommend not.

#10 Lancem

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Posted 30 September 2013 - 08:14 PM

Or purchase a Denkmeier Power Switch Diagonal and have three magnifications at your fingertips for each eyepiece! Add a filter switch and have two filters at your fingertips to slide into the light path. This was one of my best investments for my scope. I agree that the SCT is a very versatile scope. I am always amazed at the variety of objects that I can observe in an evening with my CPC 1100.

#11 NorthernSoul man

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Posted 01 October 2013 - 06:24 AM

I Think the majority of posts seem in favour of the SCT, I really enjoy using mine and find that, ok, yes, the richfield short tube refractors are nice if you have the truly dark skies, even with photo/imaging (not done any)I think you really need dark skies to make the most of the views.
One thing to point out though, I've tried a focal reducer (.63)on the 1100 and don't really find the "increased" larger field that I expected - I think this may be due to the way the mirror/lens system works - so be careful when wanting much over a degree or so for those widefield views - your not going to get the "wow" you were expecting!!!






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