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celestron C8 as a visual scope?

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#1 pesa

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Posted 01 August 2014 - 07:50 PM

I'm considering using an old C8 SCT as a (reasonably) wide field visual instrument.
My idea is to buy a 60-70 degree ~40 mm eyepiece and use with it.
perhaps GSO superview ERFLE 42mm or TS optics paragon.The exit pupil shouldn't be a problem, right?
Is there any reason that wouldnt work? Like SCT not suitable for that wide an eypiece...
pelle

#2 Brian Carter

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Posted 01 August 2014 - 08:25 PM

Oh it will work, no problem. I love my C8, and cheap eyepieces work a lot better in it. Check out the Cat forum for more discussions on it, pretty much everyone has had one at some point.

#3 Brian Carter

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Posted 01 August 2014 - 08:30 PM

A C8 is not really a widefield scope though, more like looking through a straw. It has a 2-meter focal length, widest field you can get is just over a degree. As I said, I think it's a fine scope and I like it, but definitely disappoints in the wide field category. I've been considering mounting a refractor on top of mine so I can get both views on one mount.

#4 areyoukiddingme

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Posted 01 August 2014 - 11:07 PM

Plus for a regular C8 you will want to get the field flattener/focal reducer to maximize FOV.

#5 mogur

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Posted 02 August 2014 - 04:33 PM

Even with a focal reducer a C8 can only achieve about a 1.5 degree FoV. Not exactly a "wide field" scope!

#6 jgraham

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Posted 02 August 2014 - 04:43 PM

I have an 8" SCT (and a 4" and two 10") and it is one of my favorite visual scopes. Bigger than a 6" and much, much lighter than a 10". I have a f/6.3 focal reducer for it, but I prefer to leave it at its native f/10. True, its not a wide field scope, but for the vast majority of objects it works fine.

#7 azure1961p

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Posted 04 August 2014 - 09:15 PM

I totally agree John.

 

the list of things that will fit is far greater than things that won't.

 

 

pete



#8 Eddgie

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Posted 05 August 2014 - 09:03 AM

The C8 works fine with wide field eyepeices up to a point, but not a point you are going to have if you use a 40mm wide field.

 

With long focal lenght 82 degree AFOV eyepeices, depending on how sensitive to it you are, coma and field curvature make it difficult for you to keep the entire field (center and edges) sharp all at the same time.

 

I think the C8 design is at its best when used with 68 degree AFOV eyepeices, but I found that for me personally, the inexpensive 40mm eyepeices, even in the f/10 SCT, did not give very good off axis performance.  I found the result to be far better when using more expenisve 40mm wide field types.

 

The Panoptic 41mm presented a far sharper off axis than a Meade S4000 SWA for example.  The S4000 SWA could not be made sharp at the center and edge at the same time.   If I focused on the center, the outside would be very unsharp.

 

So, I think that the C8 itself can indeed give a very nice, sharp 1.1 degree field, but don't expect to get this from just any old 40mm eyepiece.

 

If you don't mind that the entire field is not sharp, then sure, any 40mm wide field will do.

 

As to whether a C8 can be considered a wide field telescope or not, well, it is all relative after all.    When I would use my C14, I was able to get the vast majority of deep sky objects into the field using a 41mm Panoptic (95x).  It was rare that I pointed it at a target that would not fit into the field.

 

I rarely use my 12" dob at less than 92x with my binoviewers, and once again, most things fit easily in the field. 

The C8 is a great telescope, and can provide wonderful 1.1 degree field of view in a small, light, inexpensive, and comfortable to use pacakge.

 

Just what a lot of consumers really like about them, which has made it perhaps the best selling telescope design ever made.

 

I think everyone should have a C8..  There are so many times when small, light, inexpensvice, and comfortable are just what one wants at the moment.

 

It's good to be a consumer...



#9 geminijk

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Posted 05 August 2014 - 10:31 AM

I love the versatility of my C8. You can use a 6.3 reducer and get a very nice WFOV. Of course, native f10 is a great visual focal ratioin for almost everything, with the proper eyepiece. 

 

John



#10 Starman1

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Posted 05 August 2014 - 02:41 PM

With or without a focal reducer, the illuminated field is about 1.1 degrees wide.

However, even for visual use, a focal reducer is desirable (the f/6.3 one):

--you achieve low powers with lighter, and less expensive, eyepieces

--coma is reduced

--the field is flattened

--nearly all eyepieces work better with the reducer in place.

--long eye relief short focal length eyepieces exist, so you can still get comfortable high powers.

--you can remove it if you want an f/10 scope.

 

I had an 8" SCT for 11 years, and observed nearly 10,000 objects in it.  About year two, I screwed the focal reducer on and only removed it when using a Lymax SCT Cooler.

Otherwise, it stayed on.

 

P.S. if you can tolerate a little more vignetting, you can sqeeze the true field to 1.2 to 1.25 degrees with no issue.

Attempting to achieve more can be frustrating.  A 35mm Panoptic (38.7mm field stop), used with the focal reducer, made vignetting so apparent it was like a textbook case, and the edge-of-field illumination was essentially zero.  Not pleasant.  But a 22mm Nagler (31.1mm field stop) only had a small amount of noticeable darkening at the edge and it was tolerable (true field 1.39 degrees.  Knowing what i know now, though, I wouldn't go larger than the field stop of the 20mm Nagler (27.4mm=1.23 degrees, 64X with reducer).


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#11 Pete-LH

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Posted 05 August 2014 - 03:27 PM

Pelle,

I have an olde 3rd, 4th, or 5th plus hand de-forked C8. That now is my favorite telescope. The optics are great. I generally go between a 35mm Pantoptic, 17mm Nagler and 11mm Nagler(1.1 deg 57X, 0.7 deg 118X and 0.43 deg 182X). I generally enjoy these views much but on steady nights 250X and even 400X might be possible but in general I enjoy lower magnification views.

For wide field views I'm looking at small refractors as mentioned above.In the attached photo i am using a Borg 60 ED(F/5.8) as the finder (32mm TV Plossl gives 11X 4.42 field of view but I push it to 100X for comparative views on double stars sometimes).

However in all this the cost of the accessories(lenses, FT focuser upgrade, Refractor, mounting accessories, etc.) probably cost up to 10X the value of the original C8.

In my case I guess it has spiraled out of control.

But after going around the refractor - SCT cycle I find this my optimum viewing combination. Plus for nights that require a quick setup or when traveling I have the small refractor(actually unfortunately that is refractor$$$).

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Edited by Pete-LH, 05 August 2014 - 03:28 PM.


#12 Magnus Ahrling

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Posted 05 August 2014 - 03:48 PM

I do love my "little" 1973 deforked orange C8, usually mounted on my CG-5 for visual observing.So comfortable to use, mount and has great optics. I always use my f/6.3 Celestron focal reducer+1.25" visualback and diagonal. Very Happy with it :) As Eddgie noted:everyone should have a C8.Very true in my opinion.

 

Magnus 57N.


Edited by Magnus Ahrling, 06 August 2014 - 03:38 AM.


#13 Patrick

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Posted 05 August 2014 - 03:58 PM

I agree with Eddgie that Panoptic eyepieces (68° AFOV) seem to work best with SCT's.  In my C11 non-Edge scope I like to use a 27mm Panoptic eyepiece with a Denkmeier S1 Power X Switch which has a 0.6x focal reducer built in to it.  That's equivalent to about a 35mm eyepiece.  In the C8 that would be equivalent to about 1.6° TFOV, give or take a little.  The extra length of the P x S adds a little to the back focus and increases the focal length slightly.

 

One other thing, as others have said, most objects except for very extended ones fit nicely in the field of view of an 8" SCT.  If you want a truly wide TFOV you really want a different instrument like a pair of binoculars or a small refractor. However, then you give up light gathering.  It's all a trade off. :-)

 

Patrick



#14 Starman1

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Posted 05 August 2014 - 05:11 PM

Ironically, my 12.5" dob has pretty close to the same maximum field as my 8" SCT did.

I didn't feel "confined" by the field in the SCT, and I don't feel confined by the true field of the dob either.

Of course, I do have another instrument that can achieve nearly a 5 degree field, and that large field is dynamite on large asterisms.



#15 Benson

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Posted 07 August 2014 - 01:42 AM

1.5 degrees is not a bad FOV, that's 3 full moons. A decent focal reducer is a really nice SCT accessory.



#16 gnowellsct

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Posted 10 August 2014 - 09:00 PM

1.5 degrees is not a bad FOV, that's 3 full moons. A decent focal reducer is a really nice SCT accessory.

 

An SCT has 1.2 degrees due to the somewhat undersized baffle tube.  

 

A focal reducer buys field at the price of magnification.  It is *not* a way to get more field of view.  A 41 Pan and gets you all the field there is to be had from a c8 or any other scope with a two inch focuser.  If you add a focal reducer you can use a different eyepiece....such as a 28 pan, to get about the same result as the 41 by itself.

 

It's like a reverse barlow.  You can get high power with a shoft fl eyepiece or with an eyepiece + a barlow.  But the result and usable magnifications will be the same.    GN



#17 TONGKW

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Posted 10 August 2014 - 09:47 PM

My C8 and modified Voyager AltAz mount on a Feisol CF tripod.
At less than 30 lb fully loaded, I can just grab the set up with both hands and go out.

 

K W TONG
C8+CG5 GT, TSA102+HEQ5 PRO, MK67+Voyager, NexStar 6SE, C5+Mizar K, WO ZS80FD+Kenko NES, Megrez 72FD+Kenko KDS, Mini Borg 50, PST.

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