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gso 8 cc or c8edge or c925?for planetary observing

planet observing
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#1 eq3d

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Posted 21 May 2020 - 09:01 AM

需要一些帮助..



#2 havasman

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Posted 21 May 2020 - 09:16 AM

对于较大的光圈,我会选择C9.25。它将需要更大的安装座。如果那是负担,C8或C8 Edge HD也可以正常工作。
欢迎来到论坛。


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#3 gfstallin

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Posted 21 May 2020 - 12:41 PM

C9.25


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#4 glend

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Posted 21 May 2020 - 07:50 PM

The GSO Classical Cassegrain is a better choice in my opinion. I recently purchased one after much research. The big pluses are: 

 

Open tube design means no fogging that is common with any SCT design,

Primary mirror does not move, as it does in SCTs including the Edge, ie mirror shift.

Longer focal length for its size, which is f12.

No coma in the CC, and flat field, something you pay a premium for in the Edge design.

GSO has a great reputation for mirror quality 1/12 wave.

 

I have owned an 8" Edge before and it was nothing but trouble, mirror flop, focuser, etc.

If your interested GSO has just released a 10" CC version as well.


Edited by glend, 21 May 2020 - 07:52 PM.

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#5 AhBok

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Posted 21 May 2020 - 10:44 PM

You could make an argument for any of the three based on specs, but having had all 3 scopes, I find the C9.25 to put up the best planetary images. It might not be a fair comparison based on the different ages of each (C9.25 been around a long time) but the 9.25 is a well known fine planetary imaging OTA and has been touted by some excellent planetary imagers. 
 

I kept the C9.25.


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#6 RAKing

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Posted 22 May 2020 - 07:30 AM

I decided to skip the 8CC, but have owned good examples of both the C925 and the C8-HD Edge.

 

The C925 gets my vote.  Mine was sharp and clear across the whole FOV and of course the extra aperture helps even more.  The only downside is the weight.  The C925 is heavier than the other two.

 

I currently own a C8-HD Edge and it's a very good scope.

 

Cheers,

 

Ron


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#7 AlienRatDog

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Posted 22 May 2020 - 08:01 AM

My 8 inch Edge HD is pretty sharp, it does have some mirror shift (not as bad as my 12 inch SCT), the CC is intriguing due to quartz mirrors and the open tube design. I cannot imagine the views being that different, the C9.25 will probably best then both (SCTs are easier to collimate as well, which will be more of a factor than subtle difference in quality). The larger aperture would help with color saturation during planetary viewing. Thermal management is an issue, either you let the C9.25 cool down on its own, use some active thermal management (cat cooler), or wrap it in insulation. Now the ten inch CC...that’s intriguing...like a poor mans Mewlon

Edited by AlienRatDog, 22 May 2020 - 08:02 AM.


#8 ATY

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Posted 22 May 2020 - 09:57 AM

I had a C9.25 and I have a C11 and CC8. For planetry viewing the CC8 is better than the C9.25 and C11 in my case. Both C9.25 and C11 have imaging shift and mirror flop problems and the SCT push pull mirror focusing is just not very precise for critical focusing for best planet sharpness unless you spend another $400 on a dual speed Crayford style focuser. The dual speed focuser on CC8 does a pretty good job of precise focusing. There is very little you can do to precisely collimate a SCT other than tuning the three screws on the secondary mirror or you can spend $500 for the Hotech laser colliminator. The secondary and primary mirror on the CC8 are adjustable and the secondary mirror is center spotted so with a simple $20 45-degree faced Cheshire eyepiece you can precisely collimate the CC8. The tubes on most if not all SCT's are just painted flat black inside however you can still see plenty of scattered light reflection inside the tube and that will affect contrast. The 11 baffles inside the CC8 tube does an excellent job in preventing scattered light from reaching the eyepiece and resulting very high contrast image. Without a corrector plate up front the CC8 requires no dew shield or heating strap. Also without a corrector plate there is one less element the light has to pass through and be scattered, distorted, and absorbed. Last night I left my CC8 outside overnight and start observing again at 4 am without any dew problem on the secondary or primary mirror. At high magnification planetary viewing the seeing is greatly affected  by the ever changing pockets or columns of air of different thermal layers of the atmosphere, and this affects telescopes of larger apertures more than smaller ones. The small 8" of the CC8 is just about right to not be affected as much by the atmosphere than the larger scopes. Finally, for those of you who have to setup and take down their telescope equipment every night the light weight CC8 is a Godsend. The included vixen dovetail bar on top of the CC8 serves as a handle to grab hold of when mounting and dismounting the scope from the EQ saddle. The Classical Cassegrain optical system consists of only two mirrors and simple is best.  As I might have posted in another thread the CC8 produces images of much higher contrast than any of my SCT's. I truly think for pure planetary viewing and the convenient features built in on this CC8 all for under $1000 it is hard to beat.


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#9 Pat Rochford

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Posted 22 May 2020 - 10:01 AM

对于较大的光圈,我会选择C9.25。它将需要更大的安装座。如果那是负担,C8或C8 Edge HD也可以正常工作。
欢迎来到论坛。

That's exactly what I was thinking.  Couldn't have said it any better.


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#10 eq3d

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Posted 24 May 2020 - 05:14 AM

thank you every one. cc seems has more contrast. maybe it better for visual
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#11 kenc184

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Posted 24 May 2020 - 07:21 PM

How do you keep the "innards" of the CC8 clean and dust/dirt free?


Edited by kenc184, 24 May 2020 - 07:21 PM.


#12 ATY

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Posted 25 May 2020 - 02:57 PM

How do you keep the "innards" of the CC8 clean and dust/dirt free?

You don't. If you look at the way the 11 baffles were arranged inside the tube they will collect and hold onto dust and dirt and would not let dust and dirt fall down onto the primary mirror. Just put the front end cover back on when done.


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#13 ATY

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Posted 25 May 2020 - 03:17 PM

Here is a height comparison between my CC8 and C11. There is a 1" extension ring installed on the CC8 and the focuser drawtube is racked all the way in. For my eyepieces the drawtube has to rack out 35mm to 40mm  so the CC8 in use i.e., with diagonal and eyepiece in focus will be 3" to 4" longer than a C11.

 

cc8c11photo.jpg


Edited by ATY, 25 May 2020 - 03:26 PM.

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#14 ewave

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Posted 25 May 2020 - 03:22 PM

Nice pic. I wonder why the extra one inch extender for the CC8? 

I really like the CC8 focuser.  No moving mirror nightmares.



#15 ATY

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Posted 25 May 2020 - 03:35 PM

Just for fun, here is a photo I took with my iPhone while my CC8 was sitting on the CEM60 observing Saturn and Jupiter at 3:15 am PST.  

 

cc8viewing1.jpg


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#16 ATY

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Posted 25 May 2020 - 03:39 PM

Nice pic. I wonder why the extra one inch extender for the CC8? 

I really like the CC8 focuser.  No moving mirror nightmares.

The CC8 has the focus point about 5 inch from the end of the drawtube so having the included 2" and 1" extension rings you can accommodate different eyepieces and equipment to be hang at the back. In my case I need one 1" extension and rack the drawtube out about 35mm to 40mm to reach focus.


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#17 glend

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Posted 25 May 2020 - 05:12 PM

My CC8 is also configured for visual use, with one 25mm extension tube ahead of the focuser, and I use a 2" Star Diagonal in the focuser, which means all of my eye pieces achieve focus without needing to rack the focuser tube out too far. In focus range is perfect. I make it a point to not rotate that focuser, leaving it locked in standard position, as these focusers have a bit of a reputation (from the RCs) of having a bit of tilt in the tube run. If I want rotation I simply rotate the diagonal in the focuser tube, gives same result. I don't recommend switching to a Moonlight focuser (which does allow tube tilt correction), because once setup it too cannot be rotated without losing precision. Ron of Moonlight has a note about this on his website where RC focuser is discussed.



#18 chuckscap

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Posted 25 May 2020 - 05:24 PM

I don't believe Classical Cassegrains have a flat field.   They do have some advantages including faster cooling (open design), non moving primary mirror (reduces focus range, but eliminates mirror flop) and smaller Central Obstruction (an f16 is a more standard CC design with a 25% CO). 



#19 Terra Nova

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Posted 25 May 2020 - 06:20 PM

I don't have either, but based on my past experience with five SCTs, all that I've read about the CCs, and that my primary interest is planetary, I would go for the CC.


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#20 glend

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Posted 25 May 2020 - 08:05 PM

I don't believe Classical Cassegrains have a flat field.   They do have some advantages including faster cooling (open design), non moving primary mirror (reduces focus range, but eliminates mirror flop) and smaller Central Obstruction (an f16 is a more standard CC design with a 25% CO). 

Flat field is subjective, it certainly is more flat than a standard SCT. Read the reviews.


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#21 ATY

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Posted 26 May 2020 - 10:59 AM

My CC8 is also configured for visual use, with one 25mm extension tube ahead of the focuser, and I use a 2" Star Diagonal in the focuser, which means all of my eye pieces achieve focus without needing to rack the focuser tube out too far. In focus range is perfect. I make it a point to not rotate that focuser, leaving it locked in standard position, as these focusers have a bit of a reputation (from the RCs) of having a bit of tilt in the tube run. If I want rotation I simply rotate the diagonal in the focuser tube, gives same result. I don't recommend switching to a Moonlight focuser (which does allow tube tilt correction), because once setup it too cannot be rotated without losing precision. Ron of Moonlight has a note about this on his website where RC focuser is discussed.

I think you solved my mystery. Every time after a meridian flip the image would be off center quite a bit. I bet it had something to do with focuser draw tube tilted in the CC8, and mirror flopped in the C11. Thank you.



#22 glend

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Posted 26 May 2020 - 06:30 PM

I think you solved my mystery. Every time after a meridian flip the image would be off center quite a bit. I bet it had something to do with focuser draw tube tilted in the CC8, and mirror flopped in the C11. Thank you.

Your welcome, but I would not be too sure that the small amount of focuser tube tilt would produce a noticeable difference on the other side of the Meridian, (as the focuser tube tilt only becomes apparent when racking in or out because of the slight mismatch between the mechanical and optical alignment), the focuser has no flop, unlike an SCT mirror. If your in focus on say the east side of the Meridian and you do a flip, the scope should still be in focus  (more or less), because it has not been racked in or out. If you have a great deal of weight on the focuser tube, then yes you may get a gravity sag in the different direction, which could account for the difference. Of course your mount could also be causing the displacement.


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#23 glend

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Posted 26 May 2020 - 06:37 PM

I would encourage people to not get too hung up on focuser tube tilt in the CC, it is a minor issue as far as i am concerned. Yes it impacts focuser rotation but if you don't rotate the focuser by loosening the lock ring, it is not an issue. Any time you unlock that lock ring with weight on the focuser you do run a risk of the beveled face between the focuser and the extension tube being slightly different anyway.


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#24 JP-Astro

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Posted 03 June 2021 - 05:09 PM

How do you keep the "innards" of the CC8 clean and dust/dirt free?

 

You don't. If you look at the way the 11 baffles were arranged inside the tube they will collect and hold onto dust and dirt and would not let dust and dirt fall down onto the primary mirror. Just put the front end cover back on when done.

I think it's quite an optimistic (an actually unrealistic) statement. Dust will deposit on the mirrors of CC just as well as it does in any open tube design. The anti-reflective baffles are not "dust magnets" in the sense they will collect all the dust that gets inside of the CC. They won't.

The CC mirrors will have to be cleaned on the regular basis (as required) similar to Newtonians for the best performance result.



#25 pweiler

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Posted 03 June 2021 - 08:59 PM

I don't believe Classical Cassegrains have a flat field. They do have some advantages including faster cooling (open design), non moving primary mirror (reduces focus range, but eliminates mirror flop) and smaller Central Obstruction (an f16 is a more standard CC design with a 25% CO).


My CC8 f/12 for visual use shows no coma - zero - down to a 56mm 52’ eyepiece.
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