Celestron G-8N Newtonian


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Subject: Re: vote by xxx.xxx.66.160
By: chipdatajeffB
In Reply to: Anonymous (xxx.xxx.66.160) (Original Vote)
Date: 07/08/2002 09:29:32 am PDT
I left this post a few months ago, after I'd had the G8N only a couple of months. There's a mistake I should point out: "limited to about 200mm" should be "limited to about 200X".

I'd like to add two things:

(1) The portability and ease of setup are the best things about this scope. I give public star parties and setup time is a big consideration when you're having to transport three telescopes to a dark-sky site and set them up. You need scopes that are compact, travel well, and set up and cool down quickly. For an 8" scope, this one's portability and ease of setup would be difficult to match. It's also got a very solid mirror cell ... I haven't had to recollimate it since I got it, and it's been lugged to dark sky sites twice a month, plus the occasional quick setup in the back yard.

(2) I've tried all sorts of tricks (noted in the original post) to "fix" the mount. Then I found a set of solid-wood legs at Hands On Optics for $79. They came with a mount base that exactly fits the CG-5 (as advertised). Vibration damping time is now between 1 and 2 seconds! I easily spent twice the $$ and lots of time trying various other schemes. Do yourself a favor if you have a CG-5 mount and get those legs! I find I'm no longer lusting after a "better" mount ...

Another tip: Hands On Optics is the only place I've been able to find additional dovetail plates. They're only $25 dollars and are exact MFR replacements for the original. I've also purchased additional counterweights there.
I'd still give this scope my original rating score ... but you can add a point if you upgrade the mount with the wooden legs. I've also noticed several Internet sites with prices down in the $500 to $600 range (with the aluminum tripod). Jeff Barton.

>I bought from a retailer ($959) and still consider it a good value. You MUST let the mirror reach working temperature or the images double. It's easy to tell when it's right (you hear a curious little "pop" if you happen to be there when this happens) and -- if you're watching at the time -- the images just magically resolve all at once. Collimation is spot on, but recollimating might be a problem if needed because both the primary and secondary adjustments are potted with silicone adhesive. I have trucked this thing all over Texas and never had to tweak it, though. Very bright views due to fast f/ratio. Jupiter and Saturn are remarkably good, even at higher powers. Best when limited to about 200mm (probably due to the large central obstruction). Very good optics, with sharply defined diffraction rings on bright stars. Sirius is so bright it actually hurts to look at it. Venus has a discernible disk but the heavy secondary spider provides distracting "star points" and a visible diffraction pattern ... curious. Star clusters (especially M13) are absolutely amazing. Weak point is the mount. It comes with 2 11-lb counterweights and you MUST carefully balance this scope. You should fill the tripod legs with something (sand or spray foam insulation) and put c-clamps on each leg between the spreader and the top of the mount. Otherwise, it takes almost 20 seconds to stop vibrating if you tap the tube. Wind is a problem because the mount is just not steady. With the above improvements (which are "dirt" cheap) you can get the vibration down to under 5 seconds. I purchased encoders and a JMI MicroMax, as well as the Celestron dual-axis drive with PEC, and this mount tracks like a demon. I can visually observe Jupiter or Saturn for a full 15 minutes without pulsing the drive. Celestron recently upgraded the bearings in the CG-5 mount and it's a big help. I like this scope. However, at about $1,500 with the drives and MicroMax there are a number of mail-order and Internet alternatives in the 8" SCT range that offer better mounts and larger computer databases. This is my "big" scope (I have a 5" Mak, a 3" Short-tube refractor, and a 4.5" Meade Newt) and I agonized over the purchase. I feel I got good value for the dollar given the excellent optics, but if I don't move up to a 12" SCT in the near future I will have to invest in a CI-700 or Vixen GP mount to be happy.


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Subject: G8N
By: Anonymous (xxx.xxx.35.82)
Date: 05/19/2006 02:07:08 am PDT
I recently bought a used G8N on astromart. This is the second G8N I've looked through and both samples have been sharp, decent scopes that give great deep sky images. Planetary images aren't as contrasty as a real newtonian due to the large central obstruction, but it is about the same as an 8" SCT. There is a mild purple fringe on Jupiter and Saturn like in an ED refractor (nowhere near as bad as a 4" achromat). I don't mind it at all since I'm used to refractor images.

Overall I'm pretty surprised at how good this scope is. Decent skies allow spiral structure to be seen through my G8n on M101 and M51 through a Speers Waler 18mm. All messier globular clusters resolve to the core in a 4.8 nagler. Saturn gives up the three rings and cassini division on a regular basis, and castor splits at 60x. The double double splits too (not quite as cleanly as in an apo refractor).

Drawbacks: the primary mirror is a spherical f/2.5 plate glass unit which is very sensitive to collimation. The supplied tripod sucks but the CG5 mount is more than enough for the 20" tube.

Summary: It's like a good 8" SCT on planets with a little purple fringe like an ED refractor. Deep sky is like a good 8" f/5 dobsonian (orion XT8 for ex.) This is a truly unique, compact and very functional newtonian reflector.


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