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Celestron 102GT revisited

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#51 epee



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Posted 04 April 2020 - 08:25 AM

My only complaint with the 102gt is the short dew shield. In an urban backyard it lets in far too much stray light.

I've come up with several temporary fixes, but I'm trying to scheme up something more permanent, deliberate-looking, and allowing a dust-proof cap.
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#52 belgrade


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Posted 04 April 2020 - 10:18 AM

I actually added rings, dovetail and a laser finder too. I wasn’t a fan of sticky, full of grease and hard to pinpoint right spot ficuser - but your mileage may vary. Overall, one of the best values ever.
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#53 FeynmanFan


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Posted 04 April 2020 - 09:30 PM

I bought 2 when OPT was closing them out.  One went straight to my granddaughters in Houston.  The other stayed with me, and was used frequently until a couple of months ago, when I gifted it to a guy who was developing a real interest in our hobby.  His wife, who was my physical therapist for a time, said he would sit in the backyard for hours enjoying the view, but with a child on the way they couldn't afford a scope.  Problem solved!  I still have my Synta 100 f/9, which will stay with me until it's pried from my cold, dead hands.

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#54 oldmanastro


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Posted 06 April 2020 - 10:52 PM

I picked up mine about three years ago locally and paid very little for it. The telescope and GOTO mount were dirty, the Nextstar untested, battery pack needed re-soldering of wires. One eyepiece was missing but the diagonal was there. Took it home, cleaned everything well including the front of the objective. The telescope ended up in pristine condition. Nexstar was working well. The greatest surprise was the good optics. Much better than I expected. Since then it has become one of my fast grab and go telescopes and it's a pleasure to use the GOTO. In fact this is my only GOTO telescope. 

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#55 flyer92



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Posted 07 April 2020 - 12:00 PM

Here's a pic of my 102GT on a (very) reinforced Twilight I mount.  Despite the weight of the mount, I still wanted a grab-and-go configuration, so I just kept everything as lightweight/simple as possible on the OTA itself.  To address the short dew shield, I just made a 12-inch extension out of 1/8" foam and velcro, and it works beautifully.

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#56 CollinofAlabama



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Posted 09 April 2020 - 03:49 PM

Interesting thread.  For the CN historians in the crowd, I started this W-A-Y back in aught 13.   I had NO intention of actually buying one (as post #299 demonstrates), but, uh, well, my resistance finally caved (post #545).  Even described how to fix the beautiful, slightly loose Explore Scientific tube ring cradle that just happened to be sold coincidentally for less than $60 at that time as well, here.  It's an absolute beauty, giving a genuine 19th Century air of real astronomy to any star party.
But when I compared the views of daylight targets to celebrate the 60th anniversary of the founding of our astronomy club back in November to my William Optics 110mm Megrez F/6 ED scope, the views didn't seem as nice, which got me to thinking about Mike's (Sarkikos') post.  Now, of course a 110mm refractor should do better than a 100mm (or 97mm, whatever) one, and that goes without saying.  Add ED class to mere crown and flint, and it's easy to see why a thousand dollar scope might outperform a sub Ben Franklin one, duh.  But according to Mike, even an 80mm ED scope can best it, so yesterday I hoped to find out more about this, side by side.

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Edited by CollinofAlabama, 09 April 2020 - 11:53 PM.

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#57 CollinofAlabama



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Posted 09 April 2020 - 04:42 PM

Although I prefer to put my C102GT on the GSO Atl-Az mount with Ho wooden legs, yesterday the AT80ED got that one, and I put the C102GT onto my CG-4 tripod-cum-AZ-4 head mount (except for the first and last shots above).  Of course, as fate would have it, clouds came in to interfere with everything, and put an end to any definitive test, but the little bit I did get in was interesting.


To try and make things even, I tried to use equivalent powers.  I figure the C102GT has a focal length of 1016mm, while the AT80ED has a focal length of 560mm, so I started out with the 9mm Nagler T6 in my C102GT and the 5mm Nagler T6 in the AT80ED (for 112.89x and 112x, respectively) on a terrestrial target, a chimney cover on a neighbor's house about 25 yards away.  Both scopes carefully displayed the metal grating (sorry, no photos), bird poop, and screws and pitch of the grating mounted on the roof quite well.  The 80ED's color did look more appealing, though a bit darker.  The chromatic aberration made the grating, well, more grating on the eyes, but the details on the dark gray chimney cover itself, bird poop, screws, etc, were equally good.    The C012GT's image also looked brighter, which might be expected with larger aperture.  Overall, though, I gave the nod to the AT80ED, darker but sharper in appearance.


Soon night fell, and I put the scopes, with the same eyepieces, onto Venus, and the beautiful, full crescent it was last night was quite nice.  Here, again, the purple chromatic aberration of the C102GT made the image more blurry.  Ironically, the AT80ED had some red fringing, but it was less obtrusive to the view.


I also tried out a different set of eyepieces, employing the AstroTech Paradigm 5mm in the C102GT, and the venerable Synta 9mm EWA 66 degree (mine was from Agena, but can be had from SVBONY, Orion, and others).  Honestly, on Venus, the main difference was the size of the field.  I didn't notice much difference in them, particularly.  The addition of an 81B filter to lessen the chromatic aberration didn't really change things here (nor the eyepieces, for that matter).  This was another win for the AT80ED.


Then, I tried out these setups on h3945, the Winter Albireo.  At first, using the higher powers, I found them about equal, I guess, but have read that this double can look better, color-wise, with lower power, so put my APM 30mm 2" eyepiece into the C102GT, and the Nagler 16mm T5 into the AT80ED for 33.87x and 35x, respectively.  Now maybe the differences in eyepieces played a difference, or maybe the aperture kicked in, or maybe the increased cloudiness gave it an advantage, but here the C102GT seemed to have a better color rendition than the AT80ED.  Both were quite good, mind you, but the C102GT just seemed nicer, overall.


I wanted to try out the 81B filter again (which I'd removed with the eyepiece change-out), but by then, the bloody clouds crowded in to obscure the night sky.  Great!  No more doubles this night.


I can say I've been very pleased in the past with the C102GT's ability to readily split Delta Cygni with the BCO 6mm eyepiece (169.33x) when the seeing's any good at all.  If I can't split Delta Cygni in the C102GT, then the seeing ain't worth diddly that evening.  I look forward to more testing of these two.  I like both scopes, but I'm not ready to accept that the C102GT's the lesser scope.  It may be, but then again ...  Been very pleased with its Delta Cygni performance, which can give my 8" dob fits on less than "stellar" nights, too.

Edited by CollinofAlabama, 09 April 2020 - 07:21 PM.

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#58 Alfonso


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Posted 10 April 2020 - 01:14 AM

I bought my C192GT back in 2013 for 58 bucks from OPT. And it is the favorite of all my telescopes. I have 7: 2 maks, 1 reflector and 4 refractors. This telescope is wonderful. The views are crisp and clear (as seeing permits) and the stars are lovely dots.


Earlier this evening shortly after sunset I took a look at Venus and she's still mysterious and beautiful. Then I visited the Trapezium in Orion and they were so nice huddled together there. Then it was on to the Moon and what a blast.

The seeing wasn't bad here in Arizona and the images were sharp and breathtaking. Using a 20mm and then 9mm and 6mm the views of these incredible craters and their miles and miles long shadows was awesome. I could spend my lifetime looking at the Moon, it's a wonderful and beautiful thing in the sky.


For 7 years now this wonderful scope has allowed me to travel through time and space, provided me with crisp and beautiful views of the cosmos and now it's my favorite, never will I part with it. 


It's mine to the day I die. 


It's the best 58 bucks I ever spent in my 60 plus years of life. I upgraded the focuser to a dual-speed crayford focuser and added a 50mm RACI and it rides on an ES twilight II mount. 


Apo schmapo, dob schmob . . . this telescope is a gem.


So there !!!


clear skies to all


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#59 precaud


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Posted 10 April 2020 - 09:25 AM

Well, I made discovery, lol


For several years I've enjoyed this scope., and never though much of it , until I had it set up side by side with a C6R refractor, switching a 35 panoptic back and forth, and just generally wondering why the stars were pin points in the C6R , and slightly bloated in the 102gt. 


Now I know its not the eye pieces fault, the 35mm Panoptic is no slouch.


So the following morning I took lens out. it has { had ) a thin plastic ring between the lenses. And it was larger than the lens, it was actually created a bump on one side. nothing huge, but enough to not allow the lens to work at is best.


So I made some very small heavy duty aluminum foil tabs for spacers. 


I stuck them in the three equal spots, and reassembled the scope. Now they stars are pin points in the Panoptic.


Interesting, Andy. I had a similar experience. I had a CR150 (the C6R's predecessor) and a C102, neither have adjustable cells. As received, the C102 had mild astig; the CR150 strong astig plus other issues.


I noticed the C102's astig changed from rotating the lens in the cell. After disassembly, I saw that the spacing ring between lenses was inconsistent around its perimeter, with bimps and flat spots. I was able to make it smooth with consistent thickness using a razor blade and fine-grit sandpaper. Astig after that was consistent. The remainder was removed by using spacers to tilt the whole lens assembly. This can be done indoors using an atificial star. After that, the scope has superb pinpoint stars, and the best DPAC test I've ever measured.


Though it took a lot more work, I was also able to remove the astig from the CR150, but was unable to get its performance to the same level as its little bro. As DPAC showed, its lens figure wasn't as good to begin with. The visual "tell" was masking down the CR150 to 4" and comparing them. The 102 handily trounced it. It's a keeper.

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#60 Jaimo!



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Posted 10 April 2020 - 10:01 AM

I wish I would have gotten a few of these $59 GT's when they were on sale, as I distribute scopes to friends and family on a routine basis. 


I feel a 4" refractor is one of the best and easiest scopes to get out a view.  Little cooling, light weight with just enough aperature...  Very little activation required to get out and view.



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#61 CollinofAlabama



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Posted 11 April 2020 - 03:52 AM

Amen, Jaimo!  My C102GT OTA weighs less than my AT80ED!  Now the AT80ED has one of THE best dual speed crayfords on it I’ve ever used, for sure, but I put an old Orion 100ED single speed (Synta) crayford on my 102GT and it’s fantastic!  It’s a lightweight crayford, which I appreciate and keeps the overall weight quite low.  Got the Baader SkyScout III red dot on the ES cradle and a RA visual finder to boot.  The thing’s light as a feather, and at F/10+, the single speed I bot used for about $55.00 in Brooklyn on vacation once years ago has done me well.  A person just doesn’t need more than that at F/10 to get extremely precise focusing.  The SystemPro TB-6 carrying case makes the whole thing a joy to move.  I really like the way this guy can split Delta Cyg.  Just no problem for a double that can give a big dob fits, if it’s not well collimated.  Maybe the AT80ED’s better — certainly has better glass — but it’s comparatively heavy, and still too long to fit on my Portamount and not be able to reach absolute zenith because it hits the base of the mount.  And the GSO Alt-Az with Ho legs rides the C102GT perfectly.  Smooth as a whistle.

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