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Equipment Discussions >> Mounts

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gdd
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: Replace GM-8 Gemini I w/ Atlas? new [Re: neilson]
      #5733028 - 03/14/13 09:27 PM

Quote:

My third worm installed in my mount got 9 arc sec total PE. I polished it and now I consistantly get 3 arc. sec. total PE.




I Neilson,

Going from 9 to 3 arcsec error by polishing the worm is quite an improvement! What did you use and how?


Thanks,
Gale


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neilson
professor emeritus


Reged: 08/22/10

Re: Replace GM-8 Gemini I w/ Atlas? new [Re: gdd]
      #5733200 - 03/14/13 10:58 PM

Hi Gale,
I was impressed by the results myself. I have a bench grinder and I install a buffing wheel made for bench grinders. I have a polishing compound thats a solid stick form. I press the compound into the wheel every so often and it grinds some off. into the cloth wheel. Then I very carefully press the worm into the wheel. I hold both ends. It will try to fly out of your hands. If it does it could damage the worm. I slowly turn the worm. It must get warm to polish the metal and smooth it. I only press it in about 15 to 25 seconds at a time. You will notice it get smooth and shiney quickly. after about 2 to 5 minutes its done. You have to clean the excess compound off the worm. You only have to polish the middle since thats the only place the gear contacts the worm. I don't advise doing this because it's easy for the wheel to pull the worm out of your hands and send it flying. You can also get seriouse injuries. I have read of others using one of those dremel tools with a little buffing wheel and polishing compound. It would be safer. I already had the bench grinder and wheel for 20 years so I dont know where to get one now.

I install my worm with very little backlash. My power supply has an amp meter and when I slew it draws about 0.40 amps, or less than half an amp. Then I slew from one end to the other in RA about a dozen times, pausing after each slew for 20 seconds although the motor stays cool. When I was done it was only drawing 0.29 amps when I slew. I feel only a tiny bit of backlash when I wiggle the counterweight bar. Almost none.

neilson


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orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Replace GM-8 Gemini I w/ Atlas? new [Re: neilson]
      #5733317 - 03/15/13 12:17 AM

If such improvements are doable with a bench grinder and polishing wheel,

1) why doesn't Losmandy do this (they could charge more for it)

2) why doesn't Ed Thomas do this?


Neilson, regarding the GoTo accuracy -

I think I am having better GoTo accuracy with my Mach1, but it's still not perfect, because with these 1-star-align mounts, GoTo accuracy is determined by both how good your polar alignment is, and how orthogonal your OTA is to the mount.

Even if your PA is perfect, if your OTA is not perfectly orthogonal, you'll still have some pointing errors. Situation is the same - need to sync on a bright star near the DSO (near meaning within about 10-20 degrees) to get a good GoTo.

In my opinion the most accurate GoTo's are had with the Celestron Nexstar.

I actually had this idea... if you could combine a Nexstar with an AP mount, you'd have a mind-boggling combination. The AP mounts all have their motors separate and external, with a normal mil-spec connector. So if somebody built a motor board that is compatible with Nexstar but set up for the gear ratios of the AP.... then this would be possible.

I know that the NexSXD folks already have done this for the Vixen mounts.


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neilson
professor emeritus


Reged: 08/22/10

Re: Replace GM-8 Gemini I w/ Atlas? new [Re: orlyandico]
      #5733445 - 03/15/13 03:08 AM

Hi'
I do a drift Polar alignment. I have checked the ota's orthoganal several times. My goto's to the alignment stars are dead center now just not to DSO's, unless they are pretty close to an alignment star. After 24 stars in a model you would think everything would be dead on. But with Gemini 2 you are expected to build additional modeling where you plan to image. The Gemini 2 alone costs $1,600. That's more than most mounts cost.

I am still having lock ups in PHd from the ethernet bugs that require me to restart my computer and handbox lockups that require me to restart my mount and start over.

After most of my DSO goto's are on the edge of my imaging chip again tonight. And two were completely off the chip. Jupiter, and a nebula were centered. The handbox locked up. PHd locked up and several times I had runaway in RA. All this tonight. And most nights.

Because of all of this I will be getting rid of my new G-11 Gemini 2 tomorrow. Mostly because of the requirement for extensive alignment modeling that give poor goto's at best. Everything should get fixed eventually with future firmware updates but they are not going to fix the poor goto problem. I will be buying another new CGEM DX with its very accurate goto's. And an aeroquest worm probably.

I don't know why polishing my worm lowered my goto's but I have tested it a dozen times over several days. Maybe I just got lucky. Polishing the worm this way is difficult and dangerouse if it gets away from you. With PEC I get 1.7 but it must be retrained each time the mount is turned on. I was really happy I got the PE so low but it makes letting the mount go very hard.

neilson


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orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Replace GM-8 Gemini I w/ Atlas? new [Re: neilson]
      #5733453 - 03/15/13 03:46 AM

PEC training disappears every time you power off?

what kind of thing is that Gemini 2?!?!?! that sort of non-permanent PEC is what you expect from early 1990s Celestrons or the CG-5. Not a pricey piece of electronics like the Gemini 2.

Having a CGEM, I would respectfully suggest that you don't downgrade to one from a G11... maybe try to get the Gemini 2 unit changed. or buy a Gemini 1.... PEC works on those things


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neilson
professor emeritus


Reged: 08/22/10

Re: Replace GM-8 Gemini I w/ Atlas? new [Re: orlyandico]
      #5733893 - 03/15/13 11:04 AM

Hi,
I already had the Gemini 2 unit changed a few weeks ago, along with the handbox. It did fix some other problems but not these. The PEC erasing is being reported by others too. You hit it on the nose, It does feel like I'm using a system from 1990. Little things like the screen where it tells you how far you are from the pole isn't in arc minutes and arc sec. its just in arc seconds. you get numbers like 14250. or 2795. If your 5 arc minutes off it reads 300 Meaning 300arc sec.

My CGEM DX was the best mount I ever owned but the electronics died a few weeks after I got it. Celestron sent me a replacement that didnt even turn on, then one with a 45 arc sec spike every second turn of the worm then one with high PE. All 3 replacement mounts were ones that had been returned defective by someone else. They were on backorder after a big sale, and instead of telling me that, they sent me used defective mounts. I can't afford an Astro Physics mount. Theres just nothing else inbetween the CGEM DX and AP mounts that I know of.

neilson


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orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Replace GM-8 Gemini I w/ Atlas? new [Re: neilson]
      #5733922 - 03/15/13 11:19 AM

Used Tak NJP? A couple sold for $4000 a few months back. You'll have to wait a while for one to pop up though. If you don't need the 70lb payload used Tak EM200's are also out there for less (but the NJP is the best "payload for the buck" used premium mount out there).

It has a primitive handset, no GoTo unless you have a PC attached, one-star align only, but the periodic error is fantastic (2.8" p-p). No PEC but with periodic error that low.. I was aiming to get one, but the CFO disallowed it on ground the NJP is a really old design - it dates to the late 1970s.

Rob Vanderbei has some really good work with the NJP
http://www.princeton.edu/~rvdb/images/NJP/ngc7635.html


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gdd
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: Replace GM-8 Gemini I w/ Atlas? new [Re: orlyandico]
      #5734006 - 03/15/13 12:12 PM

Gemini-2 permanent PEC works for some, must be a recently introduced bug. Neilson is already getting low PE, so I don't think the NJP would solve the problem, it is accurate reliable GOTO's that are wanted. I think he really needs both which is why I don't think we will be satisfied with an upgraded CGEM DX which has a high 8/3 gearbox error that defeats the PEC.

However, getting back to the original question, since the OP is happy with the GM-8, he should be happy with the GOTO performance of the G11/Gemini.

Gale

Edited by gdd (03/15/13 12:18 PM)


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neilson
professor emeritus


Reged: 08/22/10

Re: Replace GM-8 Gemini I w/ Atlas? new [Re: orlyandico]
      #5734067 - 03/15/13 12:44 PM

I really want goto's on the handset. If I was willing to use the computer I could just keep the G11 and use other software for my alignments. And I couldn't spend that much for something used. But thanks for the suggestion. I wish there was something else in the G11 price range or less.

Gale,

I'm not the only one reporting the PEC bug. I have been testing it many times each night and every time its erased.

neilson

Edited by neilson (03/15/13 12:57 PM)


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jrcrillyAdministrator
Refractor wienie no more
*****

Reged: 04/30/03

Loc: NE Ohio
Re: Replace GM-8 Gemini I w/ Atlas? new [Re: neilson]
      #5734094 - 03/15/13 12:56 PM

Quote:

I really want goto's on the handset. If I was willing to use the computer I could just keep the G11 and use other software for my alignments. And I couldn't spend that much for something used. But thanks for the suggestion. I wish there was something else in the G11 price range or less.

neilson




The new goto handbox for the Tak Temma units will be released (finally!) next montn at NEAF. A good used EM-200 plus a new handbox might run you $4000 or so; not much more than a new G-11/Gemini. A more economical alternative to a G-11 is a CGE. Those are available only used but at least that way they are less expensive. Permanent PEC (with real worm index switches), orthogonality compensation in the controller, ASPA, ...


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orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Replace GM-8 Gemini I w/ Atlas? new [Re: jrcrilly]
      #5734306 - 03/15/13 03:11 PM

... and homing switches!!!

Yes, a CGE is a good alternative at $2000. You'll even have some money left over after selling the G11 for the Bennett mod.

and if the periodic error on the CGE is not to your liking, Aeroquest sells a replacement ring gear for $250. Not sure why Jim doesn't sell a worm though..

Overall a superior choice to the CGEM DX.

John - I bet that new Tak handset will be $1000. The old one was..

So $6000-odd for an EM200, another $1000 for the handset. No thanks, I'd rather have a Mach1..!


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neilson
professor emeritus


Reged: 08/22/10

Re: Replace GM-8 Gemini I w/ Atlas? new [Re: orlyandico]
      #5734392 - 03/15/13 03:52 PM

Yes the cge would be a great choice but even at $2,000 Its hard for me to buy a used mount site unseen. My luck it would be trashed out. And I dont believe in that saying "all those scratches and dents dont effect seeing." I got cheated once already buying a used mount/scope setup on line a few years back. I will call the telescope stores, they might have a used one.
Thanks for the suggestions

neilson


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orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Replace GM-8 Gemini I w/ Atlas? new [Re: neilson]
      #5734400 - 03/15/13 03:56 PM

Odd, I've had almost zero bad experiences buying used stuff sight unseen. I've been 12 years on a-mart and longer than that on ebay...

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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Replace GM-8 Gemini I w/ Atlas? new [Re: Ranger Tim]
      #5734469 - 03/15/13 04:45 PM

You can probably get a used G11 Gemini I for about the same as you'd pay for a new Atlas. I dumped my GM-8 (non Gemini) for an Atlas and could't be happier for visual use. Flexure problems with the GM8 start with the flimsy stock tripod. You might fix that by moving the head to a G-11 tripod (lost get sold used when folks move their G-11 head to a permanent pier) or to an A-P pier.

But if you're sure that you don't like Gemini for imaging, the Atlas wouldn't be a terrible option. Avoid the Sphinx, though. On the value front it Stinx.

Have you considered a used Takahashi EM-200? You could probably get one of those used for what you'd pay for a new Sphinx plus the Nexstar mods, and it'd be a heck of a lot more precise mount. You can even drive the EM-200 with Sky Safari and a smart phone/iPad.

Regards,

Jim


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Raginar
Post Laureate
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Reged: 10/19/10

Loc: Rapid CIty, SD
Re: Replace GM-8 Gemini I w/ Atlas? new [Re: jrbarnett]
      #5736000 - 03/16/13 11:12 AM

I think none of your options will fix his complaints minus PEC (not if he buys a TAK obviously). He wants accurate GOTOs, and my Nexstar still required me to use "precise GOTO" or syncing to get the object exactly where you want it for AP. I think what he's looking for isn't really possible with any of the mounts except maybe through Starlock/Sky Prodigy. CGEs have mechanical issues and, from what I've read, the large electronics pier is frustrating. As Orly pointed out, the 1-star with a AP mount isn't great. Personal experience tells me a CGEM will not meet your expectations either without a precise GOTO (or platesolving).

Neilson, what about just running a laptop and using Astrotortilla or Pinpoint? You'll find it to be much less painful.

Chris


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gdd
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: Replace GM-8 Gemini I w/ Atlas? new [Re: Raginar]
      #5736136 - 03/16/13 12:08 PM

If you are looking for a low-tech solution, how about using your finder or guide scope to take a wide field image to detect the DSO and then get it centered and framed. The star patterns you see on the wide field image can be used to help you fine tune the framing on the narrow field shot.

Gale


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neilson
professor emeritus


Reged: 08/22/10

Re: Replace GM-8 Gemini I w/ Atlas? new [Re: gdd]
      #5736450 - 03/16/13 02:44 PM

Hi
I owned 3 CGEM DX's with their newest firmware for a few weeks each and it hit everything centered every time on all three.
I dont want to use a computer all the time. I am very careful with balance and leveling and checking and adjusting ota perpendicularity. I use an illuminated reticule and center every star perfectly.

I no longer own my G11 Gemini 2. It was very hard to let it go but after emails with Rene I felt it was best. I will have to build lots of models for the DSO's or buy Tpoint or the Sky. Or Astrotortilla.
I think that mount is best mounted permanently so you can build all the models one time and be set. Building all those models every night is time consuming and I don't know the names of the stars next to most DSO's to add to the models.

I'm thinking the DX then that new device that does the alignments automatically. I think its going to be released. They claim it will make your goto's even more accurate. The tripod is rock solid on the DX. I already have experience doing the hypertune myself. After the Hypertune I had about 7 arc sec total PE and with PEC it was under 2 arc sec. And I only had to record the PEC one time and it stayed in the mount after I turned power off. I am not in a rush so I will still consider other options. In my opinion the regular CGEM is not the same. The tripod, larger weight bar, and improved power chip makes a difference on the DX.

I think the Atlas would probably be better than the DX if it came with a heavier tripod.
Thanks for the suggestions, I'm still deciding.

neilson


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orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Replace GM-8 Gemini I w/ Atlas? [Re: neilson]
      #5736559 - 03/16/13 03:58 PM

am surprised you could get 2" after PEC... on most of these CGEMs the 8/3 ranges from 6" to 20" plus...

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Raginar
Post Laureate
*****

Reged: 10/19/10

Loc: Rapid CIty, SD
Re: Replace GM-8 Gemini I w/ Atlas? [Re: orlyandico]
      #5736622 - 03/16/13 04:42 PM

Orly, I agree. That's phenomenal with a CGEM! I'd love to see the graphs. It might convince more people to go that route if the data was available. I liked my CGEM; I just didn't like it's internals for the weights I was using it at.

Neilson, you thought about an MI250 with a Gemini2? Native PE of around 5" and it has a conservative weight limit around 90#.


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neilson
professor emeritus


Reged: 08/22/10

Re: Replace GM-8 Gemini I w/ Atlas? [Re: orlyandico]
      #5736635 - 03/16/13 04:50 PM

Hi,
I didn't have any of the 8/3 problems on my first mount. That I know of. On the second one I had a 45 arc second spike about every 2nd or 3rd turn of the worm. I'm not sure which. I thought it was the 8/3 but Celestron kept denying that it even exists. Then I showed them the statement in the Beta testers group at the time and they claimed that 8/3 was in Dec only. That mount went back that day. Do you think that was the 8/3.

I also read they were supposed to have a fix for the 8/3 around the first of the year. I wonder if the new mounts have that fix. I guess I better find out before I buy one. Or save up for the CGE Pro. I think I'm still in the Team Celestron Beta testers group, I will see if it's mentioned.

As of Feb 22 Derek started working on it again if I read correctly. In my opinion they might be planning to put the motors from the advanced VX in the CGEM and DX mounts. Saving for the CGE PRO is looking better but the sale will be over soon and it will take me longer to save enough.

neilson

Edited by neilson (03/16/13 05:27 PM)


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