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Equipment Discussions >> Mounts

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tomo
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Reged: 06/14/10

Loc: Shelby Twp, Mi
CG5 Polar Alignment
      #3864563 - 06/14/10 03:47 PM

What would be considered a good polar alignment (point of diminishing returns) for a CG-5? I've gotten AZ -00:43:55, AL -00:13:35 and I think i'm pretty happy.

By the way I'm new to CN so hello to everyone and I'm sorry if I've posted this wrong.


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Fred1
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Reged: 09/19/07

Loc: Somewhere in the Orion Spur
Re: CG5 Polar Alignment new [Re: tomo]
      #3864596 - 06/14/10 03:58 PM

Welcome to Cloudy Nights.
Ya know? I rarely check the offset in my CG5's. But I consider that I have a good Polar Alignment when my tracking keeps my objects centered for 10-15 minutes before I notice drift. I'm visual only, so the APer's will likely chime in with a more technical reply.


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tomo
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Reged: 06/14/10

Loc: Shelby Twp, Mi
Re: CG5 Polar Alignment new [Re: Fred1]
      #3864614 - 06/14/10 04:07 PM

Cheers Fred. Perhaps you wouldn't mind taking a second during your next session, looking them up and posting back. My sense if these are good and I don't want to redo and make it worse (if that is possible).

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Fred1
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Re: CG5 Polar Alignment new [Re: tomo]
      #3864805 - 06/14/10 05:33 PM

I'll try to remember to do that, tomo, but please don't scold me if I forgot to post back. You can always PM me if you like.

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tomo
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Reged: 06/14/10

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Re: CG5 Polar Alignment new [Re: Fred1]
      #3879829 - 06/22/10 02:18 PM

After having to redo my alignment after a "wild slews" issue, I now have PA accuracy of just less than 15 arc-min on both AZ & AL. Don't know if I should try and do better next time or I run the risk of making it worse. Can anybody help?

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Chris Rowland
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Reged: 02/28/05

Re: CG5 Polar Alignment new [Re: tomo]
      #3880178 - 06/22/10 05:08 PM

I think that's pretty good but if you have a permanent setup I'd try to improve it.

First, do a really good 2+3 or 4 * alignment using a reticle EP. If you haven't got a reticle EP forget it.

Then do a polar alignment but pay attention to the amount of change the scope expects, if it's more than 20 to 25 arc minutes don't fix it.

Hope that helps,

Chris


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vlad10
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Reged: 04/20/06

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Re: CG5 Polar Alignment new [Re: Chris Rowland]
      #3881377 - 06/23/10 10:20 AM

Use a polar alignment program and a ccd. I use WCS to measure drift with a camera, in less than an hour I get within 3-4 arc minutes in either axis. The HC values are not always correct, you have to actually measure it yourself to realize how close/far you are

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tomo
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Reged: 06/14/10

Loc: Shelby Twp, Mi
Re: CG5 Polar Alignment new [Re: vlad10]
      #3881827 - 06/23/10 02:03 PM

thanks vlad10, that sounds like a plan.

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tomo
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Reged: 06/14/10

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Re: CG5 Polar Alignment new [Re: Chris Rowland]
      #3881828 - 06/23/10 02:05 PM

Thanks Chris. What is the ploar alignment accuracy you have achieved?

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Chris Rowland
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Reged: 02/28/05

Re: CG5 Polar Alignment new [Re: tomo]
      #3882381 - 06/23/10 07:03 PM

5 to 10 arc minutes

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tomo
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Reged: 06/14/10

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Re: CG5 Polar Alignment new [Re: Chris Rowland]
      #3918717 - 07/13/10 10:31 AM Attachment (48 downloads)

So last light my perseverance paid off. Now I have a PA of 2 arc mins in both axes. I simply did 2+4 align then ASPA and repeated. The only difference was that I made the mount level according to the bubble level (which I previously ignored because I can eyeball that it isn't level and an electronic level confirms). To confirm I did a quick DSLR drift 2m exposure at +1 movement and only a few pixels out. I'm sure be tempted to tweek but I'll try not to. Now what PEC or Backlash?

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dwitek
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Reged: 07/03/08

Loc: White Lake, Michigan
Re: CG5 Polar Alignment new [Re: tomo]
      #3918890 - 07/13/10 11:57 AM

The CG5 doesn't have PEC and backlash is a matter of your individual mount. A lot has to do with the adjustment of the mesh of the worm gear. I recently went through a supercharging exercize with mine and now have the backlash settings around 5, others may need upwards of 50. Depends on how much play there is.

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rmollise
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Reged: 07/06/07

Re: CG5 Polar Alignment new [Re: tomo]
      #3919060 - 07/13/10 01:31 PM

Quote:

So last light my perseverance paid off. Now I have a PA of 2 arc mins in both axes. I simply did 2+4 align then ASPA and repeated. The only difference was that I made the mount level according to the bubble level (which I previously ignored because I can eyeball that it isn't level and an electronic level confirms). To confirm I did a quick DSLR drift 2m exposure at +1 movement and only a few pixels out. I'm sure be tempted to tweek but I'll try not to. Now what PEC or Backlash?




Levelling has no effect on polar alignment, though it may make it easier for some folks to do a drift alignment.

The CG5 does not have PEC.


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tomo
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Loc: Shelby Twp, Mi
Re: CG5 Polar Alignment new [Re: rmollise]
      #3919296 - 07/13/10 03:29 PM

Thanks Rod. I knew leveling has no effect but it just "seemed" to work for me. Know I'm on the button I'll try not to mess it up again. I'm thinking that PA alignment, with all the factors taken in, has variability that means you will never know how right it is. (I'd love to know what this +/- margin of error is). So I chose to do the DSLR method to confirm in a non-computational way.

Is their any software out there that does PEC correction on top of a CG-5 even though there is no internal routine or should I just concentrate on guiding?


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mikewayne3
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Re: CG5 Polar Alignment new [Re: tomo]
      #3919401 - 07/13/10 04:34 PM

howdy Tomo I set my backlash in the hand control to 01 pos and neg RA as well as Dec other wise it seems to jump when you move it 1 way or the other and I've seen it happen wile I'm guiding so I just make sure of the setting and I'm off and imaging
Mike


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rmollise
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Reged: 07/06/07

Re: CG5 Polar Alignment new [Re: tomo]
      #3919588 - 07/13/10 06:22 PM

Quote:

Thanks Rod. I knew leveling has no effect but it just "seemed" to work for me. Know I'm on the button I'll try not to mess it up again. I'm thinking that PA alignment, with all the factors taken in, has variability that means you will never know how right it is. (I'd love to know what this +/- margin of error is). So I chose to do the DSLR method to confirm in a non-computational way.

Is their any software out there that does PEC correction on top of a CG-5 even though there is no internal routine or should I just concentrate on guiding?




Not that I know of. With a good polar alignment, autoguiding should be enough.


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Foehammer
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Reged: 06/29/06

Loc: Cyprus
Re: CG5 Polar Alignment new [Re: rmollise]
      #3920460 - 07/14/10 03:29 AM

Best numbers I've seen on mine with 2+3 and ASPA (using illuminated reticle ep) were 02" in azm and 01" in alt. this morning...so I guess its good enough?

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rmollise
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Reged: 07/06/07

Re: CG5 Polar Alignment new [Re: Foehammer]
      #3920551 - 07/14/10 07:40 AM

Quote:

Best numbers I've seen on mine with 2+3 and ASPA (using illuminated reticle ep) were 02" in azm and 01" in alt. this morning...so I guess its good enough?




Not sure what you're referring to with 02" and 01"...the polar alignment error the HC reports? It may be good enough...but I wouldn't take what it says too seriously.


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tomo
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Reged: 06/14/10

Loc: Shelby Twp, Mi
Re: CG5 Polar Alignment new [Re: rmollise]
      #3923278 - 07/15/10 01:38 PM

Agapios,
You sure have a fine tuned mount if you have PA correct to 2 arc seconds. Next chance I get I will measure the true error with a WebCam and some software.
Thanks,
Tommy


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Foehammer
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Reged: 06/29/06

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Re: CG5 Polar Alignment new [Re: tomo]
      #3924612 - 07/16/10 03:08 AM Attachment (61 downloads)

Here's what the HC reported that night. Its a cell phone pic hence the bad quality. Too bad I dont keep the mount permanently set up!

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Chris Rowland
professor emeritus


Reged: 02/28/05

Re: CG5 Polar Alignment new [Re: Foehammer]
      #3924616 - 07/16/10 03:15 AM

That's the residual error of a multi star alignment. You need to do a fresh alignment to get the an indication of the true polar align error.

Chris


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tomo
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Reged: 06/14/10

Loc: Shelby Twp, Mi
Re: CG5 Polar Alignment new [Re: Foehammer]
      #3924873 - 07/16/10 09:39 AM

I found this very informative and authorative website on the Polar Alignment topic.
http://celestialwonders.com/articles/polaralignment/
Great stuff, although a little high brow for me.

From what I can determine, once you get PA down to +/- 2 arc-minutes, the PEC kicks in as a bigger factor. So there will be little point doing further tuning.

Clear Skies, Tommy


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nemo129
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Reged: 01/03/10

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Re: CG5 Polar Alignment new [Re: tomo]
      #3924942 - 07/16/10 10:22 AM

Tommy,
Thanks for the link. It will always make good reading!!


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dickbill
scholastic sledgehammer


Reged: 09/30/08

Re: CG5 Polar Alignment new [Re: Chris Rowland]
      #3924962 - 07/16/10 10:35 AM

If you are thinking about astro photo, the CG5 is well enough polar aligned when you get 45 seconds unguided at ~1100 mm focal lenth (a C9.25 f/5) with a DSI or with a camera with equivalent field. Of course you can stack many of these 45 seconds exposures. The polar alignment required for this level is easily done with the CG5 PA routines, no need to further improve (drift align) because at this focal lenght, the Periodic Error is the limiting factor anyway. For example I bought the WSC polar align software (it's based on drift) to get a better PA, but that still doesn't improve much the exposure time: 45 seconds regularly or 60 seconds but with many bad exposures.

At f/10, the combo C9.25 (2300 mm)/CG5 well aligned/meade or orion DSI should give 20-30 seconds unguided with no wind. That's still good for many small and bright objects. I've never been able to guide a C9.25 at f/10 on a CG5. At f/5, the inertia of OTA is still too much, in my opinion, to be efficiently guided by whatever program, sometimes the software can correct, sometimes it can't (I have PhD). At the same focal lenght, a lighter set up like a C8 or below is probably better. Since guiding = quickly moving the OTA after the mistake is made, the heavier the OTA, the higher the inertia, the longer it takes to move it and the less accurate.
Personnaly, I think that the performance of a mount is better defined by the limits of its capabilities, that is, the best it can do in the maximum set up sold by the vendor, rather than in a minimal set up. Knowing that a mount can track a single 15 minute exposure unguided at 600 mm focal lenght doesn't tell you which mount it is, could be a cg5, could be an AstroPhysic1200, but if you tell me a C9.25 gets you 1 minute unguided at 2300 mm, I'll bet this C9.25 is not on a typical CG5.


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tomo
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Reged: 06/14/10

Loc: Shelby Twp, Mi
Re: CG5 Polar Alignment new [Re: dickbill]
      #3939414 - 07/23/10 12:32 PM

Found this: http://leq.one-arcsec.org/e/Gallery/framGallery_e.html

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