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General Astronomy >> General Observing and Astronomy

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sam81
member


Reged: 07/16/11

Star selection for aligning CGEM
      #5758462 - 03/26/13 03:29 PM

For my CGEM mount, I usually pick 2 stars in the west, and 4 calibration stars in the east. I pick them randomly and I've been getting ok alignment so far.

Is there a more correct or better way of selecting stars for aligning? Would you want to pick stars closer to the horizon? Stars that are more spread out from each other?

If so, what's the theory behind picking specific stars for aligning? Just curious because sometimes I overhear people aligning with specific stars, rather than what's available.

Thanks


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GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Star selection for aligning CGEM new [Re: sam81]
      #5758543 - 03/26/13 04:06 PM

In theory, if your mount's polar axis is coincident with the celestial pole, the RA and Dec axes are mutually perpendicular, and there is no cone error (the scope's optical axis passes through the pole when swung in declination), a single star sufficices to align on the sky.

In the real world, cone error at the least conspires against a really good alignment, forcing upon us software solutions to compensate if accuracy is desired. Hence the modelling routines using multiple stars to characterize the various mechanical errors.

The more stars used the better. And the more widely distributed on the sky the better. Relatively tight clusterings of stars avail little if other parts if the sky are poorly sampled. A simple example; four stars, one in each quadrant, are better than 2 in one quad, one each in two other quads, but none in the fourth. And worse yet; 3 stars in one quad and one in another (leaving 2 quads not represented.) Note: I use 'quadrants' for ease of visualization; you need not think of the sky quite in this manner, although the concept is a useful foundation. Best to avoid stars nearer to the horizon than about 10 degrees (some would say closer to 15-20), for atmospheric refraction induces position errors of more than an arcminute.


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sam81
member


Reged: 07/16/11

Re: Star selection for aligning CGEM new [Re: GlennLeDrew]
      #5758575 - 03/26/13 04:19 PM

Glenn, thanks, that's useful.

Question about your reference to quadrants. From my understanding, the CGEM selects two initial stars in the West, then four in the East, unless I'm wrong. Wouldn't my four East stars be limited to the two quadrants that are in the East, thus 2 stars per quadrant? So for the Eastern stars, they'd have to be limited to just those two quadrants, correct? It would work for the Western stars though, as I could pick one for each quadrant.


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Madratter
Postmaster


Reged: 01/14/13

Re: Star selection for aligning CGEM new [Re: sam81]
      #5758636 - 03/26/13 04:50 PM

Are you doing this for visual goto, or are you going to use this for actual polar alignment afterward? If you are doing polar alignment I would be more careful about choosing stars, although in my experience, finishing with a drift alignment still considerably improves the alignment. If for visual goto, then these mounts are really pretty good with it selecting the stars.

By the time I get to the 3rd and 4th calibration stars on my CG-5, they are landing well inside the field of view of my 12.5mm. reticule.

Edited by Madratter (03/26/13 04:50 PM)


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Star selection for aligning CGEM new [Re: sam81]
      #5758661 - 03/26/13 05:01 PM

My comments are based more on theoretical principles, and hence are general. I don't own a GoTo system, and have only fiddled with a couple of friends'. As to the specifics of your mount, I'm sure there's a (reasonably) good reason the engineers structured the procedure as they did. Do you have the option to further refine alignment by using even more stars?

The specifics of the methodology may be dictated by what one is striving to achieve, and the firmware/software capabilities of the system. Is it an accurate polar axis alignment which is desired, or a 'compromised' axial alignment which nonetheless facilitates the best overall GoTo pointing accuracy?

I have read somewhere in these forums not long ago where a member quoted a Celestron engineer as stating that one if their GEM mounts, which used multiple stars to refine alignment, could be expected to get its RA axis no closer to the pole than 5 arcminutes. From my own experience. a polar borescope allows to achieve a 2' alignment, with rather less potential anguish.

Perhaps the borescope (if your mount has one) could be used to polar align, then a run through the computer's routine is used to obtain good pointing. If the alignment routine using stars requires to physically adjust the mount, the axial pointing will not be as good. But this will only be if potential concern when engaged in imaging, where long exposures are not actively guided. Otherwise, for visual work only the pointing accuracy matters; the axial alignment can be several *degrees* off, as long as the mount has modelled this error.


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mich_al
Carpal Tunnel
*****

Reged: 05/10/09

Loc: Rural central lower Michigan ...
Re: Star selection for aligning CGEM new [Re: GlennLeDrew]
      #5759340 - 03/26/13 10:42 PM

This may help.

http://www.nexstarsite.com/Downloads.htm

Second from the bottom of the list is a download -- Select Alignment Stars


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De Lorme
professor emeritus
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Reged: 12/30/08

Re: Star selection for aligning CGEM new [Re: mich_al]
      #5761607 - 03/28/13 12:40 AM

Have you tried using the sync option yet? After going to say M42, press enter until your back where you can pick
what data base {NGC,M}you want. Then just press align,use the arrow keys until you get to sync. Press sync, then it will tell you put object in the finder scope ,press enter.
Then it will tell you to center object in the eyepiece
and press align. This improve centering objects in the
eyepiece. Your 4 star alignment will be accurate for a couple of weeks so when it ask you which alignment you want
to use just select previous alignment. Really convienant,saves so much time. Your going to like your Cgem.
Clear Skies! De Lorme


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Maverick199
Postmaster
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Reged: 02/27/11

Loc: India
Re: Star selection for aligning CGEM new [Re: sam81]
      #5761712 - 03/28/13 03:08 AM

Quote:

For my CGEM mount, I usually pick 2 stars in the west, and 4 calibration stars in the east. I pick them randomly and I've been getting ok alignment so far.

Is there a more correct or better way of selecting stars for aligning? Would you want to pick stars closer to the horizon? Stars that are more spread out from each other?

If so, what's the theory behind picking specific stars for aligning? Just curious because sometimes I overhear people aligning with specific stars, rather than what's available.

Thanks




Allow the H/C to select the initial two align stars and then do a four star calibration, again selecting stars as shown on your H/C. This should be enough for very good alignment for visual observing. If you want to refine the process even more, do the Polar alignment routine where after aligning with a star, you adjust the star with your Alt/Az knobs. You should now have enough to even do some imaging without autoguiding for up to a minute.


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Ron (Lubbock)
sage


Reged: 08/17/12

Loc: West TX
Re: Star selection for aligning CGEM new [Re: Maverick199]
      #5767715 - 03/30/13 10:03 PM

There a nice way of doing iterative alignment with the CGEM that is particularly useful for photography. If done properly, the results are impressive. It takes me about 45 minutes to complete if I hustle and don't mess anything up.

First, the normal procedure with two alignment stars and four calibration stars is followed. For the calibration stars, I try to spread them out in terms of declination, and also in terms of altitude. (I believe some of the alignment calculations take into account flexure of the scope and mount as altitude is changed.) If aligning for photography, the stars should be located in the main imaging scope, not in a guide scope due to differential flexure.

Next, the polar alignment procedure is followed using a star in the south that is near the meridian.

Using the controller buttons, the scope is then returned to the index positions on the mount (so the scope is pointing at the north pole). The power is turned off and then back on again after a few seconds.

The whole 2+4 calibration process and polar alignment are now repeated, but this time around, I like to use six different stars, preferably on the other side of the meridian. This suggestion is unconventional, but I get great results with it, so take it FWIW.

I have been doing 15 min. guided subs with my CGEM at 1280 mm focal length using this protocol, and I have absolutely nailed the polar alignment every time I did it. It is a very significant improvement over just doing the 2+4 alignment once.


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