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One more CGEM DX question

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#1 Harry.C


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Posted 26 November 2012 - 10:51 AM

Apologies in advance for yet another newbie question about this mount, and thanks to you all for the past help.

I went out recently with this mount and successfully (I think) aligned using 2-star + 3 calibration stars. After alignment the hand control said "CGEM Ready", so I chose M31 as my first test and the mount did successfully slew to it. I assumed at this point the mount was tracking the object (note that I did not have an auto-guider setup yet).

I took a series of 10 30-second widefield pictures of M31, and in each I could see star trails and the position of M31 was different in each frame. After the 10 pictures I again chose M31 using the control and the mount kicked in and moved once again to centre M31 in the frame. Given that it was already supposed to be tracking M31 at this point what could I have done wrong?

After choosing M45 as a test I again picked M31 and took another 30 second exposure:
Posted Image
No star trails in this picture which is good, but I have no idea what I did differently between my first attempt and second.

Once alignment is complete I should be able to choose any object and have the mount slew there and track, correct?

Any advice would be most appreciated.

Harry C

#2 mich_al



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Posted 26 November 2012 - 11:35 AM

I'm no expert in the area but for accurate tracking over time you'll need to include another alignment in your setup. Research 'all star polar alignment'
(ASPA) and 'drift alignment'. I prefer the later but others claim good results with the former on a CGEM.


#3 orlyandico


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Posted 26 November 2012 - 12:18 PM

looks like your polar alignment is off.

the nexstar software can compensate for polar mis-alignment when pointing (doing GoTo slews) but when tracking, only the RA motor is turning so if you have polar misalignment you would get trails.

and for the drift to be visible in wide-field images (that looks like a 50mm lens or thereabouts) and 30-second images means you must be grossly polar misaligned.

#4 Raginar



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Posted 26 November 2012 - 04:11 PM

Your polar alignment is off. Recommend you practice drift aligning; there are tons of tutorials if you google it. Also, if you haven't already, do a Polar Alignment using the HC. While the reticle is "OK", it's perfect and the ASPA can do a slightly better job. Run through your normal alignment (2+4), center everything via 'up and right', and finally slew to a star in the S that is around the meridian. Sync to that star and hit the alignment button till you get down to the All Star Polar Align Funciton.. it'll do the rest. Follow the prompts! :)

It's still not good enough for unguided... 30 seconds is about as good as you can expect depending on your focal length, camera pixel size, etc... The CGEM just isn't a beast in that regards. If you're interested in AP, I'd recommend getting a guider package of some type. They're cheap and will deal with any small mis-alignments you have.

Good luck!

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