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CGEM tracking issues

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#1 Alex Post

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Posted 23 September 2012 - 09:28 AM

I have had my CGEM for almost a year. It is pier mounted. I have attempted to polar align, align it very carefully numerous times. The only way I can keep it tracking ANYWHERE close is with Autoguiding and PhD. The problem got worse recently:

My GOTOs got worse and worse. I decided to re-polar align last night. I did a rough alignment, punched in Polar align and did best I could. So far so good, GOTOs seemed to be better (not perfect). Then I decided to realign, turned power off, back on and started normal alignment procedure. I chose Two-Star align with Algol as first star. Wheeeeee , the mount went upwards Cassiopea.

I tried a few other stars - all were going WAY off. I checked, tripple checked my time, location, everything looked fine. I wanted to re-polar align, but it required mount to be aligned first somewhat close first. After many attempts, I gave up.

I reset to Factory Settings. Punched in time, location - phewww the scope went roughly towards Algol this time, got in 25mm EP FOV. Alignment procedure went normally from that point. Since I was taking it now very personally and the time was approaching 3AM I decided to do a super fine Polar align.

I was working alignment knobs very very carefully, secured final alignment with a solid twist with Algol (that was my polar align star as well) still in dead center. The pier mount is extremely solid, I tried shaking the whole thing with my weight - alignment star barely shakes in center.

I did another power cycle and realignment. Things looked not bad, but no improvement to where I started. Here is a 60 second unguided frame in the vicinity of Bubble Nebula. This is 1/4 frame crop from image center, through C8 scope with about 0.5 degree field of view.

So the question is - is this expected of the CGEM? Is one minute too long for unguided? Thank you in advance for help.

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#2 dickbill


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Posted 23 September 2012 - 10:37 AM

I am a bit new to the cgem to make strong statements here. However, last night I tried to image at fl 4.7 meters for 1 minute and yes, 1 minute is too long.
I was not in optimal conditions though, my polar alignement was not perfect and i din't train the mount for PEC the best i should.
I am sure my station can vary quite a bit too(tripod on grass), but i still used a 2 days old night's station...
So, i am going to say that polar alignement using the routine is good, but 1) can vary over thime and maybe this has something to do with the azimuth screw 'knobs pushers' that can accumulate lots of tension.
About that, when I finish aligning the mount with the routine, i always unscrew a bit those knobs to release the tension, which doesn't affect the alignment, or so it seems... I have to check on that. Don't forget that with the mount on a standard tripod, those pushers demand to slightly untight the central knob under the mount which itself can affect the alignment when you tight it again.
But none of this seems to apply to you, neither the grassy soft ground or the tripod.

2)like for the cg5, we should assume the polar align routine is only as good as the sky model permits. If an error reoccurs, the misalignement will reiterate too.
That means we can only verify the alignement with the only independant method we know for sure: the good old drift align. Painful, but necessary sometimes.

#3 WarmWeatherGuy


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Posted 23 September 2012 - 12:09 PM

Did you use 6 alignment stars (2 + 4 calibration stars)? I always do 6 stars and then do the All Star Polar Align (ASPA) on a star in the south. Algol might be too high. I'm not sure about Iowa.

I use an eyepiece that has cross hairs. I can't imagine getting any kind of accuracy without them.


http://www.telescope...inated eyepiece

I do 6 stars and then check my polar alignment accuracy. Then I do the ASPA and then power down. Then I do 6 stars again and check my polar alignment accuracy. Repeat as needed.

The reason for the power down is that after doing the ASPA the hand controller will report zero error. It can only assume that you did everything perfectly, but you never do. The only way to find out the error is to start over.

If you've ever done the PEC you will notice that it doesn't just go back and forth sinusoidally over the 8 minute period. It may stay quite still for several minutes and then move abruptly. A one minute exposure might get lucky and be in that part of the cycle when it is still, or it might get unlucky and have that abrupt move happen during the exposure.

If you are removing your mount from the pier each night then the biggest error would be in azimuth. It would be better (for azimuth) to use a star near the horizon in the east or west rather than nearly overhead. The overhead star is good for elevation errors. A star low in the south is good for both errors.

#4 orlyandico


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Posted 23 September 2012 - 12:25 PM

Alex, your initial issues were due to runaway RA slew. Those got fixed by factory reset.

Now your second issue - star trailing even when perfectly polar-aligned - I'm afraid that's due to the CGEM's periodic error.

My CGEM as an example has 40" p-p of periodic error over 8 minutes. So the maximum error over a 1-minute period would be 5" - which would show up as trailed stars even at 700mm focal length.

Now even if your C8 was reduced to 1280mm you would see trails. Much more if un-reduced.

To give you an idea, my AP600 has 10" of uncorrected periodic error. With a reduced C9.25 (1410mm focal length) I am getting trailed stars at 2 minutes. The AP's periodic error is 1/4th of my CGEM's.

And periodic error correction won't help.. the CGEM has a non-integral periodic error (the famous 8/3) which in my CGEM is 22" in magnitude - it's actually larger than the fundamental. Only the fundamental is correctable with PEC.

So.... yeah you need to guide.

#5 Alex Post

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Posted 23 September 2012 - 12:45 PM

Thank you all for quick and useful responses!

#6 YetAnotherHobby


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Posted 25 September 2012 - 10:23 PM

As far as the need to reset to factory defaults - I was experiencing similar issues where the initial gotos for alignment were tens of degrees off, requiring a factory reset. I was in the habit of just doing that prior to any alignment, as it forced me to re-enter all of the position data. It seemed that even if the location data appeared to be correct, the initial slew was improved if I typed it in again, or if I changed it to something incorrect and then corrected it. I have since updated the firmware on the mount to the latest available, and this issue has completely disappeared. Initial gotos are consistently good. The motor control update appears to have smoothed out operation of the motors as well, as they sound less strained during operation. I am enjoying the mount much more now that these annoying issues appear to have been resolved.


#7 Patrick


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Posted 26 September 2012 - 12:10 AM

Here is a 60 second unguided frame in the vicinity of Bubble Nebula. This is 1/4 frame crop from image center, through C8 scope with about 0.5 degree field of view.

So the question is - is this expected of the CGEM? Is one minute too long for unguided? Thank you in advance for help.

Yes, you're expecting too much to think you can get one minute subs unguided without star trails using a 2000mm C8. At that focal length, (and even with a focal reducer) you will need to guide. It's not so much the mount as it is the focal length. If you put a 500mm or less focal length scope on the mount you could probably get away with 1 minute unguided exposures.

One other thing, as mentioned above, a good polar alignment will not fix periodic errors in the mounts drive gears. PE has to be compensated for with guiding or PEC (periodic error correction).


#8 Alex Post

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Posted 29 September 2012 - 03:07 PM

Thanks again, I now understand limitations of this mount. My previous mount was way cruder (NexStar), so no complaining here. One clue that made me post in this thread was the trails were not always in same direction and they were all different amount. So, as you guys explained it's PE at 2000+ mm. Cheers.

#9 Raginar


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Posted 29 September 2012 - 07:05 PM

Just get a guider setup and you'll be OK :).

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