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CG5 seems to drift, not steady tracking at all

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#1 geminijk

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Posted 27 March 2013 - 05:13 PM

I have an older CG5 mount head that seems to be having tracking issues. I have noticed that when video streaming, I get some drift in the views, even on a lowly setting of 8 secs with my SCB2k. That should not be my polar alignment, as I understand it, and hope someone can correct that idea if untrue.

Now I also see this drift in my SkySafari app. I'll align to say the Sun, align the app it, and over time, SkySafari will show the scope point well away from the Sun, even though it is still in my camera view?

I would really like to get another CG5 that are on sale right now, because I can use the tripod on this older mount head, and put the new mount head on my pier. I want to do limited AP, mostly video. My expectations for this mount were up to 2 min exposures, and hopefully w/o guiding. This issue however with this CG5, and not able to even to a few seconds without seeing drift, is really pushing me away.

Any ideas on what the issue is??

John

#2 mclewis1

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Posted 27 March 2013 - 05:29 PM

John,

95% of the time it will be your polar alignment. Video cameras with their smaller fov and corresponding "high power" do make tracking inaccuracies quite visible.

If you can verify that your polar alignment isn't causing the problem then look at things like tight clutches. Changing the balance of the scope can sometimes highlight a clutch issue.

Any idea which direction the drift is in? This will tell you what specific areas of the mount to start looking at.

#3 geminijk

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Posted 27 March 2013 - 05:53 PM

I'll try to determine the drift direction tonight Mark, its actually clear, and with a full moon, a good time to mess with this stuff.

thanks for the suggestions.

John

#4 rmollise

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Posted 28 March 2013 - 07:54 AM

Why do you think it's not polar alignment? How did you polar align?

#5 hottr6

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Posted 28 March 2013 - 10:11 AM

RU using fresh batteries?

My CG5-clone tracking "slows down" when the batteries get weak. The mount will track in RA for a few more hours then quit all together, and also will not slew. However, the DEC axis will still slew!

#6 geminijk

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Posted 28 March 2013 - 02:33 PM

Clouds rolled in last night, so I couldn't do much to determine direction of drift.

Unc...I used the ASPA. It selected Jupiter, then I adjusted the mount physically.

Hottr6...I have it on a pier and with AC power all the time.

I'll have to wait for my next round of clear skies to determine the direction of drift. From what I recall, it seemed to be in a single direct.

John

#7 mclewis1

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Posted 28 March 2013 - 05:18 PM

John,

ASPA selected Jupiter ... really?

I would use a suitable named star ... suitable would be close to celestial equator and high in the sky but not right on the meridian.

#8 RTLR 12

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Posted 28 March 2013 - 05:32 PM

Jupiter was selected because that was the last goto that was selected in the HC. Best to use a star close to the meridian and the celestial horizon.

Stan

#9 rmollise

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Posted 28 March 2013 - 05:33 PM

Clouds rolled in last night, so I couldn't do much to determine direction of drift.

Unc...I used the ASPA. It selected Jupiter, then I adjusted the mount physically.

Hottr6...I have it on a pier and with AC power all the time.

I'll have to wait for my next round of clear skies to determine the direction of drift. From what I recall, it seemed to be in a single direct.

John


I would not advise selecting Jupiter for a polar alignment...

#10 geminijk

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Posted 29 March 2013 - 07:11 PM

I'll try the actual stars than next time its clear Unc.

John

#11 cn register 5

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Posted 30 March 2013 - 04:18 AM

Whilst we use drift in dec to set the polar alignment it doesn't follow that the drift is only in dec. It can be in Ra as well. It depends on the direction the scope is pointing relative to the polar align error.

Chris

#12 Patrick

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Posted 30 March 2013 - 07:53 AM

My expectations for this mount were up to 2 min exposures, and hopefully w/o guiding.



Hi John,

Are you planning on using your 8" SCT as the imaging scope? If so, I think your expectations may be a little too high. I've had several (3) CG-5's and found through experimentation that the best I could do consistently unguided were 1 minute exposures at a focal length around 300mm. Since you're operating at 2000mm (or ~1260 with an f/6.3 focal reducer), you're going to start to see periodic error problems with even short exposures.

With guiding, the numbers can be much better, although I found that even with a C6 operating at 960mm f/l I had to throw out quite a few subs (approx 30%). The problem you face with guiding with an 8" SCT is that you'll need a second scope which adds to the weight placed on the CG-5. If you operate with a focal reducer (highly recommended) you can probably get away with a 9x50 finderscope guide scope setup. Otherwise, at 2000mm f/l you will want a guide scope with a little longer focal length.

I agree with the others that you need to make sure you have a good polar alignment, but I don't think that's your main problem...yet. I say 'yet', because once you get your periodic error sorted out, that will be the next issue to tackle.

Patrick

#13 dickbill

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Posted 30 March 2013 - 11:20 AM

yes, 2 min is too much to ask, expect more like 1 min unguided at around 1000 mm focal lenght, IF and only IF you are well polar aligned, your mount is well balanced AND you have a good power supply.

Even with that, GOTO mounts can be misleading somehow: a cg5 that has been correctly put in station, but barely polar aligned, will have good, sometimes even very good GOTOs, which might let you believe that your polar alignement is good too. That is why people in this forum endelessly repeat that polar alignement and GOTOs accuracy are separate things.

Nobody has ever quantified the thing, that is, how far off the north polar pole can you be and still get good GOTOs?
My guess is you can probably be 1 degre off the north celestial pole and still have good GOTOs, but your tracking, while OK visually, will be horrible on a ccd.

The sure thing is that without good GOTOs, you can't expect to have an accurate POLAR ALIGN routine.
So i will say like others: check your battery, balance well in RA and Dec, then do a 2 or 3 STAR ALIGN, stay away from the Meridian and then test you goto accuracy with stars or objects away from the meridian. Only then attempt a POLAR ALIGN routine.
NOTE here: the polar align routine of the cg5 originally used Polaris and was different of the cgem-related ASPA routine. But now you can have both for the cg5.
Which one is best, or lets say 'safer' (as opposed to 'easyer') for a cg5? I don't know.

I would say anyway that if your polar alignment routine shows a big error in alignment, you are probably safer to iterate the entire process once more.

#14 rmollise

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Posted 30 March 2013 - 12:13 PM

Quantify? OK. One night at the club dark site one of our members, Joe Novice we'll call him here, came over to ask for Unk's help. He'd got his new C8/CG5 setup OK, but he was having a problem: "Unk, the go-tos are great, but everything drifts out of my eyepiece." Turned out he'd "polar aligned" on Kochab... ;)

#15 mclewis1

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Posted 30 March 2013 - 03:52 PM

NOTE here: the polar align routine of the cg5 originally used Polaris and was different of the cgem-related ASPA routine. But now you can have both for the cg5.
Which one is best, or lets say 'safer' (as opposed to 'easyer') for a cg5? I don't know.

A minor point that is really semantics ... ASPA isn't related, oriented or in anyway specific to the CGEM. It was released in firmware gem4.15 which was common across the CG-5, CGE, and the then new CGEM (the CGEPro hadn't been released at that time). There's nothing about ASPA on the CGEM that's any better or worse than on the CG-5 (other than the mechanicals of the mounts being less or more accurate).

#16 cn register 5

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Posted 30 March 2013 - 04:05 PM

An alt az mounted scope can be up to 90 degrees off being polar aligned and still do gotos accurately. The conversion, after a two star alignment, is the same. The difference is that the AltAz mount tracks in both axes while the polar aligned scope tracks in hour angle only.

Chris

#17 rmollise

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Posted 30 March 2013 - 04:32 PM

That's true, but _it depends on the software_ quite a few GEMs' go-to accuracy ain't worth pea-turkey without a good polar alignment... ;)

#18 SkipW

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Posted 30 March 2013 - 10:12 PM

Whilst we use drift in dec to set the polar alignment it doesn't follow that the drift is only in dec. It can be in Ra as well. It depends on the direction the scope is pointing relative to the polar align error.

This is true, but with small alignment errors (a degree or two) the drift in RA is so small it's typically swamped by other errors, so we don't notice it.

Drift in Dec is order sin(alignment error).
Drift in RA is order 1-cos(alignment error)

sin(1 degree) is about 0.01745 (1.7%). cos(1 degree) is about 0.9998, so 1-cos(1 degree) is about 0.0002 (0.02%), about 100 times less.

If the alignment error is 90 degrees, the drift in RA (sin(90) = 1) could be as large as the drift in dec (cos(90) = 0, so 1-cos(90) = 1)!

If we're pointing to the wrong pole (error is 180 degrees), all the drift is in RA.

#19 geminijk

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Posted 30 March 2013 - 11:43 PM

Still clouds, so further clarification to which direction is taking longer. But I'll keep you all in the loop.

John

#20 geminijk

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Posted 01 April 2013 - 04:52 PM

Ok, from what SkySafari shows today when broadcasting on NSN, the drift is in BOTH RA and Dec. That REALLY surprises me. When a GEM, the only motor used during normal tracking is RA isn't it? now if SkySafari shows drift in both, that may mean my DEC is off as far as polar goes perhaps?

I'm thus open to suggestions? Oh, and tonight I plan on doing an ASPA on something OTHER then Jupiter, so I'll test again tonight and post up the results.

John

PS. I pulled the trigger on another CG5 since they are on sale. I needed a mount for outreach that I can setup 2 scopes on, so I'll use the new tripod on this older mount head I have, and put the new head on my pier for my AP and broadcasting.

#21 rmollise

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Posted 01 April 2013 - 04:57 PM

What do you mean by SkySafari showing that it drifts? What happens with an object in the eyepiece at high power?

#22 WadeH237

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Posted 01 April 2013 - 06:09 PM

Nobody has ever quantified the thing, that is, how far off the north polar pole can you be and still get good GOTOs?
My guess is you can probably be 1 degre off the north celestial pole and still have good GOTOs, but your tracking, while OK visually, will be horrible on a ccd.


Speaking for the CG5 (and other NexStar mounts), I believe that goto is independent of polar alignment. For quick and dirty visual observing, I frequently just plop the mount down aligned somewhat north and eyeballed level (I do have my latitude dialed in on the mount). Certainly, I am way outside of your arbitrary 1 degree guess. After doing this, gotos work fine and the tracking is "good enough" for casual visual observing.

As Uncle Rod mentioned in another post on this thread, results could vary with another mount. My Astro-Physics mount, for example, syncs to just one star and assumes that the mount is well aligned to the pole. In that case, gotos are quite dependent on polar alignment.

-Wade

#23 dickbill

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Posted 01 April 2013 - 08:05 PM

Align on Kochab, almost 26 degres away from Polaris! and still get good GOTOs....wow.
I'd say to John, if you can't see Polaris, or be sure it is Polaris, use your elevation/latitude and a compass and be done with it.
If everything fails (broken gears etc), next step will be Ed Thomas or a new mount i guess. I'd take this as an excuse, like...'Oh, my cg5 is broken, I ABSOLUTELY need a VX...'
Quick! before the money got wasted in who knows what useless item.

#24 DaveJ

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Posted 01 April 2013 - 08:15 PM

Align on Kochab, almost 26 degres away from Polaris! and still get good GOTOs....wow.



Well, actually Kochab is 16.6° away from Polaris and 15.8° away from the NCP.

#25 jrcrilly

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Posted 01 April 2013 - 08:28 PM

Align on Kochab, almost 26 degres away from Polaris! and still get good GOTOs....wow.



Well, actually Kochab is 16.6° away from Polaris and 15.8° away from the NCP.


While I do approve of the good goto accuracy Celestron's combination of alignment and calibration stars can achieve even with a really bad polar alignment, I feel that I must comment that they are just catching up with what the Vixen Sky Sensor 2000 did very well twenty years ago. The SS2K could also correct the declination drift caused by poor polar alignment so the issue discussed in this thread wouldn't be there. ASPA works just like the SS2K polar alignment routine did, also.






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