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CG5: DEC backlash issues (after hypertune!)

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#1 gezak22

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Posted 13 February 2011 - 02:09 PM

Hi guys,

I used to have all sorts of DEC guiding issues with my CG5. Eventually I got fed up with it, so I had it hypertuned and I switched to a polar alignment that uses PHD guiding. My guide graphs improved significantly. This is in large part due to the hypertune.

While my guide graphs are spectacular these days (only occasional bumps in DEC), my mount still has a big problem. When PHD clears the DEC backlash, it takes > 10 iterations (@ 400mm fl, 1600ms steps). This means that there is still a significant amount of backlash in my DEC axis.

In a perfect world DEC backlash would not be an issue because once the mount is perfectly aligned, DEC could be turned off. However, in the real world wind for example will occasionally bump the axis one way or another. This is why I need my mount to be able to send both 'S' and 'N' guide corrections. I have to get rid of the DEC backlash.

Given that my mount was just hypertuned, where do you think this amount of backlash comes from? Does the DEC lock level have any effect on backlash? Can I use the mount's backlash settings to help PHD? Should I try a different guide program?

Thanks for reading.

Edit: The hypertune was performed by Deep Space Products and I think it was totally worth the money.

#2 jrcrilly

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Posted 13 February 2011 - 02:25 PM

When PHD clears the DEC backlash, it takes > 10 iterations (@ 400mm fl, 1600ms steps).


16 seconds to clear backlash is definitely excessive. Unless the DEC guide rate is set to something extremely low there's a bunch of slop in there. That almost always comes from the worm-worm gear engagement being set too loose or end play on the worm itself. You can detect these by trying to rotate the DEC axis with no power applied. If it moves, that's worm slop (a worm drive prevents any backlash present elsewhere from appearing during this test). It can come from loose transfer gears but on that model the shafts have flats on them and the setscrews don't tend to work loose. It can come from worn reduction gears inside the motor housing but I've not seen this on that model. In a pinch you can swap the RA and DEC motors to isolate that.

#3 broca

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Posted 13 February 2011 - 02:28 PM

Try setting your Guide Rate settings on the hand controller to RA 50% and Dec 99%. Then you must recalibrate a guide star with PHD. See this thread. I hope this helps. I too have a CG-5 that I'm using for AP. I had the same issue you are having. I had the mount HyperTuned by Deep Space Products have am VERY happy with the results. Just keep in mind that the DEC will hiccup from time to time, it's just the level of mount we have. I imaged last night and captured 47 subs using an exposure time of 6 min and had to throw away only 4 frames.
Steve

#4 gezak22

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Posted 13 February 2011 - 02:48 PM

Try setting your Backlash settings on the hand controller to RA 50% and Dec 99%. Then you must recalibrate a guide star with PHD. See this thread. I hope this helps. I too have a CG-5 that I'm using for AP. I had the same issue you are having. I had the mount HyperTuned by Deep Space Products have am VERY happy with the results. Just keep in mind that the DEC will hiccup from time to time, it's just the level of mount we have. I imaged last night and captured 47 subs using an exposure time of 6 min and had to throw away only 4 frames.
Steve


You mean guide rate, not backlash, right? Those have been at 50% for RA and 99% for DEC for over a year now.

What are the criteria for throwing out a frame. Do you measure the eccentricity of a star or do you just eyeball it?

John: Thanks. The DEC axis does indeed move a tiny amount. I'll investigate.

#5 broca

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Posted 13 February 2011 - 03:09 PM

Excuse me yes, Guide Rate. I throw out a frame if the star is obviously malformed, just eyeball it and let DSS do the rest...

#6 avarakin

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Posted 14 February 2011 - 08:53 PM

I have LXD75 and it also has quite a bit of slop in DEC.
What really helps, is some slight polar misalignment, so DEC motor has to push in one direction.

Alex

#7 Ian Robinson

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Posted 15 February 2011 - 04:52 AM

Have you checked how well the gear wheel meshes with the worm ?
I had lots of slop in the RA on my CG5 until checked this and tweeked the gap. Problem went away then.

#8 gezak22

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Posted 16 February 2011 - 01:55 AM

I have LXD75 and it also has quite a bit of slop in DEC.
What really helps, is some slight polar misalignment, so DEC motor has to push in one direction.

Alex


I know what you are talking about but no, I will never do this. During the calibration process, PHD can mistake the drift of the star to be due to guide pulses.

Take look at the best astrophotos out there. What do those photographers preach? Align the mount well. So that's what I'll do.

Ian Robinson: I did check it before the hypertune, but not after. I still have to contact Ed (who did the hypertune) and ask why the DEC axis was left like this. I have a feeling it was because the motor was (and still is) very noisy and any further improvements in the gear mesh would have made the motor even noisier.

Again, the issue is pretty minor given that I shoot at 200mm fl but it severely limits future upgrades.

#9 rmollise

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Posted 16 February 2011 - 08:27 AM

Take look at the best astrophotos out there. What do those photographers preach? Align the mount well. So that's what I'll do.


Some preach that. Some don't. If you're using an inexpensive mount with some dec backlash, it is very helpful to leave some polar misalignment so declination corrections are always in the same direction. Of course, if you get close enough to the pole you won't need many/any dec corrections. If you must operate in portable fashion, you may get tired of doing a drift alignment every time. I did. And never saw _any_ difference in guiding quality for 5 - 10 minute subs if the CG5 was slightly off, that is, aligned using the HC routine instead of drifting.






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