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Old 01-09-2013, 07:23 AM
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PRejto (Peter)
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Guiding Problem

Hello,

I'm having an issue with my PMX that I cannot seem to resolve. In this setup I'm guiding with an ST-i through an 80 mm APO mounted firmly on top of my TEC140. I seem to get good results in RA but DEC is a problem. I have adjusted the cam stop but the behavior persists. As the picture shows below RA is roughly aligned with "X." Guiding usually starts off well in both RA and DEC, but after ca 2 minutes I will get a relatively large error in DEC that does not correct for a long time. Exposures were 6 seconds long and correction does not begin until after a minute of error (or so it seems to me). Aggression was set at "6." When I increase aggression to "7" guiding in DEC improves but the corrections in RA are then too aggressive. Through experimentation it does seem that I generally need to increase aggression when I have longer exposures. With dim guide stars I need longer exposures and this is when my problems start.

If I restart guiding with DEC (Y) corrections turned off I will track very well in DEC for about 3 minutes followed by errors in the +Y direction, so that is the way DEC is drifting with time.

Is there an adjustment that anyone might recommend to improve DEC guiding without sacrificing RA guiding? Would a change to motor speed in DEC through the Bisque TC have a bearing here? Do I need "backlash" compensation? (I would think not because during calibration the mount appears to respond immediately in DEC.)

I cannot find any free play in the guide scope though I suppose that possibility cannot be discounted entirely. It does seem odd to me that the error(s) are only in DEC however. I would imagine that movement of the guide scope would be more random and less predictable. I can try to eliminate that possibility by re-setting up the ONAG guider, but would prefer not to at this time.

Any help will be very much appreciated!

Peter
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Last edited by PRejto; 01-09-2013 at 01:44 PM.
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  #2  
Old 01-09-2013, 10:57 AM
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Joshua Bunn (Joshua)
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Hi Peter,
A couple of differebt things to try. Try a different brighter guide star and recalibrate the guider,

This does sound like your polar alignment could be off - how is the PA? mmmm... but then the Dec graph wouldnt go +ve after it being -ve.

Oscilations in Dec... this does sound like backlash in Dec.

Josh
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Old 01-09-2013, 11:25 AM
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My PMX initially guided beautifully. I was using 1-2 seconds guide exposures.

After about 10 uses and a gap of use of about 2 months it suddenly produced a bad spike in PE after about 1-2 minutes into the imaging.It would not last long but it sent errors to 3 on the guiding. This would cause a double star in the image.

It was either one of 2 things in the end. It might've been a bit of crude from the original crap black drive belts that SB got that are defective and shed material. There are several posts on this site about getting the later grey belts otherwise these black ones can cause trouble.

SB sent me a new worm. When I replaced the worm all this stopped. I was wondering at the time that perhap the worm got damaged because of the several times the mount slipped on its incorrectly factory set gears and perhaps caused some damage. After replacing though the original worm looked faultless. But there was a little bit of black crud on the pulley that the belt rides on.

So a bit of crud that lifts the belt when it rotates to that point could explain it. Especially if it was working well at some point and now does not.

This reminds me I have to get SB to send out replacement belts to me.

I am assuming the usual has been done here like a good polar alignment, no cable drag, not unbalanced especially at the imaging angle. I don't like piggyback setups as they can be nicely balanced horizontally but badly unbalanced at a typical imaging angle. I guess that would have something to do with not being centred properly.

Once I replaced the worm block it guided beautifully but the old belts remain so the possibility exists it could happen again if that were indeed the cause.

Greg.
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Old 01-09-2013, 01:49 PM
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Quote:
Originally Posted by Joshua Bunn View Post
Hi Peter,
A couple of differebt things to try. Try a different brighter guide star and recalibrate the guider,

This does sound like your polar alignment could be off - how is the PA? mmmm... but then the Dec graph wouldnt go +ve after it being -ve.

Oscilations in Dec... this does sound like backlash in Dec.

Josh
Hi Josh, Thanks for your input! My PA is quite good (or it was a few months ago...have not checked for a while) and certainly good enough to expect very good guiding (though maybe not unguided for more than a few minutes. I have re-calibrated numerous times, and guiding does the same thing in DEC at various points in the sky. When I calibrate the mount seems to move correctly and repeatably in DEC, so it's quite confusing as to why this is happening.

Peter
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Old 01-09-2013, 01:53 PM
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Quote:
Originally Posted by gregbradley View Post
My PMX initially guided beautifully. I was using 1-2 seconds guide exposures.

After about 10 uses and a gap of use of about 2 months it suddenly produced a bad spike in PE after about 1-2 minutes into the imaging.It would not last long but it sent errors to 3 on the guiding. This would cause a double star in the image.

It was either one of 2 things in the end. It might've been a bit of crude from the original crap black drive belts that SB got that are defective and shed material. There are several posts on this site about getting the later grey belts otherwise these black ones can cause trouble.

SB sent me a new worm. When I replaced the worm all this stopped. I was wondering at the time that perhap the worm got damaged because of the several times the mount slipped on its incorrectly factory set gears and perhaps caused some damage. After replacing though the original worm looked faultless. But there was a little bit of black crud on the pulley that the belt rides on.

So a bit of crud that lifts the belt when it rotates to that point could explain it. Especially if it was working well at some point and now does not.

This reminds me I have to get SB to send out replacement belts to me.

I am assuming the usual has been done here like a good polar alignment, no cable drag, not unbalanced especially at the imaging angle. I don't like piggyback setups as they can be nicely balanced horizontally but badly unbalanced at a typical imaging angle. I guess that would have something to do with not being centred properly.

Once I replaced the worm block it guided beautifully but the old belts remain so the possibility exists it could happen again if that were indeed the cause.

Greg.
Hi Greg!

Appreciate your help here. I have replaced my black belts with gray some time ago. There was little if any shredding. Perhaps I just need to re-lubricate my worms. Guiding seems quite good in RA. The mystery is why DEC is so slow to respond to an error. I've checked my balance at the guiding position and it seems as close to spot on as I can imagine.

I'll post this at SB and see if something new comes out of this.

Thanks,
Peter
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Old 01-09-2013, 04:33 PM
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Odd behaviour going on there. What do star shapes look like Peter?

Some observations though. If your PA is good then you will hardly ever need to make corrections. I find normally I get movement in RA due to PE and nill if any in Dec. The RA movement is very minimal so I would be looking at .25 of a pixel off the center line.

Just really take the time to check balance over entire sky. You might have something odd going on. Probably not but worth checking. Goes back to the conversation we had last Saturday about hunting for issues.

Is the Dec truly locked off. Does the dec need locking knob need some adjustment? Most likely not, but worth checking.

Cables need lots of attention, grabbing can cause this issue.
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Old 01-09-2013, 05:03 PM
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Hi Paul,

Thanks for your reply. I think it is an issue; my stars are about the shape of that guiding graph!! Certainly I shouldn't be seeing that in DEC should I? If not for those excursions that don't correct I would have terrific guiding. Thanks for the suggestion that I check the through the mount cables! I had not considered that obvious possibility!

I probably should redo my t-Point model and check PA, but I doubt this is the reason for my DEC issues, though it certainly wouldn't hurt to have everything spot on. Just takes time and tme is often short when it comes to clear sky!

Peter
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Old 01-09-2013, 09:34 PM
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Depending on your setup even piers can go off over time due to wet weather, dry weather affecting soil and it can create stresses in the soil that could shift a slab slighty or a pier.

I have not particularly noticed that with my setup but my pad is 1 metre deep x 800 square of concrete with lots of steel and a large flange of concrete on the top level.

But if my guiding started to go off the first things I would check apart from the usual guiding settings would be balance, cables, polar alignment - the physical side of the setup.

A quick 30 point t-point model would show if PA is off. I think you can save the existing model if it is large and reinstate it if PA turned out to be fine.

Greg.
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Old 02-09-2013, 05:21 AM
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Quote:
Originally Posted by gregbradley View Post
Depending on your setup even piers can go off over time due to wet weather, dry weather affecting soil and it can create stresses in the soil that could shift a slab slighty or a pier.

I have not particularly noticed that with my setup but my pad is 1 metre deep x 800 square of concrete with lots of steel and a large flange of concrete on the top level.

But if my guiding started to go off the first things I would check apart from the usual guiding settings would be balance, cables, polar alignment - the physical side of the setup.

A quick 30 point t-point model would show if PA is off. I think you can save the existing model if it is large and reinstate it if PA turned out to be fine.

Greg.

Thanks Greg, this is exactly what I will do. I did read on SB just today that PA can be of by as much as 5 arcmin and one should still be able to guide just fine, but who knows, maybe the bolts holding my pier down have loosened. I'm also going to investigate through the mount cables...

Peter
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Old 04-09-2013, 06:28 AM
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PROBLEM SOLVED!!!!

I can't quite believe it was so simple, but I (finally) looked at the settings in the Bisque TCS and somehow the DEC motor speed had been set back to .29 rather than the default of .5. I don't know how I missed this, but now that it is restored all is working as expected.

Peter
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Old 04-09-2013, 10:15 AM
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Thats great Peter.
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Old 04-09-2013, 10:18 AM
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Gee, quite a hidden problem. That could've gone unnoticed for ages.

Nice find.

Greg.
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