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Guiding better pointing north

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#1 dally

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Posted 21 January 2021 - 01:11 AM

I been struggling to understand why my guiding is almost twice as good when my target is in the northern sky as opposed to pointing south. Tonight I tried a few tests with an asi120mm mini, one test using my Celestron OAG and another using a 60mm guidescope. Both had similar results.

 

Target #1 The Orion Nebula

The OAG gets on average about 1.05" total RMS

The 60mm guidescope... about .92" total RMS

 

Target#2 M81

The OAG about .87" total RMS

The 60mm about .53" total RMS

 

As you can see there's a drastic difference between what the OAG and guidescope get when pointing to M82. But there's also a big difference depending on where the scope is pointing. 

 

The main scope was rebalanced for all tests to be east heavy and dec balanced even or very slightly tail heavy. 

 

Thoughts?

 

Edit: I left PHD running tracking m82 and it's now averaging .47" total rms...

 

 

 

-Darryl

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Edited by dally, 21 January 2021 - 01:31 AM.


#2 Tapio

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Posted 21 January 2021 - 02:47 AM

There's a logic in there that rotation speed is slower near pole.

But one question - do you have ST-4 guide connection or pulse guiding ?

If you are using ST-4 guiding then you have to recalibrate guiding in every target you slew to.



#3 dally

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Posted 21 January 2021 - 08:37 AM

There's a logic in there that rotation speed is slower near pole.

But one question - do you have ST-4 guide connection or pulse guiding ?

If you are using ST-4 guiding then you have to recalibrate guiding in every target you slew to.

I'm using pulse guiding via CPWI/ASCOM. I've also recalibrated based on where I'm imaging and the results are unchanged. I realize, at least for me, the oag results are based on the fact that I've not been able to get a star with good enough snr as compared to the guidescope so I'm not blaming the oag. Just odd how guiding can improve or worsen by .5" if not more depending on what part of the sky ones imaging.



#4 Der_Pit

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Posted 21 January 2021 - 01:18 PM

It is the "slower speed" Tapio mentioned.  If your mount produces errors in RA (periodic, other erratic) it's real effect in arcsec on the sky skales with the cosine of the declination you point at.  That is, an 2" error of the mount refers to 2" at the celestial equator, 1" at declination 60 degrees and only 0.5" at 76 degrees.

So with the same "performance" of the mount you'll get much better guide RMS closer to the pole.


  • jdupton, David Boulanger and jonnybravo0311 like this

#5 jonnybravo0311

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Posted 21 January 2021 - 03:38 PM

Nothing constructive to add to the conversation, but just thought it neat that OP is about a 15 minute drive from my location and we've both recently had our cameras/scopes pointed north to M81 :D



#6 David Boulanger

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Posted 21 January 2021 - 03:47 PM

Me too.  Picked up 7 hours of images the last two nights.  My problem at the moment is my latitude is 26 degrees so it has been lower in the sky.  



#7 fewayne

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Posted 21 January 2021 - 04:03 PM

Did you run a PHD calibration for this session? Where in the sky?

 

As the others pointed out, if PHD can read where your mount is pointing (via ASCOM or whatever, but not ST-4), it can do maths on the corrections needed for different parts of the sky. So that shouldn't have an effect. After all, unless you're imaging on the equator near the meridian, calibrating near your target is not optimal.

 

However, you can run the Guiding Assistant on each of your targets and note the differences in recommendations, if any. You could also do a star-cross test in each location and see if there's something mechanically different that's messing up the mount's response when pointed one way or another.



#8 dally

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Posted 21 January 2021 - 09:38 PM

Nothing constructive to add to the conversation, but just thought it neat that OP is about a 15 minute drive from my location and we've both recently had our cameras/scopes pointed north to M81 laugh.gif

Cool what town?



#9 dally

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Posted 21 January 2021 - 09:43 PM

Did you run a PHD calibration for this session? Where in the sky?

 

As the others pointed out, if PHD can read where your mount is pointing (via ASCOM or whatever, but not ST-4), it can do maths on the corrections needed for different parts of the sky. So that shouldn't have an effect. After all, unless you're imaging on the equator near the meridian, calibrating near your target is not optimal.

 

However, you can run the Guiding Assistant on each of your targets and note the differences in recommendations, if any. You could also do a star-cross test in each location and see if there's something mechanically different that's messing up the mount's response when pointed one way or another.

Yes, thank you, I've learned to run the GA when I'm pointing to different parts of the sky and tweak the settings accordingly. It helps sometimes. Usually I just tweak minmo/aggr one at a time until I see a change for the better, keep going until it negates then back off a bit. I let it settle then move on to the aggr or predictiveWT etc.

 

I did do a star cross test and it looks pretty spot on. 

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#10 jonnybravo0311

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Posted 21 January 2021 - 10:14 PM

Cool what town?

Voorhees.




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