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High Precision - for more than finding?

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#1 rsworden

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Posted 04 July 2021 - 01:26 AM

Meade DS-114AT with Autostar 497. During alignment and some basic observations the stars may drift slightly over time, and sometimes wiggle with the motor motion, so I have settled for taking images of no more than 10 seconds exposure.

 

I was recently reminded of High Precision mode and have used it to find galaxies very easily. Then the scope seems to hold the galaxy centered very solidly. It looks like I will be able to take longer exposures. Tonight I got a few at 15 seconds that kept stars nicely round instead of letting them wiggle. Unfortunately the sky clouded over before I could test much.

 

So my question is: does High Performance mode somehow help with tracking as well as with finding? I also have used the Sync function - sometimes two or three times in a row - and that seemed to help, but not nearly as much as High Precision seems to have helped. 



#2 OzAndrewJ

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Posted 04 July 2021 - 02:41 AM

Gday rsworden

does High Performance mode somehow help with tracking as well as with finding

Nope.

High precision mode is simply going to a nearby bright star and doing a synch,

ie it is no different to doing a manual synch before going to a target.

It has absolutely no effect on tracking.

My guess is your mount has relatively bad PE in places so when in that region

of the worm, it wiggles and drifts, in the good areas of the worm, it is nice and stable.

Doing an unguided error log using PEMPro or PHD etc will show you that.

 

Andrew Johansen Melbourne Australia


Edited by OzAndrewJ, 04 July 2021 - 02:41 AM.

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#3 rsworden

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Posted 06 July 2021 - 09:56 PM

And since I’m currently shooting galaxies fairly near the North Pole, the motion is slower than in other regions of the sky. So maybe the worms are turning very slowly, minimizing the wiggle and maximizing the time between major PE errors.



#4 OzAndrewJ

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Posted 06 July 2021 - 10:20 PM

Gday rsworden

So maybe the worms are turning very slowly, minimizing the wiggle and maximizing the time between major PE errors.

Nope, the worm is going at the same speed as if you were pointing at the equator

and any "mechanical" errors in the drive have the same magnitude ( at the axle ).

What is happening is the "RA" movement you see on the sensor/in the EP

is in true sky arcsecs, not axle arcsecs.

true sky arcsec error = axle error * cos(DEC)

sooo, the higher the DEC you image at, the smaller any RA axle error will "appear" in tracking on the sensor.

eg

at DEC = 0,   1.0 RA axle arcsec = 1.0   true sky arcsec

at DEC = 30, 1.0 RA axle arcsec = 0.87 true sky arcsec

at DEC = 60, 1.0 RA axle arcsec = 0.5   true sky arcsec

at DEC = 80, 1.0 RA axle arcsec = 0.17 true sky arcsec

 

Andrew Johansen Melbourne Australia



#5 barnold84

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Posted 07 July 2021 - 06:06 AM

And since I’m currently shooting galaxies fairly near the North Pole, the motion is slower than in other regions of the sky. So maybe the worms are turning very slowly, minimizing the wiggle and maximizing the time between major PE errors.

Keep in mind, the higher the declination is, the more accurate your polar alignment needs to avoid star trails from field rotation under long term exposure.

 

CS!

Björn



#6 rsworden

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Posted 09 July 2021 - 10:37 PM

This is an Alt-Az mount. I think the motor speed *is* different depending on the location. The extreme case would be observing Polaris: the tracking rate of both motors should be zero. 



#7 barnold84

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Posted 09 July 2021 - 11:27 PM

I'm sorry. I totally missed that point! 



#8 OzAndrewJ

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Posted 09 July 2021 - 11:40 PM

Gday rsworden

 

Just noticed you were altaz, ( i was thinking GEM when i replied )

so yes, the motor speeds will change based on where you are pointing.

Being AltAz its not just near the pole that it happens, so you need to look

at the specific location to see whatthe rates will be.

 

That said, Bjorns comment re image rotation does become valid

and again changes based on where you are pointing.

 

Lots of online apps out there that can simulate what happens.

 

Andrew Johansen Melbourne Auustralia



#9 rsworden

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Posted 11 July 2021 - 12:28 AM

Yes, field rotation would be an issue. But since I'm taking lots of relatively short exposures, I'm relying on stacking software to deal with that.At least, I hope it should. I imagine I should look to see if my "keepers" tend to be clustered in time or spread throughout my sequence. If I have images that can be registered in both the early and late times, the registration must be handling the rotation OK. I'm trying M81 tonight since the clouds are finally gone... we'll see.




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