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RA/DEC vs GoTo

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#1 Danzup77

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Posted 26 December 2012 - 04:19 AM

I asked this before but for some reason it was moved to the astrophotography forum so allow me to attempt again lol...

long story short I was doing some AP with my C8 and CGEM and I noticed that my GoTos were spot on while my RA/DEC were low (they were just UP and out of the EP) so I kept having to bring them down into the EP. Anyone have a reason this would happen so I dont keep doing it lol... :foreheadslap:

Objects were about 40 deg-ish in the west and I was getting ra/dec from stellarium.

#2 orlyandico

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Posted 26 December 2012 - 05:19 AM

this is probably due to slack in the gear train.

make sure that, when you did the initial GoTo alignment, only use the up and right buttons for the final align on each star. This will ensure that all of the slack is taken up.

#3 cn register 5

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Posted 26 December 2012 - 12:23 PM

This could be due to precession, the difference between the coordinates for now and those for J2000. The difference varies across the sky but can be 10 to 20 arc minutes.

Some scopes use J2000, some the current epoch. I don't know what Stellarium reports but you need to use the same as the mount.

Hope this helps,

Chris

#4 Danzup77

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Posted 27 December 2012 - 08:30 PM

Orlyandico thanks for that info!

Chris, I think Stellarium displays both if I remember correctly. Can you elaborate a little more as to the difference? Thats something I know NOTHING about lol....

#5 cn register 5

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Posted 28 December 2012 - 04:54 AM

Precession is the change of the direction of the Earth's rotational axis, this change causes the Ra and Dec coordinate system to change because it's based on the Earth's rotation axis. The effect is that the positions of objects change by about one arc minute a year.

Star positions in charts are usually converted to a particular date or epoch and the current one is usually the start of the year 2000, J2000.0.

The difference in position from 2000 to now will be of the order of 13 arc minutes and this is enough to move the position of an object by a noticeable amount.

Hope this helps, it's a very simple explanation and there are much fuller ones on the web.

Chris

#6 jrbarnett

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Posted 28 December 2012 - 11:06 AM

Sounds like either cone error or polar alignment error to me.

Use 3 calibration stars during alignment to correct cone error. Check your polar alignment too. For visual, centering Polaris in the polar scope is adequate. For imaging, you may want to drift align.

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