Re: [ap-gto] 1200 GTO pointing readout accuracy

Oct 5, 2017



#58769 Oct 5, 2017

Hi all,When I select a named star from the GTO keypad database then GOTO that target, the star centres nicely in the EP or camera frame.. Nearby named stars do likewise.. I am curious as to why the RA/Dec readout on the keypad does not agree with the target's actual coordinates.. RA is about an arc-minute out, but Dec is more variable and can be at least 5 arc-minutes out..

This is never a practical issue if I'm targetting objects just from the internal database.. The problem arises when I manually enter RA/Dec coordinates via the keypad; the scope will point that same amount (eg. 5 arc-minutes) off the target, which can make it nearly out of the FoV.. .

Even if I enter the coordinates of a star from the internal database (using the very same database coordinates), the scope will not point accurately to that star...

I am aware of things such as the inbuilt atmospheric refraction compensation, mirror flop and scope flexure, but none of these would seem to have any influence because the scope points correctly at any object from the internal database - it's just that the reported coordinates are wrong.

Can anyone throw any light on this for me please?..

Could a small misalignment of the scope's optical axis with respect to the mount cause this?

For what it's worth, the scope is a 14" SCT (with locked mirror) and my observatory-mounted 1200 GTO hasn't missed a beat for the six years I've had it...

Cheers, Chris


#58770 Oct 5, 2017

Is it possible that you are entering J2000 coordinates?. I believe that the keypad uses JNow. ��When you pick objects from the keypad database, it probably precesses the coordinates to the current epoch.. But if you enter the RA and dec manually, it probably assumes that you are using the current epoch. ��If you have an electronic planetarium, like SkySafari or TheSkyX, you can configure them to give you the current epoch coordinates.. Try using those coordinates, and I suspect that it will hit it directly. ��-Wade ��From: [] Sent: Thursday, October 5, 2017 7:22 PMTo: ap-gto@yahoogroups.comSubject: [ap-gto] 1200 GTO pointing readout accuracy


#58771 Oct 6, 2017

Thanks for your reply Wade.

Yes, I was using J2000 coordinates.. .I don't have either of the planetarium programs you suggested, so will need to find a way of converting from J2000 to JNow...

I had a look in Carte du Ciel, and it lists Apparent Coordinates, which I suspect might be the same as JNow.

Cheers, Chris

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