#### Re: Daylight Savings Keypad change

Ted Mickle

Howard,

As always, I appreciate your professorial yet clear explanations of the technical basis of the hobby we enjoy so much.

Thanks,

Ted

Ted Mickle

tedmickle@me.com

Cell/Text: 405 464-9589

From: <ap-gto@yahoogroups.com> on behalf of "howard@astro-physics.com [ap-gto]" <ap-gto@yahoogroups.com>
Date: Friday, March 9, 2018 at 15:56
To: <ap-gto@yahoogroups.com>
Subject: Re: [ap-gto] Daylight Savings Keypad change

Is it safe to say that pointing accuracy is a function of latitude and longitude, and if one were to set his/her laptop and keypad to UTC, pointing accuracy would not be affected?

Also, if the keypad is set to UTC, does that automatically set the GP4 to UTC as well?

Pointing accuracy is a function of polar alignment and the orthogonality of the telescope. Latitude, longitude and time only come into play on the first slew of the session, in that they are necessary to determine the initialization RA from the stored shut-down data (Actually, Latitude isn't really relevant either, but it is needed to calculate the position of the horizon). Once you have made the first GoTo, and centered and ReCalibrated, ALL subsequent pointing is is achieved through simple arithmetic converting the distance between current coordinates and target coordinates. Latitude does not come into play at all. Time and longitude are only used to calculate the LST, which is the RA value of the meridian so the servo knows if a meridian flip is required.

This is a common misconception that arises from people who started with typical alt/az SCT setups. An alt/z based system must convert alt/az coordinates into RA and Dec, and to do that with any accuracy requires all three: time, latitude and longitude to be as accurate as possible. A German Equatorial that is already based on RA and Dec does NOT need those parameters for pointing once it is calibrated.

Whatever device initializes the mount will send it both the current time (in its current time zone state - UTC in this case) and the net GMT offset, which is always zero for a UTC system. The GTOCP4 does not have a time reference until it is initialized.

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