Mount RA/DEC slew math

deonb

I'm trying to correct my pier's N/S and E/W positions in my dome, so I stuck a camera in the middle of my scope's lens cap and ran a tape marker down the length of the dome shutter. I'm then trying to align one axis at a time.

I thought that if I slew from Alt:0 Az:90 to Alt:0, Az:270, the mount will only slew DEC without moving RA. So if the dome alignment is correct, I should be able to move the lock the RA axis and move the DEC axis manually bit by bit, mark a path on the dome (waiting for the dome to re-slew after each scope movement), then after Meridian flip follow that path down again and the up-and-down path markers should be in alignment.

That's the theory... In practice what happens is that the RA axis also moves during this slew. Not a lot, but enough to make my doubt my hypothesis. I obviously can't move 2 axis's by hand.

Why is that? Isn't Alt:0 Az:90 and Alt:0, Az:270 on the same RA plane for every Lat?

Ray Gralak

Why is that? Isn't Alt:0 Az:90 and Alt:0, Az:270 on the same RA plane?
Does the RA axis actually move, or are you relying on where the camera is aiming to detect RA movement? If it is the latter and the camera is not orthogonal, then a 180-degree flip will not be exactly 180 degrees offset from its starting point.

-Ray Gralak
Author of PEMPro
Author of APCC (Astro-Physics Command Center): https://www.astro-physics.com/apcc-pro
Author of Astro-Physics V2 ASCOM Driver: https://www.siriusimaging.com/apdriver

-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of deonb
Sent: Saturday, December 26, 2020 11:39 PM
To: main@ap-gto.groups.io
Subject: [ap-gto] Mount RA/DEC slew math

I'm trying to correct my pier's N/S and E/W positions in my dome, so I stuck a camera in the middle of my scope's lens
cap and ran a tape marker down the length of the dome shutter. I'm then trying to align one axis at a time.

I thought that if I slew from Alt:0 Az:90 to Alt:0, Az:270, the mount will only slew DEC without moving RA. And if the
dome alignment is correct, it should move in a curved track around the dome and come to land in the middle of the
shutter tape on the other side.

That's the theory... In practice what happens is that the RA axis also moves during this slew. Not a lot, but enough to
make my doubt my hypothesis.

Why is that? Isn't Alt:0 Az:90 and Alt:0, Az:270 on the same RA plane?

deonb

The RA axis actually visually move. Not by a lot, but much more than can be explained by sidereal.

I don't rely on the camera for anything yet since I can't rationalize the mount movement yet.

I've just looked up the RA/DEC coordinates as the mount report it when it's at those 2 Alt/Az points:
Alt 0, Az 90 translates to RA: 20:25:25 Dec: 0
Alt 0, Az 270 translates to RA: 8:25:25 (+ sidereal) Dec: 0

So delta the meridian flip, that's the same RA plane - at least I'm not completely going crazy. But it doesn't seem to travel on that plane from point A to point B when it's slewing...

This is not technically an issue for me, since I want to move the mount by hand anyway. The issue is that I don't understand why it's doing this during an automated slew, so I don't know if my tests are valid.

Roland Christen

Is there an atmospheric refraction calculation happening? If so, then the difference would be about +- 1/2 degree from one side to the other.

Rolando

-----Original Message-----
From: deonb <deonb@...>
To: main@ap-gto.groups.io
Sent: Sun, Dec 27, 2020 10:21 am
Subject: Re: [ap-gto] Mount RA/DEC slew math

The RA axis actually visually move. Not by a lot, but much more than can be explained by sidereal.

I don't rely on the camera for anything yet since I can't rationalize the mount movement yet.

I've just looked up the RA/DEC coordinates as the mount report it when it's at those 2 Alt/Az points:
Alt 0, Az 90 translates to RA: 20:25:25 Dec: 0
Alt 0, Az 270 translates to RA: 8:25:25 (+ sidereal) Dec: 0

So delta the meridian flip, that's the same RA plane - at least I'm not completely going crazy. But it doesn't seem to travel on that plane from point A to point B when it's slewing...

This is not technically an issue for me, since I want to move the mount by hand anyway. The issue is that I don't understand why it's doing this during an automated slew, so I don't know if my tests are valid.

--
Roland Christen
Astro-Physics

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