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Rotation shift after meridian flip (no electronic rotator)

Meridian Flip Help Needed Troubleshooting
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ValeryL avatar
ValeryL avatar

Hello,

I recently noticed that during my last two sessions of shooting that I experienced a shift in the field rotation after the meridian flip.

All images taken before the meridian flip are same orientation and matches with framing assistant, and all after the flip are of course flipped but also with a shift in the rotation (something like 3-5°).

I’m pretty sure this was not the case before. I did not recently unmounted the OTA, I just changed target and rotation of the camera for framing (manual rotation).

I already cheched if the grub screw from the manual rotator was well tight (it is) and if the camera & adaptors were well screwed (there are).

I also checked that the OTA was aligned with the axis of the saddle (roughly but sight) and that it was tight enough.

Here is my equipment :

  • AM5N (carbon tripod ballasted with 8kg)

  • Redcat51 WIFD (with ZWO EAF, deepskydad FP2 flip flat panel)

  • Minicam8

  • Guiding : uniguide 32 and ASI 224 MC

  • Mini PC attached to the scope

I use NINA, I checked the time in the software and in the ASI ascom driver of the mount and they match. I was thinking about this because we just changed from summer time to winter time in my location.

I tried to read all forums about similar issues, but i cannot figure out what’s the issue, it seems it happened to other people but the cause is not clearly identified in those posts.

One other odd thing that I noticed :

I use TPPA to align, my routine is : I roughly point the mount toward NCP, then I slightly move the mount in NINA to the EAST (Because I read somewhere that crossing the meridian during TPPA was not good), I check “start from current position” and I choose a rotations of 20°C between frames (going EAST). I do the alignment down to 10 arcsec total error. If I’m too off in azimuth at first, I manually change tripod azimuth untill below 1°C and redo the routine.

My guiding in last two sessions was really good (0,4 to 0,6 RMS arcsec), both before and after meridian flip.

What is weird is that two days ago, after PA routine, I thaught it would be fun to do a 600s exposure (home position no tracking) just to see field rotation with stars around NCP. The resulting picture showed that NCP was not in the middle of the frame, it was in the frame but not in the middle… I’m not sure it’s related to my rotation issued after meridian flip though, but I would have thought that after PA, NCP would be in the middle of the frame?!

Any ideas?

Thanks in advance,

CS

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Oscar avatar
Oscar avatar
(deleted)
Quinn Groessl avatar
Quinn Groessl avatar

I’ve had a similar issue and never really cared enough to try and figure it out. Now I’m trying to gather data for a collaboration and it’s important that the rotation be better so I care now.

To me the simplest fix would be if someone could add to NINA’s “center after drift” trigger a “fix rotation” trigger.

EDIT: I realize I read this wrong and thought it said you did have a rotator. Sorry about that.

andrea tasselli avatar
andrea tasselli avatar
Cone error does that for you. Causes: difficult to ascertain for sure but few degrees may amount to a fraction of a mm so difficult to spot visually. I'd check whether there is any flexure between tripod and mount, for starters.
ZigZagZebraz avatar
ZigZagZebraz avatar

ValeryL · Oct 28, 2025, 08:33 AM

What is weird is that two days ago, after PA routine, I thaught it would be fun to do a 600s exposure (home position no tracking) just to see field rotation with stars around NCP. The resulting picture showed that NCP was not in the middle of the frame, it was in the frame but not in the middle… I’m not sure it’s related to my rotation issued after meridian flip though, but I would have thought that after PA, NCP would be in the middle of the frame?!

Any ideas?

Thanks in advance,

CS

You can recalibrate the home position for AM5 (https://www.cloudynights.com/forums/topic/886794-zwo-am5-help-with-home-position/)

In NINA - Equipment, Mount Tab, can also put in AZ = 00 and Alt = Your latitude, for your Star Trails experiment, and see how it goes. If the polar alignment is close enough, the NCP will be close enough to the center of the field of view of the camera.

With all advancements in electronics and related things, still the computer and the mount think that the Home position is the position at which the mount is, when it is powered ON. Typically, a relative error in the Home position is not an issue anymore with plate solving.

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ValeryL avatar
ValeryL avatar

andrea tasselli · Oct 28, 2025, 11:00 AM

Cone error does that for you. Causes: difficult to ascertain for sure but few degrees may amount to a fraction of a mm so difficult to spot visually. I'd check whether there is any flexure between tripod and mount, for starters.

Hello Andrea,

Thanks for your answer, I read a bit about cone errors and indeed, given what I experience, it seems like a good explanation.

I’ll check the mounting of the OTA on the saddle and also the fixation of the dovetail to the OTA.

What is strange is that I did not do any change on the rig recently, but I’ll check anyway.

Maybe I tightened too much one of the screws of the dove tail and it caused OTA to point higher or lower compared to the mount dovetail.

I’ll start with tripod and mount as suggested.

I’ll let you know

CS

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