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INDI Library v1.9.0 Released (23 Apr 2021)

Major INDI Library release v1.9.0 bring significant internal changes championed by @pawel-soja to modernize core INDI Library drivers and clients. New drivers for DeepSkyDad Flat Panel & Pegasus devices plus further improvements to PCM8 drivers.

Figuring out how meridian flip works

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Hi all,

Some doubts about the underlying logic of meridian flip process.
I thought that it was based on the 'side of pier' that mount driver reports (actually, talking about eqmod with a eq6r pro). But I noticed that, afaik, side of pier really reports where the scope is pointing taking only into account the declination value. That is, if mount declination is between North-East, the driver reports 'west (scope pointing to East)', and if dec is between North-West, the driver reports 'east (scope pointing to West)'. Is this right? If so, if I'm initially pointing some object to the East very near of the meridian, the object will traverse the meridian with the (obviously) same dec value; so 'side of pier' would not change. However, the meridian flip does occur. So, the meridian flip is not based on 'side of pier' information, or I'm wrong in some assumptions.
Supposing that meridian flip is not based on side of pier info, I can fully understand that the mount (well, the indi driver) does know where the meridian is, and where is pointing actually; so no problem. But my question goes beyond this and would like to understand how does it work under the hood. So, to perform the meridian flip, does eqmod driver simply issue a goto command and the mount by itself "knows" that has to perform a meridian flip? I guess my question is more focused on how skywatcher protocol works, cause I guess the handset performs the same operations than indi driver. Well, actually don't know

Thanks!

Miguel
1 month 2 weeks ago #70917

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It's confusing because some of this logic is in the mount firmware, and some is in Ekos. This means the specifics are different for different mounts.

At least for my mounts (iOptron, 10Micron), it's not "side of pier" but rather "hour angle" that triggers the flip. There are two interacting pieces of info:

1) The mount firmware (not Ekos) normally slews to locations from the west side or the east side (of course!) depending on where in the sky its target coordinates currently are (azimuth). There is a "gray zone" very close to the meridian which ca be set in the hand controller to allow a slew-from-the-west to a point just before the meridian. I usually set this gray zone to be very small -- for me, 1 degree. If Ekos triggers a slew to any point west of that gray zone, the mount will approach it from the west.

2) In Ekos there's a setting to perform a meridian flip at some hour angle past the meridian. This must be set AFTER the "gray zone" set in the hand control. In y case I set this for 1.5 degrees past the meridian.

The usual series of events is
a) Mount tracks past the hand control gray zone (hour angle = +1 degree) while exposing an image
b) Mount tracks past Ekos "Meridian Flip" limit (hour angle = +1.5 degrees) while STILL exposing image, sends message "Meridian Flip Waiting"
c) Ekos completes current image and downloads from camera
d) Ekos sends command to slew mount to current target RA/DEC
e) Mount flips in order to approach target from the west because it's west of the "gray zone"
f) Ekos does an "alignment" step using plate solving to re-center target
g) Ekos begins exposing next image in sequence

The critical thing for me is to set the gray zone in the and paddle to be smaller than the meridian flip angle in Ekos.
The following user(s) said Thank You: Miguel
1 month 2 weeks ago #70923

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Thanks for the reply!

I cannot find any setting in the hand controller to define that "gray zone". All I can see is to force the flip or leave it in auto:

On an equatorial mount, SynScan hand control automatically decides whether to perform a meridian flipping at the begining of a GOTO, to prevent telescope from bumping on the tripod. In some occasions, astrotographers can choose whether to do a meridian flip or not.
Access to menu “Setup\Flipping Mode “,
 • Scroll to “Auto Flipping“ and press ENTER key to let the hand control decide automatically.
 • Scroll to “Force Flipping“ and press ENTER key to force the mount to perform a meridian flipping for the next GOTO ONLY.
 • Scroll to “No Flipping“ and press ENTER key to forbid the mount from performing a meridian flipping for the next GOTO ONLY



I also cannot see any relevant setting in eqmod.

Any other thoughts on skywatcher mounts?

m
Last edit: 1 month 2 weeks ago by Miguel .
1 month 2 weeks ago #70936

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Hmmm. I've never used a skywatcher mount. Look for something like "meridian limit."

You can do all the testing during the day when you can see what's happening.

Try to slew to different sides of the meridian and see what happens. If you are looking east and you try to slew to a location just west of the meridian, what happens? If you slew to a point *just barely* west of the meridian when you're looking east, does the mount just nudge a few degrees, or does it flip around to come to that point from the west side?

Maybe you don't need to define any kind of meridian limit for your mount? Try just setting Ekos to flip at HA = +1 degree (see screenshot below).

Then (again during daytime is fine for testing) slew to a point just east of the meridian and track. Watch the hour angle in Ekos get closer and closer to zero and then creep into the positive. What happens when you reach the HA = +1 degree?  Ekos should send a slew at that point and the mount should flip.

  
 
Last edit: 1 month 2 weeks ago by Scott Denning.
1 month 2 weeks ago #70939
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Alternatively, maybe that "let the hand control decide" means you should just leave it up to the skywatcher?

But then Ekos won't know it's time to recenter your target on the west side, you won't be able to decide whether you want to recalibrate your guiding, refocus, etc.

Probably best to get skywatcher to sit tight and let Ekos handle the flip if you can.
1 month 2 weeks ago #70940

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well, I have to test deeper during the daylight, but yes. If I'm looking east and slew to just west the meridian, the mount flips. I'm not completely sure if it happens always, or if effectively there is a sort of gray zone.
And, by the way, definitely there is not more information in the user manual; or I'm not able to find it.

Meridian flips are working in ekos, normally. Sometimes I get a 'failed' message, but it usually retries after 4 minutes and performs it. Maybe those 'failed' messages made to popup these questions into my mind :-) Only asking just for curiosity

m
1 month 2 weeks ago #70941

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