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Meridian flip with GM1000 and NINA

N.I.N.A. Users •8 replies•147 views
Jerry Gerber avatar
Jerry Gerber avatar
=contentHi all,

I have a 130mm refractor at a remote site on a GM1000 mount. I am a bit confused about how to set the meridian flip settings as I've not been out to the site yet and I can't see what's happening when the mount does the flip.

1. Should I use Pause before meridian? My telescope is about 42 inches (1062mm) in length (not including camera, filter wheel and OAG) and is on a pier, not a tripod. I am not sure whether I need to input a value in Pause before meridian. Not using it will save time but I don't want that at the expense of safety.  Does it depend upon how high in the sky the object I am imaging is? If an object is say at zenith, does this mean there's a chance the scope will hit the pier or the plate the mount is sitting on? 

2. When the flip happens and NINA plate-solves, the plate-solving image tonight had star trails even though there was supposedly a 30 second settling time. Not sure why this happened. Last night, the first sub after the flip also had star trails but after that they were all good.

Thanks for any insight into how to get the best out of this mount!

Jerry
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Francesco Meschia avatar
Francesco Meschia avatar
Regarding 1.,you need to use “pause before meridian” if your scope can’t reach the meridian without hitting the pier at some declination. It needs to be set for the worst case. Unfortunately it’s probably something that needed to be determined before sending off the equipment to the remote site, but maybe the remote site technicians will be able to help.

For 2. you may want to examine the NINA logs to see what happened in the two instances.
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Jerry Gerber avatar
Jerry Gerber avatar
Thank you Francesco.
Christian Großmann avatar
Christian Großmann avatar
Jerry Gerber:
2. When the flip happens and NINA plate-solves, the plate-solving image tonight had star trails even though there was supposedly a 30 second settling time. Not sure why this happened. Last night, the first sub after the flip also had star trails but after that they were all good.


Hi Jerry,

I can't really help with the first question, because it is hardware dependent. Even for the second question I have no solution, But I'd like to tell that you are not alone with this problem. I experience the same issue from time to time with my EQ5 and one of my EQ6s as well. The first one is quite sensitive to this kind of problem and it happens about every second flip. The EQ6 shows this very rarely. I tried several things to prevent this, but no matter what I've done, I couldn't solve the issue. I set the time to reapeat the plate solve step in NINA to 2 minutes and live with the problem.

But I have a guess where this comes from, although I don't know exactly. I have another effect in PHD2 and they seem to be related. If I manually slew the mount to a point in NINA, the tracking rate of the mount is set to a higher speed, of course. If I then start a calibration run in PHD2, the selected star jumps out of the monitoring field for the first time. The distance may be comparable to the length of the star trails in those images after the meriadian flip. I have to reselect the guiding star and start the process again. Then it works. After manual slewing (and the tracking rate is set in a meridian flip as well to fast) NINA, PHD2 or the driver has to set the tracking rate back to "siderial". I assume, that this sometimes takes a bit too long or something went wrong with this. There are no error messages and with a second try everything works. Because both issues happen after changing the tracking rate back to the normal use case, I am pretty sure it's caused by the same issue.

I haven't realized this until your question here. Maybe this helps both of us to find a solution. I don't know if this is caused by NINA, PHD2 or the ascom driver. I would assume the third option is right.

CS
Christian
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Frédéric Ruciak avatar
Frédéric Ruciak avatar
Hi, first of all I would invite you to go through the documentation of NINA which is well done and explains precisely the parameters with clear drawings.
In brief without the drawings of the documentation ;-) Francesco is right "pause before meridian" is related to the impossibility for your equipment to track safely through the meridian, if you have no problem you do not need to put something here.
Regarding the other parameters minutes after / max minutes after they should be tuned according to your exposure time to secure a "window" for nina to operate a flip without loosing time.
Regarding star trails, IMHO it is related most probably to the fact that meridian flip is reversing the motor in RA which happens only after a flip. One never experiences RA backlash in normal operation as tracking increases or decreases the RA speed but never changes the direction so the gears continue moving in the same direction. BUT after a flip, you reverse the RA motor direction and you are seeing the backlash of the RA. The tracking will come back at sidereal ONCE the backlash will be compensated by the right amount of rotation gear train in the sidereal direction. I suspect that as everything in a GEM is rotated 180° all the play even microscopic in the roller bearings, etc need the mount to start in the new direction with everything up side down to settle. To avoid to see star trails you need to setup the parameter "scope settle time after flip" according to some experiment. On EQ6R for example users (as I am) know that backlash can vary widely according to backlash tuning from several seconds to 200ms. The reasonable advice is that you better avoid gear binding with too much of backlash rather than willing to reduce backlash and trigger gear binding as binding is ruining tracking. At least for me, I know that I have some RA backlash and I give a large amount of time even 30 to 40s after meridian to let everything restart in the good direction in a night of several hours of capture ;-)
Hope this helps, Frédéric
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Francesco Meschia avatar
Francesco Meschia avatar
The GM1000 has absolute encoders, I think backlash can be ruled out safely.
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Tom Marsala avatar
Tom Marsala avatar
Great post.
I have been experiencing the trails during the flip and successive plate solve on my EQ6R since I updated to version 3.  I've never seen this before.  But great idea to expand settle time.  I will try this tonight.

And Frederick is totally right as binding ruined my session two nights ago.  I'll take the minimal RA backlash.

Tom
Timothy Martin avatar
Timothy Martin avatar
I spent a lot of time–and actually, a lot of money on a custom-machined riser bar for the TOA ($700)–here at home to prevent this exact problem. As pointed out, the possibility of a collision is really target-dependent, so you, unfortunately, do need to set it up for the worst case. I've no idea how to judge what that might be or how long a pause you would need to cover it. You may also need to account for some time after the flip to avoid the same issue on the other side of the pier. I agree that help from observatory staff on this issue is in order.

The good news is that it's very unlikely that any collision will harm your mount. 10Micron mounts are built so that the clutches will slip when they encounter resistance. And even in a collision scenario, the mount will not lose its place because of the absolute encoders. The bad news is that your camera and other gear involved in any collision may not fare as well. That's a complete unknown. 

Note that it may also be that there was no collision at all. I had a similar experience here at home with my RedCat the other night during a meridian flip. It's a target I've been shooting for a couple of weeks with no issues. And since that night, I've had no issues with the flip. It turns out that on the night in question, I just got really lucky and some thick clouds blew through my target's zenith right as the scope was flipping. I had two subs with star trails after the flip and then everything was fine.
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Jerry Gerber avatar
Jerry Gerber avatar
Chris, who is the tech at DSPR, is going to "meet" me at the scope (I'll be 900 miles away at home) and he's going to watch the scope while slewing and let me know at what altitude there's a risk of hitting the pier.   Then I can set the mount to stop x minutes before and x minutes after the meridian flip and not waste any more imaging time than necessary.

Thanks all for your comments and sharing your experiences!

Jerry
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