Shortest Slew Path

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hatflyer
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Shortest Slew Path

#1

Post by hatflyer »


I use NINA and EQMOD, with a SW AZ-GTi.

When I slew from 1 location to another that is just 10 degrees to the west, the scope instead slews 350 to the east. How can I control the path the scope takes so that it takes the shortest and quickest way? I think the 2 targets may be on opposite sides of the Meridian, in which case, is it doing a Meridian flip? If so, can I avoid a flip by unchecking Mount Limits in EQMOD set-up?

If it matters, I'm not changing the DEC very much.

Thanks.
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KathyNS Canada
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Re: Shortest Slew Path

#2

Post by KathyNS »


Certainly it will do a meridian flip if the targets are on opposite sides of the meridian. De-selecting slew limits would prevent a flip when tracking a single object across the meridian. You cannot prevent a flip when going a goto to a new target: a goto will always send the mount to whichever side has the counterweights down. (In fact this is how software does a flip when tracking: it waits until the target is past the meridian and then just does another goto to the same coordinates. The mount knows to go to the correct side.)

Sending the mount to a counterweights-up position, which is what you get on the wrong side of the flip, is not recommended. You have to be very careful not to allow the scope to strike the tripod. The scope will get closer to the tripod as it tracks, increasing the risk of a strike with nothing to stop it.

You might want to deliberately send the mount to a counterweights-up position if an object is a short, safe distance before the meridian, which would allow it to track across the meridian without a flip. On that case, the tracking will return it to the counterweights-down orientation by itself, making it a safe thing to do.
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DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
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Re: Shortest Slew Path

#3

Post by hatflyer »


KathyNS wrote: Tue Feb 21, 2023 11:46 pm Certainly it will do a meridian flip if the targets are on opposite sides of the meridian. De-selecting slew limits would prevent a flip when tracking a single object across the meridian. You cannot prevent a flip when going a goto to a new target: a goto will always send the mount to whichever side has the counterweights down. (In fact this is how software does a flip when tracking: it waits until the target is past the meridian and then just does another goto to the same coordinates. The mount knows to go to the correct side.)

Sending the mount to a counterweights-up position, which is what you get on the wrong side of the flip, is not recommended. You have to be very careful not to allow the scope to strike the tripod. The scope will get closer to the tripod as it tracks, increasing the risk of a strike with nothing to stop it.

You might want to deliberately send the mount to a counterweights-up position if an object is a short, safe distance before the meridian, which would allow it to track across the meridian without a flip. On that case, the tracking will return it to the counterweights-down orientation by itself, making it a safe thing to do.
This is shooting Orion, from Az=170 to Az= 240. I could wait until it gets to 180? Or is the range small enough that not flipping would seem ok? Altitude is below 45 degrees.

Thanks.
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Re: Shortest Slew Path

#4

Post by KathyNS »


hatflyer wrote: Wed Feb 22, 2023 12:11 am This is shooting Orion, from Az=170 to Az= 240. I could wait until it gets to 180? Or is the range small enough that not flipping would seem ok? Altitude is below 45 degrees.

Thanks.
I am not sure now what you are trying to accomplish. I thought you had two different targets and were slewing from one to the other.

If you are finished with the first target at az=170, then you are finished with it. If the second target is at az=240, then let the mount go there. It will do it the proper way.

Not flipping is only an option when tracking a single target. And then, only for a very short time near the meridian. If you are switching targets, not flipping is not an option.
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DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
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Re: Shortest Slew Path

#5

Post by hatflyer »


KathyNS wrote: Wed Feb 22, 2023 1:43 am
hatflyer wrote: Wed Feb 22, 2023 12:11 am This is shooting Orion, from Az=170 to Az= 240. I could wait until it gets to 180? Or is the range small enough that not flipping would seem ok? Altitude is below 45 degrees.

Thanks.
I am not sure now what you are trying to accomplish. I thought you had two different targets and were slewing from one to the other.

If you are finished with the first target at az=170, then you are finished with it. If the second target is at az=240, then let the mount go there. It will do it the proper way.

Not flipping is only an option when tracking a single target. And then, only for a very short time near the meridian. If you are switching targets, not flipping is not an option.
Sorry...I usually 3-star align before the meridian due to limited skies and light pollution. Once done, then I would start tracking Orion that is also before the meridian. Over the ensuing few hours, Orion crosses the meridian. So, tracking 1 object. I was hoping to avoid slewing so far around with a flip. I have some limitations that can make that flipping an issue. I guess I will try to do my alignment with stars past the meridian and start tracking once Orion is also past the Meridian, which is only about 30 minutes anyway.

I know I am likely not being clear, but I think I get it now. Thanks for the help.
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Re: Shortest Slew Path

#6

Post by JayTee »


It sounds like waiting for Orion to get to the west side of the meridian is the correct thing to do for you.
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Re: Shortest Slew Path

#7

Post by Juno16 »


Are you plate solving in NINA?
Jim

Scopes: Explore Scientific ED102 APO, Sharpstar 61 EDPH II APO, Samyang 135 F2 (still on the Nikon).
Mount: Skywatcher HEQ5 Pro with Rowan Belt Mod
Stuff: ASI EAF Focus Motor (x2), Orion 50mm Guide Scope, ZWO 30 mm Guide Scope, ASI 220mm min, ASI 120mm mini, Stellarview 0.8 FR/FF, Sharpstar 0.8 FR/FF, Mele Overloock 3C.
Camera/Filters/Software: ASI 533 mc pro, ASI 120mm mini, Orion SSAG, IDAS LPS D-1, Optolong L-Enhance, ZWO UV/IR Cut, N.I.N.A., Green Swamp Server, PHD2, Adobe Photoshop CC, Pixinsight.
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Re: Shortest Slew Path

#8

Post by hatflyer »


Juno16 wrote: Wed Feb 22, 2023 12:33 pm Are you plate solving in NINA?
Yes I am. I enter coordinates for the first star in NINA and it slews to it.
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Re: Shortest Slew Path

#9

Post by Juno16 »


Do you have the sync switch on?
Jim

Scopes: Explore Scientific ED102 APO, Sharpstar 61 EDPH II APO, Samyang 135 F2 (still on the Nikon).
Mount: Skywatcher HEQ5 Pro with Rowan Belt Mod
Stuff: ASI EAF Focus Motor (x2), Orion 50mm Guide Scope, ZWO 30 mm Guide Scope, ASI 220mm min, ASI 120mm mini, Stellarview 0.8 FR/FF, Sharpstar 0.8 FR/FF, Mele Overloock 3C.
Camera/Filters/Software: ASI 533 mc pro, ASI 120mm mini, Orion SSAG, IDAS LPS D-1, Optolong L-Enhance, ZWO UV/IR Cut, N.I.N.A., Green Swamp Server, PHD2, Adobe Photoshop CC, Pixinsight.
Dog and best bud: Jack
Sky: Bortle 6-7
My Astrobin: https://www.astrobin.com/users/Juno16/
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Re: Shortest Slew Path

#10

Post by hatflyer »


Juno16 wrote: Wed Feb 22, 2023 3:16 pm Do you have the sync switch on?

Not sure. Where is it, and what does it do?
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Re: Shortest Slew Path

#11

Post by Juno16 »


hatflyer wrote: Wed Feb 22, 2023 3:52 pm
Juno16 wrote: Wed Feb 22, 2023 3:16 pm Do you have the sync switch on?

Not sure. Where is it, and what does it do?
The Sync switch is on Imaging - Plate Solving tab. Sync synchronizes NINA with the mount.

Just curious. Why do you do a star alignment at all?
Assuming NINA is set up (equipment information, location, etc), you can set-up a target using the Sky Atlas. Then send it to Framing (if custom Framing is desired). From Framing, add the target to a sequence and set the sequence Target options to Slew to Target and Center. That way, you will not to do any alignment at all. NINA will send the scope to where it thinks the target is then plate solve (with sync selected). Plate solving will adjust the mount to the target (based on the plate solve), then sync the mount to the target. The sequence can then be set to autofocus (if used) and then start imaging. All in one click.
I don't have the best cable arrangement, so when my scope just passes the meridian, I stop the sequence and send the scope to the Home position. I reset the sequence and start it again. NINA slews the scope the target (on the west side on the meridian) and plate solves, centers, focuses, and starts imaging again. Takes about one minute and I can watch for cable snags while the scope is slewing.
Hope that this helps.
Jim

Scopes: Explore Scientific ED102 APO, Sharpstar 61 EDPH II APO, Samyang 135 F2 (still on the Nikon).
Mount: Skywatcher HEQ5 Pro with Rowan Belt Mod
Stuff: ASI EAF Focus Motor (x2), Orion 50mm Guide Scope, ZWO 30 mm Guide Scope, ASI 220mm min, ASI 120mm mini, Stellarview 0.8 FR/FF, Sharpstar 0.8 FR/FF, Mele Overloock 3C.
Camera/Filters/Software: ASI 533 mc pro, ASI 120mm mini, Orion SSAG, IDAS LPS D-1, Optolong L-Enhance, ZWO UV/IR Cut, N.I.N.A., Green Swamp Server, PHD2, Adobe Photoshop CC, Pixinsight.
Dog and best bud: Jack
Sky: Bortle 6-7
My Astrobin: https://www.astrobin.com/users/Juno16/
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Re: Shortest Slew Path

#12

Post by hatflyer »


Juno16 wrote: Wed Feb 22, 2023 4:20 pm
hatflyer wrote: Wed Feb 22, 2023 3:52 pm
Juno16 wrote: Wed Feb 22, 2023 3:16 pm Do you have the sync switch on?

Not sure. Where is it, and what does it do?
The Sync switch is on Imaging - Plate Solving tab. Sync synchronizes NINA with the mount.

Just curious. Why do you do a star alignment at all?
Assuming NINA is set up (equipment information, location, etc), you can set-up a target using the Sky Atlas. Then send it to Framing (if custom Framing is desired). From Framing, add the target to a sequence and set the sequence Target options to Slew to Target and Center. That way, you will not to do any alignment at all. NINA will send the scope to where it thinks the target is then plate solve (with sync selected). Plate solving will adjust the mount to the target (based on the plate solve), then sync the mount to the target. The sequence can then be set to autofocus (if used) and then start imaging. All in one click.
I don't have the best cable arrangement, so when my scope just passes the meridian, I stop the sequence and send the scope to the Home position. I reset the sequence and start it again. NINA slews the scope the target (on the west side on the meridian) and plate solves, centers, focuses, and starts imaging again. Takes about one minute and I can watch for cable snags while the scope is slewing.
Hope that this helps.
No alignment needed? I never heard of that. If u just slew to target and center, u are aimed correctly, a single plate solve is enough? No need to iterate through 3 point PA and ajusting the DEC and RA until the error is very small? Sounds too good to be true. ;)
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Re: Shortest Slew Path

#13

Post by Juno16 »


Absolutely! A single plate solve is enough. That is set in my sequence as the "Center" selection.
If you are imaging a target, the only point in the sky that you need to have the mount accurately pointing to and tracking is that target. There is no better way of centering a target than plate solving. Of course, the sync is part of it.

I haven't done a star alignment in years (many years). That is the way this type of software is intended to be used (NINA, APT, and I am sure that SGP and others too).

What method do you use for polar alignment? The NINA plugin Three Point Polar Alignment works extremely well.
Jim

Scopes: Explore Scientific ED102 APO, Sharpstar 61 EDPH II APO, Samyang 135 F2 (still on the Nikon).
Mount: Skywatcher HEQ5 Pro with Rowan Belt Mod
Stuff: ASI EAF Focus Motor (x2), Orion 50mm Guide Scope, ZWO 30 mm Guide Scope, ASI 220mm min, ASI 120mm mini, Stellarview 0.8 FR/FF, Sharpstar 0.8 FR/FF, Mele Overloock 3C.
Camera/Filters/Software: ASI 533 mc pro, ASI 120mm mini, Orion SSAG, IDAS LPS D-1, Optolong L-Enhance, ZWO UV/IR Cut, N.I.N.A., Green Swamp Server, PHD2, Adobe Photoshop CC, Pixinsight.
Dog and best bud: Jack
Sky: Bortle 6-7
My Astrobin: https://www.astrobin.com/users/Juno16/
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Re: Shortest Slew Path

#14

Post by hatflyer »


Juno16 wrote: Wed Feb 22, 2023 5:08 pm Absolutely! A single plate solve is enough. That is set in my sequence as the "Center" selection.
If you are imaging a target, the only point in the sky that you need to have the mount accurately pointing to and tracking is that target. There is no better way of centering a target than plate solving. Of course, the sync is part of it.

I haven't done a star alignment in years (many years). That is the way this type of software is intended to be used (NINA, APT, and I am sure that SGP and others too).

What method do you use for polar alignment? The NINA plugin Three Point Polar Alignment works extremely well.

Yeah, I use NINA 3 point PA. It can take 15+ minutes to go through making small adjustments on the base wedge to get the alignment error < 1'. So why is there 3PPA at all?
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Re: Shortest Slew Path

#15

Post by Juno16 »


Polar alignment is aligning the mount to the celestial North Pole. Accurate PA is necessary for accurate tracking. I sometimes go with up to 3 arcmin and let guiding take care of it.
Jim

Scopes: Explore Scientific ED102 APO, Sharpstar 61 EDPH II APO, Samyang 135 F2 (still on the Nikon).
Mount: Skywatcher HEQ5 Pro with Rowan Belt Mod
Stuff: ASI EAF Focus Motor (x2), Orion 50mm Guide Scope, ZWO 30 mm Guide Scope, ASI 220mm min, ASI 120mm mini, Stellarview 0.8 FR/FF, Sharpstar 0.8 FR/FF, Mele Overloock 3C.
Camera/Filters/Software: ASI 533 mc pro, ASI 120mm mini, Orion SSAG, IDAS LPS D-1, Optolong L-Enhance, ZWO UV/IR Cut, N.I.N.A., Green Swamp Server, PHD2, Adobe Photoshop CC, Pixinsight.
Dog and best bud: Jack
Sky: Bortle 6-7
My Astrobin: https://www.astrobin.com/users/Juno16/
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Re: Shortest Slew Path

#16

Post by hatflyer »


Juno16 wrote: Wed Feb 22, 2023 6:19 pm Polar alignment is aligning the mount to the celestial North Pole. Accurate PA is necessary for accurate tracking. I sometimes go with up to 3 arcmin and let guiding take care of it.
Ok, I'm confused. ;) Don't u have to use 3PPA to PA? You made it seem like you just slew to target, center and solve and sync and that's it.
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Re: Shortest Slew Path

#17

Post by Juno16 »


hatflyer wrote: Wed Feb 22, 2023 6:23 pm
Juno16 wrote: Wed Feb 22, 2023 6:19 pm Polar alignment is aligning the mount to the celestial North Pole. Accurate PA is necessary for accurate tracking. I sometimes go with up to 3 arcmin and let guiding take care of it.
Ok, I'm confused. ;) Don't u have to use 3PPA to PA? You made it seem like you just slew to target, center and solve and sync and that's it.

You must polar align prior to slewing to a target.
Polar alignment is totally different from aligning your scope to targets in the sky.
I assumed that PA’ed before you did a star alignment before slewing to a target for imaging.
Jim

Scopes: Explore Scientific ED102 APO, Sharpstar 61 EDPH II APO, Samyang 135 F2 (still on the Nikon).
Mount: Skywatcher HEQ5 Pro with Rowan Belt Mod
Stuff: ASI EAF Focus Motor (x2), Orion 50mm Guide Scope, ZWO 30 mm Guide Scope, ASI 220mm min, ASI 120mm mini, Stellarview 0.8 FR/FF, Sharpstar 0.8 FR/FF, Mele Overloock 3C.
Camera/Filters/Software: ASI 533 mc pro, ASI 120mm mini, Orion SSAG, IDAS LPS D-1, Optolong L-Enhance, ZWO UV/IR Cut, N.I.N.A., Green Swamp Server, PHD2, Adobe Photoshop CC, Pixinsight.
Dog and best bud: Jack
Sky: Bortle 6-7
My Astrobin: https://www.astrobin.com/users/Juno16/
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Re: Shortest Slew Path

#18

Post by hatflyer »


Juno16 wrote: Wed Feb 22, 2023 6:28 pm
hatflyer wrote: Wed Feb 22, 2023 6:23 pm
Juno16 wrote: Wed Feb 22, 2023 6:19 pm Polar alignment is aligning the mount to the celestial North Pole. Accurate PA is necessary for accurate tracking. I sometimes go with up to 3 arcmin and let guiding take care of it.
Ok, I'm confused. ;) Don't u have to use 3PPA to PA? You made it seem like you just slew to target, center and solve and sync and that's it.

You must polar align prior to slewing to a target.
Polar alignment is totally different from aligning your scope to targets in the sky.
I assumed that PA’ed before you did a star alignment before slewing to a target for imaging.
Ok, got it. I do do the slew to target and pain-in the-butt plate solve step after 3PPA. ;)
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Re: Shortest Slew Path

#19

Post by Juno16 »


hatflyer wrote: Wed Feb 22, 2023 6:34 pm
Juno16 wrote: Wed Feb 22, 2023 6:28 pm
hatflyer wrote: Wed Feb 22, 2023 6:23 pm

Ok, I'm confused. ;) Don't u have to use 3PPA to PA? You made it seem like you just slew to target, center and solve and sync and that's it.

You must polar align prior to slewing to a target.
Polar alignment is totally different from aligning your scope to targets in the sky.
I assumed that PA’ed before you did a star alignment before slewing to a target for imaging.
Ok, got it. I do do the slew to target and pain-in the-butt plate solve step after 3PPA. ;)
Sure. There is no way around the polar alignment. Heck, the NINA TPPA is really quick especially compared to drift aligning!
After that though, you can set up a sequence to handle the slew to target, plate solve to center the target, then off to firing away. Good luck!
Jim

Scopes: Explore Scientific ED102 APO, Sharpstar 61 EDPH II APO, Samyang 135 F2 (still on the Nikon).
Mount: Skywatcher HEQ5 Pro with Rowan Belt Mod
Stuff: ASI EAF Focus Motor (x2), Orion 50mm Guide Scope, ZWO 30 mm Guide Scope, ASI 220mm min, ASI 120mm mini, Stellarview 0.8 FR/FF, Sharpstar 0.8 FR/FF, Mele Overloock 3C.
Camera/Filters/Software: ASI 533 mc pro, ASI 120mm mini, Orion SSAG, IDAS LPS D-1, Optolong L-Enhance, ZWO UV/IR Cut, N.I.N.A., Green Swamp Server, PHD2, Adobe Photoshop CC, Pixinsight.
Dog and best bud: Jack
Sky: Bortle 6-7
My Astrobin: https://www.astrobin.com/users/Juno16/
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KathyNS Canada
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Re: Shortest Slew Path

#20

Post by KathyNS »


hatflyer wrote: Wed Feb 22, 2023 4:45 pm No alignment needed? I never heard of that. If u just slew to target and center, u are aimed correctly, a single plate solve is enough? No need to iterate through 3 point PA and ajusting the DEC and RA until the error is very small? Sounds too good to be true. ;)
You absolutely do need to polar align. If your preferred technique is one that requires a 3-point star alignment first, then you still have to do that. What you don't have to do if you are plate-solving is the 3-star goto alignment that is done after polar alignment.
Image
DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
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