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Incorrect scope slewing when selecting an imaging target

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#1 Chmielski

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Posted 26 March 2020 - 12:52 AM

Last weekend while starting an imaging session, I used Starry Night software to select a target (M86 in Markarian's Chain) and slew the scope.  When finished, Starry night placed the scope icon right on target, yet when I took an image, M86 was no where to be found.  This has happened before and I work around it by running a blind plate solve, syncing the scope and slewing again. Usually this puts the target dead-center in the image.  However; I was having problems with the plate solve, and just couldn't get it to point to M86.  The interesting part was that I could target Polaris just fine, and when I tried M86 again it would slew to the exact same incorrect spot as before (I could tell because there were two tiny galaxies in the image), so whatever was going on was repeatable.  I'm pretty new at this, so it's probably something bone-headed on my part, but I would appreciate any advice on what would cause this behavior.  I'm imaging with a Stellarvue sv80 on a CGEM DX mount, using MaximDL for scope control, Starry Night for positioning. and a Polemaster for alignment.  

 

Thank you for the help!



#2 einarin

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Posted 26 March 2020 - 01:59 AM

After polar align you have to do 1,2 or 3 star align to get better accuracy of target slewing.

Alternative is plate solving which works.



#3 ThatsMyCoffee

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Posted 26 March 2020 - 11:03 AM

Sounds like the mount just wasn't aligned correctly.  Write how you're doing that and maybe somebody can help.



#4 kathyastro

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Posted 26 March 2020 - 11:33 AM

You either need a very good goto alignment or you need to plate solve.  If your plate solving isn't working, then the goto alignment is required. 

 

After doing the alignment, pick some identifiable stars new your target, centre them and sync to them.  If you have three synced stars in a triangle surrounding the target, your slew to the target will be very accurate. 

 

If you can identify your target in an eyepiece, centre it, sync to it, then install the camera and focus it.  If you are using a Bahtinov mask to focus, you can slew away to a bright star, do the focus, and slew back to the target.  Because you synced the goto to the target, it will be centred and you are ready to start imaging.



#5 Chmielski

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Posted 26 March 2020 - 01:43 PM

Thanks everyone for the advice, here's my process:

 

1.  Level, balance, polar align with the Polemaster

2.  I do a "Quick Align" with the Nexstar controller

3.  I take an exposure at the scope's home position plate solve, then sync the position to Starry Night

4.  Select a target in Starry night, and slew the scope.  

5.  Usually after step 4, I do a blind plate solve, resync and select the target again.  This usually gets me to where I want to be provided the plate solving works.

 

I'll give a 2-3 star alignment a shot tonight and see if that works better!



#6 kathyastro

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Posted 26 March 2020 - 02:01 PM

I am not sure there is anything to be gained by plate-solving the home position.

 

Why a blind solve at the target?  Since you have the putative coordinates, a non-blind solve will be much faster.


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#7 Chmielski

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Posted 27 March 2020 - 11:39 PM

I am not sure there is anything to be gained by plate-solving the home position.

 

Why a blind solve at the target?  Since you have the putative coordinates, a non-blind solve will be much faster.

That's the kicker -- after selecting a target and slewing, the scope often doesn't end up at the target I selected.  It's close, but still far enough off that I need to do a blind plate solve, (it's usually too far away for a non-blind solve to work) resync, and select the target and slew again.  After this second slew, it's usually right where it's supposed to be.  The weird thing I noticed last weekend, was that it kept going back  to the same wrong spot on the first slew.  If I selected M86 as the target, it would end up a ways north of that on the slew, but if I moved away, then back to M86, it ended up at the same spot, a little ways north of M86.  I was looking forward to trying a better alignment tonight, but alas, it's cloudy out.  D'oh! 



#8 kathyastro

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Posted 28 March 2020 - 06:58 AM

That's the kicker -- after selecting a target and slewing, the scope often doesn't end up at the target I selected.  It's close, but still far enough off that I need to do a blind plate solve, (it's usually too far away for a non-blind solve to work) resync, and select the target and slew again.  After this second slew, it's usually right where it's supposed to be.  The weird thing I noticed last weekend, was that it kept going back  to the same wrong spot on the first slew.  If I selected M86 as the target, it would end up a ways north of that on the slew, but if I moved away, then back to M86, it ended up at the same spot, a little ways north of M86.  I was looking forward to trying a better alignment tonight, but alas, it's cloudy out.  D'oh! 

So this is not a random error: your goto system is being consistent.  In other words, its sky map does not match the actual sky.  By definition, that means that your goto alignment is off. 

 

Your task should be to find out why it is off and to fix it.  It is possible that it is a hardware failure.  Possible, but unlikely.  More probable is some systematic error in the setup, typically an incorrect date, time, latitude or longitude.

 

An workaround if you still want to image before you troubleshoot your goto alignment is to sync the mount to known targets.  So if you slew to M86 and it ends up a distance away from the target, manually slew to get the target centred, then sync the mount.  That adds an extra data point to the mount's sky map.  Instead of the original three from your alignment, you now have four data points.  It doesn't alter the fact that the original three points are wrong, but it stretches the map so that the synced points are right.


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