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Trouble With SE Mount GoTo

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#1 CynicalRat

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Posted 13 June 2024 - 12:06 AM

Hello,

I own a NexStar 8SE, and really want to use it for galaxies this year, it being an SCT and all. However, the mount has recently been acting up, particularly evident with the GoTo. The position the mount slews to is consistently off from the target by a country mile, requiring large adjustments. Now, this has happened on and off throughout my time using it, the difference now is that the errors seem completely random. In the past, no matter how far the scope was off from its target, it was always roughly the same distance away, and in the same direction, making it easy to compensate for. But tonight when I took it out to look at double stars, the scope was all over the place. When I tried to get my bearings on Spica, it appeared above-right of the target. Then, when I slewed to Mizar, it was pretty much pointing at the horizon, requiring a large adjustment upwards. I tried many experiments to try and root out the cause, different alignments, other battery, etc., but none worked. I then studied the offset in different parts of the sky, was the scope pointing in the same position relative to stars on one side of the zenith vs. another for stars on the other side? Apparently not. Obviously, this is a larger issue for galaxies and fainter objects than for double star observing, so I want to get it sorted out before driving to dark skies. Hoping to get some input here.

Possible, untested, error causes:
1.) Both batteries, despite reading full charge, somehow did not have enough juice to power the rig

2.) Attempting to point at a star close to the Zenith exceeded the pitch limit of the scope, leading it to believe it was pointing at the star when it was in fact much lower. My thought though, is that this one would produce a predictable error pattern, rather than the observed random one

3.) User error during some input of alignment parameters. I double checked everything, but could have missed something

4.) The more sinister, but much less likely, hard/software error

 

 

Thanks 

 



#2 PNW

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Posted 13 June 2024 - 12:12 AM

Ditch the AA's and get a Talentcell battery. SE mounts are voltage sensitive.


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#3 CynicalRat

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Posted 13 June 2024 - 12:15 AM

Ditch the AA's and get a Talentcell battery. SE mounts are voltage sensitive.

I have two batteries, never used AA except for the very start. One is the apertura power bank, the other is indeed a Talentcell 12V


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#4 Choccy

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Posted 13 June 2024 - 03:20 AM

Is your dovetail clamp knob pointing upwards by any chance? The SE mount does not work "upside down", I have had these exact seemingly random goto results when trying out another SCT in the mount and rotated the altitude axis 180 degrees as that particular scope's dovetail / finder position on the ota didnt suit easy usage.

other thoughts are backlash or loose alt axis, both can be adjusted, backlash in software, loose axis by mechanical adjustment behind the Nexstar controller



#5 mlord

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Posted 13 June 2024 - 06:47 AM

Possible, untested, error causes:
..

3.) User error during some input of alignment parameters. I double checked everything, but could have missed something

About 99.9% of the time, the issue is that.  Simple ways around it, include using a smartphone or tablet over WiFi/Bluetooth instead of the Celestron hand-controller, or adding a GPS module to the mount for use with the hand-controller.



#6 spongebob@55

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Posted 13 June 2024 - 09:47 AM

I have an SE and have a couple ideas.  I use both the HC and the wi-fi dongle with skysafari at times.

With the HC, I'll always use 3 star alignment.  I use Polaris as the first star, then pick 2 others, not to high or low, and evenly spaced apart.  I always use a level on the tripod, even though its my understanding that that is not needed with this method.  Always use up and to the right with each fine alignment.  

Then, again with the HC, when I want to do a fast observation, say, just the moon, I'll use the 'one solar system' alignment.  Then I really work hard, using a good level, on the tripod top.  I use a four line alignment eyepiece.   I have amazing alignment accuracy with this fast deployment method.   If you do this, then see if you get close to your other objects.   Be aware that alt/az go to systems tend not to be the best with near zenith objects.

Then you could buy a wi-fi dongle with skysafari 7.  I use plus.  That could lead you to more clues, e.g. its your HC or maybe, something different.  Just always, throughout the night, use the final approach as up and to the right, otherwise this error will build up.  It doesn't sound like its what you have, but jic.  Also check the cable connector from your battery to the plug, along with the plug contact.  I have my battery being held to the arm with a strap/velcro so there's no chance of cord wrap.

Good luck

SB


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#7 CynicalRat

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Posted 13 June 2024 - 10:40 AM

I have an SE and have a couple ideas.  I use both the HC and the wi-fi dongle with skysafari at times.

With the HC, I'll always use 3 star alignment.  I use Polaris as the first star, then pick 2 others, not to high or low, and evenly spaced apart.  I always use a level on the tripod, even though its my understanding that that is not needed with this method.  Always use up and to the right with each fine alignment.  

Then, again with the HC, when I want to do a fast observation, say, just the moon, I'll use the 'one solar system' alignment.  Then I really work hard, using a good level, on the tripod top.  I use a four line alignment eyepiece.   I have amazing alignment accuracy with this fast deployment method.   If you do this, then see if you get close to your other objects.   Be aware that alt/az go to systems tend not to be the best with near zenith objects.

Then you could buy a wi-fi dongle with skysafari 7.  I use plus.  That could lead you to more clues, e.g. its your HC or maybe, something different.  Just always, throughout the night, use the final approach as up and to the right, otherwise this error will build up.  It doesn't sound like its what you have, but jic.  Also check the cable connector from your battery to the plug, along with the plug contact.  I have my battery being held to the arm with a strap/velcro so there's no chance of cord wrap.

Good luck

SB

I have heard the "up and to the right" a lot when I started out. Does this mean to press the up and right buttons on the controller, no matter which way the scope moves? Or do you press the buttons in such a way that the scope moves up and to the right? The controls are reversed at low speeds.



#8 mlord

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Posted 13 June 2024 - 11:10 AM

I have heard the "up and to the right" a lot when I started out. Does this mean to press the up and right buttons on the controller, no matter which way the scope moves? Or do you press the buttons in such a way that the scope moves up and to the right? The controls are reversed at low speeds.

That information is in the user manual for the Nexstar+ hand-controller.  Have a read!

 

It does refer to the final movements of the skyward end of the optical tube, not necessarily which buttons one presses to achieve that.

 

More specifically, one is trying to mimic the final movements of the optical tube when it is later commanded to "Go To" an object.  Usually that is UP/RIGHT, but I have seen some set-ups when it was UP/DOWN instead.  So pay attention to how your particular gear does a "go to" operation.

 

Getting this correct will automatically compensate for the varying amounts of gear slop in the mount, giving more accurate alignment/go-to operation.


Edited by mlord, 13 June 2024 - 11:13 AM.


#9 mclewis1

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Posted 13 June 2024 - 11:26 AM

A few suggestions in the order of impact on your pointing accuracy.

 

1) consistent use of up/right to finish your alignment movements

2) picking good/appropriate alignment stars for your skies

3) do the Backlash function to help take up/compensate for some/most of the backlash in your particular scope's gears.

4) pay a bit of attention to the balance of the ota, really just make sure it's not way off. If you have a variety of eyepiece weights just approximating things will be ok. Doing your alignments with one type of eyepiece and later swapping to a heavier or much lighter eyepiece causes some scopes to increase the pointing error.



#10 Minuam

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Posted 13 June 2024 - 11:27 AM

I think it is much easier to do it during daytime.

 

One star align can be done during daytime and then pointing the scope towards some building and then see/find(in the eye-piece) the final movement and the direction of the scope moving(the skywards side) when doing the final align at the slow speed.

 

For my next star evolution, if I am not wrong then UP is UP before the slow slew speed and DOWN is DOWN(right and left remain correct). But for final star alignment, UP is DOWN(the Skywards direction of the scope) and DOWN is UP.

 

So my two final buttons for UP and RIGHT would be.............the DOWN button and the RIGHT button on the hand control(for my scope).

 

Very Best


Edited by Minuam, 13 June 2024 - 11:49 AM.


#11 mlord

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Posted 13 June 2024 - 12:29 PM

The directions of movement in response the the buttons on the hand-controller vary, depending upon settings in the hand-controller, as well as where one is in the alignment routine.  Blame Celestron for taking something that should be so simple, and making it so complex and mysterious!


Edited by mlord, 13 June 2024 - 12:29 PM.


#12 CynicalRat

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Posted 13 June 2024 - 12:48 PM

A few suggestions in the order of impact on your pointing accuracy.

 

1) consistent use of up/right to finish your alignment movements

2) picking good/appropriate alignment stars for your skies

3) do the Backlash function to help take up/compensate for some/most of the backlash in your particular scope's gears.

4) pay a bit of attention to the balance of the ota, really just make sure it's not way off. If you have a variety of eyepiece weights just approximating things will be ok. Doing your alignments with one type of eyepiece and later swapping to a heavier or much lighter eyepiece causes some scopes to increase the pointing error.

I use the same eyepiece pretty much always (Bader zoom). The balance of the OTA is something I did not consider, but shouldn't that also create a predictable pattern in the error? Nevertheless, what should the optimal position be, say, in relation to the celestron logo? 




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