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An AVX Curiosity

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#1 desertstars

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Posted 09 May 2021 - 06:36 PM

In 2019 I upgraded the Three-legged Newt to a Celestron AVX mount. While the alignment process ultimately proved easy enough to learn, I never seemed to achieve especially impressive accuracy when using that alignment to locate objects. Selected objects (including calibration stars) were consistently at the very edge of the field of view (using a finder eyepiece with a 1.4 degree TFOV) I've done enough polar alignments for visual observing to discount polar alignment as the source of the this displacement. A minor annoyance, but an annoyance all the same.

 

Because I use the Newt in more than one location, I decided to purchase a Celestron SkySync GPS device and let technology handle a couple of set-up and alignment chores - time and place. (Yes, I'm that lazy. wink.gif ) The persistent displacement during alignment and calibration was immediately reduced to an amount I now consider trivial. Selected objects for observation land well within that 1.4 degree TFOV, requiring a modest tweak at most to center the object. I've done nothing else different, so feel safe assuming cause-and-effect here.

 

So, I'm assuming the use of the SkySync brought this about. Am I off base thinking that my distance from the standard meridian for Mountain Standard Time (105 degrees west for MST, 110.98 W. for my location in Tucson) is sufficient to create the displacement I first saw? And that a GPS fix for where (and when) I am when I observe did, in fact, provide a measure of correction?

 

The kind of stuff you don't absolutely need to know to use the gear, but can't help wondering about. 



#2 ButterFly

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Posted 09 May 2021 - 07:18 PM

Time, location, and polar alignmnet are irrelevant for star alignment.  Time and locaion matter for planets.  Polar alignment matters for tracking.  Whatever it was, it was something else.

 

You should be aiming for less than 15 minutes of error.  Entirely reasonable with the AVX if you aim at the correct stars and nothing slips.  Check the tightness of your clutches.


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#3 WadeH237

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Posted 10 May 2021 - 10:57 AM

If I had to guess, I would say that SkySync is doing a better job of aligning to the sky.

 

The first thing that comes to mind for me, is that there is a technique to get the stars centered correctly for alignment.  Specifically, you need to always to the final centering of the alignment and calibration stars using only the "up" and "right" keys on the hand controller.  When I align a NexStar mount, I always first center the star in the eyepiece, and then I use the "down" and "left" keys to push it to the edge of the field.  I can then use only "up" and "right" to center it.  This ensures that any backlash (and the AVX has quite a bit) is always taken up in the same way when you center the star.

 

If you don't center the stars that way, pointing accuracy will suffer.


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#4 desertstars

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Posted 10 May 2021 - 02:03 PM

If I had to guess, I would say that SkySync is doing a better job of aligning to the sky.

 

The first thing that comes to mind for me, is that there is a technique to get the stars centered correctly for alignment.  Specifically, you need to always to the final centering of the alignment and calibration stars using only the "up" and "right" keys on the hand controller.  When I align a NexStar mount, I always first center the star in the eyepiece, and then I use the "down" and "left" keys to push it to the edge of the field.  I can then use only "up" and "right" to center it.  This ensures that any backlash (and the AVX has quite a bit) is always taken up in the same way when you center the star.

 

If you don't center the stars that way, pointing accuracy will suffer.

That trick was explained to me when I bought the mount at Starizona. It's SOP when I use the mount. cool.gif


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#5 KJL

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Posted 10 May 2021 - 02:21 PM

That trick was explained to me when I bought the mount at Starizona. It's SOP when I use the mount. cool.gif

And, besides, SkySync doesn't do alignment, just time and location.

Out of curiosity, how did you enter your location into your AVX prior to acquiring the SkySync?


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#6 desertstars

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Posted 10 May 2021 - 03:49 PM

 

And, besides, SkySync doesn't do alignment, just time and location.

Correct. There are numerous one star reviews out there for this devise because people somehow don't get this. Go figure. I thought the product description was clear enough on the matter. shrug.gif

 

 

Out of curiosity, how did you enter your location into your AVX prior to acquiring the SkySync?

When it was set up for me, local time from someone's watch was used in the store, and I believe a location was selected from a menu. (I may not be remembering the source correctly.) The location was a nearby Air Force base. In learning how to use the mount, I redid the location using coordinates for my home obtained via Google Maps and used local time displayed on the laptop I sometimes use for astronomical purposes. Did the same to change the location on the first trip to a dark sky site, half an hour north of home. I had comparable results in all three cases.



#7 KJL

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Posted 10 May 2021 - 04:28 PM

When it was set up for me, local time from someone's watch was used in the store, and I believe a location was selected from a menu. (I may not be remembering the source correctly.) The location was a nearby Air Force base. In learning how to use the mount, I redid the location using coordinates for my home obtained via Google Maps and used local time displayed on the laptop I sometimes use for astronomical purposes. Did the same to change the location on the first trip to a dark sky site, half an hour north of home. I had comparable results in all three cases.

And in all three cases, you then used something like the Two Star Align for the calibration? Not, in particular, Quick-Align (which doesn't use stars at all).

In other words, even though you had centered the calibration stars during whichever alignment procedure you chose, the scope would still put objects to one side of the 1.4-degree FOV?


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#8 desertstars

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Posted 10 May 2021 - 06:26 PM

And in all three cases, you then used something like the Two Star Align for the calibration? Not, in particular, Quick-Align (which doesn't use stars at all).

In other words, even though you had centered the calibration stars during whichever alignment procedure you chose, the scope would still put objects to one side of the 1.4-degree FOV?

Yes. Two star alignment, with two calibration stars. I also made sure - having been advised to do so - that there were stars both east and west of the meridian used in the process. The results were consistent, with the objects invariably turning up just inside the described FOV. I accepted it because I was still getting where I needed to be, and tracking seemed unaffected.

 

In an earlier conversation on the forum, someone even told me this was simply to be expected. I found that odd, quite frankly. But I was able to work with situation, so I let it go at the time.


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#9 WadeH237

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Posted 10 May 2021 - 06:37 PM

And, besides, SkySync doesn't do alignment, just time and location.

Sorry, my eyes read "SkySync" and my brain registered "StarSense"...

 

I can't offer any explanation for improved pointing with it.  The location and date/time information is used for pointing at the first two alignment stars, and also for pointing at the sun, moon and planets.  Pointing at deep sky objects is only dependent on the alignment and calibration stars.  The time and location could be completely wrong, but if you do the 2+4 correctly (understanding that you may need a big slew to hit the first alignment star), it will still point fine at deep sky objects.


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#10 desertstars

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Posted 10 May 2021 - 06:45 PM

Sorry, my eyes read "SkySync" and my brain registered "StarSense"...

 

I do that all the time, when reading a post here or on Facebook. smirk.gif  

 

Of course, this matter isn't exactly a worrisome situation, a problem to be solved. I'm just at a loss to explain why the change I experienced occurred at all. My hypothesis does not seem to being holding up all that well.  



#11 KJL

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Posted 10 May 2021 - 06:46 PM

A mystery.

 

By any chance is your AVX up-to-date? Perhaps there was a location/time bug in the firmware that was papered-over when you started using the SkySync?

 

For example, do you still have the issue when you remove the SkySync?


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#12 desertstars

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Posted 10 May 2021 - 06:50 PM

A mystery.

 

By any chance is your AVX up-to-date? Perhaps there was a location/time bug in the firmware that was papered-over when you started using the SkySync?

 

For example, do you still have the issue when you remove the SkySync?

The last time I checked Celestron, no new firmware updates were waiting for me. I have not, however, used the AVX without the SkySync installed since I bought the device. I'll have to run the AVX without it next time out and see what happens. Next time out will be Thursday night, if the weather cooperates.


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#13 Wildetelescope

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Posted 10 May 2021 - 07:15 PM

The last time I checked Celestron, no new firmware updates were waiting for me. I have not, however, used the AVX without the SkySync installed since I bought the device. I'll have to run the AVX without it next time out and see what happens. Next time out will be Thursday night, if the weather cooperates.

betting your time is more accurate than reading off the computer and entering it.   Simplest explanation I can think of.  In any event, you are rocking and rolling now!  Have fun!

 

JMD



#14 mikewayne3

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Posted 12 May 2021 - 11:59 AM

I have a question

Why don't you do a full 2+4 star calibration?

I love my AVX but I always do a 2+4 star alignment and with each star the pointing get better.

By the 3rd cal. Star it's close to center and after all 4 my goto's are dead on.

Mike


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#15 desertstars

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Posted 12 May 2021 - 04:31 PM

I have a question

Why don't you do a full 2+4 star calibration?

I love my AVX but I always do a 2+4 star alignment and with each star the pointing get better.

By the 3rd cal. Star it's close to center and after all 4 my goto's are dead on.

Mike

I know I used more than 2 calibration stars early on, but I don't recall that it made much difference. I left it at 2+2 when I saw the change in results described above. I was getting objects in the field of view, after all. Even improved as things are, goto results have never been "dead on." I'll have the scope out under dark skies tomorrow night, and will do a 2+4 to see what happens. 



#16 mikewayne3

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Posted 12 May 2021 - 05:19 PM

Just to let you know.

First I do polar alignment with a Meade DSI then I switch to a DSLR, focus the camera and then do the 2+4 star alignment and by the 2nd calibration star I don't need to look through the finder scope and so on good luck tomorrow.

Mike



#17 KJL

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Posted 12 May 2021 - 09:43 PM

I know I used more than 2 calibration stars early on, but I don't recall that it made much difference. I left it at 2+2 when I saw the change in results described above. I was getting objects in the field of view, after all. Even improved as things are, goto results have never been "dead on." I'll have the scope out under dark skies tomorrow night, and will do a 2+4 to see what happens. 

I look forward to the results of both 2+4 and your original method of 2+2 with SkySync out of the loop.

An interesting puzzle.



#18 ButterFly

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Posted 13 May 2021 - 07:00 AM

I know I used more than 2 calibration stars early on, but I don't recall that it made much difference. I left it at 2+2 when I saw the change in results described above. I was getting objects in the field of view, after all. Even improved as things are, goto results have never been "dead on." I'll have the scope out under dark skies tomorrow night, and will do a 2+4 to see what happens. 

If it persists, do a factory reset from the handcontroller.  I've had that solve quite a few mysteries with the AVX.  You have the GPS unit anyway, so it's not too much of a hassle.  Backup you PEC curve, if any, just in case.



#19 desertstars

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Posted 15 May 2021 - 03:37 PM

Had this rig out under dark skies a couple of days ago, and did a couple of experiment.

 

The usual 2 star + 2 calibration stars definitely works better with the SkySync module active. Targets sought after alignment land about a third of the way from the center to the edge of the FOV with the module, and very near the edge without it.

 

I obtained moderately improved results repeating the experiment with a 2+4 alignment. The targets were never completely centered, but the improvement was such that I'll go with the 2x4 from now on.

 

As for other bits of advice, bear in mind that the polar alignment I do involves eyeballs and a compass. Being a visual observer, rather than an imager, I've never bothered with anything more precise.

 

By that point in the evening it was dark enough for serious observing, so I never got around to a factory reset. Perhaps next time out.

 

"Curiouser and curiouser!"



#20 ButterFly

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Posted 15 May 2021 - 08:07 PM

 

As for other bits of advice, bear in mind that the polar alignment I do involves eyeballs and a compass. Being a visual observer, rather than an imager, I've never bothered with anything more precise.

Same here.  I just plop it down Northishly, then use the latitude scale.  It gets me tracking for a few minutes in the eyepiece, so it's fine.  But again, the star alignment knows how far off your polar alignment is, and takes that into account when pointing.  You can read off the measured polar alignment error and the local sidereal time, after a star alignment.  You can also use that to polar align the mount after a star alignment.  The better your star alignment, the better that polar alignment can get.

 

In practice, if I'm several degrees off on both axes, I'll bother to fix it.  One or two degrees on each is managable at the eyepiece.  Go through the handcontroller's all-star polar alignment just once to see what's involved.  Then decide for yourself what sort of errors are worth your trouble to fix.  The longer the focal length of the scope, the more it becomes worth it.


Edited by ButterFly, 15 May 2021 - 08:08 PM.


#21 desertstars

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Posted 16 May 2021 - 08:35 AM

In practice, if I'm several degrees off on both axes, I'll bother to fix it.  One or two degrees on each is managable at the eyepiece.  Go through the handcontroller's all-star polar alignment just once to see what's involved.  Then decide for yourself what sort of errors are worth your trouble to fix.  The longer the focal length of the scope, the more it becomes worth it.

That's really the bottom line for me. The results I'm getting are a little better than good enough, so I'm satisfied. Just curious about the way things work, and why. Sometimes there's an "Oh, of course!" answer to these things. Others, you just get back to observing. 


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#22 drt

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Posted 20 May 2021 - 10:06 AM

Does anyone used Meade LX85 AC power adapter with Celestron AVX?



#23 DuncanM

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Posted 20 May 2021 - 11:37 AM

In 2019 I upgraded the Three-legged Newt to a Celestron AVX mount. While the alignment process ultimately proved easy enough to learn, I never seemed to achieve especially impressive accuracy when using that alignment to locate objects. Selected objects (including calibration stars) were consistently at the very edge of the field of view (using a finder eyepiece with a 1.4 degree TFOV) I've done enough polar alignments for visual observing to discount polar alignment as the source of the this displacement. A minor annoyance, but an annoyance all the same.

 

Because I use the Newt in more than one location, I decided to purchase a Celestron SkySync GPS device and let technology handle a couple of set-up and alignment chores - time and place. (Yes, I'm that lazy. wink.gif ) The persistent displacement during alignment and calibration was immediately reduced to an amount I now consider trivial. Selected objects for observation land well within that 1.4 degree TFOV, requiring a modest tweak at most to center the object. I've done nothing else different, so feel safe assuming cause-and-effect here.

 

So, I'm assuming the use of the SkySync brought this about. Am I off base thinking that my distance from the standard meridian for Mountain Standard Time (105 degrees west for MST, 110.98 W. for my location in Tucson) is sufficient to create the displacement I first saw? And that a GPS fix for where (and when) I am when I observe did, in fact, provide a measure of correction?

 

The kind of stuff you don't absolutely need to know to use the gear, but can't help wondering about. 

It's essential to use a crosshair EP to precisely centre each of the 2+4 alignment stars. Accurate PA is not required for good pointing accuracy. Here's what's possible:

 

https://www.cloudyni...t/#entry7099140



#24 WadeH237

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Posted 20 May 2021 - 02:07 PM

It's essential to use a crosshair EP to precisely centre each of the 2+4 alignment stars.

I don't use an illuminated reticule eyepiece.

 

I just defocus the alignment or calibration star so that it's a big donut (or big circle, in the case of an unobstructed scope).  If the star is defocused enough to fill a significant part of the field of view, then it's pretty easy to center it.  The key is that your eye is much better at detecting concentric circles (the circular star, centered in the eyepiece's field stop) than it is at detecting whether a pinpoint is centered in a large field.

 

Certainly, there is nothing wrong with using a reticule eyepiece.  It's just not necessary to buy one solely to align the mount.


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