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Suddenly the alt in AVX GoTo is off

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#1 JP-Astro

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Posted 17 August 2019 - 09:22 PM

I just started using my AVX with the Celestron C8. This is basically still a new mount with less than 4 nights of use.

I'm in an apartment and I do 'Quick Align'. For the first 2 nights stars were centered nicely in the 25 mm EP with just a minor error but on the third night some noticeable error crippled in for no apparent reason and now a star when requested by name is above the optical axis of the telescope and out of the 25 mm EP view field. I checked everything meticulously but the error persists. In other words the requested location is now always undershot in alt. In terms of the finder it's not off by too much, maybe 1/4th of the view field radius off center.

 

- I reset the HC to Factory Defaults (although I didn't play with any advanced settings).

- I went through the complete setup and entered correct data

- I checked the mount's latitude angle setting and it's spot on 60 deg

- I checked the OTA balance and did my best to set it to near perfect

Nothing helps...

The mount works without any suspicious behavior as far as I can tell. I have a new 12 v 5 Amp power supply that I bought for the mount.

 

Sorry if my terminology is lame, I'm currently getting used to and learning it.


Edited by JP-Astro, 18 August 2019 - 06:55 AM.


#2 TelescopeGreg

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Posted 18 August 2019 - 12:15 AM

"Quick Align", I believe, just assumes the mount is aligned properly to the North Celestial Pole, and that the mount is level, and that it's started exactly on the index marks.  My guess is that you're close, but not exactly there.  This can be due to a number of possible causes, but if you take a step back, there are actually two kinds of alignments that you need to do:  Polar Alignment, and Sky Alignment.

 

So the first thing, before you even turn on the mount, you need to get the axis of the scope aimed exactly at the NCP.  It's near (not exactly at) the star Polaris.  With the overall telescope aimed north, loosen the upper clamp and turn the telescope to the side (90 degrees).  Remove the cap in the front, and unscrew the cover in the back, and you'll see a hole that runs up the middle of the mount.  That hole needs to be aimed at the NCP.  Normally a Polar Alignment Scope would go in there, but I'm assuming you don't have one yet.  For now, try to adjust the knobs on the mount to put Polaris in the middle of the hole, as you sight through it.  (In reality you need to be more accurate than that, but this should get you close.)  If you have a smartphone, there are a number of apps dedicated to helping with a Polar Alignment, and will show you where Polaris should be, such that the NCP ends up in the exact center.

 

Since none of the positioning is exactly correct (microns count in this hobby), you still need to give the scope a dose of reality as to where things actually are in the sky.  That's the second alignment.  Once you do the Quick Align, tell the scope to go to a bright star you recognize in the east or west, about half way up in the sky.  It's going to miss, as you noted.  Use the hand controller's arrow buttons to move the scope so it's pointing exactly at that star (centered in the eyepiece), then go to the "Align" button and set it as an alignment star.  Do that again with another star on the other side of the sky.  That lets the hand controller update its model of where things are, and now a goto operation should be a lot more accurate.  If you have a smartphone, find one of the several night sky apps that let you aim your phone at the sky, and it shows you what you are looking at.  I'm using "SkEye" on my Android phone, but there are several others.  (Essentially what you're doing in this is a "Two Star Align", but you pick the stars that you recognize, instead of the telescope picking stars that you don't.)

 

Best is to do this yet a few more times, adding those stars in as "Calibration" stars, under the Align button's menu.  Two alignment stars, and 2-4 calibration stars should get you a goto that lands pretty much dead center in the eyepiece.  It does take a little practice, but in time you can do this pretty quickly.

 

A challenge in this is determining where the "center" of an eyepiece is.  If you can, get an Illuminated Reticle eyepiece, and use it's cross-hairs to exactly position the star. 

 

It helps to use the Motor speed control ("9" on the hand controller) when moving the scope by the hand controller.  I use speed 7 for initially moving the scope, and speed 4 when fine-tuning it through the eyepiece.

 

Another thing I recommend is to get a "Telrad" finder.  It's a lot easier to use than the magnified one that comes with the telescope.  A Polar Alignment Scope also helps getting the mount exactly aligned with the Earth's axis, instead of guessing at it through the hole.

 

I started out just as you are, and came to realize that finding stuff in the night sky requires a degree of precision that's hard to get without additional tools, and especially some practice using them.  But have fun with the new scope! 


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#3 JP-Astro

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Posted 18 August 2019 - 06:52 AM

Greg - I know what you are talking about but my problem is different. The AVX worked fine but suddenly something happened on the 3rd night and the GoTo error quadrupled in value which has thrown targets out of the field of view in terms of altitude. I repeat, it was fine and I changed nothing in my setup.

 

Since my view angle from the apartment is limited (for instance it's only 35 deg in altitude) and currently no 'alignments stars' come into view I did my best to align the mount to the North.

I observe from out of my room window and nothing changes here: the floor is level, the house doesn't turn around or changes its orientation in respect to the NCP, etc. Everything is firm and stable.

My mount is fully assembled and I only remove/install the diagonal from/on it. I checked the mount legs retraction height and it didn't change and the screws are firmly tightened.

 

Here is the exact procedure I used for the first 2 nights that suddenly began to fail on night 3.

- I did 'Quick Align' which means 'The NCP is where I point my mount's North'. It's pretty accurate in my case.

- I then used the GoTo to find a non-alignment star like Sheratan for instance and did fine adjustment of the mount legs positioning to center it in the azimuthal plane. The altitude was fine, with just a little off-center in the alt scale.

- After that the stars were pretty accurately positioned on request for the first 2 nights. On the 3rd night something happened and the stars are now positioned above the telescope's optical axis so that they are out of the view field of my 25 mm EP.

 

That is the AVX suddenly started to undershoot the targets in altitude.

 

...I want to add that my DST and time zone are correct and all other date/time/location settings also.

By the way, the tracking works firm and steady when a target is centered.


Edited by JP-Astro, 18 August 2019 - 07:47 AM.


#4 SkipW

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Posted 18 August 2019 - 12:56 PM

Try a factory reset and re-enter your location and time settings.



#5 TelescopeGreg

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Posted 18 August 2019 - 02:49 PM

Greg - I know what you are talking about but my problem is different. The AVX worked fine but suddenly something happened on the 3rd night and the GoTo error quadrupled in value which has thrown targets out of the field of view in terms of altitude. I repeat, it was fine and I changed nothing in my setup.

 

Since my view angle from the apartment is limited (for instance it's only 35 deg in altitude) and currently no 'alignments stars' come into view I did my best to align the mount to the North.

I observe from out of my room window and nothing changes here: the floor is level, the house doesn't turn around or changes its orientation in respect to the NCP, etc. Everything is firm and stable.

My mount is fully assembled and I only remove/install the diagonal from/on it. I checked the mount legs retraction height and it didn't change and the screws are firmly tightened.

 

Here is the exact procedure I used for the first 2 nights that suddenly began to fail on night 3.

- I did 'Quick Align' which means 'The NCP is where I point my mount's North'. It's pretty accurate in my case.

- I then used the GoTo to find a non-alignment star like Sheratan for instance and did fine adjustment of the mount legs positioning to center it in the azimuthal plane. The altitude was fine, with just a little off-center in the alt scale.

- After that the stars were pretty accurately positioned on request for the first 2 nights. On the 3rd night something happened and the stars are now positioned above the telescope's optical axis so that they are out of the view field of my 25 mm EP.

 

That is the AVX suddenly started to undershoot the targets in altitude.

 

...I want to add that my DST and time zone are correct and all other date/time/location settings also.

By the way, the tracking works firm and steady when a target is centered.

Ha!  This is spooky...  Sounds very similar to what I was doing when I first got my scope.  I used the clutches to reposition the scope, instead of the legs, but same effect.  Great minds, I guess.  But it didn't work for me, either.

 

My logic was that, if I had perfectly set everything up mechanically - perfectly level, perfect polar alignment, aligning to the exact index marks, etc. - that the GoTo operation would also be perfect.  If it missed, a mechanical re-alignment of the mount would fix it.  That was my logic.  The problem was that you've got multiple interacting mechanical things, and that can't be fixed by a simple adjustment of just one of them.

 

The mount knows this, and has algorithms and such to deal with it.  Readjusting things for one object doesn't fix the mount's model of the mount and sky, so everything else is going to be wrong, and possibly worse than if I'd just left things alone.  I (we) were fighting the scope's ability to work.  Once I stopped doing that, and instead used those first few stars as actual alignment stars, things started to improve.

 

My last point of confusion was that the user guide wasn't clear (to me) about the difference between Alignment and Calibration stars.  I'm still not totally sure about them, but I believe they are related to two different pieces of the alignment puzzle - where the mount and its motors are aimed vs where the telescope's optical train is aimed.  Modeling both is needed for accurate GoTos.

 

As I said, microns count in this hobby.  Best to let the mount work the way it was designed.  If you have a limited view of the sky, you might read the manual's "All star polar alignment" section and try to follow that procedure.  It's not as accurate as some other methods, but should get you close enough.  (All Star means "only using stars", and nothing to do with sports heroes.)  Then use a couple of stars to set the sky alignment, and you should be set. 


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#6 JP-Astro

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Posted 18 August 2019 - 03:49 PM

Greg - sounds logical to me and I see what you are getting at.

I wonder if the AVX latitude setting has any sensor feedback to the mount electronics.

And then again - how did I manage to align it so well on the first 2 nights.

I actually tried to use even 1 star alignment but the stars in the Menu are out of my field of view.



#7 TelescopeGreg

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Posted 18 August 2019 - 04:07 PM

No, there is no feedback for any of the mechanical adjustments, which essentially is part of the problem.

 

I think the initial success you had was more a matter of luck, or perhaps something that the hand controller hadn't yet "learned" about your setup, that became inconsistent and out of date when used later on.  I'm a little puzzled that a full factory reset didn't clear that back to your initial success, but you never know what's going on inside the hand controller's little mind.

 

The important thing about the internal star alignment procedures is that commanding a "quick align" and then adding the alignment stars yourself is essentially the same as using the internal 1-star or 2-star procedures.  The big difference is that with Quick Align you get to pick stars that a) you can actually see, and b) that you can identify.  They may not be as "good" a star choice as what the hand controller might prefer, but at least you can get something.  My first experience with the internal alignment procedures was that they kept calling out stars that I had no clue what to do with.  Quick Align also starts the rotisserie running, so it's easier to center the star in the eyepiece during the alignment procedure.  Oh, and you can also use the planets (Jupiter and Saturn are great right now) in place of a star for alignment.


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#8 JP-Astro

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Posted 19 August 2019 - 10:01 AM

No, there is no feedback for any of the mechanical adjustments, which essentially is part of the problem.

 

I think the initial success you had was more a matter of luck, or perhaps something that the hand controller hadn't yet "learned" about your setup, that became inconsistent and out of date when used later on.  I'm a little puzzled that a full factory reset didn't clear that back to your initial success, but you never know what's going on inside the hand controller's little mind.

 

The important thing about the internal star alignment procedures is that commanding a "quick align" and then adding the alignment stars yourself is essentially the same as using the internal 1-star or 2-star procedures.  The big difference is that with Quick Align you get to pick stars that a) you can actually see, and b) that you can identify.  They may not be as "good" a star choice as what the hand controller might prefer, but at least you can get something.  My first experience with the internal alignment procedures was that they kept calling out stars that I had no clue what to do with.  Quick Align also starts the rotisserie running, so it's easier to center the star in the eyepiece during the alignment procedure.  Oh, and you can also use the planets (Jupiter and Saturn are great right now) in place of a star for alignment.

Greg - thanks for staying with me on the subject.

I have to learn what 'alignment stars' is. Where is it located in the Menu? I know I read earlier about 'calibration stars'. Is it the same thing?

 

Planets (Jupiter and Saturn) are out of my view already, they were available last Spring. I would be hesitant to calibrate by Neptune...

Currently what I get at around 10 pm when I start to observe are some low Pegasus stars and... that's it. Even Andromeda is out of my reach, too high as it's obstructed by the top wall above the window. The stuff below Pegasus is too light polluted to discern any stars at all.

A little later I get Aries in my view filed with Hamal and Sheratan (bringing Uranus with them at a lower altitude) and then much later I can use Aldebaran and El Nath.

By the Dawn time when it's too late to continue observations Orion raises in its glory at the East and later on in the Autumn and on I could use its bright stars of course.


Edited by JP-Astro, 19 August 2019 - 10:03 AM.


#9 SkipW

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Posted 19 August 2019 - 01:44 PM

The one-star and two-star alignment procedure defaults to using stars west of the meridian. If you select One Star or Two Star alignment, note the 'W' at the upper right of the handset display. Press MENU, and that changes to 'E' and you get a different list of stars.

 

Similarly, Calibration defaults to stars east of the meridian. You can switch between east and west using the MENU button there, too.


Edited by SkipW, 19 August 2019 - 01:45 PM.

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