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Mount Help reqiured. AVX tracking is way out

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#1 MHD

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Posted 04 August 2020 - 04:28 PM

Hi, all. after some help with my AVX mount I recently purchased.  It's unguided tracking is appalling. To give an example I get start trails at 1 to 1 1/2 seconds and planets will disappear from a 30mm eye peace in about 5 minutes. 

 

Background

 

Mount is a an early build unit, (6 pin connectors electronics). Mechanically on purchase it was OK., but hadn't been used for a number of years.  After my first night with it, where it didn't track, I overhauled both RA and DEC bearings.. RA now has tapered bearing upgrade and you can spin it like a propeller with an 8SE fitted.(scary to watch)  DEC is really good. Back lash in both RA and DEC is fine (both spur and worm drives). I am mech engineer by profession, setting up gears is something I am pretty comfortable with.

 

Set up. This is my first GEM. So getting a southern polar alignment has been a challenge from Perth given how faint the stars are you have to use. Having said that last night after 3 hours I felt confident i had it aligned. Is there some way once you have done a GOTO  alignment for it to tell you how close your polar alignment is?  

 

GOTO performance is fine using a 2 star alignment plus 4 calibration stars, items appear either within a 13mm eye piece or on my camera view area. (canon 80D live view) . 

 

Balance. I played for hours with RA balance it see if minor changes improves it, and it is very touchy to even 5mm movement from neutral in the position of weight as to how fast it tracks off the target. Best I can get is a planet on screen for 15 minutes. DEC balance, I do pencil balance my gear and mark them on my scope, due to the known stiction in the Dec  but I did notice last night, after I finished it was a touch out for the configuration. (Dec now spins pretty freely)..

 

Funny thing is it seems to track the moon fine when I set it to lunar or even  in sidereal, both before and post overhaul. 

 

Is there any way to determine/ measure what rate the RA sidereal is rotating at?

 

Backlash compensation has been set, and is good. Mount moves instantly and smoothly in both direction. Really nice in that area. 

 

Is there any other checks I can do etc to see what the problem is? Sending to Celestron from  Australia, is not an option!! 

 

What is a realistic expectation  for a AVX to track unguided at that focal length? Having said that it I don't expect it to lose a planet like it does now! My one arm Alt- Az bandit tracks better than this thing does.

 

Settings:

 

Mode - South EQ

tracking sidereal

 

 I run the mount calibration before use it

 

Scope: 8SE. all up weight is 14 pounds with camera, 12.5  for visual viewing. 

 

pic is Jupiter and surrounds showing  trails for all objects at 5.4 seconds 

 

Thanks 

 

Matthew

 

star trails 5.4 seconds

 

 



#2 Nippon

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Posted 04 August 2020 - 04:39 PM

Does not sound like a balance issue. Even with a mount out of balance if the clutches are locked it will track accurately. Are you sure you have it set for your hemisphere. I've done things like that and puzzled over it until the duh moment came


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#3 WadeH237

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Posted 04 August 2020 - 06:05 PM

Honestly, it sounds like a rough polar alignment.

 

Have you confirmed whether the drift is in RA or declination?  If the drift is in declination, then you can rule out tracking speed and direction.  The easiest way to determine this is to get any object centered in the eyepiece.  Hit the up and down direction buttons to move the object in back and forth (or up and down, or whatever).  The direction that the object moves with the up and down buttons is declination.  The left and right buttons on the hand controller are right ascension.  If the drift corresponds with the movement you get with the left and right buttons, then it could potentially be tracking speed.

 

Seriously, though, I doubt that it's a problem with tracking speed.  In all the years that I have been using AVX mounts (and CG-5s before then), I've never seen one that tracks at the wrong speed.  I have seen the tracking direction get reversed (or simply disabled) by software running on a computer connected to the mount.  I've also seen plenty of drift due to polar alignment problems.

 

On edit:  Oh, and I have seen one other issue that sounds similar to this.  Make sure that there is nothing plugged into the mount's autoguide port.  There are some cameras out there that will close the contacts on the autoguide port to initiate a continuous guide - even (especially) if the camera is powered off.  For troubleshooting, the only things that should be plugged into the mount are the hand controller and power - nothing else.


Edited by WadeH237, 04 August 2020 - 06:08 PM.


#4 Tom Stock

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Posted 04 August 2020 - 09:26 PM

On screen is likely a very narrow field of view.

You will need better polar alignment.

Is your polar scope correctly aligned with the RA axis?

Do the crosshairs stay on target when the scope is rotated in RA?

How accurate are go to's?

5 seconds way too long for Jupiter

Edited by Tom Stock, 04 August 2020 - 09:28 PM.


#5 MHD

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Posted 05 August 2020 - 05:43 AM

Honestly, it sounds like a rough polar alignment.

 

Have you confirmed whether the drift is in RA or declination?  If the drift is in declination, then you can rule out tracking speed and direction.  The easiest way to determine this is to get any object centered in the eyepiece.  Hit the up and down direction buttons to move the object in back and forth (or up and down, or whatever).  The direction that the object moves with the up and down buttons is declination.  The left and right buttons on the hand controller are right ascension.  If the drift corresponds with the movement you get with the left and right buttons, then it could potentially be tracking speed.

 

Seriously, though, I doubt that it's a problem with tracking speed.  In all the years that I have been using AVX mounts (and CG-5s before then), I've never seen one that tracks at the wrong speed.  I have seen the tracking direction get reversed (or simply disabled) by software running on a computer connected to the mount.  I've also seen plenty of drift due to polar alignment problems.

 

On edit:  Oh, and I have seen one other issue that sounds similar to this.  Make sure that there is nothing plugged into the mount's autoguide port.  There are some cameras out there that will close the contacts on the autoguide port to initiate a continuous guide - even (especially) if the camera is powered off.  For troubleshooting, the only things that should be plugged into the mount are the hand controller and power - nothing else.

 

Honestly, it sounds like a rough polar alignment.

 

Have you confirmed whether the drift is in RA or declination?  If the drift is in declination, then you can rule out tracking speed and direction.  The easiest way to determine this is to get any object centered in the eyepiece.  Hit the up and down direction buttons to move the object in back and forth (or up and down, or whatever).  The direction that the object moves with the up and down buttons is declination.  The left and right buttons on the hand controller are right ascension.  If the drift corresponds with the movement you get with the left and right buttons, then it could potentially be tracking speed.

 

Seriously, though, I doubt that it's a problem with tracking speed. 

 

On edit:  Oh, and I have seen one other issue that sounds similar to this.  Make sure that there is nothing plugged into the mount's autoguide port.  There are some cameras out there that will close the contacts on the autoguide port to initiate a continuous guide - even (especially) if the camera is powered off.  For troubleshooting, the only things that should be plugged into the mount are the hand controller and power - nothing else

Will redo the drift on Friday night ( next forecast clear night) but thought they were in RA.  Only other thing plugged in was the  gps module


Edited by MHD, 05 August 2020 - 05:48 AM.


#6 MHD

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Posted 05 August 2020 - 05:47 AM

On screen is likely a very narrow field of view.

You will need better polar alignment.

Is your polar scope correctly aligned with the RA axis?

Do the crosshairs stay on target when the scope is rotated in RA?

How accurate are go to's?

5 seconds way too long for Jupiter

Polar scope was aligned that day, aims at the same point through 180 degree rotation, will need to redo drift to confirm what I thought last night, but go to’s are good. Like very near to dead centre on a 13mm eye piece. 



#7 MHD

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Posted 05 August 2020 - 10:25 AM

Sorry guys, had some clear skies tonight, but unable to get a polar alignment I could trust. Between the full moon and light pollution, was unable to confirm alignment. Did do one sky align when I thought I had a good polar, and it aligned fine. Goto was excellent, and tracking on alpha cent. was better, but drifting perfectly along the up RA path , i.e press the right button to bring it back into the center of view. Time to drift of about 30 seconds, but the truth was I had no confidence that I was right.

 

Will wait till I have darker  skies and try again. 

 

Matthew 



#8 Nippon

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Posted 05 August 2020 - 01:33 PM

That sounds like the RA set for sidereal is running slow or not at all. You said earlier that lunar rate runs fine which would indicate that for some reason the motor is not running at the correct sidereal rate or is not running when the mount is just tracking. Very odd. Perhaps put your ear to the RA motor to try and hear if it's running.



#9 Nippon

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Posted 05 August 2020 - 01:36 PM

I just remembered when I had an AVX I messed with some of the limit settings and the mount would stop tracking in RA at a certain point in the sky. I reset everything to default and it worked fine after that. There may also be a terrestrial mode where you can move the mount with the direction buttons but it won't track but just stay where you pointed it.


Edited by Nippon, 05 August 2020 - 01:39 PM.


#10 Nippon

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Posted 05 August 2020 - 02:01 PM

I found this in the mounts manual

 

• EQ North – Used to track the sky when the telescope is
polar aligned in the Northern Hemisphere.
• EQ South – Used to track the sky when the telescope
is polar aligned in the Southern Hemisphere.
• Off – When using the telescope for terrestrial (land)
observation, the tracking can be turned off so that the
telescope never moves.

I think the terrestrial thing is for the mount turned on and you are able to move it with the buttons but won't track. Obviously you can leave it off and point it by hand and it won't move from a land based object but since it has a tracking off for land based objects that they intend that the mount can be on and the direction buttons work but it just doesn't track. Thirty  seconds until the object drifts out of the field really sounds like the tracking is off or isn't working. 



#11 SkipW

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Posted 05 August 2020 - 02:59 PM

 

Hi, all. after some help with my AVX mount I recently purchased.  It's unguided tracking is appalling. To give an example I get start trails at 1 to 1 1/2 seconds and planets will disappear from a 30mm eye peace in about 5 minutes. 

 

Background

 

Mount is a an early build unit, (6 pin connectors electronics). Mechanically on purchase it was OK., but hadn't been used for a number of years.  After my first night with it, where it didn't track, I overhauled both RA and DEC bearings.. RA now has tapered bearing upgrade and you can spin it like a propeller with an 8SE fitted.(scary to watch)  DEC is really good. Back lash in both RA and DEC is fine (both spur and worm drives). I am mech engineer by profession, setting up gears is something I am pretty comfortable with.

 

Set up. This is my first GEM. So getting a southern polar alignment has been a challenge from Perth given how faint the stars are you have to use. Having said that last night after 3 hours I felt confident i had it aligned. Is there some way once you have done a GOTO  alignment for it to tell you how close your polar alignment is?  

Yes! From the "Ready" prompt, press ALIGN, scroll to Polar Align and press ENTER

 

Scroll to Display Align and press ENTER

 

The alignment errors in Az and El are displayed.

 

GOTO performance is fine using a 2 star alignment plus 4 calibration stars, items appear either within a 13mm eye piece or on my camera view area. (canon 80D live view). 
Settings:

Mode - South EQ
tracking sidereal

I run the mount calibration before use it

Scope: 8SE. all up weight is 14 pounds with camera, 12.5  for visual viewing.

pic is Jupiter and surrounds showing  trails for all objects at 5.4 seconds

Let's see... the 8SE has a focal length of 2032 mm and the Canon 80D has an APS-C size chip (22.5×15 mm). This gives a FOV of 0.63° × 0.42°.

 

A 13 mm eyepiece produces 156X magnification. If the EP has 70° AFOV, the TFOV is 0.45°. 

 

If your go-tos are accurate enough to place objects close to center of these FOVs, then your alignment is good enough to produce good results using the ASPA ("Any Star Polar Alignment") routine built into the hand controller (scroll to Align Mount instead of Display Align above, press ENTER, and follow the prompts, but read the instructions in the manual first). I recommend you try that. Note that ASPA assumes that the mount base is level; if it's sufficiently off level, the accuracy of ASPA's alignment will be compromised, but "reasonably close" to level is likely more than sufficient.

 

Balance. I played for hours with RA balance it see if minor changes improves it, and it is very touchy to even 5mm movement from neutral in the position of weight as to how fast it tracks off the target. Best I can get is a planet on screen for 15 minutes. DEC balance, I do pencil balance my gear and mark them on my scope, due to the known stiction in the Dec  but I did notice last night, after I finished it was a touch out for the configuration. (Dec now spins pretty freely)..

Funny thing is it seems to track the moon fine when I set it to lunar or even  in sidereal, both before and post overhaul.

Is there any way to determine/ measure what rate the RA sidereal is rotating at?

Probably the easiest way to confirm the the tracking rate is to point at a star close to the celestial equator and time how long it takes it to drift a known angle. Rotate your camera so that movement in Dec is parallel to one of the edges and RA is parallel to the other and use the angular size of the FOV as a reference. You can check those numbers above by turning tracking off and timing how long it takes a star to drift from one edge to the other.

 

Approximate formula (star close to equator):

 

A ≈ t × Ω

 

where A is the angle of drift, t is time and Ω (Omega) is sidereal rotation rate. Ω = 7.2921150×10−5 radian/second = 0.004178°/second.

 

More accurately, it's 

 

Angle = t × Ω × cos(Dec)

 

but if you use a star within a couple of degrees of the equator, the added term is effectively 1, and you don't have to worry about it (and the process takes less time!)

 

If you turn sidereal tracking back on, the mount is tracking at some rate ω (omega). Ideally, ω = Ω. If you have the wrong hemisphere set in the tracking mode, ω = −Ω.

 

The angle of drift after a given time t with tracking on becomes:

 

A' = t (Ω − ω) cos(Dec)

 

at Dec = 0

 

A' = t (Ω − ω) 

 

ω = Ω − A' / t 

 

Using your numbers and assuming the problem is an incorrect tracking rate, with an object at the equator moving from the center of your 30 mm FOV to the edge in 5 minutes...

 

A 30 mm EP with AFOV of 50º results in 68X with 0.74º TFOV. Center to edge is half that: 0.74º / 2  = 0.37º.

 

ω = Ω − (±0.37º / 300 sec)  [± because we don't know if it's too fast or too slow.]

 

 = 0.004178°/sec − 0.001230°/sec = 0.002948°/sec (about 30% slower than sidereal)

 

-or-

 

 = 0.004178°/sec + 0.001230°/sec = 0.005408°/sec (about 30% faster than sidereal)

 

Neither of these seem very likely.

 

Backlash compensation has been set, and is good. Mount moves instantly and smoothly in both direction. Really nice in that area.

Is there any other checks I can do etc to see what the problem is? Sending to Celestron from  Australia, is not an option!!

What is a realistic expectation  for a AVX to track unguided at that focal length? Having said that it I don't expect it to lose a planet like it does now! My one arm Alt- Az bandit tracks better than this thing does.

 

 

My first guess would be that you aren't correctly polar aligned and it's drifting significantly in declination. [But a later reply says otherwise]

 

That's a long focal length. IMO it's not realistic to expect to track unguided for very long at all using an AVX with a FL > 2m. Most likely less than a minute.

 

The trailing in your embedded image looks like it could be due to shake, not tracking problems, though. 

 

I have no clue why this would not be apparent for the moon, too. The difference between mean Lunar and Sidereal rates is only about 3%. Something is inconsistent here. 

 

[Edit to correct notation error.]


Edited by SkipW, 06 August 2020 - 09:36 AM.

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#12 MHD

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Posted 05 August 2020 - 11:21 PM

Thanks all, I have a fair bit to work through and a plan now how to do it. Well let you know how it goes, might take me a few weeks to work through it all to ensure the results are repeatable

#13 MHD

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Posted 12 August 2020 - 09:43 AM

Ok, had a chance to do a bit of testing. 
 

firstly polar alignment. Have a bit of work to do, but testing done tonight with the following errors alt -4’50“, az -5’44”

 

tracking results. Over the 3 nights with various alignments in that ball park, had the same issue with poor tracking and correction all in RA. Dec error was present but dwarfed by RA error. Didn’t do the time tracking as between clouds and dew issues kept getting wiped out. In frustration Did a reset (no change) then  reloaded both motor and hand piece drivers. Tonight did three polar align runs with the above being the best (my default aligns most night are with usually -10’00” / -1’before apsa) 

 

started tracking alpha C and all good. Didn’t get a very long run due to clouds, but no major movement in eye piece with equal correction in dec and RA. Moved to Jupiter and got 15 minutes with no real movement in the eye peace before the clouds wiped it out. Can only put it down to a corrupted motor driver in RA. 
 

thanks for your help and moving in the right direction now

 

Matt




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