How does EQ Goto mount alignment work?

[johngwheeler]

I've just bought a Celestron AVX goto mount, but don't yet have a telescope, so haven't been able to try it out fully.  I have some basic questions about how the setup and alignment process works.

1) If the mount is only very roughly polar-aligned (i.e. just setting latitude scale and moving tripod to point roughly to pole - South in my case), what function does the "one star" or "two star" alignment have if it doesn't physically change the mount position?

It seems to me that this may allow the mount to become aware of it's orientation, and maybe calibrate its go-to ability, but wouldn't fundamentally allow it to track objects because the polar alignment is not accurate. Does the mount do some adjustment to compensate for poor tracking by using both RA and Dec motors, in the way that I imagine Alt-Az go-to mounts work (which I assume must use both motors as they are not polar aligned at all)?

2) it makes sense that the mount must be physically aligned to the celestial pole by moving its altitude and azimuth settings, but the manual is pretty bad in explaining this. I gather that the physical alignment can be done by centring a known star multiple times and offsetting drift according to direction prompts on the hand controller. When the drift is zero over time, then the mount is correctly aligned. Is this how it works?

I haven't been able to experiment with the above without a scope, so any clarification you could give would save me time when I actually get the scope!

Thanks and best regards,

John.

[spaceman_spiff]

Hi John,

You are right that a roughly polar aligned equatorial mount will not accurately track objects. You can still perform a 1,2 or 3-star alignment and this information allows the mount to  set the true position of stars in relation with its internal celestial maps. 

I imagine that if the mount is poorly aligned, the accuracy of the star alignment will deteriorate throughout the night. However, if I perform an accurate 3-star alignment, I can use the mounts polar alignment routine to improve my PA (my SW EQ5 pro has this, hopefully your does as well). This may need to be done multiple times. 

What you are describing in 2) is a drift alignment, and this can be very accurate, there are other threads on this forum that explain this in better detail. 

Hope this helps! 

[joecoyle]

HI,

Yeah I have this mount and manual is rubbish.

To answer your questions:

1) 2 star align creates a map of the sky in its database so your Goto's know where they're going - the mount position changes in 2...

2) Drift alignment is tough - especially if your scope is not a permanent setup.

The AVX has something called All Star Polar Align... I will briefly explain:

• Do a 2 star align as mentioned.
• Press back several times
• Choose Stars button and Go To a star that is 40 degrees up, close to the meridian. I check this in Stellarium beforehand. All star is a bit of a misnomer - you are quite limited in choice.
• Press back several times.
• Choose align - scroll to polar align - scroll to align mount
• The mount will slew to your star above. Then you centre in finder and eyepiece as usual.
• Press align.
• It will say "Beginning process to polar align. Press enter to continue"
• Then it will slew away from your star again. 
• This is the important part - this is where you move the mount using the Alt Az knobs. Align the star in the eyepiece and press enter.
• Then you can choose
• Align - polar align - display align
• This will display your alignment (It takes several attempts to get right (all 0s) - i speak from experience!)
• Celestron recommend turning the mount off and doing the whole thing multiple times to improve go to accuracy, but i never get the time.

Then your mount is polar aligned. Bit long winded I know - but you get there... do you have a polar scope?

There is a really helpful video on youtube which explains it better than i can!!

https://youtu.be/coXGvcgHOMA

HTH

Joe

[johngwheeler]

Hi John,

You are right that a roughly polar aligned equatorial mount will not accurately track objects. You can still perform a 1,2 or 3-star alignment and this information allows the mount to  set the true position of stars in relation with its internal celestial maps. 

I imagine that if the mount is poorly aligned, the accuracy of the star alignment will deteriorate throughout the night. However, if I perform an accurate 3-star alignment, I can use the mounts polar alignment routine to improve my PA (my SW EQ5 pro has this, hopefully your does as well). This may need to be done multiple times. 

What you are describing in 2) is a drift alignment, and this can be very accurate, there are other threads on this forum that explain this in better detail. 

Hope this helps! 

This was really useful - thanks! I've watched a couple of YouTube videos and now understand that the alignment process has the two distinct steps that I described. Some people don't seem to bother with the fine-tuning of the PA, and just do the 1, 2 or 3 star alignment so that the go-to works OK, but it seems that any precision tracking (e.g. for photography) needs the mount to be physically aligned multiple times until the tracking error is minimised. 

This is all much more high-tech than my previous alt-az mount without any slow-motion controls. I'm really looking forward to trying it out for real! I did think that a go-to mount was somehow "cheating", but I'm coming to the conclusion that it's a great way to learn the sky and discover new things. 

John.

[ronin]

First is to seperate the mount from the scope. Once "aligned" the mount itself does not change, the bit/arm on which the scope is attached moves but not the alignment of the mount.

One star alignment is either odd or incorrect.

Using a Meade: On a Meade you set the mount and scope up Level and North, if you then pick One Star Align the scope goes to where it expects a know star to be and you centre it. If you think about it that is actually a 2 star alignment. The start of Level and North is a virtual star, maybe better a starting reference point.

A true 1 star alignment would assume that the scope is level and "perfect" but the orientation is unknown, by going to one star you are simply saying assume all else is OK and you are now pointed at Arcturus and use this RA/Dec as the start position to move to whatever next. This is used on scopes that are pillar mounted quite often.

Subsequent tracking is based then on assumption, and depending on how good you initially set it up, this may be good or bad. I set a Meade up for solar observing, daylight so no stars to align on, but I levelled it and aimed it North as best I was able and it managed to track the sun for the next 2 hours with no recentering required.

When you add a scope and align the goto feature the software works out how much the position of the scope is different to the expected ideal and then determines and applies a compensation to the scope movement.

No idea about (2) that sounds like drift alignment.

[joecoyle]

EQ mounts do take a bit of getting used to, but like you say - absolutely necessary for photography. You'll have great fun learning, as I am currently doing - and as always theres loads of advice and friendly people on here to ask...

Joe

[johngwheeler]

HI,

Yeah I have this mount and manual is rubbish.

To answer your questions:

1) 2 star align creates a map of the sky in its database so your Goto's know where they're going - the mount position changes in 2...

2) Drift alignment is tough - especially if your scope is not a permanent setup.

The AVX has something called All Star Polar Align... I will briefly explain:

• Do a 2 star align as mentioned.
• Press back several times
• Choose Stars button and Go To a star that is 40 degrees up, close to the meridian. I check this in Stellarium beforehand. All star is a bit of a misnomer - you are quite limited in choice.
• Press back several times.
• Choose align - scroll to polar align - scroll to align mount
• The mount will slew to your star above. Then you centre in finder and eyepiece as usual.
• Press align.
• It will say "Beginning process to polar align. Press enter to continue"
• Then it will slew away from your star again. 
• This is the important part - this is where you move the mount using the Alt Az knobs. Align the star in the eyepiece and press enter.
• Then you can choose
• Align - polar align - display align
• This will display your alignment (It takes several attempts to get right (all 0s) - i speak from experience!)
• Celestron recommend turning the mount off and doing the whole thing multiple times to improve go to accuracy, but i never get the time.

Then your mount is polar aligned. Bit long winded I know - but you get there... do you have a polar scope?

There is a really helpful video on youtube which explains it better than i can!!

https://youtu.be/coXGvcgHOMA

HTH

Joe

Joe,

That was a fantastically clear explanation - thank you so much! I'd actually found the video you linked about 20 minutes ago, and have just finished watching it :-) But your explanation was extremely clear and succinct. The manual seems to jump about the different alignments without explaining the sequence clearly.

I don't have a polar alignment scope (it isn't included with the mount), but from the video, it does seem that this can be a time saver to avoid having to make large adjustments to the mount alt-az. My setup will probably be (just about) moveable by one person (about 30kg all-up), but I wouldn't want to move it too often :-). PA is a bit a job at my location (about -33 South) because pretty much none of the stars near the SCP (in Octans) are naked eye visible with the light pollution and low elevation)

Cheers!

John.

[joecoyle]

John,

Yeah the manual is horrendous - much better stuff on the net - especially here. I ordered a polar scope (about £40) and do use it to help with alignment. It saves time, although like the video says, I usually stop at 0s and start imaging - turning off and on several times will only delay me - I live in Manchester, so cloud rolls in every 10 minutes! Have to take whatever time i get

Joe