Polar alignment derping...

[frugal]

I have been having problems with my guiding. Reading around the subject a couple of common causes are the mount being too well/poorly balanced and poor alignment. I figured that as I was having issues with the guiding in both RA and Dec that my alignment needed improving. As tonight was too cloudy for imaging, but clear enough to see bright alignment stars, I thought I would have a go at drift alignment.

So I set everything up and roughly polar aligned the mount (on the theory that a poor alignment would show the results of drift experiments more obviously). So after a quick 2 star alignment to see how far I was out (25' az, and 6° alt, so a good amount to show drift easily), I found a star to the south as low as I could get to the celestial equator and centred it, and waited... and waited...

The stupid star stayed rock solidly in the centre of the screen... hmm. Ok, let's try a star in the Eastern sky to validate the altitude, which I knew was much more out of true. A combination of the moonlight, thin cloud and light polution meant that I had to choose Capella, not ideal, but it should still work I think. So I centre on Capella and wait for itto drift out of the cros hairs... and wait... and wait...

It took me about 39 minutes of frustration to figure out whatwas wrong. Yup, I am sure you have guessed already... When I did the 2 star alignment to see how out of alignment I was, the mount started to compensate for my error. The star was not drifting because the mount knew how out of alignment it was and was compensating...

By the time I had realised what I had done wrong the clouds had started to thicken (despite what the MetOffice and Clear Outside predicted) , so I hadto bring it all back inside again.

The moral of this story: drift align _before_ you do a star alignment [emoji1]

[StuartJPP]

I don't know much about Skywatcher products but isn't the 2-star alignment only a GOTO pointing alignment routine (in EQ mode)? I.e. it isn't a polar alignment routine. If you ran a 2-star alignment that then the calculated polar alignment error is presented on the handset in a menu somewhere (showing polar alignment error).

What focal length did you use to drift align and how long did you wait? The longest is best.

I can't see how doing a 2-star alignment can affect any sort of tracking, only internal pointing...

When you say "The star was not drifting because the mount knew how out of alignment it was and was compensating..." the only "compensation" the mount will make is to its internal GOTO model when slewing from one object to another from what I understand. It doesn't speed up/down RA/DEC during tracking to compensate for bad polar alignment, it only repositions its internal map of where it is pointing.

The manuals are pretty rubbish with respect to "alignment" as the mount and Synscan manuals appear to interchange both types of alignment as though they are the same thing...Polar alignment is totally different to the alignment of its internal map of where it it pointing to.

But I know nothing about these mounts so I can't really comment...but polar alignment always involves making adjustments to the alt/azi knobs (unless it is already spot on). GOTO alignment is only necessary for slewing to an object, but once polar alignment is achieved, you can release the clutches, manually point to an object lock the clutches and it should then still track that object properly. (Of course your AZ EQ6-GT mount has got encoders so it should update what it is pointing at...)

[msh1]

did you disable guiding outputs in the brain settings in phd to check for drift you can drift align after star alignment

[cosmojaydee]

My guess is you used your screen to view a "star" bottom dollar it was a dead pixel NOT a star you was viewing, thus no movement

[Steve Ward]

As Stuart says , the mount will not compensate for bad polar-alignment unless instructed to do so by something like a Synguider or software such as PHD.

1 , 2 or 3 Star alignment and Polar-alignment are two entirely different beasts .

You can have a perfectly polar-aligned mount that will not "Go-To" anything you ask it to due to incorrect data fed to it .... but it will track the object it ends up pointing to perfectly ... !

And vice versa , if the mount is pointing somewhere roughly in the region of the NCP ( or SCP ) but the Location , Time , Date etc are accurately inputted then the mount will "Go-To" the object it's asked to without fail .... but the object will drift from view either slowly or rapidly depending on just how far off the polar-alignment was to start with.

The easiest method of drift-aligning I've come across is the D.A.R.V. method using a camera rather than an eyepiece , very easy to see just how far things are drifting without all the squinting ... 

http://www.cloudynights.com/page/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760

[kalasinman]

Of course you remembered to turn off tracking when drifting........

[Steve Ward]

Why would you turn off the tracking ... ?

[Atreta]

Steve,

if i'm in the

As Stuart says , the mount will not compensate for bad polar-alignment unless instructed to do so by something like a Synguider or software such as PHD.

1 , 2 or 3 Star alignment and Polar-alignment are two entirely different beasts .

You can have a perfectly polar-aligned mount that will not "Go-To" anything you ask it to due to incorrect data fed to it .... but it will track the object it ends up pointing to perfectly ... !

And vice versa , if the mount is pointing somewhere roughly in the region of the NCP ( or SCP ) but the Location , Time , Date etc are accurately inputted then the mount will "Go-To" the object it's asked to without fail .... but the object will drift from view either slowly or rapidly dependingThe easiest method of drift-aligning I've come across is the D.A.R.V. method using a camera rather than an eyepiece , very easy to see just how far things are drifting without all the squinting ... 

http://www.cloudynights.com/page/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760

Steve,

as i'm in the southern hemisphere do i need to reverse anyhow the directions mentioned in the article or can i do it the same?

[Steve Ward]

I assume that it works the same ... 

[frugal]

What focal length did you use to drift align and how long did you wait? The longest is best.

I can't see how doing a 2-star alignment can affect any sort of tracking, only internal pointing...

When you say "The star was not drifting because the mount knew how out of alignment it was and was compensating..." the only "compensation" the mount will make is to its internal GOTO model when slewing from one object to another from what I understand. It doesn't speed up/down RA/DEC during tracking to compensate for bad polar alignment, it only repositions its internal map of where it is pointing.

The bizarre thing was that after I discovered my error I turned off the mount to clear any alignment model, and then turned it back on again and just entered the location and date/time. I could hear the motor moving so it sounded like it was tracking in RA. This time however the star that was previously stable for 5 minutes wandered off the centre of the screen in 15 seconds. So I can only assume that the mount was doing some kind of corrections. Unfotunately it was at this point that the clouds really rolled in.

I will need to experiment some more. All of the descriptions of Drift Alignment tell you how to drift align, but do not make it clear to muppets like myself when in the setup process to actually do it

[frugal]

did you disable guiding outputs in the brain settings in phd to check for drift you can drift align after star alignment

I was using BackyardEOS to help with the driftalignment rather than PHD. I wanted to make sure it was not tryingto guide, so I did not even have PHD running at the time. I had not realised that PHD had a drift alignment feature until I spent last night reading every article ondrift alignment that i could find.

[Steve Ward]

As soon as you power up the mount it will start tracking at sidereal rate , before any kind of alignment .

[lensman57]

I have been having problems with my guiding. Reading around the subject a couple of common causes are the mount being too well/poorly balanced and poor alignment. I figured that as I was having issues with the guiding in both RA and Dec that my alignment needed improving. As tonight was too cloudy for imaging, but clear enough to see bright alignment stars, I thought I would have a go at drift alignment.

So I set everything up and roughly polar aligned the mount (on the theory that a poor alignment would show the results of drift experiments more obviously). So after a quick 2 star alignment to see how far I was out (25' az, and 6° alt, so a good amount to show drift easily), I found a star to the south as low as I could get to the celestial equator and centred it, and waited... and waited...

The stupid star stayed rock solidly in the centre of the screen... hmm. Ok, let's try a star in the Eastern sky to validate the altitude, which I knew was much more out of true. A combination of the moonlight, thin cloud and light polution meant that I had to choose Capella, not ideal, but it should still work I think. So I centre on Capella and wait for itto drift out of the cros hairs... and wait... and wait...

It took me about 39 minutes of frustration to figure out whatwas wrong. Yup, I am sure you have guessed already... When I did the 2 star alignment to see how out of alignment I was, the mount started to compensate for my error. The star was not drifting because the mount knew how out of alignment it was and was compensating...

By the time I had realised what I had done wrong the clouds had started to thicken (despite what the MetOffice and Clear Outside predicted) , so I hadto bring it all back inside again.

The moral of this story: drift align _before_ you do a star alignment [emoji1]

Something is not right here. Star alignment has nothing to do with PA alignment. The mount after a star align  routine and  centred on a Star will enter the sidereal track mode but it is not locked onto the star as you are not guiding so I would expect it to show drift after a 20~30 minutes period. If your scope is very short and the EP wide then the movement is not as obvious as using a narrow EP. You may wish to check your PA routine.

A.G

[bogdog]

i was under the impression that drift aligning you used a camera to take a 1 minute exposure?

you take 20seconds on a star then using slowest tracking speed move to left for 20seconds then another 20 seconds to right

finish exposure and you should have a perfect straight trail if not adjust from there?

maybe i'm wrong though

[Steve Ward]

i was under the impression that drift aligning you used a camera to take a 1 minute exposure?

you take 20seconds on a star then using slowest tracking speed move to left for 20seconds then another 20 seconds to right

finish exposure and you should have a perfect straight trail if not adjust from there?

maybe i'm wrong though

Well sort of ... the DARV method is fully explained in the link I posted earlier , here again as you obviously missed it ... http://www.cloudynights.com/page/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760 ...