Calculating polar alignment error

[jambouk]

Canon 6D at prime focus on a Celestron C14; unguided.

80 x 60 second exposures stacked without alignment. Is the drift mostly poor polar alignment? FoV is 32 arc minutes by 21 arcminutes, f/l 3910mm. (Ignore artefact bottom left and vignetting, no flats or darks used in this stack).

Thanks. James.

Edited January 15, 2022 by jambouk

[scotty38]

Is that with no guiding on a C14?

[jambouk]

Unguided yes. Sorry.

[almcl]

I think that to answer the original question we need to know what direction the drift is in.  Is it along the RA axis, the Dec, or some of both? 

Plate solving a single sub should provide this information, but a trailed one is a bit too difficult.

Edited January 15, 2022 by almcl

[michael8554]

It would be easier if you image pointing south, with the 6D parallel to the horizon.

Then up/down drift would be PAE.

Your image scale is 0.34 arcsec/pixel.

You could measure the Dec drift in pixels in a 1 minute exposure.

And then applying the image scale, work out the drift in arcmins in 1 minute

But I don't have the maths to convert that to the polar positional error of X arcmins.

Michael

 

[Stuart1971]

That is a lot of drift for 60 second exposures even at that FL, so yes you PA is not good at all and needs to be much better….on my EQ8 with an 8” SCT with 2.5x power mate so 5000mm FL I get a small bit of drift in a 2 min sub, but only small, PA done with pole master on a permanent steel pier…👍🏼

[almcl]

29 minutes ago, Stuart1971 said:

That is a lot of drift for 60 second exposures even at that FL, so yes you PA is not good at all

I don't think you can draw that conclusion, the drift is from 80 exposures each of 60 seconds, or 4800 seconds (80 minutes) in total and, until we know the direction of drift, it isn't possible to say if it's polar alignment, incorrect tracking speed or something else. 

[jambouk]

Looking on astrometry.net and using those coordinates in Stellarium, I've worked out... If I turn tracking off in Stellarium the stars differ in the direction of the blue arrow. In my subs, the stars drift off in the direction of the yellow arrow, so I suspect it is imperfect polar alignment. The stack is a stack of 80 x 60 seconds so 80 minutes in total.

[Stuart1971]

25 minutes ago, almcl said:

I don't think you can draw that conclusion, the drift is from 80 exposures each of 60 seconds, or 4800 seconds (80 minutes) in total and, until we know the direction of drift, it isn't possible to say if it's polar alignment, incorrect tracking speed or something else. 

Yes my mistake, I was thinking the drift was the same in all the images, but was forgetting it will have moved hence the longer drift lines….👍🏼Sorry

[michael8554]

If Blue is the direction of drift with RA tracking off, then the Yellow-Blue direction is RA.

So your PA is good enough to show acceptable drift in individual subs, but enough to cause image framing drift over multiple exposures.

Michael

Edited January 15, 2022 by michael8554

[newbie alert]

I'm shocked that it stacked, and also shocked that the subs wasn't checked before letting it run for 80 mins, and checked before stacking.....

[michael8554]

3 hours ago, newbie alert said:

I'm shocked that it stacked,

Why ? Your guiding appears to have been good.

Michael