Odd results with polar alignment,

[Mac]

Having had one of my rare nights off and coinciding with it being clear, I decided to tackle head on the problems i've been having with
guiding, I know the main problems was my polar alignment. So i decided to tackle this head on,
by using the iterative method.

This means getting everything set up, slewing to a star a reasonable distance away from Polaris using goto.
centering the star,
syncing on the star and then using goto, go back to Polaris.

then by using the mount adjustments ONLY NOT the handset controlls, half the distance from Polaris to the center mark.

Then repeat untill the goto's are pretty spot on, normally 5-6 itterations.

This normally results in a very accurate polar alignment.

Having done this many many times before, last night I had some bizzare results.
Having avoided the stars that dont work.

I duly synced on Caph in cassiopeia, A star i have used before!
synced and tweeked about 6 times, using polar align max to generate a cross hair on my  H9 image,
so i knew that the star was exactly centered.

I then stuck it on Andromida and set about on a 180 sec image, which should have produced very little if no drift over that time.
but the image was very bad.

After a little bit of testing, I  found it was producing noticeable drift after about 10 secs, which was far worse then when i started.

So checking that i polar aligned on Polaris, Carole
and found that it was Polaris in the camera, the slew to Caph was bang on the money, I was stumped.
I checked that the polar scope was roughly aligned with polaris and it was no where to be seen.

So any ideas
1) The mount was roughly polar aligned to start with, Not brilliant but near enough.
2) The mount was aligned and synced on Caph with no problems.
3) The goto on Polaris was reasonable.
4) the Error in polaris was slowly removed using the iterative method.
5) The goto on Caph  and Polaris are now bang on.
6) The polar alignment is way way off as per the polar scope. Proven with a 180s image Drift of about 1cm on the image.
7) Finderscope is a Laser which is Aligned with the telescope. So finderscope is pointing at the correct star that is visualized on the camera.

At which point I closed the Dome and went to bed.

Mac.

[mickw]

Is the polar scope aligned/centered ?

[Mac]

Is the polar scope aligned/centered ?

I hope so, its built into the mount. EQ6 Pro.

Mac.

[mickw]

It's been a long time but I seem to remember needing to check the polar scope alignment on distant object by flipping 180 deg. to centre the cross hairs (HEQ5 Pro).
I'm assuming your EQ6 Pro is new

[RobertM]

The polar scope being off would make a big difference and I've not been impressed with the initial accuracy other than with the NEQ6 I have.  I have however had good results using Alignmaster though it can be difficult getting alignment stars if there is a restricted field of view.  I use Micks method when doing the PS alignment, anything that you can reach focus on is good enough and it doesn't have to be too far away.

HTH
Robert
 

[Mac]

I'm assuming your EQ6 Pro is new

Nope its at least 5 years old,
and i've never had an issue with it when setting up polaris as a rough alignment.

even if it was off.
the Itteritive method should align the scope up perfectly with polaris,
it has in the past many a time.

Its just weird that this time, it's miles off for some reason.

I'll look into it in more detail on friday and saturday, assuming they are going to be clear.

Mac.

[The Thing]

Hi Mac,

Get a copy of Astrotortilla from Sourceforge. It puts Astrometry.net on your laptop. Get it solving by tweaking the field size to match your FoV + a bit and the sky area to search - default is 180deg, I use 15deg - and fiddle with the sigma value to get around 20-40 sources (it will be obvious when you have played with it for ten minutes). The point is on the menu there is a polar alignment accuracy check which will tell you how far you are off the pole in Az and Alt. It does this by you pointing east or meridanwards (doesn't need to be exact or needs a star to be centred), then it solves a plate, moves the scope, solves again and then computes the Alt or Az error in deg, min, secs. You then tweak the axis knobs and do it again, takes 25 secs per iteration.

Hope that helps and BTW the reticle in the polar scope moves as I found, I thought I had centred it but the three grub screws work loose... There is an Astronomy Shed video on YouTube showing how to do it.

Duncan

[Mike]

I agree that either the use of AlignMaster or Astrotortilla will give accurate results whereas the polar scope unless very accurately calibrated is only going to give you rough results.

Still doesn't account for the huge error you were experiencing though.

[Mac]

Ok, Update.

Polar scope is still accurately calibrated.
How do i know that you might ask,
Well I hung a spirit level on the wall so that it was vertical,
and i aligned the position of Polaris, so that when hanging vertical it was inline with the
spirit level at the end of the garden.

Let me explain that a bit better.
The RA of any star is the sidereal time at which it transits the celestial meridian.
Polaris's at the moment is 2h 48m
so when the siderial time is 2h 48,
looking through the polar scope

the circle for polaris should be vertical and at the bottom (its inverted through the view).
So by aligning the scope vertical i know one thing, its vertical.
I now need a new reference point.

Looking at the image above everything is adjustable, except the RA Index mark.
Knowing this, i had set the Date/longitude mark, to also be vertical, inline with the spirit level.
This is now on top of the polar scope when the alignment is vertical.

Now i adjust the longitude scale so that the 0 lines up with the index mark,
this extends the zero towards my RA Index mark.

I know know exactly where 2h 48m is on the time scale (RA scale above)
I turn the RA scale so that the 2H 48 aligns with the index mark, and i now know that
my Polaris is aligned correctly with the time scale.

Polaris is vertical and the RA Time is correctly showing 2h 48m

All i do is look up the local sidereal time, and turn the mount so that the local time aligns with the RA Index mark,
and voila, Polaris is in the correct position, and i can align my mount.

and boy was it off.

Its now roughly aligned, Polaris is in the correct position and rotates around the circle.

I now need to just run Pole align max to get it pretty spot on.

I quickly tried the goto  with the veil, Andromeda and the dumbell
all were in the field of view for the camera, and a 180s of Andromeda showed virtually no drift.

Mac.

[Mac]

Update.

Having stripped down my EQ6 in July last year I started on what I thought was going to be a  quick
bearing change and tune up.
Alas this was thwarted by family problems, which due to their nature have finally sorted themselves out. 

So now having some spare time on my hands after my operation, i have managed to get all of the bearings replaced
everything cleaned and re-lubed.

Having set the EQ6 up on the pier, I decided to check the alignment of the polar scope, a vertical object hung on a wall and checked at 0 and 180,

Well some thing had changed, dont know when or where, but the polaris recitle has shifted and was not center, not by a long way,
after many a twiddling, the polaris overlay is now, more or less center, and verticle, with reference to my lines. (See below)

The only other problem I now have is that during the past year the trees over the back of me have grown a few feet larger, and i can no longer see polaris,
Just a large expanse of foliage .

So its going to have to be a very rough alignment and then an iterative one, followed by pole align max.

Fingers crossed, 1st light will be soon.

Mac

[MarkS]

It's good to see you're on the road to imaging again.

Methinks you need to shin up the offending tree with a small saw

[Mac]

I had thought of that, but its now a ..kin big tree.

Mac.