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Drift alignment

Posted: Mon May 13, 2013 10:52 am
by bchamberlain
Ross advised at the last meeting that now I can set up on a pier, I should us drift alignment. I'm sure some of you will have used the software in this link and I'd appreciate any opinions. It costs around £14

Re: Drift alignment

Posted: Mon May 13, 2013 4:41 pm
by DRatledge
I guess you are leaving your mount on the pier and just bringing the tube inside? In which case it is worth doing a proper polar alignment. There is an excellent description of drift alignment in wiki - no software required. You do need a crosshair eyepiece though.
Wiki uses south and west - I've always used south and east but it makes no odds. Just work throught it methodically. It doesn't have to be accurate to arc-seconds.

Re: Drift alignment

Posted: Mon May 13, 2013 7:12 pm
by rwilkinson
DRatledge wrote:You do need a crosshair eyepiece though.
But if you've not got one of those, fit a webcam and switch on the crosshairs in the wxAstroCap or SharpCap display.

Re: Drift alignment

Posted: Mon May 13, 2013 10:15 pm
by bholmes

I have used Alignmaster a couple of times, the first time was a disaster, mainly my fault for having my laptop in the shed and the scope about 10ft away. The second time worked quite well when the laptop was positioned close to my mount but you still need to be a contortionist at times. It would have been easier with someone to hold the laptop but that was never going to happen at midnight.

You can download it for a trial period so why not give it a go before buying. The software itself is quite easy to follow and there is a tutorial on Astronomyshed but you do need EQMOD installed on your computer which is free.

I found that on second attempt I tried to get my initial polar alignment through the polarscope as accurate as possible and as a result was making quite small adjustments. I have a 10mm illuminated reticle eyepiece and the target star was comfortably in the field of view. I did the alignment twice and was then impatient to get on but a third one would have been useful. I was only imaging for 60 second exposures but my stars were round and no trails which is a first for me.

Good luck


Re: Drift alignment

Posted: Thu May 16, 2013 1:24 pm
by bchamberlain
Thanks Bernie. Encouraging comments. I intend giving it a try. Bill

Re: Drift alignment

Posted: Thu Nov 21, 2013 7:13 pm
by bholmes
Could someone please advise how often I need to drift align my EQ6 mount which is on one of Brian's piers. For example, do I need to check the alignment every time I use a different scope, or should it be good for any scope? The alignment seems to be really good for a while but then I need to drop down the exposure time as it seems to drift after a few weeks - of late that is probably no more than 5 sessions.

Many thanks

Re: Drift alignment

Posted: Thu Nov 21, 2013 8:03 pm
by DRatledge
Once per decade.

Most drifting is not caused by polar alignment issues. No mount can really be trusted to track unguided except with short camera lenses. Balance changes will effect tracking. My mount tracks best when slightly out of balance and the motors are working "uphill" but I would always guide.

Re: Drift alignment

Posted: Thu Nov 21, 2013 9:13 pm
by bholmes
Thanks David,

That means that I can do it on the same day that I tidy my shed, its roughly the same timescale.

Looks like I will need to bite the bullet on guiding then!


Re: tracking errors

Posted: Fri Nov 22, 2013 3:48 pm
by rwilkinson
Bernie, you can go back and measure your mount's tracking performance from each series of images you've taken...

Once you've loaded your images into IRIS, draw a rectangle around a prominent isolated star and then from the Processing.. Stellar Registration menu use the One star method. This will generate a file called SHIFT.LST in your IRIS Working folder, which is just a text-file listing the measured shifts (down to a thousandth of a pixel) of each of your images in the X and Y directions.

Then if you paste this data into a spreadsheet you can plot it to aid further analysis:
If your polar alignment is out, then you'll see a monotonic drift in both axes, whereas periodic errors in your mount's motor and gears show up as a cyclic variation in the RA axis (Blue trace).
RA & Dec tracking errors from a series of 30 images using unguided CG5 mount.
cg5drift1.gif (5.23 KiB) Viewed 10171 times
RA & Dec tracking errors from a series of 12 images using unguided HEQ5 mount.
heq5drift1.gif (5.22 KiB) Viewed 10171 times
The first example shows only RA periodic error, whilst the second shows a significant mis-alignment of the polar axis.

And if you make an X/Y plot of this data, you can see how each star would be trailed over the course of your total exposure time. Of course the aim of an auto-guiding control loop is to minimise this error, ideally down to within one pixel.
X/Y plot of tracking errors, showing how each star would appear to move over the 30 images.
cg5drift2.gif (9.54 KiB) Viewed 10171 times
X/Y plot of tracking errors, showing how each star would appear to move over the 12 images.
heq5drift2.gif (9.58 KiB) Viewed 10171 times

Re: Drift alignment

Posted: Thu Dec 12, 2013 4:30 pm
by cstone
Ross, within the SHIFT.LST file, which column is the RA and which is the DEC. Left or Right