Polar Alignment

[Sam]

Last night I conducted a bit of an experiment regarding drift alignment:

The theory:

If I find two bright stars that are both at the same RA and put one in the centre of the eyepiece then moving the scope only in DEC towards the other it should cross the centre as well, meaning the RA of the telescope matches the RA of the sky, the movement of the DEC of the telescope is parallel with the line between the two stars. If this is the case then the mount is aligned. If the scope isn't aligned then the star won't line up so the azimuth and latitude needs to be adjusted until it does.

The practical:

Step one:

Find two bright stars that have the same RA. Using Stellarium the two left most stars of Auriga are pretty close to each other in RA - not perfect but close enough for this experiment, 16h 0m 17s and 16h 0m 8s.

Step two:

Move the scope only in DEC axis to move between the two stars like in the picture making sure you start with one of the stars in the centre of the eyepiece, I actually used my finderscope because it's got crosshairs which makes it easier to line the star up.

(click to enlarge)

If they don't line up then you'll need to adjust the az screws on the mount until you get it as parallel as you can. It might take a combination of adjusting the lat screws as well. I found my lat was pretty good anyway so the two stars lined up very quickly with just a small adjustment to the az. The whole process took less than three minutes! So when I moved my scope only in DEC the two Auriga stars both passed through the centre of the crosshairs in the finderscope meaning the line between those stars was parallel with the dec of my mount - meaning the scope is aligned pretty well.

Step three:

The final step is to test it on an image - to see if there's any star trails. It worked great. I took 47 shots at 45sec each and only lost 6 to stars moving to much - mainly from the wind gusts. Even after a couple of one minute exposures there were no star trails.

Notes:

If the lat is off as well then I imagine you would have to do the same process but in the West or East (much the same as drift aligning) and it's probably true to say that the whole thing should be done in the East/West first for lat changes then South for az changes. What makes this method quick is that you don't have to wait for a star to drift.

I will refine this a bit more over the next few clear nights to get a logical process articulated but in the mean time I think the gist of it is pretty clear.

cheers

Sam

[centroid]

An interesting concept Sam.

I'll 'watch this space' with interest.

Dave

[Sam]

Further on the from the above, I tried this method again last night and again with success. This time I use two stars with the same DEC, Algieba and near the top of the Beehive Cluster in Cancer with the aim to lining up the RA - and to make matters tricky I moved the mount to a slightly different place to ensure I would have to adjust the latitude setting on the mount.

Here's a picture of what I mean (I just used the finderscope again as it's got the crosshairs):

(click to enlarge)

It started out quite a long way of but after a quick adjustment of the lat screws it was lining up really well. The method was to put Algieba in the centre of the crosshairs and then move the scope in RA until intersecting with the Beehive cluster. If it didn't intersect at the right place then a small adjustment is made to the lat screws and then process repeated until they lined up really well. The star on the Beehive cluster used was near the top of the cluster.

Using this method the larger the gap between reference points the more accurate the adjustments are.

After the Lat was sorted then the az was adjusted using the stars in Auriga as described above. Job done! scope aligned and ready to go (and done so quickly it still wasn't yet dark enough). The whole process took about five minutes. To test it I did 1 minute exposures of M94 (useless image though) and the stars look pretty trail-less - so I guess it was a success:

(click to enlarge)

I am quite surprised as this seems to work really well, you just have to choose the right things to line up (thanks to Stellarium and using the EQ mount setting, that was easy).

cheers

Sam