Re: An easier Pasillx N & S Polar Alignment method

Roland Christen

In a message dated 9/19/2008 5:41:18 AM Central Daylight Time,
docsquic@... writes:

With the right camera and fine enough control over the reticle brightness
this could be a really neat comercial product for those of us who hate to
kneel before our AP1200's ;-)
Howard and myself set up a 3600 mount last night from scratch, polar aligned
it and began imaging by 8pm, just after evening twilight. We were able to do
unguided imaging with a focal length of 3810 mm (12" F12.5 Mak-Cass), achieving
FWHM stars of 1.8 arc seconds on M13. The seeing wasn't the greatest and
there were thin high clouds, but the real achievement was to do the polar
alignment without using a polar scope or drift alignment, and still be able to get the
alignment dead nuts accurate for unguided imaging.

We started off by adjusting the altitude axis with a simple protractor, set
the axis to 42 degrees. I then placed the tube assembly in the Park1 position
and used a cheap and cheerful $5 bubble level from Ace Hardware. I turned on
the power and started the mount from Loc1, Resume from Park1, then slewed to
Vega. I used only my 8x50 finder to do the Meridian Delay "Quick Drift Method".
The advantage of using the 8x50 finder was that i could quickly adjust it via
the thumbscrews to be perfectly orthogonal to the mount (unlike the Big Mak
which was off about 15 arc minutes). It took only 3 iterations back and forth on
Vega to get the star to fall right on the finder crosshairs on either side of
the mount.

Now that the Altitude was set with Vega conveniently almost straight up, we
entered Arcturus, which is presently deep in the west. The star was off about 1
degree in the N-S direction. We adjusted the Azimuth axis until it was right
on the crosshair and slewed back to Vega. As expected, Vega was still dead
nuts on the crosshair because of its location straight up (turning the azimuth
axis simply rotated it but did not displace it on the crosshairs which makes
this alignment a bit easier and faster).

Now that we had two stars in the E-W direction on the crosshairs, plus I had
the finder orthogonal, we slewed to Deneb and Altair on the other side of the
Meridian. Both of those stars were almost exactly on the crosshairs also.
Finally we slewed to Polaris as the acid test. It was also very close, not quite
on the crosshairs, but probably off only a tiny fraction in the N-S direction,
so I decided to let it be. The best polar alignment is actually to be on the
refracted pole, and this alignment was close to that point. The entire polar
alignment routine took maybe 20 - 30 minutes, at a leisurely pace.

Our first target, M13 was acquired and after doing careful focusing we were
getting around 1.5 arc sec FWHM, and as anyone who does imaging knows, this is
very good resolution. The production worm on this 3600 mount is made with a
new gear cutting method that we developed here after much testing, and it
produced a raw periodic error of under 2 arc seconds without PEM in a 5 minute worm
cycle. With this low an error, together with the above polar alignment we were
able to do 30 second unguided imaging at 3810mm focal length on M13 and get
no drift in either axis, with nice round stars. Average star sizes were 1.8 arc
sec FWHM at the beginning of the imaging session. As it got darker and M13
began to drop down in the west, the air began to get more unsteady and the
seeing deteriorated to around 2 arc sec FWHM. (we will post the image of M13 of 6
minutes total exposure LRGB, unguided taken with 12" F12.5 Mak Cass at prime

I would encourage all of you imagers to try this method, especially now that
Vega is so well placed, and can be seen just after sunset. In fact, if you can
pre-align your axes using my famous Park1 - Park 2 method, you will be able
to pick up these stars while the sun is still up, and finish polar alignment at
sunset. This method of alignment is powerful, almost foolproof, and very
accurate. It beats the drift alignment in terms of speed, and does not require a
computer of CCD camera to work. You just need a crosshair finder, and has the
added bonus of getting your finder aligned orthogonally.


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