Sat, 22. December 2012
Astrophotography Without Telescope - 9
I wrote previously about the Vixen Polarie mount, which effectively turns the camera/tripod setup into an equatorial mount, with an R.A. drive motor. Long exposures then produce pin-point star images instead of star trails.
Due mainly to my hip replacement operation, subsequent convalescence and continual cloudy weather, I have not had enough opportunities to use it. My last field night with the Polarie mounted camera was spent trying to image Orion & Taurus through breaks in the troublesome clouds but the experience was enough to convince me that I needed to take the guesswork out of polar aligning and improve the alignment accuracy. I had no accurate method of alignment - and I am unfamiliar with the dim "starfield" in the Octans constellation, where the SCP is located. I didn't do too badly but on the way home I felt there was a lot of room for improvement, although Octans was behind the clouds all evening.
I had a choice between buying a polar meter and a polar scope to assist with polar alignment. Both are also supplied under the Vixen brand. I chose the polar meter - not just because it was the cheaper option - but because, for me, it would still be necessary, even if I do eventually choose to purchase the polar scope to give me 100% accurate alignment.
Canon EOS 60D, Vixen Polarie Mount & Tripod, Vixen Polarie Polar Meter.
In theory, the rough polar alignment technique requires you to satisfy these conditions:
- Set the altitude to 34 degrees (the southerly latitude here in Macarthur).
- Level the instrument.
- Point the instrument towards true south (by finding magnetic south and adjusting 11 degrees East).
There is plainly room for error with this method (because we have no Polaris to aim at in the Southern Hemisphere) and making an exact magnetic adjustment with a hand-held compass is difficult. Enter the Polar Meter.
Top view of Polarie (centre) and Polar Meter (left)
Set up the tripod and Polarie. Install the Polar Meter in the Polarie equipment shoe. Adjust the Polarie Meter elevation to 34 degrees latitude (for Sydney) and aim the Polarie roughly South.
Two fine adjustments are now required to aim at the South Celestial Pole and I found it best to do them both together: rotating the Polarie to 169 degrees East (11 degrees from South), whilst centering the bubble in the spirit level chamber. Keep doing one and then the other alternately until confident that aligment is as accurate as possible. Then secure tightly and pack the Polar Meter away safely and attach the camera.
This method should get the Polarie very close to alignment with the SCP. Now, wait for darkness, switch the Polarie on, aim the camera and start shooting!