Sat, 16. March 2013
Astrophotography Without a Telescope - 19
I have been a member of Macarthur Astronomical Society since the day it was founded and its value has been demonstrated to me a number of times. Whilst I often prefer to try and work things out for myself, sometimes it is the advice or a tip from another member which produce that 'eureka' moment that gets you out of trouble.
Over recent months, instead of using my telescope, I have been using my camera on a tripod, with a Polarie mount to follow the sidereal motion of the stars. I have been aligning it with a Vixen polarmeter up until now. To align it even more accurately with the Earth's axis, I recently purchased a Vixen polarscope:
Rear view of Vixen Polarie with removable polarscope fitted.
Front view, showing polarscope protruding from the Polarie mount.
I had never tried an accurate visual polar alignment before, so I was a little apprehensive at my first attempt last week. Would I waste the entire evening trying to polar align and leave myself no time for imaging?
Whilst the rear view of the polarscope looks somewhat complex, all the dials are there for Northern hemisphere observers aligning on a single star (Polaris). Here in the Southern Hemisphere it is much more simple. We need to rotate the engraved template inside the polarscope to align on a trapezium of four stars in the Octans constellation. Easy!
Souther sky chart from the Vixen Polarie manual. Sigma Octantis is the closest "bright" star to the South Celestial Pole.
Well, not quite! This trapezium of four fifth magnitude stars in Octans (right on the edge of dark sky visibility) rotates around the South Celestial Pole about once a day and its orientation changes with the seasons.
To accurately polar align the Polarie I did the following:
1. Roughly aimed at the SCP (see the above chart).
2. Used the Vixen polarmeter to more accurately align using its spirit level, compass and latitude setting. This produced an aim that was fairly close to the SCP.
3. Inserted the polarscope and located the four trapezium stars of Octans through it (note: the polarscope inverts the image).
4. Aligned the four stars with the reticle template of the four stars inside the Polarscope.
Wait a minute! Doing this last item for the first time, I realised that (a) I needed darkness to see the four stars; and (b) once it was dark I could not see the black reticle!
Panic! Help!
This is where being an MAS member paid off, because my good friend John Rombi, a very experienced amateur astronomer, had the answer straightaway. "Just shine a red torch down the polarscope barrel to see the template, then take it away to see the four stars," he told me. "Just alternate between the two."
I did and it worked!
I estimate that I aligned to within about five arc-minutes of the SCP - and I will need some practice at fine adjusting the Polarie ball joint support to get it exact. So my first attempt at polar aligning surprisingly paid off, thanks to a little help from my friend!
Me, my Canon 60D and my Polarie.
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Macarthur Astronomical Society:www.macastro.org.au
Vixen Polarie: http://www.vixenoptics.com/mounts/polarie.html