Pleiades overnight

A clear, cold night with excellent seeing was the forecast for Monday, 28th November, so I set up in the back yard for an overnight session. Messier 45 “The Pleiades” would clear the tree line at 21.33 (south-east, elevation 53°) and disappear behind the roof line at 02.29 (south-west, elevation 45°) the following morning. Perfect conditions for leaving the system running on its own.

The initial polar alignment of the mount was accurate to 7 arcmin, reducing to 3 arcmin after one iteration of the handset’s feedback routine. Something wasn’t right, as the feedback routine wouldn’t display all the required information to make the proper adjustment, so I had to resort to twiddling the knobs “about the right amount” from an earlier manual calibration session.

3 arcmin of alignment error (with the focal reducer at 636mm focal length) means one pixel of trailing in about 135 seconds, so I set the timer for 90-second exposures with 30-second pauses for the sensor to cool down between frames. 30 frames per hour over 5 hours would net me 150 frames.

My alarm clock went off at 2.30, when I switched off the mount, screwed the end cap on the telescope, set the camera timer to take another 65 “dark” frames and went back to bed. The battery-operated dew heater kept the telescope objective above freezing, but the camera was frosted over the next morning.

150 frames take some processing, and the cooling fan on the Mac was up to full speed by the time I’d finished. Some of the nebulosity has been sacrificed in the processing, as there was a most unwelcome flare in the centre of the photo – probably an urban light source? This target never climbs more than 60° above my local horizon: at its highest elevation, it is due south right across town, and something was the right wavelength to get through my IDAS D1 light pollution filter.

Still a pleasing result for a session devoted to one of the classic targets.

M45 "The Pleiades" Nikon D90 on Altair Wave 115/636 ISO800, 150 x 90 sec.

M45 “The Pleiades”
Nikon D90 on Altair Wave 115/636
ISO800, 150 x 90 sec.

Messier 33

For some reason I never thought I would be able to photograph this galaxy.  Wasn’t it supposed to be too faint, wasn’t the surface brightness too low, wasn’t it a target for much better systems than mine?  I’ve no idea why, but that barrier had stuck in my mind.

Looking for targets on my Sky Safari App, I noticed that the declination (the celestial version of latitude) of M33 gave it a very long track across the otherwise restricted view of the sky from my back yard.  It would rise above the trees to the east and then move quite high across the open space to fall below the roofline to the west some five and a half hours later.

Time for an experiment.  On a night when cloud-free skies cannot be guaranteed, set the system running and leave it to its own devices: choose an exposure length that is short enough to run unguided but take as many frames as can be captured in five and a half hours (that’s a longer session than I’ve ever attempted).

Why unguided?  Well, a guided system has to be attended all the time.  If a cloud passes across the field of view of the guide scope, the system loses the guide star and stops guiding.  All subsequent frames probably have to be thrown away, so a guided session needs a greater guarantee of clear skies if it is to be left running while the operator sleeps.

On the 2nd of October I set up the AZ-EQ6 mount and polar aligned to one arc minute.  All other things being equal,that should allow my system’s resolution of 1.41 arcseconds per pixel to take unguided exposures of 300 seconds without star trailing.  I set the timer to 150 seconds, giving a plentiful margin to absorb other imperfections in the tracking system.  With a 30-second pause between frames, allowing the sensor time to cool down, I’d be taking 20 frames per hour.

Next morning, all had gone as planned.  That doesn’t always happen!  Clouds had interfered as expected, so I took the best 58 frames from the session to stack and process.  Despite the relative humidity of 80-90% throughout the night, the result is a most pleasing rendition of Messier 33 “The Triangulum Galaxy”.  It is about 3 million light years away and is the third largest of the “Local Group” of galaxies which includes the Milky Way and the Andromeda Galaxy.

M33 The Triangulum Galaxy 58 frames of 150 sec, ISO 800, Nikon D90 through 805mm focal length telescope, f/7

M33 The Triangulum Galaxy
58 frames of 150 sec, ISO 800, Nikon D90 through 805mm focal length telescope, f/7