NGC7331 and Stephan’s Quintet

It’s hard to believe that a whole year has passed since my last really satisfying session of Astrophotography.  Cumbria, in the north west of the England, only seems to get one or two evenings per month of clear sky with no Moon, and I have given up several of these to organising stargazing with the Eddington Astronomical Society.

This month I let everyone know that Observing Evening would take second place to Astrophotography, and I had it in mind to try and devote an entire session to this galaxy NGC7331 and surrounding objects including “Stephan’s Quintet” – a compact group of galaxies some 300 million light years away.  The Astronomy Gods must have been smiling on me because we got two clear evenings last weekend. Full darkness at 6pm and no Moon for four or five hours, meant that I could run the system until all three camera batteries had drained.  Setting up at dusk is a bonus: Polaris is clearly visible at 5pm and the usual alignment stars of Vega, Deneb and Capella are ready and waiting.

Iterative polar alignment using star alignment feedback on the AZ-EQ6 mount gave me polar alignment error of about one arc minute, easily good enough for 10-minute guided subframes.  Guiding was courtesy of PHD2, using default settings for the QHY5L-II through an Altair 60mm finderscope.  Dithering was done manually, which was no bother for ten-minute subframes.

Nothing ever runs quite to plan in Astrophotography, so only eight clear frames came from the first night with NGC7331 just past the meridian at the start of imaging.  Eleven more frames at the second session made a total of 3 hours 10 minutes.  Two sets of flat frames were gathered for calibration, together with my library set of bias and 10-minute darks at ISO 400.

Processing was entirely with Pixinsight.

NGC7331 and Stephan’s Quintet
Nikon D90, Altair Wave 115/805, Skywatcher AZ-EQ6
11 x 588 sec, ISO 400

The final image is a crop of 2,600 x 1,950 pixels from the original 4,310 x 2,868, giving a 1.02° x 0.76° field of view.  The Moon, to the same scale, would occupy half the length and two-thirds of the width of this photo.

Quite aside from the adventure of taking the photo, there is plenty of fascinating material in this image for a short talk on astronomy.  The core of NGC7331 itself appears to rotate counter to its spiral arms, and observations of the interactions between the galaxies in Stephan’s Quintet have been used to question the viability of redshift as a gauge of galactic distance.


Quick dash after astronomy meeting

Monday night was forecast to be clear until midnight, but Monday night was also this month’s meeting of the Eddington Astronomical Society, where I was scheduled to be a contributor.  The second hour of the meeting was a presentation by another member and it would have been rude to cut and run, so I arrived at my dark location at 9.30 pm.  It took until about 10.30 to set up.

My old notebook computer can just about run the guiding program PHD2 and the camera control program Backyard Nikon at the same time, but it is prone to falling over.  Those two programs can be set to talk to one another, so that the whole imaging system is able to “dither” the frames between exposures, with the result that each exposure is captured on a slightly different part of the sensor.  That helps with elimination of noise in the post-processing.  It took a couple of reboots and re-calibrations to persuade the system to run smoothly, but eventually I could start a sequence of five-minute dithered subframes of galaxy IC342 in the constellation of Camelopardalis.

I was also experimenting with using higher ISO settings, following the analysis available on the sensorgen website.  The Nikon D90’s read noise curve flattens out at ISO 800 and then again at ISO 3200 – a somewhat confusing result – so ISO 3200 was the choice for the evening.  That’s higher than I would normally use, and the loss of dynamic range was noticeable.  So was the amount of amp glow in the dark frames!

My four-hour plan for the whole session might have been optimistic, as the notebook battery wasn’t happy at -5° celsius, and only endured for about an hour.  The mist didn’t roll in until about 1am, but there is so much water in the ground in Cumbria that low level humidity is a real problem for imaging – even on an otherwise clear night.

So not the best session, but there have been so few this season and it was good to be catching a few photons under a dark sky.

Galaxy IC342 in Camelopardalis, magnitude 9.1 7 frame sof 300sec, total 35min @ f/7, ISO3200

Galaxy IC342 in Camelopardalis, magnitude 9.1
7 frames of 300sec, total 35min @ f/7, ISO3200