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.


Spring galaxies

Not sure why I haven’t posted for a while, as there was a spell in April when we had a string of back-to-back clear nights.  There were opportunities galore to capture springtime galaxies, and I found new locations such as Elterwater carpark to take advantage of dark skies.

17 April, with some cloud in the east, I headed to the centre of the Lake District in pursuit of “Markarian’s Chain” using my new protocol of guided 10-minute exposures.   Elterwater is a small village in the Langdale valley, but has a good accessible hard standing carpark where I set up my system.  The netbook computer guides for only a couple of hours on its old battery, so once it’s up and running I have to get on with it.

The target was Markarian’s Chain of galaxies just east of Leo, and I managed six frames of 10 minutes each before the clouds chased me down.  One frame was a little hazy, so the result is a stack of 5×10 minutes.  Darkness was good, but disadvantages of Elterwater include some very bright outside lights on tourist rental properties, and large camper vans using the carpark for overnight stays.  Also it is low altitude and a bit of a bowl for mist and condensation.

That said, there are 25 or more galaxies easily seen in this photo, and the stats are mind-boggling.  Most objects in this frame are over 50 million light years away, and the galaxy in the bottom left corner (M87) is a million light years from M84 at the right hand end of the chain.  Galaxy NGC4388 at bottom right (the “smile” on the face at the front of the chain) is 11th magnitude, which makes it less than 1% of the brightness of the faintest object visible to the naked eye from a dark location.  The galaxies are gravitationally bound to one another, despite the incomprehensible distances involved.  All that with a Nikon D90!

Markarian's Chain: M64, M86 and others in the Virgo Cluster. Nikon D90 on Altair Wave 115/805 (f/7), ISO 400 50 minutes: 5 frames of 10 minutes each.

Markarian’s Chain: M84, M86 and others, plus M87 in the lower left corner, all in the Virgo Cluster
Nikon D90 on Altair Wave 115/805 (f/7), ISO 400
50 minutes: 5 frames of 10 minutes each.

On the night of 18 April, M101 was at zenith, captured here in 9 frames guided at 10 minutes each.  That’s a total of an hour and a half on one galaxy, which is a lot for me (until now!).  10 minutes is fantastic for light gathering, but pushes the boundaries of long exposure v potential for disturbance.  If a car drives past, or if a strong gust of wind nudges the system – or even if the guiding software momentarily loses sight of the guide star – then the whole 10-minute frame is spoiled.  These were taken in the long layby on the A591 outside Staveley, which is sheltered but by no means unused, and I was fortunate that there was almost no traffic that night.

M101 Nikon D90 on Altair Wave 115/805 (f/7), ISO 400 90 minutes: 9 frames of 10 minutes each.

Nikon D90 on Altair Wave 115/805 (f/7), ISO 400
90 minutes: 9 frames of 10 minutes each.

On 21 April, I was back on the Shap Road with fellow members of the Eddington Astronomical Society, and my target this time was M106, again at zenith.  Thirteen frames of 10 minutes were reduced to nine on closer inspection (gusts of wind spoiled four), so this is also a stack of 9×10 minutes.  In calm conditions I seriously wonder whether the DSLR could cope with 15 minutes per frame; with the ISO at a comfortable 400, there is very little by way of glow from the sensor.  What level of detail might be possible with three or four hours total exposure?

Nikon D90 on Altair Wave 115/805 (f/7), ISO 400 90 minutes: 9 frames of 10 minutes each.

Nikon D90 on Altair Wave 115/805 (f/7), ISO 400
90 minutes: 9 frames of 10 minutes each.

Finally, on 26 April, a quick experiment from my own back yard.  With high buildings and trees obstructing a clear view of the sky, it’s not possible to go through the star alignment process on the GOTO handset, so I’m restricted to polar-aligning the mount by eye.  I hoped that with 5-minute frames, the guiding system might smooth out the alignment error.  Here’s 20×5 minutes on M51, with the ISO wound up to 800, suggesting this has some promise.  There’s light pollution from next door’s uncurtained landing window, so a light pollution filter might be a useful addition to the armoury.

Nikon D90 on Altair Wave 115/805 (f/7), ISO 800 100 minutes: 20 frames of 5 minutes each.

Nikon D90 on Altair Wave 115/805 (f/7), ISO 800
100 minutes: 20 frames of 5 minutes each.