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.

 

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.

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

Mercury transit

On 9 May 2016, shortly after 12 noon, the planet Mercury passed across the face of the Sun and the whole transit, lasting over seven hours, was visible from the UK.  Resistance is futile, and the prospect of the transit screamed “TIME LAPSE VIDEO!” even though Mercury would be only 9 pixels in diameter on the 12MP sensor of my Nikon D90.

I reckoned that about 15 seconds of video would balance the attraction of seeing Mercury crossing the edge of the Sun against the potential tedium of watching a black dot crawl across a white disc, so I settled on 400 frames at 10 per minute – 40 minutes of transit – rendered at 30 frames per second for a smooth video of a tolerable 13 seconds.

Mindful of the seasickness-inducing bounces of the Sun in the viewfinder during my solar eclipse time lapse, and the hours I spent realigning some 800 frames by hand, I set up the mount the previous night and aligned it as well as possible using the limited view of the stars from my back yard.  After the third iteration, the handset reported polar alignment to below one arcminute of error.

Focusing on the Sun would be the next challenge, so I decided to pre-focus and leave the camera in place overnight.

This is the focusing frame through the Bahtinov mask and…

 

 

…a quick five-minute unguided frame of Arcturus.  I went to bed happy.

 

 

 

 

Next morning, the sun rose in a cloudless sky and the kit was waiting exactly as I left it.  Restarting the mount from the “Park” setting was faultless, and the GOTO put Mercury (and therefore the Sun) right in the frame first time.  The temperature change from night to day meant that the focus was awry, so I spent time refocusing using a sequence of trial-and-error photos and a felt tip pen on the focusing knob.

 

 

The reminder alarm went off at 12:10 and I started the sequence on the timer.  This device will take up to 399 frames, which suited me perfectly.  With the mount and the camera running on mains adapters rather than batteries, there was little to do but relax and enjoy the sunshine.  I watched the black dot creep onto the white disc through the viewfinder, so I can say absolutely that I observed the Mercury transit as it happened.

After 400 frames, conditions were so good and the quality of the images was so strong that I swapped the memory card in the camera and reset the timer for a second batch.  40 minutes later, same again.  And again.  And again.  By the time the Sun moved behind the house, I had accumulated 1,887 frames, so the resulting video covers over three hours of the transit and lasts 63 seconds.

There are two versions of the video – the first is sharpened using Adobe Camera RAW then the frames are cropped and the Sun is centered in each frame using an excellent piece of software called PIPP.  The second version includes a digital zoom created with further cropping.  There’s still quite a shimmy on the Sun, but I can’t process that out – much of it is down to atmospheric conditions.  It is tempting to try and align all the subframes by reference to the sunspots, but the Sun can be seen to be rotating over the course of the video so that wouldn’t work.

 

 

What can be achieved with relatively modest off-the-shelf kit and a little planning never ceases to amaze and delight me.

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 frames of 300sec, total 35min @ f/7, ISO3200

New Year’s Day Comet While Changing A Wheel

Sometimes it’s really hard to motivate yourself to get up at 3am for astronomy.  I’m more of a late night person myself, and early mornings are not my style.  One way to make it happen, to get that motivation going, is to share your intention with a friend – preferably one who is relying on you for a lift.  So it was that I set my alarm for 02:50 on New Year’s Day.  Clear skies and the knowledge that a fellow enthusiast was doing the same, meant that I was up and out in about ten minutes, including making a flask of coffee.

When I picked Stuart up in the centre of Kendal, he’d already bagged the Aurora Borealis – the Northern Lights, if you prefer – at around midnight that same night.  His dedication to the cause is beyond all understanding, and puts my meagre efforts to shame!

We had two options: a site at the top of Shap Road – dark, clear, cold and very exposed (ideal for stargazing) – or the church car park at Old Hutton – not quite so dark, low lying (so prone to localised humidity), still cold but nowhere near as exposed.  Stuart reported very strong winds across Shap Fell at midnight, as had been forecast, so we set off on the short drive to Old Hutton.

About a mile before we arrived, we found the road closed at a barrier, so turned off to the left on a tiny side road and navigated round a big loop to rejoin the main road on the other side of the diversion.  The side roads were a real mess, and somewhere in the darkness I hit a massive pothole.  By the time we arrived in the church car park, the front tyre was completely flat.

Keeping to the right priorities, we both set up our systems and started taking photos.  There followed some stargazing, some musing on the recent awful weather, some discussion of my much-anticipated observing evenings for the Eddington Astronomical Society, a review of Stuart’s aurora photos from earlier that night, some coffee drinking, and of course much delight in the images now coming off the backs of the cameras.  Eventually, though, we just had to attend to the wheel change.

Everything went smoothly until the nearside front wheel was jacked up in the air and the wheel nuts were off.  The wheel itself just wouldn’t budge, and our efforts to dislodge it by repeated and ever-more-vigorous kicking came close to waking the neighbours and/or sending the car toppling off the jack.  5am, New Year’s Day, remote village in Cumbria, no signal.  Hmmmm….  Only a joint effort, one pulling the wheel while the other aimed kicks at the opposite side, eventually freed it.  Phew!  Suffice to say that the rest of the session went very well indeed.  The conditions looked good – but they can be quite deceptive at Old Hutton as the local shape of the land collects moisture in the air – so the photo sets were as good as we could have expected.  Some hazy cloud lingered high in some frames, but nothing to stop us until about 6.30 when mid-level clouds started to spoil the view.

I took a series of 2-minute frames and 3-minute frames (all unguided) of C/2013 US10 (Catalina) at its closest approach to Arcturus, the brightest star in the constellation of Bootes.  Given the moisture in the air, the close proximity of the bright star, the Moon just 38° from the comet and all that other stuff going on, I was pleased with the result.  Given also that the weather in Cumbria has spoiled all plans for astrophotography for the last eleven weeks, it just felt good to be out again.

Comet C/2013 US10 (Catalina) passes half a degree from a very hazy Arcturus
Nikon D90 through Altair Wave 115/805 reduced to f/5.5
11 x 3 minutes, ISO 400

I hope we find the opportunity for  a few more of these Stuart!