Processing…

No-one is getting out much for astrophotography in Cumbria at the moment, as we are seeing endless cloudy skies.  Plenty of time to practise processing.

I made it up to my favourite dark lay-by near Tebay on 23 November and captured an hour of the Andromeda Galaxy M31.  29 frames of two minutes each.  If you’re wondering what a single two-minute frame looks like, here is one.  This is what you get if you open the shutter on your camera for two minutes.  The darkness of the background shows what an excellent location it is on the remote country road.  I’ve not done any processing on this frame, just converted it to jpeg for display here.

_DSC0020

300mm lens @ f/5.6 on Nikon D90 @ ISO 1600

Stacking the 29 frames and applying the usual processing tricks of adjusting the background, stretching the histogram, and a little colour saturation boost, gives this:

integration_DBE_Stretch

M31: 300mm @f/5,6, ISO 1600, 58 minutes
29 frames of 2 minutes.

That’s a shot I’ve wanted for a long time.  M31 nicely framed through a 300mm lens.  What more can we do with this?  Well, today’s lesson is all about High Dynamic Range “HDR” transforms: re-stretching the brightest parts of the image to bring out further detail.  Pixinsight, my chosen processing software, permits this:

integration_DBE_Stretch_HDR6

M31: 300mm @f/5,6, ISO 1600, 58 minutes
29 frames of 2 minutes
HDR transform

The effect of this adjustment is noticeable at the centre of the galaxy, where previously the detail had been hidden by the excessive brightness.

I’m also on a mission to use every scrap of data I can gather on M31, and Pixinsight also allows me to re-scale last year’s frames (taken through a 200mm lens at 5 minutes per exposure) and amalgamate them with the current set of 300mm frames.  Here’s the combined set, processed as normal:

200+300

M31: 300mm @f/5,6, ISO 1600, 58 minutes
29 frames of 2 minutes
Plus: 200mm @ f/f.6, ISO 200, 40 minutes
8 frames of 5 minutes

 

Next, the combined set with the HDR transform pushed hard to enhance the detail in the spiral arms of the galaxy:

Transformed

M31: 300mm @f/5,6, ISO 1600, 58 minutes
29 frames of 2 minutes
Plus: 200mm @ f/f.6, ISO 200, 40 minutes
8 frames of 5 minutes
HDR transform

 

I could be happy with that, but it’s quite hard on the eye, so I layer the original and the transformed versions together in Photoshop, which gives us the best of both worlds.

200+300layered

M31: 300mm @f/5,6, ISO 1600, 58 minutes
29 frames of 2 minutes
Plus: 200mm @ f/f.6, ISO 200, 40 minutes
8 frames of 5 minutes
HDR transform layered over original

There’s still a long way to go with this.  I’d like to take longer exposures without the stars trailing, and so get finer detail in the outer reaches of the galaxy’s arms.  Even at two minutes through the 300mm lens, the stars trail by a couple of pixels or so.  A couple of pixels doesn’t really matter for stars (they just appear slightly elongated rather than perfectly round) but it matters a great deal for the fine detail in the arms of the galaxy.

Patience and perseverance…

The forecast for the night of the 26th of September looked very promising, so I packed the car and set off for my Tebay Road site about at about 7.30 pm.  Full darkness was due at 9pm, so that would give me half an hour to drive and set up, plus a full hour for fine tuning the polar alignment.  It still needs practice.

Sadly the clouds had other ideas, and after a frustrating couple of hours trying to polar align through cloud gaps – in anticipation of clearing skies later – I decided to cut my losses and pack up shortly after 10pm.  Of course, that’s when the skies started to clear, in a tempting, optimistic kind of way, so I unpacked and started again from scratch.

From 11pm onwards, there followed about three hours of good conditions, occasionally excellent, in which I took about a hundred and twenty frames of various objects.  Afterwards, while the camera sat in the back of the car taking dark frames for calibration, I sat at the telescope eyepiece and manually calibrated the mount’s Periodic Error Correction system.

Good to be out under a clear dark sky!

First up, an attempt at Comet C/2014 E2 (Jacques), discovered in early March this year and now moving away from the Sun at about 32 kilometres per second (71,461 mph) at the time of writing.  Elevated 40º above the southern horizon, it makes a challenging target at Mag 10.0, given its distance of 175 million kilometres from Earth.  That part of the sky “moves” fastest, accentuating the slightest misalignment of the mount, so I restrict the exposure of each frame to just 45 seconds and push the ISO to 3200.  Here is the comet against half an hour of star movement:

C/2014 E2 (Jacques)  Nikon D90 on Altair Wave 115/805, 38x45 sec, ISO 3200

C/2014 E2 (Jacques)
Nikon D90 on Altair Wave 115/805, 38×45 sec, ISO 3200

Not my best comet photo to date, but it’s something.  Processing out the star movement:

Comet C/2014 E2 (Jacques) Nikon D90 on Altair Wave 115/805, 38 x 45 sec, ISO 3200

Comet C/2014 E2 (Jacques)
Nikon D90 on Altair Wave 115/805, 38 x 45 sec, ISO 3200

With the comet in the bag, I look to the zenith for promising targets.  NGC7789_WideThere have been a few to look forward to recently, and I am tempted by NGC 7789, a beautiful looking star cluster in Cassiopeia known as “Caroline’s Rose”.  It was discovered by William Herschel’s sister Caroline in 1783, and I have only seen it before in my wide-field photos of Cassiopeia.

Through the telescope, the camera’s field of view is only 1.68º x 1.12º and the cluster is indeed a beauty.  This shot is slightly cropped from full frame:

NGC 7789 "Caroline's Rose" Nikon D90 on Altair Wave 115/805, 20x45 sec, ISO 3200.

NGC 7789 “Caroline’s Rose”
Nikon D90 on Altair Wave 115/805, 20×45 sec, ISO 3200.

Between Cassiopeia and Perseus, the “double cluster” is next, NGC 869 and NGC 884:

Double Cluster in Perseus Nikon D90 on Altair Wave 115/805, 20x45sec, ISO 3200

Double Cluster in Perseus
Nikon D90 on Altair Wave 115/805, 20x45sec, ISO 3200

Then a very challenging edge-on spiral galaxy in Andromeda, NGC 891, showing angular size of only 11.7 x 1.6 arc minutes and Mag 9.9.  This galaxy is about 30 million light years away:

NGC 891 Nikon D90 on Altair Wave 115/805, 20x45sec, ISO3200

NGC 891
Nikon D90 on Altair Wave 115/805, 20x45sec, ISO3200

With the telescope in the region of Andromeda, it would be wrong not to take a peek, wouldn’t it?  Even with minimal processing, the Andromeda galaxy M31 makes a very big impression through the telescope and deserves a full session to itself on another night.

Andromeda Galaxy through the telescope Just 6 frames of 45 sec @ ISO 3200

Andromeda Galaxy through the telescope
Just 6 frames of 45 sec @ ISO 3200

M31 and PANSTARRS

When the Cumbrian skies cleared on 1 April we had the opportunity for several nights of catching PANSTARRS in glorious juxtaposition with the Andromeda Galaxy M31.

First up, some test frames on 1 April which turned into a nice capture that I’d had in mind for some time.

50mm f/5.6, ISO1600, 2min 4 x 30-sec frames stacked in DSS

50mm f/5.6, ISO1600, 2min
4 x 30-sec frames stacked in DSS

With a standard 50mm lens and some judicious cropping, this letterbox format shows Mirach (one of the guide stars used when finding M31: “from Mirach, hop right one star, then again, then down to the fuzzy blob”) with M33 faintly visible on the left, M31 on the right and PANSTARRS making its way in from the bottom.

Quite a windy evening, so I cranked up the ISO to 1600 and took just four good frames at 30 seconds.

On to the following night…

200mm f/5.6, ISO 1600, 15 min. 30 x 30 sec. frames stacked in DSS.

200mm f/5.6, ISO 1600, 15 min.
30 x 30 sec. frames stacked in DSS.

Quite breezy again, so I kept the exposures down at 30 seconds, but on the 200mm lens.  30 frames stacked for a 15 minute total exposure.

PANSTARRS is only about 7 degrees above the horizon here.  That presents a whole new set of problems.  At this angle, the line of sight goes through about ten times as much atmosphere as at zenith, multiplying the effect of water vapour on the incoming light.  From this location, Helsington Church, the view North-West also passes over the lights of Windermere and Ambleside, giving a street light skyglow to the bottom of the frame.  One of the beautiful features of PANSTARRS is its fan tail, glowing by reflected sunlight, so any attempt to process out the skyglow tends to eliminate the tail too.

300mm f/5.6, ISO 400, 40 min. 20 x 120 sec. frames stacked in DSS.

300mm f/5.6, ISO 400, 40 min.
20 x 120 sec. frames stacked in DSS.

Another shot I had framed in my mind, estimating there would be just enough room in the 300mm frame to put both M31 and PANSTARRS.  As the wind had dropped, I could get exposure up to 2 minutes, and grabbed 23 frames of which 20 were acceptable.

Finally on 3 April…

200mm f/6.3, ISO 200, 60 min 12 x 5 min. frames stacked in DSS

200mm f/6.3, ISO 200, 60 min
12 x 5 min. frames stacked in DSS

The birthday fairy brought me a polarscope this year, which dramatically reduces the time taken to polar-align the EQ3-2 equatorial mount.  In  a couple of minutes I can align more accurately than I used to get from 30 to 40 minutes of drift alignment using the camera.  Certainly it is good enough for 5 minute exposures at up to 300mm.  I must find time to test the alignment with longer focal length.  Anyway, this stack of 12 frames at 5 minutes each has cropped nicely.  M31 is not as clear as I would have liked (see M31: up to 200mm) but it was too low in the sky for that.

There was another very pleasing alignment on 5 April but the Cumbrian clouds had closed in.

M31 Andromeda @ 300mm

Chuffed to bits with the eight-frame stack of M31 at 200mm, I couldn’t resist a quick grab at 300mm when the new lens arrived.  Fixed focal length Nikon 300mm f/4.5, manual, donkey’s years old, bought second hand on ebay, it has that most precious of attributes – a focus ring that stops at infinity.  That means you can focus on the night sky without needing to see what you are photographing.  Priceless.

M31 300mm

300mm f/4.5, ISO 3200, 180 sec.
Single frame of 3 minutes at 300mm.

This was at the end of a session experimenting with drift alignment alignment and imaging M42, I quickly turned the camera to M31 and took a single 3-minute frame as I was tidying up.

This lens is brilliant.  There is almost as much detail in this frame as in the stack of 8 x 5 minutes taken earlier in the month.  I will do more work on M31 with this lens when I get the opportunity.

M31: up to 200mm

Experience with tackling M31 using the fixed tripod and the 50mm lens made me realise that these objects (let’s not forget M31 is one of the most accessible) need longer exposure and longer focal length lenses.

That means using the equatorial mount, and all the issues of polar alignment and motor drives that go with it.  The first kit upgrade is therefore a set of motor drives, one for each axis of the EQ3-2 mount.

200mm @ f/6.3, ISO 6400, 330 sec.11 x 30 sec. frames stacked in DSS.

200mm @ f/6.3, ISO 6400, 330 sec.
11 x 30 sec. frames stacked in DSS.

Polar alignment here was by eye (using the hole where the polar scope would go if I had one), followed by several phases of 2-minute drift alignment using the 500mm reflex lens.  The whole process takes just over half an hour.  This gives acceptable results with 30 seconds exposure at 200mm.

200mm f/6.3, ISO 200, 5 min.

200mm f/6.3, ISO 200, 5 min.
Single frame of 300 sec.

Made bold by success with 30 seconds, I push the capabilities of the new motor and my skill at aligning to try five minutes per exposure.

200mm f/6.3, ISO 200, 40 min. 8 x 300 sec. frames stacked in DSS.

200mm f/6.3, ISO 200, 40 min.
8 x 300 sec. frames stacked in DSS.

After a few tweaks in Photoshop, this stack of 8 frames of 5 minutes each looks pretty good.

M31 Andromeda first attempt: fixed tripod

The moment of revelation that turned me on to the possibilities of amateur astrophotography occurred on the evening of 5th March 2012, when Jeremy Hunt (not that one) came from Cockermouth Astronomical Society to present some examples of his work in Kendal.  All his slides included a note in the corner, giving details of the focal length of the lens used.  In the Q&A, I commented that there must be something amiss – if only with my understanding of the process.  If he had taken that photo with, say, the 300mm lens as noted, then he must have enlarged or cropped it because I knew the object in the sky simply wasn’t that big.  Jeremy’s answer turned my understanding on its head: “It is that big, it just isn’t that bright”.

Some of the night sky objects are visually enormous.  The reason that we cannot see them is not that they are small, but that they are faint.  Next time you look at the full Moon, imagine 12 of them arranged six by two like eggs in a dozen box.  That’s the visual size of M31, the “Andromeda” galaxy.

The key to photographing faint objects in the night sky is not magnification, but exposure.  You don’t need a long lens, but you do need to hold the shutter open for a long time.  Unfortunately for the astrophotographer, the sky moves (the Earth rotates) so long exposures of stars appear as lines rather than dots.  Holding the camera stationary for long exposure against the rotating Earth requires technology which I didn’t have for my first attempt at M31.  However, using a 50mm lens and 5-second exposures meant I could get satisfactory results from a fixed tripod.

50mm f/1.8, ISO 3200 5 sec.

50mm f/1.8, ISO 3200 5 sec.

This is a 5-second frame using a standard 50mm manual lens at full aperture.  There is nowhere for lens defects to hide in astrophotography, and the distorted shape of the stars towards the edge of the frame is a fine example of coma.  Coma is most noticeable at wide apertures, and can be fixed by stopping the lens down.  I took 40 frames of 5 seconds and stacked them using the free software “Deep Sky Stacker“.

50mm f/1.8, ISO 3200 5-sec40 frames stacked in DSS

50mm f/1.8, ISO 3200 5-sec
40 frames stacked in DSS

This is the stacked result, cropped to pick out the galaxy.  It’s not brilliant, but it is:

  • an object that is almost invisible to the naked eye
  • photographed with a standard lens from a fixed tripod
  • stacked using free software

From here, as the song says, the only way is up.