The Hockey Stick and the Whale...

NGC 4656-7 and NGC 4631

Objects: NGC 4656-7 (the Hockey Stick galaxy) and NGC 4631 (the Whale galaxy)
Type: Barred Spiral Galaxies (NGC 4656-7 morphological classification SB(s)m pec), NGC 4631 SB(s)d
Constellation: Canes Venatici
Distance: 30 million light years

Date: March 26th, 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 10 x 600s luminance, 6 x 300s (2x2 binned) each for RGB, flats, bias as darks (hot pixel removal in Astroart). Above is an 80% crop of the original image.

As the winter turns to spring, so the night sky turns from a host of local galactic nebulae to the dim and chillingly distant realm of faraway galaxies. The pair of interacting galaxies pictured above can be found in the obscure constellation of Canes Venatici, tucked away beneath the handle of the Plough (see map below).

Field of NGC 4656-7 and NGC 4631

Both galaxies are classified as "barred spirals", that is, spiral galaxies that contain a central bar. However, gravitational interactions between the pair (separated by about half a million light years) have warped and twisted the smaller galaxy (NGC 4656-7, bottom left) into its contorted shape, giving rise to its nickname.

Discovered on March 20th. 1787 by William Herschel, the Hockey Stick was classified as two separate nebulae, long before their true identities as a single remote galaxy was known.  NGC 4656, the blade of the hockey stick, is a bright starburst region, originally observed as a separate entity from the main body of the galaxy, NGC 4657.

The eagle-eyed Herschel discovered NGC 4631 on the same evening.  We see this galaxy inclined only 5 degrees from edge on.  Radial velocity data suggests that if we were able to see this object from above, it would be a late-type barred spiral with very clumpy and loosely-wound arms.  Its gravitational interactions with NGC 4656 and the small satellite galaxy lying just above it (NGC 4627) have triggered a wave of star formation within NGC 4631 giving rise to its mottled appearance, with bright starburst regions standing out from surrounding galactic dust clouds.

NGC designations for the NGC 4631 group

The NGC designations are shown opposite.

The evening of March 26th was blighted by air pollution, despite the whole country being on coronavirus shut-down (plus side: no aircraft trails!) resulting in rather poor backgrounds a and noisy subframes. I also had some problems with the Avalon mount, which seemed to be "twitching" in RA, though it didn't seem to degrade image quality too much.

I have tightened up the RA clutch and I hope that problem has gone away!

M101 - The Pinwheel Galaxy...

M101 full frame, north is up.

M101, selective stretch to show outer spiral arms (north is to the right)

Object: M101 (NGC 5457)

Type: Spiral Galaxy (morphological classification SAB (rs) cd)
Constellation: Ursa Major
Distance: 20.9 million light years

Date: March 22nd, 23rd, 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 30 x 600s luminance, 10 x 300s (2x2 binned) each for RGB, flats, bias as darks (hot pixel removal in Astroart). Above is a 70% crop of the original image.

I have had a couple of previous attempts (see here and here) at imaging this well-known and oft-imaged galaxy, always with mediocre results.  My Bortle 5 suburban sky makes LRGB imaging of this low surface brightness object a challenge, peering through a murk of light pollution that narrowband filters otherwise suppress. Spring weather in the Medway Valley is also a challenge, with skies often blurred by high cloud or a faint mist over the river.  

Nevertheless, I decided that I would try once again to do this famous galaxy justice given a rare couple of reasonably clear and moonless nights. The air was fairly unsteady and the autoguiding varied anywhere between 0.7 and 1.2 RMS, but fortunately this did not give rise to any weird star shapes (although it probably cost me some detail).

The Aurora Medwayalis always seems to put a nasty gradient of light pollution across any subframes I take towards my north-eastern outlook, even when quite high in the sky.  Fortunately the gradient removal plug-in for Astroart does a good job of flattening that out, although it is difficult to get a consistent background in each of the RGB component stacks. This gives rise to some blotchy colour noise in the dark sky background that has to be crudely hammered out with noise removal and careful background colour adjustments in PaintShop Pro. In the end, I cheated a bit by using the "magic wand" in PSP to carefully select the background without leaving behind stars, applying a "flood fill" at around 20% transparency of 20/20/20RGB, then using curves to make sure the final background comes in at around 20/20/20. This seems to get rid of the worst of the coloured blobs lurking in the dark bits without totally wiping out faint background stars.  It does make the background a bit black for IPad screens, but cranking up the brightness then starts to show colour noise again, so I've left it as is. It looks fine on my (carefully calibrated!) PC screen.

The image above is my best effort to date though, and I am satisfied enough to leave this one alone now. Actually, I'm not really. I think the raw data is probably workable, but maybe I need to look at better processing.

There are also several faint background galaxies in the full camera field of view. I have tried to identify some of these on the luminance frame below:

Annotated luminance frame of M101 showing other objects in field

A star atlas also shows the disc of M101 to be strewn with numerous NGC objects. These are bright star fields in the galaxy itself, some initially observed in 1851 by Bindon Stoney using the Birr Castle reflector, with others being noted a few years later by William Herschel, long after the recognition of M101 as a celestial object by Pierre Mechain in 1781.  Mechain had observed the 7" core of what was eventually found to be an object some 28" across (the full moon is about 30" in diameter). It took the advent of larger telescopes and astrophotography to realise that the various observations were of parts of the same extremely faint object.

Further information about this galaxy can be found in an earlier post here. 

The Spider and the Fly...

NGC 1931 and IC 417

Objects: NGC 1931 (star cluster with emission/reflection nebula Sharpless 2-237, IC 417 (Sh2-234, Spider nebula, with star cluster Stock 8)
Type: Emission Nebulae 
Constellation: Auriga
Distance: 7500 light years

Dates: 12th. March 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 3 x Red (300s, 2x2 binned), 3 x Green (300s, 2x2 binned), 6 x Blue (450s, 2x2 binned), 9 x 600s H-alpha, flats for each channel, no darks (hot pixel removal in Astroart).

Stellarium map showing field of IC 417

Between Theta Aurigae and Iota Aurigae (the two southernmost stars of the “kite” of the constellation of Auriga) lies a string of emission nebulae. The above image shows another two faint areas of nebulosity lying between the brighter and better known nebulae IC 410 and IC 405 and the "Pumpkin Patch".

Two large OB associations have traditionally been identified in the constellation of Auriga, aligned one behind the other. The first is indicated with the abbreviation Auriga OB1.  Twelve stars of the spectral classes O and B were initially designated as effective members of the association, to which are added three red supergiants at the end of their life cycle. The suggested distance was about 1750 parsecs and was therefore placed on the galactic Arm of Perseus.  The second association is indicated as Auriga OB2 and eight class O and B stars were associated to it, mostly on the main sequence; its distance was indicated at least 6300 parsecs, i.e. in a remote area of the outer disk of the Milky Way, on the Cygnus Arm.


The estimated distance of IC 417 suggests it to be close to the Auriga OB1 association.

The image itself is fairy noisy, as clouds cut short the imaging session and I was only able to grab nine Ha frames to use as luminance: not that I would have been able to get to many more anyway, as the object was sinking into the western horizon.

Moon...

Moon, 88% of full phase

Object: Moon
Constellation: Cancer
Distance: 367980 km (228652 miles)

Dates: 6th. March 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 100 x  H-alpha (0.001s), 100 x OIII (0.001s)

The H-alpha and OIII frames were stacked in Registax and wavelet sharpened. The histograms of each were carefully stretched to match each other, and then combined as HOO for RGB.  Unsurprisingly, the image is not particularly colourful: some images of the moon you see have lurid blues, greens and reds but such colours are largely artefacts.

I used the Ha and OIII filters largely as light rejection filters, as the moon is otherwise so bright it would overwhelm the sensitive CCD camera.  There was little visual difference in appearance between the Ha and OIII stacks.