 |
| 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
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:
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.
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.Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD
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.
No comments:
Post a Comment