Messier 63...

Messier 63

Object: Messier 63 (The "Sunflower Galaxy", M63, NGC 5055, PGC 46153, UGC 8334)

Type: Spiral galaxy (classification SA(rs)bc)
Constellation: Canes Venatici
Distance: 30 million light years

Date: March 19th 2021
Equipment: Vixen VC200L with x0.71 focal reducer, SX694, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 60 x 60s (2x2 binned) for luminance, 20 x 60s (2x2 binned) each for red, green and blue, flats, no darks (hot pixel removal in Astroart).

Messier 63 (M63, also known as the Sunflower Galaxy) was discovered in 1779 by the French astronomer Pierre Méchain and was the first of 24 objects that Méchain would contribute to Charles Messier’s catalogue.

High overhead on late Spring evenings, M63 is tucked away under the tail of the Great Bear in the obscure constellation of Canes Venatici.  It can be seen in binoculars as a small, hazy patch of light or an out-of-focus star.  The Stellarium sky map below shows M63’s location.

As seen in the sky from Earth, the galaxy occupies an area of 12.6 by 7.2 arc minutes, which corresponds to a spatial diameter of 98,000 light years. This makes it roughly the same size as the Milky Way, having a mass around 140 billion times that of the Sun.

Messier 63 has a distinctive appearance that gives rise to the “Sunflower” nickname, with a yellowish central disc and a number of short spiral arm segments dotted with starburst regions and dust lanes.

M63 is a prototype for a class of galaxies known as “flocculent spirals". Such galaxies seem to have many spiral arms that appear patchy and discontinuous, although infrared observations indicate that M63 is in fact a two-armed spiral structure.

Messier 63 is one of the members of the M51 Group, a group of gravitationally bound galaxies located in Canes Venatici, named after the brightest member of the group, Messier 51 (the Whirlpool Galaxy).

In 2011, astronomers discovered a tidal stellar stream in the galaxy’s halo. The faint giant arc-loop feature had been detected as early as 1979, but not connected to a minor merger with a dwarf satellite galaxy, disrupted as a result of interaction with M63. The stream of stars originated from the accretion of the smaller galaxy within the last 5 billion years. The fate of the dwarf galaxy is unknown, but the colour of the stars indicates that it was probably a galaxy belonging to the Local Group.

The recent and unprecedented run of overcast weather means that this winter’s nebula season has been pretty much clouded out. I have therefore packed away the refractor and decided to try and go for some galaxies using my ancient Vixen VC200L and a x0.71 focal reducer.  I am hoping to capture some of the brighter spring galaxies, using short exposures so that I can complete imaging projects in a single night and not have to hope that I can get two or three clear nights to collect enough data.

This was a test run of the concept and it seemed to go OK.  The subs appeared slightly vignetted and there were some horrible dust doughnuts, but a combination of flat fields and Astroart’s gradient removal tool seemed to clean up the worst of it all.

The exposures were not deep enough to clearly reveal M63’s tidal halo referenced above, but it was nice to get enough data for a colour image within three hours.

References: 

1)         https://www.nasa.gov/feature/goddard/2017/messier-63-the-sunflower-galaxy

2)              https://www.cosmotography.com/images/small_ngc5055.html

3)         https://www.messier-objects.com/messier-63-sunflower-galaxy/

Sharpless 2-273

Sharpless 2-273

Object: Sh2-273 (centred on NGC 2264, the "Christmas Tree Cluster" and Cone Nebula)

Type: Emission and reflection nebulae, open cluster 
Constellation: Monoceros
Distance: 2400 light years

Date: February 26th., March 5th., 6th., 7th., 2021
Equipment: ATIK 460EX with EFW2, Samyang 135mm lens @f2.8, Vixen GPDX mount, guiding with Lodestar X2/PHD

Subframes: 10 x 600s each for Ha, red, green and blue, flats, no darks (hot pixel removal in Astroart).

Map showing image field of view

The dim winter constellation of Monoceros is overshadowed by its more brilliant neighbours, with the twins of Gemini to the north and Orion to the west.  Nevertheless, within it can be found several interesting deep sky objects, one of which is the huge nebula complex Sharpless 2-273, an area approximately 5 degrees across. The Stellarium map opposite shows the location of the field of view of the main image above.

At the centre of the nebulosity is the 4th-magnitude open cluster NGC 2264, which was discovered on Jan 18, 1784 by William Herschel.

The surrounding nebulosity is a complex consisting of dark absorption nebulae, emission nebulae, reflection nebulae, and the stars that illuminate or outline their structures. At one end of the brightest central area, the “Cone Nebula” is a dense cloud of gas and dust sculpted by stellar winds from an extremely hot, bright star which is completely hidden in visible light by the gas and dust in front of it. Scattered across the central area are a number of bright stars which look like lights strung on a Christmas tree (hence the cluster’s popular name of the “Christmas Tree cluster”) with the Cone Nebula at the apex of the tree, and the bright star S Monocerotis and the “Fox Fur Nebula” near the base. The Fox Fur nebula is not generally considered a part of NGC 2264, but is certainly an extension of the gas and dust filling the region, as all the stars and clouds of gas and dust lie at about the same distance from us. 

I took a narrower field image of the area back in 2019, which shows this colourful area in more detail.

A Hubble Space Telescope view (below left) of the Cone Nebula shows the dense clouds of gas and dust in a region only a couple of light years across. The overall size of the Cone is about 7 light years.

Cone nebula in visible light (left), NGC 2264 in IR (right)

 The Spitzer Space Telescope image (above right) shows open cluster NGC 2264 in infra-red. The brilliant star near the Cone nebula is NGC 2264 IRS, the source of the stellar winds sculpting the Cone. Despite its brilliance, this star is completely hidden by the gas and dust in front of it. Only infrared images can penetrate the dust and reveal the star; but when they do, its brilliance dwarfs that of the other stars in the region.

Just to the southwest of the Cone Nebula lies a tiny fan of nebulosity designated as NGC 2261.  It appears as a tiny comet-shaped blob on the image above (enlarged below).

Although discovered on Dec 26 1783, by William Herschel, the nebula is named after the American astronomer Edwin P. Hubble, who carried out some of the early studies of this object.

It is a fan-shaped cloud of gas and dust which is illuminated by R Monocerotis (R Mon), the bright star at the bottom end of the nebula. Dense condensations of dust near the star cast shadows out into the nebula, and as they move the illumination changes, giving rise to the variations first noted by Hubble. The star itself, lying about 2,500 light-years from Earth, cannot be seen directly, but only through light scattered off of dust particles in the surrounding nebula. R Mon is believed to have a mass of about 10 times that of the Sun, and to have an age of only 300,000 years. There is probably a symmetrical counterpart of the fan-shaped nebula on the southern side of the star, but it is heavily obscured from view by dust lying between this lobe and our line of sight.

This image was my first in over 3 months, thanks to almost continuous cloud cover in my corner of the world.  As always seems the way, the evening of February 26th. was plagued by a high haze and a 100% full moon, but was adequate for H-alpha imaging. Of the 12 x 600 sub-frames I collected before the haze became too dense, I had to discard two due to bright aircraft trails (what lockdown travel restrictions are they subject to, then?). Fortunately, a later bonus (and completely un-forecast) string of clear, moonless evenings in early March allowed me to get some RGB colour data as well. 

The subs for each channel were sigma stacked in Astroart, with the result treated with AA’s gradient removal tool and then given a log stretch. Each stacked channel was then run through Starnet and the resultant starless image given an edge preserving smooth and “clarified” in Paint Shop Pro.  This proved particularly effective for the R, G and B stacks, where the stars were otherwise overpowering: Starnet allowed the dog to see the rabbit when attempting to pull out nebulosity, especially in the blue channel.

The starless stacks are shown below:

Ha channel - stars removed with Starnet

Red channel - starless

Green channel - starless

Blue channel - starless

The R(R = 70:30 R/Ha), G and B channels were colour combined in PaintShop Pro to give this rather striking RGB starless image...

RGB starless combination

The starless Ha layer was pasted back over the above as a luminosity layer at around 30%: this helped to improve detail in the central Cone and Fox Fur area. 

A less aggressively stretched version of each of the R, G and B stacks to exclude nebulosity was given a Gaussian blur to tidy up the rather under-sampled stars, then RGB combined, star-reduced, then given a hefty saturation tweak to produce a colour star layer...

RGB star layer

This was pasted back over the starless HaRGB image in screen mode, with some final selective sharpening and contrast/colour adjustment in PSP to give the final image.   

References:

1)           https://cseligman.com/text/atlas/ngc22a.htm#2264

2)           https://hubblesite.org/contents/media/images/1999/35/904-Image.html

Pleiades (M45) and Mars...

Conjunction of M45 and Mars

Object: M45 (the Pleiades or Seven Sisters), Mars

Type: Open cluster and planetary conjunction 
Constellation: Taurus

Date: March 5th., 2021
Equipment: ATIK 460EX with EFW2, Samyang 135mm lens @f2.8, Vixen GPDX mount, guiding with Lodestar X2/PHD

Subframes: 20 x 10s each for luminance, red, green and blue, flats, no darks (hot pixel removal in Astroart).

This image was an unplanned shot; I was attempting to gather 600 second subs for my Sh2-273 project, but clouds began to intermittently drift over, making long subs impossible. Rather than waste a rare clear, dark and moon/free evening, I swung the rig around to M45 and its conjunction with Mars. The planet is long past opposition but it was still as bright as nearby Aldebaran, and was only about 2 degrees from the cluster.  As it won't be this close to the Pleiades until March 2036,  I thought it was worth capturing.

The short and numerous exposures allowed me to reject the cloud-fogged ones. I was pleasantly surprised by the amount of cluster nebulosity captured given the very short exposure length.

Each of the R, G and B channels were stacked in Astroart, and a nasty gradient removed with the AA plug-in.  An RGB image was prepared in PaintShop Pro. No stretch was applied, but a mild Gaussian blur was used to smooth out the rather blocky stars (the 4.54u pixels are rather under-sampling at 135mm).

The luminance stack was similarly gradient-scrubbed then blurred, and star reduction applied in PSP (background selected with "magic wand", inverted to select stars, selection expanded and feathered by a few pixels, then "eroded").  A mild edge-preserving smooth was applied to reduce noise, and a selective mild stretch applied to bring up the Pleiades luminosity was applied.  This was then pasted over the RGB one in luminance mode to given the final image. 

Moonrise over Borstal...

 

Moonrise...

A quick shot of this evening's full moon rising, taken using a Sony HX90 in twilight mode...

Lunar halo...

 

In the continued cloud-enforced absence of any deep-sky stuff, this was a shot of a lunar halo taken this evening with a hand-held Sony HX90 in shutter speed mode, combination of 1/1600th and 1/8th second exposures...

The Gamma Cassiopeiae nebula...

 

IC63 and IC59

Object: IC59, 63 (Sharpless 2-185)
Type: Emission and reflection nebulae 
Constellation: Cassiopeia
Distance: 600 light years

Date: November 2nd, 3rd, 4th, 15-16th. 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 47 x 600s Ha, 10 x 600s each for RGB (2x2 binned), flats, no darks (hot pixel removal in Astroart).

IC63 (the bright, pointed object near the middle of the image) and IC59 (the fainter bluish area above and to the upper right) are the brightest areas of the nebulosity known as Sharpless 2-185 (Sh2-185). They were discovered independently by Max Wolf (Dec 30, 1893) and Edward Barnard (Feb 2, 1894). 

Together, the nebulae occupy an area of space approximately 10 light-years across. 

The bright, hot star Gamma Cassiopeia (seen at the bottom right of the above image) is located only 3 or 4 light-years from the nebulae, and it may have shed this nebulous material into the space around it. The edges of the nebulae glow brightly from this intense radiation that is slowly evaporating and lighting up these flowing shapes of gas and dust 

Gamma Cassiopeia has a radius 14 times greater than our Sun and is 55,000 times more luminous, 19 times more massive, and rotates at about 300 km/hour, or 150 times more rapidly than our Sun. It is known as an eruptive blue-white sub-giant variable star. (Eruptive variable stars vary in brightness because of violent processes and flares in their coronae and chromospheres.) 

This star is an erratic variable that reached a maximum brightness in 1937, but then unexpectedly dropped in surface temperature from 12,000°K to 8500°K. It is encircled by a surrounding gaseous disk of material thrown off by its rapid rotation. Mass loss is apparently related to the brightness variations. 

Stellarium map showing image field

The constellation of Cassiopeia can be found virtually overhead during late autumn evenings (see map opposite). IC 63 (the brighter of the two and slightly closer to Gamma Cassiopeia than IC 59) is a combination of an emission and reflection nebula. Unlike a reflection nebula which appears blue, the glowing hydrogen gas appears red. IC 59 is primary a refection nebula, showing much less red hydrogen, and is appearing blue of dust reflected starlight that is passing through it. 

As can be seen above, the two nebulae have very different visual appearances. IC63 can be referred to as a "cometary cloud", is pointing toward Gamma Cas, and is narrower and more sharply defined than IC59. Spectral measurements suggest that IC59 is slightly cooler at 590K and less dense than IC63 at 630K. They are not actually separate nebulae, but are part of a much larger nebulous region surrounding Gamma Cas based upon the WHAM (Wisconsin H-alpha Mapper) survey. IC63 has bright filaments, visible in the image above, that are believed to be ionized fronts of gas created by Gamma Cas, and seen by us as nearly edge-on. 

Both nebulae exhibit spectroscopic evidence from the mid-infrared of molecular hydrogen and polycyclic aromatic hydrocarbons (PAH). The contrast in appearance between IC63 and IC59 is consistent with a difference in actual distances from Gamma Cas and small differences in temperature and column density..

There is some discussion in professional circles as to whether the H-alpha signal that we pick up in our images is actual emission from the nebula, or a reflection of the H-a emitted from Gamma Cas that is scattered by the dust in IC59 and IC63. This light scattering and reflection is called ERE, (Extended Red Emission). Gamma Cas is the prototype B0 IV star, emitting significant H-a. It is above the main sequence with a more extended atmosphere. Being somewhat cooler than a BO V star, it is only marginally capable of ionizing molecular hydrogen in its vicinity. Thus, it is possible that the H-a we pick up in our images is a mixture of both processes; direct H-a emission from ionization, and ERE. 

H-alpha data was gathered on the bright moonlit evenings of 2-4 November. The bright (and damp!) conditions were not ideal for the imaging of what is quite a faint nebula, but given that the previous month’s skies had been completely clouded out, it seemed best to make do.  All three evening sessions were ended at around midnight due to high haze and mist setting in, and the resultant image stack suffered from background noise. 

Application of Starnet++ to the Ha stack allowed the nebulosity to be stretched and smoothed, with stars from the “unstretched” stack being pasted back in in “screen” mode to avoid star bloat. 

Colour data was acquired on 15-16^th November, with the moon mercifully absent: the imaging session was still stop-start due to clouds and was finally clouded out at around 2.00 am. 

The blue reflection nebulosity was pretty faint but once again Starnet++ allowed separation of the nebulosity from stars for processing purposes.  The Ha data was blended in with the red channel and the blend used in an RGB composite in PSP.  A partial luminance overlay of Ha was used to sharpen the image up a bit. 

References: 

1.         Astrodon imaging 

2.         NASA APOD 

3.                Anne’s Astronomy News 

4.         https://cseligman.com/text/atlas/ic0a.htm

Mars...

 

Mars, showing rotation over 90 minutes

Object: Mars
Type: Planet 
Constellation: Pisces
Distance: 45 million miles

Date: November 7th. 2020
Equipment: Phillips TouCam Pro, x2 barlow, Celestron C9.25, Vixen GPDX mount

Subframes: 15 images, each compiled from 1000 frames shot at 30 fps and aligned/stacked/wavelet processed in Registax v2.

The above gif is made from a series of images of Mars, and shows the rising of the feature of Martian geography known as Sirtis Major over the eastern limb of the planet over a period of about 90 minutes.

The evening of November 7^th was a foggy, soggy one, but Mars was very visible above the vapours and the evening represented one of the few opportunities I had to try and photograph the planet during its current opposition. 

Mars made its most recent close approach to Earth on October 6^th, when it was 38.5m miles away and appeared around 23 arc-seconds across in the sky. It won’t get that close again until September 2035, an event I am unlikely to witness. 

The closest Mars ever gets to Earth is about 34.5 millions miles and thus never appears to be more than around 25 arc-seconds across, effectively still looking like a bright star. Bear in mind that the full Moon is around 30 arc-MINUTES across and you can see how stupid social media stories of “Mars as large as the full moon” really are. 

Although a month past opposition, Mars was still a respectable 18.8 arc-seconds across and at magnitude -1.9, was brighter than any star and nearly as bright as Jupiter, which was setting in the west. 

To photograph Mars, I set up my old Celestron C9.25 SCT, and used an ancient (c. year 2000) Phillips TouCam Pro webcam as the image capture device. 

Planetary details are extremely small and views are constantly distorted by atmospheric fluctuations. Using a video camera to record 20 or 30 frames a second for a minute or so gives a bank of thousand or so pictures that can be processed by software (I use Registax) that selects the sharpest few frames and stacks them together to give a detailed result – in theory. In addition, Mars has a day that is about the same length as Earth’s and it therefore rotates quite quickly: a video longer than about a minute will suffer from blurring caused by the movement of planetary features as the planet spins on its axis. 

In practice, achieving a sharp focus on an image that is bouncing around your laptop screen like a demented ping-pong ball is a pretty tall order, and conditions have to be exceptional to get images that show much detail. This wasn’t one of those evenings, and indeed, I gave up at around 8.30 because the dew was dripping off of everything and was worried it would get into the electrics! 

Nevertheless, the individual processed image stacks showed up recognisable features when compared with the BAA’s Mars mapper…    

 

Mars at 19.30 compared to BAA map (click on image to enlarge)

My image shows a hint of the Martian phase at the time (97%) which is not shown on the Mars map above.

NGC 6960: The Western Veil Nebula...

NGC 6960 in HOO

Object: NGC 6960 
Type: Supernova remnant
Constellation: Cygnus
Distance: 1,470 light years
Date: September 23rd. 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD
Subframes: 10 x 300s Ha, 10 x 300s OIII, no flats, no darks (hot pixel removal in Astroart).

Stellarium map showing location of field of view

Tucked under the eastern wing of Cygnus, NGC 6960 (also nicknamed the “Witches’ Broom”) is a remnant of a supernova that is believed to have occurred approximately   8,000 years ago.  It is part of a larger nebula covering nearly 3 degrees of sky, and which includes the Eastern Veil nebula NGC 6992-5.  The link to my earlier image of the other half of this nebula gives more information about the object.

The image above is an HOO composite. Both seeing and transparency were not good during the evening of September 21st. and the resultant sub-frames were rather noisy, a sure sign of high altitude haze. I was also too lazy to take more or longer ones, although I may revisit this object in the near future. After grappling with a series of objects with very weak OIII signal, at least that of NGC 6960 is reasonably strong and facilitates image processing.

NGC 7000: The North America Nebula...

NGC 7000/IC 5070 in H alpha

Objects: NGC 7000 (North America nebula), IC 5070 (Pelican Nebula)
Type: Emission nebulae
Constellation: Cygnus
Distance: 1,500 light years
Equipment: SX Pro 694, Samyang 135mm lens@ F2, Vixen GPDX mount, guiding with Lodestar X2/PHD
Date: September 14th. 2020
Subframes: 6 x 300s Ha, no flats, no darks (hot pixel removal in Astroart).

Stellarium map showing field of view

On October 24, 1786, William Herschel, observing the area of sky around Deneb (the alpha star of the constellation Cygnus) from Slough, England, noted a “faint milky nebulosity scattered over this space, in some places pretty bright.” The most prominent region was catalogued by his son John Herschel on August 21, 1829. It was listed in the New General Catalogue as NGC 7000, where it is described as a "faint, most extremely large, diffuse nebulosity.” 

On December 12, 1890, the German astrophotographer Max Wolf noticed the characteristic shape of the eastern part of the nebula on a long-exposure photograph, and dubbed it the North America Nebula. In addition, in a paper of June 10, 1891 he described the region near that nebula as photographed on a 3 hour plate taken on June 1 of that year.  However, an accurate position of the Pelican Nebula had to wait until Sep 7, 1899, where the object was described by British astronomer Thomas Espin. 

In his study of nebulae on the Palomar Sky Survey plates in 1959, American astronomer Stewart Sharpless realised that the North America Nebula is part of the same interstellar cloud of ionised hydrogen as the Pelican Nebula, separated by a dark band of dust, and listed the two nebulae together in his second list of 313 bright nebulae as Sh2-117.  American astronomer Beverly T. Lynds catalogued the obscuring dust cloud as L935 in her 1962 compilation of dark nebulae. Dutch radio astronomer Gart Westerhout also detected the HII region Sh2-117 as a strong radio emitter, 3° across, and it appears as W80 in his 1958 catalogue of radio sources in the band of the Milky Way. 

One of the most famous bright nebulae in the heavens, the North America Nebula is shaped very much like its namesake. Despite its relative brightness, its large size and low surface brightness make it undetectable with the unaided eye except in very dark skies, and even then only by using special filters to increase the contrast of its line radiation. The North America and Pelican nebulae (IC 5070) are part of an approximately 100 light year-wide ionised hydrogen region. Their shapes and apparent separation are due to clouds of obscuring dust lying between us and them. 

What star or stars are responsible for heating the gas has long been unknown, but recently the 2MASS infrared telescope, concentrating on the area obscured by dust, has shown that there is a massive O-type star in the general area of the nebulae, which is the most likely source of their radiation. Estimates of the distance of the North America and Pelican nebulae vary considerably, ranging from as little as 1500 light years to as much as 2200 light years. 

The image above maps the hydrogen alpha emissions of the region.  This represents “first light” of my new wide-field imaging system, comprising of a Samyang F2 135mm focal length lens coupled to a Starlight Xpress SX PRO-694 camera.  This gives a 5 x 4 degree field of view. 

To carry the lens and camera, I refurbished my old Vixen GPDX mount, re-greasing it and carefully adjusting the RA and declination worm drives to try and eliminate the horrendous backlash that had always plagued it. I also (reluctantly) retired its old and increasingly unreliable Skysensor control unit, replacing it with a Skysensor EQ5 upgrade kit which allows me to control the mount via EQASCOM.

Although such a short focal length system probably doesn’t need auto-guiding, I had a spare guide camera and a Vixen 450mm focal length guide scope, so I thought I may as well use them. 

The set-up is intended to be a portable one, although I will be setting it up in the same position in my garden.  I Araldited three steel washers to the hard-standing where I would be setting up the tripod, and used the GPDX’s excellent polar-scope to polar align. 

It all seemed to work pretty well first time. PHD reckoned the polar alignment error was only around 1.5 arc-minutes, with an RMS guiding accuracy of around 0.6”, way better than needed (and better than my observatory Avalon mount!), so I think the mount refurb went pretty well. 

At the moment I am manually focussing the lens but at F2, it is extremely sensitive and I may need some engineered assistance.  Spacing between the lens and the CCD camera is also critical. Fortunately I managed to find an assembly of various adaptors that connected the lens to the camera via my old ATIK manual filter wheel that manage to land the Ha focus point exactly on the “infinity” point of the lens. 

Manual focussing required a deft touch but was relatively easy using the focus indicator on the Astroart camera control module. For starters, I shot 6 x 300 second exposures in Ha at F2 (via an old 12nm Ha Astronomik filter) and was very pleased with the sharpness and detail in the single subs. There was a small amount of flaring around the brighter stars that I attributed to the filter and some small distortion of the stars in one corner of the image field, but nothing disastrous. 

OIII flares around stars

Attempting to use an old Baader filter for OIII imaging was a complete washout however, as the flaring around all stars made the sub-frames unusable. That filter will have to be replaced, hence the mono image above. 

I used Starnet to remove the stars from the stacked Ha data as I found the snowstorm of the Milky Way to be distracting. This allowed some minor selective sharpening and stretching of the nebulosity, although the data was pretty good even though I only had 30 minutes-worth of subs.  I restored the stars by layering a “de-stretched” and slightly Gaussian-blurred version of the original stack over the Starnet version in “blend” lighten mode. 

References: 

1. https://apod.nasa.gov/apod/ap171201.html 

2. https://en.wikipedia.org/wiki/North_America_Nebula 

3. https://cseligman.com/text/atlas/ngc70.htm 

4. https://cseligman.com/text/atlas/ic50a.htm#ic5070

Sharpless 2-115: The Troll Nebula...

Sharpless 2-115

Object: Sh2-115 (containing star cluster Berkeley 90)
Type: Emission nebula with open cluster
Constellation: Cygnus
Distance: 7,500 light years

Date: September 13th/14th. 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 12 x 600s Ha, 12 x 600s OIII (3x3 binned), flats, no darks (hot pixel removal in Astroart).

High overhead during early autumn evenings flies the constellation of Cygnus. The Milky Way forms a star-strewn backdrop to the celestial swan, where many deep-sky objects can be found. One such object is Sharpless 115, an emission nebula that can be found 2 degrees north-west of Deneb, the alpha star of Cygnus the Swan and easternmost star of the Summer Triangle.

Stellarium map showing location of field of view

Noted in his eponymous 1959 catalogue by astronomer Stewart Sharpless, this faint nebula lies along the edge of one of the outer Milky Way's giant molecular clouds, about 7,500 light-years away. Fluorescing with the light of ionized atoms of hydrogen, sulphur and oxygen, the nebulous glow is powered by hot stars in star cluster Berkeley 90 (the cluster of small stars just below left of centre in the field of view above). The cluster stars are thought to be only 100 million years old or so and are still partly embedded in its parent nebula. 

To the northwest of Sh-115 can be seen dim streaks of nebulosity catalogued as LBN 362 (the LBN designation referring to the Lynd’s Bright Nebula catalogue). Embedded within that is a small bright circular emission nebula given the designation SH2-116. This object was first classified as a planetary nebula (hence its alternative designations of Abell 71 or PK85+4.1) but recent studies show it to be an HII emission region instead. 

The evening of September 13^th offered good “seeing” (the air was steady) but less than perfect transparency. Hydrogen alpha data of this faint object showed up well, but was slightly blurred by what seemed to be very faint high-altitude haze. The stars were a little bloated, even though guiding and focus seemed good. Nevertheless, the miracle of Starnet allowed me to remove the stars from the image so that I could polish up the nebula (mild stretch, denoise, despeckle, unsharp mask) and then add the stars back in afterwards.

(I used a “destretched” version of the Ha stack that just showed the brighter stars, although I did paste in a bit of stretched stack to show up the Berkeley 90 cluster).

"Starnet" version of Ha data, stars restored

Needless to say, anything less than perfectly clear air hamstrings any attempt to gather OIII data on such weak sources such as Sh2-115.  Indeed, only a trace of signal could be found on 600 second exposures binned at 3x3. As a result, even a stack of 12 of such frames looked pretty awful. 

Raw OIII data stack

I ran this though Starnet and was surprised to find that it worked though, just leaving behind some of the more bloated stars that I could manually clean up. A combination of the 3x3 binning and weak signal still left a very grainy stack, so I applied a strong Gaussian blur (radius 5) to the starless frame before adding a star layer back (made from the original OIII stack, with brightness and contrast strongly adjusted to leave just the brightest stars, which were then Gaussian blurred, radius 2 to remove the binned “blockiness”) in “blend lighten” mode.

Final OIII stack

An HOO composite from the Ha (as red), OIII (as blue) and a 70/30 OIII/Ha blend was then used to give an RGB colour image, which I sharpened up a bit by adding the Ha data back over as a 50% luminance layer. At first I couldn’t turn a strong magenta hue in the nebula into the more pleasing OIII-related blues without getting weird colours elsewhere. Eventually, I took the OIII layer and pasted it over a blue background in “hard light” mode, that gave blue OIII nebulosity and white stars on a dark background. This got pasted over the colour frame in “dodge” mode at around 20%, which succeeded in bringing out the blue without overly affecting the rest of the colour balance.  

A bit of selective tweaking in curves gave the final colour image above, such as it is. 

After I put this post up, an acquaintance showed it to their astronomy-mad grand-daughter. She thought that the nebula looked like a troll, a horned and bearded beast who was trying to grab the Berkeley 90 cluster in two giant paws. 

I can see where she was coming from, hence the amended title of this post… 

Sh2-115 with extra added troll...

References: 

https://apod.nasa.gov/apod/ap130614.html 

http://www.astromaster.org/oggetti/sharpless_data/Sharpless_r.pdf 

https://jthommes.com/Astro/SH2-115_116.htm 

https://heasarc.gsfc.nasa.gov/db-perl/W3Browse/w3hdprods.pl