The Cygnus Loop...

The Cygnus Loop (Sharpless 2-103)...

Objects NGC 6992-5, IC1340 (the Eastern Veil), NGC 6960 (the Western Veil or "Witches Broom"), "Pickering's Triangle" (wedge of nebulosity at "one-o-clock" position above): also NGC 6974 and NGC 6979 (knots of nebulosity to the left of Pickering's Triangle)

Type: Supernova remnant
Constellation: Cygnus
Distance: 2,400 light years

Equipment: Atik 460/EFW 2, Samyang 135mm lens@ F2, Vixen GPDX mount, guiding with Lodestar X2/PHD
Date: August 8th. 2022

Subframes: 18 x 300s for Ha, 12 x 300s for OIII, flats, no darks (hot pixel removal in Astroart). 

High overhead on a summer’s evening can be found the constellation of Cygnus (The Swan), whose brightest star (Deneb) is the top left star of the isosceles triangle of bright stars that dominates the UK summer night-time skies called the Summer Triangle.

Tucked under the Swan’s eastern wing is an enigmatic object, a supernova remnant called the Cygnus Loop. It consists of a bubble of dust filaments and fluorescent gas over 120 light years in diameter, or about 3 degrees across (6 moon diameters) as viewed from Earth, the remains of a supernova that occurred some 15,000 years ago.

The shock wave from that event is ploughing through and compressing the patchy interstellar medium of gas and dust that surrounded the exploding star, causing the rarefied gasses to fluoresce, hence the nebula we see today.

The brighter sections of the Cygnus Loop, NGC 6992-5 (the Veil Nebula) and NGC 6960 (The “Witches Broom”) are just visible through a large (8-inch aperture or more) telescope on clear, moon-free nights. Details of the central area of the bubble can only be detected on long exposure photography.

The five-minute subframes I used here weren’t really long enough to show up all of the available detail, and the stacks required a hefty tangential stretch to show the wispy central areas of nebulosity.

I used Starnet+ to remove the fog of stars in the stacked hydrogen-alpha and OIII images, which allowed some selective stretching and sharpening of the nebulosity. I then used the PSP “erode” function (“magic wand” to select background > invert > feather (3 pixels) > effects > edge > erode) to reduce the star sizes on the “normal” images, and combined the “starry” and “non-starry” images in “blend lighten” mode to give sharp nebulosity and small stars.

To make the colour image, I used the Ha channel as red, the OIII channel as blue and made an artificial green channel of 60/40 OIII/Ha. Combining the three as an RGB image gave the result at the top of this post.

Eastern Veil Nebula (NGC 6992-5)

Eastern Veil Nebula (NGC 6992-5)

Object: Eastern Veil Nebula (NGC 6992-5)
Type: Emission Nebula (supernova remnant)
Constellation: Cygnus
Distance: 1470 light years

Date: September 7th, 2019
Equipment: ATIK 460EX, Skywatcher f5.5 Espirit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 16 x 300s H-alpha, 3 x 300s OIII, no flats/darks (hot pixel removal in Astroart).

The nebula was discovered telescopically on 1784 September 5 by William Herschel.  He described the western end of the nebula as "Extended; passes thro' 52 Cygni... near 2 degree in length", and described the eastern end as "Branching nebulosity... The following part divides into several streams uniting again towards the south."

The Veil Nebula is part of a much larger supernova remnant, believed to have formed when a star 20 times more massive than our Sun, exploded around 8,000 years ago.  The remnants have since expanded to cover an area of the sky roughly 3 degrees in diameter (about 6 times the diameter, or 36 times the area, of the full Moon).   The distance to the nebula is not precisely known, but Far Ultraviolet Spectroscopic Explorer (FUSE) data supports a distance of about 1,470 light-years.

The nebula appears as a streamer of fine filaments. The standard explanation is that the outgoing shock wave from the original supernova is heating a shell of residual hydrogen gas and the wave-front is visible only when viewed exactly edge-on, giving the shell the appearance of a filament.  Given a distance of 1470 LightYears, this gives the radius of the entire nebula as 38.5 Light Years (totalwidth, 77 Light Years). At 1/50,000th of the radius, this places the thickness of each filament at around 4 billion miles, or roughly the distance to Pluto.  Undulations in the surface of the shell leadto multiple filamentary images, which appear to be intertwined.

Even though the nebula has a relatively bright integrated magnitude of 7, it is spread over so large an area that the surface brightness is quite low.  It is very hard to see visually from suburban locations, although use of an OIII filter is said to help.

The brighter segments of the nebula have the New General Catalogue designations NGC 6960, 6974, 6979, 6992, and 6995. The easiest segment to find is the Western Veil Nebula NGC 6960, which runs behind the naked eye star 52 Cygni. NGC 6992 and 6995 are also relatively bright objects on the eastern side of the loop. NGC 6974 and NGC 6979 are visible as knots in an area of nebulosity along the northern rim.  Pickering's Triangle, a triangular area of nebulosity between the east and western nebulae, is much fainter and has no NGC number.  It was discovered photographically in 1904 by Williamina Fleming (after the New General Catalogue was published), but credit went to Edward Charles Pickering, the director of her observatory, as was the custom of the day.

Over ten years ago, I managed to photograph the whole of the "Cygnus Loop" using a 135mm camera lens and my old SXV-H9 CCD camera.  The result can be seen here.

The Eastern Veil Nebula is easy to find, tucked just in front of the eastern wing of Cygnus the Swan. The constellation of Cygnus flies high in the southern sky in late summer, with its brightest star Deneb being one of the three bright stars that form the "Summer Triangle".

I had planned to take 16 300 second exposures through H-alpha and OIII filters, but clouds rolled in and I only got three OIII frames.

The separate channels were stacked in Astroart and then RGB combined, using the H-alpha as the red channel and the OIII data as the green and blue channels. The image was a bit noisy due to the lack of subframes, but using the H-alpha data as a luminance overlay to the smoothed RGB channel helped sharpen up the final image.

This was the first test run of my new Esprit 100ED refractor, coupled with a second-hand Avalon Linear mount. The mount performed pretty well after some initial problems with PHD settings, settling down to give an RMS figure of around 0.6, which was much better than my old NEQ6, which usually could only manage 0.8 at best.

I cannot say that the Esprit performed any better than my old Vixen ED114 in narrowband, however. I had hoped that the relatively small field of view offered by the Atik 460 would mean I didn't need to use a field flattener, with all of the critical requirements of camera spacing that requires. Unfortunately, there was a fair bit of star distortion in the corners of the final image, that I manually processed out.

I will see how the scope performs in RGB, but I could well be reinstating my old Vixen...

IC 443: The Jellyfish Nebula...

Object: IC 443
Type: Supernova remnant
Constellation: Gemini
Distance: 5000 light years

Date: February 10th. 2019
Equipment: ATIK 460EX, Vixen 114mm f5.3 ED114 refractor, NEQ6 mount, guiding with Lodestar X2/PHD

Subframes: 16 x 600s H-alpha, 6 x 150s each for RGB (2x2 binned), no flats, hot pixel removal in Astroart (no main frame darks).

Discovered on September 25th, 1892 by German astrophotographer Max Wolf, IC 443 is a supernova remnant in the constellation Gemini, produced by a supernova explosion thought to have occurred between 3,000 and 30,000 years ago.  Physically, the nebula is about 70 light years across. Visually it appears to be approximately 50 arc-minutes across, an area of sky just under twice the apparent size of the full moon.  It is not readily seen in backyard telescopes.

The location of IC 443 is shown in the Stellarium sky chart below, just to the east of the third magnitude star Eta Geminorium (named Propus, the bright orange star to the right of the image).

Observations from the Chandra X-ray observatory show that the explosion that created the Jellyfish Nebula may have also formed a peculiar object located on the southern edge of the remnant, called CXOU J061705.3+222127, or J0617 for short. The object is likely a rapidly spinning neutron star, or pulsar, representing the end stage of a star with between 1.4 and 3 solar masses  

When a massive star runs out of thermonuclear fuel, it implodes, forming a dense stellar core called a neutron star.  The outer layers of the star collapse toward the neutron star then bounce outward in a supernova explosion.  

A spinning neutron star that produces a beam of radiation is called a pulsar.  The radiation sweeps by like a beacon of light from a lighthouse and can be detected as pulses of radio waves and other types of radiation.

The comet-like shape of the diffuse X-ray emission suggests motion towards the lower right of the image.  The structure is also evidenced at visible wavelengths, as highlighted in the hydrogen alpha image below:

The orientation of the “line” is about 50 degrees away from the direction expected if the pulsar was moving away from the centre of the supernova remnant in a straight line.

This misalignment has cast some doubt on the association of the pulsar with the supernova remnant. 

However, it is thought that this misalignment could also be explained by movement towards the left of material in the supernova remnant pushing J0617's cometary tail aside.



Seeing conditions and atmospheric clarity were excellent on this particular evening. I was able to grab 16 hydrogen alpha frames before the dreaded "meridian flip" (where the object crosses the east/west line, requiring the telescope to be spun round to prevent it hitting its own pier).  This is a pain and requires the telescope to be re-targetted, although fortunately my set-up usually gets the object back on the CCD chip after the flip, requiring only minor adjustments to re-frame the object on the CCD chip.

Given that the weather forecast was threatening clouds, I decided to grab the RGB frames rather than get more H-alpha data, although 16 subs isn't really enough to beat the background noise down.  My luck held, and I got the last of the blue subs just as a few clouds started to roll in at around midnight.

Colour processing was conducted using the same methods as I used for the Cave Nebula.

M1 - The Crab Nebula...

Click on image to enlarge

Object: Messier 1 (NGC 1952)
Type: Supernova remnant
Constellation: Taurus
Distance: 6500 light years

Date: 27&28 December 2017
Equipment: SXV-H9, Vixen 114mm f5.3 ED refractor, guiding with Lodestar X2/PHD

Subframes: 30 x 300s H-alpha, 20 x 300s OIII, 20 flats for each channel, no darks

This object is clearly visible in my (200mm aperture) VC200L.  With averted vision some structure can be seen, even from my light-polluted sky. Visually, a dark notch on the eastern side of the nebula does give the object a passing resemblance to a crab's claw, although this is much less apparent on filtered CCD exposures than it is at the eyepiece.  I previously imaged this object in "white light" and the feature shows up quite well.

Subframes for this effort were acquired and preprocessed in Astroart.  I have started to use the hot pixel removal function rather than muck about with dark frames.  My ancient SXV-H9 is not actively cooled so the camera temperature (and hence dark noise) probably varies during an imaging session.  This seems to lead to odd artifacts over long sequences of exposures when using conventional dark frames.  Just hitting any hot pixels with the filter as part of preprocessing seems to a better job and is a lot less effort than making master darks.

For imaging M1, I decided to try colour imaging just based on H-alpha (as red channel) and OIII (as blue channel) data.  The first hurdle came when I looked at my OIII images.  I had not used this filter for CCD imaging before, and the images showed what seemed to be horrendous reflection rings around the stars (section enlarged below).

I contacted Astronomix, the filter manufacturer. They replied that the filter I had was for visual use only, and that the rings were due to it not being IR/UV blocked.  I am not so sure, but will try again sometime with a blocking filter in place.  For the purposes of this image however, I found that the situation was not irretrievable.

When I layered the bloated OIII image stack over the much sharper Ha data and blended in "darken" mode in PSP, this effectively subtracted out the halation, as below, giving a "clean" OIII channel.

To try and avoid the odd pink and blue stars that two colour-only channels seem to give in some instances, I decided to make an additional artificial green channel by layering the above image over the Ha stack and selecting the "multiply" option in PSP.  This allowed me to generate an RGB colour image, using the fake green channel, plus the above OIII image as blue and the Ha stack as red.  I then added the Ha stack back in as the luminance channel to give the final LRGB image.  A bit of tweaking in PSP (curves, cropping) gave the final image as shown at the top.

Quite how "valid" any of this is in terms of true colour is questionable of course, but I think it looks OK and not too unrealistic compared to what else is out there on the web.

The Crab Nebula is perhaps the most intriguing object in the deep sky realm.  Information about it can be found here.