NGC 246 Skull Nebula HaOIII + RGB stars (12.2 hours)

[Amaranthus]

NGC 246, "The Skull", is a eerie-looking planetary nebula in the constellation Cetus, about 1600 light years distant. It has an angular diameter of 4.5'. The central white dwarf (part of a binary) was mag 9 in 1930 but has since dimmed to mag 12. The complex braided structure is a result of shock waves as the dying star expelled its outer layers.

The image was captured in narrowband from my Adelaide surburban backyard, using an Skywatcher 120 mm achromat refractor (at f/4), mounted on an AZ-EQ6. This telescope is excellent for narrowband work as it has a decent aperture, fast focal ratio, and under these conditions, doesn't suffer from CA. The CCD camera was an Orion StarShoot G3 mono (dithered and drizzled), guided with PHD2 using a thin OAG and an ASI120MM-S guide camera.

The image is composed of the following:
Ha = 18 x 20 min
OIII = 18 x 16 min
R, G & B = 28 x 60 sec for each channel

Total integration time = 12.2 hours.

Synthetic luminance channel was created by blending an exposure-based weighting of the Ha, OIII, R, G and B. All subs were unbinned. Nebula colour was mapped as Ha:Ha:OIII.

Captured over the period 19 to 22 Sept 2014. Pre-processed with flats (light box), bad pixel map (based on 70 darks) and bias in Nebulosity. Aligned and Drizzled in DSS. Post-processed in StarTools.

Full capture/processing details and higher rez version here: http://www.astrobin.com/125368

[Camelopardalis]

Beautiful Barry :thumbsup:

[RickS]

Nice work, Barry!

[Amaranthus]

Thanks guys - I'm pretty happy with how it turned out, given that it was my first go at NB with the achro, and most other people's attempts at this sucker have been a little... underwhelming! I worked hard to bring out the skeletal look ;)

[Geoff45]

Nice work Barry. You've got to give the difficult stuff a try sometimes. As someone once said: "Life wasn't meant to be easy"

[Amaranthus]

Exactly right Geoff.

And to paraphrase a certain Skywalker... "One day, I'm going to image them all!"

For reference, below is the equivalent version from the book "Imaging the Southern Sky" - in RGB rather than narrowband.

[MortonH]

Whoa! That thing is creepy!

Amazing what these fast achros are capable of in narrowband. Great result. Much better than the RGB one above.

[cometcatcher]

That's excellent Barry!

[Amaranthus]

Thanks Morton & Kevin! I agree that it's one of the weirder DSOs out there - one of the things that attracted me to try and image it.

Hey Kevin, remember when I was asking advice about what rich-field visual scope to get, and you recommended the SW120? Glad I took your advice - it's not just a one-trick pony thanks to narrowband filters and a field flattener...

[LightningNZ]

That's really cool Barry! Which flattener do you use? I tried one on my 102mm F/5 Celestron but it didn't work - it looked like massive astigmatism resulted. I'm really keen to get simultaneous narrow-band and one-shot colour going.

[Amaranthus]

Cam, I use the Tele Vue 0.8x FR/FF (TRF-2008). Works great for both the SW120 and ED80T. It would suit your 102 f/5 too, I reckon - just make sure you get the spacing (56 +/- 2mm) correct.

[LightningNZ]

Thanks, it's a nice idea, but it costs twice what the original scope cost! :eyepop:

[Amaranthus]

True. I got mine for the apo, and then used it opportunistically for the achro!

[Placidus]

A challenging object for the aperture - well done. Thanks for the info on the dimming of the white dwarf - that was interesting. Wonder if it's rapidly cooling, or if it's being progressively obscured by dust.

[atalas]

Good catch!

[Amaranthus]

Thanks Mike & Louie!

The fading is definitely not due to the white dwarf cooling - it is estimated to take approximately 1 trillion (1000 billion) years to reach the black dwarf stage, so the luminosity change would be imperceptible over a few milllennia. Obviously, there are none in the Universe right now. I think your dust/gas envelopment theory is probably correct.

[Ross G]

Great capture Barry.

Ross.

[Amaranthus]

Thanks Ross, I love this fast achro for narrowband work.