The extended halo of NGC 253

[RickS]

NGC253 captured over two nights last weekend at the AAQ Dark Sky Observing weekend at Glen Aplin (props to John Salini for being such a gracious host!)

Scope: Ceravolo C300 @ f/9 = 2760mm FL, Atlas focuser
Mount: AP900
Camera: U16M
Filters: Astrodon E series Gen 2 LRGB
Guiding: Lodestar / MMOAG
Image scale: 0.67 arcsec/pixel
Exposures: 7x900s R, 7x900s G, 7x900s B, 30x600s Lum (10+ hours)

Inspired by Mike Sidonio's attempts to show the extended halo of NGC 253 I have attempted to do this too. Not sure if the experiment has been entirely successful but you have to try these things. Unfortunately, my FOV is too small to include the Peter Ward S-bend :)

Constructive criticism appreciated...

Higher res version here: http://rickstevenson.smugmug.com/Astro-Photography/2013-09/i-scDL4PW/0/X3/NGC253v4psr-med-X3.jpg

Thanks,
Rick.

[Larryp]

That's superb, Rick :thumbsup:

[batema]

Stunning. Well done.

[ChrisM]

Nice work Rick. You must be pleased with the Ceravolo.
Chris

[DavidTrap]

Love the star colours Rick! (And the galaxy full stop)

DT

[gregbradley]

Wow, high impact image. The image really shows the glow coming off that galaxy and its luminosity.

The red rings around the yellow stars could be minimised a few ways. The best would be to run deconvolution on the red subs to reduce the size of the stars. Perhaps you may need to refocus when doing the red subs or seeing was worse then or the object lower in the sky. Another way is to select an eliptical lasso around the yellow star, feather it 1-2 pixels and lower the red using colour balance tool or selective colour set to red. Then do the next star etc.

This is a great setup you've put together. High quality image.

Greg.

[RobF]

Sure is pretty cool stuff Rick. I'd enjoy it a bit more if the star halos had a bit of erosion to smooth, but heck its already 11/10 :)

[jjjnettie]

So nice...plenty to explore in this one Rick. :)

[RickS]

Thanks, Larry & Mark!



Thanks, Chris. It's a very nice scope and I'm pleased I didn't penny pinch ;-)



Thanks, David.



Thanks very much for the suggestions, Greg. I feel a repro coming on.



Thanks, Rob. I did do a fair amount of work on the stars but bringing up the halo made them bloat again. I think I should have masked them for the final stretch. Hmmm... why didn't I think of that earlier? :P

[RickS]

Thanks, JJJ!

[ozstronomer]

Great image Rick, looks like you had a very productive weekend.

loads of detail and love the halo around the galaxy, well done :thumbsup:

[RickS]

Thanks, Geoff. I only got one image in two days but I think it was worth it.

[alpal]

Hi Rick,
A great image - a whole new look.

That Ceravolo telescope seems to be really sharp.
It is however a Cassegrain & not a Ritchey Chretien so
I am wondering about it's spot sizes off axis in comparison.
Do you have any comparison charts?

cheers
Allan

[RickS]

Thanks, Allan. The Ceravolo is a Dall-Kirkham design. Peter doesn't publish spot diagrams AFAIK.

Cheers,
Rick.

[Stevec35]

I'm a bit late with this one Rick. A spectacular image to be sure and one of the best images of this galaxy I've seen.

Cheers

Steve

[alpal]

Yes Rick,
I noticed that when I went to the website.
It all seems quite confusing but your image is so sharp
that I start to wonder about the worth of spot diagrams.

cheers
Allan

[RickS]

Thanks, Steve!



Allan, I've been reading the Telescopes, Eyepieces, Astrographs book by Smith, Ceragioli and Berry. They analyze a lot of designs in detail including Ritchey-Chrétien and Dall-Kirkham. Both need a corrector to cover a wide image circle (RC has field curvature and DK has coma) but with a corrector both designs are capable of very good performance.

The book shows spot diagrams for all the designs they analyse and ray-fan plots are provided as well. These are really useful - you can actually "read" the aberrations from a ray-fan plot and get a much better feel for the (theoretical) performance of a design.

Cheers,
Rick.

[multiweb]

Beaute shot Rick. Sharp as and not over processed like many I've seen before. Great details. :thumbsup::thumbsup::thumbsup:

[John Hothersall]

That image jumped out at me so rich and bright, I thought you had used Ha as a few red patches have come out. Only criticism is the bright stars and diff spikes are a bit too dominant but you have brought out the halo very clearly.

John.

[peter_4059]

Nice going Rick. Heaps of detail and sure is high impact. Agree with what others have already said re the red stars. They seem to have a dark ring on my monitor.

[strongmanmike]

Well :whistle: being a blueaphile :P I'm gunna say it is way too red/magenta and I recon there needs to be more brightness variation from centre to edge of the galaxy buuuut otherwise....man, a pretty damn cool image that Ricki :thumbsup: With plenty of exposure you have certainly revealed the faint outer halo and display it quite nicely too. Yes, a real shame you weren't able to reveal the S-Bend :doh: :shrug: guess it takes incredible stretching skills to be able to show that, oh well, next year mate :lol:. The limiting magnitude looks to be pretty faint too.

Very well done :thumbsup:

Mike

[Bassnut]

Gee, spectacular 3d detail there Rick, top work. Who cares about the stars . The diffraction spikes add drama.

[gregbradley]

I don't know if Rick is willing but it would be cool if he uploaded his ata to Dropbox and invited others to have a go at processing. It could be a fast track way of gaining some processing info if those who process it post what they did.

This is potentially the most interesting data I have seen for a while.

Greg.

[RickS]

Thanks, Marc!



Thanks, John. Good points. The stars did blow out when I stretched the halo despite some attempts to control this. I need to work on my mask generation.



Thanks, Peter. That's another thing I will try to correct.



Thanks, Mike. I did deliberately pick that colouring based on an eXcalibrator colour calibration and a bit of a survey of NGC 253 images that I liked and didn't like, but perhaps I'm a magentaphile :) I did flatten the dynamic range of the galaxy a bit to bring out the halo and I agree I could probably do better with some more work. I also promise to do better on the S-bend next year :lol:



Thanks, Fred!



Greg: I don't think I'd want to make a public release but I'd be happy to make the data available on request.

Cheers,
Rick.

[allan gould]

Rick
Another superb image. Just love how well you have brought out all the faint halo.
Allan

[RickS]

Thanks, Allan! I've had some great feedback for a repro but I think it only needs a bit of tweaking. It might be a bit OTT for some people, of course :D

[astronobob]

Wow, EyePoPPin image Ric, Dig that Lots :thumbsup: :thumbsup:

[Shiraz]

Excellent image Rick - got a lot of really dim stuff and at very high resolution. Top stuff.

FWIW, I think it looks just like a deep image of 253 should look - the dust lanes have a fair bit of red even in much shorter exposures and as you go deeper, they should look redder. I would not have used quite as much saturation, but that is just my personal taste and not anything to do with your fine image.

Regards Ray

[broca]

Fantastic work! The time you put into this has paid off well!

[RickS]

Thanks, Bob!



Thanks, Ray. I may tone it down just a little when I address some of the other issues. Part of the problem is my fantastic calibrated 27" NEC monitor. It makes everything look great!



Thanks, Steve.

[marco]

congrats on the attempt Rick, the halo is well visible. I too find the overall look a bit reddish but that doesn't take much out of the "impact" of the image :thumbsup:

Regards
Marco

[Tandum]

Yes, definitely an original colour scheme. Never seen anything like it before.

[jase]

Great work in picking out the extended halo, Rick. Sure is an incredible sight to see. Equally impressive are the details you've obtained throughout the dust lanes. I don't mind the colour presented. The HII regions really pop so you've on to something. Well done.

[RickS]

Thanks, Marco.



Is that good or bad, Robin? :)



Thanks, Jase...

[White Rabbit]

Lovely shot Rick.

Would you mind explaining how you came about your image plan ie the number of subs and length of subs for each channel? I understand it's a pain but would help us newbs out here in wilderness lol.

Your image plan was 7x900s R, 7x900s G, 7x900s B, 30x600s Lum would mind if I ask why the lum channel is only at 600s but 30 subs? I understand that 30 subs would make it a smoother image than 7 but I dont understand why the colour channels are a full 1/3 the length longer than the lum?

Cheers.
Sandy

[RickS]

Thanks, Sandy.

The image plan is a bit of a wet finger thing. I could be more scientific about it and probably should be, but I have a reasonable idea of what works for my scope/camera combination.

Some of the factors governing my choice of exposure time are:
- imaging at a fairly slow f/9 -> long exposures to collect significant data
- sensor has a reasonable amount of read noise (8 e-) -> long exposures to overcome read noise
- a sensor with deep wells -> long exposures won't tend to saturate stars too quickly
- dark skies mean less sky glow and associated noise -> long exposures
- Luminance collects data much faster than RGB (the colour filters block approx 2/3 of the incident photons) so shorter exposures make sense for L

If possible, I always like to have at least half a dozen subs for each of the RGB filters so that the rejection algorithm has some data to work with.

I believe the theory that says our eyes are more sensitive to Luminance than Chrominance so I always go for lots of Lum data. I have managed to salvage a couple of images where I had very little colour data. Stacking a good number of subs always seems to produce good results.

I tend to play with the data during the day when I'm out for a few nights. I check the S/N ratios and can make a call on how much more data I think I need. After the first night imaging NGC 253 I realised I didn't need much more colour, so I mostly concentrated on Lum for the second night.

I also put a lot of effort into good calibration data. I redo my library every few months and normally use 500 bias frames and at least 24 hours worth of dark frames for each sub length. For flats, I always get enough to give me approximately half a million electrons which is the point of diminishing returns on my sensor.

So, that's a few of my thoughts. Hope they are helpful...

Cheers,
Rick.