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Whale Galaxy & Elephant Trunk Nebula

antimorris

By antimorris
in Imaging - Deep Sky

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antimorris

antimorris

Posted
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As those of you who know me probably know, I prefer nebulas to galaxies. No real reason, I just do. Not that I dislike galaxies but I enjoy imaging nebulae a lot more. So as galaxy season goes into full swing what do I pick for an object to image: the Elephant Trunk Nebula :jump: Being at a nice northern latitude I am day by day getting less "dark" to work with not to mention the infamous cloudy skies of England so when I saw on the forecast that I was supposed to get not one, but two perfect nights in a row I set up my scope and accepted the fact that I wouldn't sleep from Friday morning until Sunday night.

I have been looking forward to doing Elephant Trunk in narrowband since I started imaging, I just had to wait until I was good enough and I finally got SII and OIII filters. Of course, the Elephant Trunk doesn't rise high enough for me to begin imaging it until about 1257am so I had to come up with something to image as a warm-up. I was randomly slewing about while trying to decide and landed on the "Whale Galaxy" and figured, why not? Off I went on my imaging marathon.

Night 1 I imaged for 5 hrs on the Whale, mostly on luminance but a little red and blue binned as well and then switched to narrowband filters and moved on over to the Elephant Trunk and continued on until sunrise.

Night 2, same except I shot more luminance and a little red, blue, and evened out the green on the Whale followed by Hydrogen-Alpha only on Elephant Trunk.

My marathon weekend is complete and I couldn't have had a better time!

First, the Whale Galaxy (and his companion), total LRGB integration time: 7 hrs (4:1:1:1), 900sec subs

5603980511_c781d86d76_z.jpg

and finally my Elephant Trunk in HST-mapped narrowband (SII, Ha, OIII). Total integration time: 10hrs (2:6:2), (I had a couple of hours of Ha from a week or two ago that I added in to this weekends data), all 1200s subs, the SII & OIII were binned 2x2, the Ha was just 1x1.

5606119984_439c9f6d6d_z.jpg

Let me know what you think!

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AndyUK

AndyUK

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Hi Anna - I'm really sorry I missed this... I only just caught up with it from this week's POW (congrats by the way!) - The whale galaxy's very good, but you're Elephant's trunk nebula capture is awesome!

This is on my list too but with the shortening nights and clear skies only seeming to coincide with full moon's at the moment, it's probably going to be a bit of a challenge (I did make a start on NGC 7023 a couple of weeks ago, but again, I haven't been able to add to it since). I'm hoping the weather might be kind next week...

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roundycat

roundycat

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I did a fairly serious binning test using the Rosette as a target and shooting in Narrowband. I came to the conclusion that with a bright target there is no worthwhile gain in binning. The increase in sensitivity depends on the readout pixels on your camera chip and is not, as many maintain, a gain of four for x2 binning. I made it about root two or 1.4x increase in sensitivity. The big gain of course is in reducing the read noise. For x2 binning you only read one pixel instead of four.

The big loss is in resolution. My results show a distinct reduction in quality and I only binned the S2.

Dennis

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rfdesigner

rfdesigner

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Nice pics.. well done. I do love the elephant trunk image, very dilicate, what did you do for a luminance image?, did you use one?

----

RE roundycat: "The increase in sensitivity depends on the readout pixels on your camera chip and is not, as many maintain, a gain of four for x2 binning."

yup that makes sense.. also I'd include sky noise.. which on a bright object is going to be high, so binning as you say might not help. I suppose if you take this to the logical extreme then it's like imaging the planets using binning.. total waste of time.

question is.. can we work out the maths that tells us when to bin and when not to bin?

Derek

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roundycat

roundycat

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I'm not sure about your comment re sky noise and bright objects. Surely you mean shot noise?

Binning is only ever effective when your sky background is high due to not being at a dark site and the long exposures necessary to get any detail 'fog' the image. Under these circumstances you would have to take much shorter exposures to avoid this 'sky fog' and that means your signal is rapidly descending into the read noise.

As for knowing when to bin, the math requires you to take measurements of the sky brightness and the signal for a given exposure. When you have done that the empirical knowledge so gained makes the math a waste of time. You should know the read noise of your camera, that is pretty much a constant. If you can get ten minute exposures before the sky background reads much above 500DU you are good to go.

Here is an excerpt from something I wrote a while ago when doing some binning tests.

Initial results were encouraging in one way. A quick 55m unbinned grab of M76 was a very close match to a 27.5m grab of the same object binned x2. Several star brightness measurements were remarkably similar indicating a sensitivity increase of x2 for x2 binning, not a x4 sensitivity increase. The downside was that when re-sizing the binned example the star FWHM figures were degraded by 40%. These figures were for non saturated stars.

(the theoretical calculations I have done so far using values of 1.5” seeing, 1.15” guiding and 1.17”/pix image scale show that x2 binning degrades resolution by 30%. At 3” seeing the figure is 13%. These figures do not take into account the extra loss incurred by re-sizing and bi-cubic interpolation).

It stands to reason that fine nebula detail will be degraded to the same extent. If you want to maximise the fine detail evident in the Veil or Jellyfish (IC443) binning will not help. A luminance channel will help if it is unbinned but it will not recover all the lost detail. The degree to which luminance fails in this respect is a matter of personal taste. There is also an argument that says, according to Fourier, if you want good acutance in an image you need to resolve spatial frequencies three times higher than you think. If you are after maximum sharpness then binning fails and that is all there is to be said.

The exposure time argument is a moot point. Binned data does not take a lot longer. In the case of my ST10 where it seems that x2 binning can reduce exposure time for a given signal level by a half I still need to take 30:30:30 +60m L compared to 60:60:60 RGB un-binned. That’s a difference of only 30 minutes. Plus I need extra darks and flats for the LRGB version. Plus more processing time.

I hope to get the data soon but in the meantime my view is that binning represents the quick way to get an image, not the quality way, even under UK skies.

I binned binning ages ago and never looked back.

Dennis

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antimorris

antimorris

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Nice pics.. well done. I do love the elephant trunk image, very dilicate, what did you do for a luminance image?, did you use one?

Derek

I used an enhanced Ha for the Luminance. By enhanced I mean I added in about 10% SII and 10% OIII into the Ha for the luminance.

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rfdesigner

rfdesigner

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I used an enhanced Ha for the Luminance. By enhanced I mean I added in about 10% SII and 10% OIII into the Ha for the luminance.

thanks.. any particular reason for the weightings?

Derek

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