The Helix Nebula

[Itchy]

Originally Posted by PhotonCollector

Why is the green interesting ? (here's some info about the colours of the Helix I have at hand; look at this AAO Image http://www.aao.gov.au/images/captions/aat015.html it says: "The greenish middle portion is evidence of excited oxygen atoms...." )

OK. It's just that when I did my helix with my 300D, that colour turned out much bluer than yours: (I think you are right about the red noise)

http://users.bigpond.net.au/itchysas...30finalweb.jpg

By sub-frames do you mean individual exposures ? Thinking that you do, then the answer is yes (otherwise it would not be green in the final image).
However it is certainly my processing which effected the stronger colours.

Yes, "subframes" refer to your original individual exposures.

Do you process 'n stack your images by hand ? or do you have some software that does it for you ?

I use ImagePlus to do the basic processing. I shoot in RAW and convert to pre bayer CFA (Colour Filter Array). IP does the calibration (darks, flats and bias) including an auto matched dark. I then covert the CFA frames to colour before I align and stack. IP gives a range of stacking options. With a large number of subframes the sigma average seems to work really well. Once stacked I use IP's digital development process to do the initial stretch and then move to Photoshop to do the levels & curves, etc.

One thing that I have noticed is that ImagesPlus seems to enhance the blue in an image a little more that my manual attempts, so perhaps that is a factor in my centre of the helix being so blue.

Cheers

[PhotonCollector (Paul)]

Originally Posted by Itchy

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OK. It's just that when I did my helix with my 300D, that colour turned out much bluer than yours: (I think you are right about the red noise)

http://users.bigpond.net.au/itchysas...30finalweb.jpg

Yes, "subframes" refer to your original individual exposures.

I use ImagePlus to do the basic processing. I shoot in RAW and convert to pre bayer CFA (Colour Filter Array). IP does the calibration (darks, flats and bias) including an auto matched dark. I then covert the CFA frames to colour before I align and stack. IP gives a range of stacking options. With a large number of subframes the sigma average seems to work really well. Once stacked I use IP's digital development process to do the initial stretch and then move to Photoshop to do the levels & curves, etc.

One thing that I have noticed is that ImagesPlus seems to enhance the blue in an image a little more that my manual attempts, so perhaps that is a factor in my centre of the helix being so blue.

Cheers

Geeday Itchy,

First may I say that is a great image you have of the Helix, really shows the outer nebula well.

Might I also say, that everything else I say might be totally wrong! :-) I'm just a guy who enjoys astronomy and producing images to show other people - so what I say here, is just my humble thoughts.

I think the green colour is lacking in your image because your individual exposures are too short to detect that colour. meaning; there are too few "green photons" to collect in that short exposure time.

My exposures are individually longer (which gives an improved signal to noise ratio than shorter images) and also bear in mind my 'scope collects around 40% more light than yours and at 1/f-stop faster (I think). These factors attribute to collecting a lot more of those green channel photons.

Finally, I will say that most of my raw images also are "too blue" when I first stack them. But I work on the basis that the EOS produces images which have too much blue and therefore you must either add one colour or increase two colours to balance the colour. But this also depends on the amount of sky pollution for that night since pollutions tends to produce more yellow(brown) glows on my images, so the colour handling is slightly different.

BTW: I'm not sure what CFAs or Sigma Averages are. something to do with the auto-stacking software?

PS. Last night I managed 40 minute exposure of NGC300 after that major storm we had (near Newcastle), then it started to cloud in again.

Regards
Paul

[Itchy]

Originally Posted by PhotonCollector

Geeday Itchy,

First may I say that is a great image you have of the Helix, really shows the outer nebula well.

Thanks Paul

Might I also say, that everything else I say might be totally wrong! :-) I'm just a guy who enjoys astronomy and producing images to show other people - so what I say here, is just my humble thoughts.

You are not alone there Paul!! I've only been doing this (DSLR imaging) for a little over twelve months. There is so much to learn and I think that is what I love about it. Image processing is an art as well as a science and I have learned the most about it by talking to others of like mind. I know that just when I think I have something figured out, I read something that makes me feel as if I know nothing!

I think the green colour is lacking in your image because your individual exposures are too short to detect that colour. meaning; there are too few "green photons" to collect in that short exposure time.

My exposures are individually longer (which gives an improved signal to noise ratio than shorter images) and also bear in mind my 'scope collects around 40% more light than yours and at 1/f-stop faster (I think). These factors attribute to collecting a lot more of those green channel photons.

You may well be right. I had no Dec drive operational when I took those frames and was limited to 30sec. Also the clouds came in and stopped me taking the extra 40 frames I wanted. I have now sorted my manual guiding and so next time I will increase the sub exposure length. (my scope is 10", f4 BTW)

Finally, I will say that most of my raw images also are "too blue" when I first stack them. But I work on the basis that the EOS produces images which have too much blue and therefore you must either add one colour or increase two colours to balance the colour. But this also depends on the amount of sky pollution for that night since pollutions tends to produce more yellow(brown) glows on my images, so the colour handling is slightly different.

BTW: I'm not sure what CFAs or Sigma Averages are. something to do with the auto-stacking software?

The CFA (Colour Filter Array) is what the sensor actually records in terms of luminosity. The light passes through an array of coloured filters, one for each pixel, called a Bayer array. So each pixel records only one colour, red, green or blue. To make a colour image, the data has to be "de-bayerised". The three colour values for each pixel are interpolated according to the values of the surrounding pixels.

In astro image processing it is more accurate to perform calibration with dark frames and flat frames before the image is de-bayerised. It is not essential however.

The sigma combine first calculates the mean and standard deviation of each pixel from each frame. Any values outside of one standard deviation are then ignored and the rest are averaged to give the pixel value in the final image. It is good at removing hot pixels that are in slightly different positions relative to the image.

PS. Last night I managed 40 minute exposure of NGC300 after that major storm we had (near Newcastle), then it started to cloud in again.

Regards
Paul

I look forward to your result!

cheers

[tornado33]

Hi Paul.
What I really love about your helix shot is the background galaxies, not just there but actually showing shape
Scott