Imaging The Heavens - Basic Processing 3

An example of tri-band colour image processing using emission line filters (Ha, OIII and SII)

This is a single dark subtracted 10 minute subframe taken with a Ha filter, 10x10 minute dark frames were taken earlier in the summer and these have been median combined before subtraction from the light frame. As can be seen the single frame is somewhat grainy (noisy) due to the lower signal to noise ratio common with single subframes, also noticable is a horizontal line going across the image which may be due to a passing satellite or an artefact from the download, both the graininess and the line will be taken care of when combined with more subframes.

Single dark subtracted Ha filtered subframe

Single dark subtracted Ha filtered subframe

Single dark subtracted OIII filtered subframe

Single dark subtracted OIII filtered subframe

Single dark subtracted SII filtered subframe

Single dark subtracted SII filtered subframe

6 x 10 minute Ha subframes combined

6 x 10 minute Ha subframes combined

6 x 10 minute OIII subframes combined

6 x 10 minute OIII subframes combined

6 x 10 minute SII subframes combined

6 x 10 minute SII subframes combined

There are several ways of producing a colour image

Use a single shot colour camera where all the RGB (Red, Green and Blue) data is obtained in one shot and then processed
Use a monochrome camera with Red, Green and Blue filters and then combine the 3 seperate images to make a colour image, sometimes an extra luminance channel using a clear, light pollution or Ha filter is taken to augment the RGB channels
Emission line imaging where Ha, OIII and SII (or Hbeta) is used instead of the traditional RGB filters. Because we don't see in "emission line colours" you can assign the emission line filters in any combination to the RGB channels when processing, the traditional method being Ha=Red, OIII=Green and SII=Blue. For this image and others that I am doing with emission line filters my preference is the Hubble Space Telescope palette which is SII=Red, Ha=Green and OIII=Blue, as with RGB imaging I have used the Ha data as a luminance channel as well

What is noticable in the combine image is that there appear to be stars of different colours, this is because the three colour channels need to be correctly aligned which will be done in the next stage

Colour combining the image

Colour combining the image

Aligning the colour image

Aligning the colour image

Colour weighting

Colour weighting

When viewing the images in Maxim DL while doing any adjustments a facility called screen stretch is used which allows the image to be seen on a monitor, as the image is 16 bit or higher it would normally be difficult to see anything of value on a monitor as the monitor is 8 bit. The image now has to be saved as a 16 bit TIFF file so that it can be transferred to Photoshop CS2 for further processing, in order to do this it has to be stretched in order to preserve the data. My preference at the moment is to use a 16 bit permanent gamma stretch with a value of 0.7 using the maximum pixel value setting, this ensures that the full 16 bit data is used. This has the effect of making the image look almost blank apart from a few faint spots (some of the brighter stars in the image), there is no need to worry about this as the data is there and the functions in Photoshop will bring it out

Stretching and saving as 16 bit TIFF file

Stretching and saving as 16 bit TIFF file

When I do my processing I usually start off with a fairly aggressive iteration of curves (steep curve) ensuring that no part of the curve touches any part of the curves box (this would clip/lose data), already as a result of this some of the detail and the nebula have started to reappear as well as the background being lightened. Also you may notice that there is a black band at the top and left of the image, this is a result of when the subframes were aligned and stacked and I will take care of this in the next stage so that they don't have any effect later in the processing stages

1st use of curves

1st use of curves

Using the crop tool I have removed the black bands mentioned earlier so that further processing can continue

Cropping image

Cropping image

1st use of levels

1st use of levels

2nd use of curves

2nd use of curves

Gradient removal with

Gradient removal with

2nd use of levels

2nd use of levels

I raise the contrast by 30 points on the brightness/contrast slider in image adjustments which helps highlight the colours and detail in the image better

Increase contrast

Increase contrast

Local contrast enhancement in

Local contrast enhancement in

Noise reduction

Noise reduction

Colour balance and levels

Colour balance and levels

Finally I made a minor adjustment to the curves just to brighten the image which has also helped show up the already present detail a bit better. This is now the finished widefield image

Final curves adjustment

Final curves adjustment

Cropped closeup

Cropped closeup