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Lifting the Fog

This photograph, taken in November 2001 during our brief but pleasant stay at COAA in the Algarve, is a tracked time exposure of the constellation Orion and surrounds. Camera Olympus OM-1; 28-80mm Hanimex zoom lens at 28mm; Fuji Super G 1600 film; exposure time 15 minutes. Scanned from a print.

The camera was mounted on a tracking platform, which compensates for the Earth's rotation. Briefly: the main axis points -- very accurately -- at the sky's north pole. Not quite at the Pole Star, Polaris, because it's actually a small distance away from the true pole position. A battery-driven motor then rotates the mount around this axis, once every day.

You'd hardly believe it was Orion, though. Mainly what you're seeing are the lights of Portimao, a coastal resort a few miles from COAA. Sky fog isn't very bad at the observatory, but there is some in this particular direction (though it's pretty much invisible to the naked eye).

The main fault was that we over-exposed the film; we should have gone for 5-10 minutes. It was also very fast film (1600), which didn't help. And the aperture was wrong, and....

Judging exposure times for wide-field astrophotography is not easy, with many variables all interacting with each other.

Anyway, we now have this picture. Can anything be done to rescue it?

Here's what we came up with. What you do is construct a 'flat field' consisting of the background glow only, then subtract it from the image. The kicker is making a good quality and accurate flat field: the method described in Correcting Camera Vignetting doesn't work very well here (the glow is unevenly distributed, and the brightness varies a lot), and other ways of constructing it were extremely laborious, too difficult, inaccurate, or all three. This method is quick, easy, accurate, and we haven't seen it described anywhere else.

• Get the above image into Photodesk and make a copy of it, named "flat" (File>Copy). All the work is done on this copy. Note that the next two steps are optional, if you're in a hurry; doing them increases the quality, but not hugely.
• [Optional] First we'll get rid of many of the star images. Select Special effects:Outline>MinMax. Set Radius to 1, select Maximum, and Apply. Fix.
• [Optional] Select the Clone tool and get rid of any very large and bright objects that remain, using bits of nearby sky. This doesn't need to be done particularly well or accurately! Just do the three or four brightest objects in this picture. Fix.
• Here's the important step. Open the Resize image window (Image>Resize... or Ctrl-S). Select Scale, and type "1/32" into the writable icon. Click on Resize. Fix. You should now have a tiny image.
• Now open the Resize image window again, and scale it back up by typing "32" in the writable icon. Click on Resize and fix. What you should get is this:
  

  

  The generated flat field

• This is an almost perfect flat field. What the scaling down and up does is a type of extreme blurring, but without the artifacts and inaccuracies that start appearing with normal blur methods. Very importantly, it also means that the stars do not contribute to the brightness levels -- they're simply destroyed by the extreme shrinkage we've applied to the image.
• The final step is to subtract this image from the original. But there's a small problem: the scaling process has meant that the flat-field picture now has slightly different dimensions due to rounding errors (it always will unless both height and width are exactly divisible by 32), and you can't combine images unless they're precisely the same size. The difference isn't huge, though, and if we simply resize the flat field it works OK. So take a note of the original dimensions, and resize the flat field to that (make sure Maintain current aspect ratio isn't ticked).
• Open the Calculation window (Ctrl-U), and set up as follows:
  • Source 1:Image - Your original image
  • Source 1:Channel - #0 RGB
  • Source 2:Image - The flat field
  • Source 2:Channel - #0 RGB
  • Destination:Image - New
  • Destination:Channel - New
  • Function: Subtract
  • Offset: 0
  • Gain: 2.5
• Click on Calculate. Here's the result:
  

  

  The flat field subtracted from the original image

Notes

• Depending on your original image, you may need to adjust the Gain factor.
• Again depending on the original, you may need a different scaling factor. The important thing is to choose it so variations in overall brightness are largely preserved, but anything smaller in size gets reduced to nothing.
• This method uses essentially three steps: shrink, enlarge, subtract. It's one of the many occasions I've yearned for Photodesk to have a simple scripting facility.
• You may have noticed the peculiar 'smearing' of stars around the edges: this is a lens aberration caused by using too wide an aperture on a wide-angle lens. It's nothing to do with tracking errors. The very bright object at lower left is Jupiter, by the way.
• This isn't a very good star picture, but it was our first attempt at a tracked shot and we're happy to have got something out of it. The original scan was about twice the size of the image here, and has many more stars. Here's a section of it, after processing and rotation:
  

  

  Orion, minus Portimao