CN FORUMS ARCHIVE Telescope Reviews: Another M31

Hi Glacy. I hope the following stuff help you and anyone else with the "blue image syndrome". I've been there!!

You have given me enough clues to say yes you *are* experiencing a WB correction problem in your work flow at the RAW file convertion step.

I think its worth some explanation and evidence as to why you and others get these blue images. I can see that your luminance data is very good and an execellent picture is in there, its that many folk don't pay any attention to the RAW file conversion step and what it actually does to the colour in your image.

The main point I'm trying to say is its much harder to colour correct after an inappropriate Wb has been applied, than to work on getting the right WB correction in the first place.

Just from my own experience and frustration in my first year of astrophotography when I constantly produced blue images. What bothered me was the lack of star colour range. You expect to se a few red stars, far more orange and yellow stars and of course white and blue stars. All I got was white and blue stars and stars with an odd green cast! Last winter I did a bit of research and asked a few questions about "over blue" images of a variety of forums.

The Images Plus forum provided the answer. White Balance correction is applied in the RAW conversion step and you can override it! AutoWB is not a satifactory way to control the colour in your image.

All the Canon RAW to TIFF/JPEG convertions in Canon, Maxim/Bear Creek, Images Plus, Adobe Photoshop etc software use the same library software supplied by Canon to do the actual conversion (I'm a software developer and program the Canon tool kit myself, so have direct experience here). The canon software either works on defaults (as set on your camera) or the hosting software may allow you to choose a overriding WB correction to apply to your image via one of many presets (autoWB, daylight, tungsten etc) or via a user selected colour temperature value.

Images Plus for example allows you to select a batch of RAW file and the overridding WB correction (amongst other parameters). It ignores the setting on your camera. (NOTE teh camera does not apply a WB correction to the RAW file, but merely attaches the WB setting to the file header information as a hint). If you shoot in a JPEG format, all colour balance bets are off as the WB is directly applied and you can't change your mind later.

On the other hand Canon's own utility software converts using the setting selected on your camera, though there is a way to override this behavour.

So my question now is - does Maxims Bear Creek driver allow you to override the camera set WB? If so you should experiment here.

The problem of astrophoto "over blue" images arises via applying autoWB. Canon have optimised autoWB for the common types of pictures that regular photographers make. Those taken at daytime! Astrophotgrahy is not considered in the WB equation Canon apply so autoWB can produce uncontrolled colour corrections. This depends on factors such as light pollution gradients and casts etc present in the original RAW image. A heavy blue cast can and often results.

If you are not convinced take a look at these images I took last winter. The first in the group where processed via using the Canon File Viewer Utility to make the RAW -> 16 TIFF conversion - my camera was set at AutoWB. Other than basic DDP, RGB level correction and curves stretching these images have had no extra colour treatment.

The second group are the same images (same RAW file originals) but this time I used the Images Plus Canon RAW converter and selected a Daylight WB override. Again DDP, levels and curves were applied but also some iterative colour saturation was applied (this doesn't affect colour balance, just colour strength)

Group 1

M33 - using autoWB correction in RAW to TIFF step. Note star colours are confined to white and blue. No obviously red, orange or yellow stars can be seen. There should be some! One of the brighter yellow stars has a strange green cast.

M31 - using autoWB correction in RAW to TIFF step. Similar comment about the star colours

Group 2

M33 with a Daylight preset WB override - note stars colours are better balanced and yellow and orange stars are present

M33 with a Daylight preset WB override - note stars colours are better balanced and the full range of stars are present

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Televue 85/TRF-2008 field flattener, Meade LX200 10", Manfrotto 055SSB tripod/410 geared head/AstroTrac TT320X-AG/056 3D head, GM-8/Gemini, 10 Micron 1000 HPS, Canon 40D (unmodded), Canon 450D (modded w/Astronomiks clip-ins - UV/IR, OWB), Coronado SM60/Lunt B1200/WO diagonal, Baader Herschel Wedge
Leeds Sky Clock Ripon Sky Clock