Australian Sky & Telescope - June 2018

62 AUSTRALIAN SKY & TELESCOPE July 2018

UNLESS OTHERWISE NOTED, ALL PHOTOS COURTESY OF AUTHOR

A

s a child on car trips with my family, I often heard

my parents ask this question about car colours: Is

that blue or green? While driving my parents to a

restaurant nearly 40 years later, from the back seat came

the same question, “Is that blue or green?” I turned to my

wife and said, “I can’t believe they’re still doing that.” Today

I engage in discussions about colour as it relates to stars,

galaxies and nebulae instead of cars.

Several years ago, I shared an image of the open cluster

M67 on an online imaging forum. A more experienced

astrophotographer commented that the stars in the photo were

too yellow, as open clusters usually have younger bluish stars.

As the discussion continued, another imager, Wolfgang Renz,

noted that M67 is a very old cluster, no longer dominated by

blue stars. Using data from the Naval Observatory Merged

Astrometric Dataset (NOMAD) catalogue, he showed that the

stars are mostly white or yellowish white.

This started my quest for a repeatable process to obtain

consistent and reasonably accurate colour balance in my

The eXcalibrator freeware program helps to

take the guesswork out of colour-balancing

your deep sky images.

STOO BLUE There are many ways to achieve a natural colour balance

in deep sky photography, though most are subjective. This image of

M31 in Andromeda was captured by the author and processed two

different ways. The right side is balanced based on comparison to other

images found online, while the left side uses eXcalibrator to establish

colour balance based on known star colours in the image.

deep sky astrophotos. The journey eventually culminated

in the development of the freeware program eXcalibrator for

Windows (https://is.gd/eXcalibrator).

Colour Is complicated

When imaging the night sky, several factors affect the colour

of your results. First, the spectral sensitivity of different

CCD and CMOS detectors varies greatly. Some are more

sensitive to blue light, while others respond better to red or

green light. This is true with both a monochrome camera

(used with individual colour filters) or a one-shot colour

camera (which incorporates tiny red, green and blue filters

over individual pixels).

blue or

green?

Is that star

IMAGE PROCESSING by Bob Franke