Take your pick! Also known as the 'Black Arrow', this spectacular open cluster in Carina covers an area of about 60' x 30', and sparkles with over 100 blue and yellow stars of even brightness. They look somewhat like a collection of coins shimmering at the bottom of a wishing well. This cluster is clearly apparent as a dense aggregation of stars to the naked eye, just below the Eta Carina nebula, but it is rarely imaged. It lies about 1600 light years away, and is only 200-350 million years old.
This is my first serious image with my new QHY22 monochrome camera. It is beautifully sensitive and clean, especially in the Hydrogen Alpha wavelength. Astrograph is an ED80T CF, on an AZ-EQ6 GT.
This image is composed of the following:
12 x Red at 305 sec
12 x Green at 303 sec
12 x Blue at 328 sec
7 x Ha at 1200 sec
Total integration is a little over 5 hours. There was no need for a separate luminance channel given the cluster's brightness, and the RGB channel durations are based on G2V calibration. In post-processing, the colours were perfect.
Captured in APT, calibrated in Nebulosity, stacked in DSS and post-processed in StarTools. I created a synthetic HaRGB luminance and used RGB only for the colour.
Now that I have my Seniors Card, it is nice to see an older open cluster with rather a goodly handful of stars on the ascending giant branch and glittering in gold instead of the usual blue-white.
I spent a fair bit of timing thinking about the best composition, so I'm glad you liked that. It's actually a challenging target, because the Ha emission nebula is difficult to draw out without overstretching the background Milky Way stars (too much and the background degrades into a graininess). I might not have got it quite right, but I'm pretty happy overall with the result.
It can look like quite a different object if one just concentrates on the cluster stars and ignores the nebula - the background gets much darker, but the traceries of gas is then lost.
Tony, if you search for other images of this target you'll find that almost everyone ignores the nebulosity, so it was definitely a motivation to capture it here.
Rolf, I've often admired the composition in your beautiful images, so that's a real compliment to me
I was happy with the colours - I'd recently done a careful G2V calibration with my filters, and with the variable R, G and B exposures, the colour balance came out perfectly in post-processing. I like it when the 'science' of astrophotography works out!
Great image Barry. I love how Startools and you have kept the nice (and scientific) colours.
Can you explain what you mean by G2V calibration? I guessing something to do with making white (in Startools) the same as G2V star (to match the sun's light/colour)
Paul, thanks - I really like StarTools' scientific colour method.
Re: G2V calibration you are right, it means finding a sun-like star (class G, stage 2V) which to our eyes emits pure white light. Against this, one can calibrate the R, G and B filters. To do this, I located a G2V star at the zenith, and took a number of exposures (of a few different durations) through each filter, and measured the ADU of the unsaturated star core. From that, I was able to work out the relative transmissivity of my R, G and B filters, and adjust the exposure duration of my subs to match this ratio. This will always hold thereafter for my filter/sensor combination.
Thanks. So not really something that can be used with a OSC or DSLR?
I tend to use a random group of stars for my colour in Startools. Sometimes I will search the field for a G2V or similar star. I use SkyTools Pro to identify stars as it has the spectral class for a lot of them. I usually only resort to that if I have a lot of trouble with the colours. Sometimes Startools gets it right with it's own calculated settings.
Well, you obviously can't change the exposure duration for a OSC, but you can split the Bayer channels in post-processing (e.g. Nebulosity will do this), and then check the relative intensities on the G2V star for each of the 3 mono 'slices' of your OSC image. That will give you the info needed to adjust the relative ratios (e.g. in StarTools Color module).
