Object: Messier 35 (NGC 2168)
Type: Open Cluster
Distance: 2800 light years
Constellation: Gemini
Date: 24 January 2008
Equipment: SXV-H9, ATIK manual filter wheel with Astronomix LRGB filters, Vixen ED114S (f.l. 600mm)
Subframes: 20 x 120 second (luminance), 20 x 60 second 2x2 binned subframes each for R,G and B channels, 16 darks, 16 flats/flat darks, calibrated and stacked in AIP4Win. Final processing in PSP7.
Using my usual "false colour" processing methods for colour renditions of open clusters doesn't work too well. To prepare the above image (click on it to enlarge if you so wish), I used data from monochrome images obtained through colour filters. To try and get realistic colours, I scanned the filters I used into a reference frame, and produced colour masks for each colour channel that were exactly the same colour as that of the filter as scanned in to PaintShop. I then layered each colour channel over the luminance frame, and adjusted the intensity of each layer based on the spectral response for the Sony ICX285AL chip in the SXV-H9.
The result was a load of odd-coloured stars against a dark green sky, so I stopped trying to be clever and just blended the LRGB channels in PSP7 until I got something that looked about right, which is what you see above. The red colour of M35's "neighbouring" cluster, NGC 2158 (a line of sight effect - it lies six times further away) has not been exaggerated compared to the bluish tinge of M35's stars: that's just how they have turned out relative to each other in processing. Looking at other images of this pair of clusters (e.g. such as Rob Gendler's effort here), the colours I obtained seem geniune enough, although "colour" in astrophotos is always a subjective thing. Fans of the "Hubble Pallette" have their very own preference for the psychedelic, after all.
This was the first time I had tried to do "proper" LRGB imaging with a resasonable number of subframes and it's worked out OK. I can see why "one shot" colour CCDs are so popular though, (although I think my skies are too sodiumed-up to allow subjective colour imaging and I prefer the increased sensitivity of the monos as well): gathering the input for separate subs and pasting them all together for a colour composite was quite time consuming.
The of M35 image below is a much earlier one from January 2004:
A single 60 second frame taken by afocal projection through a 25mm eyepiece using a Casio QV-3500EX digital camera gives a reasonable impression of what the cluster looks like under dark skies through an eyepiece, although a hint of star colour is seen in the image. NGC 2158 is barely discernable. Click on the image to enlarge.
Type: Open Cluster
Distance: 2800 light years
Constellation: Gemini
Date: 24 January 2008
Equipment: SXV-H9, ATIK manual filter wheel with Astronomix LRGB filters, Vixen ED114S (f.l. 600mm)
Subframes: 20 x 120 second (luminance), 20 x 60 second 2x2 binned subframes each for R,G and B channels, 16 darks, 16 flats/flat darks, calibrated and stacked in AIP4Win. Final processing in PSP7.
Using my usual "false colour" processing methods for colour renditions of open clusters doesn't work too well. To prepare the above image (click on it to enlarge if you so wish), I used data from monochrome images obtained through colour filters. To try and get realistic colours, I scanned the filters I used into a reference frame, and produced colour masks for each colour channel that were exactly the same colour as that of the filter as scanned in to PaintShop. I then layered each colour channel over the luminance frame, and adjusted the intensity of each layer based on the spectral response for the Sony ICX285AL chip in the SXV-H9.
The result was a load of odd-coloured stars against a dark green sky, so I stopped trying to be clever and just blended the LRGB channels in PSP7 until I got something that looked about right, which is what you see above. The red colour of M35's "neighbouring" cluster, NGC 2158 (a line of sight effect - it lies six times further away) has not been exaggerated compared to the bluish tinge of M35's stars: that's just how they have turned out relative to each other in processing. Looking at other images of this pair of clusters (e.g. such as Rob Gendler's effort here), the colours I obtained seem geniune enough, although "colour" in astrophotos is always a subjective thing. Fans of the "Hubble Pallette" have their very own preference for the psychedelic, after all.
This was the first time I had tried to do "proper" LRGB imaging with a resasonable number of subframes and it's worked out OK. I can see why "one shot" colour CCDs are so popular though, (although I think my skies are too sodiumed-up to allow subjective colour imaging and I prefer the increased sensitivity of the monos as well): gathering the input for separate subs and pasting them all together for a colour composite was quite time consuming.
The of M35 image below is a much earlier one from January 2004:
A single 60 second frame taken by afocal projection through a 25mm eyepiece using a Casio QV-3500EX digital camera gives a reasonable impression of what the cluster looks like under dark skies through an eyepiece, although a hint of star colour is seen in the image. NGC 2158 is barely discernable. Click on the image to enlarge. 
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