NGC 1664 in Color

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Location: New York

NGC 1664 in Color

Post by oopfan »

NGC-1664_W12-45x60s_R-70x60s_G-35x60s_APP_S15-3-25_SA-35-25_G96_NEAT.jpg (412.47 KiB) Viewed 825 times
Last week I imaged this delicate star cluster in Auriga in monochrome with my new friend, the Wratten #12 filter (minus blue):


Shortly thereafter I developed a color imaging technique that I borrowed from narrowband for synthesizing blue from only the red and green stacks. That led to my first patient, the Perseus Double Cluster. Unfortunately the image suffered from poor focus and tracking:


So, last night, it was time to revisit NGC 1664. I tuned the tracking, and spent the time needed to achieve the best focus in each channel (see attachment.) To me, this cluster looks like a heart-shaped balloon on a string.

The 40% Moon was about 120 degrees away and 14 degrees above the horizon when I began the 70-minute run of red frames. Periodically I sampled the ADU count of the sky background, but I didn't notice any significant impact from moonlight. That was followed by 35 minutes in green, and 45 minutes in Wratten #12. The frame rejection rate peaked at about 20% as the scope approached the meridian. I kept track of bad frames, and captured more to achieve the desired ratio of 70 : 35 : 45. According to my calculations, that achieved SNR 100 for a magnitude 16.0 star at 3.0" FWHM. I compared the image to a star catalog, and I can easily see stars down to 17th magnitude.

It is good to finally have a color imaging system that consistently produces good quality images every time. The only gripe I have is that blue stars are cyan and red stars are orange, but besides that I am very happy. Tackling the blue-focus issue of my Achromat refractor has been a real challenge. Here are some of my previous efforts when trying to do conventional LRGB astrophotography. Notice the large blue stars:


Technical Details:

William Optics ZenithStar 71 Achromat
Atik 314E
GSO Wratten #12
Optolong Red and Green
3D-printed Newtonian Mask


W12: 45x60s
R: 70x60s
G: 35x60s
Darks: 30x60s
Flats: 3x50
Bias: 100x1ms

Color Combine:

67% of Red stack => Red channel
33% of Red stack + 33% of Green stack => Green channel
67% of Green stack => Blue channel

No color calibration needed due to precise balance of frame counts.
SNR of W12 stack == SNR of Red stack + SNR of Green stack (added in quadrature)

Astro Pixel Processor (APP): Calibrate, Register, Stack, Crop, Rotate, Saturation, Black Point, Default Stretch.
NEAT noise reduction.

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