NGC 6791 (2026)
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July 20, 2026
NGC 6791, in Lyra, is one of the most studied open clusters in the sky. It has a mass of about 5,000 times that of our Sun. It is old, at about 8 billion years, but contains some unusual younger stars (4 and 6 billion years old). It is very metal-rich (in astronomy, “metals” are elements other than hydrogen and helium); old open clusters are usually metal-poor. It covers an area about half the width of the Moon. NGC lies around 13,300 light years away.
It is interesting to compare open clusters with globular clusters such as M3. Open clusters contain hundreds to a few thousand stars; the latter contain hundreds of thousands to millions of stars. Open clusters lie in the spiral arms of the Milky Way; globular clusters lie in a halo around the main nbody of our galaxy.
I often shoot open clusters while waiting for my main target to rise, or after the main target has set. This image was taken in a couple of nights after my main target had set. I really like how each telescope gives this cluster a different flavour. With the big C14 used for this photo, the cluster almost completely fills the field. The 2020 version shows the cluster surrounded by a relatively uncluttered sky. The stars are sharper in the 2024 version, and there are a few galaxies visible in the background too. This isn’t surprising, since the C14 has about three times the resolving power of the setup I was using in 2015.
You’ll note I used a combination of 1-minute and 5-minute exposures for this image. I was evaluating the unguided tracking performance. Stars were good in the longer exposures but excellent in the shorter ones.
Tekkies:
Acquisition, focusing, and control of Paramount MX mount with N.I.N.A., TheSkyX. Primalucelab low-profile 2″ Essato focuser, ARCO rotator and Giotto flat panel. No guiding. Equipment control with PrimaLuce Labs Eagle 4 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Average transparency and above average seeing. Acquired on May 16 and 26, 2026.
Celestron 14″ EDGE HD telescope at f/11 (3,931 mm focal length) and QHY600M-SBFL camera binned 2×2 with Optolong filters.
15 x 5m and 45 x 1m Red = 1hr 04m
15 x 5m and 44 x 1m Green = 1hr 05m
15 x 5m and 41 x 1m Blue = 1hr 07m
Total: 5hr 55m
Preprocessing: The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration, local normalization, integration and Drizzle integration of all frames.
RGB master: A master RGB image was made from the Red, Green and Blue masters using ChannelCombination in RGB mode.
Gradient Removal: Spectrophotometric Flux Calibration follwed by Multiscale Gradient Correction was used to remove gradients from the RGB image.
Colour Calibration: SpectrophotometricColorCalibration was used to calibrate the RGB master.
Deconvolution: BlurXterminator was applied to the RGB master with Automatic psf , star sharpening set to 0.5, and non-stellar set to 0.5.
Stretching: MultisaleAdaptiveStretch was applied to the RGB maaster, with the approximate background level after stretch at 0.1.
Nonlinear Processing
Nonlinear Noise Reduction: NoiseXterminator was used to reduce noise in the background areas of the RGB master with Amount=0.9 and Detail=0.15.
Star Removal: StarXterminator was used to remove the stars from the SynthLRGB master, with default settings, except Large Overlap was selected. The RGB and SynthL stars-only images were retained.
Re-stretch: HistogramTransformation was used to boost contrast by moving the dark point to the toe of the histogram and slightly decreasing the mid-point slider.
Contrast, Brightness and Colour: Background brightness, contrast, and saturation were adjusted in several iterations using CurvesTransformation.
Stars-only steps: The saturation slider of CurvesTransformation was used to boost colour in the stars. A mask was used to limit the change to the stars. The mask was removed and ColorSaturation was used to slightly boost the blue in the stars. The mask was removed and SCNR was applied.
Star Restoration: PixelMath expression combine(starless, stars, op_screen()) was used to combine the starless RGB image with the stars-only image.
Final Steps: Background and star brightness, contrast, hue, and saturation were adjusted in several iterations using CurvesTransformation with masks as required. ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving as a jpg. The finder chart was made using the FindingChart process.
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