text: NGC 6791 (2024)
  May 7, 2024
  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 (“metals” in astronomy refers to elements other than hydrogen and helium); old open clusters are usually metal-poor. It covers an area about half the width of the Moon. This image was taken in a single night after my main target had set. I last imaged this object in 2015 and reprocessed that data in 2020. 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. I think the stars generally look sharper in the 2024 version. This isn’t surprising, since the C14 has about three times the resolving power of the setup I was using in 2015.
  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.
  Acquisition, focusing, and control of Paramount MX mount with N.I.N.A., TheSkyX. Guiding with PHD2. Primalucelab low-profile 2″ Essato focuser and ARCO rotator. Equipment control with PrimaLuce Labs Eagle 4 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Data acquired under excellent transparency and seeing in a moonless sky on May 7, 2024.
  Celestron 14″ EDGE HD telescope at f/11 (3,912 mm focal length) and QHY600M camera binned 2×2 with Optolong filters.
  Preprocessing: The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration, local normalization, and integration of all frames.
  RGB master: A master RGB image was made from the Red, Green and Blue masters using ChannelCombination in RGB mode.
  Synthetic Luminance: A SynthL was made by integrating the three four masters, weighted by SNR, with no pixel rejection
  Gradient Removal: Graxpert was used to remove gradients from the RGB and SynthL masters.
  Colour Calibration: SpectroptometricColorCalibration was used to calibrate the RGB master.
  Deconvolution: BlurXterminator was applied to the RGB and SynthL masters with Automatic psf , star sharpening set to 0.5, and non-stellar set to 0.5.
  Linear Noise Reduction: NoiseXterminator was applied to the RGB and SynthL masters with settings Amount=0.9 and Detail=0.25
  Stretching: HistogramTransformation was applied to the RGB and SynthL masters to make pleasing images. Approximate background level after stretch was 0.1 for SynthL and 0.08 for RGB.
  Combinining SynthL and RGB: LRGBCombination was used to replace the lightness channel of the RGB image with the SynthL.
  Nonlinear Noise Reduction: NoiseXterminator was used to reduce noise in the background areas of the SynthRGB master with Amount=0.9 and Detail=0.15.
  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.
  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.
  Contrast, Brightness and Colour: Background brightness, contrast, and saturation were adjusted in several iterations using CurvesTransformation.
  Stars-only steps: The SynthL stars-only image was used as a mask on the SynthLRGB stars-only image. The CIE c* slider of CurvesTransformation was used to boost colour in the stars.
  Star Restoration: PixelMath expression combine(starless, stars, op_screen()) was used to combine the starless SynthLRGB starless 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.