1/27/2024 0 Comments Imagemagick wordpress![]() I will use them."Įcho "To clean this directory, do: rm -f. I use GIMP for that, a FOSS “Photoshop” with a perfect price/quality ratio, which can be dowloaded from calibrate_photo.sh #!/bin/bash # Calibrate a photo, applying dark, bias and flat-field corrections # Copyright (c) 2013 AstroFloyd - Įcho -e "\nCalibrate a photo, applying dark, bias and flat-field corrections"Įcho -e " If a bias frame is not provided, the dark frame is used to calibrate the flat field\n"Įcho -e " light frame: actual photo with desired focal length, ISO and exposure"Įcho -e " dark frame: cap on lens, same ISO, same exposure as light frame"Įcho -e " flat field: same f as light frame, unfocussed on uniform, white background"Įcho -e " bias frame: cap on lens, same ISO as light frame, shortest exposure"Įcho -e "\nSome existing calibration files were found. The above operations are coded up in the bash script calibrate_photo.sh, which can be found at the bottom of this page.Īfter calibrating your images this way, you will still want to play with the colour levels, but that should be done by eye, as your desires will change from picture to picture. convert light-flat.jpg -auto-level flat-final.jpg.composite light-dark.jpg -compose divide flat-bias.jpg light-flat.jpg.composite light.jpg -compose subtract dark.jpg light-dark.jpg.composite flat.jpg -compose subtract bias.jpg flat-bias.jpg.In ImageMagick language, this comes down to: 1 – 3 = -2 + 256 = 254) so that this gives a weird outcome (as may be expected when rounding off intermediate results).) Secondly, at least ImageMagick wraps values around 255 (e.g. Firstly, I’d think that (light – bias) – (dark – bias) = light – dark, so that the outcome should be identical. (Actually, some people suggest to subtract the bias from the light and dark as well. optionally, stretch the colour levels of the final image.divide the corrected light by the corrected flat to remove field distortions.subtract the bias from the flat, to do the same for the expposure.subtract the dark from the light, to get rid of hot pixels and other noise in the long expposure.The theory for post-processing the images is rather simple: convert -orient Top-left input.jpg output.jpg.One way of doing this is by enforcing the default orientation (losing the original orientation of your light) for all of your frames: However, if you forgot to take any of the additional frames, taking them later is probably better than doing nothing at all (dead pixels usually don’t move around very fast).īefore you start, ensure that all images have the same orientation. the temperature, battery voltage, and other conditions that can influence the CCD are roughly the same. The dark and bias frames can be taken between the lights to ensure that e.g. In principle you should take these frames under the same circumstances as much as possible. Either way, in the following I’ll refer to all four files as light.jpg, dark.jpg, flat.jpg and bias.jpg. less lazy than I), you can take many images for each of these and combine them. the cap on lens, using the same ISO as for the light frame, but with the shortest exposure time available unfocussed white computer screen at ~75% of optimal exposure time), using the same ISO and focal length as the light frame the flat field, a uniform, white field ( e.g.the cap on lens, and using the same ISO, same exposure time as the light frame ImageMagick is free and open-source software (FOSS) and can be found on The package contains several commands, from which we will use the composite and convert programs.Īpart from your normal image, the light frame, you’ll need (at least) three more images: Here is how to do that from the command line, using ImageMagick. Also, if you live as close to a city as I do, you’ll want to subtract background and correct for field distortions, to improve the (contrast-enhanced) images. On high ISO and long exposures, you’ll find plenty of noise. I am not a very dedicated (or experienced) astrophotographer, but every now and then I feel like taking a photo of the sky.
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