Using Flats
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Perhaps wrong group to post this to, but I assume most people here image the sky.
When ever I use darks I always, without fail, get ugly dark spots in my calibrated images. I use Darks, Flats, Bias, and Lights in CCD Stack v2.95. When I disable using Darks I do not get the dark spots. Within the Calibration Manager I specify a dark master and check the Subtract from flat option using a Bias Master. In the Flat Field tab I select the correct color flat and the same Bias Master. When I use the Darks I'm able to obtain a 'flatter' calibrated image but get those nasty black spots. I do dither my images and some have said not to use the Darks, but when I don't use the Darks the calibrated image is not as 'flat' as when I use Dark Master. Any suggestions why the dark spots appear? See attached as an example. Kenn |
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I'm inclined to agree with Paul...you could have mis-labelled a Flat file as a Dark and get dust mote shadows improperly subtracted from (or divided by) your Light images.? But it could also be an error in your FlatDark files.? Or it could be that your software package did not get the Master Flat right, or did not divide/subtract the master Flat correctly from each Light raw file.?? Do here goes a quick tutorial about the imaging process, and what to look for to find out where the dust shadows are coming from. You must have 4 types of files.... Lights: File images of the target. Say 300 sec. each.? (If you use an OSC camera, these must be Raw un-debayered images.? De-Bayering is done later.) Darks: Completely covered camera sensor, usually by completely covering the telescope.? Also exposed the same duration as the Light files, say 300 sec. Because there is zero light coming to the image sensor there can be no dust shadows.? Look at each Dark image to see if you have a bad or mislabelled Dark file.?? Flats.? Image files completely de-focussed, of a white flat surface.? Often done by stretching a clean white translucent cloth over the front of the scope and exposing the dawn sky, or making an illuminated translucent plastic sheet in a box (a " light box") over the front of the scope.? Because the illumination is often much stronger than the Light exposures, the Flat exposure might be like 0.1 sec.? The flat files will show dust on the image sensor or on the corrector plate or lenses as shadows ("dust mites").?? FlatDarks.? These are like the darks but at the exposure of the Flat files, say also 0.1 sec, with the scope or camera covered.? Again these must have no dust motes because there should be no light hitting the image sensor.? Look at each FlatDark image to see if you have a bad or mislabelled file.?? The software must do these 10 steps properly. 1. Make a Master Dark file.? Done by averaging all the Dark files, at each pixel location.? If you have even a single bad file (mislabelled Dark, but really a Flat) you will have dust images, and they will appear in your final image. 2. Make a master FlatDark file?by averaging all the FlatDark files at each pixel. 3. Make an initial master Flat by averaging all the Flat files at each pixel. 4. Make a Final Master Flat by subtracting the Master FlatDark from the initial master Flat.? 5. Subtract the Master Dark from each Light file separately, making a Light intermediate file.? These are still Raw.?? 6. Divide each Light intermediate file by the Final Master Flat.? This operation is critical as if not properly fine, the dust mote shadows will still be present.? This operation results in a 2nd intermediate Light.? These files should have no dust shadows ideally.?? 7.? Create a Final Light File for each Light file.? DeBayer to color each 2nd intermediate Light file, if you used a color imaging chip.? ("OSC one shot color).? These give individual Final Light files.?? If the image chip is monochrome then step 6 is this Final Light file. 8. Create a Master Final Image by doing an overlay of the Final Light files by shifting and rotating the Final Image files. This is the stacking process.?? 9. Denoising, color balance, stretching the Master Final Image.? ( Most computer displays use 8 bits per image, giving 2^8 = 256 colors per Red, Green, and Blue pixel. That is 256^3 colors or? 16.777 million colors.? However, the center of bright stars will be very bright, and the faint nebulae will be very dim.? So stretching the light intensity is needed to reveal the nebulae in a similar intensity to the stars.? ) 10. Save the final file image in 8 bit (.jpg) or better: 16 bit formats (like 16 bit TIFF) or high bit depth formats.? You can then play in photoshop or another program.?? 12. If your camera was Mono, the final R, G, and B filed are then combined into a color RGB image.? Variations include adding in Halpha etc to ref, or false color mapping (Hubble pallette for example). "Many a slip twixt the cup and the lip," as the saying goes.? ?Good luck chasing down the processing error. Happy imaging! Michael? On Wed, Nov 25, 2020, 11:37 PM Paul Homer <phomer60@...> wrote:
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Kenn
Michael Herman