Drizzle vs flats in removing pattern noise from image

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bobharmony
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Drizzle vs flats in removing pattern noise from image

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Post by bobharmony »


As I am prone to do, I sometimes wonder if drizzle is more or less effective that darks in eliminating the pattern noise associated with DSLR imaging. If dithering is spreading that signal around so it can be averaged out in stacking will that provide a cleaner result than doing both drizzling and subtracting dark frames in calibration?

I am running a quick experiment with some M51 data I collected earlier this year and will report on results, but am curious about others experience if they have tried to omit darks from their process in favor of lots of drizzled lights (in my experiment I am dealing with 46 180" light subs.

The floor is open, please let me know what you think, and what you have observed.

Bob
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STEVE333 United States of America
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Re: Drizzle vs flats in removing pattern noise from image

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Post by STEVE333 »


Hi Bob - I believe you are talking about three different techniques that address different problems.

Darks are simple images taken with the telescope completely blocked so no light enters. Over time each pixel will slowly build up signal even though no light is present. The amount each pixel builds up varies linearly with time (a 2 min Dark exposure will have two times the Dark Signal that a 1 min Dark exposure would have). It also varies with the sensor temperature. For a given exposure time, a warmer camera (thus a warmer sensor) will produce more Dark Signal than a cooler camera. To make matters worse, each pixel will, in general, have a different Dark Signal than its neighbors. Because the Dark Signals are different for each pixel it creates "pattern noise" across the sensor. The longer the exposure the larger the "pattern noise". Because the Dark Signals depend on both exposure time and sensor temperature, DSLR users often create a Library of Darks to cover their normal exposure times and camera operating temperatures. As exposures get longer there is no substitute have having properly matched (exposure and sensor temperature) Darks to correct the pattern noise. By the way, each image is Calibrated (corrected) with Dark Frames BEFORE the images are aligned and stacked. It should be noted that each Dark Frame will have some nise of its own. Thus, many Dark Frames are usually added together to reduce the noise "added" by the Dark Frame corrections.

Dithering slightly changes the telescope aiming point for each capture so that the same point on the target doesn't fall on the same pixel in every image. Say N images are to be combined. During integration, after the images are aligned, there will be N values for each pixel (one value from each image). The integration program will find the average value for all N samples for a given pixel, compute the Standard Deviation, and the reject any pixels falling more the 3 Standard Deviations (or whatever limit has been chosen) from the mean. Thus any remaining "fixed pattern noise" after Darks have been applied will be rejected and not averaged.

Drizzle is a technique for improving the resolution of an image that is undersampled. When done at 1x1 it can slightly reduce the overall noise in the imager as compared to standard stacking in PixInsight. However, it doesn't remove pattern noise. Drizzling is done after the images are Calibrated.

In general Darks and Dithering are always used to improve image quality and reduce noise.

Hope this makes some sense.

Steve
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bobharmony
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Re: Drizzle vs flats in removing pattern noise from image

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Post by bobharmony »


Thanks for the details Steve. I will ponder these things.

I also realize I need to edit myself better. Everywhere I wrote "drizzle" in my post ( including the title), I meant to say "dither". I wasn't concerned about drizzle at all, not sure how that got in there!

Nob
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Re: Drizzle vs flats in removing pattern noise from image

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Post by JayTee »


bobharmony wrote: Tue Aug 20, 2019 2:03 amI wasn't concerned about drizzle at all, not sure how that got in there!
Is it drizzling in Connecticut right now?

JT
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Re: Drizzle vs flats in removing pattern noise from image

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Post by Stuart »


Dithering improves signal to noise by moving the camera around so that hot pixels don't pile up on the same spot.
Flats improve image quality by helping get rid of vignetting, dust motes, etc.
But you have to calibrate your flats with bias frames to make them work correctly.
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