{"id":48426,"date":"2023-08-06T07:31:50","date_gmt":"2023-08-06T07:31:50","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=120849"},"modified":"2023-08-06T08:32:31","modified_gmt":"2023-08-06T08:32:31","slug":"reducing-noise-in-dslr-images-via-dark-frame-subtraction","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/reducing-noise-in-dslr-images-via-dark-frame-subtraction\/","title":{"rendered":"Reducing noise in DSLR images via dark frame subtraction"},"content":{"rendered":"

Use this vital technique to help improve your DSLR astrophotography. <\/p>

By Pete Lawrence\n \t\t<\/p>

Published: Sunday, 06 August 2023 at 07:30 AM<\/p>

\n\n

If you\u2019re having trouble with noise in your long-exposure astro images captured using a DSLR camera, you\u2019re not alone!<\/p>\n

There is a good technique you can use to avoid noise spoiling your DSLR images, and we\u2019ll cover that in this guide.<\/p>\n

The techniques we\u2019ll cover include frames and stacking, but you can also read our guide to astrophotography for beginners<\/a> and image processing<\/a> for more advice.<\/p>\n

Fixed noise<\/strong><\/h2>\n

Fixed noise, such as a \u2018hot pixel\u2019, can be removed by a technique called dark frame subtraction.<\/p>\n

Following a normal exposure, called a light frame, take an image using exactly the same settings but with the lens cap on.<\/p>\n

Then subtract this frame from the light frame to remove the fixed noise.<\/p>\n

Find out more about this in our guide to flat frames<\/a>.<\/p>\n

\"A<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
A stacked master dark frame. Credit: Steve Richards<\/span><\/figcaption><\/div>\n

Thermal noise<\/strong><\/h2>\n

However, the most common type of noise is thermal noise.<\/p>\n

This is reduced by averaging light frames in a process called stacking<\/a>.<\/p>\n

Typically, multiple dark frames are also averaged before subtracting them from light frames.<\/p>\n

A program such as DeepSkyStacker<\/a> will perform these stacking procedures automatically for you.<\/p>\n

Long exposure noise reduction<\/b><\/span><\/h2>\n
\"How<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Credit: Gajus \/ iStock \/ Getty Images Plus<\/span><\/figcaption><\/div>\n

Some DSLR cameras have a function called \u2018long exposure noise reduction\u2019. Does this mean you don\u2019t need to take dark frames?<\/span><\/p>\n

This function is essentially an automatic dark frame subtraction routine.<\/span><\/p>\n

When it\u2019s switched on, each long exposure you take is followed by another of the same length taken with the shutter closed.<\/span><\/p>\n

This gives you a record of the static noise (eg hot pixels) present in the main image.<\/span><\/p>\n

The camera automatically subtracts this noise from the main shot and, hey presto, the static noise disappears.<\/span><\/p>\n

In theory this means you don\u2019t have to take separate dark frames\u2026 but there are several reasons why you still should. <\/span><\/p>\n

The normal method of deep-sky astrophotography<\/a> is to take lots of exposures of your subject, which are then calibrated (including dark frame subtraction) before being stacked to reduce random noise.<\/span><\/p>\n

With the camera\u2019s automatic routine active, you\u2019re forced to wait for the dark-frame to be exposed and subtracted for every shot you take. This effectively halves your imaging time. <\/span><\/p>\n

A single dark frame image also contains random noise as well as the predictable static noise it\u2019s designed to remove.<\/span><\/p>\n

Subtract just one dark frame and you may well introduce additional noise in the main image.<\/span><\/p>\n

Taking several dark frames and stacking them together helps reduce this effect. <\/span><\/p>\n

The routine is a good idea, but for astrophotography, taking your own dark frames gives you more overall control and allows you to make better use of your imaging time.<\/span><\/p> <\/body><\/html>\n

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