{"id":45420,"date":"2023-05-27T08:04:14","date_gmt":"2023-05-27T08:04:14","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=119142"},"modified":"2023-05-27T08:32:32","modified_gmt":"2023-05-27T08:32:32","slug":"how-to-use-siril-free-software-to-process-your-dslr-astro-images","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/how-to-use-siril-free-software-to-process-your-dslr-astro-images\/","title":{"rendered":"How to use Siril free software to process your DSLR astro images"},"content":{"rendered":"

Using Siril freeware to stack, extract the background and calibrate colour. <\/p>

By Charlotte Daniels\n \t\t<\/p>

Published: Saturday, 27 May 2023 at 12:00 am<\/p>

\n\n

A<\/span>strophotography can be an expensive hobby and one where we constantly seek perfection from our data.<\/span><\/p>\n

While processing is a crucial step to achieving image excellence, many processing software packages come with the cost of monthly subscriptions.<\/span><\/p>\n

But that\u2019s not the case with astrophotography<\/a> freeware Siril<\/a><\/span>.<\/span><\/p>\n

\""Our\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Our original stacked image, with the Andromeda Galaxy, M31 looking no more than a dark smudge.<\/span><\/figcaption><\/div>\n

Siril performs many functions including image stacking<\/a> and autostretching, and it interfaces with external programs such as StarNet++ to remove stars too.<\/span><\/p>\n

This lessens the need to jump between various programs and simplifies the workflow for beginners.<\/span><\/p>\n

We won\u2019t focus on these functions, however, and will instead run though a few key features of Siril\u2019s \u2018Image processing\u2019 suite.<\/span><\/p>\n

Siril is available across Linux, Windows and macOS platforms. For this article, we installed the Windows 64-bit version. <\/span><\/p>\n

To process our image of the Andromeda Galaxy<\/a>, M31, we started by opening our stacked TIFF file (see image below).<\/p>\n

By clicking the dropdown menu at the bottom of our screen (highlighted in the image below and defaulted to Linear<\/strong><\/em>) and selecting AutoStretch<\/strong><\/em>, Siril performed a histogram stretch on our data, bringing the galaxy out from the initially dark and blank stacked image.<\/p>\n

\""The\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
The initial stacked TIFF image, opened and stretched using Siril\u2019s handy \u2018AutoStretch\u2019 function that highlights the areas that are going to need attention.<\/span><\/figcaption><\/div>\n

We should note that this step doesn\u2019t actually perform an official auto-stretch; it is done for the purposes of showing us how much data is there and helping to inform our next processing steps.<\/p>\n

The actual stretch is applied later on.<\/p>\n

We were happy with the galaxy data, but the mock-autostretch showed significant background gradients.<\/p>\n

Next, we clicked on the Image processing<\/strong><\/em> icon (highlighted) and selected Background extraction<\/strong><\/em>.<\/p>\n

A pop-up window appeared (highlighted in the image above). Leaving the side bars as they were, we clicked the Generate<\/strong><\/em>\u00a0icon and red squares appeared across the image, showing where to focus our gradient removal.<\/p>\n

Naturally, we didn\u2019t want to remove details from our target, so we removed any squares that covered Andromeda by right-clicking on them.<\/p>\n

Once we were satisfied that they only covered the background, we clicked Compute background<\/strong><\/em>. This improved our background significantly.<\/p>\n

We then performed a really neat trick that Siril has up its sleeve.<\/p>\n

Clicking back onto Image processing<\/strong><\/em>, we chose Colour calibration<\/strong><\/em> and selected the Photometric colour calibration<\/strong><\/em>\u00a0(PCC) option.<\/p>\n

\""To\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
To balance the colour channels, \u2018Photometric colour calibration\u2019 is your friend. Input your target\u2019s catalogue name and your scope\u2019s focal length on this screen<\/span><\/figcaption><\/div>\n

An adjustment menu appeared (see image above). This tool is designed to balance colour channels in order to achieve the \u2018true\u2019 colours of a target as shown in catalogues, and does this through a process called plate-solving.<\/p>\n

In order to work, PCC must be performed before a histogram stretch, as this part of the processing workflow can unbalance your colour channels.<\/p>\n

This is why we have the handy fake autostretch function available to us at the start of the workflow.<\/p>\n

In the adjustment menu window, we added the Messier Catalogue<\/a> designation for our target (M31) and clicked Find<\/strong><\/em>\u00a0(highlighted, image above).<\/p>\n

Next, we added the focal length<\/a> of the telescope we used for our image and the pixel size of our camera model (we simply looked this up online).<\/p>\n

We then clicked OK<\/strong><\/em>\u00a0to start the calibration process, which took a couple of minutes to run.<\/p>\n

Our colour channels were automatically adjusted to provide a subtle, better representation of the galaxy.<\/p>\n

If afterwards there is any notable green left in the background of your image, you can run the Remove green noise<\/strong><\/em> function (under Image processing<\/strong><\/em>) at this stage.<\/p>\n

Finally, we performed the histogram stretch.<\/p>\n

\""The\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
The final step was the true histogram stretch using Siril\u2019s \u2018Histogram transformation\u2019 function and, last of all, tweaking colour saturation and reducing noise<\/span><\/figcaption><\/div>\n

Clicking back to linear mode on the dropdown menu (highlighted, image above), we clicked Image processing<\/strong><\/em> and selected Histogram transformation<\/strong><\/em>.<\/p>\n

Clicking the furthermost right icon performed the autostretch.<\/p>\n

Our final image, which you can see below, also had colour saturation and noise reductions performed (both under Image processing<\/strong><\/em>).<\/p>\n

\""Our\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Our Andromeda Galaxy image after using Siril\u2019s \u2018Image processing\u2019 suite for common processing steps like enhancing the background and bringing out the galaxy\u2019s \u2018true\u2019 colour.<\/span><\/figcaption><\/div>\n

3 quick tips<\/span><\/strong><\/h2>\n

1. <\/b><\/span>Use the \u2018AutoStretch\u2019 option at the very start of your process to assess your data and help guide your adjustments.<\/p>\n

2.<\/b><\/span> If stacking files from multiple imaging sessions, crop your image straight after stacking, to avoid colour distortions.<\/p>\n

3.<\/b><\/span> Use the preview function within your adjustment window before deciding whether to apply a change.<\/p>\n

Are you an astrophotographer? Don\u2019t forget to send us your images<\/a>.<\/strong><\/em><\/p>\n

This guide originally appeared in the June 2023 issue of <\/strong><\/em>BBC Sky at Night Magazine<\/strong>.<\/strong><\/em><\/p> <\/body><\/html>\n

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