{"id":31103,"date":"2022-04-21T00:00:00","date_gmt":"2022-04-21T00:00:00","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=31103"},"modified":"2022-04-29T14:13:20","modified_gmt":"2022-04-29T14:13:20","slug":"processing-improving-sky-backgrounds-in-astro-images","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2022\/04\/21\/processing-improving-sky-backgrounds-in-astro-images\/","title":{"rendered":"Processing: Improving sky backgrounds in astro images"},"content":{"rendered":"\n
PROCESSING<\/span><\/h5>\n\n

Improving sky backgrounds in deep-sky astro images<\/h2>\n\n

How a Photoshop plug-in can reduce light pollution and vignetting around a deep-sky target<\/p>\n\n

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LEFT: The Jellyfish Nebula region, before the gradient control process has been applied
RIGHT: In the final image the background is more even and the vignetting has been removed <\/figcaption><\/figure>\n\n

Astrophotography processing is about more than just stretching data and reducing noise, those unwanted artefacts. It is also important to create an even, realistic-looking background for your deep-sky objects. A flat, dark background will enhance the appearance of the target, but depending on the conditions you are imaging in, this can be a challenging thing to achieve.<\/p>\n\n

Generally, you don\u2019t want to focus on making the background sky completely black, as this can look artificial. Thanks to light pollution and imagevignetting (where an image appears darker around the edges), captured images will often consist of uneven gradients \u2013 different patches of brightness or colour \u2013 that become more obvious as we stretch the data. These gradients can be difficult to correct evenly, which is where a software plug-in to help with gradient control can be particularly useful.<\/p>\n\n

In Adobe Photoshop, gradient control can be carried out via basic functions such as \u2018Curves\u2019, but a Photoshop plug-in that can swiftly flatten gradients and restore an even background colour saves a lot of time. In this article we will show you how to use Russell Croman\u2019s useful GradientXTerminator Photoshop plug-in to adjust the gradients in a wide-field image of the Jellyfish Nebula, IC443. The plug-in is downloaded from his website, <\/span>bit.ly\/3tsJNHr<\/a>, and a 30-day free trial is available before purchase. Once the plug-in is downloaded, you need to locate it \u2013 for Windows computers it is found in the ‘C:\/drive > Downloads’.<\/p>\n\n

The file needs to be transferred to your Photoshop plug-ins directory, the location of which depends on the version of the software you are using. For Photoshop 2022 on a Windows computer, click and drag the file to ‘C:\\Program Files\\Common Files\\ Adobe\\Plug-Ins\\CC’.<\/p>\n\n

Stand-out nebule<\/strong><\/h5>\n\n

It\u2019s best to apply GradientXTerminator after a \u2018stretch\u2019 of the data using the \u2018Levels\u2019 function (click \u2018Image > Levels\u2019), as we\u2019ve done in our \u2018Before\u2019 image (see above, left). So in the ‘Levels’ window, drag the anchor points closer in towards the histogram, allowing it to widen and details to emerge. Although this step was repeated several times to make the nebulae stand out in our ‘Before’ image, there is still a vignette disrupting the image, so it’s now time to remove these gradients.<\/span><\/p>\n\n

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Screenshot 1: Prepare the regions that you want the plug-in to ignore by using Photoshop\u2019s \u2018Lasso\u2019 tool and the \u2018Select > Inverse’ function<\/figcaption><\/figure><\/div>\n\n

Before you use the Gradient XTerminator plug-in, remember that you only want to apply it to the background, rather than the target. If you allow the plug-in to change the target appearance too, it can remove some colour or nebulous detail. To avoid this happening, use the \u2018Lasso\u2019 tool in Photoshop: select the icon from the Toolbar on the left-hand side of the screen and then drag the \u2018Lasso\u2019 tool around the objects. Although this action automatically \u2018selects\u2019 regions within the \u2018Lasso\u2019, you want to \u2018deselect\u2019 these regions to ensure GradientXTerminator ignores them. To do this click \u2018Select > Inverse\u2019 to focus on the regions outside the \u2018Lasso\u2019 (see Screenshot 1).<\/p>\n\n

Now it\u2019s time to apply the plug-in, which is accessed via the \u2018Filters\u2019 menu, by clicking \u2018Filters > RC Astro > GradientXTerminator\u2019. For the first gradient adjustment, in the plug-in window, select \u2018Detail > Medium\u2019 and \u2018Aggressiveness > Low\u2019 (see Screenshot 2). Although this applies a broad correction to the background, some gradients are still present.<\/p>\n\n

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Screenshot 2: Apply a ‘Medium’ gradient adjustment to the background sky using the GradientXTerminator plug-in<\/figcaption><\/figure><\/div>\n\n

To make a finer adjustment to complete the gradient removal, without the risk of over-flattening our background, select the \u2018Magic Wand\u2019 tool from Photoshop\u2019s Toolbar. A menu appears on the top of the screen, allowing you to set the strength. Set the \u2018Tolerance\u2019 to \u20187\u2019 on this occasion, but the best value will depend on the image. Generally, the best value will range between \u20185\u2019 and \u201810\u2019.<\/p>\n\n

Background checks<\/strong><\/h5>\n\n

Now click on an area in the background, which depending on the \u2018Tolerance\u2019 setting chosen should select most of, or all of the background (but not the object). It is important to ensure the target is left untouched by GradientXTerminator, so this process takes a little trial and error. Make sure you experiment with the ‘Tolerance’ setting until you have most of the background selected minus the target(s) of your image.<\/p>\n\n

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Screenshot 3: With GradientXTerminator selected, use the \u2018Tolerance\u2019 levels in the top menu to make a finer gradient adjustment to the background<\/figcaption><\/figure><\/div>\n\n

Then click on GradientXTerminator again, and this time set the \u2018Detail\u2019 to \u2018Fine\u2019, and the \u2018Aggressiveness\u2019 to \u2018High\u2019 (see Screenshot 3). This will ensure finer gradients are dealt with, giving a smoother result. In the ‘After ‘ image, the background now looks more even, with colour gradients and vignetting removed. All that is left is to make some final Photoshop adjustments by using \u2018Selective Color\u2019 and \u2018Brightness\u2019 (click \u2018Image > Adjustments\u2019) and \u2018Dust and Scratches\u2019 (\u2018Filters > Noise\u2019), before saving it.<\/p>\n\n

<\/div>
\n

3 QUICK TIPS<\/h4>\n\n\n\n

1. <\/strong>The best \u2018Tolerance\u2019 settings depend on the deep-sky object \u2013 ensure dusty details are not highlighted.<\/p>\n\n\n\n

2. <\/strong>Apply GradientXTerminator in stages: use the \u2018eye\u2019 icon for that layer to view each change.<\/p>\n\n\n\n

3. <\/strong>For Magic Wand, the \u2018Alt\u2019 key allows you to add or subtract regions. A plus or minus will appear next to the icon.<\/p>\n<\/div><\/section>\n\n

\n
\n
\"\"<\/figure><\/div>\n<\/div>\n\n\n\n
\n

Charlotte Daniels <\/strong>is an amateur astronomer, astrophotographer and journalist<\/p>\n<\/div>\n<\/div>\n\n

Photos: CHARLOTTE DANIELS<\/p>\n","protected":false},"excerpt":{"rendered":"

How a Photoshop plug-in can reduce light pollution and vignetting around a deep-sky target<\/p>\n","protected":false},"author":24,"featured_media":31097,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"ub_ctt_via":"","purple_page_number":"78","purple_custom_meta_purple_page_number":"78","purple_seq_number":"1","purple_custom_meta_purple_seq_number":"1","purple_source_article":"article_78-1.xml","purple_custom_meta_purple_source_article":"article_78-1.xml","purple_source_issue":"May-2022","purple_custom_meta_purple_source_issue":"May-2022","purple_external_id":"May-2022-78-1","purple_custom_meta_purple_external_id":"May-2022-78-1","purple_issue_code":"|0000086552||","purple_custom_meta_purple_issue_code":"|0000086552||","purple_android_product":"com.im.skyatnight.204","purple_custom_meta_purple_android_product":"com.im.skyatnight.204","purple_ios_product":"com.im.skyatnight.204","purple_custom_meta_purple_ios_product":"com.im.skyatnight.204","purple_web_product":"","purple_custom_meta_purple_web_product":"","purple_publication_id":"075fab74-0a21-4201-866a-899d6c41c40c","purple_migrated":"","kt_blocks_editor_width":""},"categories":[34],"tags":[14],"featured_image_src":"https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/9f73d071-bb8d-47dd-94cb-68d796e4d72d.jpg","author_info":{"display_name":"importmanagerhub@sprylab.com","author_link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/author\/importmanagerhubsprylab-com\/"},"acf":{"readingTimeMinutes":"5","apple_news_title":""},"uagb_featured_image_src":{"full":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/9f73d071-bb8d-47dd-94cb-68d796e4d72d.jpg",1216,857,false],"thumbnail":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/9f73d071-bb8d-47dd-94cb-68d796e4d72d-150x150.jpg",150,150,true],"medium":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/9f73d071-bb8d-47dd-94cb-68d796e4d72d-300x211.jpg",300,211,true],"medium_large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/9f73d071-bb8d-47dd-94cb-68d796e4d72d-768x541.jpg",768,541,true],"large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/9f73d071-bb8d-47dd-94cb-68d796e4d72d-1024x722.jpg",800,564,true],"1536x1536":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/9f73d071-bb8d-47dd-94cb-68d796e4d72d.jpg",1216,857,false],"2048x2048":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/9f73d071-bb8d-47dd-94cb-68d796e4d72d.jpg",1216,857,false]},"uagb_author_info":{"display_name":"importmanagerhub@sprylab.com","author_link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/author\/importmanagerhubsprylab-com\/"},"uagb_comment_info":0,"uagb_excerpt":"How a Photoshop plug-in can reduce light pollution and vignetting around a deep-sky target","_links":{"self":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/31103"}],"collection":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/users\/24"}],"replies":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/comments?post=31103"}],"version-history":[{"count":6,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/31103\/revisions"}],"predecessor-version":[{"id":34003,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/31103\/revisions\/34003"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media\/31097"}],"wp:attachment":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media?parent=31103"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/categories?post=31103"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/tags?post=31103"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}