{"id":48756,"date":"2023-08-27T07:36:00","date_gmt":"2023-08-27T07:36:00","guid":{"rendered":"http:\/\/a10aa2c1-4179-41f9-92a8-f939ac28bac3"},"modified":"2023-08-27T09:32:32","modified_gmt":"2023-08-27T09:32:32","slug":"how-to-master-flat-frames-in-astrophotography","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/how-to-master-flat-frames-in-astrophotography\/","title":{"rendered":"How to master flat frames in astrophotography"},"content":{"rendered":"

<\/p>

By Charlotte Daniels\n <\/p>

Published: Sunday, 27 August 2023 at 07:36 AM<\/p>

\n\n

In this guide we’ll look at calibration flat frames and how you can master them to improve your astrophotography<\/a>.<\/p>

Getting the most out of our deep-sky data is a constant learning process.<\/p>

Not only do we need to master the complexities of capturing the data in the first place, but the art of image processing<\/a> is just as important. <\/p>\n

What a flat frame looks like<\/figcaption><\/figure>\n
Between the data capture and processing stages lie calibration frames.<\/p>
The two most common types of calibration frame in astrophotography are dark frames<\/a> and flat frames<\/a>.<\/p>
They are integral as they allow us to overcome some of the limitations in our imaging setups, such as sensor noise, dusty lenses and vignetting.<\/p>
Reducing noise and vignetting<\/a> allows us to stretch our data further.<\/p>
While dark frames reduce the noise in an image, flat frames help subtract anomalies in the image train, such as dust bunnies and vignette.<\/p>\n
$\"A$
A typical example of vignetting, taken through a 3-inch refractor\nCredit: Steve Richards<\/figcaption><\/figure>\n
Solving vignetting<\/strong><\/h2>
Vignette presents as dark edges in the field of view (FOV) or an uneven field illumination<\/p>
It can be due to lens\/mirror design or something interfering with the light path in the image train.\u00a0<\/p>
It\u2019s good practice to add calibration frames to all astro images.<\/p>
First, we\u2019ll explain how to take these using a DSLR and then we\u2019ll explore how flat frames improve an image of the Sadr (IC 1318) and Crescent Nebula<\/a> (NGC 6888) region of Cygnus<\/a>.<\/p>
There are three main ways to take flat images:<\/p>
\n
Using a flat panel<\/li>
Using a white T-shirt<\/li>
Using a Word document<\/li>\n<\/ul>
The best time to take them depends on the method that you use.<\/p>
Flat frames need to be taken at the exact same focus point as the light frames and with the DSLR set to \u2018aperture priority\u2019 (AV).<\/p>\n
$\"The$
The Celestron RASA with a temporary cover to capture flat frames. Credit: Gary Palmer<\/figcaption><\/figure>\n
How to take flat frames<\/strong><\/h2>
If using a **flat panel<\/strong>, these flat frames can be taken directly after focusing and before you start the imaging run.<\/p>**
If using the **Word document<\/strong> method, take these once you\u2019ve finished imaging.<\/p>**
**Carefully taking the OTA off its mount, carry it indoors, set the DSLR to AV and position the telescope in front of a blank Word document on your computer screen.<\/p>**
**Fire off 20\u201330 frames. Your flat frames should display a bright ring and dark corners.\u00a0<\/p>**
The **white T-shirt<\/strong> method returns similar results, but the frames need to be taken the morning after the imaging session.<\/p>**
**Again, leave the focus in place overnight and wait until there\u2019s a bright early-morning sky.<\/p>**
**Gently place a white T-shirt over the front lens and secure it with an elastic band.<\/p>**
**Using the Sun as a light source, point your telescope towards it and take your flat frames.<\/p>**
**Flat frames in practice<\/strong><\/h2>** $\"flat$ <\/figure>
Turning to our example image of the region in Cygnus, we started with an unprocessed image without flat frames, stacked using DeepSkyStacker<\/a> (DSS).<\/p>
We opened the image in Photoshop (File > Open > stacked file location<\/em><\/strong>) and used the Levels<\/em><\/strong> function to stretch the data (***Image > Adjustments > Levels<\/em><\/strong>).<\/p>***
In ***Levels<\/em><\/strong>, we used the dropdown menu to select between red, green and blue channels, dragging the anchor points until they met the histogram (see in the image above).<\/p>***
***Data began to emerge, but the corners were clearly dark and there was a bright zone in the middle of the image.<\/p>***
**Adding flats to the stack<\/strong><\/h2>**
$\"flat$ <\/figure>
It was time to re-stack the original images, but this time using our flat frames.<\/p>
Heading back to DSS, we added our previously captured light frames, then clicked on ***flat files<\/em><\/strong> (see image above).<\/p>***
We navigated to our saved DSLR flat frames and selected them all (clicking Ctrl + A<\/em><\/strong> simultaneously), then clicked ***Check all<\/em><\/strong>.<\/p>***
***Our calibration files appeared as registered in the bottom window of DSS. We then used DSS to register and stack the images and saved the final result as a TIF file.<\/p>***
***With the calibration frames added, we could process our new TIF image.<\/p>***
Reloading in Photoshop, we performed the data stretch again, using the ***Levels<\/em><\/strong> function.<\/p>***
$\"flat$ <\/figure>
The FOV of our image was now noticeably flat, with the corners as bright as the middle of our image (see image above).<\/p>
Final adjustments included colour balance (Image > Adjustment > Colour balance<\/em><\/strong>), noise reduction (***Filter > Noise > Reduce noise<\/em><\/strong>)<\/p>***
And the colour range (***Select > Colour range<\/em><\/strong>) to reduce the size of the stars and produce our final image, which you can see at the very top of this article.<\/p>***
**3 tips for mastering flat frames in astrophotography<\/strong><\/h2>** \n
If using a white T-shirt, bright clear skies without cloud will ensure an even field of view<\/li>
Try the Word document and white T-shirt to see which works best for your setup and images<\/li>
Make it a habit to capture flat frames as part of every imaging session<\/li>\n<\/ul>
***Are you an astrophotographer? Whether beginner or expert, don’t forget to*** ***send us your images<\/a>.<\/em><\/strong><\/p>***
***This guide originally appeared in the September 2023 issue of BBC Sky at Night Magazine.<\/em><\/strong><\/p> <\/body><\/html>\n***
","protected":false},"excerpt":{"rendered":"
By Charlotte Daniels Published: Sunday, 27 August 2023 at 07:36 AM In this guide we’ll look at calibration flat frames and how you can master them to improve your astrophotography. Getting the most out of our deep-sky data is a constant learning process. Not only do we need to master the complexities of capturing the […]<\/p>\n","protected":false},"author":24,"featured_media":48757,"template":"","categories":[34],"acf":{"readingTimeMinutes":"5"},"uagb_featured_image_src":{"full":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/08\/how-to-master-flat-frames-in-astrophotography.jpg",1200,801,false],"thumbnail":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/08\/how-to-master-flat-frames-in-astrophotography-150x150.jpg",150,150,true],"medium":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/08\/how-to-master-flat-frames-in-astrophotography-300x200.jpg",300,200,true],"medium_large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/08\/how-to-master-flat-frames-in-astrophotography-768x513.jpg",768,513,true],"large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/08\/how-to-master-flat-frames-in-astrophotography-1024x684.jpg",800,534,true],"1536x1536":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/08\/how-to-master-flat-frames-in-astrophotography.jpg",1200,801,false],"2048x2048":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2023\/08\/how-to-master-flat-frames-in-astrophotography.jpg",1200,801,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":"By Charlotte Daniels Published: Sunday, 27 August 2023 at 07:36 AM In this guide we’ll look at calibration flat frames and how you can master them to improve your astrophotography. Getting the most out of our deep-sky data is a constant learning process. Not only do we need to master the complexities of capturing the…","_links":{"self":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/rss_feed\/48756"}],"collection":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/rss_feed"}],"about":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/types\/rss_feed"}],"author":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/users\/24"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media\/48757"}],"wp:attachment":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media?parent=48756"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/categories?post=48756"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}

Flat frames in practice<\/strong><\/h2> <\/figure>

3 tips for mastering flat frames in astrophotography<\/strong><\/h2> \nIf using a white T-shirt, bright clear skies without cloud will ensure an even field of view<\/li> Try the Word document and white T-shirt to see which works best for your setup and images<\/li>