{"id":36286,"date":"2022-09-15T14:27:00","date_gmt":"2022-09-15T14:27:00","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=36286"},"modified":"2022-09-21T10:26:27","modified_gmt":"2022-09-21T10:26:27","slug":"diy-astronomy-8","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2022\/09\/15\/diy-astronomy-8\/","title":{"rendered":"DIY Astronomy"},"content":{"rendered":"\n

Practical astronomy projects for every level of expertise <\/p>\n\n

DIY Astronomy <\/span><\/h2>\n\n
\n\n

Create a 24-hour polar star trail photo<\/h4>\n\n

Merge a series of photos to make a spectacular image showing Earth\u2019s rotation<\/p>\n\n

\"\"
The final merged picture made from a year\u2019s worth of images <\/figcaption><\/figure>\n\n

Star trails are relatively simple to do yet produce very striking results, especially when centred around the north or south celestial poles. These images capture the movement of Earth as it rotates on its axis, producing star trails that are comprised of fragments of concentric rings. <\/p>\n\n

If you are in the polar regions during the winter and have clear, dark skies for 24 hours, the trails will form complete circles, including the pole star Polaris because it is not perfectly aligned with the celestial pole. Most of us will never experience a polar winter, but it is still possible to create a 24-hour star trail photograph by merging together a stack of many images taken with the same setup, from exactly the same spot, on different nights throughout the year. <\/p>\n\n

Although taking a star trail image on a single night is straightforward, replicating it exactly on multiple dates, then merging them together successfully, is the challenge here. First, the camera location must be identical. We placed marks on the ground to ensure the tripod was in exactly the same spot, but attaching the camera to a fixed structure would be even better. Focus must also be the same, otherwise the width of the star trails will differ, and they won\u2019t line up perfectly. Circumpolar star trails can be created under moonlight, but doing so will affect the fainter stars, so try to ensure the Moon\u2019s brightness \u2013 its phase \u2013 is consistent between sessions. Also, when you blend your final stacked images, the overlapping regions will be brighter, so only stack what\u2019s necessary to complete the full circle.<\/span><\/p>\n\n

<\/div>
\n

What you\u2019ll need <\/strong><\/h4>\n\n\n\n

\u27a4 A DSLR camera with a widefield lens and a high-power battery or mains power lead.<\/p>\n\n\n\n

\u27a4 A remote shutter release cable that locks in place.<\/p>\n\n\n\n

\u27a4 A sturdy tripod.<\/p>\n\n\n\n

\u27a4 A dew heater to prevent the lens from fogging up during long imaging sessions.<\/p>\n\n\n\n

\u27a4 Software for image stacking and processing, eg StarStaX for stacking, and Photoshop or GIMP for merging and processing the stacked images.<\/p>\n<\/div><\/section>\n\n

Pick your foreground wisely <\/strong><\/h4>\n\n

Another consideration is the foreground. It\u2019s important to have something in the foreground for context and scale. I had a large tree in mine, which grew during the year and then required an extra layer-masking step at the end to compensate for the different tree size. You might want to opt for a non-organic foreground object! <\/p>\n\n

Choose some potential imaging dates that are spread across the whole year and if you have a clear sky forecast, be ready on those nights. I took images on seven nights during the year, but only used the photos taken in February, April, July and November. During the winter months you will have many more hours of imaging available to you, but in the summer you need to grab every minute of darkness. <\/p>\n\n

If thin cloud moves through your field of view, don\u2019t worry; the stars will still shine through. Issues such as aircraft lights can be omitted from the image stack by using a program called StarStaX, which can fill the gaps. Before you attempt to merge the stacked images, adjust the brightness and colour balance to make them as similar to each other as possible. You may also have to apply a lens distortion correction to successfully line up the images. But all this effort is well worth it for the end result. <\/p>\n\n

Step by step <\/strong><\/h4>\n\n

Step 1 <\/strong><\/h4>\n\n
\"\"<\/figure><\/div>\n\n

Position your tripod then mark the ground where your tripod legs lie. We used acrylic paint because it\u2019s weatherproof but can be removed with a kitchen scouring pad and water. Alternatively, screw the tripod shoe to a wall or sturdy object. <\/p>\n\n

Step 2 <\/strong><\/h4>\n\n
\"\"<\/figure><\/div>\n\n

Fit the camera lens with a dew heater. Set the lens to manual focus, then focus the camera on a bright star or planet using the live view and 10x zoom. It\u2019s vital to get good focus for each session, so don\u2019t rush this step. <\/p>\n\n

Step 3 <\/strong><\/h4>\n\n
\"\"<\/figure><\/div>\n\n

Point the camera, with the celestial pole in exactly the same place in the frame for each session. Set the camera to continuous mode, shutter speed to 30 seconds, ISO to 800 and aperture to f\/3.5. Lock the remote shutter cable so it takes a sequence. <\/p>\n\n

Step 4 <\/strong><\/h4>\n\n
\"\"<\/figure><\/div>\n\n

After each session, put the lens cap on then shoot 20 to 30 dark frames. This will help to remove dark signal noise and hot or cold pixels from your images in processing. Repeat Steps 2 and 3 during different months throughout the year. <\/p>\n\n

Step 5 <\/strong><\/h4>\n\n
\"\"<\/figure><\/div>\n\n

Stack each night\u2019s data separately. Open StarStaX, drag and drop your images, then add any dark frames. Untick any images with aircraft in them. Select \u2018Gap filling\u2019 mode and, if needed, tick the \u2018Subtract dark frames\u2019 box. Click \u2018Stack\u2019, then save the image. <\/p>\n\n

Step 6 <\/strong><\/h4>\n\n
\"\"<\/figure><\/div>\n\n

In Photoshop, paste each session\u2019s image as a layer on top of another. For each layer, set the blend mode to \u2018Difference\u2019, use \u2018Free transform\u2019 to align, and set the blend mode back to \u2018Normal\u2019. Adjust the opacity until each layer\u2019s stars have equal brightness. <\/p>\n\n

<\/p>\n\n

\n\n

Mary McIntyre is an outreach astronomer and teacher of astrophotography<\/p>\n\n

<\/p>\n\n

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

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