Using a Webcam for Astronomical Imaging

By Glen Rowe

I have been interested in astronomy for more than 30 years and during that time have photographed various objects in the sky. I never attempted photographing any of the planets because of the difficulties involved - these targets appear very small due to their distance from Earth and require the use of high magnification. Imagine taking a picture of a 10c coin at a distance of 200 metres - a picture good enough to show some of the detail on the coin, not just the outline shape.

To take photographs of the planets, astronomers use a telescope as if it were a (very) long telephoto lens - focal lengths greater than 5 metres are not unusual. By attaching a camera, without its lens, to the telescope in place of the eyepiece, the telescope becomes the telephoto lens.

Glen Rowe ready for a night at the telescope.

However, all too often these high power telescopic views are blurred by atmospheric turbulence. Shimmer on a summer's day and the twinkling of the stars are examples of the effects of turbulence. Astronomers prefer a steady atmosphere as this gives the best views. Unfortunately such conditions are experienced very seldom so most attempts at planetary photography produce poor results. But this need not be the case so often any more.

Sun 25 October 2003 showing one of the large sunspot groups, region 486, visible at that time

Sun 25 October 2003 showing one of the large sunspot groups, region 484, visible at that time

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Now, thanks to digital imaging methods combined with clever processing software the problem described above can be largely overcome.

Since we can't actually stop the atmospheric turbulence, the idea is to capture those fleeting moments when the image is steady and then use software to digitally combine the best images to make the final result.

A webcam is able to take many images every second for a period of time and some of these images (maybe only 10 - 20%) should show the object clearly enough to be usable. The proportion of good images will be greater during periods of better observing conditions.

The real power of this technique lies not in the webcam but rather in the capabilities of the software that is used to process the images made by the webcam.

The software is firstly used to align the image in each frame with respect to a reference frame and rank all frames in terms of quality. With hundreds of frames (or over a thousand in longer sequences) there is quite a lot of processing involved and this can take some time.

Because webcams are not sophisticated imaging device, their raw images tend to be noisy, or grainy, in appearance. But with hundreds of frames of an object the software can automatically 'stack' the best frames to significantly improve the quality of the final image. This process reduces noise and other artefacts (such as dust specks on the chip) and smoothes the resulting composite image.

The software stacks the aligned images starting with those ranked highest by quality and finishing at a cut-off level decided by the user. Stacking 100 images improves the S/N ratio by a factor of two over a stack of 25 images.

Having obtained a single image from the video sequence some further enhancement can be made using graphics-editing software. This process usually includes sharpening, adjusting the tonal range, etc.

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So, with relatively modest equipment, amateur astronomers can now produce planetary images that rival what was once achievable only by professional astronomers with their large telescopes.

I imported my webcam, a Philips ToUcam Pro 740K, from the United States. The webcams available in our stores use CMOS technology that does not have the low light sensitivity that is characteristic of the CCD chip used in the ToUcam.

My telescope is a Celestron 8 with an aperture of 20-cm diameter. It has an electric motor that, with the telescope mounting aligned to the south celestial pole, allows objects to be followed as they track across the sky. For the planets I configure the telescope to have a focal length of 6000mm. For extended objects like the Moon I use a focal length of 2000mm, but even this allows only a small portion of the Moon to be included in one image.

This telescope is quite portable and I set it up on a tripod outside my home in Lower Hutt as I do not have the luxury of an observatory. I put my computer on a table next to the telescope so that I can operate both from one position. If I had a notebook computer I would be able to take the whole system to other locations with ease.

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The process of capturing video images involves 3 main steps.

Firstly, I set up all of the equipment and aim the telescope at the target by placing the object in the centre of the eyepiece's field of view.

Next I remove the eyepiece and replace it with the webcam (minus its own small lens) that has a special adapter to allow it to take the place of the eyepiece. The video image of the object should be visible on the computer screen - if not then these two steps need to be repeated. Then I work on getting the focus of the image right - this is very important but can be quite tricky when the conditions are turbulent. I then use the controls in the image capture software to adjust image brightness, contrast and gain. Next I specify the exposure time, frames per second and recording duration.

Finally the video sequence is recorded on the computer (as an .avi file). Whilst recording is in progress, I am able to make small corrections to the telescope's tracking mechanism to ensure that the target does not drift out of the frame.

Mars 12 September 2003

Mars 30 August 2003

Mars 18 August 2003

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For planets I use an exposure time of 1/25th second, but the Moon requires shorter times depending on its brightness which changes according to the phase. All of my videos are recorded at 10 frames per second and the duration can be between 30 and 180 seconds, but mostly I use 60 seconds. A one-minute long video comprises 600 images and creates a file that is 250 - 400 Mb in size, so plenty of disk space is a necessity for a night's imaging.

The processing software that I use for aligning and stacking the images is available as freeware. I use Photoshop to enhance the final image, but there are several good freeware options available that will do this also.

To date I have imaged Mars, Jupiter, Saturn, the Moon and sunspots (using an appropriate filter). Mars was well placed in the sky for Southern Hemisphere observers during its close approach last year.

Jupiter 17 January 2004 with two of its moons, Ganymede to the left, Io on the right. The Great Red Spot is visible near the right hand side of Jupiter.

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Jupiter is becoming better placed in the night sky so I will be making more images of this giant planet in the coming months. Unfortunately Saturn is in the northern part of the sky and does not rise very high in the sky making it more susceptible to atmospheric turbulence. Over the next few years Saturn will become increasingly better positioned.

Saturn 17 January 2004

I am interested in creating time-lapse animations of the rotation of Jupiter and the motion of its moons, and a sequence showing the progression of the day/night boundary across the Lunar surface as the Sun rises.

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23.7 day old Moon 17 January 2004

Last year I made an animation showing Mars making a half revolution - it is interesting to see events that in reality occur over many hours take place in less than a minute. Other projects are sure to be attempted, it is just a matter of thinking of the ideas (and then getting fine weather and good observing conditions).

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Mars 21 September 2003, single raw image

Mars 21 September 2003, stack of 250 images

Mars 21 September 2003, final enhanced image

Since I already had the telescope and computer, I have been able to explore the remarkable capabilities of webcam imaging for the modest outlay of around $200.

I have been amazed at how such relatively simple technology is able to produce results that have previously been beyond the capability of amateur astronomers' telescopes. It is all the more astounding when we remember that the humble webcam was not designed with this type of use in mind.

It has been exciting and very rewarding to be a part of this electronic imaging and digital processing revolution.

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