Introduction

In the last few years imaging planets has become popular using commercially available web cameras. Philips released the popular ToUcam 740k a few years ago. A little over a year ago the ToUcam 840k was released which offered superior low light performance.

Webcam imaging requires the acquisition of a movie sequence of a planet, which may contain hundreds or, thousands of frames. Webcam frames are of short duration and subsequently little detail is acquired in a single frame. A movie sequence is imported into a program such as Registax, which decomposes the movie into its individual frames. These frames are aligned, optimised and stacked by the Registax software. (Other image processing software such as AstroStack may be used.) The stacking process yields a frame which has a vastly improved signal to noise ratio over that of any individual frame in the movie.

With the success of the ToUcam and other similar units in the hands of amateur astronomers, Meade soon launched the LPI (Lunar Planetary Imager) to be followed some months later by Celestron’s NexImage system. This article will describe the use of the Philips’ToUcam and Registax image processing software.

The Camera

The Philips ToUcam 840K offers a maximum resolution of 640 x 480 pixels where each pixel is 5.6um x 5.6um. The camera is lightweight and provides excellent low light performance. The 840K is supplied with a screw in lens which is removed before imaging. A suitable screw-in adaptor is needed to connect the webcam to the telescope.

The CCD pixel size must match the telescopes focal ratio to avoid undersampling. ‘(Undersampling is characterised by star images which appear blocky or square shaped.) Undersampling is avoided by choice of pixel size that provides sky coverage equivalent to half the “seeing” conditions or less. Backyard observing sites typically achieve “seeing” conditions of 3 to 5 arcseconds. The following formula defines the relationship between the three variables “seeing”, focal length and pixel size: (sampling in arcsecond)=206.265*(pixel size in microns)/(focal length in mm).

Image Acquisition Software

Philips provides a software bundle, VLounge with the camera. VRecord the image acquisition software is a component of the bundle. VRecord allows the user to capture an *.avi of the planet being imaged. VRecord includes controls to set the camera gain, brightness level and saturation.

The Computer

A laptop computer is ideal, it offers mobility and battery operation. A Pentium 4 CPU is ideally suited to the task of aligning, optimising and stacking frames. The laptop must have a USB port for fast image data download from the camera. A USB extension cable is also typically required.

Image Capture

A planet’s image is captured as a movie sequence, a *.avi file. Given the camera is set for a high frame resolution and 10 to 20 frames per second are being acquired for some number of seconds the file size grows quickly. Consequently a large hard disk is required to save the file. A 20 gigabyte hard disk is the minimum with 40 gigabyte preferred.

The image is downloaded to the computer over a USB connection. Given the large amount of data to be downloaded a fast CPU is desirable. To avoid “frame dropping” a Pentium 4 CPU is recommended.

Image Processing

One of the best available programs for processing the image sequence is Registax. Registax is a freeware program available to download from the Internet. Simply type Registax into a search engine such as Google to find a download site. The program allows a *.avi to be decomposed into a discrete number of frames. The user selects a frame from the *.avi which offers sharp focus captured at a moment of good “seeing”. This frame becomes the datum to which all other frames in the *.avi are referenced.

The user now performs an alignment procedure. All frames in the *.avi are aligned to the datum and are ordered according to their “quality” with respect to the datum. The highest quality frames appear at the top of the order. The user now sets a quality cut-off point so only a certain percentage of frames are further processed. A cut-off point of 80% is normally sufficient so only 20% of frames in the *.avi progress to the next phase – optimisation. The optimisation phase attempts to improve the quality of the remaining 20% of frames.

The stacking phase is next and allows the user to manually select how many frames they wish to stack by adjusting two slider controls. Having selected the desired frames the user presses the stack button. The stacking phase yields a single frame which has a vastly improved signal to noise ratio.

Registax also includes quite powerful supplementary functions including unsharp mask to “sharpen” image features. If required the user can “touch up” the result using image processing programs such as Photoshop.

Guiding And Optics

For imaging planets exposure times are typically 30 to 80 seconds. A telescope with at least an RA drive and reasonable tracking performance is ideal. The telescope does not have to be accurately polar-aligned. A telescope of focal ratio f9 and X2 or X3 Barlow gives an effective focal ratio of f18 or f27, which is fine for planetary imaging. This arrangement reduces the field size sufficiently to offer a good size planetary image. An aperture of 200mm affords adequate light gathering power.

Focusing

Focusing is almost certainly the hardest aspect of CCD imaging. The Earth’s atmosphere is constantly changing. As a result planet image presented to the observer is going in and out of focus. On some evenings it is almost impossible to achieve good focus, particularly if the planet is low in the sky. Finding good focus is easier with a rack and pinion focuser than with a mirror shift focuser.

An example focusing procedure is:

• Start the image capture software.
• Adjust the telescope controls until the planet disk appears in the centre of the computer screen.
• Adjust the focuser so the image looks sharp on the computer screen.
• Reduce the camera gain so features in the disk are apparent.
• Adjust the focuser so these features are clearly defined.
• Initiate a sample image capture.
• Process the sample image capture using Registax and qualify the result.

Results

That the above process yields outstanding results is demonstrated here. Figure 1 is an individual frame from a *.avi movie of the planet Jupiter. Note the poor quality of this frame, very little detail is apparent.

Figure 1: A Single Frame Of Jupiter

Registax processing of this *.avi yields Figure 2. Note the vast improvement in the quality of this image. Detail such as storm cells and equatorial banding is clearly visible.

Figure 2: Registax Processed Image Of Jupiter

Both images by Brent Joyce.

There are number of tutorials on the web to help in using the Registax program. The AAQ also has a number of webcam users who can assist those interested starting webcam imaging.