How to image the planets
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We show you how to capture stunning images of the planets.
In part three of Astrophotography, The Complete Guide, we show you how to photograph the planets. It's an incredibly rewarding pastime, which can, even today, lead to discovery. Often the announcement of an impact on Jupiter or a storm of Saturn comes from an amateur and is inveriably recorded by a planetary imager.
MASTERCLASS
Photograph Mars with a colour planetary camera
Mars comes into opposition every 2.1 years. This is when the planet is best for imaging since its orbit puts it opposite the Sun from our point of view, making it appear at its largest and brightest. Things next start to get interesting from the end of 2011. There’s an opposition on 3 March 2012, when the planet will present a 13-arcsecond disc and reach an altitude of close to 50º from the UK.
Through a telescope there’s plenty to observe on and above the Martian surface. The reddy-brown deserts of Mars are interrupted by dark albedo (shaded) markings that rotate with the planet.
Mars also has seasons and the effects of these can be seen in the growing and shrinking of the planet’s polar ice caps. There’s weather too, and the appearance of bright clouds or dust storms all add to the excitement of imaging this fascinating world. High surface features such as giant volcanoes affect the Martian atmosphere. Here you may find bright ‘orographic’ clouds forming as the atmosphere is
forced above the volcanoes.
It takes slightly more than an Earth day for Mars to rotate – 24.6 hours – which means that the planet looks very similar from one night to the next, changing more noticeably over the course of several weeks. Detail is subtle and finely structured, so a high-contrast telescope with a large aperture and a long focal length is best for imaging. Large reflectors or catadioptric scopes such as Schmidt-Cassegrain Telescopes (SCTs) are a popular choice for imaging Mars.
Although a monochrome camera with filters will produce the best images of Mars, a colour camera has the advantage that a one-shot full colour capability helps keep the capture time to a minimum – useful since the planet rotates relatively quickly. There’s also less equipment to set up and you won’t need to spend as long processing colour into the image after you’ve captured it.
The planet holds up well, even under average seeing conditions, so don’t be afraid to pile on the magnification power by using an optical amplifier like a Barlow lens. Aim to keep your scope’s focal ratio in the region of f/25 to f/45. Following the methods in the ‘Technique’ section, you should end up with an image of the planet in the capture software that shows as a bright and tangible disc. If your camera has a gamma control option, keep this at the default level, adjusting exposure and gain to get the level right.
An IR-blocking filter is essential for good results and some colour cameras have this built in by default. If yours doesn’t, you can get a separate filter for around £30 that normally screws into the front of the eyepiece adaptor.
It can be tricky to get the right colour balance with Mars. Before manually adjusting your camera’s colour settings, if you’ve managed to get a large and bright planetary disc in the frame, try using the camera’s auto colour-balance function. If necessary, push the gain up high to get a bright enough signal. This should also alleviate the so-called ‘onion-ring’ effect, which can occur after registration and stacking has been applied. Under certain seeing conditions you may get a ‘false edge’ effect on processed results; however, you’re at the mercy of the sky.
The capture file will need to be processed with registration and stacking software like RegiStax or AviStack. These pick out the best frames, align them and then stack them together automatically to reduce frame noise.
In order for this to work well, you do need a good number of frames to start with. High-frame-rate cameras can easily generate several thousand frames during a capture run. A webcam operating at a more sedate 10fps over a typical three-minute run will net 1,800 frames which, although towards the low end, should be enough to produce an acceptable result.
When passing the capture file through stacking software, expect the number of frames that make up the final stacked image to be just 10-20 per cent of the full frame count. If the final result shows colour fringing, it can be corrected by re-aligning the colour channels either in a graphics editing program or using the RGB colour-align function that some programs have.
Sharpen your images of Mars with Registax
1. Reference Frame
RegiStax analyses your capture file to assess how sharp and well defined each frame is. Load your capture file (typically an AVI movie file) using the ‘Select’ option and then adjust the lower slider so that a particularly sharp frame is displayed. This will be the reference against which the others are compared.
2. Placing the Alignment Box
An alignment box should be placed over a region of high contrast in the reference frame. Typically this will be the main planet’s disc, so choose an appropriate ‘Align box’ size to do this, but don’t worry if some of the disc falls outside the box. Move the cursor over the image and left click to place the box.
3. Align
Check the ‘Use pre-blurring’ option and then click on the ‘Align’ button. RegiStax runs through each frame, comparing it to the reference frame. The end result is a graph (‘RegiStrationgraph’ as it’s called). A red line shows the image quality and a green one shows the movement shift between frames.
4. Limit
There’s a quality bar in the graph, which can be moved using the slider at the bottom of the main window. Dragging the bar right adds more frames to the final stack and improves noise but also brings more poor quality images into the process. Drag it to the point where the red line indicates 80-90 per cent quality.
5. Limit and Process
With your decision on the placement of the quality bar made, click ‘Limit’. This tells RegiStax to ignore any frames to the right of the bar. Follow this with a click on ‘Optimize & Stack’, which will instruct RegiStax to align all of the images to each other and then combine them in a stack to reduce noise.
6. Wavelets
The Wavelet’s control sliders allow you to sharpen different levels of detail in the processed image. Click on each slider’s ‘Preview’ button to reveal the type of detail that will be affected by adjusting the slider. Choose the level that you find works on detail rather than noise and move the slider to the right.
SOFTWARE WALKTHROUGH
Our software walkthrough video shows you how to merge red, green and blue filtered images taken with a monochorome planetary camera to create a full-colour final shot.
The full version of Astrophotography, The Complete Guide, part 3 can be found in the June 2011 issue of Sky at Night Magazine
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