Much has changed in the last few years regarding the way in which planetary images are taken. Planetary imaging used to be one of the most challenging aspects of astronomical imaging, but new technology has now made capturing high-resolution pictures easy and inexpensive.
The Keys to Good Planetary Imaging
The most important factor in planetary imaging is atmospheric stability, or good seeing conditions. If the atmosphere is turbulent, no amount of equipment can compensate for it. The planets are small and require very high magnifications in order to obtain high resolution images. Unsteady air will smear the image of a planet, even during a brief exposure, blurring any fine details.
Even on nights of good seeing there will be moments when the conditions are better than average. Ideally, you want to capture an image during the moment of greatest clarity.
The reason this is normally difficult — and was almost impossible in the days of film photography — is that you never know when this moment will come. With film, exposures are longer than with CCDs, so there is more likelihood of the image blurring during the course of the exposure. Another drawback is that to capture the proper moment of good seeing, you have to continue taking picture after picture hoping to grab one during a steady instant of seeing. With film, this was a problem because you only got 24 or 36 shots in a roll of film. Then you were out $6 plus processing (and scanning if you wanted digital pictures). And out of 36 shots you may not have gotten more than 1 or 2 good ones, if that.
Even with a CCD camera, the situation is little improved. The increased sensitivity of a CCD chip allows shorter exposures, resulting in more good images for a given amount taken, but taking these images still consumes a lot of time and effort. Ideally, hundreds or even thousands of images will be captured, because stacking multiple good images results in a better final image.
Webcams are ideal for planetary imaging because they allows those hundreds or thousands of pictures to be captured in a matter of seconds at a very high frame rate (usually around 10-20 frames per second). The images are captured as a video file which can then be broken down into individual component frames. The latest software packages, such as Registax, allow the individual frames to be analyzed and sorted for sharpness. The worst images are rejected, while the best files are combined for processing. This is a completely automated procedure. In a matter of minutes, planetary images can be captured with more resolution than anything ever taken with even the best CCD cameras!
Note: More specific imaging techniques for webcams are covered on the Webcam page, but most of the details below apply equally well to imaging with CCDs or webcams.
Imaging the Planets
In addition to a telescope and CCD camera or webcam, it is necessary to have some means for magnifying the image on the imaging sensor. This is needed due to the very small apparent sizes of the planets. To capture sufficient detail, the disk of a planet must cover quite a few pixels on the chip.
|
Planet |
Apparent Size at Best Visibility* |
|
Mercury |
7.5″ |
|
Venus |
25″ |
|
Mars |
25″ |
|
Jupiter |
45″ |
|
Saturn |
21″ |
|
Uranus |
3.7″ |
|
Neptune |
2.3″ |
|
Pluto |
0.1″ |
* Best visibility is opposition for planets outside Earth’s orbit and greatest elongation for planets inside Earth’s orbit.
The most common accessories for amplifying the image are Barlows and eyepiece-projection adapters. If your telescope has a fairly long focal length (2000mm or more) a Barlow will probably be sufficient. The exceptions to this might be if you have a CCD with very large pixels (16-microns or more), or if you are seeking the most detail possible and have very good seeing conditions. For smaller (shorter-focal-length) telescopes, or for large-pixel CCDs, the usual method of magnification is eyepiece projection. shooting through an eyepiece, more magnification is provided than a Barlow can give.
Note: Most Barlows provide 2x magnification, but there are some high-quality 3x Barlows available as well. Also, products such as TeleVue’s Powermates and Meade’s TeleExtenders provide up to 5x magnification. These are also ideal for planetary imaging.
How Much Power?
What amount of magnification should be used when imaging the planets? Often, an image scale of about 0.25 arcseconds/pixel has been recommended. This should reveal the most detail possible under good seeing conditions without being too much magnification. However, in excellent seeing conditions, much higher image scales have been used with much success. A scale of 0.1 arcseconds/pixel might be a better recommendation for exceptional atmospheric conditions. The required focal length depends on the size of the pixels in your CCD camera. Below is a chart showing the necessary focal length to achieve a 0.25″/pixel scale and 0.1″/pixel scale with various common pixel sizes. Formulas are given as well.
0.25″/pixel: Focal Length = Pixel Size * 825
0.1″/pixel: Focal Length = Pixel Size * 2060
|
Pixel Size |
Focal Length (0.25″) |
Focal Length (0.1″) |
|
3.5 microns |
2900mm |
7200mm |
|
5.6 microns (typical webcam) |
4600mm |
11,500mm |
|
6.8 microns |
5600mm |
14,000mm |
|
7.4 microns |
6100mm |
15,200mm |
|
9 microns |
7400mm |
18,500mm |
|
13 microns |
10,800mm |
26,800mm |
|
16 microns |
13,200mm |
33,000mm |
|
18 microns |
14,800mm |
37,100mm |
|
20 microns |
16,500mm |
41,200mm |
|
24 microns |
19,800mm |
49,400mm |
It should be clear that the best combination for planetary imaging would be a camera will small pixels (such as a webcam) and a telescope with a long inherent focal length and large aperture. The reason for having a long inherent focal length is to make the task of increasing the focal length easier. For example, achieving a focal length of 11,500mm with a scope having an inherent focal length of 2000mm requires a 5.75x magnification factor, greater than most Barlow lenses. However, a scope with a 4000mm focal length will require only a 3x Barlow, a standard accessory. Also, the larger the aperture, the faster the focal ratio. While a fast focal ratio is more important for deep sky imaging, it is still important for planetary shots since the shorter the exposure time, the less problems you will have with atmospheric turbulence. So, an 8″ SCT at 11,500mm focal length has a focal ratio of f/58, while a 14″ SCT at the same focal length has a focal ratio of f/32. Equivalent exposures are 3.3x shorter with the 14″ scope. (Of course, there’s no limit to this. A 45″ f/10 scope has a native focal length of 11,500mm and requires no extra magnification, but good luck toting such a scope out to your favorite observing site.)
If the seeing is less than perfect, or if there are other factors preventing you from realistically using such a high pixel scale, shorter focal lengths are still quite suitable for planetary imaging. The above numbers are starting points, and greater magnification is not always better. The observing conditions must always be taken into account. The image of Jupiter at the top of this page was capturing using a 10″ f/9 scope and 2x Barlow, yielding a focal length of 4600mm, or an image scale of 0.25″/pixel. While the scope being used was certainly capable of greater magnification, the average seeing conditions limited the useful power. The resulting image is still quite nice.
From the above chart and the one at the top of the page, it can be seen that even at these long focal lengths the planets cover very little of the CCD chip. Venus, Mars, and Saturn cover only about 100 pixels, and even giant Jupiter spans only 180 pixels at this scale. Making Pluto even 2 pixels wide requires a focal length of 14,000mm even when using a tiny pixel size. (It also requires seeing conditions about 10 times better than that at the best mountaintop observatories, plus it is 2 million times fainter than Jupiter, but if you like a challenge….)
Note: Be sure to also visit the Planetary Imaging Equipment page for more info and a JavaScript magnification calculator.
Exposure Time
As with all imaging, exposure time depends on the CCD camera being used, which (if any) filters are in place, the focal ratio of the telescope plus Barlow or eyepiece, and the planet being imaged. In many cases, the necessary exposure time is actually shorter than the camera is possible of taking. Many CCDs will only expose as briefly as 0.1 second. For bright planets like Venus or Jupiter, or when imaging the Moon, this may be too long. The use of a neutral density filter or polarizing filter is recommended with CCD cameras. Webcams can have their frame rate and gain adjusted to alter exposure time. Usually a filter is not required since webcams are capable of shorter exposure times and are less sensitive than CCDs.
In general, exposure times will be less than 1 second, even for very long-focal-ratio scopes imaging dimmer targets like Saturn. Experimentation is the best procedure. The great thing about CCDs is that you can take a ton of bad images and just throw them away without any worries!
As with deep-sky objects, a good signal-to-noise ratio is desirable, especially to get the most out of an image during later processing. This requires using a longer exposure time, but not so long that the planet is overexposed (causing a loss of detail in the highlights) or that the atmospheric conditions blur the image.
Another factor to consider is the rotation of the planet itself. In a single brief exposure, this will not be a problem. This is most noticeable on Jupiter, which rotates once on its axis every 9 hours and 50 minutes. Jupiter is about 45″ in apparent diameter at opposition. This means that a feature 0.25″ wide moves its own width in just 60 seconds. While each individual exposure might be only a few tenths of a second long, the time a large number of exposures has been taken, the rotation of the planet can blur some of the fine details. This is even more true with CCDs which often have a noticeable delay for download time.
The necessary readout time for a CCD came be reduced. CCD camera control programs allow you to select a small portion of the frame for downloading. This is ideal for planetary imaging as even a planet as large as Jupiter will not fill the entire frame. Downloading a smaller portion of the frame results in faster downloads, essential for imaging rapidly rotating planets. For example, Jupiter covers, at most, 180 pixels at a scale of 0.25″/pixel. Taking a 250×250 pixel subframe (Jupiter plus room to spare) requires reading out only 62,500 pixels. A CCD camera with an array size of 1600×1200, which requires 4.5 seconds to read out a full-frame image, needs less than 0.2 seconds for such a subframe. taking a sequence of exposures using a subframe, hundreds of images can be captured in just 1 minute.
|
Planet |
Time Required to Rotate 0.25″ at Opposition |
|
Mars |
280 seconds |
|
Jupiter |
60 seconds |
|
Saturn |
150 seconds |
Other Considerations
Precise polar alignment, a stable mount, and accurate tracking seem more like considerations for deep-sky imaging, but they are equally important for planetary shots. Since the field of view is so small when imaging tiny solar system targets, any tracking errors or drift from polar misalignment can quickly cause the planet to leave the field of view. This is frustrating during the imaging process and can make combining the images later more difficult than necessary.
Seeing — the stability of the atmosphere — is probably the single most important factor is getting good planetary images. Excellent planetary images are routinely obtained from Florida and coastal Texas, low-lying, humid, hazy regions which deep-sky imagers would avoid like the plague. But these conditions make for excellent seeing and very good planetary imaging. Remember that dark skies does not necessarily equal steady seeing. In fact, the two are most often mutually exclusive. For example, the mountainous regions of Colorado have very dark skies, the air is often very unstable, making for lousy planetary imaging conditions. On the other hand, the soggy regions of Texas and Florida that are so ideal for planetary imaging are often hazy and unclear. Any region is eventually blessed with a bout of steady skies, so patience is as critical as anything.
Always consult a Jet Stream forecast map !!
