Webcam Imaging the Giant Planet

Compared to Venus and Mars, Jupiter is not a planet with a high surface brightness. Yes, it is bright in the evening sky, but that is mainly by virtue of its physical size. However, it is much brighter than Saturn and, crucially, more than bright enough for a webcam to easily record, even with brief 1/10th second exposures that freeze the seeing. The biggest problem with Jupiter is the speed with which it rotates. As we saw earlier, the formula to calculate the 0.5 arc-second drift time window of the center of the disc when the planet spans 45 arc-seconds, is: 0.5/((3.14 x 45)/590) = a mere 2.1 minutes. Unlike Saturn, whose globe is less than half the angular size and whose features are very subtle, we really do have a serious problem here in keeping to the time margin if we are imaging with filters. With a color webcam, changing filters is not necessary, but when aiming for the highest quality results, amateurs often use red, green, and blue filters. Splitting the 2.1 minutes into three gives just over 40 seconds for each run. However, I cannot stress just how tight things become in the cold, damp, and dark in such a frantic scenario. Typically, you may be observing through gaps in cloud. Your Schmidt-Cassegrain may be dewing up and a hairdryer is needed to de-dew it. Then you have to wait for the corrector plate (or Newtonian secondary mirror) to cool down again. As soon as things are at ambient they start dewing again. You are then expected to change filters every 40 seconds and may have to refocus and re-center the planet every time, as even knocking the tube slightly when you change the filters, will cause an alignment problem; remember, the field of view may only be an arc-minute wide. Unless you have a very slick motorized filter wheel and a rigid telescope mounting, changing filters three times within a 2-minute window can be very fraught indeed. Jupiter shows considerable detail in an infrared filter (700-1000 nanometers) and one option with this planet, especially when low down, is to just take monochrome IR images, as shown in Figure 13.10. Specialist imagers like Antonio Cidadao often concentrate on narrow-band IR Jupiter imaging at the methane band wavelengths of 619, 727, or 890 nanometers; the 890 nanometer band is where methane gas absorbs light the most and is the most important region to study. With photographic film this spectral region was invisible to the amateur.

Another ingenious option, as frequently practiced by Damian Peach, and also by this author, is to just image Jupiter through infrared/red and blue filters and synthesize green by adding the red and blue channels together (see Figure 13.11).

Figure 13.10. Images taken in the infra-red (700-1000 nm) by Antonio Cidadao with a 35-cm SCT from Portugal. Opposite sides of the planet are shown with the Great Red Spot on alternate limbs. Note the fine details visible at this wavelength. Image: A. Cidadao.

Figure 13.11. When Jupiter is low down and blurred, good images can still be obtained with just two filters: an IR filter and a blue filter. The diagram shows infra-red (700-900-nm) and blue filtered images of Jupiter taken by the author on March 28, 2005. A green image synthesized by adding infrared and blue data is shown in the top right. The lower image is an LRGB image where 50% of the luminance has been derived from the IR image (and the other 50% from R,G,B.) The RGB color info is from the IR, synthesized green, and blue images. Images stacked and sharpened in Registax and LRGB combined in Maxim DL. Image: M. Mobberley.

Figure 13.11. When Jupiter is low down and blurred, good images can still be obtained with just two filters: an IR filter and a blue filter. The diagram shows infra-red (700-900-nm) and blue filtered images of Jupiter taken by the author on March 28, 2005. A green image synthesized by adding infrared and blue data is shown in the top right. The lower image is an LRGB image where 50% of the luminance has been derived from the IR image (and the other 50% from R,G,B.) The RGB color info is from the IR, synthesized green, and blue images. Images stacked and sharpened in Registax and LRGB combined in Maxim DL. Image: M. Mobberley.

Remarkably, this does work well, and the red-filtered image can produce some very fine detail, too. However, correcting the color balance to make the planet look natural can be a major challenge. In the infrared, Jupiter's moons appear very bright and so much redder than normal in the color image. Unfortunately, although seeing and contrast are improved substantially in I band there are a couple of instances where I band images lose out. The brown NEB barges do not show up well at infrared wavelengths and neither do some of the orange features like the NNTB, because these features are not as dark at IR wavelengths as they are in the red and green. The synthesized green technique is a very useful one, especially for a low-altitude Jupiter where seeing and contrast are normally poor. However, a true green filter really brings out the best contrast in the NEB and gives it the deep brick red color that simulated green fails to capture.

Ironically, when the planet is far away from opposition, or conditions are poor, you have a larger time window. If Jupiter has just emerged from solar conjunction it may only be 30 arc-seconds across, in which case, with the same 0.5 arc-second drift limit, you have three minutes to collect the images. If Jupiter is low down and blurred and there is no hope of getting sharp images, even a four-minute imaging run can produce a very pleasing image, capturing all the detail conditions allow. If your color image is the result of a red/infrared luminance and a blue image, with green synthesized, you only really need to worry about the red AVI being two minutes long. The human eye only notices luminance sharpness anyway and when aligning the blue/synthesized green with the red detail on the planet (i.e., aligning the detail, not the limb), the alignment error will only affect the planetary limb. An alternative solution to swapping filters is to swap webcams; one can be used for luminance (B&W) and one for color (a standard commercial webcam). If you have a Barlow or a Powermate on each, fine ... just remove one and slide the other in. But there are problems even with this approach. The B&W and color webcams will invariably have slightly different focus positions. This, in turn, can mean a slightly different focal length and a slightly different image size for the B&W and color information. Another problem with this approach is that PC's and software can lock up when switching from one USB webcam to another. The last thing you want to have to do in a two- or three-minute imaging window is to reboot the PC! If you have a large-aperture telescope and a site with frequently good seeing, a standard color webcam can produce good, strong, high signal-to-noise Jupiter images at exposures less than the usual 1/10th of a second. Remember, it is a good plan to keep to 10 frames per second to avoid compressing and corrupting the image with a USB 1.1 webcam, but reducing the exposure time will still give you a shorter exposure (even if the actual exposure time indicated is not reliable). In Florida, Don Parker has taken spectacular Jupiter images with a ToUcam Pro webcam at a relatively bright (f/14) on his 0.4 meter f/16 Newtonian. Florida is a site renowned for its good seeing. Reducing the focal length may lose you some resolution on the best nights, but on mediocre nights it will dramatically improve the finding and ease-of-centering of the planet.

Being a gaseous world, Jupiter will never look good on the raw webcam output unless you are imaging in the infrared. If you are using an IR filter, the IR image should be focused first as it will be the sharpest and you can usually ignore refo-cusing when the blue (and optional green, if green is not being synthesized) filters are used. Two other advantages of not using a green filter and, instead, synthesiz ing green as an average of "IR + blue" are as follows. Firstly, Jupiter is usually a similar brightness in the near infrared to the blue, so you do not waste time turning the webcam gain down when the bright green image comes up. Secondly, certain features, especially the moons, are very bright in the IR image and very dim in the blue. If green is synthesized from bright Moon red and dark Moon blue, the resulting satellite images look orangey and not too bright. If a real green is used in the mix, the moons usually end up looking bright red. Remember, when using the LRGB method, the luminance image does not need to be the 100% luminance contributor. An LRGB image composed from 100% IR luminance and with its red color derived from the IR data, too, may look very artificial. In packages like Maxim DL, the luminance contribution can be altered in the LRGB mix. The most natural ratio seems to be one where the luminance weighting is 50%, i.e., 50% from the IR and 50% from the normal RGB values, even when the R is really IR and the green is synthesized. So, in a package like Maxim DL you would simply set red as IR, blue as blue, green as the synthesized IR-blue average image, and luminance as IR with a 50% weighting. Basically, you need to do plenty of experiments to see what looks the most natural.

Only the moons and their shadows will ever look sharp in a commercial color webcam; the rest of the planet will look drab and washed out as you image it. It is a good idea to reduce the gamma value in the webcam properties window when imaging Jupiter. A high gamma may make Jupiter's zones too bright and they will wash out. We want mid-range brightness values to be slightly dimmer than normal with Jupiter so that the zones will not swamp the fine detail, such as bluish projections and festoons entering the bright equatorial zone. This swamping of detail can partially be compensated for later in other packages, too, if the original webcam gamma is untouched. In this case, a gamma adjustment from 1.0 to around 0.6 seems to work well. With a basic color webcam like the ToUcam Pro, Jupiter seems to come out with rather a strange greenish tint. Tweaking the color to increase the red and decrease the green seems to work well with the giant planet. Alternatively, use the auto white-balance tool in Photoshop as a good starting point. A final tweak worth noting with any planet, especially Jupiter or Saturn, is, as a last stage, to adjust the brightness such that the very brightest part of the planet's globe (i.e., the equator/meridian central region) is almost whiting out. This may seem counterintuitive when considering what I have just said about gamma, but it will ensure that your final image has a good dynamic range.

With Jupiter, the problem for all photographers used to be the planet's limb-darkening. It is not really a problem with the powerful image-processing tools we have today, but making the brightest parts of the image reach 100% will ensure that Jupiter's limb does not make the planet look smaller than it is. This is essential if the images are going to be measured by students of the planet. If you are submitting your images for analysis, make sure you know exactly what time the mid-point of your exposure was. This can easily be determined using Windows Explorer and right clicking on the AVI file name and then selecting properties. The "created" and "modified" dates will tell you the start and end times of your webcam run. Bear in mind that it is worth checking that your PC clock is accurate when you do this. If a PC has locked up, and needed re-booting in recent weeks, the clock can be very inaccurate.

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