There are optimum times to image the planets, and features on the Moon and surface of the Sun. We need to do a little research to learn when these times are.
For example, we may decide it would be nice to shoot the planet Saturn. But it just happens that when we decide this, Saturn is in conjunction with the Sun and not visible. Or we may want to shoot the lunar crater Clavius, but it is full Moon and not the best time to shoot it. Planetary features such as the Great Red Spot on Jupiter, or Syrtis Major on Mars are best placed for photography only at certain times because of planetary rotation, even when the planet is visible all night long. In general, the best time to shoot a particular planetary feature is when it is crossing the planet's meridian. Using the programs listed below can tell us exactly when this is. These programs can also tell us many other interesting things. For example, using JUPOS, I found that Io would transit across the face of Jupiter starting at about 7 p.m. on February 28, 2012. Io was also going to be very near the Great Red Spot as it transited. At 8 p.m. Io and the Great Red Spot would be ideally placed as they transited Jupiter's central meridian. About 5 minutes later, Io's shadow would begin a transit. For the next hour, both Io and its shadow would be on the face of the planet. At about 9:10 p.m. Io would egress, but for the next hour Io's shadow would chase and almost catch up to the Great Red Spot. All of this would happen conveniently early in the evening starting when Jupiter was still 40 degrees high in elevation above the horizon. We can use these programs to run through the next few days, weeks, or even years in the future, to see what is going on with planets. They can tell us at what time a particular object transits our local meridian when it will be highest in the sky and best placed for observing. Here are some programs and internet applications (web apps) that are very helpful in seeing what is going on in the sky and on the Sun, Moon and planets. General Tips for Shooting the Planets Shoot when they are highest in the sky, which is when they transit the meridian. The meridian is an imaginary line that runs from due north to due south and passes through the zenith. It is best to shoot a planet when it is highest in the sky because that is when we are looking through the least amount of atmosphere which reduces atmospheric extinction and usually improves atmospheric seeing. The outer superior planets in the solar system, and Mars in particular, are largest when they are closest to us at opposition. Opposition means they are opposite the Sun in the sky from our viewpoint on the Earth. At opposition, a planet will rise around sunset, transit at midnight, and set around sunrise. At opposition, a planet will be up all night.
The inner inferior planets can't be opposite the Sun in the sky because their orbits are between the Earth and the Sun. They appear largest when they are closest to the Earth at inferior conjunction, but unfortunately this is when they are nearest to the Sun in the sky. Occasionally this can make for a spectacular transit when a planet like Mercury or Venus passes directly in front of the Sun from our vantage point. When Mercury or Venus is at superior conjunction we can't see them because they are behind the Sun. When Mars, Saturn, Jupiter, Uranus, Neptune or Pluto are at conjunction, they are also behind the Sun. The inferior planets of Mercury and Venus are farthest away from the Sun, and easiest to shoot, when they are at greatest eastern and western elongation. Jupos JUPOS is an acronym for JUpiter POSitions. It is a great planetary program written originally by Hans-Jörg Mettig, but mostly coded by Grischa Hahn, both of Germany. It is freeware that shows all kinds of information about the planets. Download the latest version of JUPOS, or find out more information.
To use JUPOS: The Ephemerides tab has all kinds of interesting information about the planet you selected. The Graphics tab shows what the planet looks like from our perspective on Earth as we view it at the time selected. If you want to see what particular features will be visible on a given planet at a given time, just input the time. Remember to convert to Universal Time (UT). For example, for the east coast of North America, for eastern daylight savings time, add 4 hours to get UT. For eastern standard time, add 5 hours to get UT. For central daylight savings time, add 5 hours, and for central standard time, add 6 hours. You can look up your time zone correction (how many hours east or west you are from the prime meridian in Greenwich England) with this NASA map of the world's time zones, and learn more information about Universal Time on Wikipedia. Weather The weather is important for planetary imaging, and not just because we can't image at all if it is totally cloudy or raining! The weather, and jet stream in particular, have a tremendous impact on the quality of the seeing and how steady or turbulent the atmosphere is. For imaging, the first thing we usually check is whether it will be clear enough to go to the trouble of setting up the telescope. We can get a general impression from the weather forecast on the news on our local television and radio stations, but there are several weather forecasts made specifically for astronomical observing. Don't forget that these forecasts are predictions based on mathematical models, and that sometimes they will be wrong. The cloud forecasts are usually pretty good, but seeing can be much more influenced by local conditions and geography. I like to look at the satellite infrared cloud loop and the water vapor loop and compare them to what the Clear Sky Chart is predicting. If the Clear Sky Chart matches the loops for the last several hours, it's a pretty good indication that the forecast will be good for the next couple of hours. Astronomical Forecasts Jet Stream Forecasts Regular Weather Forecasts You can find hourly forecasts and visible, infrared and water vapor cloud loops at the above web sites. I like to check them all and see how well their forecasts agree. If they do, you can feel more sure about their predictions. If they vary wildly, and sometimes they do, then you know that the forecast models are probably not in agreement and that the forecasts are much less certain. The most helpful is the Clear Sky Chart by Attilla Danko. The data for the Clear Sky Chart comes from a numerical forecast model specifically developed for astronomers by Allan Rahill of the Canadian Meteorological Center. It gives specific predictions on factors such as cloud cover, seeing, humidity and wind for more than 1,900 observing locations in North America.
A very useful feature of the Clear Sky Chart is its ability to put the prediction maps into motion into the future. We can do this for cloud cover as well as seeing. Here's how to animate the maps: You can also animate the seeing prediction charts, although they are only available for night-time hours. I have found these predictions to be incredibly helpful, especially in the animated images. For example, we can see if we are going to be in a large area of clear skies, or good seeing and how that area will move over the time we will be observing. If the area is small and surrounded by areas with a poorer forecast, we can make a pretty good guess that we have less of a chance of a good prediction lasting. Similar predictions for other parts of the world can be found at MeteoBlue (click on the "sky" tab) and SkippySky. Seeing Seeing, the steadiness of the atmosphere that allows fine details to be seen in celestial objects, is probably the single greatest factor that influences the quality of high-resolution planetary images. If the seeing is not good, details in an image will be blurred. We discussed seeing in general in the atmospheric effects section of Chapter 1, but here we will deal with some more specific recommendations. General Seeing Observations Many observers have noted generalizations about the quality of the seeing and weather and related factors. Note that these are not rules set in stone. You have to learn what specific conditions affect the seeing for your observing location. Here are some of the generalizations: Directly Observing the Seeing You can directly observe the seeing and learn a lot about it by simply aiming the telescope at a bright star and defocusing it.
Look at the out-of-focus image. If you see it boiling, and rapidly shimmering, that is not good. This bad seeing can be caused by the telescope, the local environment, or the atmosphere. We can get a pretty good idea of what the cause is if we observe the out-of-focus image more closely. Irregular ripples, spikes dancing and moving around on the outer diffraction ring, and obvious heat plumes rising off one side of the out-of-focus star are indications of poor seeing inside of the telescope almost certainly caused because the primary mirror has not reached thermal equilibrium with the ambient environment, as can be seen in the video at right. Let the scope cool down some more and examine the image again. For a large SCT it may take several hours for it to cool down. Once the scope has cooled down, if the out-of-focus star image has a fast moving "river of air" going across it, this is usually an indication of poor seeing in the upper atmosphere. Frequently you can even bring the turbulence into better focus by starting with the star in focus at infinity and then racking the focus out slightly. This brings the point of focus from infinity to high up in the atmosphere. If the out-of-focus star image has a very slow ripple, or is steady, this indicates good seeing. Once your telescope has cooled down, you can accurately judge the quality of the seeing. If it is poor we can be patient and hopefully wait for periods of better seeing. Seeing will vary over short time periods on any given night. However, if you have seen a forecast and know that the jet stream is going to be overhead all night long, and then you see terrible seeing from this in the scope after it has cooled down, then you might decide to pack it in. The jet stream is really a killer. We can think of it like this. It's probably not worth wasting the time trying to do really high-resolution work on a night of bad or awful seeing. On those nights we should shoot wide-field shots of the entire lunar disk, or if the moon is not up, we can try some long-exposure deep-sky astrophotography. We can use nights of bad seeing to practice our craft if we are just learning. We can practice things like achieving accurate polar alignment, determining the correct exposure for a given focal ratio on various planetary subjects, and figuring out how the different parts of our planetary photography equipment work together. We may find we need extra extension tubes to reach focus with our particular setup. It's better to go through these kinds of processes on nights of bad or average seeing, rather than waste time figuring this out during a night of good seeing. To be honest, nights of great seeing are rare and precious. My Personal Experiences With Seeing The best seeing I've ever experienced was in New Orleans in the summer, in the middle of the city when it was hot, hazy and humid. One of my best friends had a 12-inch f/8 Newtonian on top of his grandfather's garage about 10 feet up in the air. This got the scope up out of the ground effects. The entire atmosphere was stable because the entire southern half of Louisiana is basically a swamp next to the Gulf of Mexico, which is a very large stable body of water. There was also little change from daytime to nighttime temperatures in the summer, so the scope did not have significant cool down problems, and the atmosphere stayed stable. Finally, the jet stream usually wasn't over head in the summer down on the gulf coast. Other great nights were also had in Picayune, Mississippi at my friend's father's farm. One night, when we had superlative views of Saturn, the air was so humid and thick that when we finally looked around in the morning twilight, we found we were in fog! I've also experienced nights of good seeing at Sunspot, New Mexico, near the solar observatory, and, of course, at the Winter Star Party in the Florida Keys. As far as bad seeing, I have experienced a lot of that here in southern New Jersey. It seems that no matter where the jet stream is in the rest of the country, in the winter time, it is always over us when it crosses the East Coast. I have been out observing with friends here in southern Jersey and they have called me over for a view in their scopes saying things like "Wow, I can't believe how good the seeing is tonight!" I will look and think that the seeing is average at best. I think that some of my friends who are occasional observers and who have grown up here may have never actually seen a night of excellent seeing. They are rare everywhere, and seemingly even rarer here in the Garden State. Finally, this isn't specifically about seeing, but another thing I should mention is that when I setup in my driveway, I have a fairly busy street less than 50 yards away. Sometimes I get obvious vibrations when a big truck passes. I wonder if it is affecting the image in the scope even when a car passes. Working at 4,000 mm to 9,000 mm of focal length for high-resolution planetary imaging puts me at a high magnification which might be picking up this sort of thing. I have found that it is better for me to set up the scope on the lawn rather than in the driveway for two reasons. The first is because of heat waves coming off the concrete in the driveway that might cause bad seeing. The concrete heats up from absorbing heat from the Sun during the day and releases it at night. The second is that the concrete in the driveway picks up vibrations and passes them along to the scope much more easily than when I set up on the lawn. I know I can see an effect when I just walk very carefully and softly on the driveway while imaging.
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