What to Shoot - Pick Your Subject Spend a little time planning your imaging session before you go out and you will save precious dark-sky time out under the stars. Think ahead of time about exactly which object you want to image, and when it will be highest in the sky. Think about the size of the object you are interested in and the best focal length with which to shoot it. Pick your subject based on several different criteria. When first starting out, it's best to try for those bright, large objects like M42 and M31. Because they are large, you can shoot them with a shorter focal length, allowing more room for error in tracking. Because they are bright, you can shoot them with shorter total exposure times. Also try to pick your deep-sky subject based on the focal length you intend to shoot with. You are not going to see much in the final image if you try to shoot a tiny planetary nebula with a short focal length instrument. For deep-sky astrophotography, you will probably be shooting with a telescope and maybe a telecompressor. That will give you two basic focal lengths to work with. The telecompressor is very tempting because it will allow shorter exposures, but it will also give a wider field of view. This is good for very large objects and a scope with a long focal length, but not good for small objects. How to Shoot It - Framing and Focal Length Decide on the focal length you need to shoot the object so that it is optimally framed. You want to have enough focal length so that the object nearly fills the frame with just enough open space around it for aesthetic considerations. This, of course, implies that you have a wide range of focal length instruments that you can use. If you don't, then you will have to shoot the object at the focal length that you have! If the object is too big for that focal length, you may need to consider an advanced technique such as shooting several over-lapping frames and creating a mosaic to encompass the entire object.
Using a desktop planetarium program such as SkyMap Pro, or MegaStar, can make your planning sessions easier. These programs allow you to input the sensor size of the chip in your camera and the focal length of your telescope or camera lens, and then plot a frame around the object so you can see exactly what you will get when you shoot. If you don't have such a planetarium program, but do have some star charts such as Tirion or Uranometria, you can calculate the field of view of your scope or lens, and then make a template on a piece of transparent material and overlay it on the chart to show the field of view for the particular scale of the atlas or charts you are using. You can calculate your field of view with Formula 8 in the appendix of this book in the formulas section. It's not a hard and fast rule, but you generally will want to align your camera so that one side of the sensor is parallel to the right ascension or declination of your scope. This will align the camera north - south or east - west. When you take images like this it is then easy to match them up exactly with your charts and tell exactly which way is north, south, east and west. This is not a law however, and you can frame a deep-sky object however you find aesthetically pleasing. However, if you only plan to shoot flat-field frames once at the beginning of the night, you can not change the orientation of the camera after you shoot them, unless you rotate the entire telescope tube and camera together. So if the object you plan to shoot is elongated in a particular direction and would best be framed that way, the camera must be oriented that way for the flat-field frames. Think about how you are going to find an object that may be too dim to even be seen on the camera's groundglass. You may be able to dial it in if you have a Go To mount, or accurate setting circles. Take a test exposure and examine it on the DSLR's LCD screen to be sure the object is correctly framed. Research a Guidestar Look for a guidestar on your star charts while you are planning the imaging session and deciding how to frame the object of interest. Finding one ahead of time can be a great help especially in areas of the sky that don't have many bright stars near choice objects, like in the Virgo cluster of galaxies. Finding a guidestar ahead of time is particularly helpful when using an off-axis guider. It tells you exactly how you will have to orient the off-axis guider in relation to the framing of the object, and can be a great time and aggravation saver. You can also calculate the distance of the pick-off mirror from the center of the image frame, and then draw a circle around the template frame that you drew earlier. This also allows you to predict the orientation of the camera frame and guider beforehand and makes it easier to acquire the guidestar when you are at the scope. Alternatively, if you use a really sensitive autoguider like a Lodestar, you will almost always have a guidestar already in the frame and you won't have to go hunting around for one. When to Shoot Try to shoot an object just before or after it transits the meridian. The meridian is an imaginary line that runs from due north through the zenith to due south. It's best to try to shoot objects when they are nearest to being highest in the sky, which is when they transit the meridian. That's when you will shoot through the least amount of the Earth's atmosphere. When objects are low on the horizon, you are looking through much more atmosphere. This not only dims, but also blurs the object compared to when it is higher in the sky. If you are planning on shooting several hours worth of exposures on an object that does not get very high above the horizon, it may be necessary to shoot half of the exposures before the object transits the meridian, and then flip the scope over to the other side of the mount and shoot the other half of the exposures. Or, it may even be necessary to shoot the object on several different nights to accumulate enough exposure time to get a really good picture of it, if it doesn't get very high above the horizon. Some mountings may also have an "dark" zone where an object cannot be observed. On my Losmandy mount, there are certain parts of the western sky near the meridian that I cannot observe because the declination lock knob bumps into the right ascension motor. This has to be taken into account when planning an imaging session. For example, if I wanted to shoot two hours of total exposure of an object that transits at midnight, you might first think to shoot one hour's worth of exposure on the eastern side of the meridian, and another hour on the western side. However, you also have to include time for re-acquiring the object after moving the mounting for the meridian flip, and possibly re-focusing, and re-acquiring the guidestar and related setup operations. If the object is in my dark zone, I would also have to budget time for this. For such an object, it might take me three hours to acquire two hours of exposure time. In such a case, if the object is high in the sky, I might consider shooting two hours of total exposure before the object transits to avoid these hassles. If you have the choice of shooting M31 when it is high overhead around midnight in the fall, or M42 when it is low on the horizon just before dawn at the same time of year, shoot M31 because you will get a much better picture when an object is high in the sky. However, if you live in Canada, and you are making a once-in-a-lifetime trip to the Texas Star Party, and Omega Centauri is only going to be 12 degrees above the horizon at its highest, go ahead and shoot it even though it is low in the sky! Different objects are also best placed in the sky at different times of the year. For example, the realm of the galaxies in Virgo and Coma transit the meridian at about 6am on January 1st, just before the start of astronomical twilight. So you wouldn't want to try to shoot them in the fall because they just are not very well high in the sky then. In September and October, they are setting in the west just after sunset, and just rising in the east just before sunrise. The realm of galaxies are best shot early in the morning in January, or around midnight in April, or early in the evening in the summer. Some people as so excited about shooting an object they have never shot before that they will attempt to do this as soon as the object rises in the east. It is much better to wait a couple of months until the object is higher in the sky. The only time it's necessary to ignore this rule of thumb is when the object you are interested in shooting never gets far from the Sun, such as with the inferior planets (Mercury and Venus) and comets. In these cases it is necessary to shoot them low in the east just before the start of astronomical twilight or in the west, just after the end of twilight. This is when they will be highest in a dark sky. A planisphere or planetarium program is a very useful tool in planning a night's astrophotography. With them, you can tell when an object transits the meridian for the time of night you will be shooting. Obviously, you will also want to try to shoot when there are no clouds around and when the Moon is not up if you are shooting faint deep-sky objects. Atilla Danko's Clear Sky Chart at is a great resource for planning observing and photography sessions. It provides forecasts for astronomical observing conditions for more than 1900 observing sites in North America. You can shoot deep-sky if the moon is up, but the same rule of thumb applies to the increased sky brightness caused by Moonlight as for light pollution. You will have to increase the total exposure time in proportion to the increased brightness of the sky. Another thing to think about if imaging from a true dark-sky location is the Zodiacal light. This wide cone of sunlight reflecting off of interplanetary dust particles can be amazingly bright when the ecliptic is at a steep angle to the horizon before dawn in the fall, and after sunset in the spring. Try to shoot when it is not windy. A long telescope tube is a very efficient lever arm and it doesn't take much wind to move the scope and prove detrimental, if not fatal, to a high-quality long-exposure image. Try to shoot on nights of relatively good seeing. Poor seeing makes focusing more difficult. Even well-focused images will have larger stars from poor seeing. Try shooting with a shorter focal length, or wide angle lenses on those nights of poor seeing. Faint deep-sky objects are best shot when the atmospheric transparency is good. This usually occurs after the passage of a front that scrubs the air and a high-pressure system builds in that brings clear dry air. This is especially true in the winter time. You can check the sky's transparency during the day with the thumb test. Hold your thumb out at arm's length and cover the Sun with it. Note how blue the sky is next to the Sun. If the sky is hazy and the transparency is not very good, the sky next to the Sun will be white. If the atmosphere is very clear and transparent, they sky will be blue almost right up to the Sun. Be very careful when doing this test not to look at the Sun when your thumb is not covering it! Also note that there is a difference between transparency and seeing. Good transparency means clear air that you can easily see through to see fainter objects. Good seeing means that the atmosphere is steady and calm and objects viewed through a telescope at high power are not jumping around distorting fine detail. Good transparency, particularly during the winter, usually means poor seeing. Good seeing usually results from stable air masses that have been in place for a while, letting the atmosphere calm down. Good seeing is critical for high-resolution planetary work. Good transparency is important for imaging faint deep-sky objects. Where to Shoot - Observing Sites Take into account factors that affect the local seeing at your observing site. For example, pavement, concrete and house roofs all absorb heat from the Sun during the daytime and radiate this heat back into the sky at night. These rising heat waves cause poor seeing. So if you are interested in doing planetary work, don't set up in a parking lot or look at the planets over any houses. Warm air rises and cool air sinks. So the seeing will frequently be worse in a valley compared to the top of a hill due to the cool air at night heading downhill into the valleys. In general, you'll be better off setting up in areas with natural vegetation such as a large grass field or clearing in a forest. While DSLRs make it possible to image from urban locations by shooting many more short sub-exposures to equal one longer exposure, there is no substitute for dark skies. You may think that your backyard is darker than under the street light in front of your house, but both are probably compromised by urban light pollution, unless you are lucky enough to live in the country, far from the city. Individual Clear Sky Clocks for the observing location closest to you also provide links to light pollution maps taken from the World Atlas of Artificial Night Sky Brightness compiled by P. Cinzano, F. Falchi and C. D. Elvidge. This is an excellent resource to find out exactly how dark your observing location is, and to find darker skies within driving distance.
The best I can do within a couple of hours drive from my home in suburban Philadelphia is to shoot from a yellow area (number 11). I can see the Milky Way from this location when it is overhead but not near the horizons. The west is severely compromised at this location from the light dome from the city of Philadelphia. To reach truly dark skies, a 6 hour drive is required. If you don't have access to a pristine dark-sky location with no man-made light pollution, then you can try to optimize your site selection in relation to the objects location in the sky when you plan to shoot it. I have severe light pollution that I must deal with here on the East coast. I also have several different observing locations that have light domes in different directions on the horizon. If I want to shoot Orion when it transits the meridian in the south, I pick a location where the southern sky has the least light pollution. Try to select an observing site that has the least light pollution in the direction that the object will be when you shoot it.
Shooting after local midnight can also help as some businesses and homes turn off their outdoor lighting around this time. Also, after mid-night, you will be less plagued with airplanes and satellites, and the seeing is usually better because the atmosphere has had some time to stabilize. Try to pick a location that is not subject to ground fog, such as the ones that I used in the swamps when I lived in Louisiana, or in the cranberry bogs where I now live in New Jersey. I find that the top of a mountain in Arizona is usually best, although wind can sometimes be a problem up there. Realistically, you have to make do with what is available to you. If you can't get to a really dark location, have no fear! You can still shoot deep-sky objects from your driveway. Exposures will be limited in duration by the sky fog from any man-made light pollution at your observing site. To create the equivalent of a longer exposure needed to capture a faint deep-sky object from a dark location, you just need to add or average together many shorter exposures. This is made possible by the linear nature of the digital sensors in DSLR cameras. Of course, you don't get something for nothing, and the price you have to pay is that you need a whole lot more short exposures than you would think to equal a longer one under dark skies. Unfortunately, ten six-minute exposures under light polluted skies do not equal one sixty-minute exposure under dark skies. You have to increase the total exposure by the same factor by which the sky is brighter. For example, let's say you need 60 minutes of exposure at a relatively dark sky site to get a good signal-to-noise ratio of a particular object. Your naked-eye-limiting magnitude there is 6th magnitude. At another observing site, such as your driveway, the naked-eye-limiting magnitude may be 4th magnitude, which is 6.3 times brighter (2 magnitudes = (2.51 * 2.51) = 6.3). That means that in your driveway, to achieve the same signal-to-noise ratio as in a 1-hour image shot at a dark-sky site, you would need to expose 6.3 times as long, or a total of 6.3 hours. Sorry, there is no free lunch. What Else to Shoot - Support Frames Decide when you will shoot your support frames. Do you plan to shoot flat-field frames first and then not change the orientation of the camera for the rest of the night? If you have a light box, you can shoot flat-field frames before or after you shoot each object and you can change the orientation for each one. Or, perhaps you don't plan on shooting flat-field frames at all if your system doesn't suffer from serious vignetting and the cover glass over the sensor is free from dust. Unless you have made a thermal frame library at various temperatures, you will also need to think about when you are going to acquire your thermal frames. It's best to acquire them at the exact same temperature as the light frames, and this usually means you have to shoot them very near in time since the temperature usually drops as the night progresses. If you are going to stay out all night shooting, look up the times of sunset and sunrise, as well as the end and start of astronomical twilight. This is when the sky will be completely dark. Miscellaneous Notes Have a plan for an emergency such as if your car battery dies. Tell someone of your specific observing plans and where you are going to be located at, and when you can be expected back, especially if you are out observing in the wilds. Take a cell phone along, but remember that despite what the phone companies say, many remote locations cannot receive cell phone service. If you let someone know when you are supposed to be back, and you run into trouble and can't call, then they know to come looking for you. Be aware of any potential hazards from wild animals and local yokels at your observing site. The general rule of thumb, in both cases, is that if you don't mess with them, they won't mess with you. However, if you are in bear country, observe common sense rules such as not leaving any food out. In mosquito and tick areas, wear insects repellant with DEET. It is the only scientifically proven repellant. This is important because of mosquito-borne West Nile Virus, and tick-borne Lyme disease. Battery Charging Make sure your batteries are maintained and charged before you plan on going out for an imaging session. Sometimes decisions to go out can be made at the last minute on the spur of the moment based on weather. Finding your main power supply battery has not been charged at a time like this can cause a potential good night to be wasted.
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