Punching Past Pollution: Target Selection Part 1
“The weather will finally be good this weekend! What should I shoot at now?” For an intermediate imager like myself, this is one of the thoughts that leads to selection of a target. Alternatively, I’ll see something like this as I browse astronomy sites and publications:
I think this object is pretty cool looking and I don’t know if I can do it justice, but I’d love to try. When and where do I aim my telescope? And if I can find it, which equipment should I use to image it?
Two free tools are invaluable for addressing these questions. Next month I’ll discuss a great piece of freeware that helps match equipment choices to specific targets. But first let’s discuss how to find invisible targets in the city or suburban sky.
For a visual observer in the city, the starting point can be just looking up and aiming the scope at the moon, Jupiter, Saturn, or other bright target. But astrophotography lets us explore things you can’t otherwise see, so sky charts are essential. Although many great “Planetarium” programs are available, with varying degrees of connectivity to telescope mounts, I will only discuss the one that I know best, which is Cartes du Ciel (“CDC”, French for “Sky Maps”). CDC is free and very robust, and can be downloaded for Mac, PC, or Linux. If you prefer to use a different program (e.g. Stellarium), that’s fine as long as you can gain good control over the features that I’ll emphasize below.
The first step to “power use” the software is setting up your observatory. If you’re bringing your scope to wide open fields, this isn’t that important. But urban and suburban backyard imagers typically don’t have 360° views. From my backyard, trees, hillside, and houses block about 180° from the southwest to the northeast. And my own roof blocks the first 30° up from the horizon. If you do the arithmetic I can only see 1/3 of the sky. But the sky is a big place! There’s plenty up there, it’s just imperative for me to know what is passing through my third and when.
CDC allows you to customize your horizon by modifying a simple text file. To create a custom horizon you just need a good estimate of the elevation of any obstructing buildings or trees, and the compass points at which the obstruction begins and ends. My lowest horizon is almost due south – at 180º on the compass, and I estimate that my neighbor’s roof blocks about 20º up from the ground. After the entry 180 20, the next entry 195 35 is for the giant maple tree a bit towards the west, and so on to due north at 0 (or 359) where everything up to 75° is blocked. The text file can include comments and looks in part like this:
0 75# neighbor house end trees
85 25# neighbor house end roof
94 25# my roof start
111 40 #chimney start
I drew mine based on rough guesses and refined it over a few weeks as I learned more about exactly where the tree-tops open towards the top of the sky.
The next helpful step is to use CDC’s setup menus to customize the symbols used to mark deep sky objects. My galaxies are little green circles; planetary nebulae are purple circles with a line through them; and big nebulae are outlined in white. As the night progresses, the program updates the positions of items in the sky as they move from east to west. With the time control, it is easy to advance or go back to any specific time. So I can easily check for what’s in the sky tonight, just by opening the program and clicking through the nighttime hours. Does every galaxy above my house represent a good target? No, some are too small or faint or devoid of interesting detail – more on this in next month’s installment. But what if I have a specific target in mind?
Thor’s Helmet is NGC 2359. Can I shoot it tonight? I use the search tool in CDC and immediately learn that by the time it is dark enough for imaging this object is well behind the trees defining my horizon.
So when do I have a clear shot at this object? CDC has a set of time controls that allow me to answer this in two steps. First, I step through one hour at a time to find the highest point the target reaches during the day. We can keep track of the time and date being displayed with the information in the black box on the upper left. With a few clicks I get this screen on CDC:
I next try going back in time 2 months and discover that the target can be found in this slot after dark in mid March.
With a few more minutes of exploration, I learn that the target can be found in this spot as early as October, but at dawn, and during reasonable imaging hours (midnight plus or minus two hours) from December through March. I’ve missed it for this year but I understand why. With other interesting targets well positioned over my house, this one, requiring a lot of attention to timing, has simply fallen through the cracks. But it can be a high priority for next year.
This is good news, however, since it gives me a lot of time to select my ideal equipment combination for the image! We’ll cover this next month.
Note: This article is appearing simultaneously in Eyepiece, the newsletter of NYC based Amateur Astronomers Association of New York and SkyWAAtch, the newsletter of Westchester Amateur Astronomers. I’m a member and supporter of both organizations.