My First Wide Field Astrophotos

After the encounter with the Pretty Good Comet of 2020, F3 NEOWISE, and not getting any good pictures of it, I got it in my head to get DSLR camera so I could do wide field astrophotography. That means wide shots of the sky that look sort of like what you see with your eyes, not a closeup through a telescope. The idea wouldn’t go away, even though the comet did. So I set out to find something used and useable that I could afford, something that would be good for a beginner and with enough capacity to grow with me for a while. After a few weeks of research and shopping online, I found a Nikon D3100 body in great condition for what seemed to be a reasonable price. After a misfire, I found an AF-S 18-55mm DX f/3.5-5.4 zoom lens, also in great shape, to go with it, and again for not too much money. So off we go.

My Photographer Cred

My dad used to do a lot of photography as a hobby. He even had his own darkroom, developing his own film and printing his own pictures in black and white. My brother was something of a photographer, too, having got in pretty early on the digital revolution with a Canon Rebel, which my daughter now has. While I’ve never owned an SLR before, being around a couple good photographers gave me a general conceptual knowledge of how they work. Plus, I have used plenty of point-and-shoots and have developed a pretty good eye, I’d like to think. Still, actually using a DSLR, and for a specialized brand of night photography, presents a pretty steep learning curve.

Not Gonna Do It

For one thing, on my first attempt, I found that my camera is finicky and stubborn in low light conditions. Even having figured how to set the aperture wide and the shutter speed long in manual mode, I couldn’t convince the D3100 to actually take a picture it was sure would be bad. This led to a great deal of frustration on my part, causing me to question whether I had made a terrible mistake. I managed not to rage quit and instead resigned myself to discover through further study how to be smarter than my camera. Reading the owner’s manual seemed a good first step, but before too long I got bored and did an interweb search for my problem.

It turns out the “AF” in the AF-S lens stands for “auto-focus,” which the D3100 takes very seriously. This line of cameras performs auto-focusing with motors in the lens rather than in the camera itself as the lens and camera discuss the shot you are trying to take. Consequently, there is a switch on the side of the lens to go between automatic focus and manual focus. When in auto mode, the camera tries to give the lens all the information it can in order to get a sharp focus. If the camera can’t see well enough to know if the shot will be in focus, it won’t flip the shutter. Putting the lens in manual focus mode solves this problem. The camera still boldly announces that the shot is too dark, but assured by the now-manual lens saying, “I got this,” it allows the picture to go ahead, even though you clearly don’t know what you are doing, in its not so humble opinion. Thanks, internet photography forums! You saved me from having to slog through the boring owner’s manual!

And so it was that, having switched to manual focus, I accidentally took a 20 second exposure of the inside of my lens cap. At first I was confused, because, as noted, the camera was still declaring the shot to be invalid. But having heard the shutter flip, I realized I had broken the code. Actually, taking a “dark frame” is an important part of astrophotograph processing, as it reveals any hot pixels and biases and stuff that the camera and lens may have that can then be subtracted from the final product, or so I’m told. I’m nowhere near ready for that level of postproduction yet. Nevertheless, that first unintentional black picture was the start of something wonderful. I hurried outside with my camera and tripod, found a mostly clear sky (not sure how this was allowed, but I’ll take it), and started taking long, dark pictures of the night sky at 1:00 in the morning.

The Pictures

You can see the whole collection except the dark frame >here< (plus all of what you just read), and I’ll put a few in here directly below. I’m pretty pleased with how they came out straight from the camera. Obviously there is so much more that can be done that I intend to learn, but for the first run, this is pretty cool.

Technical

Most of these shots are 25 seconds long, with a couple at 20 seconds. If you zoom in you can tell there is just a bit of a trail on the 25 second stars, but not so much that you notice it much when zoomed out. As expected, the 20 second exposures have less of that effect, but they are significantly darker. I suppose about 20-25% darker. The 20-25 second figure is a product of what is called the “500 rule” that I read about on a number of websites. Divide 500 by the lens focal length to get the max shutter speed without trailing. For certain types of digital sensors, such as the one in the D3100, you have to adjust for the architecture by using 1.5 x the lens focal length. So I was shooting at 18mm zoom, times 1.5 is 27, and 500 / 27 = 18.5. So I was really going too long at 25 seconds by that calculus. Some folks I read recommended more like 8 seconds, take lots of images, and stack them in processing. That’s a lot of work for a beginner. The other option is to get a tracking mount of some sort that would essentially remove the limit altogether. By following the stars as the earth turns under them, you never get any trails. Well, we may get there eventually, but for now I’ll play with the math.

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