Image Capture Workflow
By Don Selle
This is the first in a series of “how-to” articles to help beginners learn astrophotography. The series will assume that long exposure imaging will be done through an OTA on a GoTo telescope mount that can be polar aligned. The camera attached to the OTA is either a cooled dedicated astronomy camera or a DSLR attached at prime focus. Since there are so many combinations of specific equipment the articles will assume that the telescope and camera have been set-up, equipment connected to a computer, powered up and checked out.
Most beginning astrophotographers (me included) were inspired by the work of others to jump into the hobby, not realizing how involved creating those beautiful pictures can be. No matter how much you know about daylight photography or how technically inclined you are when starting out, learning astrophotography can seem like a huge steep hill to climb.
If you can develop a consistent imaging workflow, a series of steps that you follow each time you go into the field, your astrophotography experience will be much more pleasant and productive. In this column we will explore at a high level the steps involved in a typical imaging workflow. Over the next several columns, we will take each of the steps and dig into them in more detail. So, let’s get started.
1. Measure your imaging system. When you get a new piece of imaging equipment, the first thing you want to do is to set it up and try it out by imaging something interesting. However, if you first take the time to test out and characterize your system, during full moon or when the sky is not so good, you will put yourself in a position to efficiently use the time you have under clear dark skies. There are a few key parameters you can calculate such as image scale, but it is usually best to actually measure to confirm them.
2. Set up your equipment in a consistent way each time you use it. This can be practiced at home in your garage if necessary when the weather is bad. Not only will this help you become time efficient, it will also help you to store your equipment in a consistent way so that it is easy to pack for the trip to the dark site, and the risk of forgetting that one critical item will be much reduced.
3. Plan your imaging session before you head out to the dark site. “Fail to plan, plan to fail” is an old adage that applies very well to astrophotography. You should know what object (s) you plan to image, and how you are going to image them (exposure time, sequence of exposures, etc). Dark time in the field is a precious commodity and it can be very easily wasted. When you finally get all the equipment working together, and learn to avoid the operator errors, you will find having a plan and working it will help you make the most of your time under the stars.
4. Take your calibration frames. Flats, darks and bias frames are necessary for you to process your images and to get the most out of them. While you can take dark and bias frames at home and use them from a library, flats must be taken every time you set up your equipment or change the rotation angle of your camera. This requires that you know the focus position for your camera with each of your filters. While there are other ways to take flats, I have found that taking twilight flats is the most consistent and least expensive method once you learn how.
5. Polar align your scope. This may be a rough alignment, or it may be an accurate one, depending on your equipment and software. If you took advantage of evening twilight to shoot your flats, Polaris should be peeking out by the time you get them done. There are many ways to polar align, and since most methods use Polaris and or the stars surrounding it, the end of Civil Twilight is a good time to get this done.
6. Establish your mount’s pointing model. Once you are polar aligned (even if only roughly) you can go through the process of getting your mount and computer set up so that you can accurately point your OTA and Camera. Some methods are mount specific and others are software driven.
7. Focus and calibrate your guide camera. Whether you are using a guide scope or an off-axis guider (OAG) you will need to get the guide camera focused. Once focused, you will need to run a software routine to calibrate your guide system, so that it makes appropriate guide corrections while you are imaging.
8. Slew to your first target. Should be no problem, right? Just like daylight photography, how you chose to frame your target can have a significant effect on how eye catching your final image turns out. This is where planning comes in. You need to know how you intend to frame your target for the resulting image, and how you will get it your camera to the right location on the sky and at the correct orientation.
9. Acquire guide star and begin guiding. If you have planned your imaging well, you should already have a good idea about where you will find a good guide star. Since you have already calibrated your autoguider, this step can be completed relatively quickly – maybe.
10. Focus your camera. You will need to focus your camera before you begin taking subs, and then at various points throughout the night. Manually or with a motorized focuser, either way takes some practice. Knowing what good focus is, and how it is measured will help you become proficient.
11. Set up and begin taking your series of subs. This is the fun part. But first, you should take the time to measure the sky brightness around your target so that you can calculate the optimum exposure time for your subs. Don’t forget to focus periodically, but not so often that you waste time for no real return.
12. Logging your imaging session. When I first started imaging, I was so busy trying to learn to make everything work that I never really considered keeping a log to track various weather, sky quality and imaging parameters. It took me a while but I finally realized that keeping track of things can help improve the quality of your images. Sometimes, it can also help you decide when conditions are such that getting some sleep is a better use of your time!