This section of our Basics of Astrophotography series outlines some of the technical skills you will need to develop. This not a tutorial on these skills – I’m still crawling up the learning curve myself, and am hardly qualified to teach them. Think of it, instead, as the table of contents for a bunch of things that you will have to learn. I’ll provide pointers, where I can, to books or tutorials that have been helping me.
Before we get into detail, let’s start with an example of the typical “workflow” for producing a single image of a deep sky object, to give an idea of where the various skills fit in the overall process. This example assumes we are going to image a deep-sky object with a CCD camera or a DSLR. Video-imaging of the moon or bright planets is similar, but somewhat simpler, so this deep-sky example will be more complete.
Here’s what we would do:
- Set up the equatorial mount and do a rough polar alignment.
- Attach the optics to the mount, both the main telescope and the guide scope if you are using one.
- If we are using an SCT, start it cooling off (open the cover, and turn on cooling fans if it is so equipped).
- If this is a “go-to” mount, do the necessary alignments to get a working sky model.
- Attach the camera to the optics (you waited until now for this so you could use an eyepiece for go-to setup), and start it cooling if so equipped.
- Set up the computer and the cables connecting it to the mount and camera(s).
- If we are using a cooled CCD camera, power up the camera and let the cooler get to work. With luck, the temperatures of the scope and camera will have stabilized by the time we have everything else ready and it’s dark.
- Fine-tune the mount’s polar alignment by using the camera and computer to do a drift alignment.
With practice, the above steps take about 60 to 90 minutes. Frequently, just about when you complete this is when the sky will suddenly cloud over and threaten rain. So you will quickly reverse all the above steps and put everything away.
Repeat that often enough, and you will find yourself thinking about having some kind of structure in which you can leave everything permanently assembled. Even a secure and weatherproof garden shed, where you can leave major components assembled to speed taking them out and putting them away, is a big time saver.
- Get the telescope pointed to approximately the right location in the sky.
- Focus the telescope carefully, using an eyepiece and flip-mirror, or using multiple exposures with your camera, and probably using some kind of focus aid.
- Find the target either manually or using the go-to mount. Take some fast, low-quality images to confirm we are pointed at the target and adjust the mount pointing until the target is framed properly.
- Calibrate the Autoguider and start autoguiding.
- Take a large number of images of the selected target. For example, we might take 2 hours of exposure by taking 24 5-minute images.
- Take multiple flat frames, using either an artificial flat field light source or the twilight or dawn sky.
- If we are using a DSLR or one-shot colour camera our image acquisition is complete. If we are imaging with a monochrome camera and filters, repeat with each needed filter. Typically we will take 4 sets of exposures: multiple frames each through clear, red, green, and blue filters. It may take more than one evening to do these.
- If using a monochrome camera and filters, take additional flat frames, so we have a set for each filter.
- Take multiple dark frames (images of complete darkness with the shutter closed or telescope covered) with the same exposure length as your main images. (Ideally dark frames should be taken at the same temperature as the main images. If we have a CCD camera that regulates the chip temperature at a known value, we can take the dark frames at a different time, such as on a cloudy night, and save this dark frame library for frequent re-use.)
Process the Image
- Calibrate all the collected image frames using the collected dark and flat frames.
- Align all the collected image frames.
- Combined the aligned frames into “master” frames (for each filter colour if doing filtered monochrome imaging).
- Combine the master frames for each colour into a single colour image.
- Do various image processing adjustments on the resulting image, expanding the dynamic range, improving the contrast, tuning the colour balance, etc.
With that example workflow in mind, let’s look at some of the necessary skills and techniques in a bit more detail. We will discuss: