Deep space astrophotography is probably the most technically demanding thing you can do with a DSLR. It poses two incredibly challenging problems… the targets are very very faint AND moving! This is about as bad as it gets but fear not. All that is required is the proper technique and equipment. Let’s talk about these two problems separately.
Problem 1 – VERY Faint Targets
In principle, the solution is simple. Simply leave the camera shutter open for a few hours and light from the deep space object will just accumulate on our sensor. Unfortunately this is practically impossible because of the large amount of noise that would also accumulate. The light from the sky background would quickly overwhelm the image after roughly 2 minutes from my backyard. Also, the thermal noise in the DSLR camera itself would slowly grow out of control. It would be impossible to tease out the deep space object from all of the other photons that had hit the sensor.
Fortunately, the practical solution is relatively straight forward. The camera shutter can be left open for many shorter spans and then the individual photos can be superimposed or “stacked”. Additionally, calibration photos with the lens cap on can be used to “subtract” out the thermal noise. For most of my photographs I take 60 individual exposures of 30 seconds each.
After you have stacked and calibrated your data into one composite photograph it will still look like garbage. The final step is to digitally enhance it. This is an art and there is no single algorithm that will automatically do this for you. Also, each software suite is slightly different in its features. In general, the photograph must have the faint features amplified and the colors further balanced. This often takes much longer than acquiring the images out under the stars!
Problem 2 – Moving Targets
The only way to address the moving target problem is to move the whole telescope and camera to follow it. An equatorial mount is the best option because it only needs to track with one axis and so can cleanly follow targets. All that is required is that the other axis is aligned to the celestial north pole. This can be tricky but it is critical that this is done properly otherwise there may be some blurring in the photographs.
An indirect way of dealing with the moving target problem is using a short focal length telescope. This has the effect of a wider field of view, thus “hiding” any errors you may have in tracking.
So this is it in a nutshell ->
- Use an equatorial mount with a short focal length refractor attached to a DSLR
- Polar align the mount, find your target and bring it into focus
- Take dozens of photographs of roughly 30 seconds each (I can control my camera through my computer and set up a script to do this)
- Take a few dozen calibration “dark” frames with the lens cap on
- At your computer digitally calibrate the images with these dark frames and stack them all together with astrophotography software
- Further enhance the stacked image in order to achieve proper contrast and color balance (not easy but fun)