Clad in the light of a million stars, I’ll never forget the first time I saw it. Just a hop away from Cassiopeia, our nearest galactic neighbor lives amidst a busy summer sky, competing for attention with the magnificent core of the Milky Way. But for those who seek out its beauty, it is every bit as breathtaking.
While in possession of the 600mm Canon lens I used for the eclipse, I sought that beauty for real this time (instead of my accidental capture a few years ago while doing wide-field of the Milky Way).
Without a tracking mount, I knew I’d have my work cut out for me; but the complexity of the situation would prove no match for sheer determination. My task involved calculating the maximum exposure time I could get away with before star trails became a problem (which at a focal length of 600mm was going to be really short) and relying on: a) tracking the galaxy manually by independently adjusting the azimuth and altitude positions of the camera every few minutes, and b) taking a massive number of short exposures that I could later stack in Deep Sky Stacker to increase apparent exposure length.
Light pollution is a perennial challenge faced by the astrophotographer, its lament only exceeded by that of clouds. So I packed up my gear and set out for darker skies. Using my oft-referenced Dark Sky Finder, I ended up in a green location, which although not ideal, was the best option given the circumstances. Conditions were further worsened by a large, uncontained wildfire near the area, creating significant air pollution. But alas.
After getting all the gear set up, the first task is locating the photographic subject. Of course, there are a million ways to skin this cat. For Andromeda, I know where it is relative to the constellation of Cassiopeia, so I just located that first and then star-hopped to Andromeda. You can also try the Google Sky Maps app, with which I’ve had variable success; it can be useful if the compass on your phone is perfectly calibrated, but for me, old-school star hopping has always just worked better.
For starters, I calculated my maximum exposure time, based on the formula I laid out in my wide field astrophotography post. I am now using a full-frame camera (6D!), so the formula was:
Max exposure = 600/focal length.
So, with a 600mm lens, the calculation is pretty simple – 1 second. But I like to live dangerously with my exposure times, so I used this as a mere guideline around which to shoot.
What I actually did
- Light frames: 823
- ISO range: 6400 – 25600
- Aperture: f/4
- Exposure range: 1/4 – 2 seconds
- Cumulative exposure: 13.5 minutes
- Dark frames: 99
- Bias frames: 95
I can’t recall how many actual photos I ended up taking of the galaxy, but after going through every single one individually to evaluate quality, I ended up with 823 usable light frames. I managed to push some of the exposures to 2 seconds, but notably these were extremely hit-or-miss in their usability. The aperture was f/4 for all shots, which is wide open for this lens, and there were happily no problems with coma distortion. I also took 95 bias frames and 99 dark frames to add into the stack, as this strategy can reduce noise in the final image.
Dark and bias frames
Dark frames are captured by taking images with the lens cap on at the same ISO, exposure length and ambient temperature you took your light frames at. Stacking these images with your light frames essentially “cancels out” the thermal and electronic noise generated by the camera. Since I shot at so many different exposures and ISOs, I ended up with a lot of dark frames.
The purpose of the bias frames is to basically zero out all the pixels, so they all have equal starting color values. Bias frames are also captured with the lens cap on and at the same ISO and ambient temperature, but at the shortest exposure time allowed by the camera.
My initial image out of Deep Sky Stacker had a distinctive red hue, I’m assuming secondary to airglow, given that the night was moonless and I was under relatively dark skies. This is really the first time I’ve so prominently captured airglow; usually, I end up with more green tint because of the Bayer filter in DLSRs that generally captures twice as much green as red or blue, coupled with the ambient yellow-green haze from light pollution. Anyway, I popped this into Photoshop and did a variety of post-processing maneuvers, mostly using Curves, which worked nicely and produced the above final image.
My post-processing technique is usually fairly straightforward. I essentially set the white balance in the camera to “daylight”, and once I take my image from Deep Sky Stacker into Photoshop, I open curves and adjust each individual color channel by moving the lower left slider to match up with the beginning of its respective color peak on the histogram.
Beyond that, I generally make a few adjustments to the RBG Curves line to increase contrast, and occasionally do some work with Levels. I usually make a pass through a noise-reducing program. I don’t have anything specific to astrophotography; I use Topaz DeNoise, because that just happens to be the noise-reducing software I have. I think there are some programs that are better for astrophotography, but I’ve never used them.