Astrophotography on film.
Work in Progress! Check back soon
It’s approximately 240 times easier to do astrophotography with a DSLR than it is with film, but if you have a passion for analog photography then you’re going to want to do it anyway. Here’s an idea of what’s involved and how to get started.
Why I use a star tracker.
For the majority of this article I’m going to be talking about photographing the Milky Way core. It stretches across the entire sky so it’s the obvious starting point - the wider the shot the easier it is shoot. I usually recommend that you start off with shooting it on a digital camera first just to get familiar with the concepts. I’m going to continue with the assumption that you know how to do that. If not, there are countless wonderful tutorials on youtube that will help you out. Guides on how to do this with film is a different story and that’s the hole I’m attempting to fill.
Let’s start with a digital exposure as a reference point so that we can understand what the challenges of film are.
A typical digital Milky Way exposure:
f/2.8, 25 seconds at ISO 3200
The main challenge with analog astrophotography is reciprocity failure. Film doesn’t maintain a linear response for exposures longer than a couple of seconds — doubling the exposure time no longer doubles the exposure result on the film. To put it differently, a 10 second exposure on a digital camera would require a longer exposure on film. How much longer? Well that depends on the film and there are charts and tables for some film stocks to work that out. We’re not going to be calculating exposures though, we are going to deal with exposures a lot longer than those charts typically list. Since high speed colour film is non-existent (unless you want find some wildly pricey Natura 1600 on ebay) we’re going to have to use a star tracker.
Film stock choices
Instead of choosing a high speed film, we’re going to choose films with good reciprocity performance and sensitivity to blue and red light (the kind of light from astrophotography targets.) Astropix has a good list of astro film stocks but unfortunately almost all of the recommend films are discontinued. Provia 100f is probably the best colour film for astrophotography that’s still in production. If you want to use a colour negative film I’ve had great results with Ektar 100. If you’re looking to take black and white images then Acros is one of the best choices - high speed black and white films such as Delta 3200 and Tmax P3200 are also good options to use without a star tracker but I’ll write a seperate tutorial about that.
Shooting the Milky Way on 35mm is a good place to start because you have much faster lenses available — at an aperture of f/1.4, you’re getting an exposure duration of around 15 minutes on Provia 100 (with some pushing). If you stop down you’ll get much sharper results of coruse.
My Setup
With a wide lens and a digital camera, all you need is a tripod to take these photos. With film and these very long exposures, the Earth’s rotation is a big problem — you’ll end up with star trails (which is a fun type of photo to take). The solution to this problem is a star tracker, a mount that precisely rotates your camera at the same rate as the Earth (known as sidereal rate). I use the Sky-Watcher Star Adventurer Pro.
The Sky-Watcher Star Adventurer Pro
The first step to getting accurate tracking is polar alignment — the axis of the tracker needs be precisely aligned with the Earth’s axis of rotation by pointing it precisely at the North Celestial Pole or South Celestial Pole. The more accurately you can align the tracker, the longer you can expose before seeing star trails. I use a piece of equipment called a QHYCCD PoleMaster to help me get this very accurate. It’s a little camera with dedicated software to monitor stars near the pole and tell you how to adjust your mount to align it.
The QHYCCD PoleMaster
When shooting on medium format cameras, the fastest lenses are typically f/2.8 or f/3.5, so you’ll be tracking exposures of greater than an hour. This is beyond the normal scope of a tracker like the Star Adventurer but there’s a way to increase its performance even more. It’s a feature called guiding, which is yet another camera with a scope (this time aligned with your imaging camera). It looks at the stars near your imaging target and monitors the error in your tracking, it then sends pulses to the tracker to correct these errors. I use a ZWO 30mm/f4 guide scope with an ASI120MC-S camera and PHD2 guiding software.