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When I got my first dedicated astronomy camera I expected my DSLR experience would help me determine the best exposure settings. Boy, was I wrong!
Astrophotography is so different from "normal" photography that those rules don't apply. Unfortunately for me, I had trouble finding instructions on how to set a correct exposure. All I found were ultra-technical articles that were more in-depth than I wanted or ones that were so vague they didn't answer my questions. This is the post I wish I could have found when I started with my first dedicated astronomy camera.
What is the goal of a "good" exposure?
What is the goal of a "good" exposure?
Simply put, it's to get the best possible stacked image at the end of pre-processing. It's not to get the best-looking subframes from your camera. There is a difference.
It helps to think of an astronomy camera as a scientific instrument, not a camera. It's just a photon counter. Photons hit a pixel in the sensor and cause an electric charge. The sensor reads that charge and converts it into a digital number, which is then stored in the image file. We're trying to get an accurate count of photons (data), but several issues cause small errors.
These errors (noise) lead to differences in brightness from one pixel to the next, which looks like grains of sand in an image. The noisier the image, the harder it is to see faint details.
Noise comes from several sources. Some noise comes from the properties of the light gathered by the telescope, while the camera itself causes other noise. For exposure settings we're concerned with the noise caused by the camera itself, generally called read noise.
Read noise is in every subframe and adds up in our final stacked image. It's like a fee you have to pay for every subframe you stack. To keep that fee as low as possible we swamp the read noise with our exposure settings. That means exposing long enough that the darkest parts of a subframe are several times brighter than the read noise.
Noise in one subframe (top) vs. noise in a stacked master (bottom).
Camera settings
Camera settings
There are three settings on an astro camera: gain, offset, and exposure length.
Gain is like ISO on a DSLR. Increasing the gain tells the camera to write a bigger number every time it counts a photon. It just amplifies what the sensor sees.
Offset allows us to make sure the darkest pixels in an image aren't equal to 0, or black. Having zeros, or even numbers close to zero, can prevent the pre-processing math from working correctly. Increasing the offset raises the minimum value a pixel can have.
Exposure length is simply the amount of time the sensor collects photons.
How to determine exposure time
How to determine exposure time
To set an exposure you pick a gain setting, decide how much you want to swamp the read noise by (swamp factor), and plug those into the following formula. We'll also need to plug in how much light pollution falls on the sensor per second. The answer to the formula is the minimum number of seconds an exposure should last to swamp the read noise. Then we set an offset to keep the minimum pixel values away from zero and we're all set.
This formula comes from the creator of Sharpcap software, Dr. Robin Glover. He has a terrific video on YouTube if you're interested in more of the technical details. This has worked very well for me and takes the guesswork out of exposure settings.
Exposure Time (s) = (swamp factor) x ((read noise)^2 / (light pollution))
READ NOISE: every gain setting has a read noise value associated with it. We just find that value on the manufacturer's chart and put that into the formula.
This is the chart for my camera, the ZWO 294 MM Pro. For this example we are choosing a gain of 50.
According to the chart, the read noise is about 6.4 for gain 50. That's the number we need.
ZWO Read Noise Chart for the ASI 294MM Pro, as posted on the ZWO website.
LIGHT POLLUTION: The creator of Sharpcap software has an online light pollution calculator to determine the light pollution figure needed for our formula
We need to input some data into the calculator:
The Bortle Number of our imaging site.
The f-ratio of our telescope.
The pixel size of our camera.
The quantum efficiency of the camera (from the manufacturer).
What type of camera it is (color or mono) and what filter, if any, we'll be using.
This image shows the calculation for imaging with my Redcat 51 and 294 MM Pro, at home (Bortle 5 Zone) and through a color filter. The result is the number we need for the formula, 3.1.
You can enter a Bortle Number or a Sky Magnitude. The calculator will determine a value for the other.
SWAMP FACTOR: the swamp factor determines how many times brighter the sky background should be than the read noise. Swamping helps to reduce the impact of read noise in our stacked image. Most people use a swamp factor of 5 or 10. 10 is a good place to start and is easy to achieve if you have light pollution. 10 may be too high if you're imaging from a dark sky site, and lead to very long exposure times. generally, light-polluted skies require short exposures, while dark skies require longer exposures.
For this example, from my suburban backyard, we'll choose a swamp factor of 10.
Exposure Time (s) = (10) x ((6.4)^2 / (3.1))
Exposure Time = 132 seconds
So, for my camera, through that scope, in my Bortle zone, with that filter, at Gain 50, I would want to take subframes that are 132 seconds. I'd round that to 130.
Determining Offset
Determining Offset
The offset setting determines what the darkest value in an image will be. We'll be subtracting a master dark from each light frame, and the result of that subtraction can't be a negative number. Adding an offset raises the value of all the pixels and gives us room for that subtraction. But raising the offset also raises the highest values, and can increase the number of pixels that reach the highest possible value and clip to white. Using the lowest offset possible let's us use the widest possible range our camera can provide.
I've used two ways to determine an offset:
Guessing- I've gotten good results just by picking 15 for low gain values and 30 for high gains. The number doesn't really matter (that much) if your minimum values remain above 0.
Next, we'll talk about why to choose a particular gain, and how to keep things simple by only using a few default exposure settings.
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