Astrophotography Calculator for Long Exposures

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Ideal Sub-exposure Times in Seconds


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Many people, especially those who are just starting out with astrophotography, would like a calculator that figures out the best lengths of sub-exposure so they can choose with more confidence how long to set their exposure and integration time before starting their imaging session. So, this is why I made this calculator for astrophotography. I hope that it will help you.

The calculator is based on information from a talk by Robin Glover of SharpCap. It tries to take into account things that change depending on your equipment and where you live, such as:

  • The amount of light pollution in your area
  • Your camera’s read noise level
  • The amount of noise you can tolerate in your image

In addition to the above, the ideal length of your sub-exposures will also depend on whether you are imaging with a colour camera or monochrome camera and the filter you are using (i.e., an RGB light pollution filter or a narrowband filter).

So, three different exposure times are given below:

  1. Length of sub-exposure for a monochrome camera
  2. Length of sub-exposure for a colour camera or using an RGB filter
  3. Length of sub-exposure for imaging with a narrowband filter

The calculator values are meant to be used as a guide or indicator of the best exposure times for the equipment you have and the environment where you are imaging. It has been simplified since the exact numbers would be highly complex to calculate due to many variables that can change from moment to moment such as seeing conditions etc.

Use these values as a starting point and try things out from there.

The calculator I have developed required quite a bit of experimenting, even after plugging in a number of formulae to work out the exposure times and overall integration times based on the required signal-to-noise ratio in the final image.

Values you’ll need to know for the calculator

1. Your light pollution on the Bortle scale

The sky where I live is rated a Bortle 5, which is a suburban sky. How do you find your Bortle level? It’s pretty easy, and you have a couple of options:

  • Here’s what I do. I use the Clear Outside app on my iPhone, and it tells me the Bortle level of my local site.
  • You can also use an online light pollution map to estimate your Bortle level by looking at the colors on the map. 
  • Another method is to go out and observe your sky at night when it is very clear and compare it to this Bortle Scale Photograph (the photographs are by astrobackyard.com).
  • More information about light pollution can be found on this excellent page.

The Focal Ratio of Your Telescope or Camera Lens

You’ll need to combine the Bortle level with your focal ratio to work out the correct figure for your light pollution rate to enter into the first input box of the calculator.

The focal ratio, which is usually written as f/4, etc., shows how much light your optics let through. If the number is higher, it means that your lens or telescope lets in less light, which is often called slower optics. If the number is lower, it means that more light gets to your sensor, making your optics faster.

If your focal ratio is f/2 or f/3, for example, you’ll need shorter exposure times than if it’s f/4, f/5, or more.

Most telescopes have an f/6 aperture, but some can go up to f/10. There are many good f/1.8 camera lenses, but anything better than f/4 is fine.

My Celestron SLT 130mm reflector OTA is f/5, which is pretty fast. My Bortle sky is 5. So, I would use 3.7 from the following chart from Robin Glover’s talk as the light pollution level. Look at the picture below to find the number that corresponds to your Bortle level and focal ratio:

richardgloverLP
Light pollution rates from a talk by Robin Glover (available on YouTube)

Take the appropriate number from the chart shown in the image above, taken from this talk given by Robin Glover of SharpCap. Here is the link to the YouTube video where you can watch the talk and learn more about where this information comes from.

2. Your camera’s read noise

Look for the model of your camera, whether you are using a DSLR or an astronomy camera, and you should be able to find the read noise value for your camera. You need a value in electrons, so it will have the unit “e.”

My camera, for example, is a ZWO ASI533 MC Pro (colour camera), and the value quoted for read noise at unity gain (or 100 level) is 1.5e. This means that each pixel’s sensor electronics produce 1.5 electrons’ worth of noise. The value is the average for 50% of the pixels, so it is not an absolute value but one that we use to compare camera noise.

3. The Amount of Noise You Can Accept In Your Image

The images you can take with your camera will always have noise, so how much noise can you accept before you are happy with the image you have taken? This is obviously a matter of opinion.

The calculator will give you the answer to “How long should my sub-exposures be?” based on a constant value you choose.

Acceptable Noise LevelConstant C
2%25
5%10
10%5

The quality of your image depends on the noise level

So now that we have understood how to find or choose the three values you need for the astrophotography exposure calculator, these values are automatically placed into the calculation to estimate the ideal sub-exposure values for the details you have entered into the calculator.

Finally, use the value that applies to your imaging session: monochrome camera, colour camera, RGB filter, or narrowband filter.

Plans for more astronomy calculators in the future

I plan to add another calculator here to answer another important question relating to long exposures for astrophotography that I think you’ll find helpful to use with the one above.

Depending on the target you choose to image, you may need to image for a longer or shorter total integration time than usual. For example, you’ll be able to get much better photos of the Orion Nebula much more easily than of other, dimmer targets such as Thor’s Nebula or something even fainter.

I’m going to do the research and make a calculator that will tell you, based on the target, how long the total exposure time should be. I imagine that we will have to look at the relationship between the brightness of different objects and how that will change the length of time needed to get a good image.

So, keep an eye out for this new calculator and other ones I may add soon.

Enjoy your astrophotography!