Astrophotography Exposure Calculator (Bortle Scale + Read Noise Method)

Last updated August 10, 2026

My astrophotography exposure calculator will help you find your ideal sub-exposure time based on Robin Glover’s SharpCap research. I found this while searching for exactly this kind of information on Youtube. His explanation was really helpful to me when I use CMOS and DSLR cameras. Enter your sky’s Bortle level, your telescope focal ratio, and your camera read noise. My calculator instantly shows you the best sub-exposure for monochrome, colour, and narrowband filters, and I’ve also added a session planner which shows you how many frames you’ll need to capture across one or more nights.

Read noise values in camera specs are measured at a specific gain setting. At higher gains the read noise drops but so does dynamic range. Use the spec value as a starting point and treat the sub-exposure result as a minimum — not a target. Most imagers go 2-3× longer than the calculator suggests and get good results, especially on a guided mount.

Calculator developed by Karl Perera MA — founder of Astroimagery, astrophotographer, British Astronomical Association member, and ZWO ASI533 MC Pro user. All values verified against real imaging sessions from a Bortle 5 suburban sky.

Not sure what to image next?

Once you know your sub-exposure time, choosing the right target for your sky and equipment is the next decision. I’ve put together a guide to the 50 best nebulae for amateur astrophotography to help you pick something achievable from your Bortle level. If you’re still deciding between a DSLR and a dedicated astronomy camera, my DSLR vs astronomy camera comparison covers the practical differences. And once your subs are stacked, my GIMP astrophotography processing guide walks through the editing workflow I use on my own images.

If you’re new to long exposure photography, here is a guide which covers the basics of long exposure photography, . You’ll need to understand this before moving on.

The astrophotography calculator is based on information from a talk by Robin Glover of SharpCap¹. 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 (depends on the camera sensor you are using).
• 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, I give three different exposure times in the calculator:

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 long exposure 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. At least the tool should give you an idea of the minimum and maximum exposure time and a point to start out from.

The long exposure astrophotography 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.

This calculator has helped guide me to capture the Wizard Nebula — 13 hours of calculated exposures from my garden in Turkey. That image is now on a phone case if you want to see what’s possible.

How Does the Sub-Exposure Calculator Work?

The calculator is based on research by Robin Glover of SharpCap — the idea being that once your sky noise per sub outweighs your camera’s read noise by a meaningful margin, extra exposure time stops helping much. Get that balance right and your stacked image will be cleaner regardless of how many frames you shoot.

The calculator uses the formula: Sub-exposure = C × ReadNoise² ÷ LightPollutionRate. The constant C is determined by your acceptable noise level (25 for 2% noise, 10 for 5%, 5 for 10%). The light pollution rate combines your Bortle scale and focal ratio using values from Robin Glover’s empirical research. Colour cameras need 3× longer than monochrome; narrowband filters allow 25× longer exposures by blocking most light pollution.

What Values Do I Need for the Astrophotography Exposure 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 Photograph² (the photographs are by astrobackyard.com).
• More information about light pollution³ 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 astrophotography calculator.

You can calculate the focal ratio of your telescope or lens by taking your focal length in mm. and dividing this by your aperture size in mm. So for a given aperture size a longer focal length will increase the focal ratio and vice versa.

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 astrophotography camera lenses with f/1.8, 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:

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. You can remove noise in image processing.

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

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 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.

Who is Robin Glover and What is SharpCap?

I’ve been using SharpCap for at least three years now — mainly for planetary and lunar captures where its live stacking and histogram tools are hard to beat, and for polar alignment which it handles brilliantly. So when I came across Robin Glover’s talk on YouTube while researching exposure times for deep sky work, I already knew and trusted the software he’d built.

Glover developed SharpCap, a very popular program for capturing astronomy targets. But the talk I’m referring to — “Deep Sky Astrophotography With CMOS Cameras” — goes well beyond the software. It’s a masterclass in the physics of why exposure time matters, presented in a way that actually made sense to me. I was aware that sky conditions and focal ratio affected my exposures, but after watching this talk, it all clicked for the first time. If you the time, it’s worth watching in full. The calculator on this page is built directly on the methodology he presents.

Watch Robin Glover’s SharpCap talk on YouTube →

What Does Part 2 of The Calculator Do?

I added the session planner, (part 2 of the exposure calculator), to answer another important question relating to long exposures for astrophotography that I think you’ll find helpful.

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.

The session planner allows you to work out how many frames you need to capture and how long you should dither to make this possible in a certain number of nights.

Enjoy your astrophotography!

Astrophotography Exposure Calculator — Frequently Asked Questions

What is the ideal sub-exposure time for astrophotography?

It’s difficult for me to answer this because it depends on three things which may be unique to you — how dark your sky is, your telescope’s focal ratio, and your camera’s read noise. From my own Bortle 5 suburban garden with an f/5 scope and my ZWO ASI533 MC Pro, I get around 30 seconds minimum for broadband colour work. Switch to a narrowband filter and that jumps to around 4-5 minutes. Use the calculator above to get the number for your exact setup. And treat it as a start, not the final word — on a guided mount I often take 3-5 minutes per sub regardless of what the minimum says.

How does the Bortle scale affect astrophotography exposure time?

Quite a lot once you get above Bortle 6. The Bortle scale measures how much artificial light pollution is washing out your sky — Bortle 1 is an absolutely dark sky, Bortle 9 is an inner city. Each step up roughly doubles the sky background noise hitting your sensor, which means your optimal sub-exposure roughly halves. I image from Bortle 5 which is pretty typical for suburban locations. If I drove 40 minutes out to a Bortle 3 site I could shoot subs 4 times longer before sky glow became a problem. The calculator accounts for this automatically once you set your Bortle level.

What is the difference between sub-exposure and total integration time?

Sub-exposure is the length of each individual frame. Total integration is all the frames added together. So if I shoot 120 frames at 3 minutes each, my total integration is 6 hours. The two are related but separate decisions. The sub-exposure calculator tells you the minimum length each frame needs to be so that sky noise dominates your camera’s read noise — below that threshold, read noise starts eating into your image quality. Total integration is a different question entirely — it determines how much faint detail you can get out of your target. Use the session planner in Part 2 to manage that.

What is the difference between narrowband and broadband filters for astrophotography exposure?

Broadband (RGB or no filter) lets in more of the light from an object which means it captures light pollution just as much as the light in your target. From a light-polluted site this limits how long you can image before the sky background washes everything out. Narrowband filters — Ha, OIII, SII — only pass a very narrow part of the spectrum, typically 3-7nm. Most light pollution falls outside that window so it gets blocked. The result is you can expose 25 times longer per sub compared to broadband from the same sky. From Bortle 5 and higher, narrowband is genuinely a game changer — it’s how we get images that look like they were taken from a dark site.

How do I find my camera’s read noise for the astrophotography calculator?

For ZWO cameras, go to the ZWO website, find your camera model and look at the sensor specifications — the read noise is listed in electrons (e) usually at unity gain (gain 100 for ZWO). My ASI533 MC Pro is 1.5e at unity gain which is very low and one of the reasons I chose it. For DSLRs, the site Photons to Photos has read noise measurements for most Canon and Nikon models. One thing to bear in mind — read noise changes with gain. At higher gain settings it drops, but so does your dynamic range and full well capacity. For most deep sky work I’d suggest using the unity gain read noise value as your input and imaging at or near unity gain.

How many hours of total integration time do I need for astrophotography?

It really depends on your target and your sky. Something bright like the Orion Nebula can give you a decent result in 2 hours even from a light polluted sky. Something faint like a distant galaxy or a dim reflection nebula might need 15-20 hours before the fine detail emerges properly. From a suburban Bortle 5 sky like mine, I add roughly 50% more time compared to what I’d need from a dark site to get the same result. As a rough guide I aim for a minimum of 4-5 hours on any target, and 8-10 hours for anything faint or challenging. The session planner above allows you to set your integration time and spread it across however many nights you want.

Can I use this astrophotography calculator for a DSLR camera?

Yes, absolutely — I originally built this calculator in its early form when I was using a DSLR myself before switching to a dedicated astronomy camera. Look up your camera model at Photons to Photos to find the read noise in electrons. Use the colour camera result from the calculator since DSLRs use a Bayer colour array just like a colour astronomy camera. The main practical difference is that DSLRs have higher read noise than modern CMOS astronomy cameras — typically 3-6e compared to 1-2e for something like my ASI533. That means shorter optimal subs from a given sky, all else being equal. A modified DSLR (IR cut filter removed) improves your camera’s performance, especially for Ha and narrowband images.

What is the 500 rule in astrophotography?

The 500 rule is a quick formula to work out the maximum exposure time before your stars start to trail when you’re shooting without a tracking mount. Divide 500 by your focal length in millimetres and the answer is your maximum exposure in seconds. So a 24mm wide angle lens gives you about 20 seconds before trailing becomes visible (500 ÷ 24 = 20.8s). On a full frame camera I’d use 300 instead of 500 to be safer — full frame pixels are larger so trailing shows up more readily. Beyond that limit stars become streaks rather than points, which you generally don’t want unless you’re deliberately shooting star trail images. For serious deep sky imaging you really want an equatorial mount with tracking — the 500 rule becomes irrelevant once you’re guiding accurately.

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Important long exposure settings for night sky photography

Here are some lesser-known settings for long exposure night sky photography that I have used for my astrophotography. Based on experience, I can say that the most important and often neglected settings are:

• Noise reduction settings in DSLR cameras. In most DSLR cameras you can set automatic noise reduction. This takes a dark frame of the same exposure length as your light frame. So, if your exposure time is five minutes, you’ll have to wait five minutes until you can take your next photo. In astrophotography we need as much total exposure time as possible, so taking a dark frame after every shot wastes too much imaging time. Turn this setting off. 
• Mirror Lockup (DSLR). When I use my DSLR I always use this setting as it reduces the camera shake because the shutter doesn’t need to open and close between shots. 
• Bulb mode (DSLR). This setting allows long exposures of more than 30 seconds to be taken with a DSLR camera. For night sky photography 30 seconds is quite low and we may need to go as long as several minutes. I normally use 6 minutes exposure time for each frame.
• Camera Gain and offset (astronomy cameras). This depends on the best settings for your camera but you can test these. The gain setting is equivalent to the ISO setting of a DSLR camera.
• Focus. The most important camera setting of all is your focus. Without sharp focus your images will be very poor. It is worth taking extra time, as I have learnt, to ensure your focus is as good as you can get it. Focus on a bright star and use a Bahtinov mask. Adjust focus until the diffraction spikes are equally spaced and you’ll be good to go. Also, tighten the locking screw to make sure the focus doesn’t move through the night. If you have big temperature differences you might need to adjust focus again so keep an eye on it.

References

[1] Robin Glover. Deep Sky Astrophotography With CMOS Cameras – YouTube. Retrieved from https://www.youtube.com/watch?v=3RH93UvP358

[2] Astrobackyard. Light Pollution (Bortle Scale 1-8) | I received hundreds of … | Flickr. Retrieved from https://www.flickr.com/photos/143103129@N03/51092671783

[3] No title. Retrieved from https://www.researchgate.net/figure/Figure-B1-Light-Pollution-Simulation-Bortle-Scale_fig45_317717659