Exposure, ISO and Gain for Night-Sky Photography with a Camera or Smart Telescope

The three settings and what they do

Exposure time

Exposure time is how long the shutter stays open or the sensor integrates light. Longer exposures collect more light from faint objects, but the sky also rotates during the exposure. Longer exposures therefore show more motion, and that motion has to be controlled. The main limit is the rotation of the sky, which is covered in the first problem below.

ISO and gain

On a camera with an ISO setting, ISO amplifies the signal from the sensor. On a dedicated astronomy camera or a smart telescope, the same job is often done by a setting called gain. Both raise the brightness of the image, and both also raise the noise. Higher settings do not add detail. They make faint signal easier to see while making random speckle more visible too.

Aperture and focal length

A faster lens, with a lower f-number, collects more light in the same time. A longer focal length makes stars drift across the sensor more quickly, which shortens the exposure you can use before stars begin to trail. Most beginners get better results with a wide lens than with a long telephoto, because wide fields give more time before trailing becomes visible.

Problem 1: Stars look like short streaks

How to recognize it. Stars are elongated, and the streaks all point in roughly the same direction. Stars near the edges may show more trailing than stars near the center. Sometimes the trailing is faint and only shows when you zoom to 100 percent.

Why it happens. The sky appears to rotate around the celestial pole. Over several seconds or minutes, a point of light moves across the sensor. Shorter focal lengths and a sensor that is physically small give more margin before the motion is visible.

How to fix it.

• Use the 500 rule as a starting point. Divide 500 by your focal length. On a full-frame camera with a 50 mm lens, the result is about 10 seconds. A 24 mm lens gives about 20 seconds. On a camera with a smaller sensor, divide the result by the crop factor. A more detailed rule that also accounts for pixel size can give slightly shorter limits, so treat the result as an upper bound.
• Use a tracking mount if you want longer exposures. A tracker turns the camera slowly to counter the rotation, which allows exposures of several minutes at moderate focal lengths. Without tracking, shorter exposures stacked together give more total light with much less trailing.
• Check focus and framing before you trust the timing. A slightly soft star can look like a trail at a glance.

Problem 2: The image is too dark

How to recognize it. The sky is black with little detail. Faint stars are missing, and only the brightest stars appear. On a histogram, the data is bunched against the left edge.

Why it happens. The exposure is too short, the aperture is too small, or the ISO or gain is too low for the light that reached the sensor.

How to fix it.

1. Lengthen the exposure, but stay within the trailing limit from Problem 1.
2. Raise ISO or gain in moderate steps. Increase it, take a test frame, and compare. Stop when the faint stars appear without the background turning gray.
3. Use a faster lens if you have one. Opening the aperture by one stop roughly doubles the light collected.
4. Take more frames. A stack of ten or twenty identical exposures, combined afterward, produces a brighter and cleaner result than one long frame at high ISO.

Be realistic about what a suburban sky will show. Bright star fields, the Moon and the brightest star clusters are good early targets. Faint nebulae and dim galaxies may barely register, even with long total exposure, and they are better reserved for darker sites.

Problem 3: The sky is bright and flat

How to recognize it. The background is gray, orange or greenish instead of dark. Contrast is low, and stars look washed out even when they are not trailing.

Why it happens. Light pollution from street lamps, buildings and sky glow scatters back into the sky and fills the frame. Moonlight does the same thing on a larger scale. A bright sky raises the background level, and a long exposure accumulates more of it.

How to fix it.

• Shorten the exposure and lower ISO or gain. This keeps the background from filling the histogram.
• Choose your observing time. A moonless night, away from the brightest part of the evening, gives a darker background. Check the Moon's phase and rise time before planning.
• Point away from the brightest part of the horizon. Even a few degrees of separation can make a visible difference.
• Consider a light pollution filter if you use a dedicated camera with a filter slot. These filters block some of the wavelengths produced by common street lighting and can improve contrast on targets they suit.
• Adjust the background in processing rather than trying to remove it during capture. Background gradients are normal in suburban frames and can be reduced afterward.

Problem 4: The image is grainy or speckled

How to recognize it. Colored speckles and grain cover the sky and the darker parts of the frame. The noise looks strongest in the shadows and in areas of smooth sky.

Why it happens. Every sensor produces random noise. High ISO or gain makes that noise more visible. Short exposures from a single frame also contain more noise relative to signal.

How to fix it.

• Lower ISO or gain, and compensate with a longer total exposure. Stacking many frames averages out random noise much more effectively than a single high-sensitivity frame.
• Take dark frames, which are exposures with the lens cap on at the same ISO, exposure and temperature as your light frames. They record the sensor's fixed noise pattern so it can be subtracted.
• Take flat frames when you use a telescope or lens with a visible dust pattern. A flat frame records the shadows that dust and vignetting cast on the sensor.
• Avoid very high gain settings as a default. Many beginners set the gain too high because the preview looks bright. A test frame at a lower setting often gives a cleaner result after stacking.

Problem 5: Bright stars are blown out

How to recognize it. The centers of the brightest stars are flat white with no visible color or structure. Their edges can look bloated. On a histogram, the data piles up against the right edge.

Why it happens. The sensor collects more light than it can record at that setting. Once a pixel is full, additional light does not change its value. The result is a featureless white disc.

How to fix it.

• Shorten the exposure or lower ISO or gain. Bright stars saturate long before faint ones show detail, so shortening the exposure protects the highlights.
• Frame the scene so the brightest stars do not dominate. Shift the target or choose a different time.
• Remember that the Moon is far brighter than any star. Photograph it with very short exposures and low ISO or gain, or it will be blown out in a fraction of a second.

Problem 6: Hot pixels and glow

How to recognize it. Fixed bright dots appear in the same place in every frame. In long exposures, the edges or one corner of the frame may glow brighter than the rest.

Why it happens. Some pixels leak charge faster than others, which produces hot pixels. Electronics near the sensor can add a soft glow, often called amp glow, that becomes visible in long exposures. Heat makes both effects worse.

How to fix it.

• Take dark frames at the same temperature, ISO or gain, and exposure length as your light frames. Subtracting them removes most hot pixels and glow.
• Dither between frames if your mount supports it. Dithering moves the image slightly between exposures, so fixed patterns do not line up when the frames are stacked.
• Use a dedicated astronomy camera with cooling, if you have one. Cooling reduces thermal noise and makes dark frames more consistent.

Problem 7: Stars are soft or out of focus

How to recognize it. Stars look like soft blobs instead of points, even when trailing is not the cause. Fine detail in bright star fields is missing.

Why it happens. Focus on a dark sky is difficult to judge by eye, and small shifts in temperature can change focus during a session.

How to fix it.

• Focus on a bright star. Use live view if your camera has it, and magnify the view until the star is as small as possible. Adjust focus slowly until it is sharp.
• Check focus again after the temperature changes, especially after the camera has been outside for a while.
• Stop the lens down by one or two stops if the corners are soft. Many lenses are sharpest a little below their widest aperture.

A simple routine for your first session

1. Choose a wide lens or a wide-field setting and a bright target, such as a star field or the Moon.
2. Focus on a bright star using magnified live view.
3. Calculate your starting exposure from the 500 rule, and set ISO or gain to a moderate level.
4. Take a test frame and check the stars at 100 percent. Look for trailing, saturation and background brightness.
5. Adjust one setting at a time. Change the exposure first if stars trail, the ISO or gain if the image is too dark, and the exposure again if the highlights are blown.
6. Once the test frame looks right, take a series of identical frames, plus dark frames at the same settings if you plan to process the results.
7. Review a few frames before packing up. Small problems are easier to fix on site than after a full night of imaging.

Checking your results

Use the histogram to judge exposure. The data should sit clear of both edges, with the sky background in the lower third and the brightest stars below the top edge. Zoom to 100 percent to check for trailing, hot pixels and focus. A frame that looks fine at fit-to-screen can show problems at full size.

Keep notes on each session. Write down the lens, focal length, exposure, ISO or gain, temperature and sky conditions. Over a few sessions, these notes show which settings work for your equipment and your site, and they save time on the next trip out.