M108 Astrophotography: Imaging Settings for a Faint Spiral Galaxy

Target and sky conditions

At magnitude 10.1, M108 is far too faint to see with the naked eye from a suburban sky, where the naked-eye limit is about magnitude 5. Its light is spread across a small area, so its surface brightness is low. Short exposures will show little more than the core. Plan for long total integration, and expect the outer spiral structure to take the most time to appear.

Your latitude controls how high the galaxy climbs. At 35° N it reaches 69° at its highest. At 50° N it reaches 84°, which is close to the zenith. At 35° S it never rises above the horizon, so it cannot be imaged from there or from any site farther south.

April and May are the best months for imaging M108, because it is well placed in the evening sky during those months. Plan your sessions around moonless nights. Moonlight raises the sky background, which adds noise to every frame and reduces the faint detail you can record.

Suburban light pollution brightens the background across the whole frame. A brighter background adds noise faster than it adds signal, so keep sub-exposures moderate and choose gain settings carefully, as described below.

Framing and focal length

The galaxy is small relative to most astrophotography fields, so the main framing question is how much surrounding sky to include. Use these ranges as a guide:

• Focal length: About 400 to 1000 mm works well for a galaxy this size. Longer focal lengths fill the frame more but make tracking and focusing more demanding.
• Field of view: Aim for roughly 30′ to 60′ across the long side of the frame. This gives the galaxy room in the frame and leaves enough background for calibration and gradient removal.
• Sensor size: A smaller sensor at the same focal length produces a narrower field. Check your field of view with a planetarium or framing tool before the session.

Center the target using plate solving with the J2000 coordinates RA 11h 11m 31s, Dec +55° 40′ 27″. Take a test frame, confirm that the galaxy sits where you planned, and then lock the framing. Rotate the camera if you want the long axis of the galaxy horizontal or vertical in the final image. Make this decision before the session, because changing rotation later means restarting the stack.

Mount, tracking and guiding

Star trailing ruins faint detail, so tracking accuracy often sets your maximum exposure length more than the camera does.

Polar align the mount before the session. A rough alignment is enough for short sub-exposures, but long subs require accurate alignment and usually a guide camera. If you are not guiding, keep individual exposures short, typically 30 to 60 seconds, and accept more frames to reach your total integration time. If you are guiding, you can usually work with 60 to 300 seconds per sub-exposure, depending on the mount's accuracy and the seeing.

If your mount is alt-azimuth, note that field rotation increases at high altitude. At 50° N, where the galaxy reaches 84°, an alt-azimuth mount will rotate the field noticeably during long exposures. An equatorial mount avoids this problem and is the practical choice for long sessions at high altitude.

Exposure length and gain

Exposure length and gain (or ISO, on a camera that uses it) work together. The goal is to make each sub-exposure long enough that the camera's read noise becomes small compared with the sky background, while keeping the core of the galaxy below saturation.

For a DSLR or mirrorless camera, a typical range is ISO 800 to 1600 with sub-exposures of 60 to 180 seconds under good tracking. For a dedicated astronomy camera with adjustable gain, use the manufacturer's gain chart to find the setting where read noise falls to a low level. Gain values in the middle of the adjustable range are often a reasonable starting point.

Check the histogram on a test frame. The sky background should sit clearly above the black point, but not so high that it crowds the highlights. The galaxy core should not clip. If the core saturates, shorten the sub-exposures or reduce gain slightly.

Filters

A broadband light pollution filter can reduce the sky background from a suburban site. It does not change the galaxy's broadband light much, so you should expect a modest improvement in contrast rather than a large one. Use the filter for the whole session and keep it in place for all light, dark and flat frames.

Narrowband filters are not useful for this target. They pass only specific emission lines, and a spiral galaxy's light comes mainly from stars across a broad range of wavelengths, so a narrowband filter would record very little of the galaxy.

Many cameras include a built-in infrared cut filter. Leave it in place for color imaging.

Total integration time

The galaxy is faint, so total integration time matters more than any single setting. Use these ranges as a guide:

• Basic image: 1 to 2 hours of total exposure. This usually shows the bright core and the inner structure.
• Detailed image: 4 to 8 hours of total exposure, often across several nights. This usually reveals fainter arms and the surrounding background structure.

Signal-to-noise ratio improves with the square root of total integration time. Doubling your total exposure improves the signal-to-noise ratio by about 40 percent, so the last hours of a session add less than the first ones. Plan a total you can realistically reach across your available nights, and stop when the stacked image stops changing in ways you care about.

Focusing

Focus is critical for a small, faint target. Focus on a bright star in or near the frame, not on the galaxy itself, which is too faint to focus on reliably.

Use a focusing mask or a software focusing aid, and check the star's sharpness at high magnification on the camera's display. Refocus after the temperature changes noticeably, and check focus every 30 to 60 minutes during a long session. Focus drift from temperature changes is common, and a soft image is often caused by focus drift rather than by seeing.

Record the focus position or the star's measured width in each frame if your software allows it. Frames with a noticeably wider star can be discarded before stacking.

Calibration frames

Calibration frames remove sensor and optical artifacts, and they are necessary for a clean image of a faint galaxy:

• Dark frames: Use the same exposure length, gain and temperature as your light frames. Take 20 or more.
• Flat frames: Use a uniform light source or a twilight sky. Keep the same optical train and filter in place. Take 20 or more.
• Bias frames: Use the shortest exposure and the same gain. Take 20 or more.

Shoot calibration frames at the same temperature as your light frames whenever possible, and store them with the same settings in your processing software.

Stacking and basic processing

Process the frames in a clear order. A typical workflow is:

1. Calibrate each light frame with the matching darks, flats and bias frames.
2. Register (align) the frames on the galaxy core or on stars across the frame.
3. Stack the frames with outlier rejection to remove satellite trails, cosmic ray hits and frames with poor tracking.
4. Remove the background gradient from the stacked image, including residual light pollution.
5. Stretch the image so the faint outer structure becomes visible. Stretch in small steps, and avoid pushing the core to white.
6. Adjust color balance if you shot in color, using the background and nearby stars as references.
7. Apply light noise reduction. Heavy noise reduction smooths away the faint arms and the fine structure you recorded.
8. Sharpen the core carefully, if at all. Strong sharpening adds halos around the bright center.

Save an unstretched copy before you process, so you can return to the linear data if a stretch looks wrong later.

Smart telescope and live stacking

A smart telescope with live stacking shows M108 clearly after a few minutes of live stacking. The bright core and the main structure of the galaxy usually appear first, and this is a useful check that the target is centered and in focus.

Expect the faint outer arms, dust lanes and background structure to take much longer. A live stack shows what has been recorded so far, so keep the telescope still during the session and do not move it to recenter unless necessary. If your device allows you to save a longer capture, use it to extend the total integration and reveal fainter detail.