M110 (NGC 205) Imaging Guide for Smart Telescopes: Framing, Exposure and Stacking

When and where to image M110

The best months to image M110 are November and December. In these months the galaxy is high in the evening sky, which gives you a longer usable window after dark. Start the session once the galaxy is above about 30° altitude, and check the altitude in a planetarium or sky-chart application before you set up.

Observers at 35° N and 50° N

At 35° N the galaxy reaches a maximum altitude of 83°, so it passes almost overhead. At 50° N the maximum is 82°. At either latitude the galaxy stays high for much of the evening during the best months. Light from the sky travels through less atmosphere at high altitude, so you get less haze, less color shift and sharper stars. Plan a window of one to three hours for total integration. Shorter sessions work if your sky is steady, but a longer window gives the faint outer light more time to build up in the stack.

Observers at 35° S

At 35° S the galaxy reaches a maximum altitude of only 13°. It stays low over the northern horizon and is never high in your sky. Light at this altitude passes through a thick layer of air, which softens stars, adds color gradients and dims the target. Choose a site with a clear northern horizon, away from buildings, streetlights and trees. Plan your session around the time the galaxy crosses the meridian, when it is highest, and expect that the window of usable altitude will be short. From a suburban site at this latitude, a useful stack may be hard to achieve, so check the horizon and the forecast before you commit to a long session.

Framing the galaxy

The galaxy's long axis is 16′, which is a small part of a degree. Choose a field of view with enough room around the oval to include a little sky for background measurement. If your instrument's field is roughly one degree or wider, the galaxy will occupy a modest part of the frame, and you can crop later. If the field is narrower, the oval can fill most of the frame, so center the galaxy carefully and leave a margin at each edge.

Orient the frame so the long axis runs across the longer side of the sensor. This uses the field efficiently and leaves more empty sky for background subtraction. Many smart telescopes can rotate the framing or use plate solving to center a target from its coordinates. Enter the coordinates from the table above and let the instrument slew to them, then check the framing on the first preview image.

Be aware that the Andromeda Galaxy lies nearby. In a wide field, its bright core and disk may enter the frame and dominate the exposure. If this happens, shift the framing slightly or plan a separate session for the larger galaxy. Trying to keep both in one frame usually makes the faint outer light of M110 harder to recover.

Exposure, gain and total integration

Most smart telescopes choose exposure automatically, but if you can set it manually, the following ranges work well for a faint galaxy under suburban light pollution:

• Exposure per frame: 10 to 30 seconds. Use shorter exposures if stars show trailing, and longer ones if your mount tracks well and the sky is dark enough to let the background build up.
• Gain: a mid-range setting, roughly one-third to two-thirds of the adjustment range. Higher gain raises noise. Lower gain can leave the core and inner structure under-exposed. Choose the setting that keeps the histogram away from the left edge without the background looking grainy.
• Total integration time: 30 to 60 minutes is usually enough for a clear core and the inner oval. The outer halo and any subtle brightness changes near the edges need two to three hours.

Total integration is the number of frames multiplied by the exposure per frame. For example, 120 frames of 20 seconds give 40 minutes. Keep frames that pass your quality check, and reject frames with clouds, trailing or satellite streaks. Losing a few frames does little harm, but a large share of rejected frames will lengthen the session noticeably.

Filters

A light pollution filter is worth considering from a suburban site. It reduces the light from sodium and LED street lighting, which raises the background and lowers contrast in faint outer regions. Because M110 is a broadband galaxy, the filter will also dim the target slightly, so compare the result with and without it on a test night. If your smart telescope has a built-in filter that you cannot remove, leave it in place and adjust the exposure and stretch to suit it.

Narrowband filters are not useful for M110. They pass only specific emission lines, and an elliptical galaxy emits mostly broadband starlight. Using one would remove most of the signal you are trying to capture.

Focusing

Focus on a bright star near the target before you start the stack. Alpha Andromedae (Alpheratz) and Beta Andromedae (Mirach) are both in the same part of the sky and are bright enough for focusing. Use the instrument's focus aid if it has one, or switch to a magnified preview and adjust until the star is as small and round as possible. Keep the focus check close to the target so you do not need to slew far.

Recheck focus at the start of the session and every 30 to 60 minutes. Temperature changes slightly shift the focal point, especially after the instrument has been running for a while. A focus error shows as bloated stars, and a stack made with soft focus will show a soft, muddy core that is hard to repair later.

Live stacking and basic processing

Live stacking combines frames as they arrive and updates a preview image. The galaxy's core and inner oval should be visible after a few minutes of live stacking. The outer halo and fine structure take longer and appear as the total integration grows.

Checking the stack

Watch these indicators while the session runs:

• Core brightness: the core should be bright without a solid white blob. If it saturates early, lower the gain or shorten the exposure on the next session.
• Background level: the background should sit clearly above the black point. A background that is nearly black means the stretch will be unreliable, so add exposure or check the light pollution filter setting.
• Star shapes: round stars indicate good focus and tracking. Elongated stars point to tracking or polar alignment issues, which should be corrected before the session continues.
• Rejected frames: a high rejection rate suggests clouds, wind or dew. Stop and check the instrument if rejection climbs past the level you saw earlier in the session.

Basic processing

After the session, process the stack with a light touch. Heavy processing can make a smooth elliptical galaxy look artificial.

1. Save the stack in the most linear format the software offers, so that you keep full control of the stretch.
2. Stretch the histogram until the galaxy's core is clearly brighter than the background, but stop before the core clips to pure white.
3. Adjust color balance gently. Elliptical galaxies are made mostly of older stars, so they appear warm and yellowish rather than blue.
4. Apply noise reduction in small steps. Stop when the background grain is reduced without blurring the faint edges of the oval.
5. Avoid strong sharpening. A galaxy with few fine features gains little from it, and sharpening tends to add halos around bright stars.