M15 Astrophotography: Timing Your Pegasus Globular Cluster Session

Choosing the season

M15 is best placed in the evening in September and October. Through these months it stands high enough in the early and middle part of the night to image through a useful amount of atmosphere, and the sky is dark once evening twilight ends. Earlier in the year, the cluster is lost in evening twilight. Later, it sets soon after dark, which leaves only a short session.

Plan your first session for late September, when the cluster is well up by late evening. Keep October as a second window, especially if the Moon or the weather rules out a September night. Outside these two months you can still image M15, but the session is shorter and the altitude is lower, which limits the detail you can record.

Pick a date within this window that also has a dark Moon, since the Moon affects the outer halo more than the core. The section on Moon phase below explains how to check this.

Time of night

The cluster reaches its highest point when it crosses your meridian, a moment called transit or culmination. The exact time depends on the date and on where you are within your time zone. As a guide, transit falls near 22:00 local clock time in late September and near 20:00 by mid-October. Clock times for transit move about four minutes earlier each night, so check a planetarium program or an almanac for your date and location before you set up.

Daylight saving time shifts these times by one hour. Use the local clock time reported by your software rather than an estimate from memory.

Plan the session around transit. Within about two hours either side of it, the cluster stays well placed. At 50° N it remains above roughly 45 degrees across that span. Shooting far outside this window means the light passes through more atmosphere, and the star images soften.

Altitude at your latitude

Altitude is one of the strongest timing factors for this target. The cluster's maximum altitude depends on your latitude:

• 35° N: 67 degrees at transit, the highest of the three cases
• 50° N: 52 degrees at transit
• 35° S: 43 degrees at transit

Higher altitude means less atmosphere between the cluster and your camera. That reduces blur, color fringing and haze near the horizon. At 35° N the cluster reaches its best position, which leaves the most room for error in timing. At 50° N the maximum is lower, so keep the session close to transit and avoid starting late in the night.

Observers at 35° S have the least favorable geometry. The cluster lies north of the celestial equator, so from the south it transits in the northern sky at 43 degrees. That is lower than the northern figures, so air quality matters more. Your field orientation will also differ from the northern charts you may be used to, so match the star pattern against your chart before you frame the cluster. The target is still reachable, but the lower maximum leaves less margin.

Moon phase

Moonlight adds glow to the whole frame. It mainly affects the faint outer halo of the cluster and lowers the contrast of the core in short exposures. A bright Moon raises the background level, which makes the stacking software's background subtraction work harder and can flatten the faint stars around the cluster.

Schedule your session within a few nights of new Moon, when the sky is darkest. If the Moon is up, check where it sits relative to the cluster. A thin crescent low in the west, after sunset in late September, has little effect once it sets. A bright Moon high in the sky during your window does the most harm. If you have a choice of nights, move the session to a darker one.

Twilight and dark-sky timing

Begin only after astronomical twilight has ended, when the Sun is more than 18 degrees below the horizon. Until then, the background is still bright enough to reduce contrast in long exposures and can wash out the outer stars of the cluster. Check the end-of-twilight time for your location, because at mid-northern latitudes in autumn the sky can take longer to darken than expected.

Once the sky is dark, the autumn night is usually long enough for a full session, so you do not need to rush the start. Finish before morning twilight begins. At mid-northern latitudes the cluster sets in the early hours in late September, so the final part of your night may bring it low in the west, where the air is thicker and the image is less steady. Stop when the cluster falls below your chosen minimum altitude or when the sky starts to brighten, whichever comes first.

Conditions that matter most

• Transparency: Clean, dry air shows the faint outer halo. Haze or high humidity lowers contrast quickly, and you may see it as a dimmer halo even when the core looks fine.
• Seeing: Steady air keeps the core sharp. Turbulence shows as shimmer in the stars. On unsteady nights, a shorter session often yields more usable frames than a long one.
• Dew: Cool, damp autumn nights can fog the front lens or the corrector plate. A dew heater keeps the optical surface clear. Check it before you begin and again during the session.
• Light pollution: A suburban sky limits the naked eye to about magnitude 5. M15 at magnitude 3.0 is well above that limit, so the core will record. The outer halo is more affected by background glow. A broadband light pollution filter can help modestly, but keep your expectations realistic.
• Wind: A steady mount matters for long exposures. Shelter the rig from gusts, which can shake the tripod and trail the stars.

Planning the session

Camera on a tracking mount

Polar align the mount before you start, then focus on a bright star near the cluster. Choose subexposures that the mount can track without star trailing. As a typical range, exposures of 30 seconds to a couple of minutes work for many setups, depending on mount accuracy and focal length. Record a set of light frames, then dark frames at the same exposure and temperature. Flat frames correct vignetting and dust shadows, and they are worth taking at the end of the session while the optics are still in position.

Smart telescope

Point the smart telescope at the cluster and start live stacking. The cluster shows clearly after a few minutes of live stacking, as the facts state. The outer halo and the fainter stars take longer to build up, so let the stack run for the full session rather than stopping at the first clear preview. Clean the lens before you start, and check for dew, because a smart telescope's optics can fog just as a camera's do.

Finding M15 before the session

Start from Enif, the bright star at the tip of Pegasus's nose. Move about 4 degrees northwest and you reach the cluster, which lies within less than half the width of a closed fist held at arm's length. Scheat and Markab, on the western side of the Great Square, help you orient the field once you know which star pattern you are looking at.

Scout the field at low magnification in a finder or wide-field setup before you begin imaging. The cluster looks like a soft, round glow with a bright core, not a single star. Once you have found it, center it in the frame and note the framing so you can return to it the next night.