M28 (NGC 6626) Astrophotography: Best Timing for Imaging This Globular Cluster

Season and time of night

M28 is best placed in July and August. During those months Sagittarius is well up in the evening sky, and the cluster is available for a long part of the night. Outside them, the constellation rises later and sets earlier, so each session gets shorter and the cluster sits lower when it is dark enough to image.

In mid-July, the cluster crosses the meridian at roughly 22:30 local solar time. By mid-August, that crossing moves to about 21:00. These are approximate figures. Check the exact transit time for your location and date in a planetarium program before you plan the session.

Start your imaging run before the cluster reaches the meridian, and continue for one to two hours afterward. Imaging the cluster near its highest point gives you the least atmosphere between you and the target, which is the main reason to center your window on transit.

Timing by hemisphere

Use the altitude figures in the table above to set your expectations. In the Northern Hemisphere at 35° N, the cluster peaks at 30°. The whole useful window lies in the hour or two around transit, and you should not expect a long run. At 50° N, the peak is only 15°, so the target is close to the horizon at its best. In the Southern Hemisphere at 35° S, the cluster reaches 80° and passes nearly overhead, which gives you the longest comfortable window of the three.

July and August are winter months in the Southern Hemisphere, so the nights are long and generally cool. This works in your favor: you have more dark hours to spread across the session.

Altitude and atmosphere

Altitude controls how much atmosphere your light passes through. Light from a low object crosses more air, and that air adds haze, color shift and extra glow from nearby light sources. This loss of contrast hurts a faint target like M28 the most.

At 35° N, the cluster never climbs above 30°. Accept that the window is short, and focus on the period when it is highest. Shoot a series of frames during that span rather than spreading your time across the whole night.

At 50° N, a peak of 15° means the cluster sits low over the horizon even at its best. Expect softer stars, more color fringing and lower contrast. Check that the ground to the south is free of street lights and buildings. If the seeing is poor, a session of a few minutes may give you more usable data than a long run in turbulent air. Do not expect the same results you would get from a higher target.

At 35° S, the cluster reaches 80°, so atmospheric effects are much smaller. This is the easiest setup for timing. Image through the whole window, from when the cluster rises above your local obstruction line until it begins to drop away from the zenith.

Moon phase and twilight

The Moon is the single biggest factor in a suburban session. Moonlight raises the sky background and lowers the contrast of faint nebulous detail. For M28, this matters because the cluster's outer halo is faint and easily lost in a bright sky.

Plan your imaging around the new Moon. Aim for the few nights on either side of new Moon, when the Moon is below the horizon during your dark hours. A thin crescent that sets soon after dusk causes little trouble. A bright gibbous or full Moon high in the sky is the worst case. If the Moon rises during your session, stop or shift to another target until it has set.

Wait until astronomical twilight has ended before you start the main exposures. This is when the Sun is more than 18 degrees below the horizon. Imaging during evening or morning twilight adds a glow to your background that stacking cannot fully remove.

In July and August, nights at mid-northern latitudes are short. Plan your session so the cluster is high during the darkest portion of the night, and set up the mount and focus before dark.

Imaging in a suburban sky

A suburban sky usually has a naked-eye limit of about magnitude 5. The cluster, at magnitude 6.9, is beyond that limit. You will not see M28 with the unaided eye from most suburban sites. Visual observation needs binoculars or a small telescope, and even then the cluster appears as a faint, diffuse patch rather than a resolved mass of stars.

Imaging changes this picture. Stacking frames builds up the faint light of the cluster and the stars within it, which is why a camera gives you much more detail than your eye can. The trade-off is that the image still depends on sky brightness. Light pollution adds background noise, and a bright Moon adds more. Image on the darkest nights you can find.

Keep the cluster's core in mind when you set exposure lengths. The core is much brighter than the outer halo. Long exposures will overexpose it while the outer stars are still faint. Take a short set of frames for the core and a longer set for the outer region, then combine them afterward.

Equipment and workflow

If you use a smart telescope, the instrument's live stacking is the fastest route to a usable view. The facts above say the cluster shows clearly after a few minutes of live stacking. Keep the instrument pointed at the target and let it accumulate frames before you judge the result. A brief wait is normal, and you should not stop early unless the sky clouds over or the Moon rises.

If you image with a camera on a tracking mount, use the following typical ranges as a starting point:

• Use subexposures of 30 to 60 seconds, and stack at least 30 of them for a clean result.
• Keep the mount tracking well enough that stars stay round over each subexposure.
• Focus carefully on a bright star near the cluster before you start, and recheck focus as the temperature drops.
• Take a separate short set of frames for the core, since its brightness will overwhelm longer subexposures.

These figures are guidance, not requirements. Adjust them to your equipment, your sky and the conditions on the night.

Session checklist

1. Check the Moon phase and rise time before you set a date. Aim for the nights around new Moon.
2. Find the cluster's transit time for your location and date. Note when it is highest.
3. Set up and polar-align the mount well before dark, so you can begin imaging when astronomical twilight ends.
4. Focus on a bright star near the cluster and confirm focus after the temperature changes.
5. Image around the highest part of the cluster's path, and keep the window as long as the altitude and sky allow.
6. Stop if the Moon rises, clouds build or the seeing deteriorates.