M5 — The Dignity of the Parking Target
Serpens · Askar FRA400 · ToupTek ATR585M · Sky-Watcher AZ-EQ6 PRO · 2026-03-19
Acquisition
| Filter | Exposures | Sessions |
|---|---|---|
| Luminance | 84 × 120s | 3 |
| Red | 36 × 180s | 3 |
| Green | 36 × 180s | 3 |
| Blue | 36 × 180s | 3 |
Total integration: 8h 12m
Astrometric Data
Field Center
15h 18m 35.8s
+02° 05′ 13″
Pixel Scale
0.745 ″/px
Orientation
91.96°
Field Radius
0.422°
Objects in Field
Main Challenges
M5 at 0.422-degree field radius is a compact target at this focal length — all the interesting structure is concentrated in the core and the sparse outer halo. The dynamic range is the familiar globular problem: the core is bright, the halo is faint, and a single processing approach can't serve both. At Dec +2°, M5 never gets high from Torria — the 40-degree peak altitude and the 2.78 airmass at session start are the permanent constraints. More depth would reveal the outer halo and the faint tidal extensions that ground-based imaging can show at sufficient integration.
From the Field
M5 was the third target of each night — the slot that opens after midnight when the Leo Triplet has set and Markarian's Chain has been handed off. NINA switched to M5 at 23:46–23:54 and ran until around 02:00, starting every session when the cluster was sitting at 20.9 degrees altitude and airmass 2.78. A low start, a modest peak at 38–40 degrees. The cluster rose through the session; the airmass improved as it climbed; and the LRGB sequence ran its cycle. Three nights, roughly two hours each. The AstroBin stack includes frames from at least one additional session not retained in the network archive — the published total of 192 frames exceeds the 137 frames found in the NINA folder.
Opening session. Guide RMS 0.23″ — clean. M5 was the third target of the night, after Leo Triplet (19:45) and Markarian's Chain (21:36).
Guide RMS 0.70″ — the same bad night that degraded Markarian's Chain (0.73″) and Leo Triplet (0.57″, one night later). All three targets running the same mount, the same guiding failure propagating through the sequence.
Guide RMS 0.24″ — full recovery. Short on G and B versus the other sessions; the sequence ended earlier.
Process Notes
The "parking target" is an astrophotography concept that sounds dismissive but is actually a form of respect for clear sky. When your primary target has set and the next one hasn't risen, you point at something that fills the gap — an object that's available, accessible, worth imaging, and not what you stayed up for. You park the mount, run the sequence, and collect photons that would otherwise be wasted on an empty sky.
M5 was the parking target for three consecutive nights in March 2026. The Leo Triplet in the early evening, Markarian's Chain through the middle of the night, then M5 from midnight until the sky closed. The cluster opened each session at barely 21 degrees altitude and rose to a modest peak of 38–40 degrees before the sequence ended around 2 AM. Not ideal geometry — but 8 hours of LRGB integration say otherwise.
M5 (NGC 5904) is 13 billion years old. It contains over 100,000 stars bound together since an era when the universe was less than a billion years old. It is 24,500 light-years away and 165 light-years across. It is one of the oldest and largest globular clusters in the Milky Way. It was here before the Sun existed. The parking target, it turns out, has a great deal of dignity.
The Cluster
What separates M5 from the other great northern globulars is the combination of density and age. The core is compact — stellar density high enough that the centre appears nearly saturated even at longer focal lengths — but the outer halo extends across the field with a population of red giant and horizontal branch stars that give the image its warm, layered colour gradient. The innermost regions burn white-blue with the concentrated mass of older stars; the halo fades through amber and gold.
5 Serpentis sits nearby in the field — a foreground star far closer than M5 that appears as a bright point surrounded by faint diffraction spikes, a visual reminder that not everything in the same line of sight is at the same depth.
LRGB for a Globular Cluster
The standard challenge — the same one as M13: the core saturates long before the halo has enough signal. The split luminance strategy used for M13 (two exposure lengths combined via HDR) applies here too, though this image runs a single luminance series at 120 seconds. The 0.745 arcsec/pixel plate scale allows individual stars in the outer halo to resolve cleanly, while the core remains a compact blaze of light that processing can only partially separate.
The colour channels add the stellar population story: the warm red giant population concentrated in the outer halo, the hotter and denser stellar types in the core, the blue stragglers — stars that appear anomalously young in a 13-billion-year-old cluster, likely formed through stellar mergers. LRGB makes the population gradients visible in a way that luminance alone cannot.
Software
Stacked and processed in PixInsight. Acquisition managed with N.I.N.A., autofocus via ZWO EAF through the ToupTek AFW-M 8-position filter wheel.
Behind the Scenes
Three nights, same pattern each time: Leo Triplet opens the session, Markarian's Chain runs second, M5 takes the midnight-to-2AM slot. NINA sequenced all three targets automatically. M5 inherited whatever guiding conditions were left at the end of the night.
Sessions
| Date | Frames | Filters | Window | Alt range | Guide RMS | Focus drift |
|---|---|---|---|---|---|---|
| 2026-03-17 | 48 | 21×L + 9×R + 9×G + 9×B | 23:54–02:01 | 20.9°–39.8° | 0.23″ | 8 steps |
| 2026-03-18 | 47 | 21×L + 9×R + 9×G + 8×B | 23:50–02:00 | 20.9°–40.6° | 0.70″ | 8 steps |
| 2026-03-19 | 42 | 21×L + 9×R + 6×G + 6×B | 23:46–01:35 | 20.9°–37.9° | 0.24″ | 8 steps |
The March 18 Night
March 18 was the worst guiding night of the March run — and it affected all three targets simultaneously. Markarian's Chain recorded 0.73″ RMS on the same mount that same night; M5 got 0.70″. The Leo Triplet's equivalent bad night was March 19 (0.57″) — one day later, the same underlying conditions degrading the same equipment.
All three rejection events trace back to the same 48-hour weather pattern. One degraded night in a run of three produced a proportional rejection rate across three different targets that happened to be sequenced back to back.
Frame Count Note
The NINA archive at \\192.168.1.165\c\NINA\M 5 holds 137 frames across three sessions. AstroBin shows 192 frames in the published stack. The 55-frame gap suggests at least one additional session — acquired and integrated, raw data not retained in the network share. The published integration time (8h 12m) reflects the complete stack including those frames.