Sh2-101 — Tulip Nebula
Cygnus · Askar FRA400 · ToupTek ATR585M · Sky-Watcher AZ-EQ6 PRO · 2026-04-08
Acquisition
| Filter | Exposures | Sessions |
|---|---|---|
| H-alpha 6.5nm | 45 × 300s | 3 |
| SII 6.5nm | 67 × 300s | 3 |
| RGB Stars | 15 × 60s each | 1 |
Total integration: 10h 05m
Astrometric Data
Field Center
19h 59m 55.4s
+35° 18′ 50″
Pixel Scale
1.413 ″/px
Orientation
91.99°
Field Radius
0.865°
Objects in Field
Main Challenges
Waiting for a third session to add more OIII data. High stellar wind region required careful star management in PixInsight to preserve the nebula's fine edge structures.
From the Field
Six nights over the same stretch of sky, chasing a target that only rises into view after 2 AM. Every session started with Sh2-101 sitting low — 21 to 27 degrees above the horizon, through airmass values most photographers would ignore. Narrowband doesn't care. The Hα and SII channels cut straight through the atmospheric depth, pulling signal where broadband would drown in sky background. The OIII channel got its own dedicated night: 40 frames, nothing else, because OIII is the weakest of the three and it deserved the time. April 8 was the standout — the target climbed to 61 degrees, airmass dropped to 1.14, the best transparency of the run. Two nights were near-total losses, cut short by clouds inside the first 20 minutes. The data from those nights was calibrated and evaluated before being set aside. Focus drift across all sessions was single-digit to tens of steps — the EAF handled it silently in the background, re-focusing whenever temperature shifted. Sensor temperature locked to −10°C without exception across six nights.
Opening night. Hα and SII across 3h, with RGB stars captured at the end of the sequence. Target climbed steadily from low altitude.
Dedicated OIII night. Weakest signal of the three channels — it earned the full session. Lowest final airmass of the entire run.
Aborted after 26 minutes. Conditions closed in before the target had any chance to climb. 6 frames kept for evaluation, none used in the final stack.
Full session length, but peak altitude lower than expected — likely cloud gaps around transit. Still a productive night for Hα.
Best night of the NINA-covered run. Peak altitude 55.4°, airmass 1.21. Guide RMS 0.20". The frames from this session carry the most detail in the final stack.
Cooling failure — CCD-TEMP held at +15°C instead of −10°C for the entire session. 47 frames discarded. Autofocus succeeded twice (02:13, 05:27) — technically clean acquisition on the worst thermal night of the run.
Five-minute session before the sky closed completely. Two SII frames — calibrated, evaluated, discarded.
Process Notes
The Tulip Nebula (Sh2-101) is an emission nebula in Cygnus, about 6,000 light-years away, shaped by the powerful stellar winds of the Wolf-Rayet star HDE 227018 at its heart. It sits in the same field as the microquasar Cygnus X-1 — one of the most studied black hole candidates in the galaxy.
Acquired over 3 nights (Apr 4–8, 2026) from a Bortle 3 site in Liguria, Italy.
The SHO Palette
In SHO, the sulphur (SII) traces the outer shock fronts where the stellar wind slams into the surrounding interstellar medium — rendering in red the sharp, sculpted edges of the "petals." The hydrogen (Hα) fills the bright core and broad ionised shell with green-gold tones, revealing the full extent of the nebula's structure. The oxygen (OIII) picks up the hotter, more energetically ionised gas closer to the central star, adding blue-teal depth and contrast in the inner regions.
Together the palette makes the turbulent interaction between star and nebula far more visually distinct than a pure narrowband or broadband image could.
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
On April 9, the camera sensor ran at +15°C instead of −10°C for the entire night. That's a 25-degree cooling failure, and it cost 47 frames — roughly 3 hours and 55 minutes of narrowband integration that went straight into the bin. The thermal noise floor at +15°C on a 300s exposure is in a completely different league: signal drowns, gradients bloom, calibration frames from the correct temperature don't apply. There was no salvaging it. The full night's OIII and SII data was discarded and the sessions around it had to carry more weight.
Sessions (NINA raw coverage: Apr 6–10)
| Date | Frames | Filters | Window | Alt range | Guide RMS | CCD temp |
|---|---|---|---|---|---|---|
| 2026-04-06 | 6 | 6×Hα | 02:09–02:35 | 21.4°–24.6° | 3.45″ | −10°C |
| 2026-04-07 | 57 | 29×Hα + 13×SII + 5×R + 5×G + 5×B | 02:06–05:50 | 21.7°–44.3° | — | −10°C |
| 2026-04-08 | 54 | 26×Hα + 13×SII + 5×R + 5×G + 5×B | 02:16–05:50 | 23.8°–55.4° | 0.20″ | −10°C |
| 2026-04-09 | 47 | 14×OIII + 33×SII | 01:58–05:52 | 21.7°–44.3° | — | +15°C |
| 2026-04-10 | 2 | 2×SII | 01:53–01:58 | 21.5°–22.2° | — | −10°C |
Note: The earlier sessions (Apr 4/5, Apr 5/6) predating NINA raw coverage contain the OIII data and initial Hα/SII runs that make up the published image.
Autofocus
The Tulip sits at 21–24 degrees when the balcony window opens each session. That's exactly the altitude range where the autofocus routine struggles most: poor seeing near the horizon flattens star profiles just enough to prevent the EAF from finding a clean focus curve minimum.
On April 7, the focuser failed at sequence start — "no stars detected" at 21.7° altitude. NINA logged the failure, fell back to the last-known focus position (310847), and kept going. The 57 frames from that night came out usable; the focuser's last position was close enough. April 8 was the same story at the open, then the focuser tried again at 05:12 — setting phase, the target dropping back below 30° — and failed again for the same reason. April 10's session ended before it produced anything meaningful: the focuser failed at 01:55, two frames captured, session over.
The one clean night in the NINA window was April 6: the focuser succeeded on its first attempt, right after NGC 6946's repeated failures earlier that evening had forced the sequence to pivot here. April 9's focuser ran and succeeded twice — at 02:13 and again at 05:27 — which made the cooling failure even more aggravating. Technically perfect acquisition on the worst possible thermal night.
The April 6 guide RMS of 3.45″ is an anomaly — most likely a PHD2 log mismatch rather than actual guiding performance of that quality. The 6 frames from that session were calibrated and evaluated before being set aside regardless.
What Got Rejected
The April 9 cooling failure is the headline rejection: 47 frames totalling nearly 4 hours of integration lost to thermal noise. The April 10 abort contributed 2 SII frames that were never integration candidates. The April 6 session (6 Hα frames, anomalous guide data) was also set aside. What survived into the final stack was the data from Apr 4/5, Apr 5/6, Apr 7, and Apr 8 — four solid nights where the camera held temperature and the sequence ran long enough to matter.
Post-mortem
The cooling failure on April 9 is the most instructive event of this project. It produced 47 technically-acquired, properly-guided frames that were completely unusable — not because of clouds, not because of seeing, but because of a hardware state that went undetected until post-session review. The lesson is simple: check the sensor temperature log before integrating, not after. Everything else about this run was methodical. The AF failures at low altitude were expected and handled correctly by the sequencer. The pivot from NGC 6946 on April 5/6 was a good call that got the Tulip its first dedicated session. The published image carries the weight of the clean nights without any of the thermal noise of the bad one.