IC 5146 – The Cocoon Nebula in the Constellation Cygnus

IC 5146, better known as the Cocoon Nebula (also catalogued as Sh2-125 and Caldwell 19), is an emission and reflection nebula with an embedded open star cluster in the constellation Cygnus (the Swan). The cluster carries the designation Collinder 470.

This image was taken over several nights between the end of May and September 2026. It is the first time my new camera, a ToupTek ATR2600M with Antlia LRGB and 7 nm narrowband filters (here only Hα), was put to work. The corrector on the Lacerta FN250 telescope with its 1000 mm focal length is a TeleVue Paracorr Type II, which extends the focal length by a factor of 1.15 to an effective 1158 mm (f/4.6). Adapting the corrector, together with a rotator inserted between focuser and camera unit, called for individually machined adapters. After a few learning steps, this is now the first light of the new setup.

The nebula has an apparent diameter of about 10 arc minutes and lies roughly 2500 light years from Earth, which puts its physical extent at a good seven light years - depending on how much of the faint outer regions one includes, values of up to about 10 light years can be found in the literature. It is a star-forming region containing ionised atomic hydrogen, and it combines emitting, reflecting and absorbing components in one object. Its apparent magnitude is 9.3 mag.

As seen from Earth, IC 5146 sits at the eastern end of the elongated dark cloud Barnard 168, and together the two are part of an extended molecular cloud. A little further west lies the reflection nebula vdB 147, which most likely belongs to the same complex.

The vigorous star formation going on inside the cloud has produced a young open cluster designated Collinder 470, with an apparent magnitude of 7.2 mag and an estimated age of a few hundred thousand years. This cluster - and in particular the brightest star at its centre, BD +46° 3474 (spectral type B0 V, listed between 9.5 and 10 mag depending on the source) - is responsible for ionising the nebula and thus for making it glow. Together they carve a "cavern" into the dust and gas of the molecular cloud and so open up our view into its interior. The nebula was discovered photographically by Edward Emerson Barnard on a plate taken on 11 October 1893, and independently later by Max Wolf (28 July 1894). (Wikipedia: IC 5146)

Exposure Data:

• Newton f=1000mm f/4 (Lacerta FN250)

• Corrector TeleVue Paracorr Type II (1.15x), effective focal length 1158mm, f/4.6

• Camera ToupTek ATR2600M with Antlia LRGB and Hα filters, Gain 100, Offset 200, HCG, Low Noise Mode, High Fullwell

• Exposure Times

  • 359x180s L

  • 48x180s R

  • 50x180s G

  • 47x180s B

  • 78x300s Hα

• Total: 31h42min

• Date: 2026-05-28 - 2026-09-23

• Location: Fuerfeld / Germany

• Mount Skywatcher EQ8R-pro / Pegasus Astro EQMod

• Guiding and Exposure Control with INDI / PHD2 / CCDCiel running on Linux

• Image Processing PixInsight (RC-Tools) and Darktable

Sources: https://en.wikipedia.org/wiki/IC_5146 https://de.wikipedia.org/wiki/Kokon-Nebel