Cocoon Nebula IC 5146

The Cocoon Nebula (IC 5146) is a newly developing cluster of stars. Nearby hot young stars have excited the hydrogen gas around the cluster and made it glow red. The wider view is generally of a busy starfield, but there also appears to be a long dark lane of obscuring interstellar dust clouds trailing off from the Cocoon towards the right (westwards). This is part of a dark nebula known as Barnard 168 (B168). The Cocoon is 4000 light years away in the constellation Cygnus, and it is about 15 light-years wide.

IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)


The latest addition to my deep-sky rig is the Pegasus FocusCube v2 autofocuser. I attached it to the Hexafoc focuser on my Explore Scientific FCD100 series ED 102 mm, as shown in the photos below. I permanently removed the central locking screw on the focuser, and removed the two screws diagonally across from each other in the Hexafoc assembly, replacing them with the longer ones provided to hold the universal bracket in place (with a washer each). The length of the screws needed ‘tuning’ with a hacksaw slightly.

Before purchasing, I was slightly worried I would not get a good fixing as the pinion housing is rounded and does not have a flat face, but actually, the two screws are able to hold it firmly without it slipping or rotating at all.

In total, I was able to install the whole thing inside the 45-minute afternoon nap of my toddler, which included a 20-minute trip to Halfords to get a 12 V DC car power socket splitter (which is how the focuser is powered). Impressively quick and easy.

The screengrab below shows the autofocuser in action using multi-star full width at half maximum (FWHM) measurements. In the routine I set up, starting from reasonably good focus, the stepper motor moves the drawtube out by 250 steps, takes the average of five multi-star FWHM measurements, and then loops moving the drawtube in with 100-step increments and with an average FWHM measurement at each focus position. When plotted, as in the screengrab below, the optimum focus position is at the minimum FWHM position, which SharpCap calculates by fitting a curve through the points.

My Hexafoc focuser has enormous backlash. It needs 2000 steps to completely take up the backlash. When moving the drawtube out, it needs to clear the backlash after every movement, which is very time consuming. I therefore only move the drawtube out once at the start, and focus by incrementing the position of the drawtube in only, which doesn’t need the backlash clearing (as the gears are always engaged).


I also used the Beta version of SharpCap 4.0 for the first time on this target. This was to access some of the new powerful features, including support for dithering (outside of live stack mode), and support for sequences.

Although dithering is extremely easy (just check Tools > Enable Server in PHD2), I have never actually got it working before in SharpCap 3.2. With SharpCap 4.0, it just worked with no stress, and I kept all the default settings, including a max dither step of 20 px.

The code below is the SharpCap 4.0 sequence I wrote. It loads my typical deep sky profile on a dark night in RAW14 with exposures of 300 s, an analogue gain of 900, and a black level of 99. It then loops, starting with the autofocus routine described above, and followed by capturing 10 light frames with a dithering step every 2 frames (every 600 s).

    LOAD PROFILE "Deep Sky (AA294CPRO(USB2.0))"
    TARGETNAME "Cocoon Nebula IC 5146"
            SET EXPOSURE TO 2
            SET GAIN TO 72000
            AUTOFOCUS OFFSET 250 TO -250 STEP COUNT 10 BACKLASH 2000
            FRAMETYPE Light
            GUIDING DITHER EVERY 600

The dither step is visible in the PHD2 guiding graph as sharp peaks in RA and Dec, labelled as “Dither”. Next time I will increase the minimum settle time parameter in SharpCap from the default of 5 s to something bigger, like 10 s. I found the stars were not as perfectly round as I would normally like them in the period immediate after dithering.


  • 39× 300-s light frames (Gain 900)
  • 3 h 15 min total exposure time
  • Full use of calibration frames (darks, flats and dark flats)


  • Explore Scientific ED 102 mm Apo f/7 refractor
  • Revelation Adjustable Field Flattener
  • Sky-Watcher EQ6-R PRO SynScan GOTO equatorial mount
  • Altair Hypercam 294C PRO colour fan-cooled camera
  • Altair quad-band one-shot colour (OSC) 2″ filter
  • Pegasus FocusCube v2
  • Altair 60mm guide scope
  • Altair GPCAM2 AR0130 mono guide camera


  • SharpCap 4.0 Beta
  • PHD2
  • DeepSkyStacker
  • Photoshop
  • Topaz DeNoise AI
  • Gradient XTerminator

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