Acquiring the Cygnus mosaic


Most of the summer and fall of 2019 was spent imaging the Cygnus region in Hydrogen Alpha. I ended up doing a 30 tiles mosaic, with one hour of integration per tile. Each tile is made of 60 subs of 1 minute each, stacked together. The image covers a very wide field of 22×19 degrees, so 420 square degrees of real estate! To put in in perspective the field width has the angular size of 45 full moons side by side.

Everything is recorded behind a narrowband filter, letting light through only around a wavelength of 656.3 nano meters, one of the emission rays of ionized hydrogen. This correspond to a deep red color. For those interested, I used a 36mm Baader H-Alpha 7nm of band width.

So why use narrow band imaging? First I have come to love the black and white feel, or, when multiple narrowband images are combined, the pure (false) colors of the Hubble palette. Then, narrowband imaging cuts through light pollution and does increase the period of acceptable moon for imaging, with some limits. When looking at faint nebulosity on long exposures, it also reduces the stars blotting. And my favorite: with imperfect optics, it removes most chromatic aberrations. This is worth lingering a bit on, as most information out there on the subject is at best incomplete. Transverse chromatic aberration (TCA) happens when each color focuses at separate points in the image plane. Longitudinal chromatic aberration (LCA) occurs when different wavelengths focus at different points along the horizontal optical axis. By going monochromatic (for all practical purpose using a narrow band filter) and refocusing, one can eliminate both TCA and LCA. This is great, almost magical! This explains why some refactors that would not be very satisfying for visual observation (say a a conventional crown-flint doublet) suddenly turn into reasonable performers in narrowband photography. The price to pay is the image scale for each band is slightly different, but this is homogenized at processing anyways. There is one caveat, though: there exist more complex chromatic aberrations that won’t be eliminated by going monochromatic. Spectrochromatism is the main example. With spectrochromatism, an objective exhibits different spherical aberrations for different wave lengths. Bear with me: spectrochromatism changes with the color, so it is a chromatic aberration. A lens may be optimized to have no spherical aberration at a particular wave length. However, as we move away from that optimized wave length, spherical aberration starts to creep in and there is no amount of NB filtering/refocusing that can fix that. Spechtrochromatism is the reason why a commercial SCT will never provide a satisfying images of the clouds of Venus, for example. The SCT has too much spherical aberrations in UV. Even if all the other colors are removed from the picture by a narrow filter passing UV only, and even if the image is refocused for UV, the image is still blurred by the spherical aberration of the system at that wavelength.

Back to the Cygnus mosaic. The optic I used is a Borg 55mm f/3.6, a modified Petzval objective with a of field corrector. With only 6 elements, the Borg does not perform as well as most photo objectives. The Samyang 135, for example, a very popular astrophoto lens with 11 elements, is much cheaper and a better performer optically, particularly on full frame. However, for the reasons explained above, the Borg 55 can be tamed to behave if used behind a narrow band filter. Then by using only the center of the image, near the optical axis where aberrations are best controlled, the image quality is further enhanced. So the relatively small image circle of the ASI 1600MM is an excellent fit for the Borg. With any bigger sensor, the quality on the edge of the field would start to suffer. Another attraction of the Borg is its nice (alas pricey) feather-touch focuser. This allows reliable and repeatable autofocusing for automatic imaging, something I had difficulty to achieve with the Samyang. The longer back focus also makes it easier to put a filter wheel and tilt corrector in the imaging train. Last the setup is both small and light, a super portable option that can be setup in minutes without lifting heavy weights. So this particular setup, although not perfect is the best compromise I have found for the job at hand. The portability and ease of use have in fact made it my favorite care free setup.

The mosaic started with a Celestron CGEM DX mount which I since sold as it was too heavy to be truly portable, and finished on an AP 1100 GTO (on which the little scope was totally over mounted). I ran Sequence Generator Pro on a laptop, everything being automatic (focusing, pointing, plate solving, etc…). At 200mm of focal length the mount alignment can be casual, no guiding is required (and none was used), no collimation needed either on this small refractor, and the setup can perform all night all the nights on its own, without supervision. It is so small I don’t worry about a pier collision, even in the (now) very rare occasions where the command program (SGP) experiences a complete crash. I usually go to bed 30mn after dark and wake up in the morning to a disk full of pictures and a mount parked after a complete night of integration.

The ASI 1600MM is behind a tilt corrector, to be able to remove sensor tilt (at f3.6 with 3.8micro pixels any miss alignment shows) and to tune exactly the distance between sensor and corrector, to which the setting is very sensitive.

Picture of the Borg 55 f3.6, with the ASI 1600MM, the filter wheel, the autofocuser and the DewBuster. Light and compact.

Here is the astrometric data for the image:

  • Resolution …………… 3.830 arcsec/px
  • Rotation …………….. -90.258 deg
  • Focal distance ……….. 204.63 mm
  • Pixel size …………… 3.80 um
  • Field of view ………… 22d 9′ 54.2″ x 19d 25′ 27.1″
  • Image center …………. RA: 20 35 18.338 Dec: +39 17 56.67
  • Image bounds:
    • top-left ………….. RA: 19 36 41.291 Dec: +49 16 21.77
    • top-right …………. RA: 19 52 50.108 Dec: +27 52 36.42
    • bottom-left ……….. RA: 21 33 16.654 Dec: +49 23 50.38
    • bottom-right ………. RA: 21 18 23.450 Dec: +27 58 01.42


  • NamedStars: 55 of 3672 objects
  • Messier: 2 of 111 objects
  • NGC-IC: 42 of 9933 objects

Here is a link to the final image, brace yourself as with 20,832 x 18,256 pixels it is kinda big!

Leave a Reply

%d bloggers like this: