North America Nebula

This picture of North America Nebula (NGC 7000, a bright nebular region located in the Milky Way area of Cynus) was taken yesterday, 2010-11-03 under good transparency skies, using a 55mm lens and Canon EOS 450d, Rebel XSi and an EQ6 mount doing the unguided tracking. This is just one shot of 1380 seconds of exposition (23 minutes).

Postprocessing done using PSP9 and Fitswork4.

NGC 7000 is an emission nebula in the constellation Cygnus, near Deneb (α Cygni), also called the North American Nebula. The dark central region called the Gulf of Mexico, as in some astronomical plates for many years resembled that region of America.

Nebula NGC 7000 is the largest covering an area equivalent to the full moon, but its low surface brightness does not normally visible to the naked eye (though, in a dark, using a UHC filter can be seen without optical aid) NGC 7000 and the nearby Pelican Nebula (IC 5070) are part of the same interstellar cloud of ionized hydrogen (HII region). The dark area in the center is a very dense region of interstellar material in front of the nebula and which absorbs light of it, giving the group its characteristic shape.

It is not known with precision the distance that separates us from NGC 7000, neither the star responsible for the ionization of hydrogen that results in the emission of light. Supposing Deneb is the star that illuminates the nebula NGC 7000 then the distance to Earth is on the order of 1800 light years.

Tamron 300mm lens on Canon EOS

Tamron SP-54B 300mm is a telephoto lens built 30 years ago, oriented to analog reflex camera market. Thanks to a special adapter for Canon EOS DSLR camera, I could attach them each other and take some shots. The field of view and the magnification are very different to a conventional lens, like Canon EF-S 18-55mm.

Here it is a comparison image:

Canon EF-S 18-55mm barrel distortion

In order to stack non-tracking subframes for widefield astrophotography, it is important to get images with no field curvature. My lens Canon EF-S 18-55mm when working at lowest zoom, that is 18mm of focal, presents a clear barrel distortion as reported in the image below. Nevertheless, it can be corrected applying a pincushion lens distortion of 15 in Paint Shop Pro. But the image border remains distorted so it is needed a crop reducing the image to an 80% of its original size.

Of course, the best solution is to stop using the 18 mm focal length for shift-and-add astrophotography. Instead longer focal is advisable like 35 mm, where the lens does not show any significant distortion in the field of view.

Another advice to get a sharp focus and pinpoint stars with Canon EF-S 18-55mm lens is to always use a focal ratio no less than F:5 and no more than F:10.  Under F:5 the lens coma aberration would mess the stars near the image border and over F:10 the diffraction would create big Airy patterns instead of pinpoint stars.

I have said above that “it is important to get images with no field curvature to do shift-and-add astrophotography”. I must correct it. Sky pictures are not like an image of a wall. A wall is a plane surface, but the sky is not. Actually, the sky is an sphere, the celestial sphere, that is constantly rotating around Polaris. So, the correct way to add images is not converting them to plane field, but to spherical field in the right proportion.

I have told you that pincushion lens distortion corrects the field. That is truth, but it is not appropriated to process non-tracking subframes. The subframes must firstly be corrected to show an spherical field. So the right filter to apply is indeed a barrel lens distortion, increasing this way the natural barrel that comes with the lens.

It is easy to verify this, just trying to overlap 2 distant subframes. The first and the last of the session may be optimal. Only after applying a barrel of 17 in Paint Shop Pro I could overlap successfully the subframes, preserving angles and distances between stars.