Framing the Veil Nebula (Cygnus Loop): whole loop or one arc

The Veil is a supernova remnant, the expanding debris of a star that NASA describes as once about 20 times as massive as the Sun and exploded several thousand years ago. What we see is a ring of filaments, brightest along two arcs on opposite sides. That shape gives two different framing problems: the whole ring, or one arc in detail.

Three names, three sizes

Object	Catalogue	Size
Cygnus Loop (whole remnant)	SIMBAD	230′ × 160′
Cygnus Loop	Sharpless (Sh 2-103)	210′
Cygnus Loop	Lynds (LBN 191)	210′ × 160′
NGC 6960, Western Veil	NGC 2000.0	70′
NGC 6992, Eastern Veil	NGC 2000.0	60′

NASA’s Caldwell pages split the remnant the same way: Caldwell 34 is the western section (NGC 6960) and Caldwell 33, the Eastern Veil, includes NGC 6992 and NGC 6995. The chart above draws the whole loop from SIMBAD and both arcs from NGC 2000.0. None of these records gives an orientation, so all three are drawn as circles of their largest dimension.

The whole loop

How far you can push the focal length depends on how much you trust the 160′ minor axis. As a 230′ circle, the loop fills 90 % of an APS-C sensor’s short side at about 210 mm, and 90 % of a full-frame sensor at about 320 mm. If you rotate the camera so the 230′ axis lies along the long side, the 160′ axis becomes the limit and APS-C reaches roughly 300 mm. The 250 mm Petzval in the setups table holds the loop at about three-quarters of the frame. At 400 mm with APS-C, the mosaic planner gives three panels for the loop plus a 10 % margin; at 550 mm, four.

One arc at a time

The two bright arcs are 150′ apart (computed from SIMBAD positions), on opposite sides of the loop. Each alone is a medium-focal-length target: the 70′ western arc fills half of an APS-C frame at about 385 mm and 90 % at about 690 mm; the 60′ eastern arc reaches those fills at about 450 mm and 810 mm. The NGC 2000.0 sizes are single values, and in long exposures the filaments continue into the fainter central parts of the loop, so most frames of a single arc leave room on the inside edge.

Emission lines

NASA’s Caldwell 33 page describes Hubble images coloured by the emission of oxygen, sulfur and hydrogen in the filaments. That is why narrowband and dual-band filters are commonly used on the Veil: they pass those emission lines and reject much of the continuum from light pollution. The narrowband filter guide lists the line wavelengths and the effect of fast optics on filter bandwidth.

When

The loop culminates at local midnight in early August. NASA puts the best time in early autumn for northern observers (early spring in the southern hemisphere). At about +31° declination it reaches 71° from 50° N, nearly overhead from 35° N and about 24° from 35° S.