Sh2-290 is a very large (angular size, as well as actual size) planetary nebula in the constellation Cancer.
Its angular size is about 16 arcminutes across, about half the angular size of the full moon. It is extremely dim; it barely can be detected in a single 30-minute exposure. It is thought to be about
2000 light years away from us; at that distance, it would be about 17 light years across. Technical Information: Ha:OIII:L:R:G:B: 690:600:345:180:150:195 (a total of 36 hours of light-frame exposure time). Here's a chart showing the various subexposures I took: Equipment: RC Optical Systems 14.5 inch Ritchey-Chretien carbon fiber truss telescope, with ion-milled optics and RCOS field flattener, at about f/9, and an SBIG STX-16803 camera with
internal filter wheel (SBIG filter set), guided by an SBIG STX guider, all riding on a Bisque Paramount ME German Equatorial Mount. Image Acquisition/Camera Control: Maxim DL, controlled with ACP Expert/Scheduler, working in concert with TheSky X. Processing: All images calibrated (darks, bias and sky flats), aligned, combined and cropped in Pixinsight. Color combine in Pixinsight. An immense amount of HDR processing was done in
Pixinsight. Some finish work (background neutralization, color calibration, NoiseXTerminator and BlurXTerminator) done in Pixinsight; some cleanup finish work, and (most importantly) blending the various intensities
of the images into an HDR composite) was done in Photoshop CC. Location: Data acquired remotely from Sierra Remote Observatories, Auberry, California, USA. Date: Images taken on many nights in December 2024 and January, February and March 2025. Image posted August 5, 2025. Date: Image scale of full-resolution image: 0.56 arcseconds per pixel. Seeing: Generally poor CCD Chip temperature: -25C Copyright 2024, 2025
Mark de Regt
A "planetary nebula" (so called because the astronomer who first identified them as nebulae noted the color was similar to the then recently-discovered Neptune) is a structure of gas resulting from the
death throes of a star about the size of our sun, when it runs out of fusable material; the color is the result of the gas being ionized by the remnant of the star, a white dwarf (ionized oxygen is the
dominant emission in this planetary nebulae, giving off the characteristic blue-green color; there also is some ionized hydrogen, which shows a reddish color).
The nebula is so large because is is very old, and has been expanding for a very long time. It is starting to dissipate into the interstellar medium, and soon (in cosmic terms; probably a few thousand years
away) it will disappear from our view.
This beautiful nebula probably would be nicely symmetric/round (as so many planetary nebulae are), except that it is traveling through the interstellar medium around it (in the direction of slightly up and to
the left on my photo). Over time, this forms a "bow shock wave," caused by the leading edge of the nebula being slowed by the matter (not quite vacuum) it is encountering. We see that as the rounded, brighter
edge of the nebula.
Luminance: 23 fifteen-minute and 20 three-minute
Red: 12 fifteen-minute
Green: 10 fifteen-minute
Blue: 13 twenty-minute
Ha: 23 thirty-minute
OIII: 20 thirty-minute
Luminance layer is a blend of the luminance-filtered images, the Ha-filtered images and the OIII-filtered images.
Red is a blend of the red-filtered images and the Ha-filtered images.
Green is a blend of the green-filtered images and the OIII-filtered images.
Blue is a blend of the blue-filtered images and the OIII-filtered images.