Spiral Galaxy in Ursa Major

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Click on image to toggle between a grayscale version and color version


M81 is an iconic 7th magnitude (that's pretty bright) galaxy in Ursa Major (the constellation that contains the Big Dipper), relatively close to us at approximately twelve million light years from Earth. It is a prominent grand design spiral galaxy, with about 250 billion stars (slightly fewer than our Milky Way is thought to have). It has a diameter of approximately 90,000 light years (quite a bit less than our Milky Way). The field of this photograph is about the angular size of a full moon.

Many galaxies have a super-massive black hole in their center; M81 has an unusually large one--a supermassive black hole with a mass of about 70 million times that of our sun. By comparison, our Milky Way galaxy, significantly larger than M81, has a supermassive black hole at its core with a mass of about 4 million times that of our sun.

The background is not as dark as I would have expected; some of that dust is in our own galaxy, and some probably is tidal debris from M81's close encounters with its galactic neighbors.

There are many other galaxies in this image, most priminently the dwarf galaxy that is a companion of M81, Holmberg IX (just above M81, to the left of center).

Unusually for a galaxy photo, I have presented two versions of this galaxy--a color version and a grayscale version. I find the structure more evident in the grayscale version, so I thought it worth sharing. Click on the image to toggle between the color image and the grayscale image.


Technical Information:

HaLRGB: 280:1005:240:240:300 (That's over 34 hours of keepers included in this image); luminance layer consists of data from 59 fifteen-minute images and 24 five-minute images; R, and G consist of fifteen-minute images; B consists of 15 twenty-minute images; Ha data consists of 14 twenty-minute images. I gathered Ha data to enhance the pink HII (star-forming) regions. All images were unbinned.)

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 AO-X, 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 Professional Edition.

Processing: All images calibrated (darks, bias and sky flats), aligned, combined and cropped in Pixinsight. Color combine in Pixinsight. Some finish work (background neutralization, color calibration, deconvolution, HDR combine/HDR Mulitscale Transform of luminance data and noise reduction) done in Pixinsight; some cleanup finish work was done in Photoshop CC.

Location: Data acquired remotely with my equipment hosted by Sierra Remote Observatories, Auberry, California, USA.

Date: Images taken on many nights in May 2020. Image posted June 1, 2020.

Date: Image scale of full-resolution image: 0.56 arcseconds per pixel.

Seeing: Generally excellent, with individual calibrated luminance frames varying from 1.5 to 2.0 arcsecond FWHM.

CCD Chip temperature: -25C

Copyright 2020 Mark de Regt

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