# How to determine the scale on astronomical images?

I've always been amazed at the many beautiful images of objects in the night sky. The Andromeda galaxy is a superb example. But it was only a few years ago that I discovered that Andromeda in those images is about six times bigger than the Moon. It is very very much fainter, of course, but it's actually a huge object in the night sky - covering more than three degrees.

The image below is NASA's Astronomy Picture of the Day for 2020 September 25 Moon over Andromeda, Composite Image by Adam Block and Tim Puckett.

Explanation: The Great Spiral Galaxy in Andromeda (also known as M31), a mere 2.5 million light-years distant, is the closest large spiral to our own Milky Way. Andromeda is visible to the unaided eye as a small, faint, fuzzy patch, but because its surface brightness is so low, casual skygazers can't appreciate the galaxy's impressive extent in planet Earth's sky. This entertaining composite image compares the angular size of the nearby galaxy to a brighter, more familiar celestial sight. In it, a deep exposure of Andromeda, tracing beautiful blue star clusters in spiral arms far beyond the bright yellow core, is combined with a typical view of a nearly full Moon. Shown at the same angular scale, the Moon covers about 1/2 degree on the sky, while the galaxy is clearly several times that size. The deep Andromeda exposure also includes two bright satellite galaxies, M32 and M110 (below and right).

I've been looking at that very recent JWST focussed image of a test star with the beautiful diffraction pattern and the faint sprinking of background galaxies. Again, there's no scale. I'm guessing from the diffraction pattern arising from that vertical boom that the largest of those background galaxies is about 5 microradians (that boom looks about 10 cm thick and at 1 $$\mu$$m wavelength will produce a pattern with $$10^{-5}/2$$ radians between peaks - about the same size as that largest galaxy).

What is the scale on the image, officially? Where is it listed?

• Mostly because these are PR images, and most will have been modified by cropping, rotating at the very least, and the scale is lost. But the actual data that gets stored would ALWAYS have the scale information stored along with it. But it wouldn't be incorporated into the actual image, some applications might display it separately or as an overlay. Mar 23 at 4:38
• Speculation: In public facing images the "scale" on a micrograph can be in units of length but on a astrophotograph the scale would have to be in units of angle, and muggles don't think it terms of angular size. Mar 23 at 7:33
• @GregMiller you should turn that comment into an anwer. Do you have an example for JWST? Is the scale typically quoted in arcseconds per pixel? Mar 23 at 18:59
• @JamesK I would vote for 'Moons', 'milliMoons' and 'microMoons'. I think that composite photo of the Moon and Andromeda side-by-side is just fantastic. Maybe I'll try to edit it into the original question. Mar 23 at 19:03
• @uhoh - appreciate the edit Mar 24 at 1:34