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Why do astronomers never put a scale on their photographs? I have been looking at images of the Bird nebula, a collision of three galaxies, but in none of the dozen or so that I have found, nor in the accompanying text, is there any scale to show how much (or how little) sky this represents. No cartographer would omit a scale of distance from, say, an isolated Pacific island, but very very seldom do I see an arc-second, or whatever is appropriate, scale on a photograph of the sky. This omission is not just for this one image, amazingly it is true for every (well 99.99...% of them) astronomical picture I look at.

The area covered by the Bird must be very small as the picture came from a clever analysis of data from Hubble and a new South African telescope, but it would greatly enrich the wonder of this achievement if we were told just how tiny it appears in the sky.

I have posted this as a question, I would really like to see, and sign up to, a petition to demand that astrophotographers correct this strange policy.

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Without defending the practice exactly, I'd note that astronomers rarely "publish" a photograph, rather they publish a paper and I suspect that the field of view data can reliably be found in the caption or the text. Of course the press generally does a less than stellar (heh!) job of presenting such material for the layman. They may leave the scale information out to avoid having to explain what the heck a "milliarcsecond" is. – dmckee Feb 11 '13 at 16:38
good question. But I think the answer is simple. To put scale on the image requires using the correct plot command to add legend to the plot. This is not easy to do in most plotting software. Given that the scientists who take these pictures are very busy (and most are not good programmers, but that is ok, they have more important things to do, which is find stars and new planets and other more important things like this than spend the time to find the legend command in some GUI library and figure how to use it), then the picture ends up with no scale added. – Nasser Feb 11 '13 at 22:47

A scale of distance would not make sense as a photograph often shows objects at vastly different distances from the observer and thus the distance of two objects on the photograph does not translate directly into a distance in real space.

Or in other words: a map of an area on earth is mostly a projection of a 2D area onto a 2D map while a astronomical picture is a projection of a 3D area onto a 2D 'map'.

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I did not think it necessary to specify an angular scale, although I did mention it in the question -- I though that would be self evident. – Harry Weston Feb 12 '13 at 10:29

Because the scale, in terms of how much of the sky the picture covers, almost never matters.

The scale/pixel does matter because it tells you if / how much you can believe the details in the image or if they are likely to be artifacts of the AO system or the image analysis.

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