Why do astronomers never put a scale on their photographs?

Question

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.

Answer 1

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'.

Answer 2

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.