Is it possible for a deep space photo to be upside-down?
September 6, 2016 8:25 AM   Subscribe

.... and, if so, upside-down relative to what?

I suppose what I'm asking is whether there's any universally-observed convention which astronomers use to ensure such photographs are issued the "right" way up? I've come across references to our own solar system's invariable plane and the right hand rule, both of which allow us to allocate a North and South pole to each of our neighbour planets in a consistent way. But is there any equivalent convention which stretches this principle into deep space?

This question was prompted by photographs of the Horsehead Nebula, which are almost always presented with the orientation of a horse's upright neck and head. I always wonder if that orientation in the photograph is merely a question of aesthetics of if it's obeying some consistent astronomical convention which I don't know about.
posted by Paul Slade to Science & Nature (8 answers total) 4 users marked this as a favorite
You may see references to "north is up" (or equivalent for other directions) in descriptions of photos. This indicates the top of the photo is oriented in the north direction - ie, you should be able to print out the photo, hold it up above you somewhere below Polaris (the north star) and "overlay" the view of the photo in question.
posted by saeculorum at 8:35 AM on September 6, 2016

This comment assumes that I correctly followed Neil Tyson's explanation of directionality in deep space. As a general convention it's based on rotation. A rotating body has a north and south in the same way that the earth spins in relation to its north and south. If you know which direction a body, or a galaxy, is rotating, you can determine a north-south axis from there.

If the Horsehead Nebula exhibits a known rotation, you can proceed from there. My guess is that the photo orientation is a likely result of the way the photo was taken.
posted by Flexagon at 8:53 AM on September 6, 2016

Short answer: no, not really. Any direction is equally 'up'...further complicating matters is that most telescopes produce a reversed image (since flipping it requires another mirror and more glass/reflections = less light and the whole point of telescopes isito bring in more light). You can even buy moon and star maps that are reversed to help navigate better when looking through a telescope.
posted by sexyrobot at 9:31 AM on September 6, 2016

Astronomical images of deep-sky objects like nebulae, star clusters, and galaxies are typically oriented with north-on-the-sky up. As saeculorum says, that means the top of the photo is pointed toward Polaris.

Imagine that the earth is at the center of a giant beach ball and all the stars and galaxies and stuff are painted on the inside of the beachball (the celestial sphere). If you're standing at the Earth's north pole, and look straight up, you find the north celestial pole (and Polaris happens to be fairly near the NCP). If you project the Earth's lines of latitude and longitude outward onto the beach ball, an image will tend to be aligned with the latitude lines horizontal and the longitude lines vertical, and north "up." (Notice that this means that if you're standing with north up, east is on your left and west is on your right.)

These images show the Horsehead nebula in context. The largest image is oriented north-up, and you can see that you have to tilt it slightly so that northwest is up to get it the "horse's head" to look upright.
posted by BrashTech at 9:31 AM on September 6, 2016 [1 favorite]

There's no convention that I know of. Scientifically, the orientation of deep space photographs doesn't matter - data is data - and the aesthetics of the presentation of a deep space picture depends on many things - the presentation of the major structures, whether it has a feature in a plane or direction that fits best in a particular orientation, and so on. Uranus's poles are where its equator 'should' be if it were oriented like the other major planets with respect to the plane of the solar system, but the Voyager pictures of Uranus are normally shown with the poles at top and bottom - not how it would actually observed from Earth - it's 'natural' for us to interpret rotation as being around a vertical axis,

But you get systems with multiple planes of rotation. The solar system's plane is around 53 degrees inclined with respect to the galactic plane, so if you're trying to illustrate something like how the solar system's magnetic field and solar wind interact with the galactic field and wind, your choice of depiction will rather depend on what emphasis you want to give to which component.

As the Hubble Deep Field images show, the large-scale universe is homogeneous and has no overall axis or symmetry. So do what thou wilt shall be the whole of the law.
posted by Devonian at 9:31 AM on September 6, 2016 [1 favorite]

Also it depends on what you're looking at...north and south within our galaxy and solar system are usually based on earth's north and south, and outside the galaxy on galactic coordinates based on the plane of the milky way. Mostly that's just used when mapping the locations of galaxies (a pretty specific astronomical field) mostly things in space are located by their ascention and declination (basically latitude and longitude projected onto the sky)
posted by sexyrobot at 9:37 AM on September 6, 2016

Wouldn't this depend upon the location of the camera? BrashTech's beachball analogy seems logical for all pictures taken from earth, but those taken from space would be a whole 'nother story.

(I am not a scientist and really have no business responding to this question.)
posted by she's not there at 1:10 PM on September 6, 2016

It is absolutely a convention among astronomers to display images with North up and East to the left in the Equatorial system, as described by saeculorum and BrashTech. (The distance between Earth and a space telescope is basically zero compared to the distance between either and most astronomical objects, so this is true even for HST.) Data are data, but you still have to know where to look if you see something interesting that is worth following up.

The "raw" images that come out of a telescope are not necessarily north up and east to the left. The "position angle" depends on the optics, and astronomers can also specify it to get multiple objects of interest in the frame. There may also be distortion in the raw images. Through a process called astrometry, astronomers figure out where how the image maps onto the Equatorial grid. They can then process the image to remove distortions and align the images to the standard orientation.

That said, it's not a law, and you can choose other coordinate systems based on what makes scientific sense.
posted by pizzazz at 2:57 PM on September 6, 2016

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