Light pollution

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By /u/Bersonic

A hugely important factor in astrophotography is light pollution. When you look up at the sky from the middle of NYC you may only see a few of the brightest stars. Drive a few hours away and you will see thousands. While light pollution won’t make or break your chances at a successful image, it sure can make getting there a pain.

Measuring light pollution

Light pollution is quantified by a special system called the Bortle Scale. The Bortle Scale is a 9 point scale measuring the brightness of the sky at Zenith (90 degrees up.) Each of the 9 levels of the scale are assigned a color. White, red, light red, orange, yellow, green, blue, grey, and black. These “zones” are projected onto services like Google Earth to create a light pollution map. White zones correspond to a Bortle scale of 8-9. These zones are found in heavily populated areas like cities and large suburbs. Black zones are rare and are usually found in the middle of deserts or oceans.

Effects of light pollution

To get an idea of what the sky looks like in different zones, here is a nifty graphic:

The biggest problem light pollution poses to astrophotos is color. If you are unlucky enough to be in a red or white zone, you will quickly realize that your images are coming out with either a orange or green tint. If you see this do not fret. Astrophotographers are a smart bunch, and have figured out a number of ways to remove light pollution “gradients” in post processing.

The other problem light pollution creates is that it brightens an image considerably. this can create problems for people with fast scopes, or with people doing long exposures. After taking your first raw frame, check its histogram (most cameras will let you do that on camera.) If your main spike is in the left left 2/3 of the graph, then you are fine. Example: ----- /\ ---- is fine. If your main spike is close to the right wall of the graph, you need to back your exposure time down, lower your iso, or go to a darker site (go down a Bortle scale).

Light pollution map

To find out your closest dark sites, use this handy map: Light pollution map