Deep Sky Astrophotography – What you need to know about Light Pollution

Many deep sky astrophotographers photograph stellar objects right from their own backyard.  There are several good reasons to photograph from your backyard:

• Astrophotography can require a large amount of equipment and set up time
• You may only have a short window to photograph
• Commuting to a dark sky site will take away additional sleep time from a hobby that already provides little time for sleep

However, many of us live in areas with a large amount of light pollution.  In this blog, I’ll discuss light pollution how it’s measured, tools to know how bad the light pollution is in your area, factors contributing to light pollution, and light pollution filters.

Why do I need to worry about light pollution? 

Light pollution washes out the starlight in the sky and makes it difficult or to photograph stellar objects.  It also causes a brown / orange tinge to our images making it harder to process.  It’s still possible to photograph deep sky objects with some light pollution, it just more difficult.

How is Light Pollution Measured?

Light pollution is measured by the Bortle Scale.  The Bortle Scale ranges from 1 to 9 with 1 being an excellent dark sky and 9 being an inner city sky.  Suburban skies can range from around 5 to 6.

Tools to Understand Light Pollution in the Area you are Photographing

Here are a few tools that will help determine the light pollution in your area:

LightPollutionMap.info has a color coded map of the light pollution areas around the world.  You can search for your location or just zoom in to find it.  Whites, reds, and pinks are the areas with the most light pollution while blues, grays, and blacks have the least amount of light pollution.

DarkSiteFinder.com will help you to locate a dark sky site near you if you do want to travel to a less light polluted area.

An app that I talked about in my last blog (Clear Outside) will tell you the Bortle Scale rating for a given location including home.

Other Light Considerations

Besides the Bortle class rating for your area, there are several other considerations that can impact the amount of light pollution in your images.

Moon Phase – A full or mostly full moon will generate a lot of light in the night sky.  Many astrophotogaphers avoid or shoot with narrowband filters during a full moon.  You can also just focus on photographing the moon during this time.

Proximity of the Moon to your Target.  If the moon is located close to your intended target for the night, it’s probably best to find another target.

Shooting close to the Horizon.  There is more light pollution closer to the horizon than further up in the sky.  Select a target that is higher in the sky if light pollution is a problem.  Use the Stellarium App to see when targets are higher in the sky during the night or months that the target is visible.

The photo below show how the quantity of light pollution is greater near the horizon. This was taken in my Bortle Class 5 backyard.

Light Pollution Filters 

There is enough material here for a separate blog, so I will cover filters at a high level. 

For DSLRs, there are Clip-in filters that fit into the body of your camera.  These can be used with your camera connected to lenses or a telescope.

There are also circular filters that fit between your telescope and camera.  These are typically 1.25” or 2” and are meant to be used with telescopes.  They can work with either DSLR / Mirrorless or CCD cameras.

There are several types of Broadband Light Pollution Filters: City Light Suppression (CLS), Light Pollution Reduction (LPS), and UHC (Ultra High Contrast) Filters.

I am currently testing a SkyTech CLS clip-in filter.  More to come on the results.

Clear Skies!

Deep Sky Astrophotography Tutorial – Table of Contents

Written by Martin Belan

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