How to carry out a dark sky survey
I have worked with a number of different groups over the past few years, helping them plan for and carry out a dark sky survey. There are many reasons why a group might want to do this:
- As part of a formal assessment of their night sky, to allow them to apply to become a Dark Sky Place under the International Dark-sky Association‘s (IDA) programme.
- To identify where the best observing site is, whether for public star-parties or for private amateur astronomy society observing
- To support a case for improved lighting, whether from a local council, private landowner, the highways agency etc
- Or just out of interest
The groups I have worked with – Forestry Commission Scotland as part of Galloway Forest Park‘s efforts to become an International Dark Sky Park (successful); the community on Sark, as they worked to become the world’s first Dark Sky Island (pending); Exmoor National Park, Peak District National Park, and Loch Lomond and the Trossachs National Park, as they all work towards becoming Dark Sky Reserves (ongoing) – have all taken more or less the same approach, as advised by me.
I have been asked over the past couple of years whether any formal guidance is available for anyone wishing to carry out such a survey, and to my mind there is not. I have decided therefore to put together this guide to carrying out a dark sky survey.
The following is only my interpretation of the method that will yield the most useful data. This is not a formal IDA procedure, however it is the method I have used on two applications to date (one successful, one pending).
How to carry out a formal dark sky survey
There are three basic components to a dark sky survey. These are:
- Quantitative measurements using a light meter or similar device. The IDA recommend using a Sky Quality Metre with Lens (SQM-L), if only so that all measurements are consistent. The simple hand-held SQM-L costs around £100.
- Photographic all-sky images to reveal sources of light pollution. The equipment needed for this is a bit more expensive; a full-frame digital SLR camera (such as the Canon 5D) plus a 180 degree fisheye lens (such as the Sigma 8mm F3.5 EX DG Circular Fisheye Lens) which combined will cost around £2300. The IDA insist on photographic evidence as a requirement for application.
- Qualitative assessment of the night sky using the naked eye, in order to rate the sky using the Bortle Scale.
You should try and carry out your survey on a clear night, with little or no cloud cover. This will not always be possible, but even slight cloud cover can impact on your readings. You should make a note of observing and meteorological conditions throughout each evening of surveying. If your survey is being carried out over several nights, this is especially important.
You should, of course, avoid carrying out the survey when the moon is in the sky, and wait until the end of astronomical twilight.
Using the SQM-L
The Sky Quality Metre is a simple device, with one button and one numerical display. Here is how to use the SQM-L:
- You will need a SQM-L, as well as a means of recording the readings. Pen and paper is fine, but a voice recorder can also be useful to allow you to make verbal notes as you go along.
- Point the SQM-L at the zenith (the point of the sky directly overhead) and push the button. Within a few seconds (if the sky is really dark it may take tens of seconds) a number will display on the readout.
- Discard the first reading, as it may be inaccurate due to thermal noise on the sensor.
- Take as many readings as possible so that you can get an accurate average reading in that location.
- Despite the fact that the SQM-L gives you readings to 2 decimal places, it’s only necessary to quote them to one decimal place, as the device is only that accurate.
Taking all-sky images
The purpose of the photographic survey is to supplement the SQM-L readings with a visual image, which will allow you to eyeball the sources of local light pollution. It is possible to carry out a quantitative analysis of an all-sky image, but that’s beyond the scope of a normal dark sky survey.
The camera settings should be:
Aperture: fully open (f/3.5)
Shutter Speed: 180s (in places with more light pollution you will need a shorter exposure time, around 120s)
Focus: Manual, infinity
The camera should, of course, be mounted on a tripod and pointed to the zenith so that the perimeter of the circular fisheye image will be the horizon. You should use a cable release or remote control.
Establishing your sky’s Bortle Class
Using the naked eye, you should assess your sky’s Bortle Class, using the table here.
How many locations should I include in my survey?
This will depend on the size of the area you wish to survey, but there are a few good rules of thumb. In general you will want to get SQM-L readings in all suitable locations, backed up with a selection of all-sky photographs, and a few Bortle Class assessments. You should:
Use an SQM-L to:
- Get two series of readings that bisect the area, i.e. at regular intervals (every km) running north-south, and east-west through the area, or along any roads / tracks through the area
- Take readings at regular intervals (every km) along all roads within the area
- Take readings from all car parks / lay-bys etc where people might stop to stargaze
- Take readings at any visitors centres / halls / sports fields etc where you might consider running a star party or stargazing event
- Take readings within any settlements / villages / towns within the area as these will probably be where visitors will come and stay
- Try and get a reading of the darkest part of the area, even if that is not easily accessible
- Try and get readings of the most light polluted parts of the area
Use the all-sky camera to:
- Take images at any visitors centres / halls / sports fields etc where you might consider running a star party or stargazing event
- Take images in the darkest part of the area
- Take images at any other locations you feel are relevant.
Assess the Bortle Class of the sky at:
- Any visitors centres / halls / sports fields etc where you might consider running a star party or stargazing event
- The darkest part of the area
- Any settlements / villages / towns within the area as these will probably be where visitors will come and stay
- Any other locations you feel are relevant.
Following this procedure you should end up with a database of SQM-L readings, a selection of all-sky images, and a handful of Bortle Class assessments. This should satisfy the IDA should you wish to apply for formal recognition for the area, and certainly provide you with more than enough information to assess the quality of your night sky. More crucially, if these guidelines are regularly followed it will allow for direct comparison between different dark sites.
Any readings should be made public so that others can see the results, and the excellent mydarksky.com aims to be the one-stop-shop for such readings.