JA
01-03-2021, 05:29 PM
Hello All,
PART 1 - Light Pollution
I thought I'd open a discussion on Light Pollution, its measurement and using filters to help reduce its effects. That might be a lot to bite off in one chunk, but when I thought of doing it in separate threads, that seemed like it would result in too much shuffling around.
Of course we all want the best views and photographs that come with a dark sky, but how do we get them? Move house? Travel to darker skies? Use light pollution or narrowband type filters?..... Any or all of the above.
When considering any of those possibilities the obvious question might be .... How much have I got to gain? That is where quality data comes in. It is possible to get light pollution (sky brightness) data for your area, suburb and even down to your street although it's somewhat questionable at that resolution.
Simply go to LightPollutionMap here.....
https://www.lightpollutionmap.info/#zoom=8.00&lat=-37.8158&lon=145.6046&layers=B0TFFFFFFFFFFFFFFFF and then enter your suburb/location and some options will come up. Choose the appropriate and then zero in on your location and you can get a light pollution reading of some sort. I prefer, in reference to the Light Blue Dialogue Box at the top right, to select the "World Atlas 2015" Dataset Overlay as this gives SQM readings, Bortle Number and the sky brightness readings in Candela/m^2, which to me are far more meaningful than those in the VIIRS datasets, more recent they may be. If I want to Update the World Atlas data to 2020, or whatever year I simply scale the World Altas 2015 reading to a "World Atlas 2020 reading" using the 2015 & 2020 radiance data from the VIIRS data. Close enough.
At my location in Melbourne LightPollutionMap reports the sky as Bortle 7 with an SQM of 18.84mag./arcsec^2 and most importantly for me, intuitively anyway, is the Sky Brightness reading = 3130 µcd/m^2. This compares with a "typical perfect dark sky site" of 171 µcd/m^2. So my background sky at the Zenith is approximately 18 times that at a perfect Dark Sky Site. I've been to a few dark sky zones, so being 18 times brighter puts things very much in to a linear perspective. I will return to that concept later.
PART 2 - Sky Brightness Measurement
So aside from getting your sky brightness data from sites such as LightPollutionMap you can measure it yourself with the benefit of it being ultra focused on your exact location and your local conditions. It is possible to use devices like the Unihedron (brand) Sky Quality Meter often simply referred to as an SQM meter for such measurements. See image below...
272622
There is a version with an approx 60degree field of view and an SQM-L model with a narrower 20 degree field of view that might be preferrable. They provide a Sky brightness measurement in units of magnitude per arcsecond squared. An excellent dark site will measure 22.0 Mag. per arcsecond squared which equates to the in my view far more meaningful sky brightness measure of 171 micro candela per square metre. Here is a conversion calculator to convert beteween these units:
http://www.unihedron.com/projects/darksky/magconv.php
INCOMPLETE......TO BE CONTINUED
PART 3 - Filter Candidates
PART 4 - Filter Initial Comparison
PART 5 - Filter Effectiveness
PART 1 - Light Pollution
I thought I'd open a discussion on Light Pollution, its measurement and using filters to help reduce its effects. That might be a lot to bite off in one chunk, but when I thought of doing it in separate threads, that seemed like it would result in too much shuffling around.
Of course we all want the best views and photographs that come with a dark sky, but how do we get them? Move house? Travel to darker skies? Use light pollution or narrowband type filters?..... Any or all of the above.
When considering any of those possibilities the obvious question might be .... How much have I got to gain? That is where quality data comes in. It is possible to get light pollution (sky brightness) data for your area, suburb and even down to your street although it's somewhat questionable at that resolution.
Simply go to LightPollutionMap here.....
https://www.lightpollutionmap.info/#zoom=8.00&lat=-37.8158&lon=145.6046&layers=B0TFFFFFFFFFFFFFFFF and then enter your suburb/location and some options will come up. Choose the appropriate and then zero in on your location and you can get a light pollution reading of some sort. I prefer, in reference to the Light Blue Dialogue Box at the top right, to select the "World Atlas 2015" Dataset Overlay as this gives SQM readings, Bortle Number and the sky brightness readings in Candela/m^2, which to me are far more meaningful than those in the VIIRS datasets, more recent they may be. If I want to Update the World Atlas data to 2020, or whatever year I simply scale the World Altas 2015 reading to a "World Atlas 2020 reading" using the 2015 & 2020 radiance data from the VIIRS data. Close enough.
At my location in Melbourne LightPollutionMap reports the sky as Bortle 7 with an SQM of 18.84mag./arcsec^2 and most importantly for me, intuitively anyway, is the Sky Brightness reading = 3130 µcd/m^2. This compares with a "typical perfect dark sky site" of 171 µcd/m^2. So my background sky at the Zenith is approximately 18 times that at a perfect Dark Sky Site. I've been to a few dark sky zones, so being 18 times brighter puts things very much in to a linear perspective. I will return to that concept later.
PART 2 - Sky Brightness Measurement
So aside from getting your sky brightness data from sites such as LightPollutionMap you can measure it yourself with the benefit of it being ultra focused on your exact location and your local conditions. It is possible to use devices like the Unihedron (brand) Sky Quality Meter often simply referred to as an SQM meter for such measurements. See image below...
272622
There is a version with an approx 60degree field of view and an SQM-L model with a narrower 20 degree field of view that might be preferrable. They provide a Sky brightness measurement in units of magnitude per arcsecond squared. An excellent dark site will measure 22.0 Mag. per arcsecond squared which equates to the in my view far more meaningful sky brightness measure of 171 micro candela per square metre. Here is a conversion calculator to convert beteween these units:
http://www.unihedron.com/projects/darksky/magconv.php
INCOMPLETE......TO BE CONTINUED
PART 3 - Filter Candidates
PART 4 - Filter Initial Comparison
PART 5 - Filter Effectiveness