Dark Sky Meter vs SQM-L Readings

 I decided to post this information because I realize I need to become a LOT better at tracking the darkness of the sites and locations I use to observe. Hopefully this will reaveal a trend in light pollution or darkness and help me and others in the area to know where to go and observe on a regular basis. I also need to enter into the SQM database or my database there the readings I have taken for the last couple of years while observing. They are found typically on my sketches and in a log I keep in a small spiral notebook I keep with the meter. I am not going to get into the meters or explain how they work here. If you want to do that I encourage you to go to the Unihdedron website at this LINK or the Dark Sky Meter website at this LINK. I have to state since upgrading to the iPhone 6 plus my Dark Sky Meter app hasn't worked correctly which is expected as the app works with the iPhone 5c and 5s models. Time to upgrade the app I believe.

Here is the data I have entered for my observing sites on the Unihedron site:

The locations on the site are Elizabeth Ridge which  I do not have a Dark Site Meter reading for; Pit n Pole which I do have readings for; Vernon Forest Road 006 Juniper Grove Site that I do have readings for; my backyard which I do have readings for and that is all. So here are the Dark Sky Meter Map readings from the map.

Three of the readings, the top three above show readings taken on September 12th, 2015 of 21.88 and 21.82.  Another reading from August 15th 2015 shows a reading of 20.71 which is an abnormality as the DSM phone app and the SQM L show readings at this site averaging in the 21.8 range.  The fourth reading of 21.77 was taken at the site FR006 Owl's Roost site as was taken on June 29th 2015 and shows the impact of the summer Milky Way.  There is also just a tad more ambient light (extremely minor) visible at FR006 Owl's Roost vs Juniper Grove since the grove of Juniper's blocks all ambient light from the town of Vernon to the north. These readings seem to agree though there are far too few to make that ensure. The Vernon Observing Site is rated as a Class 1 Bortle area (SQM lists it as a Class 1), though depending on conditions it could be upper Class 2 Bortle. Link 1 Link 2 Personally, I don't use the Bortle System but it provides a guide to what your seeing at a dark site. I believe sites transition to what they are, as in FR006 a site 1 to a level below, site 2 depending on sky conditions, weather patterns etc.For me, the FR006 has allowed me to see on a regular pattern the following items in terms of visibility; the zodiacal light with color and up to 60 degrees; the gegenschein, and the  "Dark Horse" section of the summer Milky Way.

Zodiacal Light Link (this is how I have seen it out at FR006) Wikipedia LINK

gegenshchein   That light band going diagnol from the Milky Way in the image. Explanation Wiki

Dark Horse Section Milky Way Example/Image LINK 1 (circled) LINK 2

Challenge: Next time you go to a Rural or darker site, see if you can see these items when they are available.

The readings above from the Faust road of 20.72 and 20.55 are slightly less than the readings I have in the 21.2 range for Pit n Pole. At Pit n Pole though my readings are taken with the SQM L pointed south, the darkest part of the sky there, while the DSM is pointed up and there is significant light pollution to the northeast and east here. The direction and narrowness of the SQM L is the difference for the reading here I believe. Pit n Pole is a Class 3 Bortle Site.

Here is my house.

The SQM L readings in my backyard are in the 19.5 to 19.6 range, low 19.6.  The SQM readings are 19.69, 19.69, 19.24, 19.15. You can get the dates above and times for that impact on the images. For me this again is consistent for the tool, though more data points would be needed. My house is a Class 5 Bortle, sometimes transitions to a Class 6 Bortle as more homes fill into the area.

Overall I prefer to use my SQM L for readings as I can measure specific parts of the sky and even at home, the southern horizon is darker than the northern or eastern because of where I live in the Salt Lake Valley. Kinda of cool though to see how an hour drive to the southwest greatly enhances the darkness of one's site. I'm spoiled.

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