Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

Sign in to follow this  

LP maps - Is it that bad?

Recommended Posts

I used to use Needless-org for to get a rough idea how good or bad possible dark sites would be but since I've been taking my own sky measurements I've discovered that Needless is way out on its estimations.

I know Needless is just a simulation but I used to use it a lot and travel up to an hours drive just to get somewhere that it said was good when I could of stayed a lot closer to home and had just as dark skies.

My local dark site Needless says its LM 5.5 but its actual readings from NELM and SQM are more like 6.3!

This winter I'II be saving a lot of money in fuel and staying closer to home!

Anyone else compared actual on the ground readings to a LP map and had a pleasant surprise? :)

Share this post

Link to post
Share on other sites

Can't say that I have Mike but would have thought you were fairly spoilt for dark skies down there in Cornwall. I'm at our caravan in Newquay in Ceredigion and the difference is chalk and cheese here to back home. Not sure what scale is relevant to the actual seeing conditions just know that if I look up the milky way is there looking back at me. Sorry if I've gone a bit off topic still marvelling at the skies down here. Just pictured yours on the coast down there being the same. :-)

  • Like 1

Share this post

Link to post
Share on other sites

I wouldn't worry about sites like this. At best they are there to make a point, rather than to give precise readings to specific locations. It's unclear where they get their basic data from, to make a LP model - or when it was last updated. Also, they don't take into account natural variation (up to ¾ of a Magnitude) in VLM, due to solar activity. The best way to get an idea of what LP will be like is simply to look at where local population centres are, and find a place well away from them.

Share this post

Link to post
Share on other sites

We've experimented with overlaying the excellent Avex light pollution map, which is basically a population map with a point spread function applied, onto our own (MyDarkSky) map, which is based on both naked eye limiting magnitude and sky quality meter data. We've found that the correlation is quite impressive. Not 100% but it certainly gives the gist - kudos to Avex.

Share this post

Link to post
Share on other sites

I was looking at the Needless Sky Simulator the other day and was thinking the same. Areas of Snowdonia looked quite poor because there was a tiny village nearby (perhaps even in the next valley).

I think most of the images of light pollution are in Infra Red, small towns will be warmer than the surrounding countryside but may be quite dark.

One mile from where I live is a very small nature reserve/park surrounded by bright city lights, the sky is much darker there.

The inverse square law works in our favour regarding LP. If I were one mile from a light source (town) then travelled a further mile away the direct light will be one quarter the brightness not half. The light hitting the sky directly above even less. edit: A bit of a simplistic statement/illustration but I'm a simpleton :)


Edited by alexog

Share this post

Link to post
Share on other sites

I don't think it's consistent, light pollution levels change dependent on skies condition's, factors such as humidity, general pollution, reflected light, due to wet surfaces, even living 4.5 miles from Manchester city centre I have had some cracking nights. I must confess I start AP because of light pollution, but generally it's not to bad if the conditions are right, saying that, that’s not very often, but hay ho, that’s what you get from living on the outskirts of a large city.

Edited by Si W

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By wimvb
      Just finished work on a sky quality meter with built in wifi. The device is based on the ambient light sensor TSL2591 and the wifi board ESP32. Communication between the two boards is through I2C. The device has a 40 degrees lens.
      The light sensor is programmable, which means you can set integration time (from 100 to 600 ms) and gain (from 1 to almost 5000, in 4 steps). I implemented automatic adjustment of these parameters to allow for the highest dynamic range (600M:1 accoring to the spec sheet).
      The device shows Sky readings as a web page. It is connected to a local wifi network, although it could also create its own access point. So far I haven't been able to calibrate the sqm yet, partly due to eternal cloud cover. But it should only require one parameter to be adjusted.
      The code is available on GitHub. Sky-Quality-Meter
      Here are som pictures.
      The components:

      The parts connected:

      The finished device:

      This is how output is presented:

    • By Captain Magenta
      Last year I was given a Unihedron SQM-L, the narrow field of view version of their gadget for measuring night-sky brightness. Since then, I’ve nipped outside to take zenith readings whenever I’ve been able, often a few times per night. As a result I now have 85 data-points, all from my back garden in Sunbury on Thames which rates a 19.04 on www.lightpollutionmap.info . As it turns out, this agrees well with the data I’ve collected.
      The darkest I’ve measured at this location has been 19.13, with 4 records better than 19.05 and 10 better than 19.00.
      Plotted against Moon altitude, it looks like:

      One thing I noticed very early on was that the reading generally gets darker and darker as the night goes on. The chart below suggests the data agrees, but how strongly I’m not adept enough yet with my statistics to work out. If anyone fancies doing this for me, I’d be grateful, I’ve attached the data .csv file I think to the end of this post.

      The data itself: each record contains date, time[GMT], SQM value, Moon phase, Moon altitude . For the purposes of my analysis, I’ve converted the time value into hoursafter6pm, which allows the intercept of the regression solution to be loosely considered as the “6pm starting point” for the darkness estimation, which is OK for this dataset as my data is all from this latest Autumn/Winter.
      I’ve done an “ordinary least-squares” regression with multiple input variables. At first glance it seems to me that the SQ vs altitude chart above should not behave well with that: there’s a clear kink, intuitively obvious I guess, at the point the Moon altitude goes negative.
      To cope with that, I divided my data into two and did three separate regressions: “Moon up” data, “Moon down”, and “All data” but treating phase and altitude as zero if the Moon is below -5 degrees (I chose -5 degrees arbitrarily).
      With Moon up, I decided the SQM value will depend on Time of Night, Moon Altitude and Phase. With Moon down, it only needs to depend on time of night.
      Thus my regression model is:
      SkyQual = a + b.timeafter6pm + c.phase + d.altitude + residual
      or rearranged
      residual = a + b.timeafter6pm + c.phase + d.altitude – SkyQual
      The analysis involves minimizing the sum of (the squares of the) residuals, by hunting around for the appropriate values of a, b, c & d which yields this minimum. I used MS Excel’s built-in Solver to do the “hunting around”.
      The following table summarizes the results:

      In words, using “Moon Up” as my subject, my Sky Quality, in magnitudes per arc-second, can be estimated as
      19.28 mags/arc-sec
      plus 0.0314 /hour
      minus 0.864 /full-phase (or 0.216 /quarter)
      minus 0.0186 /degree above horizon (or 0.186 /10 degrees).
      This is a pretty simple analysis. I’m sure there’s theory and formulae available relating Moon-altitude and -phase to extra sky brightness, but I haven’t used any of that here. And the “error model” I’ve used implicitly assumes that the relationships between SQM-reading and the variables are linear.
      If anyone is curious and wishes to do their own analysis, my raw-ish data is available as a .csv file attachment at the end of this post.
      A note about the data collection: each reading is an average of a few readings at a given time, with outliers rejected. For instance, often the first press yields an outlier, and over the following few seconds subsequent ones tend to settle down. So the series of readings 19.05 (me getting excited), 18.85, 18.86, 18.86 , which is a quite typical pattern, would cause me to record 18.86. My highest recorded reading, 19.13, was indeed where it settled down.
      Other “one-on-one” charts:
    • By Leon-Fleet
      Looking at getting a light pollution filter, for my camera as I'm in a bortle  5 area and not currently using any filters. 
      I've been looking around and the clip in filters are way out of budget, for now, so I've been looking at the Hoya filters and the red intensity filters. Then I saw these from Globe  (Link Below), and thought maybe it's too good to be true? All those filters for the price of one Hoya? 
      Has anyone used these before what do you think? 
    • By impactcrater
      Of interest to us all-
    • By Evie
      So I am not quite total beginner to astro images, but I am not hurrying to get to telescope range due to various factors and I did not find similar topic on several first pages of search, so this topic might be best suitable here.
      I have a camera that has a possibility to track astro-images up to 5 minutes, which is enough for me to take the Milky Way shots and even some Nebulae, clusters and similar level images. I have moved to the location that is a more light polluted than the one I lived before (from http://darksitefinder.com/maps/world.html : I lived close to the edge of yellow and all the dark orange, the city was light orange, now I live in the middle of red, the city is gray/white). It would be not so bad, as there are some empty fields around, so no very close light sources (same as in previous location), but the Milky Way is now directly above the city with way bigger light pollution opposed to the location where I had the MW in opposite side from the city. As nearest dark location is not for everyday drive (around an hour for a bit darker place), and even in red zone I get some recognizable images, I want to continue working on my technique and started looking to Light Pollution filters (I travel to some darker areas several times a year, but not too often).
      I found several 100mmx100mm filter brands, and several round ones, but I found no comparisons between them and very little to no sample images or reviews:
      PureNight Premium Light Pollution Reduction Filter by Lonely Speck – Lonely Speck - Lonelyspeck filter at the moment unavailable
      NiSi NiSi Natural Night Filter for Nighttime Light NIP-100-NGT Nisi filters
      Haida 100x100mm/4x4" Nanopro MC Optical Glass/ HD3702 B&H Haida filters
      IDAS Filters and Accessories IDAS LPS-D1 (round filter)
      Astronomik CLS Filter  (round filter)
      I would prefer getting 100x100mm filter, as I can use it for multiple lenses (I have 49-86 filter thread lenses), but I could live with 77mm, if it is really worth it and possible to use with step-up/down rings. Any recommendations - is it worth to get any of these, or did I miss some good one, or any reviews? Are they worth buying? Clip-in filters are not an option because of camera brand (Pentax).
      Also, in the future I am considering modifying this camera after I will get a newer one (http://www.spencerscamera.com/store/store_product_detail.cfm?Product_ID=25&Category_ID=1) - any recommendations which modification type to choose and why (this is totally green part for me)? Also, will the chosen LP filter work with the modification?
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.