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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
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
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:
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?
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?
After getting my new Altair Astro 102mm f11 scope out briefly over the last few nights for brief views between some cracks in the clouds, I realise that because of the length of the scope I cannot easily get into the few dark places I use when I just use my shorter f5 ST120 frac, and am sadly exposed to the full glare of the street lamps in front and behind my house. So after taking inspiration from Nick (Cotterless45) and others on here on SGL I decided to buy some heavy duty black backdrop so I can drape a curtain around my scope to shield from the pain in the b** light pollution that those blumming street lamps give off. I've ordered up an initial 3 metres length of the stuff (the drop of the material is also 3 metres), and debating whether to hang this cloth out on some ropes or strong washing line, or to build a light framework to go around the front and sides of the scope. The framework might be the best idea as I could also use it in the front garden too as there are no close structures to hang rope or washing lines on to, but would also want to be able to break it down for easy storage in my house. Need to get a few ideas on how I might build this frame perhaps. Tent poles spring to mind perhaps, so need to look into that maybe. Anyone else built any similar framework around their scope, other than attaching it onto any fencing to block out the light?
Hope you dont mind but edited the title, nightfisher