I live in Herriman, Utah which using Dave L.'s light pollution maps, found at this link, and his maps put me in an orange zone, not far from a yellow zone. I will openly admit that the light pollution in my town as gotten worse in the 7 years I have lived here because of the growth and the street light designs that are used. I expect that to continue. Overall though, if I stay up into the wee hours of the morning, I can get decent seeing and observing in from my backyard. My average SQM reading here is around 19.8 to 19.84. If I use this NELM chart based on Ursa Minor found at this link, I get in the range of 5.45 which matches closely to the SQM. That puts me in what some would call a Bortle 6 which fits the description of my home. Let me state though that I caution people to make comparisons between the SQM and the Bortle. My findings is some dark sites can go from green to blue in the Bortle system depending on the time the reading takes place, and conditions. I make the correlation here for the simple reasoning that I some may relate to the SQM and some to the Bortle Scale. If your not familiar with the Bortle Scale, go to this link. The other issue with rating a Bortle Scale on a site is my eyes are not your eyes, and your eyes are not my eyes and as I age, my eyes are not the same as they use to be. Variance occurs and so a site that may sit at a boundary between Bortle 3 and 2 may come down to the observer or sky conditions.
My dark sites are actually quite dark. One is in a blue zone on the map, and I get a SQM reading on average of 21.88 to 22.2 depending on the night, the season and conditions. That matches being in a blue zone on the map and in the Bortle system. It's typical to do down to around magnitude 6.6 to 6.8 at this site depending on conditions. My other two frequent year round sites come in around SQM of 22.3 to 22.5 and 22.8 or slightly better. I would rate them both as definite Bortle 2 sites, possibly Bortle 1 sites. So that gives some background on my observing sites, my backyard vs. my dark sites.
Now that's a lot of information to put out and read, so I'll get to the meat of the project which is two parts. The first part will be to simply share the objects I've sketched at a dark site and the objects I have sketched in my backyard. The purpose will be to compare and contrast each object so the reader can see for themselves the difference between an object seen in light pollution, and one observed at a dark site. Please note that on some objects, it will take experience and training of the eye(s) in order to see the object and details in either zone. I remember not seeing spiral arms when I first started observing, but then with experience, and looking at several hundred galaxies, it became easier and easier to pick out the details. The second part of this post will be to discuss observing techniques you can use to learn to pick out details while observing, and which techniques are good for both sites, and which may work better at either a dark site or a light polluted site. Here we go.
All light polluted objects here were observed on March 27th, 2012 in my backyard in Herriman, Utah. Conditions were Antoniadi IV at first and then improved to Antoniadi III. Conditions were clear, mild for Utah for this time of the year, with lots of twinkly except at zenith. The jet stream is moving over us right now, typical of spring in Utah. Instrument used was my 14" dob and my 27mm Panoptic as a finder, and then a 10mm and 7mm Pentax XW in most cases. Again, click on the images to see them larger.
The top sketch is one I did in February of Messier 81 at one of the dark sites. In it you can observe I have more stars sketched in that I observed, the core is brighter and the stellar nucleus is also brighter. Perhaps the biggest thing here is I was able to observe some actual dark areas that separated the arms and provided structures to the galaxy in the observation.
Below you can see last nights sketch from my observation of Messier 81 from my light polluted backyard. Fewer stars are observed, really only the brighter ones and the structure of the arms is lost and the core and stellar nucleus are not as defined as in the dark site sketch and observation. Though still a fine object to observe, there is a significant difference here.
Here is Messier 82 or Bode's Galaxy in Ursa Major. At the dark site the galaxy had a mottled appearance and fainter stars were easily to identify. With averted vision I could see the slight breaks in the galaxy from dust. In my backyard which is the bottom example, the breaks were hinted at and that is all. No real faint stars were seen though they were hinted at. There was a hint of mottling as well, and for the most part, the surface brightness seem more consistent across the galaxy.
In the sketches above of the Leo Triplet, Messier 65, 66 and NGC 3628 I think the sketches speak volumes in the differences of the views of these galaxies. At the dark site which is the top one, again more stars are viewable, the bright cores stick out, the stellar nucleus are obvious and NGG 3628 shows its structure and its dark lanes. In the one done on the bottom from my light polluted backyard, the two Messier galaxies are there, with a hint of brightening of the core, but no nucleus can be discern. NGC 3628 is there, though really faint, faint that just looking at the sketch as I type this I cannot see it. Yet it is there, as it was in the scope, barely. It is good to point out though that were I new to observing or when I was new to observing, I could not see NGC 3628. I had to get to a dark site to see it and then when I returned to my backyard, I was able to use averted vision to see this object. I also realize that for many of you, seeing NGC 3628 is easy from an orange zone. My eyes are not that good at home.
These sketches represent Messier 105 and NGC 3384 with a very, very faint NGC 3389 in the bottom sketch from light polluted skies. Besides the difference in orientation, the stars are mostly lost in the light polluted skies on the bottom sketch as the bright cores are dimmer or lost. NGC 3389 you'll have to search for, but it was there, barely as the sky conditions cleared up for a few seconds from time to time.
These are three separate sketches of the galaxy Messier 95. I've posted all of the recently. The top one was done on February 17th I believe from a dark site. The bar was showing that night nicely as was some structure in the arms. Then on March 21st, I sketched it again because of SN2012aw that I wanted to capture. Again, the bar is there as is a stellar nucleus and the arms were observable not just by me, but by my friend Jorge and Shahid who looked in the scope. Shahid saw them the west with his early twenty something eyes.
The bottom sketch was last night in my light polluted backyard. First thing besides the reduction in stars is the loss of any hint or sign of the arms. There was nothing there and in truth, the galaxy looked like the fainter outward details were also lost. The supernova is still there, though fainter, as are the main stars. What a contrast here I felt!
I realized looking at my directions, that once again I may have marked my cardinal directions wrong. I'll have to check that. Anyway, here is the biggest difference I have seen. The top one is Messier 96 from a truly dark site, a Bortle 1 site and the bottom is the same galaxy from my light polluted dark site. Now I did have some fun with the top sketch with some of the fainter stars, but overall it reflects what I saw from my dark of darkest sites. The bottom one is just a wash in light pollution. Not much there to view. The core region is nice and bright in the top with a stellar core that is small. The LP sketch shows a brightening in the core region but I could not see a stellar core.
So there we go for now. A simple comparisons among objects that many amateurs go after from light pollution zones or in their backyards. I have attempted to show a contrast in the level of details that one sees by going to a dark site versus being in a light polluted zone. I encourage you to get out from your light polluted backyards and travel to a true dark zone and take it in. Take in the set up of your equipment, making sure you have time to enjoy twilight. Take in the time to transitions from day to night vision. Use a red light as little as possible and make sure when you do use one, to patch your observing eye and make sure your red light is dim, very dim. I'll post some spring ones up if the weather is clear in April and I can get out. Not sure if anyone will find this useful but I think it is a good idea and provides a good contrast. May your heart and eyes look up to the wonders of the sky.
PS I may have to do this once Carl gets done with my mirror and I have the scope working with it to show the differences in the two mirrors.