August 31, 2016: Plato craterlet! and LPR filters are doomed

Monsoon season continues here in Denver practically unabated.  Note the use of the word practically there.  Just over a week ago, as the moon was about three days past full, there was a slight break in the unending parade of clouds one night.  I had a nice look at the moon from off of my balcony.  It was well past midnight, and the moon had risen up past 30 degrees above the horizon.

 As I usually do, I had a good look at the floor of Plato to catch the elusive craterlets that lurk there.  These are very difficult to see – you need a telescope with enough aperture to resolve them.  The largest craterlet, just offset from the center, is just about 1.5 miles wide, and the angular resolution on a 5″ Mak is about 1.1 miles, so that largest one should just barely be within my grasp.  You also need some excellent seeing, or at least a window of excellent seeing.  It’s just this weird little thing I do, for, you know, “fun”.  I’ve been trying to find one of the craterlets for many, many months now, without success.  Yet.


And see one I did!  Or at least, I’m pretty darned sure I did.  People have told me that I should be able to resolve up to about 4 of the larger craterlets with my scope.  I only saw the very largest one, the one that’s just off-center.  I definitely saw something in exactly the right place:  close to the middle, on the side of the floor that’s further away from the bright triangular feature in the wall of the crater.

I went in and got out my lunar atlas and made sure it was the right place.  By the time I confirmed it and got back to the scope, it was gone and not coming back.  However, in that brief window of excellent seeing, I did see it.  It was just a brighter albedo feature on the floor, a distinctly lighter spot among the lunar gray.  I couldn’t resolve it into a crater (or craterlet), but I know it was there.  Cool!

In the meantime, however, observing off of my balcony is not so hot.  I have a southern exposure, but I’m on the fourth floor, and the twin building directly south of me in my complex is six stories high, blocking most of the sky.  Plus, the high-intensity lights used to illuminate the parking lot in between the two buildings are exactly at my eye level here, preventing any and all possible dark adaptation anyway.  (And coming in through my windows at night.  Oof.)  So that just leaves moon observing from the balcony.  Oh, well.

I’ve been leaving my scope out on the balcony under a thick plastic bag (and under the roof of the balcony on the fifth floor, right above mine) so that it would always be acclimated to the outside temperature and ready to use – no cooldown time.  For lunar observing, anyway.  Fortunately, between the bag and the balcony roof, the scope hasn’t been getting wet in these sometimes violent late summer Front Range downpours.  I do worry about the strong gusts of wind hitting the bag and making it into a sail to topple the scope, but so far, so good.

I thought the roof of the apartment building might be unlocked so I could get up there with my scope.  I had a good look around one night while up there.  The fence on the edge of the roof completely blocks out all of the ambient light from the parking lot lights and street lights below all around, so that it was pretty dark up there – really good for dark adaptation.  And the views of the eastern and southern skies were almost completely unblocked with a little maneuvering around to different spots on the roof.  Alas, since my brief sojourn up there, the building has now seen fit to firmly lock the roof access door.  Sheesh!  Don’t they trust people?

So, that leaves me to hoofing it out with the scope almost half a mile to a semi-dark spot in a park just north of the Cherry Creek Trail.  Fortunately, I do have one of those foldout laundry carts to carry the scope and stuff, and I’m about to pull the trigger on an astronomer’s chair.  As soon as my credit card rewards clear, that is.  The sky is pretty wide open there (except for these two tall office buildings directly east of the spot), and the light pollution there doesn’t seem too bad.  I’m hoping to get down to 4th magnitude there visually, as opposed to 3rd magnitude from my apartment building’s roof in Manhattan.  That should add another magnitude onto my scope’s ability to see as well, getting me down to about mid-10th magnitude.  If that’s true, I’ll take it!

Like the AAA, the DAS also has seminars for its members and their guests on various topics of astronomical interest.  Recently there was just such a talk given by Dr. John Barentine from the International Dark Sky Association.  The IDA is a terrific organization, promoting awareness about how human activity is causing dark skies to recede all over the developed world, and what we can do to reverse that trend.

Dr. Barentine informed us that in a few years, as municipalities complete the switching out of their street lamps to LEDs, light pollution reduction filters will become useless.  This is because LEDs give off light across the entire visible spectrum. This is as opposed to mercury vapor or sodium vapor (the bluish and yellowish street lights we all grew up with) that give off their light in just a narrow slice of the spectrum. These narrow slices are what the LPR filters are designed to block.  With their full spectrum light, LEDs are unblockable.

Well, Dr. Barentine did tell us about a filter that would effectively block the blue and violet end of the LED’s spectrum.  This would do two things – one, it would shift the spectrum to make the LED light more yellowish, and much less harsh on the eyes.  But more importantly for astronomy, cutting out the blue end would mean that the light from LEDs would cause significantly less light pollution.  This is because the atmosphere is most effective at scattering the blue wavelengths of light; which is exactly why the sky is blue.  Cutting off the blue end of the LED spectrum would reduce the extent of that scattering and therefore reduce the impact from the LEDs.  Unfortunately, these filters cost money, and the whole reason that municipalities are making the switch is to save money, not spend it on filters to please a few astronomers.

So if your community has made the switch, is in the middle of making the switch, or is about to make the switch, these LPR filters are done and dusted. However, if your community does not have plans to switch over, you might buy one of these filters now and hope to get a couple/three years’ use out of it before the inevitable switch.

On a side note, anyone wanna buy a used Orion Ultrablock filter? Cheap?  (Just kidding.)

6 thoughts on “August 31, 2016: Plato craterlet! and LPR filters are doomed

  1. I’ve heard that some LED streetlights are nicer to astronomers than other LED streetlights and for some reason, probably cost, the less nice ones are usually the default ones installed. Would anyone care to comment on that ?


    1. The IDA guy explained to us that there are some LED lights that are not full spectrum, but are skewed towards the red end of the spectrum where they have less of a light-polluting effect.

      Then there are those that are installed with full hoods so that their light is directed down toward the ground, and not up at the sky.


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