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New Mexico: Mag/sq arcsec and Bortle

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#1 hottr6

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Posted 09 January 2013 - 08:33 AM

I have the incredibly good fortune of living in the Magdalena Mountains in central NM. I've estimated that I live in the Bortle grey zone as I cannot see shadows cast by the Milky Way.

I picked up a SQM-L amd measured sky brightness at nautical twilight in the evening and morning, taking several readings and averaging. In the evening I was recording 21.50 mag/sq arcsec, and this morning, 21.90. Pretty dark.

I would have thought that these readings would be more appropriate for a Bortle black zone.

As an aside, don't pack your bags for Mag (as we call it). The mountains entail hefty scintillation. The air here is pretty busy, though it settles down by early morning.

#2 knightware

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Posted 09 January 2013 - 08:53 AM

How fortunate you are to live beneath such dark skies!

As I'm sure you know, there isn't a rigorous, quantitative relationship between the subjective Bortle scale and SQM readings. At best, there is a suggested relationship. For that reason, properly acquired SQM readings should be preferred as they are objective. Still, observers use both scales rather widely. You may be discovering why the relationship isn't perfect.

Here are a few things you might consider in assessing your skies:

1) Why not take SQM readings during astronomical darkness? That is, after astronomical twilight in the evening, before astronomical twilight in the morning and during no moonlight. This would also apply in assessing your site's darkness on the Bortle Scale.

2) Conditions change over a given night due to local weather, aerosols and particulate. Readings really should be taken periodically through an observing session. You can get aerosol measurements and forecasts at http://www.star.nesd...pb/aq/index.php
and particulate measurements and predictions at http://airnow.gov/index.cfm

#3 George N

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Posted 10 January 2013 - 12:16 AM

.......Here are a few things you might consider in assessing your skies......


You also need to adjust for the Milky Way, or take readings that do not include it.

For comparison: At Kopernik Observatory on the NY/PA border, the best SQM readings I have gotten are 21.0 – on nights when the Susquehanna River valley (and 90% of the lights) is fogged up, but the mountain tops are still clear. At Cherry Springs State Park in PA and at my camp in the central Adirondacks, the best I’ve gotten with the SQM is 21.7. Others have reported higher (darker) readings.

#4 Tony Flanders

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Posted 10 January 2013 - 07:53 AM

I have the incredibly good fortune of living in the Magdalena Mountains in central NM. I've estimated that I live in the Bortle grey zone as I cannot see shadows cast by the Milky Way.


I will say it again for the record -- the color zones from the Light Pollution Atlas have nothing to do with the Bortle Dark-Sky Scale, which is rated by number rather than color. See my extended discussion of skyglow.

The ability to see shadows cast by the Milky Way probably depends more on you and on the position of the Milky Way than on skyglow -- within reason. I wouldn't expect the gray and black zones to be different in this regard. But the winter Milky Way is feeble indeed compared to the summer Milky Way.

I picked up a SQM-L amd measured sky brightness at nautical twilight in the evening and morning, taking several readings and averaging. In the evening I was recording 21.50 mag/sq arcsec, and this morning, 21.90.


There's a HUGE difference between 21.50 and 21.90. Remember that the SQM is measuring total brightness, both artificial and natural. Depending on numerous factors, the natural skyglow is typically 21.9 or brighter, so an SQM reading of 21.9 may or may not mean negligible artificial light pollution. But 21.5 is always considerably brighter than the natural skyglow. That's a fairly typical reading in the green zone.

Because of the enormous variation in natural skyglow, I don't think the SQM can distinguish between the gray and black zones.

I'm skeptical that you actually took these at the end of nautical twilight. I would expect the sky to be closer to 20.5 at that point. Are you sure you didn't mean astronomical twilight?

#5 Mr. Bill

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Posted 10 January 2013 - 11:05 AM

A few comments on using SQMs...

Rich V and I have observed several times together in central Nevada where the summer MW indeed casts shadows.

I have the older wide beam version and he has the new narrow beam. We typically see differences of 0.2 to 0.3 between them with mine reading darker.

I see good repeatability but really wonder about absolute accuracys as there is no built in calibration technique.

As far as the MW influencing readings, I typically see about 0.2 difference in pointing towards and away from the summer MW.

I live in rural NE Calf in a green/blue zone and see SQMs of 21.5 on a good night and 21.65 on exceptional nights.

:cool:

#6 hottr6

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Posted 10 January 2013 - 05:11 PM

Thanks for the good read. I'm learning! :)

I'm skeptical that you actually took these at the end of nautical twilight. I would expect the sky to be closer to 20.5 at that point. Are you sure you didn't mean astronomical twilight?

I made my observations about 100 mins after sunset, and before sunrise, in early Jan.

#7 derangedhermit

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Posted 11 January 2013 - 12:59 AM

Because of the enormous variation in natural skyglow, I don't think the SQM can distinguish between the gray and black zones.

Yes, it must be so, since they defined gray as only a 1% change from the reference natural sky brightness (black).

#8 Tony Flanders

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Posted 11 January 2013 - 05:21 AM

Thanks for the good read. I'm learning! :)


I'm skeptical that you actually took these at the end of nautical twilight. I would expect the sky to be closer to 20.5 at that point. Are you sure you didn't mean astronomical twilight?

I made my observations about 100 mins after sunset, and before sunrise, in early Jan.


That's probably completely out of twilight.

Rule of thumb is that civil twilight ends 30 minutes after sunset, nautical twilight ends 60 minutes after sunset, and astronomical twilight ends 90 minutes after sunset. So by 100 minutes, glow from the Sun should be completely absent.

Twilight lasts a smidge longer around the winter solstice and much longer around the summer solstice. See my blog on astronomical twilight.

#9 knightware

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Posted 11 January 2013 - 09:46 AM

Hi Mr. Bill,

Meters with Ethernet, USB or 9-pin serial connections can be calibrated. If you have the SQM and SQM-L models, you may have to send them back to Unihedron for calibration.

Two and three tenths of MPSAS difference in readings is a significant difference, but as others have pointed out, how you take the reading is very significant. I see differences between my 2 SQM-LUs and SQM-LE of a few hundredths MPSAS. I would encourage you to explore the difference further and address it.

#10 Mr. Bill

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Posted 11 January 2013 - 10:41 AM

I think I'll buy the narrow beam and play with that.

Sending my wide beam unit in next week to be calibrated.

Another comment about SQMs...they don't address the issue of transparency...some of my best readings are on moonless overcast nights.

;)

I find a much better indicator of darkness/transparency is observing detail visible in MW nebulosity such as around Gamma Cygni or low surface brightness objects such as 6822 (Barnard's Galaxy)

#11 knightware

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Posted 12 January 2013 - 08:56 AM

You are quite right about transparency - it is a different beast. I mentioned the particulate and aerosol measurements as a factor in determining transparency on cloudless nights. They are contributors to light scattering and haze, meaning bad for transparency.

Observers really should assess transparency at every observing session using some familiar extended object such as those you mentioned.

#12 Mr. Bill

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Posted 12 January 2013 - 11:33 AM

One "trick of the trade" I use is to occult the sun with my fist and look at the surrounding halo. This will tell you a lot about light scattering and is a good indicator of conditions of the upcoming night.






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