1.20
#...
·
 |
|---|
|
As far as I understand, the main function of the SQM meter of the eagle computer (Eye sensor) is to detect if the sky conditions degraded, not only because of the moon but due to clouds passing by. If so, considering a bortle 2 sky, what SQM value would be safe to assume that clouds are in? |
0.00
#...
·
·
1
like
|
|---|
| I think bortle 2 would normally give you 21.5+ sqm. If sqm dropped below 19 if would most likely be cloud caused. Best regards - Gene |
0.90
#...
·
·
1
like
|
|---|
Gene: A cloudy sky at a dark site will give you a higher SQM since there are no light sources to reflect off the clouds. |
 #...
·
|
|---|
Dan Watt:Gene: That was one of the surprising things when I visited a Bortle 1 site. Not used to seeing earth clouds look dark, just blotting out the stars. No hint of light reflecting from them. I believe the Bortle designation includes the appearance of clouds, if present, in the determination. And I believe B2 will have clouds be dark. I know that B1 the clouds were as dark as the clear patches of sky. |
|
1.20
#...
·
·
1
like
|
|---|
Dan Watt:Gene: That’s interesting. A cloud in a dark site will cover the stars, so actually there will be less lights sources, as there isn’t any other source of light. The sky will get darker, and the SQM will rise. Is that what you mean? |
#...
·
|
|---|
Rafael Sampaio:Dan Watt:Gene: That may be true. However, I was just basically quoting the "verbal" description of the Bortle Scale and that my experience is aligned with that. Bortle 2 from Wikipedia:  Note the third item listed. Of course the same is for B1. For B3, light creeps into the area and clouds near the horizon (away from Zenith) start to become noticable from reflected light. Where I live, ~B4.5, clouds even at Zenith are visible from reflected light and become obviously lighter than the background. Clearly, these are just rules-of-thumb. Depending on the altitudes of the clouds, the higher clouds will be able to "see" over the horizon many more miles, and they may be subject to reflected light from cities hundreds of miles away, whereas clouds close to the ground will not have this issue. Where I live, B4+ and greater, is probably typical for the experience of most people who live on this planet. So most of us can see clouds move across the sky as lit from below. To see clouds as simply holes in the sky is an experience most have never seen. Given the fact that many astro objects become naked eye in B1&2, one understands that the the typical eye becomes better adjusted to dark in such situations and starlight may illuminate the surroundings. For example, Bortle 1 descriptions state that certain parts of the Milky Way (eg. Sagittarius) cast shadows because of their brightness. Because of this, your statement of the effect of clouds on SQM may be true. But I am no expert on how true SQM is derived. |
