12 replies to this topic

[Spaceman 56]

I was looking up the cloud forecast in Astronomy Tools website to see how long before I could image.

 

I noticed a Sky Quality reading came up, for where I live,  and magnitude 21.99 ?

 

then some more technical stuff I don't understand.

 

 

can someone please explain what these technical numbers mean, and also how do they know all this ?

 

I do know its pretty dark here at night, and that helps my imaging.

 

any help understanding this would be appreciated.  

 

Spaceman 56

 

 

 

 

[Luke24592]

Hello Spaceman 56

I can't tell you how they know that exactly, but it means you are a very fortunate observer/imager. 

 

If you scroll to the lower half of this page there is a chart which gives some insight into those numbers.

 

https://www.cleardar...ml?Mn=telescope

 

Luke

[Mike7Mak]

I believe the numbers are measurements of natural and artificial sky glow per square meter.

 

This site sells a really nifty sky quality meter and probably explains more than you want to know about sky quality. I have one and it works great.

 

http://unihedron.com/projects/darksky/

[ngc7319_20]

I was looking up the cloud forecast in Astronomy Tools website to see how long before I could image.

 

I noticed a Sky Quality reading came up, for where I live,  and magnitude 21.99 ?

 

then some more technical stuff I don't understand.

 

Sky quality Takahue.png

 

can someone please explain what these technical numbers mean, and also how do they know all this ?

 

These are just rough estimates of the sky brightness due to natural and artificial sources.  The website

 

https://www.cleardar...lution_map.html

 

attempts to model the sky brightness.  They assume natural sources give 22 magnitudes per square arcsecond in the V band (visual green light).  This is only a very rough guess, and ignores factors like solar cycle, air glow, weather, moonlight, etc.  The natural sky light of 22 mag / arcsec squared corresponds to about 169 micro-candles per square meter.

 

Light from artificial sources is estimated from satellite photos showing the brightness of artificial lights on earth, and then converted to zenith reflected light based on some model of atmospheric scattering.  So at your location the model for artificial light gives 1.53 micro-candles per meter squared.  Adding this to the natural light gives the total brightness of 170 micro-candles per square meter or 0.170 milli-candles per square meter. 

 

Your ratio of artificial light to natural sky light is about 0.01 which corresponds to Bortle class 0 on the Bortle scale.  This is indicative of very dark skies with almost no light pollution.

 

If you want to measure the actual night sky brightness at your sire, you can buy a Sky Quality Meter from these folks.  This would be much more reliable than the estimates.

 

http://www.unihedron...ojects/darksky/

Edited by ngc7319_20, 06 February 2023 - 10:21 AM.

[Drothgeb]

It’s a scale of artificial lighting, per square arc seconds I think. The scale stops at 22.0. I think only 21.99 and 22.0 qualify as Bortle 1. You definitely have good dark skies. By comparison, my Bortle 5 backyard is on the very darkest nights is 19.9 SQM. Not very dark, I can easily see to walk around.

 

I have a dark sky location in the mountains, it’s Bortle 3 at 21.7 SQM. But it’s always cloudy when I go there. In 5 nights this year, I only have 2 hrs of data.

Edited by Drothgeb, 06 February 2023 - 09:49 AM.

[bobzeq25]

There are a number of ways of measuring sky brightness.  Bortle level is simple and "good enough".

 

Knowing what the other numbers mean may be interesting, but it doesn't help your imaging.

 

Note that the Moon changes sky brightness.  A lot.

Edited by bobzeq25, 06 February 2023 - 10:17 AM.

[pedxing]

+1 for the unihedron sqm.

Actual measurement of a current quantifiable property in meaningful units, unlike the bortle estimate.

[Robert7980]

I was looking up the cloud forecast in Astronomy Tools website to see how long before I could image.

 

I noticed a Sky Quality reading came up, for where I live,  and magnitude 21.99 ?

 

then some more technical stuff I don't understand.

 

Sky quality Takahue.png

 

can someone please explain what these technical numbers mean, and also how do they know all this ?

 

I do know its pretty dark here at night, and that helps my imaging.

 

any help understanding this would be appreciated.  

 

Spaceman 56

It means you’re luckier than 99.999% of us 

Edited by Robert7980, 06 February 2023 - 12:07 PM.

[Andros246]

I was looking up the cloud forecast in Astronomy Tools website to see how long before I could image.

 

I noticed a Sky Quality reading came up, for where I live,  and magnitude 21.99 ?

 

then some more technical stuff I don't understand.

 

Sky quality Takahue.png

 

can someone please explain what these technical numbers mean, and also how do they know all this ?

 

I do know its pretty dark here at night, and that helps my imaging.

 

any help understanding this would be appreciated.  

 

Spaceman 56

Any land out there for sale? 

[Spaceman 56]

There are a number of ways of measuring sky brightness.  Bortle level is simple and "good enough".

 

Knowing what the other numbers mean may be interesting, but it doesn't help your imaging.

 

Note that the Moon changes sky brightness.  A lot.

yes I noticed that last night. the moon was up and bright after midnight, and visually I had a much more restricted number of visible stars.

so I am guessing that from high bortle areas, as well as the sky glow/pollution , you also actually visually see less stars ?

 

I'm interested because I am planning a trip to a differant location, and want to estimate how much longer it might take to image from there.

 

I found this graph in another thread, and it had SQM on the left side, and Dob17.5 said it was a sky quality meter. thats why I asked what a SQM was.

 

 

the chart was someones attempt, to estimate the time required, for equivalent imaging, at different Bortle zones.

[Andros246]

yes I noticed that last night. the moon was up and bright after midnight, and visually I had a much more restricted number of visible stars.
so I am guessing that from high bortle areas, as well as the sky glow/pollution , you also actually visually see less stars ?

I'm interested because I am planning a trip to a differant location, and want to estimate how much longer it might take to image from there.

I found this graph in another thread, and it had SQM on the left side, and Dob17.5 said it was a sky quality meter. thats why I asked what a SQM was.

sky quality meter..png

the chart was someones attempt, to estimate the time required, for equivalent imaging, at different Bortle zones.

I have no information to back this up but I saw somewhere someone said that in bortle 2-3 with a moon is equivalent to bortle 7-8

Edit:
Here’s some good info

https://www.cloudyni...ring-full-moon/

https://www.cloudyni...or-a-full-moon/

Edited by Andros246, 07 February 2023 - 02:20 PM.

[dciobota]

If you have a DSLR and don't want to spend money on an sqm meter, here's an article posted by Dr Samir Kharusi, a very helpful nice gentleman, sadly passed on:

https://pbase.com/sa.../image/37608572

As far as the moon effect. It varies not only with the phase of the moon, but also angular distance from the moon to where you image, and also transparency. It's hard to say the bortle change, but bortle 3 to bortle 7 for a new to full moon sounds about right if you're looking in the vicinity of the moon.

Edited by dciobota, 07 February 2023 - 06:47 PM.

[17.5Dob]

If you have a DSLR and don't want to spend money on an sqm meter, here's an article posted by Dr Samir Kharusi, a very helpful nice gentleman, sadly passed on:

https://pbase.com/sa.../image/37608572

As far as the moon effect. It varies not only with the phase of the moon, but also angular distance from the moon to where you image, and also transparency. It's hard to say the bortle change, but bortle 3 to bortle 7 for a new to full moon sounds about right if you're looking in the vicinity of the moon.

I had forgotten about that method. Samir was a fairly frequent poster in the dSLR forum many years ago.