How does the 'clarity' of the sky translate to telescope performance?

[Martin Pugh]

I guess what I am asking, is that if you look at the Cloud Sensor data at Siding Springs, it states that the sky 'ambient' temp (which is a measure of the clarity of the sky) is a rather fantastic -50! Then looking at River Dingo, their Cloud Sensor data reported -30, still very good.

But how do you think this difference in temp and therefore the difference in the clarity of the sky would show up in an exposure? Doesnt really matter about the telescope or camera, but assuming a 30 minute exposure was taken of the same object using the same equipment, how much better do you think the image from Siding Springs would appear?

cheers
Martin

[glend (Glen)]

Baseline it Martin, would not be hard to do, just take some sample images of the same area and measure the sky glow ADU at the various sky temperatures- that should give you the relationship. Would it not?

[Martin Pugh]

Yes of course, but that means taking the gear to each site Which would be ok if I had portable gear. I could probably do that in the future.

[algwat (Alan)]

This may help, water vapor content, black is none , white is cloud/rain/cyclone?

http://tropic.ssec.wisc.edu/real-tim...mswv_loop.html

Best sky seeing in black regions. Can then be matched to temperature?

kind regards, Alan.

[Shiraz (Ray)]

Hi Martin.
I think that the answer to your basic question is probably no, sky temperature is not a direct measure of clarity, so will not necessarily indicate how good the imaging conditions are.

a few notes that might possibly be useful:
- the sensors respond to a wider spectrum than just the water vapour window, so local temperature radiation from air that is close to the sensor is also important. Thus, the most useful measure is not absolute sky temperature, but the difference between ambient and the sky.
- the relative sky temperature only tells how much water is up there (vapour or clouds), not how clear the sky is in the visible band. You can have a sky with lots of visible scattering (light dust, aerosols) that would be useless for imaging, but it will still look cold in a thermal sensor (indicating clear sky). Alternatively you could have a clear humid atmosphere that would be OK for imaging, but would appear to be relatively warm to a thermal sensor.
- cloud temperature can vary a lot since it depends on local temp at the cloud (ie altitude) and high clouds could conceivably appear cold enough to look like clear sky. However, clouds also reflect ground radiation and that apparently makes them appear warm enough to be sensed as clouds.
- sensor field of view and pointing angle can affect the results.
- there is presumably some relationship between humidity (sky temperature) and turbulence (seeing) - not sure and interested if anyone has any info.

useful reference maybe:
http://onlinelibrary.wiley.com/doi/1...2JD017881/full

regards Ray

[Martin Pugh]

Interesting. Thanks for the info.

[Paul Haese]

I concur with Ray. I have found over the years that sky temp changes not only daily in clear conditions but also seasonally. For instance during summer the sky temp at my location is generally around -6 during hot weather and then about -10 during days of 25 degrees. Whereas during the peak of winter the sky temp can measure as high as -25. Each of those instances are when the sky is perfectly clear.

Also something to consider is the equipment being used to measure the sky. There are a few brands out there and each would no doubt differ a little in measuring the temperature.

[Phil Hart]

Interesting thread.. and thanks Ray for the comments.

I'd concur that I don't think there's a simple correlation with sky temperature. I'm still using an AAG.. getting the parameters for clear sky right are a little tricky.. humid nights definitely confuse it but not related to transparency.

In general transparency indexes are based on humidity/precipitable water. The idea being that if you have lots of pollutants (aerosols) then in humid atmosphere they help condense moisture and degrade transparency. That might work ok as a correlation in Texas, but in Oz in southerly airflow you can have high humidity but very clear air off southern ocean. So humidity does not equal poor transparency here. Conversely you could have very dry northerly air flow with dust/smoke and poor transparency but very low humidity.

Hope that helps confuse the situation even more..

Phil

[markas (Mark)]

Two pieces of direct evidence in support of Phil's comments:

1. Shot a surprisingly good image in 99.9% RH (even managed to avoid dew problems). The local air temp was 19C, so the moisture loading was very high. SQL 20.8. From the subs, seeing was ~2.5" - a little better than average for the venue.

2. Had a perfect looking night - low RH, 21+ SQL - ruined by what proved to be high level smoke haze which was not obvious at the start of the run and escaped detection by my RH and SQL monitors (I don't have a cloud sensor).

The only solution I can see for the second example is to be there watching the subs deteriorate. (I WAS there, but not watching)

Mark

[Shiraz (Ray)]

Originally Posted by markas

Two pieces of direct evidence in support of Phil's comments:

1. Shot a surprisingly good image in 99.9% RH (even managed to avoid dew problems). The local air temp was 19C, so the moisture loading was very high. SQL 20.8. From the subs, seeing was ~2.5" - a little better than average for the venue.

2. Had a perfect looking night - low RH, 21+ SQL - ruined by what proved to be high level smoke haze which was not obvious at the start of the run and escaped detection by my RH and SQL monitors (I don't have a cloud sensor).

The only solution I can see for the second example is to be there watching the subs deteriorate. (I WAS there, but not watching)

Mark

A cloud sensor would probably not have picked up the smoke either - from the reference linked to earlier: The integrated sky temperature over the considered wavelengths (5.5 to 50μm) changes by a maximum of 3°C as we change from the tropospheric aerosol type with visibility = 50 km to the urban aerosol type with visibility = 5 km. We conclude that under normal conditions, the aerosol particles in the atmosphere have little influence on the atmospheric radiation.

[Martin Pugh]

Hi folks

returning this thread, I think perhaps the image linked here, possibly answers the question I initially posed. Note this is a calibrated FIT (31mb), no other processing.

I am so impressed with this image given that on the night in question, the sky temp as recorded by my Cloud Sensor was 14.5 and humidity at 88%.

I have my Cloud Sensor configured to trigger a cloudy condition when the sky temp gets to 12, so at 14.5, it was not far off. Admittedly, the scope used is f3.8 but a 5nm Ha filter was also used.

cheers
Martin