I was going to take part in the CPRE dark night sky campaign by counting
stars in between Orion's arms.

https://www.cpre.org.uk/what-we-care...ar-count-2021/

However, during this cold snap I have noticed that even when the evening
sky is brilliant clear blue at sunset by the time it is properly dark
there is a thin high cloud layer enough to take the edge off faint stars.

My sky is good to mag 5.5 but the past few clear nights the limiting
magnitude has hovered between 3.5 and 4 (much worse at low elevations).

I am reluctant to give them an answer that doesn't truly represent my
actual sky conditions. What is it about this cold snap that is making
the sky go so hazy after dark? I have known very clear conditions at
this time of year. There can't be all that much water vapour about but I
guess what there is must be supersatured once the temperature crashes.

In the dawn morning sky it is clear that there is a thin layer of high
translucent cloud and then when the sun comes up a spectacular low
surface fog forms on the snow fields and drifts downhill.

The blue morning sky is basically a pale ice blue until mid morning when
the sun burns it off.

Ambient late last night and early this morning here was -7C.

--
Regards,
Martin Brown

On 12/02/2021 17:50, Martin Brown wrote:
I was going to take part in the CPRE dark night sky campaign by counting
stars in between Orion's arms.

https://www.cpre.org.uk/what-we-care...ar-count-2021/


However, during this cold snap I have noticed that even when the evening
sky is brilliant clear blue at sunset by the time it is properly dark
there is a thin high cloud layer enough to take the edge off faint stars.

My sky is good to mag 5.5 but the past few clear nights the limiting
magnitude has hovered between 3.5 and 4 (much worse at low elevations).

I am reluctant to give them an answer that doesn't truly represent my
actual sky conditions. What is it about this cold snap that is making
the sky go so hazy after dark? I have known very clear conditions at
this time of year. There can't be all that much water vapour about but I
guess what there is must be supersatured once the temperature crashes.

In the dawn morning sky it is clear that there is a thin layer of high
translucent cloud and then when the sun comes up a spectacular low
surface fog forms on the snow fields and drifts downhill.

The blue morning sky is basically a pale ice blue until mid morning when
the sun burns it off.

Ambient late last night and early this morning here was -7C.


Most of what I know of astronomy comes from the S@N, nothing practical .
I'm aware that you have to allow any optical telescope to reach the
ambient air temperature before using. But say you could somehow get air
interchange and get some humid air inside the internal optics, is there
a way of definitively determining that you have a very fine film of
condensation on a lens or two?
Other than swapping out an eyepiece, to check that part of the optics.


--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm

On 13/02/2021 11:26, N_Cook wrote:
On 12/02/2021 17:50, Martin Brown wrote:
I was going to take part in the CPRE dark night sky campaign by counting
stars in between Orion's arms.

https://www.cpre.org.uk/what-we-care...ar-count-2021/

However, during this cold snap I have noticed that even when the evening
sky is brilliant clear blue at sunset by the time it is properly dark
there is a thin high cloud layer enough to take the edge off faint stars.

My sky is good to mag 5.5 but the past few clear nights the limiting
magnitude has hovered between 3.5 and 4 (much worse at low elevations).

I am reluctant to give them an answer that doesn't truly represent my
actual sky conditions. What is it about this cold snap that is making
the sky go so hazy after dark? I have known very clear conditions at
this time of year. There can't be all that much water vapour about but I
guess what there is must be supersatured once the temperature crashes.

Just to be clear this is a citizen science experiment that anyone can do
- basically count the number of stars you can see with the naked eye
inside the four bright stars that make up the arms and legs of Orion.

The thing is I know what my fairly dark North Yorkshire sky is really
capable of delivering and these recent nights have all been well below
par. Comparable with the sort of thing I'd see from central Manchester!

When I look at the morning dawn sky it is a very pale blue and there is
a translucent cloud deck fairly high up which the sun burns off
completely after a couple of hours. Today the cloud deck is thicker and
still not burnt off. Temperature here didn't go much below -3C last
night and then warmed up to -2C as the clouds thickened.

In the dawn morning sky it is clear that there is a thin layer of high
translucent cloud and then when the sun comes up a spectacular low
surface fog forms on the snow fields and drifts downhill.

The blue morning sky is basically a pale ice blue until mid morning when
the sun burns it off.

Ambient late last night and early this morning here was -7C.

Most of what I know of astronomy comes from the S@N, nothing practical .
I'm aware that you have to allow any optical telescope to reach the
ambient air temperature before using. But say you could somehow get air
interchange and get some humid air inside the internal optics, is there
a way of definitively determining that you have a very fine film of
condensation on a lens or two?

This is definitely a weather phenomenon. The sky at sunset most days is
pretty well deep blue with a hint of haze low on the horizon and the odd
cloud. But although it starts clear as the air temperature falls rapidly
the previously clear sky at sunset visibly deteriorates with time.

I'm mainly interested to know what it is about the present weather
conditions is causing this hazing effect at high altitude from a couple
of hours after sunset. I presume it is a rapid cooling dew point thing.

Other than swapping out an eyepiece, to check that part of the optics.

No optics involved beyond a Mk 1 eyeball. My telescope has resistive
strip heaters on the optics but sometimes even they are not enough.

http://www.nezumidemon.co.uk/astro/sct/dew.html

I also have a passive dew shield (not shown on that photo).

--
Regards,
Martin Brown

On 13/02/2021 14:53, Martin Brown wrote:
On 13/02/2021 11:26, N_Cook wrote:
On 12/02/2021 17:50, Martin Brown wrote:
I was going to take part in the CPRE dark night sky campaign by counting
stars in between Orion's arms.

https://www.cpre.org.uk/what-we-care...ar-count-2021/

However, during this cold snap I have noticed that even when the evening
sky is brilliant clear blue at sunset by the time it is properly dark
there is a thin high cloud layer enough to take the edge off faint
stars.

My sky is good to mag 5.5 but the past few clear nights the limiting
magnitude has hovered between 3.5 and 4 (much worse at low elevations).

I am reluctant to give them an answer that doesn't truly represent my
actual sky conditions. What is it about this cold snap that is making
the sky go so hazy after dark? I have known very clear conditions at
this time of year. There can't be all that much water vapour about but I
guess what there is must be supersatured once the temperature crashes.

Just to be clear this is a citizen science experiment that anyone can do
- basically count the number of stars you can see with the naked eye
inside the four bright stars that make up the arms and legs of Orion.

The thing is I know what my fairly dark North Yorkshire sky is really
capable of delivering and these recent nights have all been well below
par. Comparable with the sort of thing I'd see from central Manchester!

When I look at the morning dawn sky it is a very pale blue and there is
a translucent cloud deck fairly high up which the sun burns off
completely after a couple of hours. Today the cloud deck is thicker and
still not burnt off. Temperature here didn't go much below -3C last
night and then warmed up to -2C as the clouds thickened.

In the dawn morning sky it is clear that there is a thin layer of high
translucent cloud and then when the sun comes up a spectacular low
surface fog forms on the snow fields and drifts downhill.

The blue morning sky is basically a pale ice blue until mid morning when
the sun burns it off.

Ambient late last night and early this morning here was -7C.

Most of what I know of astronomy comes from the S@N, nothing practical .
I'm aware that you have to allow any optical telescope to reach the
ambient air temperature before using. But say you could somehow get
air interchange and get some humid air inside the internal optics, is
there a way of definitively determining that you have a very fine film
of condensation on a lens or two?

This is definitely a weather phenomenon. The sky at sunset most days is
pretty well deep blue with a hint of haze low on the horizon and the odd
cloud. But although it starts clear as the air temperature falls rapidly
the previously clear sky at sunset visibly deteriorates with time.

I'm mainly interested to know what it is about the present weather
conditions is causing this hazing effect at high altitude from a couple
of hours after sunset. I presume it is a rapid cooling dew point thing.

Other than swapping out an eyepiece, to check that part of the optics.

No optics involved beyond a Mk 1 eyeball. My telescope has resistive
strip heaters on the optics but sometimes even they are not enough.

http://www.nezumidemon.co.uk/astro/sct/dew.html

I also have a passive dew shield (not shown on that photo).



Semeru volcano eruption, late Jan, led to aircraft ash warnings, perhaps
2 weeks is enough to distribute around the N Hemisphere

--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm