星閃與視寧度的關係

[社長]

在討論區上看到三段關於星星閃動，與視寧度的關係，持有相反意見，值得大家細讀：

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Posted by: "Stan" stan_ccd@yahoo.com stan_ccd
Sat Jan 21, 2012 9:22 am (PST)

Unfortunately, naked eye star twinkle is a useless indicator. Bad seeing often looks steady to the naked or low power eye; the image may be so blurred that it suppresses all twinkle. And good seeing can twinkle; there may be very little blur and so image motion translates to twinkling.

A high alt "jet stream" is very destructive to seeing. Such winds are mapped and predicted by weather sites. So if the maps show a jet stream anywhere near you then forget about hi-res imaging.

But a calm upper atmosphere does not guarantee good seeing. A local inversion can be very destructive and often not appear on weather maps.

Also, "ground seeing" can ruin an otherwise good night. Trees, buildings, and concrete/asphalt can release "heat waves" that cause the images to jump around and blur. Pre-dusk watering the local grounds can improve ground seeing.

OTA or mirror seeing can destroy hi-res. OTA/optic/ambient temperature differences > 1-2 deg can set-up destructive air currents. OTA fans can accelerate cool-down and break-up tube currents. Closed tubes can impede/suppress heat chimneys but they also retard cool-down.

Heat from up-wind observer, computer (etc.) can create bad seeing.

Low-alt imaging looks thru more disturbances and degrades resolution.

There are many causes and ways to degrade resolution! <g>

Stan


之後另一位愛好者的回覆：
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Posted by: "Chris Peterson" cpeterson@earthlink.net cloudbait
Sat Jan 21, 2012 9:41 am (PST)


Naked eye star twinkle is far from useless. Depending on the nature of
your specific location, it can be highly diagnostic. I routinely monitor
my seeing instrumentally, and it very closely correlates with visual
scintillation of stars. Most of the time, I can predict the instrumental
seeing to withing a half arcsecond simply by looking at the stars.

My impression is that it is of lower diagnostic value in places which
regularly experience very good seeing. Such sites seem to maintain good
seeing even when the stars are twinkling. Sites with habitually poor
seeing, however, tend to have their best seeing in the absence of
scintillation.

Chris


之後第三位愛好者的回覆：
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Posted by: "Tom Harrison" twharrison@manoprietoobservatory.com stargazertwh
Sat Jan 21, 2012 10:14 am (PST)


That's correct. If you know your site sky characteristics and seeing
patterns, visual inspection can be very accurate. When visual
inspection is indeterminent, I will open the observatory and do a star
test to confirm what visually seems to be the case. Almost always, if
the stars are twinkling at a certain rate over my site, I know that
good guiding and resolution at f9 is not possible, however, hating to
miss a good night, I will keep checking for hours unless weather graphs
also confirm what I'm seeing will be around for the night. Last night,
I imaged from about midnight when seeing was predicted to get better
until astronomical daylight, however, the seeing didn't get really
excellent until about 2am, so those raws taken before that will likely
be tossed upon inspection. Never have I seen a instance when the stars
were twinkling at a high rate over my site and the seeing was even
good, much less very good to excellent.

Best, Tom

Tom Harrison
Mano Prieto Observatory