Jump to content

Banner.jpg.b83b14cd4142fe10848741bb2a14c66b.jpg

Quantitative measurement of "seeing"?

Macavity

By Macavity
in The Astro Lounge

Recommended Posts

Macavity

Macavity

Posted
Posted

Quantitative measurement of "seeing"? Until recently, I was convinced that

this could be monitored by a (commercial or DIY!) Sky Quality Meter. But a

little thinking, reading, and my own observation suggests (obviously): This

is a measurement of "sky brightness" (sic). Indeed some authors note that

a "dark sky" might be misty or even totally cloudy (And devoid of stars)! :p

One intrepid chap tried to use infra-red monitoring at various wavelengths? 

But that gave even more "random" numbers. The "ancient sages" seem to

rely mostly on their eyeballs and experience - Perhaps that is the real way?

BUT maybe indeed the sky brightness is the "good number"? It would be

nice to record some sort of *number* to accompany my VIDEO images! 

Notwithstanding (heaven forbid sales plummet) an SQM seems useful? :)

I do need some sort of monitoring though. An all sky camera too? Remote 

control can leave you caught out by cloud - Until the screen goes white! :D

Thoughts? Comments? 

Link to comment
Share on other sites
pete_l

pete_l

Posted
Posted

If you want to measure the brightness of the sky, you really need to concentrate those measurements on bits of sky that have no visible stars in them.

I have an SQM and it's handy for giving a basic number for the sky. But the answer it gives is variable - up to about +/- 0.5 on the device's readout. It's also important to remember that darkness is only one attribute: clarity and turbulence are just as important and more difficult to measure.

And probably the most significant measurement of all is the number of cloud-free nights in a year!

Link to comment
Share on other sites
coulthamst

coulthamst

Posted
Posted

I use the "which stars of known magnitude can you see in constellation X?" method. Best for me is Ursa Minor, which has stars of a range of magnitudes in a comparatively small field that is circumpolar, and therefore always accessible (weather permitting!)

Link to comment
Share on other sites
ronin

ronin

Posted
Posted

Don't the big observatories fire a big laser at one of the atmospheric layers - think that for some reason one layer has sodium in it and the laser excites that then they analyse the image. The excited sodium creates an artifical star in effect. Used for adaptive optics. :eek: :eek: :eek:

Bit over the top for an amateur set up and I doubt that you can easily get a laser to get up there with sifficent power. :grin: :grin: :grin:

Link to comment
Share on other sites
sharkmelley

sharkmelley

Posted
Posted

As a previous poster said, astronomical seeing measures the turbulence of the atmosphere i.e. how stable are moon craters, Saturn's rings and Jupiter's red spot etc. through the scope.

For imaging faint galaxies etc you are really interested in a mixture of sky brightness and transparency.  I do have a SQM and it faithfully measures the sky brightness but sky brightness doesn't tell the whole story.  I have measured very dark skies where few stars are visible and imaging is a waste of time because of lack of transparency e.g. caused by a veiled  layer of cloud.

In the end, for imaging purposes, the best measure is good old-fashioned NLM (naked eye limiting magnitude)

Mark

Link to comment
Share on other sites
jnb

jnb

Posted
Posted

To quantitatively measure seeing, if by seeing you mean atmospheric stability, then an exposure of fixed duration at a long focal length would allow you to measure FWHM of a star image which would be an objective measure. That of course will vary by altitude and magnitude and obtaining a sufficiently long focal length might mean adding high power barlows in which case you might be measure optical defects in the imaging rather than the atmospheric seeing

Link to comment
Share on other sites
Macavity

Macavity

Posted
  • Author
Posted

Belated thanks, ALL. Certainly provoked / clarified some of my own thoughts. :)

To quantitatively measure seeing, if by seeing you mean atmospheric stability, then an exposure of fixed duration at a long focal length would allow you to measure FWHM of a star image which would be an objective measure. That of course will vary by altitude and magnitude and obtaining a sufficiently long focal length might mean adding high power barlows in which case you might be measure optical defects in the imaging rather than the atmospheric seeing

That was my (almost independent) thinking. :p

This seems to be the way such things are monitored "professionally"? I was vaguely dreaming of twin optics monitoring the magnitude (pixel width) of a bright star and subtracting annular - or simply local, sky background. But I sense this is a tad beyond (my!) funding and technology. And probably overkill too?  ;)

Nevertheless, I am now moved to build a TSL237 + Arduino Sky Quality Meter.

Reading past threads, I have a few (Baldrick) "cunning plans" up my sleeve...  

And, when that doesn't work too well, I can buy a commercial SQM sometime! :D

Link to comment
Share on other sites
pete_l

pete_l

Posted
Posted

To quantitatively measure seeing, if by seeing you mean atmospheric stability, then an exposure of fixed duration at a long focal length would allow you to measure FWHM of a star image which would be an objective measure.

There's a Mag 10 star only 10 arc-min away from the NCP. So if you've got a scope+CCD with a 20 arc-min FoV you could set that permanently pointing at the NCP and the star would always be in view.

An  ATIK 314+  with a  1000mm FL refractor would do the job. What exposure time would you need to get a reasonably noise-free image of such a star at (say) 80mm aperture?

Link to comment
Share on other sites
Macavity

Macavity

Posted
  • Author
Posted

There's a Mag 10 star only 10 arc-min away from the NCP. So if you've got a scope+CCD with a 20 arc-min FoV you could set that permanently pointing at the NCP and the star would always be in view.

An  ATIK 314+  with a  1000mm FL refractor would do the job. What exposure time would you need to get a reasonably noise-free image of such a star at (say) 80mm aperture?

My general thought (along these lines?) was still a fairly state-of-the-art INTEGRATING video camera. 

You can certainly generate a substantial "blob" on a half inch chip from a second magnitude star! ;)

I sense "calculations" are better implemented in software than trying such things in hardware...

Link to comment
Share on other sites
jnb

jnb

Posted
Posted

There's a Mag 10 star only 10 arc-min away from the NCP. So if you've got a scope+CCD with a 20 arc-min FoV you could set that permanently pointing at the NCP and the star would always be in view.

An  ATIK 314+  with a  1000mm FL refractor would do the job. What exposure time would you need to get a reasonably noise-free image of such a star at (say) 80mm aperture?

Or could you do it in real time with a guiding scope?

Link to comment
Share on other sites
michaelmorris

michaelmorris

Posted
Posted

Another strategy would be to have a guide camera set up pointing at a set altitude above the horizon.  Set the guiding exposure to a really short figure so that the guide scope attempts to chase the seeing.  If you then measure the average guiding deflection in RA and Dec you would get a measure of the seeing.

I guess there could be issues to overcome, like making sure that the guiding set up didn't overshoot on corrections.

One way around this might be to take two measures. 

- The first set of very short exposures, this would produce a model of deflections due to tracking + deflections due to seeing

- A repeat set of measures with a very long exposure would produce a model of deflections due to tracking

Take one from the other and you have a model of deflections due to seeing

Whether this would work in practice is another thing!

Also this might only work for low frequency seeing effects.  High frequency seeing effects (general blurring) caused by the jet stream could be modelled by comparative contrast in the image (although this would also be effected by high cloud, fog or poor focus).

My apologies for the stream of consciousness!

Link to comment
Share on other sites
knightware2

knightware2

Posted
Posted

Chris,

Measuring seeing is mostly a subjective matter for amateur astronomer, but as a previous poster mentioned, it can be done objectively using a CCD camera. There is a very thorough discussion of measuring atmospheric seeing  here: http://www.handprint.com/ASTRO/seeing2.html

I believe that the most commonly used (subjective) seeing metrics used by amateurs are:

Seeing can change according to the area of sky being observed, so the measurement should be made in the area of interest. In practice, measuring at the beginning of an observing session after optics have stabilized thermally is perhaps more practical.

As others have pointed out, SQM measures sky darkness. There are other metrics for sky darkness, but SQM is the objective one. Lastly, one might measure sky transparency. The only objective measurements I know of for transparency are Particulate Matter (PM2.5), and Aerosol Optical Depth (see http://www.gmes-atmosphere.eu/d/services/gac/nrt/nrt_opticaldepth/)

Link to comment
Share on other sites
niallk

niallk

Posted
Posted

I don't have a SQM but go off Naked Eye Limiting Magnitude using star counting and a chart. Once you get familiar with the method it gives a pretty accurate method of quantitativly judging the sky conditions.

This is the chart I use. limiting_mag.pdf

I like that!! Cheers

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.