Astronomical seeing

Predicting the seeing (over Holland) is that possible?!

When looking through a telescope, celestial objects often look blurred and are deforming rapidly.
This is caused by the Earth's atmosphere.

One kilogram of air is resting on each square centimeter of our planet's surface: this is equivalent to 10 meters of water!
So the sky often looks like the bottom of a (filled) swimming pool ....

The effect of atmospheric turbulence on quality of astromical observation is referred to as 'seeing'.

Examples of (very) bad seeing: Moon (June 5, 2006 and December 17, 2005),
Saturn (April 14, 2006), Mars (Nov. 21, 2005), Venus (Dec. 25, 2005,
note atmospheric dispersion (rainbow colors) resulting from Venus' low position above the horizon):

Examples of "relatively good" seeing (Saturn on March 15, 2006, and March 15, 2007;
Copernicus on March 28, 2007, Plato on December 21, 2007):

Amateur astronomy in Holland is a somewhat masochistic activity, because the seeing is usually bad.
This is caused by the unstable atmosphere. Predominantly western winds push humid air in from the Atlantic.
This process is strongly linked to jetstream winds.

Rule of thumb: only with low wind speed at ALL altitudes, can one hope for good seeing.
This usually only happens near the center of a high-pressure area.

Experienced amateurs continuously keep track of the weather, to be able to pick those rare moments of good seeing.

A very informative article about prediction of seeing (in UK) was published by Damian Peach.

For seeing prediction over Holland, I found the following weather sites to be very useful:

1. Bolam weather charts (learned about this from Damian's article),
     with a.o. jetstream forecast:

2. GFS jetstream forecast.
3. More jetstream information (animation) (and still more, more).
4. Jetstream forecast, animation (view with IE).
5. Vertikal cut-through, with forecast.

6. MeteoBlue astronomical seeing prediction tool for Europe (but also other parts of the world). Requires registration.
7. SkippySky astronomical seeing prediction tool for Europe and other parts of the world.
8. 7Timer astronomical seeing prediction tool for world (for Southern Holland).

     Twice each day, a sounding balloon is released over De Bilt (near center of Holland), collecting data over an altitude
     range of ca. 10 kilometers. Especially the wind speed information is useful: if all flags (at all altitudes) are weak
     and if they point to the same direction (little wind shear) then we may hope for good seeing.
     See also: explanation of sounding data, and this wikipedia site.
9a. Site with sounding data of De Bilt (0h00, 12h00).
9b. Alternative site for sounding data (western Europe).

10. Actual (surface) weather situation over Holland (KNMI) and weather charts.
11. Meteorological chart.
12. Big satellite image.
13. Movie based on satellite images.
14. USAF weather prediction (Europe, surface animation, 5 day forecast).

The following table lists dates on which I experienced best seeing (applicable to Eindhoven, Southern Holland):

Date	Surface wind (1)	Surface wind (2)	Jetstream	Weather chart	Sounding
March 15, 2006	chart	chart	chart	chart	chart
May 4, 2006	chart	chart	chart	chart	chart
June 10, 2006	chart	chart	chart	chart	chart
June 12, 2006	chart	chart	chart	chart	chart
September 12, 2006	chart	chart	chart	chart	chart
March 13, 2007	chart	chart	chart	chart	chart
March 15, 2007	chart	chart	chart	chart	chart
March 28, 2007	chart	chart	chart	chart	chart
April 11, 2007	chart	chart	chart	chart	chart
October 14, 2007	chart	chart	chart	chart	chart

The table indicates that a weak jetstream is a 'hard' requirement for good seeing. I don't bother
to set up the telescope any more when jetstream is strong (except when high resolution is not required).
However, weak jetstream is not a 'sufficient' requirement: seeing often still is bad even if jetstream is (very) weak.
For instance, during April 2007 Holland enjoyed several weeks of fine weather (blue skies), during which the
jetstream was constantly weak. Wind was predominantly from the east. Several Dutch amateurs reported
that seeing was mostly bad during this period.
Anyone understand why?

Ground seeing, mirror seeing.

Next to atmospheric seeing, there is also the issue of "ground seeing": turbulence in the surface air layer
(5 meters or so) can be detrimental (that's why professional telescope are usually built on elevated structures).
Stay away from open doors, windows, chimneys, through which warm air is rising, and stay away from sun-heated
surfaces like asphalt.

Furthermore, the telescope must be in thermal equilibrium with it's surroundings.
The movie below shows Mars, strongly defocused (C11, focal reducer; January 6, 2008). Thermal convection currents are visible.
The telescope had reached thermal equilibrium during storage (in the house, T = 16C), after which it was placed outside,
where T = 2C. The movie was made after only ca. 30 minutes of cooling, so the tube and corrector plate of the C11 had
cooled a lot, but the heavy primary mirror was still at a temperature close to 16C. So the currents are most likely caused by
warm air circulating inside the telescope, along the surface of the primary mirror. Very bad mirror seeing!

This is a (spherical) panorama of my backyard. How bad is that tree for the seeing?
Click on the image to open the corresponding Flash panorama.
In such a panorama you can view in any direction.
Use the mouse or left/right/up/down arrows. Use SHIFT to zoom in, CTRL to zoom out.

Lunar astrophotography with a DMK camera

Planets, stars, etc.

Wide field astrophotography (low budget)

[© P. van de Haar. Page started December 23, 2006. This page was last updated on September 30, 2011]

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