Seeing

Seeing

Focusing with FocusMax is fairly straight forward as it should be. However, achieving your best possible focus requires more than simply clicking and running. Other considerations should be taken into account for tweaking FocusMax parameters. Your optical system and local conditions are major items for tuning FocusMax. Study of documentation and close attention to what a focus run tells you can lead to a small parameter change which can produce surprising results. The following are some subjects which better understanding can lead to better focusing.

Focusing and CFZ, by Don Goldman:

"In Perfect Focus Article Preprint. This was submitted and published in an abbreviated format in Sky and Telescope entitled "In Perfect Focus", August 2010, page 72 (with Dr. B. Megdal). .It explains why the conventional Critical Focus Zone (CFZ) is too large to get precisely focused stars, and that a new derivation using David Suiter's book guided by data from the freeware program, Aberrator, provides more accurate, but much smaller values. For f/10/, f/7, f/5 and f/3.5 optical systems at 500 nm (green), these new CFZ values become ~ 40, 20, 10 and 5 microns in one direction, respectively.  These values are incredibly small! A human hair is 50 microns thick to provide some perspective. Using a high-precision electronic focuser becomes essential for optimum focus."

PDF - http://www.astrodonimaging.com/docs/GetFocusedPreprint.pdf

Additionally, Don has some good on-line image processing tutorials:

http://www.astrodonimaging.com/tutorials/

Seeing and Air Mass:
"In astronomy, air mass (or airmass) is the optical path length through Earth's atmosphere for light from a celestial source. As it passes through the atmosphere, light is attenuated by scattering and absorption; the more atmosphere through which it passes, the greater the attenuation. Consequently, celestial bodies at the horizon appear less bright than when at the zenith. The attenuation, known as atmospheric extinction, is described quantitatively by the Beer-Lambert-Bouguer Law.

"Air mass" normally indicates relative air mass, the path length relative to that at the Zenith at Sea Level, so by definition, the sea-level air mass at the zenith is 1. Air mass increases as the angle between the source and the zenith increases, reaching a value of approximately 38 at the horizon."

http://en.wikipedia.org/wiki/Air_mass_(astronomy)

http://spiff.rit.edu/classes/phys445/lectures/atmos/animloop.gif

Air Mass not only provides an indication of deterioration but also to some extent atmospheric turbulence or Seeing. The more Air Mass light passes through the worse the effects.

http://spiff.rit.edu/classes/phys445/lectures/atmos/single_anim.gif

When star light encounters turbulent air cells these cells can act as lenses to distort and deflect incoming photons on very short timescales. As more and more turbulent air is encountered distortions become compounded leading to madly twinkling and highly refracted colorful stars near the horizon.  When this occurs accurate focusing becomes more difficult and usually poorer. Sirius near the horizon:

http://epod.usra.edu/.a/6a0105371bb32c970b0168e5bf0ad1970c-750wi

To minimize these effects it is advisable to focus and image within a few hours of the Zenith, both RA and DEC, where Air Mass and turbulence minimum. Imaging locations at higher elevations than Sea Level by nature reduces Air Mass some allowing greater angles from the Zenith before Air Mass and Seeing effects becomes a problem. To obtain best images, one should avoid taking exposures at an Air Mass of 1.5 or higher, (40 degrees elevation or less).

Air Mass Extinction Correction Factors

Wm. Keck Observatory Air Mass JAVA tool,shows Air Mass for objects. Enter Object, RA & DEC, press "Add/Update Object", select Air Mass or Elevation.

http://www2.keck.hawaii.edu/software/obsplan/obsplan.php

Seeing, Ground Layer:
Usually 0-25ft, is caused mostly by thermal radiation or vortices around or over objects .
http://www.footootjes.nl/Astrophotography_Seeing/Astrophotography_Seeing.html

Seeing, Boundary or Mixing Layer:
From the Earth's surface to ~1,000ft. Wind flow near the surface encounters obstacles reducing wind speed introducing horizontal and vertical turbulence which interacts with the atmospheric layer above. This interaction is the primary transport of smoke and dust.
http://en.wikipedia.org/wiki/Planetary_boundary_layer

NOAA's ADDS Aviation Weather Center:

This is NOAA's ADDS Aviation Weather Center site showing Flight Level Turbulence regions with 12hrs Forecast.

http://www.aviationweather.gov/adds/turbulence/turbnav

Seeing, High Altitude:
There are several Jet Streams flowing around the globe. They reside at a level between the Stratosphere and Troposphere, a mixing level several kilometers wide and thick, where temperatures change from rising with altitude to decreasing with altitude. Jet Streams meander,and change in altitude, split and merge, with velocities ranging from calm to 250mph.

Although high altitude wind speeds don't buffet our telescopes as low level winds they still contribute to observable Seeing because of vertical turbulence. These fast moving rivers form turbulence at boundaries of fast to slow windspeed layers. Which, just as low level convective turbulence, distorts incoming light in a similar manner.

http://spiff.rit.edu/classes/phys445/lectures/atmos/animloop.gif

National Jet Stream Map, updated every 12hrs:

Stream Lines
http://weather.unisys.com/upper_air/ua_cont.php?plot=str&inv=0&t=cur

300spd plots

http://weather.unisys.com/upper_air/ua_cont.php?plot=300&inv=0&t=cur

Animated 60hr 300mb Wind Speed Forecast:
http://weather.unisys.com/nam/loop/nam_300_loop.gif

Radiosonode, weather balloon, readings updated every 12hrs:
...Choose your Region and City code close to your location.
...Click Text and scroll to the 'second' section, "Wind Level Data"
...Scroll to the 300mb line, the fourth column is wind speed in Knots, 1kt = 1.15mph.
http://weather.unisys.com/upper_air/skew/ua_sound.php?type=no&city=ktlh&region=se&t=cur

Because Jet Streams also change altitude,as they meander horizontally, be sure to scan the Speed column above and below the 300mb line for the highest wind speeds in your area.

Over time I have come to the conclusion wind speeds greater than 80kt are not good imaging weather for my 3300mm FL telescope. Star diameters are enlarged and reliable focusing becomes most difficult. Short FL telescopes can tolerate higher wind speeds.

"May You Go Among The Imperishable Stars"
Joe Mize  www.cav-sfo.com
Chiefland Astronomy Village (CAV), Fla
StarFields Observatory, (SFO).

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"New Critical Focus Zone"

Dr. Jeff Winter has proposed an alternative critical focus zone that takes into account:

•Seeing

•Telescope aperture Telescope focal ratio

•Acceptable focus tolerance

See New Critical Focus Zone for details