Sharper telescope images with video astronomy : an undergraduate laboratory Michael Dubson Physics Department, University of Colorado at Boulder

The Problem

The resolution of ground-based telescopes is usually limited by "seeing" (atmospheric turbulence) rather than by the diffraction limit of the optics. So bigger telescopes produce brighter images but not clearer images. Professional astronomers are trying to overcome this limit with a new technology called adaptive optics, but this is extremely expensive, and only works over a very small field of view ("isoplanatic patch") – a few arcsec in the visible.

Often, seeing limits resolution to 1 ~ 3 arcsec ("), which is the diffraction limit of a 3-inch telescope. How can small observatories with limited budgets (or students or amateurs) overcome the limitations of poor seeing?

A Partial Solution

A new technique called Video Astronomy, largely developed by amateur astronomers over the last 10 years, can dramatically improve the resolution of ground-based telescopes. The basic procedure is this:

•take a video through the telescope at 30 Hz,

•sort the frames according to sharpness

•stack the better frames with the center-of-light motion corrected

•deconvolute the stacked image with a point spread function

This technique only works with bright objects (Moon, Sun, planets, brighter stars, etc) – bright enough so that a good image can be captured at 1/30 s exposure.

Diffraction limited

= wavelength

Airy disk = / D

D = aperture diameter

Turbulence limited

/ D

turbulent air layer

ro =seeing cell size

D ~ ro D > ro

"dancing disk" / D

/ro

wind velocity

"speckle pattern"

Image Resolution Primer

Good seeing – location of the jet stream is key:

Jet stream overhead means turbulent air, bad seeing.

Bad Good

A telescope – not too large – 5 to 10 inch aperture OK – with a sturdy mount.

A CCD video camera: either analog like this one or a digital camcorder is fine.

Free software to sort and stack the images

Two freeware packages are available:

1) http://registax.astronomy.net/

2) http://www.astrostack.com/

What is needed?

Results

Best single frame from a 500-frame 16-second video clip. Diameter of Jupiter is ~ 45". Note shot noise.

Stack of best 50 frames Image shift removed before stacking. Note shot noise averaged out.

After deconvolution with guessed point spread function. Note satellite Io and its shadow.

Images of Jupiter taken with a 5 inch aperature telescope on night of good seeing.

diameter D2.4 in5 in18 in2.4 m (HST)10 m (Keck)

1.8"0.88"0.25"0.05"0.010"

Diffraction Limit

Stack of 80 frames

Saturn with 5 inch aperture.

References

“Video Astronomy” Sky Publishing Corp.by Steve Massey, Thomas A. Dobbins, Eric J. Douglass

Equipment Suppliers:Adirondack Video: http://www.astrovid.com/http://www.scopetronix.com/

Lunar crator Clavius, stack of 50 frames

1o = 60 arcmin(') = 3600 arcsec(")

Moon diameter = 0.5o = 1800"

Jupiter diameter = 40" = dime at 100 m