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Castor A and B under very high magnification, the effect of the AO-7:

     

 

Stacked images without the AO-7 (larger FWHM, left image) and with the AO-7 (smaller FWHM, right image).

 

The image scale is 0.15 arc seconds / pixel.  The stellar FWHM without the AO-7  = 1.8 arc seconds or 12 pixels.  The stellar FWHM with the AO-7 = 1.35 arc seconds or 9 pixels.  Use of the AO-7 results in about a 25% reduction in overall size of the stellar profile.

 

 

    

 

Blinking between images without and with the AO-7.

 

 

  

 

The animation shows the individual 0.11 second exposure frames that make up the stacked images described above.  Notice to the left, the stellar image is "jumping around" from frame to frame without the AO-7.  Over a long exposure, the stellar profile is spread out over a larger area due to normal seeing conditions.  Notice to the right, the stellar images are stabilized from frame to frame with the AO-7 activated.  Over a long exposure, the stellar profile is not spread out as much and thus produces a smaller and sharper stellar image even under normal seeing conditions.

 

Santa Barbara Instrument Group (SBIG) makes an electro-optical device that makes images sharper.  The A0-7 is an Adaptive-Optics (AO) device for the amateur astronomer.  The device can be thought of as a high-speed guider that is capable of guiding much faster than a feedback loop through the drive motors and gears of the mount.  The A0-7 is capable of correcting for X, Y shifts of stellar images at up to about 20 Hz by a simple tip-tilt of a diagonal mirror.  Wave front errors are not corrected, that would require a laser guide-star system, like the ones coming online at several major observatories described on page 17 of the February 2005 issue of Sky & Telescope.  The animation on the left is an example of what "real time" stars would look like without the A0-7.  Notice the X, Y positional shifting due to normal seeing.  The animation on the right is an example of how the A0-7 works.  Notice the centroids are stabilized.  The effects of diffraction limited optics and wave-front errors are still present.

 

These animations are of Castor A and B in Gemini.  The stars are separated by only 3.2 arc seconds.  The individual frames are short exposures and are at very high magnification using eyepiece projection.  Several images were captured over a short time interval.  The images have been layered into these GIF animations.  These animations serve to illustrate several things.  1) the diffraction limiting optics of the Schmidt Cassegrain Telescope (SCT),  2) Oversampling with the CCD camera, 3) the effects of seeing conditions on the Airy disk and diffraction rings and 4) How the A0-7 stabilizes the stellar images.

 

Double Star:  Castor A & B (Alpha Gemini).  Castor is the bright star that forms the right head of the Gemini twins.  Use high power to see that Castor is really  a double star.  It was the first double that was described as a binary system by William Herschel in 1804.  Each frame is a 0.11 second exposure “freezing” the atmospheric disturbances which form the “starfish like arms” which change moment by moment, frame by frame.  All images were made with the 10” LX200 and ST-7 with eyepiece projection at f/49 unfiltered.

 

Rotation of the centroid of Castor B:

 

This animation is a 3-D illustration of brightness vs. position of an image of the star Castor B in Gemini.  This animation server to illustrate the effect of seeing on a stellar image.  Momentary seeing conditions captured in a fast exposure show that the surface of a centroid is usually not smooth but irregularly shaped.  The distortions in the centroids shape near the base are called wings.

The following images show Castor A and B at different field scales.  The four images on the left have a field scale of 0.15 arc seconds / pixel.  These were taken as noted above with eyepiece projection at f/49.  The field scale at a prime focus of f/6.3 is simulated in the four images at 1.16 arc seconds / pixel on the right.  This shows what stellar images look like with near ideal Nyquist sampling on the right.  The images on the left illustrate oversampling.