Mike Simmons wrote:
Hi Don,

I'd just like to double-check something. It seems that you would then
agree that the SBIG AO-7 "adaptive optics" unit -- a tip-tilt mirror --
is truly "adaptive" optics. Is that right? I've asked this of others
working in the field -- astronomers and optical engineers -- and get
varied responses. Some say tip-tilt is "adaptive", others (a minority,
I think) say it's not. Most, however, give me a quizzical look and say
something like "Well, you could say..." (fill in the blank). So I'm
very interested in the view of someone not just building a particular
unit or using AO but having spent many years in the business.

Hi,

The SBIG AO-7 is a good example of an active optics system, but some
people might also consider it an adaptive optics system. In use, it
might correct a small amount of atmospheric "tilt", but I believe most
of the improvement comes from correcting slower drive error.

Adaptive optics is useful only over a very small field of view -
typically a few arcseconds in the visible spectral region, for full
adaptive optics, and up to an arcminute for tip-tilt only. Since most
users of the SBIG AO-7 use a much wider field of view, true adaptive
optics would actually degrade the image more than an arcminute from the
guide star. Since the real improvement seen with this device is evident
over the entire image, the improvement is due to telescope drive errors,
not atmospheric turbulence. With an update rate of 10 samples per
second, the bandwidth is close to 1 Hz; this is too slow for most
atmospheric turbulence correction.

This depends on your definition of "active" vs "adaptive" - I tend to
use the more restrictive definition of adaptive. A fast visible
tip-tilt system is a true adaptive optics system, if it can keep up with
the atmosphere and is used only over a field of view less than an
arcminute. The Stellar Products AO-2 did exactly this.

Don Bruns