active vs adaptive optics

Hi,

Can someone explain me the difference between active and adaptive optics?

Thanks,
Ante (albireo @ vip "dot" hr)
--
http://astro.fdst.hr

On 9 Nov 2003 12:56:46 -0800, (Ante Perkovic) wrote:

Hi,

Can someone explain me the difference between active and adaptive optics?

Usually, "active" refers to simple first order tip/tilt correction, and
"adaptive" refers to higher order corrections that actually involve modification
of the shape of the wavefront. But these terms aren't formally defined, so you
will see some variation in usage.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Ante Perkovic wrote:
Hi,

Can someone explain me the difference between active and adaptive optics?

"Active optics" usually refers to optics that can be slowly moved, while
"adaptive optics" usually refers to optics that can compensate
atmospheric turbulence. Active optics can range from a
temperature-programmed focuser to automatic primary recollimation to
compensate tube sag as the telescope is pointed at different altitudes.
Flexing a spherical primary to a parabolic shape as the temperature
changes can be considered active optics. "Adaptive optics" usually uses
light from the target area to determine the effects of atmospheric
turbulence, then moves the optics rapidly to compensate. These optics
may be a tip-tilt mirror or a deformable mirror.

Don Bruns

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.

Best regards,

Mike Simmons

Don Bruns wrote:

Ante Perkovic wrote:
Hi,

Can someone explain me the difference between active and adaptive optics?

"Active optics" usually refers to optics that can be slowly moved, while
"adaptive optics" usually refers to optics that can compensate
atmospheric turbulence. Active optics can range from a
temperature-programmed focuser to automatic primary recollimation to
compensate tube sag as the telescope is pointed at different altitudes.
Flexing a spherical primary to a parabolic shape as the temperature
changes can be considered active optics. "Adaptive optics" usually uses
light from the target area to determine the effects of atmospheric
turbulence, then moves the optics rapidly to compensate. These optics
may be a tip-tilt mirror or a deformable mirror.

Don Bruns

Mike,

I would describe a tip-tilt mirror as "first order" adaptive optics as it
rapidly responds to real displacements of the image via optical methods.
Obviously, adaptive optics can become quite sophisticated but I would not
say that it requires that glass be bent.

This is kind of like arguing whether or not Pluto is a planet.....

Del Johnson



"Mike Simmons" wrote in message
...
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.

Best regards,

Mike Simmons

Perhaps we can get someone from Lowell Observatory or Stewart observatories,
the former in Flagstaff and the latter in Tucson, to answer this, although I
wonder how definitive the answer would be. It seems to me that this is
another area of human knowledge where DEFINING what you mean by adaptive and
active would clarify what the situation is immensely.

As for my two cents . . .

I don't know what is meant by "active." Is tracking an object automatically
active? Is changing the optics to "correct" for something active?

Adaptive to me means a method by which the optics can be changed to reduce
or eliminate seeing effects. By this definition, and by the fact that
SBIG's tilt-mirror system reduces low frequency "waves" seeing, it is an
adaptive system.
--
----------------------------------------------------------------------
There is a fifth dimension beyond that which is known to Man.
It is a dimension as vast as space and as timeless as infinity.
It is the middle ground between light and shadow,
Between Science and superstition
And it lies between the pit of Man's fears
and the Sunlight of his knowledge.
It is the dimension of imagination.
It is an area that might be called. . . The Twilight Zone.


----------------------------------------------------------------------
"Mike Simmons" wrote in message
...
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.

Best regards,

Mike Simmons

Don Bruns wrote:

Ante Perkovic wrote:
Hi,

Can someone explain me the difference between active and adaptive
optics?

"Active optics" usually refers to optics that can be slowly moved, while
"adaptive optics" usually refers to optics that can compensate
atmospheric turbulence. Active optics can range from a
temperature-programmed focuser to automatic primary recollimation to
compensate tube sag as the telescope is pointed at different altitudes.
Flexing a spherical primary to a parabolic shape as the temperature
changes can be considered active optics. "Adaptive optics" usually uses
light from the target area to determine the effects of atmospheric
turbulence, then moves the optics rapidly to compensate. These optics
may be a tip-tilt mirror or a deformable mirror.

Don Bruns

David Nakamoto wrote:

Perhaps we can get someone from Lowell Observatory or Stewart observatories,
the former in Flagstaff and the latter in Tucson, to answer this, although I
wonder how definitive the answer would be. It seems to me that this is
another area of human knowledge where DEFINING what you mean by adaptive and
active would clarify what the situation is immensely.

As for my two cents . . .

I don't know what is meant by "active." Is tracking an object automatically
active? Is changing the optics to "correct" for something active?

Adaptive to me means a method by which the optics can be changed to reduce
or eliminate seeing effects. By this definition, and by the fact that
SBIG's tilt-mirror system reduces low frequency "waves" seeing, it is an
adaptive system.

I was one of Steward Observatory's first three people working on the MMT
active optics. The goal was active focus and alignment of the outer 6
telescopes with respect to central (guide/alignment) telescope. We were
to do this with less than 10E-12 watts of laser power out of the
guide/alignment telescope. I left the project in 1974 I think but the
concept was complete then.

Chuck
--
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons

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

David Nakamoto wrote:

....

I don't know what is meant by "active." Is tracking an object automatically
active? Is changing the optics to "correct" for something active?

Adaptive to me means a method by which the optics can be changed to reduce
or eliminate seeing effects. By this definition, and by the fact that
SBIG's tilt-mirror system reduces low frequency "waves" seeing, it is an
adaptive system.

The key question here is whether or not "seeing" include telescope drive
errors. This certainly degrades image resolution, but I would apply the
term "active" optics to this improvement, and "adaptive" to
corrections of atmospheric turbulence.

Don Bruns

Hi David,

David Nakamoto wrote:

Perhaps we can get someone from Lowell Observatory or Stewart observatories,
the former in Flagstaff and the latter in Tucson, to answer this, although I
wonder how definitive the answer would be. It seems to me that this is
another area of human knowledge where DEFINING what you mean by adaptive and
active would clarify what the situation is immensely.

Well, I've talked to people at a few observatories about this
definition, including Mount Wilson and Keck. One optical engineer I
know at Mount Wilson who is familiar with the natural guide star systems
used there (and worked on spy satellites which I understand use AO) says
the AO-7 *isn't* "adaptive optics". A physicist/astronomer at Mount
Wilson who's been in the field from the beginning of its appearance in
astronomy, and who's currently building the laser guide star system on
the 100-inch says it *is* "adaptive optics", though (albeit without a
great deal of conviction). IIRC, the Keck AO engineer (who built the
earliest AO system in astronomy outside of DoD) thought it isn't but was
kind of equivocal.

But Don is actually in the industry that creates the parts and systems
the rest use rather than being an end user of a system or two, which is
why I was interested in his perspective. Don's been around this stuff a
very long time -- I think much longer than any astronomers that didn't
work for DoD in the earliest years. If there IS a definitive answer Don
would probably know what it is. If not, it's still another view that's
usually missing in astronomy circles (or is that ellipses?).

Mike Simmons