Starship Asterisk*

neufer wrote:None of them super hi-res :!:

Sorry, that's a technical term I'm unfamiliar with.

You asked for long exposure examples from the telescope, which is what I provided. Perhaps you are confusing the Magellan 6.5 meter telescope with the VisAO camera, one of many instruments used on that telescope?

Large apertures with adaptive optics permit super hi-res but not for the faintest objects!

(They also permit hi-res for the faintest objects!)

The AO system is perfectly capable of imaging extremely faint objects at very high resolution. That's what the large aperture provides. The limitation with AO is that you need a bright reference object within the region of atmospheric coherence, typically a few arcseconds. This can be achieved with a natural guide star (which is what is used with the VisAO) or an artificial guide star. The latter option allows for many more accessible targets. In either case, however, the limiting magnitude of imaged objects is primarily determined by the telescope aperture, and is the same whether or not the AO system is enabled.

Chris Peterson wrote:

neufer wrote:
None of them super hi-res

Sorry, that's a technical term I'm unfamiliar with.

I.e., resolution as good or better than Hubble.

Chris Peterson wrote:

neufer wrote:
Large apertures with adaptive optics permit super hi-res but not for the faintest objects!

(They also permit hi-res for the faintest objects!)

The AO system is perfectly capable of imaging extremely faint objects at very high resolution. That's what the large aperture provides. The limitation with AO is that you need a bright reference object within the region of atmospheric coherence, typically a few arcseconds. This can be achieved with a natural guide star (which is what is used with the VisAO) or an artificial guide star. The latter option allows for many more accessible targets. In either case, however, the limiting magnitude of imaged objects is primarily determined by the telescope aperture, and is the same whether or not the AO system is enabled.

The limiting magnitude of imaged objects is primarily determined by exposure time and sky darkness. Advantage Hubble.

neufer wrote:

Chris Peterson wrote:

neufer wrote:
None of them super hi-res :!:

Sorry, that's a technical term I'm unfamiliar with.

I.e., resolution as good or better than Hubble.

Apples and oranges. HST resolution over anything larger than a few arcseconds is currently not possible to achieve from the ground. Higher resolutions than the HST are routinely obtained from the ground at many telescopes, but only for very small fields. Again, this resolution does not depend upon exposure time, and high resolution in AO systems is not necessarily limited to bright objects.

The limiting magnitude of imaged objects is primarily determined by exposure time and sky darkness. Advantage Hubble.

Not always. Another important factor is dark current. Ground based professional cameras usually have insignificant dark current, because they are cryogenically cooled. The HST camera is thermoelectrically cooled, and as a consequence has a much higher dark current level (180 e-/hour, or a dark current noise of 13 e-/hour), high enough to affect limiting magnitude. Other important factors include subexposure time- the WFC3 has higher readout noise than newer ground-based instruments like MegaCAM, so the shorter subexposures required to avoid localized saturation reduce the S/N (and therefore the limiting magnitude) more with HST than with modern ground-based cameras. And while sky background is an important factor, it varies significantly with photometric band, so the disadvantage isn't always as large as you might expect.

Chris Peterson wrote:

neufer wrote:
None of them super hi-res

I.e., resolution as good or better than Hubble.

Apples and oranges. HST resolution over any [FOV] larger than a few arcseconds is currently not possible to achieve from the ground. Higher resolutions than the HST are routinely obtained from the ground at many telescopes, but only for very small fields. Again, this resolution does not depend upon exposure time, and high resolution in AO systems is not necessarily limited to bright objects.

It is generally limited to 1 minute exposure times which do not permit viewing the faintest objects.

Chris Peterson wrote:

neufer wrote:
The limiting magnitude of imaged objects is primarily determined by exposure time and sky darkness. Advantage Hubble.

Not always. Another important factor is dark current. Ground based professional cameras usually have insignificant dark current, because they are cryogenically cooled. The HST camera is thermoelectrically cooled, and as a consequence has a much higher dark current level (180 e-/hour, or a dark current noise of 13 e-/hour), high enough to affect limiting magnitude. Other important factors include subexposure time- the WFC3 has higher readout noise than newer ground-based instruments like MegaCAM, so the shorter subexposures required to avoid localized saturation reduce the S/N (and therefore the limiting magnitude) more with HST than with modern ground-based cameras. And while sky background is an important factor, it varies significantly with photometric band, so the disadvantage isn't always as large as you might expect.

All very interesting, Chris, but I am still waiting for one example of HST resolution from the ground with a 10+ minute exposure time.

neufer wrote:

Chris Peterson wrote:Apples and oranges. HST resolution over any [FOV] larger than a few arcseconds is currently not possible to achieve from the ground. Higher resolutions than the HST are routinely obtained from the ground at many telescopes, but only for very small fields. Again, this resolution does not depend upon exposure time, and high resolution in AO systems is not necessarily limited to bright objects.

It is generally limited to 1 minute exposure times which do not permit viewing the faintest objects.

Sorry, but where are you getting this idea of exposures limited to 1 minute?

neufer wrote:All very interesting, Chris, but I am still waiting for one example of HST resolution from the ground with a 10+ minute exposure time.

Such images are taken all the time, although the images themselves frequently go unpublished, since they have little aesthetic value. But there are many papers about deep AO observations (try Googling it), with HST or better resolution and exposures so long they may require multiple nights.

One thing to be careful of is to avoid confusing subexposure times, which may be limited to a few minutes by sky background considerations, and total exposure time, which is the sum of all the subexposures (this isn't the same as lucky imaging, which utilizes exposure times of a few milliseconds; the shorter subs for conventional imaging are merely to avoid saturation, not to compensate for seeing).

Chris Peterson wrote:

neufer wrote:

Chris Peterson wrote:
Apples and oranges. HST resolution over any [FOV] larger than a few arcseconds is currently not possible to achieve from the ground. Higher resolutions than the HST are routinely obtained from the ground at many telescopes, but only for very small fields. Again, this resolution does not depend upon exposure time, and high resolution in AO systems is not necessarily limited to bright objects.

It is generally limited to 1 minute exposure times which do not permit viewing the faintest objects.

Sorry, but where are you getting this idea of exposures limited to 1 minute?

From the examples given.

Besides: adaptive optics must be monochromatic which greatly limits the amount of light for stars, planets, moons, etc.

Advantage Hubble.

neufer wrote:

Chris Peterson wrote:Sorry, but where are you getting this idea of exposures limited to 1 minute?

From the examples given.

Those were hardly more than first light examples (early light, maybe), the first scientific results from a new instrument. They were looking at bright objects, which necessitated short exposures (as would be the case with any camera on a large telescope). I think it would be a mistake to draw any assumptions about limitations of this instrument from a couple of examples. AO systems in general don't depend on short exposures, and there's no reason at all to think this one does.

neufer wrote:Besides: adaptive optics must be monochromatic which greatly limits the amount of light for stars, planets, moons, etc.

Virtually all astronomical cameras are monochromatic in the sense that they only acquire a single channel of luminance data in a single exposure. That doesn't mean they are constrained to only collecting light in a narrow wavelength band. The example images from the VisAO camera were collected through the same broad photometric filter that would have been used with a more conventional camera. Neither VisAO nor AO systems in general are narrow band imagers. Except for any loss associated with light diverted to the wavefront analyzer, AO cameras have the same sort of sensitivity that non-AO cameras have. VisAO is capable of collecting color information just like most cameras are, by multiple exposures through different filters. We wouldn't get true color, because the shortest wavelength this camera collects is 600 nm, but we get multichannel color images from other near-IR cameras just as we can with this one.

VisAO is actually a bit unusual in the degree to which it is coupled with the AO component. Usually, the AO is a separate subsystem of the telescope, and can be utilized with many instruments.