Starship Asterisk*

Wikipedia wrote:HgCdTe or mercury cadmium telluride (also cadmium mercury telluride, MCT, MerCad Telluride, MerCadTel, MerCaT or CMT) is narrow direct bandgap zincblende II-VI ternary alloy of CdTe and HgTe with a tunable bandgap spanning the shortwave infrared to the very long wave infrared regions. The amount of cadmium (Cd) in the alloy (the alloy composition) can be chosen so as to tune the optical absorption of the material to the desired infrared wavelength. CdTe is a semiconductor with a bandgap of approximately 1.5 eV at room temperature. HgTe is a semimetal, hence its bandgap energy is zero. Mixing these two substances allows one to obtain any bandgap between 0 and 1.5 eV.

geckzilla wrote:I just read something really cool about JWST's detectors. They are not CCDs and they can partially correct for cosmic rays. So far I don't know what the detectors are called. They are just calling them detectors or the focal plane assembly, which isn't very specific.

Chris Peterson wrote:

geckzilla wrote:I have often imagined that some cleverly-written software could selectively reject areas of obvious cosmic ray hits if the data were somehow constantly streaming instead of delivered all at once. This is probably a manifestation of my lack of real understanding about how CCD's work, though.

I can certainly imagine something like that, although not with a CCD. But you could have some hypothetical new type of detector that output a signal each time a photon hit it, containing the time and coordinates of that event. With such a rich dataset, all sorts of interesting processing would be possible. With a CCD (and other current spatial detectors), however, most of the time information is lost. At best, you can determine that a certain number of photons hit a specific pixel between two known times, where the difference in those times is typically large compared to the time between photon strikes.

Chris Peterson wrote:

Anthony Barreiro wrote:Such a detector is no longer purely hypothetical.

http://www.skyandtelescope.com/communit ... 70371.html

Indeed, detectors that work this way have been experimented with for a while now. They would also allow for interferometry using a pair of telescopes (which can currently only be done by tying those scopes together optically).

Still, I think it will be a while before we see optical detectors with a high spatial resolution and a large dynamic range that truly give information about each photon (time, position, energy). But I'm sure it will happen. It's "just" an engineering problem, not one of any fundamental physics. It's also something that would be immensely valuable for ordinary consumer cameras, and that's always good for driving development.

Anthony Barreiro wrote:Such a detector is no longer purely hypothetical.

http://www.skyandtelescope.com/communit ... 70371.html

Nitpicker wrote:Intuitively (because I've got nothing else) it sounds like a job for lots of short exposures, with the new zero readout-noise sensors you mentioned a few weeks back.

Chris Peterson wrote:

geckzilla wrote:I have often imagined that some cleverly-written software could selectively reject areas of obvious cosmic ray hits if the data were somehow constantly streaming instead of delivered all at once. This is probably a manifestation of my lack of real understanding about how CCD's work, though.

I can certainly imagine something like that, although not with a CCD. But you could have some hypothetical new type of detector that output a signal each time a photon hit it, containing the time and coordinates of that event. With such a rich dataset, all sorts of interesting processing would be possible. With a CCD (and other current spatial detectors), however, most of the time information is lost. At best, you can determine that a certain number of photons hit a specific pixel between two known times, where the difference in those times is typically large compared to the time between photon strikes.

Chris Peterson wrote:

geckzilla wrote:I have often imagined that some cleverly-written software could selectively reject areas of obvious cosmic ray hits if the data were somehow constantly streaming instead of delivered all at once. This is probably a manifestation of my lack of real understanding about how CCD's work, though.

I can certainly imagine something like that, although not with a CCD. But you could have some hypothetical new type of detector that output a signal each time a photon hit it, containing the time and coordinates of that event. With such a rich dataset, all sorts of interesting processing would be possible. With a CCD (and other current spatial detectors), however, most of the time information is lost. At best, you can determine that a certain number of photons hit a specific pixel between two known times, where the difference in those times is typically large compared to the time between photon strikes.

geckzilla wrote:I have often imagined that some cleverly-written software could selectively reject areas of obvious cosmic ray hits if the data were somehow constantly streaming instead of delivered all at once. This is probably a manifestation of my lack of real understanding about how CCD's work, though.

http://en.wikipedia.org/wiki/Airglow wrote:
<<Airglow (also called nightglow) is the very weak emission of light by a planetary atmosphere. This causes the night sky never to be completely dark, even after the effects of starlight and diffused sunlight from the far side are removed. The airglow phenomenon was first identified in 1868 by Swedish scientist Anders Ångström. The airglow at night may be bright enough to be noticed by an observer and is generally bluish in colour. To an observer on the ground it appears brightest at about 10 degrees above the horizon, because very low down, atmospheric extinction reduces the apparent brightness of the airglow.

Even at the best ground-based observatories, airglow limits the sensitivity of telescopes at visible wavelengths. Partly for this reason, space-based telescopes such as the Hubble Space Telescope can observe much fainter objects than current ground-based telescopes at visible wavelengths. For an 8 m unit Very Large Telescope telescope one needs 40 hours of observing time to detect a V=28 magnitude star through a normal V band filter, while the 2.4 m Hubble only takes 4 hours. Reducing the view field size can make fainter objects more detectable against the airglow; unfortunately, adaptive optics techniques that reduce the diameter of the view field of an Earth-based telescope by an order of magnitude only as yet work in the infrared, where the sky is much brighter.>>

geckzilla wrote:Thanks, Chris. To clarify one thing I said about things not getting lost in noise I just thought that given enough exposure time a space telescope would eventually be able to detect almost anything as opposed to noise hopelessly drowning things out no matter what.

geckzilla wrote:Cosmic rays are transient so it is very easy to take several exposures and eliminate them completely. They only limit visibility if there's not enough exposures to eliminate them.

They're not like noise at all.

I don't think things really get lost in the noise.

Bigger telescopes would definitely have an advantage.

geckzilla wrote:
Cosmic rays are transient so it is very easy to take several exposures and eliminate them completely. They only limit visibility if there's not enough exposures to eliminate them.
They're not like noise at all. I don't think things really get lost in the noise.

http://en.wikipedia.org/wiki/Gurgen_Askaryan#Cosmic_rays_and_sound_waves wrote:
<<Gurgen Askaryan discovered and investigated in details various effects accompanying passage of high energy particles through dense matter (liquids or solids). He showed that hadron-electron-photon showers and even single fast particles may produce sound pulses. Ionization losses are quickly converted into heat, and the small region adjacent to trajectory undergoes quick thermal expansion thus generating sound waves. These results gave a new approach to the study of cosmic rays. Before, investigations of cosmic rays were based on direct interaction of cosmic ray particle with a detector. Askaryan’s results made it possible to detect showers and single particles using sound receivers situated at some distance from the event. Several years ago, the registration of energetic particles and showers with sound detectors in sea water was planned as an important part of global monitoring.>>

Anthony Barreiro wrote:

geckzilla wrote:Cosmic rays, Mark. As in, they aren't actually objects.

Cosmic rays are objects -- very tiny objects, and very far away from the stars in the globular cluster.

geckzilla wrote:Cosmic rays, Mark. As in, they aren't actually objects.

neufer wrote:2) or "free range" so as have no sun and be in permanent night.

geckzilla wrote:
But the extra light would seem normal to any hypothetical life form on any hypothetical planet in a globular cluster. Curiously, if light levels were high enough, such a planet might not have such a sharp divide between nocturnal and diurnal creatures. Sleep might be very different and much less frequent.

• 1) so close to their sun as to be tidally locked into a permanent dayside or nightside
  
  2) or "free range" so as have no sun and be in permanent night.

Guest wrote:What would the night sky look like from a planet orbiting one of the center stars?

neufer wrote:There would be dozens of stars brighter than the brightest historical supernova
and perhaps a thousand stars brighter than Venus.

Guest wrote: Sounds like it would be hard to sleep at night

neufer wrote:Probably not much brighter than a full moon.