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Cosmological Problems



 
 
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  #11  
Old December 17th 18, 09:25 PM posted to sci.astro.research
Phillip Helbig
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Posts: 33
Default Cosmological Problems

In article , (Steve
Willner) writes:

[Moderator's note: The James Webb Space Telescope is more like a
traditional telescope in space, the successor to HST in some sense, but
with more emphasis on the infrared.


JWST wavelength range is roughly 0.6 to 27 microns


Visible light is about 0.4 to 0.7 microns (400 to 700 nm (nanometers),
4000 to 7000 Å (Ångström)), so there is a bit of overlap between HST and
JWST. JWST is more or less a normal reflecting telescope, with a CCD as
detector.

The CMB is observed at lower frequencies


much longer wavelengths than JWST.


You can easily search for "Planck focal plane" on the web and find an
image showing lots of horns and other radio-astronomy stuff. Planck has
a wide frequency range, with frequencies from 30 GHz to 857 GHz,
corresponding to wavelengths between a centimetre and about a third of a
millimetre, the latter being about 300 microns. Typical traditional
ground-based radio astronomy is in the GHz range and below, so
wavelengths from centimetres to metres.

All ground-based and balloon telescopes study relatively high
multipoles, i.e., relatively small angular scales.


To do the large angular scales, one has to observe a large part of the
sky.

I think it's only
the lower multipoles that carry information on H, but I may be wrong.


Here are some movies where one can get a feel for how changing a
parameter changes the CMB power spectrum:

https://space.mit.edu/home/tegmark/movies.html

In the plots, as usual, larger angular scales are on the left, smaller
ones on the right.

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  #12  
Old January 2nd 19, 07:09 AM posted to sci.astro.research
Steve Willner
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Posts: 1,161
Default Cosmological Problems

In article ,
(Phillip Helbig (undress to reply)) writes:
Visible light is about 0.4 to 0.7 microns (400 to 700 nm (nanometers),


That range is what the human eye can see. In practice, the term
"visible" often refers to light detectable by instrumentation
suitable for visible light, say from 300 nm (the atmospheric cutoff)
to 1000 nm (the intrinsic silicon limit).

there is a bit of overlap between HST and JWST.


JWST's short-wavelength limit is 600 nm. Its prime range is roughly
1000 to 3000 nm, and the long limit is 28000 nm (=28 microns).

JWST is more or less a normal reflecting telescope, with a CCD as
detector.


Detectors are actually infrared hybrid arrays, not CCDs. They are
based on the "HAWAII-2RG" technology:
http://www.teledyne-si.com/products-...d-fpa-products

You can easily search for "Planck focal plane" on the web and find an
image showing lots of horns and other radio-astronomy stuff. Planck has


I think "had" for that last word above.

a wide frequency range, with frequencies from 30 GHz to 857 GHz,
corresponding to wavelengths between a centimetre and about a third of a
millimetre, the latter being about 300 microns. Typical traditional
ground-based radio astronomy is in the GHz range and below, so
wavelengths from centimetres to metres.


Frequencies up to 15 GHz (wavelength 2 cm) were pretty common even
when I was in school. Nowadays, the VLA
https://public.nrao.edu/telescopes/vla/
makes images up to 50 GHz, and ALMA
https://public.nrao.edu/telescopes/alma/
goes up to 950 GHz (though I don't think the highest frequencies are
100% operational yet).

https://space.mit.edu/home/tegmark/movies.html
In the plots, as usual, larger angular scales are on the left, smaller
ones on the right.


Yes, very nice.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #14  
Old February 5th 19, 03:44 PM posted to sci.astro.research
Steve Willner
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Posts: 1,161
Default Cosmological Problems

I wrote:
In practice, the term
"visible" often refers to light detectable by instrumentation
suitable for visible light, say from 300 nm (the atmospheric cutoff)
to 1000 nm (the intrinsic silicon limit).


In article ,
Jos Bergervoet writes:
(In the same fashion, this silicon is a 'metal' of course!)


Heh. (Silicon is a semiconductor, not a metal, for anyone who is
confused.) I reported how the language is used in practice. Human
language is not always logical. I don't think I've seen silicon
described as a metal, but it wouldn't shock me. Actually, come to
think of it, doesn't silicon become a metal at very high pressure?

950 GHz sounds like an interesting LNA design problem!
Is there any pointer to the solutions they use? (Other metals than
silicon, undoubtedly..)


There must be some design documents, but I don't know where. There
should also be descriptions in the literature. Try an ADS search.

My courses in radio astronomy were a long time ago, and the
technology has changed. I don't think there is any amplification at
the incoming frequency, though. In most radio telescopes, the signal
is mixed down to an intermediate frequency and amplified there. In
the old days, they would have used klystrons or something, but I
doubt they do now.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #15  
Old February 11th 19, 07:11 PM posted to sci.astro.research
Edward Prochak
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Posts: 3
Default Cosmological Problems

[[Mod. note -- I apologise for the delay in posting this article
(the delay was caused by a mistake on my part). This article was
received by the s.a.r moderation system on 2019-02-06.
-- jt]]

On Tuesday, February 5, 2019 at 10:44:44 AM UTC-5, Steve Willner wrote:
I wrote:
In practice, the term
"visible" often refers to light detectable by instrumentation
suitable for visible light, say from 300 nm (the atmospheric cutoff)
to 1000 nm (the intrinsic silicon limit).


In article ,
Jos Bergervoet writes:
(In the same fashion, this silicon is a 'metal' of course!)


Heh. (Silicon is a semiconductor, not a metal, for anyone who is
confused.) I reported how the language is used in practice. Human
language is not always logical. I don't think I've seen silicon
described as a metal, but it wouldn't shock me. Actually, come to
think of it, doesn't silicon become a metal at very high pressure?


I suspect Jos was hinting at the astro view of metals,
IOW, there is hydrogen and helium, while the rest are metals.

Ed
 




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