This space telescope can see much longer IR than 14um

[Ben321]

It's called Spitzer and I remember reading that it can see all the way to 100um. That's a VERY long wavelength. I've seen pictures of the telescope and I've read that that large flat piece of metal on one side of it is a light reflector. The actual IR imager needs to be kept very cold and any direct sunlight hitting the main body of the telescope will raise the imager to a temperature too  high for it to work at the intended wavelengths. So that large metal plate reflects away all the sunlight to keep the sun from heating the telescope. It's always positioned so that it is between the sun and the telescope's main body.

I wonder how cold a microbolometer array needs to be kept though to successfully image at 100um? And I wonder what material the microbolometer array is made of on that telescope? It's hard to find such technical specs on NASA tech though (probably national security issue).

[Vipitis]

You can image beyond 14µm. The FIR part of the spectrum 14µm - 1mm(different categorizations exist) is not found in any consumer products. But also Microwave 1mm - 1m can be imaged as well as radio 1m+(but you are entering single pixel territory, so you need scanning or other tricks to "image")

Most of the infrared detector research was founded by the US military, but not all of it is Classified. Spitzer for example has 3 sensor modules with multiple sensors each. You can find very detailed descriptions about them via Caltech, as they developed said sensors for NASA. I can link you some resources meant for astrophysicists who work with the data, so it goes in depth about calibration but also has a section about the detectors themselves.

https://irsa.ipac.caltech.edu/data/SPITZER/docs/irac/iracinstrumenthandbook/IRAC_Instrument_Handbook.pdf

https://irsa.ipac.caltech.edu/data/SPITZER/docs/mips/mipsinstrumenthandbook/MIPS_Instrument_Handbook.pdf

https://irsa.ipac.caltech.edu/data/SPITZER/docs/irs/irsinstrumenthandbook/IRS_Instrument_Handbook.pdf

You won't have a good time reading these documents, but the internet has answers when you look for it. There are more IR telescopes and even some ground based instruments. SOFIA is also hosting lot's of IR instruments and those are more modern.

E#1: number

[Ultrapurple]

@Vipitis - a good, thorough answer as usual - thank you. And to some extent you're right about "entering single pixel territory" with the longer wavelengths. In my mind I considered a 640x480 sensor for 1m wavelengths - think in terms of an array the size of 36 football pitches. The lens would be interesting too!

(Actually, such things really do exist, for radio astronomy, satellite / meteor / ICBM surveillance, and the 'lens' is formed by using a combination of 'directional pixels' (typically Yagi antennas or dishes) and careful attention to the phase of received signals. Ask your neighbourhood radio astronomy expert for more details).

[Vipitis]

ALMA can be up to 66 atennas in an array reaching 14km but only meant for 35μm to 10mm wavelengths. Interferometry goes beyond just synthetic aperture, as the earth rotates and orbits the sun.
FAST goes up to 4.3m wavelength and is 500 meters across(multiple football fields).

[Ultrapurple]

On a more serious note, I recall seeing a microwave power meter for something like 75-100GHz. It was basically a short piece of waveguide, open one end and with the other terminated in a sensitive bolometer. The seller demonstrated how well it worked by waving their hand in front of the aperture - there was enough VLIR radiation coming off their hand that the rather sensitive bolometer was easily able to detect it. (I'm not saying the radiation was necessarily in the 75-100GHz range, just that the detector was sensitive to something that was being radiated by the hand).

[Ben321]

You can image beyond 14µm. The FIR part of the spectrum 1µm - 1mm is not found in any consumer products.

A CCD or CMOS camera without an IR blocking filter, and instead using an IR passing filter (such as a 950nm long-pass filter) is capable of seeing wavelengths up to about 1.1um (at which point its sensitivity has dropped to about 1% of its maximum sensitivity). You of course need to turn up the gain and/or exposure settings in such a case, because silicon based sensors aren't very sensitive at these wavelengths. So yes, it is indeed possible to get the longest NIR wavelengths that are also considered to by the shortest SWIR wavelengths.

I notice you call 1um part of the FIR spectrum. However, I think that FIR refers to all wavelengths longer than LWIR. So basically FIR is longer than 14um. In fact, I think that that Spitzer space telescope is designed to see such long wavelengths. I remember reading about it a few years ago, and it was described as being designed for FIR imaging, which was why it needed that solar shield.

[DaJMasta]

It also seems normal for space telescopes to have some capability beyond LWIR - the Webb telescope can see into the visible spectrum up to about 600nm, but then down to about 28.5um - and this is basically because this capability is not foreign to even the sensors that are used in other applications.... but the CO2 and H2O in the atmosphere effectively blocks 14um+ wavelength for a ways, so it's never going to be in the usable specification for an in-atmosphere camera.

[Ben321]

It also seems normal for space telescopes to have some capability beyond LWIR - the Webb telescope can see into the visible spectrum up to about 600nm, but then down to about 28.5um - and this is basically because this capability is not foreign to even the sensors that are used in other applications.... but the CO2 and H2O in the atmosphere effectively blocks 14um+ wavelength for a ways, so it's never going to be in the usable specification for an in-atmosphere camera.

As mentioned in another thread, it is possible to get above 14um for military thermal scopes. So, at least for short enough distances, it is possible to see above that, assuming the LWIR-passing lens is made out of a material that doesn't block wavelengths longer than 14um. I wonder how many miles away you could see using a band from 20um to 30um, in the air?

[Max Planck]

Because of carbon dioxide contained in the air, it is pretty useles to operate infrared cameras in the atmosphere beyond 14-15 um. 15um is the second strongest CO2 attenuation band.

http://www.sarracenia.com/astronomy/remotesensing/physics060.html

It is also worth to note, that astronomers are using a different IR spectrum band naming scheme than thermographers. For example the mid-infrared is the 5um to 25um (sometimes 40um).

Max 

[Fraser]

Somewhat off topic, but I would love a thermal telescope like those pictured in the attachments for my FLIR SC4000 MWIR camera

Things of beauty. The 'small' 10" telescope has an SC4000 or SC6000 hanging off the back of it. The 20" telescope is for a military customer.

Pictures come from here: https://www.rcopticalsystems.com/telescopes/10military.html

Fraser

[SilverSolder]

^looks like pretty serious kit!

[CatalinaWOW]

Be very cautious with band names.  Different countries and different disciplines have different abbreviations and different interpretations of the abbreviations.  These have also changed over time, though in many places they have stabilized somewhat over the last twenty years.

If it really matters to you, be sure to check band edges for the equipment or document you are working with.

The practice of various military applications to classify information is a silly excuse for not doing your homework.  Almost all the information about operations in any band is widely available.  The are bookshelves full of reference books, published performance characteristics of components and systems and literally thousands of research and survey papers.  Classification usually applies only to specific characteristics of specific equipment.  Things like actual band edges, operating frequencies and sensitivity.  You can actually find fairly detailed descriptions of how to build and operate military systems published by the US military.

Commercial barriers to information are far more limiting.  Strangely enough people want to be paid for their hard work.  But if you are willing to pay the bill they are more than happy to work with you.  This goes for everything from textbooks all the way up to personalized instruction.

[Vipitis]

correcting my mistake above.

Those tracking scope must be real fun, sometimes you see their footage on the SpaceX streams. I have no doubt you will eventually get across a bargain for something like this.

[Spirit532]

Cameras that see into the THz range don't operate as direct microbolometers. They add another step onto this Pile Of Detection™ - RF(ish) to heat conversion, directly on the pixel. They do this by depositing an antenna on each pixel, with what effectively is a dead short across it. The radiation is then focused onto the sensor(believe it or not, you can focus it using some polymers!), at which point your sensor isn't looking at emitted heat, but rather an "afterimage" of what the RF is heating up on the sensor. Essentially, operating in what normally is a fault mode, where a hot object leaves a long lasting(minutes) mark on the sensor.

[Ben321]

Essentially, operating in what normally is a fault mode, where a hot object leaves a long lasting(minutes) mark on the sensor.

If the mark lasts for minutes, then the camera's frame rate is VERY low. That's useless, especially if it's to be used for THz imaging of airplane passengers at an airport to check for weapons. I assume the heat generated by that antenna is actually transferred to the microbolometer by its normal thermal image function, except that instead of being heated by IR photons, it's heated by direct contact transfer with the RF antenna.

[Ben321]

Because of carbon dioxide contained in the air, it is pretty useles to operate infrared cameras in the atmosphere beyond 14-15 um. 15um is the second strongest CO2 attenuation band.

Not entirely true. The military does use wavelengths longer than 14um, to see through thick-particle smoke from smoke grenades that has particles thicker than normal combustion smoke. Yes, CO2 may be a problem of looking through tens of miles of air up into space, but at the distances in a typical military engagement where you need to see enemy troops to shoot at them, you are talking about maybe up to a mile away for a sniper rifle if it's being used by an expert sniper. Enemy troops will likely conceal their exact positions by using smoke grenades, that normal thermal imagers (which see up to 14um) CANNOT see through. If that sniper is using a thermal IR scope on his rifle, it will have to be able to see over 14um (probably up into the 20um to 30um range). And at such short distances (only 1 mile away) CO2 absorption will have very little impact. So yes, such VLWIR imagers do exist for military purposes. So I've got a couple quesetions. Do such beyond-14um thermal imagers also exist in the civilian market? And if so, what companies sell them, and what material are they using for the lens (because I know germanium only works up to 14um)?

[Max Planck]

I'm affraid the source of confusion are some manufacturers advertising their cameras operating in the 12-15um (or similar) band as VLWIR. There is really no need to be a military user to get an MCT "VLWIR" camera, i.e. operating above 14um (up to 15um). Regarding atmosphere transmittance, there is no need of miles and miles of optical path length to get a strong absorption. You can check it easily using a database, such as Hitran.

Max