Astronomy and Light Gathering

[GarrisonChisholm]

I started thinking about analyzing light from distant stars ([spectroscopy] comments took me to wikipedia and the Extremely Large Telescope and the canceled-but-brilliantly-backronymed Overwhelmingly Large Telescope), and realized it all came down to photons.  Which got me to think-  if one were analyzing the atmosphere of ProximaB 4 light-years away, how many photons are you *actually* talking about that pass through the detector each second?  Millions?  Thousands?

Furthermore, how would this change, if I were looking at a - say supernova in the Andromeda Galaxy 250,000 times farther away?  My layman's thinking of light says the farther away you get - like a fire hose- the less light is emitted at an angle which touches your detector.

Do astronomers count, or care about, photons?  - & do you need a minimal quantity to do spectroscopy, or (i wouldn't believe it) is *one* photon enough?

[Steel]

At the end of the day, all astronomy boils down to getting photons onto a photographic plate or CCD. In that regard - and to greatly simplifying an entire field of study into one sentence - to successfully do any astronomy you need enough photons that you can distinguish what you're observing above background noise.

How many that is depends hugely on what you're observing, where you're observing, the quality of instrumentation and about a thousand other factors. Of course, the more accurate you want your measurement, the more photons you want, so there is no real answer to how many you need other than "as many as possible".

Edited May 15, 2017 by Steel

[Green Baron]

9 hours ago, GarrisonChisholm said:

I started thinking about analyzing light from distant stars ([spectroscopy] comments took me to wikipedia and the Extremely Large Telescope and the canceled-but-brilliantly-backronymed Overwhelmingly Large Telescope), and realized it all came down to photons.  Which got me to think-  if one were analyzing the atmosphere of ProximaB 4 light-years away, how many photons are you *actually* talking about that pass through the detector each second?  Millions?  Thousands?

An amateur chip already has pretty a decent quantum efficiency (that's what your looking for i think :-)). In principle a single photon triggers a current, but the chip itself as well as the amplifier/electronics behind produce noise. The art is to part the noise from the signal.

The overwhelmingly (ridiculously and subsequent ludicrously) large telescope will not be built because it makes no sense right now (and money matters too). We are technologically not able to actually use its potential. When the E-ELT was planned it wasn't even sure whether the aperture could be transformed into resolution, but advances in active and adaptive optics where expected. And so it came, it is now expected that the diffraction limit can actually be achieved.

I can tell you that with a simple 12cm aperture amateur telescope (apo refractor) without any gizmos one is limited rather by the atmospheric conditions than the equipment.

The atmosphere of an extraterrestrial planet cannot be observed directly with current generation telescopes. What can be measured is the light that passes through such an atmosphere (if it was present) when the planet passes in front of the star. Few actually do us the favour, but it has been tried with some. How many photons you collect depends on the aperture, exposure time and brightness of the object. The big fat GTC needs less for a decent signal/noise ratio than my poor little tube with a glass bottle bottom ....

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Furthermore, how would this change, if I were looking at a - say supernova in the Andromeda Galaxy 250,000 times farther away?  My layman's thinking of light says the farther away you get - like a fire hose- the less light is emitted at an angle which touches your detector.

Inverse square of distance applies. But a supernova in M31 will so bright that youll see it with the eye. I guess you can see M31 itself (in summer again), here this is no problem. The detector is perpendicular to the path of the rays, even a slight tilt would get you a nasty distortion in the picture.

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Do astronomers count, or care about, photons?  - & do you need a minimal quantity to do spectroscopy, or (i wouldn't believe it) is *one* photon enough?

No and yes. It's not the number of photons, it is the signal to noise ratio that is the interesting thing to practical astronomy. Astronomers themselves don't care about th earthly subtleties, they submit an application, if it is accepted a technician programs the telescope, the telescope does its work and if it screeches another technician leaves a few drops of oil in the cogs :-)

btw. you can visit the websites of the telescopes around the world, i have seen online application forms or at leasts calls for applications for a season or period for the instrument. The GTC, ALMA, the VLT might be nice places to start ...

 

Edit: i totally forgot to tell about focal ratio. This is of course and by far the most important indicator for the light gathering capabilities of any optical device ! E-ELT primary mirror will have 0.9 (but other foci like cassegrain, nasmyth may be possible) ! Which is one mind boggling photon collector. The GTC is actually not that sporty. My stuff has 7 or 5.8 depending on mods :-) Others here lie between 4 and 10. Old world amateur telescopes usually where around 10 - 30.

 

Edited May 16, 2017 by Green Baron

[Green Baron]

Spectroscopy is a wide field. For an amateur and the visual spectrum there exist echelle, prism or slit spectroscopes to part the light into its wavelengths by refraction or interference. The recording instrument usually is a photographic chip, a ccd is more sensitive and the electronics more sophisticated than a consumer type dslr, that's why they have their prices though they are comparably simple. An echelle ("ladder") or prism let more light pass than a slit so exposure times on the recording instrument are shorter. Nevertheless, if you want a decent bright spectrum you must wait several times as long as if taking a photo of the whole spectrum at once, or a single colour like R, G or B.

How many photons ? As @Steel said, the more the better ...

 

[_Augustus_]

5 hours ago, Green Baron said:

I can tell you that with a simple 12cm aperture amateur telescope (apo refractor) without any gizmos one is limited rather by the atmospheric conditions than the equipment.

Edit: i totally forgot to tell about focal ratio. This is of course and by far the most important indicator for the light gathering capabilities of any optical device ! E-ELT primary mirror will have 0.9 (but other foci like cassegrain, nasmyth may be possible) ! Which is one mind boggling photon collector. The GTC is actually not that sporty. My stuff has 7 or 5.8 depending on mods :-) Others here lie between 4 and 10. Old world amateur telescopes usually where around 10 - 30.

 

But for photography one doesn't necessarily need adaptive optics/a space telescope. Lucky imaging by taking video and then stacking it can overcome atmospheric conditions.

[Green Baron]

2 minutes ago, _Augustus_ said:

But for photography one doesn't necessarily need adaptive optics/a space telescope. Lucky imaging by taking video and then stacking it can overcome atmospheric conditions.

Amateurs certainly not. Though the first active systems are coming on the market. But to actually use the capabilities of a professional telescope yep, otherwise they'd always be limited to atmospheric conditions (comparable to a very nice 20-30cm telescope under good conditions).

Discoveries of the last years were only possible through the use of active and now as well adaptive optics. Space telescopes play a role as well, but they aren't that many. The bulk is handled by terrestrial telescopes and that won't change in the foreseeable future.

 

[GarrisonChisholm]

This is a fascinating conversation, and boy would I love working at a telescope if it were practical (it is not; children, life, et.al. ), but I think my very esoteric question doesn't seem like it can be answered.  It doesn't seem there is a "photons/sec" metric for telescopes, one simply continues gathering light until you have gathered enough, whether that is a 12 minute exposure in the back yard or a 40 day exposure like Hubble Deep Field.

Concerning KSP and astronomy, I added a mod that places a star system 10 trillion km away, and then didn't add any tech to my game that would let me explore it-  and just "looking" in the tracking station didn't feel right. My solution?  I orbited a massive "radio telescope" to posit that I *could* see the detail in the tracking center.

I really look forward to JWST's first assessment of the local stellar neighborhood.  If only somewhere we find a single pixel of the color green...

[YNM]

We care about numbers - but if your numbers of photons are similar across the field, what for ? (well yes unless you're doing something like that ex. CMB or anything.)

It's a combination of intensity/flux of light ("brightness") and signal-to-noise ratio, SNR ("contrast"). Though, generally, low brightness means less SNR, but there are points where adding more frames is just not worth it.

Edited May 16, 2017 by YNM

[Green Baron]

E-ELT construction has just begun. The mountain was already decapitated and levelled.

Site with webcam: http://www.eso.org/public/teles-instr/elt/

TMT hurry up !

[YNM]

7 hours ago, Green Baron said:

E-ELT construction has just begun. The mountain was already decapitated and levelled.

TMT hurry up !

I guess the Hawaiians won't be happy at all... it was supposed to be limited number of telescopes only ! The Andes, however... plenty of it.

[Green Baron]

It's still in court. Last year they said that they want a decision until May this year or they'd go to La Palma. But then the term was extended until next April (2018). Meanwhile some parts are being made, like mirrors, instruments, material science, tools, control units,  .... This can take many years until all the research is finished and results can be transformed into something that makes a noise when it is dropped (like a tank lid, NASA :-)).

As far as i am concerned they can leave the peak of Mauna Kea to the deities and spirits and build that popeye right here. I would of course report from the construction site if the forum is still running by then.

First light for E-ELT is planned for 2024.

 

[Green Baron]

... don't know where to put this and i don't want to disrupt dedicated threads on exoplanets. Here is a nice and comprehensible summary of the current state of exoplanet research. While reading it my coffee got cold:

http://www.sciencemag.org/news/2016/07/forbidden-planets-understanding-alien-worlds-once-thought-impossible