Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
Lecture 11
Lecture 11, 12, 13
| Question | Answer |
|---|---|
| Photographic techniques using photosensitive emulsions were | particularly inefficient, resulting in only 0.5-5% of incident photons being effectively recorded. |
| Modern solid-state detectors can reach up to | 95% efficiency of photons that make it to the detector. Filters, mirrors, absorption, etc can all remove photons from the source before they make it to the detector |
| In practice, there is some | Poissonian uncertainty in counting photons incident on a detector |
| Poisson statistics describes | the probability that a given number of events will occur in a given amount of time. |
| A real detector will detect fewer than | 100% of incident photons, have extra sources of noise or both. This results in a less than perfect SNR |
| SWIR | short wavelength IR |
| MWIR | medium wavelength IR |
| LWIR | long wavelength IR |
| VLWIR | very long wavelength IR |
| In the near infrared, the most common type of detectors are made of | indium antimondide, or a combination of mercury-cadmium-telluride (MCT). Both types can reach into the MIR but the properties of MCT can be variable |
| Modern cameras are based on | complementary metal-oxide semi-conductor (CMOS) technology, and are gradually replacing CCDs in astronomical detectors |
| Current IR detectors combine | Si-based CMOS technology for readout and IR sensitive photodiodes in a back-illuminated design |
| Current IR detectors | incident photons illuminate the InSb- or MCT-based photodiodes, which are deposited/ grown on a stronger IR transparent substrate. Electrodes are attached to the diodes |
| IR detectors, Separately the CMOS readout array is | manufactured, and output electrodes are attached via "indium dots" |
| IR detectors: since each pixel | attaches to its own CMOS readout unit, there is no inter-pixel crosstalk, and saturation doesn't result in blooming |
| IR background levels in detectors are | high, so exposure times must be kept short. Often several readouts will be read into memory while continuing an exposure and then combined into a single image read out to permanent memory. |
| In IR detectors what is a notorious problem | Dark current if cooling is not adequate |
| For IR, atmospheric transparency drops to zero beyond | wavelengths around 25 micrometers and is only marginally transparent (during very dry conditions) shorter than this |
| Thermal IR (MIR-FIR, wavelength greater than or around 3 micrometers) requires | cooling below 77K, so very few large format detectors exist on the ground. Most common type at these wavelengths are antimony or arcsnic doped silicon detectors (Si:Sb, Si:Ar on JWST/MIRI) |
| At all wavelengths, but especially long wavelength IR | bolometers use a thermistor |
| Thermistor | is a combination of a thermometer and an absorber, such that incident photons are absorbed, which heats the device up. The temperature change then follows the number of incident photons |
| TAC | Time allocation committees receive, read, deliberate and rank proposals. If ranked high enough you're awarded observation time |
| Design observations: seeing | how fine a resolution do you need in your images/spectra? |
| Design observations: cloud cover | how much signal can you afford to lose to clouds (large telescopes can observe through thin cirrus)? What accuracy do you need in your final images/spectra |
| Design observations: moonlight | what wavelengths are you observing in? Can you handle a full moon (NIR) or do you need dark time (optical) |
| Design observations: water vapour | how dry do you need the conditions to be? NIR needs dry conditions, but optical doesn't matter |
| Classical observing | One would be assigned a collection of set nights/half-nights to carry out your observations, physically at the telescope. Weather was up to luck, if you got clouded out try again next year. |
| In general, we always want to minimize the airmass, and observe targets when they're nearest maximum elevation | This means trying to average out their air masses, catching them on their way DOWN at the beginning of the night (setting targets) or on their way UP at the end (rising targets) |
| We want to schedule high priority targets at | optimal times (prioritize their airmass ahead of lower priority targets) |
| Which only two observations can be done in moonlight | red-optical and near IR |
| The night plan also takes into account | that some programs have calibrators that need to be done right before/after the science observations. Use all of the time possible, even twilight can be useful to observe calibrations, standard stars, or even some near IR targets. |
| Targets of opportunity | SNe, transient phenomena almost always take precedence. |
| IRAF | image reduction and analysis facility was the long time standard but is quickly fading away |
| IDL | Interactive Data Language is used by some. Good community support, but expensive. |
| Python | is by far the most common language used for astronomical data analysis now. Many packages exist and are still being developed. Astropy, photultils, specutils,matplotlib. ALL JWST data is reduced by python in Space Telescope Science Institute. |
| Why are modern telescopes built where they are | the higher up the less atmosphere to look through, remote sites away from urban centers reduce light pollution and have darker skies, high altitude windward of flat space provides smooth non turbulent airflow, dry locations=fewer clouds less water vapour |
| On September 14, 2015 Einstein's prediction | of the existence of gravitational waves was verified |
| Advanced LIGO | the world's first successful gravitational wave detector |
| We can consider objects on the sky to be | in projection so that any coordinate corresponding to depth is superfluous to locating where an object is on sky |
| At any point within a sphere, we can define | planes (or great circles) passing through the center, and define coordinates on the surface by the spherical angles between the planes |
| Meridians | Planes perpendicular to the equator necessarily pass through the poles |
| By connecting three sides, each of which is defined by segments of the planar edges, we can define | a spherical triangle |
| celestial equator is | the plane of earth's equator projected onto the celestial sphere |
| The ecliptic | is the plane of the solar system projected onto the celestial sphere |
| equatorial system: lines drawn in either direction perpendicular to the plane of the celestial equator point towards | the North and South celestial poles. This then sets up a model in which, rather than Earth rotating, the entire celestial sphere rotates around Earth |
| Equatorial system | Under this model, since celestial objects are pinned (more or less) to the celestial sphere, they follow circular paths on the sky centered on one of the celestial poles, and travelling from East to West |
Created by:
user-1996284