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 14-16
Lecture 14+
| Question | Answer |
|---|---|
| Space telescopes need support | no need to compensate for Earth's rotation or torque induced gravity but constant orbital motions cause complications. Extreme resolution requires extreme pointing/tracking accuracy |
| Let the observer at O be at a location of 60 degrees north latitude | The the altitude/elevation of the North Celestial pole will also be 60 degrees. This holds true for any latitude on Earth. The observer will watch stars trace out circles of constant declination centered on the N Celestial pole |
| At a location of 60 degrees north latitude, which stars will never set | any star with declination higher than ns (Northern horizon) They are circumpolar stars |
| stars with declination between nr and ns (Northern horizons both in Northern and Southern hemisphere?) | will rise and set and the observer will only witness circular arcs centered on the NCP. They will transit across the observer's celestial meridian |
| The observer at location of 60 degrees N latitude will never see which stars | stars with declination lower than nr, and will only barely see stars with declinations near nr rise above the horizon. |
| A sidereal day measures the | time taken for Earth to undergo a full rotation with respect to stars (23 hours, 56 minutes)=360 degrees |
| A solar day measures the | time for Earth to rotate with respect to the sun (24 hours)=261 degrees |
| Solar and sidereal days differ because | Earth's progress in orbit around the sun means it has to rotate a little bit more for the sun to return to the same point from our perspective |
| A sidereal zero point is given by when | the vernal equinox (RA=0) crosses the observers meridian then, sidereal day=time between meridian transits of vernal equinox |
| The North Celestial Pole (and thus equatorial coordinates of objects in the sky) | slowly changes over time due to precession |
| Distant objects can have their coordinates change due to | proper motion, precession and nutation |
| During precession, the | ecliptic plane remains fixed, but the celestial equator follows the precession of the NCP |
| Over a 26000 year period, the NCP | traces out a circle centered on the North Ecliptic Pole and the vernal equinox shifts along the ecliptic plane |
| Nutation is a shorter term effect, arising from | an 18.6 year oscillatory pattern. During this cycle, the vernal equinox oscillates by around 9.21 arcsec ahead of and behind the precessional position. This is nutation in longitude |
| Nutation in obliquity | changes the angle between the celestial equator and the ecliptic by + or - 6.86 arcsec |
| Why Barycentric? | If one defines coordinates using a transit telescope, the coordinates will be defined in a non-inertial reference frame |
| If using a transit telescope | Earth's spin and orbital motion mean that the exact coordinates of an object depend on when and where they were defined. Effects such as stellar parallax and aberration of starlight make these values uncertain |
| Aberration | causes the apparent position of an object to appear up to 20.5 arcsec offset |
| ICRS | International coordinate reference system is barycentric, with the x axis roughly coincident with the vernal equinox in epoch J2000 (the location of the vernal equinox at 12:00UT on 1 January 2000) |
| ICRS y and z axes | approximately align with the Earth's equatorial plane in J2000 |
| Radio reference frames | ICRS. The exact directions of the axes were first determined through very long baseline radio interferometry of distance radio sources (quasars) |
| Optical reference frames | ICRS. The system is defined for optical sources originally using sources observed with the Hipparcus satellite |
| Objects in the Solar System are close enough that we can use | radar to bounce radio signals off of them and use the time for the return signal to infer the distance to an object |
| To measure the distance to the sun | we wait until Venus is at greatest elongation- the point where its angular separation from the sun is a maximum and forms a right triangle with Earth |
| An object moving away from an observer will | have its spectral features redshifted towards longer wavelengths |
| An object moving toward an observer will | have its spectral features blueshifted towards shorter wavelengths |
| The universe is expanding, with one consequence being that | at cosmological distances, objects that are further away are moving away faster than more nearby objects. As we look at the more and more distant universe, we see that galaxies are moving faster and faster away from us according to Hubble's law. |
| Early attempts to create positionally based designations were based on | the positions catalogued by Tycho Brahe in the late 1500s |
| Nx X Ny full array is | the digital image/frame/exposure, R |
| R | the digital representation of the full array of pixel responses, r |
| To form a human-interpretable image, we map | the digital image to a colour scale and apply stretch (log, power) and scaling (setting max/min values) to highlight features of interest |
| r[x,y] is converted to | the pixel value R[x,y] |
| In reality, R is a combination of the source signal as well as intervening effects arising from | interstellar light, atmospheric/environmental effects, telescope effects, and detector effects |
| Non-linear effects can also come into play at | low or high signal levels, e.g. saturation arising from a chosen gain |
| If these additive, multiplicative, or non-linear effects affect each pixel consistently, predictably, and repeatedly, then | they are relatively easy to correct for with digital image manipulation |
| Digital image manipulation techniques | underpin all of modern astronomical imaging. Commercial image manipulation software essentially apply these techniques under the hood of a fancy user interface. These techniques are also very applicable to other fields |
| Image manipulation is essentially applying | image arithmetic or linear algebra to your image array |
Created by:
user-1996284