1. “ Who will I blame my mistakes on. ” Dr
“ Who will I blame my mistakes on?” Dr. Forrester on the death of his assistant Mystery Science Theater OAAC meeting Tuesday, Sept. 5 at 6:30pm in Kemper 101. Updated reading list. HW1 on-line now. Due next Wednesday.

2. Time. UT: The same as GMT. No daylight savings.
JD: continuous days since Jan. 1, 4713 BC with noon as the start of the day.
MJD: modified JD. Subtracts 2,400,000.5 days from JD.

3. When their signals reach us is important.
Time is relative
Many celestial bodies (stars, transits, AGNs, etc.) undergo variations.
When their signals reach us is important.
But we are in motion!

4. Earth’s orbit is ~16 light minutes across
Earth’s orbit is ~16 light minutes across. If we keep time in Earth’s frame, then astronomical events all have a period variation of 16 minutes per year.

5. Time is relative Time systems:
Earth-Sun distance is about 8 light minutes.
Heliocentric time corrects time to the Sun’s center.
This corrects the Earth’s orbit around the Sun.

6. But the Sun is not at the center of our solar system.

7. Barycentric time corrects to the mass center of our solar system
Barycentric time corrects to the mass center of our solar system. This can differ from HJD by up to 4 seconds.

8. Time is relative Time systems: Geocentric (Earth-based: UT, JD)
Heliocentric (Sun-based: HJD)
Barycentric (mass center: BJD)

9. Sidereal versus Solar
But because we are orbiting the Sun, a solar day is not the same as a day according to the stars.

10. A solar day is one full rotation according to the Sun.
A sidereal day is one full rotation according to the stars.

11. So a sidereal day is nearly 4 minutes shorter than a solar one.
Because the Earth is in motion around the Sun, it takes longer for the Sun to get back to the same position in our sky.
So a sidereal day is nearly 4 minutes shorter than a solar one.

12. Mercury is really freaky!
1 sidereal day = 59 Earth days,
1 solar day = 176 Earth days,
1 orbit = 88 Earth days.

14. Sidereal time
If we have solar days and sidereal days, then we can have solar time (UT) and sidereal time (ST).

15. Sidereal time Sidereal time is time according to the stars.
And in a standardized way, it corresponds to the same longitude as UT (Greenwich).
So if you're at Greenwich and it is 4am sidereal time, then stars with an RA=4 are on your meridian.
If it's 10pm, then stars with an RA=22 are on your meridian.

16. Sidereal time
But just like we don't set our house clocks to GMT, we usually use local sidereal time (LST). In that way, stars on our meridian at any time have the same RA as LST.

17. So if LST is 16:13:10, then stars on your meridian have that same RA.
Sidereal time
But just like we don't set our house clocks to GMT, we usually use local sidereal time (LST). In that way, stars on our meridian at any time have the same RA as LST.
So if LST is 16:13:10, then stars on your meridian have that same RA.

18. Hour Angle
In more common usage: It is the angle between an object and the meridian (East or West, whichever is closest).
HA = LST - RA

19. So what is the HA for a star with RA=14:32:27 at LST=19:52:48?
Hour Angle
So what is the HA for a star with RA=14:32:27 at LST=19:52:48?
HA = LST - RA

20. So what is the HA for a star with RA=14:32:27 at LST=19:52:48?
Hour Angle
So what is the HA for a star with RA=14:32:27 at LST=19:52:48?
HA = LST – RA
19:52:48 – 14:32:27 = 5:20:21 = 5.339hrs = o

21. Detectors: Important terms: Quantum efficiency (QE): amount of photons detected compared to incoming photons. Detected/incident. Linearity: a single photon makes a single detection. Dead/read time: the amount of time between images/detections. Pixel size. Limits resolution. Saturation: beyond which no signal can be measured.

22. Detectors: The eye Photographic plates Photomultiplier tubes CCDs CMOS

23. Detectors: The eye: original instrument
Detectors: The eye: original instrument. A non-linear (logrhythmic base ~2.5) instrument. Hipparchus introduced the magnitude system using the eye as a guide. This horrible system continues to this day.

24. Detectors: Photographic plates Also non-linear in sensitivity
Detectors: Photographic plates Also non-linear in sensitivity. But allowed brightnesses to be recorded.

25. Can be digitized using plate scanners.

26. Detectors: Photomultiplier tubes Single pixel, but have a linear range and extremely fast readout.

27. Detectors: Photomultiplier tubes 1, 2, or 3 channels
Detectors: Photomultiplier tubes 1, 2, or 3 channels. Target, or target & sky or target, sky, & comparison

28. Detectors: CCDs High QE, Linear over a large range.

29. CCDs Array of pixels. The pixels are shifted down and then each row is read to the left. This takes time.

30. CCDs Array of pixels. The pixels are shifted down and then each row is read to the left. This takes time. Binning. Decreases time, but increases noise and lowers resolution.

31. CCDs Array of pixels. The pixels are shifted down and then each row is read to the left. This takes time. Frame transfer: 2nd CCD

32. CCDs Arrays of CCD. Kepler array: each ‘module’ has 2 CCDs with 2 readouts.

33. CCDs LSST; 3.2 Gpix/2 seconds.

34. QE: eye~1-2%, photographic plates ~1-2%, PMT 20-40%, CCD: 70-97%, IR array 30-50%

35. Detectors: CCDs What information do you get when you take a CCD image?

36. CCDs The CCD is like several layers of information:
1) The base is the noise you get when you run electricity through an amplifier. This is called the bias level. You get this regardless of how long the exposure time is. Even an image of 0 length has this component.

37. The CCD is like several layers of information:
1) Bias
2) The 'hum' of the electronics. This is called dark current. This accumulates with time. It is a constant rate (counts/second) but the longer you expose, the more you get. To account for this, we take images without opening the shutter.

38. Some CCDs have bias strips. These are covered pixels
Some CCDs have bias strips. These are covered pixels. The advantage is that you bias+dark with every image.

39. 3) Pixel-to-pixel sensitivity variations
3) Pixel-to-pixel sensitivity variations. Some CCD pixels will accumulate counts (light) easier than others. This could be caused by a property of the pixel or because there is a speck of dust covering it. To correct this, we take 'flat' images, which are pictures of nothing (a blank screen).

40. CCD calibration: Because no 2 pixels in any 2 images (including biases) are the same, to reduce noise of calibration images, multiple calibration images are obtained and combined into a “master” frame.

41. Bias frames correct the dark frames which correct the sky frames, which correct images.