williyamshoe
commented
5 years ago I double checked the calculations by comparing the derived LST from the MJD on the website and the real LST found on the website. I get approximately the same answer. I also derived an equation that converted D into GMST then into LST, and got approximately the same equation as yours.
However, when I calculate the hour-angles for each exposure, they seem to range from [-60, 80], not [-60, 60]. However, it is worth noting that only a small fraction of exposures are above 60.
This is the code I have for adding the hour-angles. Is there anything wrong with it?
D = exposures["MJD"] - 51544.5
LST = (168.86072948111115 + 360.98564736628623 * D) % 360
exposures["HOURANGLE"] = LST - exposures["RA"]
def change_range(i):
if i > 180:
return i - 360
if i < -180:
return 360 + i
return i
exposures["HOURANGLE"] = [change_range(i) for i in exposures["HOURANGLE"]]
sbailey
Add code for calculating the "hour angle" of each exposure, which is basically a measure of how far off you are from observing each tile as it passes through the meridian going directly overhead from south to north. Background:
The "local sidereal time" (LST) is a measure of time relative to the distant stars instead of to the sun, i.e. LST depends upon the longitude of where you are on earth and what is overhead right now. The hour angle of a particular exposure = LST - RA, where RA is the Right Ascension of wherever the telescope is pointing.
Detailed routines are available in astropy, but the approximations at https://aa.usno.navy.mil/faq/docs/GAST.php are good enough for here. Converting units from hours to degrees, using MJD instead of JD, and using the longitude of Kitt Peak, I get:
Please double check me on those numbers, cross checking to http://www-kpno.kpno.noao.edu/Info/Mtn_Weather/ while noting that MJD = JD - 2400000.5 .
Calculate the hour angle for every exposure and include histograms of that on the summary page and timeseries and histograms on the nightly pages. Coordinate with @ana-lyons and issue #24 for adding placeholders for those plots while @williyamshoe works on adding the calculations to enable those plots.
Watch out for the RA and LST wraparound around 0/360. The hour angle of a DESI exposure should always be between [-60, 60], not something like 359.
Old-school "hour angles" are measured in hours (thus the name...) but let's do everything in degrees.