Questions tagged [units]
Use for questions about standards of measurement used for different types of quantities, and for conversions between units.
54
questions
3
votes
1
answer
142
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Conversion to Jy of Hubble WFC3 units (electrons/s) and of Spitzer IRAC (MJy/sr)
I am trying to figure out how to convert the units of some FITS files to Jy. These files refer to observations in different bands using different instruments. I list them below with my way of ...
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2
votes
1
answer
44
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Units in distance parallax relation
Relation between distance d and paralax plx is:
$$d (pc) = 1 / plx(arc)$$
When I want to express plx in mas, will it be
$$d (pc) = 1/[plx(mas)*1000]$$
or
$$d (pc) = 1000/plx(mas)$$
Thank you very much
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4
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1
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What does using these units (c=G=M=1) in MHD simulations mean?
I was reading some papers on numerical HD/MHD simulations of black hole accretion disks, and most of them scale their physical quantities to this form of units: c = G = M = 1. I know c = 1 and G = 1 ...
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vote
0
answers
36
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How do I create a comoving quantity using YT?
I can assign a unit to a scalar using:
x = YTQuantity(scalar, 'kpc')
but how would I specify that the unit should be kpccm? Perhaps it doesn't even make sense to do so? Something as simple as this is ...
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7
votes
1
answer
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What does par. lines mean in relation to a telescope aperture in 19th century German astronomical publications?
I am trying to understand what is meant by par. lines in an 1867 article "The Aberration of the fixed stars after the wave theory" by Prof. W. Klinkerfues of Royal Astronomy Works of ...
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5
votes
1
answer
704
views
Astropy units issue
hope you're all doing well.
I'm not very familiarized with AstroPy and I'm having a problem computing the conversion between angular momentum units.
If I define the ...
- 605
2
votes
1
answer
57
views
How to calculate an infrared SED from the specific intensities?
I have values of total infrared fluxes (SED) in literature expressed in Jy (Jansky) for galaxies. I want to assure that my infrared radial profiles of the same galaxies in MJy/sr (and other in Jy/...
- 23
7
votes
0
answers
326
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Convert from Jy/beam km/s to W/m$^2$
I need to compare some ALMA observations of a protoplanetary disk to disk-integrated fluxes obtained from a model.
The ALMA observations are upper limits of non-detected spectral lines, where the RMS ...
- 1,384
2
votes
0
answers
75
views
Flux Density Unit Conversion [closed]
I am confused how to convert from ${\rm cm^{-2}\,s^{-1}\,Å^{-1}}$ to ${\rm erg\,cm^{-2}\,s^{-1}\,Å^{-1}}$. How do I do that?
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1
vote
1
answer
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Rayleighs units conversion
I have a question regarding the units conversion from photon counts on a detector to Rayleighs. I am pretty new to the subject but I would like to know more.
The problem is the following :
I know that ...
1
vote
1
answer
249
views
RA expressed in degrees
Using the Python package Skyfield I thought that I could get the RA expressed in degrees simply by doing ra.degrees, but I get the following message
this angle is usually expressed in hours, not ...
3
votes
1
answer
101
views
Units and some equations
I'm a grad student and I feel like this is an extremely elementary question that I should know, but I admit that I don't. Occasionally in science I find myself solving an equation for some answer ...
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3
votes
1
answer
284
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Mathamatica vs Sympy and Astropy to solve basic astronomy problems: solving linear equations and unit conversion
I realize this may be slightly off topic, but I also believe this is probably the best way to get an answer to my question:
What is the best combination of software packages / programming languages to ...
- 299
1
vote
2
answers
105
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Unit problem regards to proton
In a textbook, we have an example:
The density of matter in the interstellar medium is of order one
proton per ${\rm cm^3}$, then the total mass of the interstellar
medium is of order (The $10^6$ ...
- 1,226
7
votes
1
answer
971
views
Converting from $\mathrm{W \: m^{-2}}$ to $\mathrm{Jy \: km \: s^{-1}}$
Suppose I observe a spectral line with rest frequency $\nu_0$ and integrated flux density $F = 10^{-26} \mathrm{W/m^2}$. Is the following line of reasoning correct?
$1 \: \mathrm{Jy} = 10^{-26} \: \...
- 95
5
votes
3
answers
2k
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What does BJD-2450000 mean?
In different papers I've encountered time indications in BJD-2450000. I know that BJD stands for Barycentric Julian Date, but I don't understand the meaning of the suffix -2450000.
This is an example ...
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1
vote
1
answer
668
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How to use spectral flux density data while using Planck's law giving spectral radiance?
I'm currently working on a fun project in my free time where I'm trying to calculate the temperature, among other things, of stars based on their spectra. Since I have essentially no prior experience ...
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4
votes
1
answer
156
views
Flux density from brightness map
This is a naive question. I have a temperature map of radio emission at 1420 MHz, in Kelvin. I want to extract the flux density (in Janskys) from an extended region of this map.
Is the flux density ($...
- 73
4
votes
2
answers
422
views
'Little h' usage in cosmological simulations
I am running a cosmological simulation and am having some trouble putting things into code units. The physical distance units in my simulation are in terms of $\text{Mpc/h}$, where $h$ is the ...
- 141
1
vote
0
answers
249
views
Units confusion while calculating flux of a star using distance and luminosity, Python
I am trying to calculate the flux of various stars in Python. I am using the information from a database containing all stars in Hipparcos, Yale Bright Star, and Gliese catalogs (almost 120,000 stars)....
user30617
0
votes
1
answer
1k
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Correct units to use while plotting mollweide projection of sky chart with RA and Dec values, matplotlib
I am trying to plot a mollweide projection of a sky chart in Python using the matplotlib package. I am using the RA and Dec values from a database containing all stars in Hipparcos, Yale Bright Star, ...
user30617
2
votes
1
answer
280
views
How to convert from Black body intensity to MJy/sr?
How does one convert from ${\rm ergs\cdot cm^{-2}\cdot s^{-1} \cdot sr^{-1}\cdot Hz^{-1}}$ to ${\rm MJy \cdot sr^{-1}}$
and from ${\rm ergs\cdot cm^{-2}\cdot s^{-1} \cdot sr^{-1}\cdot cm^{-1}}$ to ${\...
- 155
2
votes
1
answer
192
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Neutrino Modelling in Friedmann Equation
I'm trying to model neutrinos in the Friedmann Equation. I've covered the case of the Benchmark Model where we have matter, radiation, curvature, and the cosmological constant, Lambda. I know my ...
3
votes
2
answers
729
views
How to convert apparent visual magnitude to SI units?
How do I convert the apparent visual magnitude of astronomical objects to SI units (lumens)?
- 171
2
votes
1
answer
203
views
How to convert from FELO-HEL to Km/s?
I am using DS9 to calculate the rotational velocity of a galaxy. I have 21cm cube and I used DS9 to plot this graph
I do not understand what is FELO-HEL! and I want to convert the units from FELO-...
- 35
1
vote
1
answer
87
views
What is "g-98" in plots of stellar spectra?
I was looking to find some example carbon star spectra and came across this page on the Harvard website which shows several plots of carbon star spectra. The horizontal axis is labelled as wavelength ...
user24157
1
vote
1
answer
659
views
What does it mean when there are two plus-or-minus signs for a measurement?
As far as I know, the plus-or-minus sign is used to indicate the statistical margin of error of a measurement. But what does it mean when there are two plus-or-minus signs in the same measurement?
...
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1
vote
0
answers
371
views
Conversion factors for flux
I have a flux value in the infrared band of 8 mJy at a wavelength of 80 microns that I want to translate into X-ray conventional ...
- 11
11
votes
1
answer
2k
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A "strange" unit radio astronomy
I'm reading up on radio astronomy, and I came across this paper from 1964. At the bottom of page 193, the author uses a unit that I've not seen before in discussing radio power emission from stars:
...
- 2,394
0
votes
2
answers
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Conversion of UBVRIJHK magnitudes to luminosity?
So, I know that the standard for conversion from bolometric magnitude to luminosity established in 2015 equates to:
L = 10^(-0.4 * magnitude) * 3.0128e28
Easy ...
- 173
2
votes
1
answer
162
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Units in optical and radio spectral data cubes: Flux vs Brightness
I have noticed that the units used in spectral data cubes coming from optical IFU instruments are usually in $erg/s/cm^2/Ang/pixel$. To my understanding this is a measure of Flux. Right?
However, in ...
- 325
4
votes
3
answers
5k
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Converting Jy/beam to Jy?
Maybe its a dumb question, but to convert Jy/beam to Jy, I just have to multiply it by the beam size in sr right?
Being $\Omega$ the beam size:
$\Omega = \frac{\pi \theta_{maj} ~~ \theta_{min}}{4 \...
- 81
3
votes
2
answers
2k
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Finding the radius of a star in arcsec
If i know the radius r of a star in cm, how can I convert it to arcseconds?
For example, if I have a star with r = 3.18e13 cm, and distance to the star d = 220 parsecs, what is the relation to ...
- 69
9
votes
1
answer
3k
views
How to convert density from physical unit to comoving unit?
I am learning co-moving units and I am interested to see how the conversion works. If Hubble constant is assumed to be 0.7, then I want to convert $1gm/cm^3$ into comoving unit. If I want to express ...
- 393
2
votes
1
answer
129
views
Units of integrated flux density (irradiance) along longitudes
If I've taken an image of the Galactic bulge with the pixel units in MJ per steradian:
and I've looked down each column of the image and added up the values:
then do the units change? My thinking is ...
- 2,394
-1
votes
1
answer
991
views
Astrodynamic Canonical Units [closed]
Maybe this is more appropriate for the Physics or Maths, but anyway.
I am struggling with applying Canonical units with formulas. I am struggling specifically with how they are used, am I to convert ...
- 1
2
votes
1
answer
111
views
Natural units of star clusters angular momentum and energy
I'm calculating the total angular momentum of a bunch of theoretical stars of a few solar masses, separated by a few light years, moving at velocities of about a few dozens of km / sec.
Mechanical ...
- 263
1
vote
1
answer
3k
views
How are people converting intensities in Janskys to Kelvin?
I'm reading some ALMA proposals and I am often seeing a conversion from Jy to Kelvin when, for example, people quote noise levels or source flux.
For example on their sensitivity calculator
(https://...
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84
votes
12
answers
22k
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Why don't astronomers use meters to measure astronomical distances?
In astronomy distances are generally expressed in non-metric units like: light-years, astronomical units (AU), parsecs, etc. Why don't they use meters (or multiples thereof) to measure distances, as ...
- 965
3
votes
1
answer
204
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wavelength-based IFU data cubes VS velocity-based IFU data cubes
I have 2 simulated IFU data cubes of the same observation, let's say A and B. The 3rd dimension of cube A is in wavelengths. In order to create cube B I just rebinned cube A in log-wavelengths and ...
- 325
1
vote
0
answers
177
views
${\rm W/m}^2$ to Jansky [duplicate]
Lets assume I have a source and receive $8 \times 10^{-20}$ ${\rm Wm}^{-2}$ in (400-750) nm filter. How do you convert this received flux to the unit Jansky, is there a formula? (Ultimately, I want to ...
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1
answer
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Why levels of radio contours maps are given in mJy/beam and what does it mean?
I know what a jansky is, but I don't understand what beam (in $\frac{1 Jy}{beam}$) is.
If jansky is a unit of flux density, what kind of unit is $\frac{1 Jy}{beam}$?
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votes
2
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Does the age of the universe take into account General Relativity / Special Relativity?
It is generally accepted that the age of the universe is approximately 12-15 billion years old based on the speed of the expansion of the universe. Since everything is moving very fast away from us, ...
- 1,032
2
votes
1
answer
1k
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Converting ångström spectral dimension to galaxy speed (km/s)
I have a spectral cube (in FITS format) whose spectral dimension is in ångströms. The sampling along the spectral dimension is 0.28A (CDELT=0.28). The observation in the cube is Ha emission of a ...
- 325
16
votes
3
answers
3k
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Is the Earth-Sun distance 1.012 AU?
Wolfram Alpha's query: Sun Earth distance in AU gives:
1.012 au (astronomical units)
They definitely use this value in other places too - e.g. Earth-Sun $L_1$ distances from Earth and from Sun ...
- 6,157
3
votes
1
answer
161
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Astrophysical "unitised" version of the Gravitational constant
This question relates formalising the gravitational constant, $G$, (which in S.I. units is usually quoted as $\sim6.67\times10^{-11}\,\mathrm{m}^{3}\,\mathrm{kg}^{-1}\,\mathrm{s}^{-2}$) in units which ...
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votes
2
answers
1k
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What is projected separation and how can I make sense of its unit (h^-1 kpc)?
I have been reading this paper on the separation of a binary quasar in which it states:
At a projected separation of 8.3/h kpc (Omega_matter = 1), CTQ 839 would be the smallest projected separation ...
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6
votes
3
answers
901
views
How to scale down solar system data to simulatable values
Okay, I am seriously ashamed for asking this especialy when I geniualy study on physics but there is something that bugs me with the simulation I am working on.
I am re-creating the solar system in an ...
- 61
2
votes
2
answers
998
views
What are the units of distance in this subtended angle calculation?
I'd like to find the distance at which a 2.4 cm coin is subtended by an angle of 10". I've drawn my diagram and found that $D=\dfrac{d}{\alpha}$ using the small angle approximation. Since $10" = 4.85\...
- 21
3
votes
1
answer
119
views
How often and over what period is Earth’s rotation averaged to compute UT1?
I understand that UT1 (and for that matter UT0, UT2, etc.) are based on averages of actual earth rotation, and serve as a form of mean solar time. However it’s not clear to me when these averages are ...
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