# Tagged Questions

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### Stretch Factor for SNIa

How do I compute the stretch factor for a Supernova Ia ? Data I have : Redshift, Points on the light curve, Rise time (∆m=2), Fall time (∆m=2), Time from the peak day for an inflection point in the ...
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### Synthetic Photometry - Calculating a colour index

I have a theoretical black body spectrum, described by plancks law. I also ave the bandpass sensitivity function for various filters. I would like to calculate a colour index from this information, so ...
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### Is it possible to have a 1m2 base at 1m from a source with an angle of 1sr?

Theoretically, a light source with a solid angle and $1\ \mathrm{candela}$ will produce $1\ \mathrm{ lux}$ at a $1\ \mathrm{m^2}$ area $1\ \mathrm{m}$ away. However, when I calculate the base of the ...
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### Best way for calculating Lux from candela vs from lumen?

Researching this topic, I came across two methods that seem to contradict each other. Need to decide which to use. I am a novice in this field so please be gentle. Also, I would have loved to use ...
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### How to begin using SDSS data?

SDSS has gone through so many updates, it's difficult to find out how to access the data today in 2015. How can I use SQL and Python to access SDSS data, e.g. the photometry and spectra of quasars, ...
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### If the Sun were smaller but had the same surface temperature, would it still have the same luminance?

Let's assume we have two stars that have the same surface temperature but very different size. I understand how luminosity depends on surface area so the two stars will have different luminosity, but ...
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### During transition of the electron from high to low energy state, is the photon released always of same energy?

Is it not possible that during transition of an electron from higher to lower energy state, it can release multiple photons of low energy instead of a single photon of the exact energy difference ...
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### Does Lambert's formula give correct results of luminous flux density?

A uniform point-source S, having luminous intensity, $I$, is located at a point at a distance $r(=OS)$ from the center O of a plane having area $A$ such that $\theta$ is the angle between the line ...
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### How does the Luminosity Function relate to the tristimulus sensitivity function?

In colorimetry, the irradiance spectrum is weighted with the luminosity function to obtain the perceived luminosity, and the tristimulus sensitivity functions to obtain the perceived RGB-...
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### Photometric surveys vs. Spectroscopic surveys

I consistently read about certain astronomical surveys which are either described as "photometric surveys" or "redshift surveys". I'm still unclear as to how these two methods differ. Photometry ...
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### Distance from redshift

I am looking for a exact derivation of a relation between redshift $z$ and distance $d$. What I know is the definition z=\frac{\lambda_{\text{observed}}}{\lambda_{\text{unshifted}}}-1=\sqrt{\frac{1+...
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### Why is the candela dimension J, not W?

According to the table at the bottom of the Wikipedia page for the candela, the dimension for candelas is J (joules). Why is this not W (watts)? The luminous intensity for light of a particular ...
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### need data-point: count rate of APD (avalanche photo-diode) for specific aperture and stellar magnitude

I hope lab / experimental physics is fair game for this web-site. If not, sorry! I'm designing a sensor system to perform specialized [astronomy and space-sciences] experiments, and need a "reality ...
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### Relation between brightness and magnitude

Let say we measure the superficial brightness of an object in the $B$ band, $I_{B}$ measured in $\displaystyle\frac{L_{*}}{\mbox{pc}^2}$ where $L_{*}$ is solar luminosity and $\mbox{pc}$ is parsec. I ...
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### Can I see the light from 10 million km away?

If light is switched ON, only for a second, and the distance between the observer and the light source is 10 million kilometers, can I still see the light spark? For example, let's assume that the ...
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### Implementing a black-body using Planck's Law in a 3D ray tracer

1_ In one of my software projects, I need to implement Planck's Law in terms of wavelength. I can't be sure if it's right because I've seen different variations of it, and all the implementations ...