# Tag Info

## Hot answers tagged photometry

20 votes

### What are optical counterparts?

Observations can be made using different parts of the electromagnetic spectrum. Many of them not within the range of the spectrum that can be made with optical telescopes. Other messengers than ...
18 votes

### Could mirrors be replaced with CCDs?

The CCD has no way of recording the direction, the point in the sky, from which a photon is coming. Say you point your mirror-less telescope at the Moon. Every point on the moon's surface would be ...
• 1,021
10 votes
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### How fast is Neptune getting brighter? When was was this first noticed and reported?

TL;DR: There was apparent 11% increase of Neptune brightness during 1980 and 2000. This could be due to multiple reasons. Recent observation suggested the reason to be change in the amount and ...
• 3,780
9 votes
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### What is the ugriz magnitude system?

"ugriz" is short for U-band, G-band, R-band, I-band, and Z-band, detailed on the Wikipedia article on photometric systems: \begin{array}{|c|c|} \hline \text{Band}&\text{Effective Wavelength ...
• 34.7k
9 votes

### Could mirrors be replaced with CCDs?

To answer your question, we need to first show the job each mirror is doing. First up, the Newtonian (lovingly called the "Newt", and invented by Sir Ike Newton): https://en.m.wikipedia.org/wiki/...
• 99
9 votes
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### How do astronomers calibrate the intensity scale of their spectrometers?

This can be done in a number of ways. There is a "theoretical" approach, where the transmission and reflection characteristics of all the components are measured in the lab and put together ...
• 131k
7 votes

### Could mirrors be replaced with CCDs?

Since neither the word "phase" nor "interference" is mentioned in any other answer here, I'll approach it from that direction. In this answer I said In an imaging optical telescope (or any imaging ...
• 31.1k
7 votes
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### How can I calculate the uncertainties in magnitude like the CDS does?

That is because what is measured is a flux and the flux errors are in the DR2 catalogue. Since magnitudes are based on the logarithm of the flux, then there is no straightforward correspondence (...
• 131k
7 votes

### Should I drop source shot noise from the SNR calculation when all I care about is detection (not photometric precision)?

Assuming you know the position of the source: You collect a certain number of counts within an aperture (which could be modelled on a point spread function). You can then test the hypothesis that this ...
• 131k
6 votes
Accepted

### Can you subtract magnitude of constant source to get magnitude of variable source?

The magnitudes don't sum and subtract that way: consider two 0.00 magnitude stars closely together. Is their combined magnitude 0.00? Actually, it is -0.75. This paper might help you with the ...
• 2,884
5 votes
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### How can I convolve a template spectrum with a photometric filter response spectrum?

Your feeling is right: You shouldn't convolve the spectrum and the filter, you should only multiply so that flux outside the bandpass is suppressed. Subsequently you integrate the resulting function ...
• 35.6k
5 votes

### How do I obtain the brightness of a star in the night sky?

I don't know about the telescope, but I know that you will need the telescope that has CCD technology to be able to convert from counts/second (as brightness) that CCD detects to quantitative values ...
• 1,406
5 votes

### Why do we use filters in telescopes for astronomical imaging?

In general, the CCDs used to capture images do not register the energy (therefore, colour) of the incident photons on them - they just count the number of photons observed by each pixel (or a value ...
• 521
5 votes
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### How bright is the Crab Pulsar's 30 Hz modulation in visible light? What color is it?

The optical pulsations of the Crab pulsar have been studied closely since 1969. The observations are actually not that difficult (I did some myself with a photoelectric photometer as a student) and ...
• 131k
5 votes

### How many photons does it take to determine the existence of a distant object?

What matters is how many photons you collect versus how many you would expect to see if the object wasn't there. Photons would be present, without a source, for a variety of astrophysical (e.g. ...
• 131k
5 votes
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### Contamination problem data

Your teacher means that when you look at the light from a star in the data from a space-based photometry mission like Kepler or TESS, you cannot be immediately sure that any variability seen is from ...
• 131k
5 votes

### How does one actually fold a light curve?

Assume I have a bunch of measurements of something like brightness or flux versus time, and I know or guess a period for these data. Pick a zeropoint in time - say the time of the first observation - ...
• 131k
4 votes

### Could mirrors be replaced with CCDs?

I you just set out a CCD in a room, each pixel will record photons from every direction. With this, you will be able to record the amount of ambient light, but you won't get an image of the room. ...
• 17.1k
4 votes

### Estimating galactic dust extinction for medium band filters

The quantity you want is basically the extinction law, and is usually called $k(\lambda)$. An extinction law is a fit to several measurements of the extinction $A_\lambda$ in some direction (or an ...
• 35.6k
4 votes

### What is the unit used for star brightness due to skyglow?

The brightness of a diffuse area is it's luminance, and can be quoted in units of $cd\,m^{-2}$. Measuring it would require knowledge of the properties of the camera: the sensitivity (at various ...
• 103k
4 votes
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### What does less than one count from an x-ray detector mean? (Swift BAT detector)

I think there is a missing piece of information. The BAT is a coded mask telescope. The imaging is done by photons passing through a mask and falling onto an array of 32768 detectors. http://swift....
• 131k
4 votes

### Has star KIC 8462852's 3 year long Kepler-fading continued after the primary Kepler mission ended?

Well "if no known astrophysical model can explain it" then nobody told Wright & Sigurdsson (2016) who, cognisant of Montet & Simon's results, explore a number of astrophysical models. They ...
• 131k
4 votes

### Why do we use filters in telescopes for astronomical imaging?

As WDC pointed out in his comment, without filters, you simply get a recording of the received irradiance as a function of the sensor's spectral response function. In other words, a normal CCD that ...
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4 votes
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### Relationship between photometric colour and redshift

The main reason is that the intrinsic spectra of galaxies are complex and therefore a redshift of their spectrum, whilst leading to a redder spectrum overall, does not necessarily lead to reddening in ...
• 131k
4 votes

### How to convert luminosity at rest frame wavelength of 1450 A to absolute magnitude at 1450 A?

The absolute magnitude of an object is defined as the brightness of the object observed at a distance of $d = 10\,\mathrm{pc}$. With this distance, you can convert the luminosity density $L_\nu$ in \$\...
• 35.6k
4 votes
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### Is 486958 Arrokoth (2014 MU69 aka Ultima Thule) the only solar-system object determined to be binary by occultation?

Actually, the very first binary asteroid was discovered by stellar occultation in 1980. The evidence was rejected by the IAU, and wikipedia credits the first binary asteroid discovery to the Galileo ...
• 15.6k
4 votes

### Converting Light curve to Luminosity

To calculate a "luminosity light curve" from a time series of V-band photoetry, you need two things. You need to know the distance. The distance to Betelgeuse is uncertain and that means ...
• 131k
4 votes

### Delta Velorum - non-observed variable star

I am the discoverer of delta Velorum's variability (along with the Galileo spacecraft) and I detected those variations visually, so yes, they can be observed, and they are really fun! If you go to the ...
3 votes
Accepted

### What is the unit used for star brightness due to skyglow?

Surface brightness of galaxies and nebulae is typically expressed as visual magnitude per square arcsecond. Skyglow is often quantified in the same way. Given the angular dimensions of the frame and ...
• 17.1k
3 votes
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### What telescopes have observed anomalies in the light curve of Tabby's star, KIC 8462852?

I do not know whether complimentary observations of the light curve are being done, but I will try to answer the rest of the question. Is it easy to observe the light curve with ground-based ...
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