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Questions tagged [photometry]

Questions regarding the measurement of the amount of light coming from astronomical objects.

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cross-matched SDSS-WISE catalog

Please where can I find cross-matched SDSS and WISE dataset for a machine learning project? (Galaxy-star-qso classification task)
Fanoela Rabemanotrona's user avatar
2 votes
0 answers
59 views

BVR photometry and Luminiscense

I have the values from B, V and R filters. How do I obtain the total luminescence of a star? My ultimate aim is to find the relative masses and luminescence of various stars/galaxies in a galactic ...
Angela's user avatar
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3 votes
1 answer
78 views

What is the difference between albedo and Bond albedo?

Recently I came across Bond albedo, but I am not sure I grasped the difference (if there is any) between it and the "classical" albedo. Thanks in advance
jack_O'Dim's user avatar
3 votes
0 answers
41 views

Why MIT's Benjamin Rackham says "nearby ultracool dwarf SPECULOOS-3" would look "purplish-red, spotted, and flaring" from an airless orbiting planet?

May 15, 2024 MIT News article Newly discovered Earth-sized planet may lack an atmosphere includes the following: “We can say from our spectra and other observations that the star has a temperature of ...
uhoh's user avatar
  • 30.7k
0 votes
0 answers
63 views

calculating Sun's J band mag via integrating the Planck’s law

...
questionhang's user avatar
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0 votes
0 answers
28 views

Errors in SDSS photometry

In plotting out the magnitude errors in SDSS photometry, I get the following pattern: Why do the errors go up to about 0.5, then down? In other words, why does it look like a high-heeled shoe (or ...
Jim421616's user avatar
  • 2,620
1 vote
0 answers
69 views

magnitude calculation if a blackbody's radius and temperature are provided

I know it is necessary to convolve its spectrum with the transmission curve of a filter. Are there any online tools or codes available for calculating a BB's magnitude in any specific band (e.g., ...
questionhang's user avatar
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3 votes
0 answers
51 views

How do you calculate the reddening corrections for photometry?

I have photometric data from Gaia for a star and am trying to de-redden the colors. I need to find the reddening corrections for the photometry. I have photometry for G, BP, and RP. To start I am ...
anna's user avatar
  • 31
1 vote
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65 views

How do you convert from the Gaia photometric system to Johnson-Cousin?

I have Gaia photometry (G, G_bp, and G_rp) and need to convert it to the Johnson Cousin system. In the Data release 3 (DR3) documentation they provide tables of how to do this but I’m very confused by ...
Snav's user avatar
  • 11
6 votes
1 answer
232 views

Human Brightness perception and contrast

This is kind of an interdisciplinary question on human brightness perception and the ability to distinguish grayscales. To be honest, I wasn't sure if the Physics, Biology or Computer graphics SE site ...
Charles Tucker 3's user avatar
3 votes
0 answers
104 views

SExtractor parameters for identifying Star Forming regions

I am using SExtractor (Source Extractor) to identify Star-Forming regions in a galaxy. The problem is I am unable to choose the parameters required for it. SExtractor is just a tool to identify the ...
Abhinna Sundar's user avatar
3 votes
1 answer
56 views

Is it possible to detect lyman alpha blobs (LABs) with the lyman-break method?

LABs are luminous extended nebulae of hydrogen gas in the early universe (z ≥ 3) found in overdensities of LAEs (lyman-alpha emitters) and LBGs (lyman-break galaxies). I was wondering if LABs could be ...
Ankit Biswas's user avatar
2 votes
0 answers
60 views

What could these LMC objects in this color-color diagram be?

I have performed a color-color and colour-magnitude plot of various sources detected in an LMC image (JWST NIRCAM F200W, F150W and F115W filters). I then also queried SIMBAD to collect relevant ...
KBo's user avatar
  • 21
3 votes
3 answers
170 views

Understanding PSF models and PFSEx output

I understand that PSFs are models for spreading of light. When I visit Hubble website (https://www.stsci.edu/hst/instrumentation/wfc3/data-analysis/psf) there is PSF model for me to download. I could ...
cassandra's user avatar
1 vote
0 answers
87 views

How do you estimate standard deviation of sky from (Hubble) image

I'm working on a simulation project with Hubble data and I need the standard deviation of the sky in ADU per pixel. Sometimes I can obtain a fits file (e.g. hdfs image), open this with astropy, and ...
cassandra's user avatar
0 votes
1 answer
274 views

Rest-frame magnitudes and fluxes

I have a question concerning rest-frame vs observed frame. In the observer frame, I can define an apparent magnitude and an absolute magnitude (as the apparent magnitude if the object were at d=10pc). ...
Benny's user avatar
  • 204
4 votes
0 answers
419 views

Calculation of average FWHM by PSF fitting of all the detected stars in the fits image

I need to calculate the FWHM of all the stars detected via the DaoStarFinder package/any other package by performing PSF fitting in Python. Here is the code for my detection. ...
Abhinna Sundar's user avatar
1 vote
0 answers
173 views

Photometry in SExtractor vs Python

I wanted to find stars in the dwarf Spheroidal (dSph) galaxy. I performed photometry using SExtractor and in Python using DaoStarFinder and other astropy packages. However, while extracting the ...
Abhinna Sundar's user avatar
1 vote
0 answers
238 views

Split Multi-Extension fits file - fits.fz format

I have CFHT/MegaCam fits.fz file on which I want to do photometry. I want to split these large fits.fz file into separate CCD detector fits and then run Source Extractor on them for photometric ...
Abhinna Sundar's user avatar
1 vote
0 answers
74 views

Effective gain when stacking N frame images

I am trying to understand how to derive the effective gain in [electrons / ADU] when stacking N frame images in different (summed, averaged, or medianed) ways. I found the effective gains for the ...
cosmozaka46's user avatar
1 vote
0 answers
56 views

Magnitude cuts in different bands/surveys

I have data on around 142,000 galaxies. Some of these galaxies have apparent magnitude data in different bands. For example, many of my galaxies have SDSS PSF data in ugriz bands,GALEX FUV, GALEX NUV, ...
Bobasheto's user avatar
2 votes
1 answer
371 views

How does one actually fold a light curve?

I'm studying the variable stars (periodic variables with brightness changes repeating over time), and to examine the shape of the light curve it is useful to fold the data points at this period and ...
Saturn.'s user avatar
  • 49
5 votes
1 answer
346 views

Surface Brightness Definition

I know the surface brightness is calculated by taking the apparent magnitude of an object and then divide by the size of the thing. But this is more of "observed" surface brightness, how do ...
JOAQUÍN HERNÁNDEZ's user avatar
2 votes
0 answers
46 views

How do I calculate the flux density of a substellar object?

I am currently trying to generate photometric passband colors for brown dwarfs and planets generated through MESA (Modules for Experiments in Stellar Astrophysics). I believe I understand the ...
Ankit Biswas's user avatar
5 votes
1 answer
126 views

Calculating background photons from flux levels

For a project, I'm trying to calculate the photon flux from the zenith galactic background that a sensor would see. From ScienceDirect I get a value of $2.6 \times 10^{12}$ photons sr$^{−1}$ s $^{−1}$ ...
Paul Blase's user avatar
4 votes
0 answers
42 views

Fitting a line or polynomial to optical and radio SEDs

I've been trying to fit functions to plots showing photometric data for AGN from both optical and radio measurements, such as: but my supervisor says that this shouldn't be done; the radio and ...
Jim421616's user avatar
  • 2,620
11 votes
1 answer
536 views

How do astronomers calibrate the intensity scale of their spectrometers?

Discussion on Strange bump in solar spectrum taken with home-made spectrograph made me wonder: How do astronomers calibrate the intensity scale of their spectrometers? I mean, how to take in ...
Luis López's user avatar
4 votes
1 answer
73 views

Contamination problem data

I do not have a source for this statement. A teacher just said it. Could you tell me what 'a contamination problem' mean? He explained that 'stellar images from a space-based photometry data have ...
Elena Greg's user avatar
2 votes
0 answers
49 views

General questions about SDSS

I'm considering doing research on SDSS spectral classification (STAR, GALAXY, QSO) with machine learning for my school dissertation. Not sure if this is an appropriate place for these kinds of ...
Robert Mifsud's user avatar
1 vote
0 answers
106 views

How has the rate of photon arrival from stars been estimated and then confirmed?

My understanding is that we can estimate the total energy emitted over a star’s entire spectrum and from that and the star’s measured spectrum estimate how many photons of a given wavelength we should ...
Todd Stellanova's user avatar
3 votes
1 answer
248 views

creating asteroid light curves and analysing them

I came across the video Tutorial: Creating a Lightcurve of a Minor Planet linked below by Tycho Tracker, which itself is a software to build an asteroid light curve from raw images and analyse it to ...
mysterium's user avatar
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1 vote
1 answer
57 views

Querying VizieR using SQL interface on their website

I'm trying to query http://tapvizier.u-strasbg.fr/adql/?%20V/147/sdss12 using the following query, built by clicking Construct your query and ...
Jim421616's user avatar
  • 2,620
2 votes
1 answer
2k views

How do I phase fold the light curve for a variable star?

I have some observational data for a star where I've done aperture photometry to get a partial period. I understand that you need to use other techniques to estimate a period for stars whose period is ...
Zachary Kennedy's user avatar
4 votes
0 answers
52 views

How does SDSS estimate photometric redshift of sources?

I'm looking through the data in SDSS DR 12: ...
Jim421616's user avatar
  • 2,620
2 votes
1 answer
198 views

Where can I find data on SDSS filter profiles?

I've seen lots of filter throughput curves for the SDSS, such as this one: from the SDSS camera page, but I'd really like to produce my own from the data. Is the filter response data per wavelength ...
Jim421616's user avatar
  • 2,620
7 votes
1 answer
397 views

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

All of the calculations for signal-to-noise ratio (SNR) of sources in astronomical images include various noise terms of relevance: read noise, quantization noise, and shot noise from each of the ...
NeutronStar's user avatar
  • 2,673
1 vote
0 answers
62 views

Angle to the line of sight

I don't understand this angle used in galaxy clustering. There is the line of sight and this angle to the line of sight which is not clear to me. Here is a transcript of a description from 1705.05442: ...
P. Ferreira's user avatar
1 vote
0 answers
104 views

Errors in photometric magnitudes from SDSS DR16

I'm playing around with the data in the SDSS DR16, but the magnitudes have huge uncertainties: ...
Jim421616's user avatar
  • 2,620
2 votes
1 answer
60 views

Query about the definition of photometric zeropoints

In stscl page, apparent magnitude is defined as $$m=-2.5\log{\frac{{\rm DN}}{{\rm EXPTIME}}}+{\rm ZEROPOINT}\ ,$$, DN is Data number (I cannot understand what is meant by 'Data number'... they said it ...
BAO's user avatar
  • 357
3 votes
0 answers
633 views

DIY Sky Quality Meter: Conversion between Illuminance and Luminance

I am trying to create a homemade Sky Quality Meter (SQM) using a TSL2591 light sensor. This sensor measures Illumination (lux), but SQMs measure Luminance (mpsas). I have seen that there are many ...
Boube's user avatar
  • 31
1 vote
0 answers
104 views

What observational data was there that comet Leonard C/2021 A1 might have been dimming and/or breaking up?

Space.com's Comet Leonard, the brightest of the year, is fading and acting strange seems to be full of inexact language, like "something's wrong" and "unhealthy" and "goner&...
uhoh's user avatar
  • 30.7k
2 votes
0 answers
23 views

Stromgren photometric data in Vizier

Is there a catalog in Vizier that provides Stromgren photometric data for stars, (the all four uvby, not just the indices b-y, m and c) I would like to try the photometric transform given in my book, ...
Cheng's user avatar
  • 342
1 vote
0 answers
60 views

How to properly compare radio to optical/NIR images?

I mainly work with radio continuum data and recently I started working on some HST hydrogen recombination line datasets to compare with the radio data that I have, but I'm very confused about what is ...
spacedog's user avatar
4 votes
1 answer
113 views

Is there a difference between Vis and V when analyzing the magnitudes of stars?

I have a dataset of a binary star system's magnitudes taken over the past decade, obtained from AAVSO. These magnitudes are reported in B,V,R,I, and so on. Some magnitudes have their V magnitudes ...
requiemman's user avatar
2 votes
2 answers
1k views

Straightforward conversion from Gaia GDR3 photometry to RGB values

I'm a non-astronomer, but I'm working on a project for which I need to produce CGI renderings of as complete a starfield as I can find. The latest Gaia catalogue release has an impressive 1.5 billion ...
Niall's user avatar
  • 21
3 votes
1 answer
80 views

Disk-resolved vs disk-averaged photometry?

I have already read this question about the difference between disk-integrated and disk-resolved photometry. But what exactly is the difference between disk-resolved and disk-averaged photometry? I am ...
lazerlini's user avatar
1 vote
0 answers
46 views

Procedure to construct a spectral energy distribution and bolometric light curve of supernovae?

Can anyone check the logic of this proposed workflow and/or point me in the direction of a guide and example to check my work? Proposed procedure to construct a spectral energy distribution and ...
ames's user avatar
  • 11
4 votes
2 answers
369 views

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

Let's say I observe a source that is actually two sources (source A and source B) where each cannot be individually resolved. Source A is a variable source and source B is a constant source. I want to ...
theta's user avatar
  • 258
2 votes
2 answers
212 views

During aperture photometry when simulating an undetected star, why must an upper limit and standard deviation need to be calculated?

I am trying to learn aperture photometry using python I noticed the following logic in the image below, where an upper limit was determined by simulating an undetected star and the standard deviation ...
Robin's user avatar
  • 41
0 votes
2 answers
65 views

How can I find more information on a certain galaxy when NED isn't helping?

The paper High Frequency Peakers: Young radio sources or flaring blazars? by S.Tinti et.al. lists some nice radio-wavelength SEDs, like these for instance: When looking for more photometric info on a ...
Jim421616's user avatar
  • 2,620