Questions tagged [photometry]

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

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3 votes
0 answers
45 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 ...
6 votes
1 answer
167 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 ...
1 vote
0 answers
42 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 ...
7 votes
1 answer
317 views

Did the late 2019 "fainting" of Betelgeuse show any spectral trends that differ from it's normal variability?

update: (August 2020) With all the newest news about including what the Dr. Becky video discusses as linked in How do magnetic fields mess with astronomers' observations? I'll bet this question ...
9 votes
0 answers
162 views

Color index B-V of the dimming Betelgeuse

Using AAVSO light curve generator I have plotted V and B magnitudes of Betelgeuse in the past 150 days to see trend of B-V color index. If dust would be the cause of the recent drop in the star ...
2 votes
0 answers
151 views

Are astronomers still using photomultiplier tubes for optical photometry?

In Time domain astronomy and fastest eclipsing binary ZTF J1539+5027 (+20 mag, 6.91 minutes) How to measure it's minimum brightness? I've written A logarithmic magnitude scale might tend to show ...
1 vote
1 answer
175 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). ...
3 votes
0 answers
77 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 ...
3 votes
1 answer
45 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 ...
3 votes
1 answer
213 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 ...
2 votes
0 answers
53 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 ...
3 votes
3 answers
130 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 ...
1 vote
0 answers
68 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 ...
6 votes
2 answers
4k views

How can I calculate the uncertainties in magnitude like the CDS does?

When you check an entry of the Gaia DR2 stellar catalogue in Vizier, for example this one, you can see that there is a value for the G magnitude (in our case 18.0733 mag) and a value for the ...
7 votes
2 answers
173 views

Observing eclipsing binaries

In a matter of weeks, I will be performing my first observation using a 25-cm Newton telescope (+CCD camera) and would be grateful for advice regarding an otherwise basic research that I have decided ...
11 votes
1 answer
520 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 ...
4 votes
0 answers
288 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. ...
1 vote
0 answers
116 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 ...
1 vote
0 answers
176 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 ...
1 vote
0 answers
60 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 ...
1 vote
0 answers
44 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, ...
4 votes
1 answer
799 views

Advantages or disadvantages of a photometer versus a CCD-camera?

The Wikipedia article on photometry says this about photometers: These have largely been replaced with CCD cameras that can simultaneously image multiple objects, although photoelectric photometers ...
5 votes
2 answers
1k views

What is the gain of a telescope?

What is the gain of a telescope? How would I go about calculating the gain from a flatfield image?
2 votes
1 answer
281 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 ...
2 votes
1 answer
57 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 ...
5 votes
1 answer
278 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 ...
2 votes
0 answers
41 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 ...
5 votes
1 answer
113 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 $^{−1}$ ...
4 votes
0 answers
39 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 ...
8 votes
2 answers
344 views

DAMIT how do they get 3D shape and rotational trajectory of a tumbling asteroid from photometry?

This question asks about an algorithm used in Project DAMIT. @RobJeffries' answer led me to Wikipedia's William Herschel Telescope which led me to Wikipedia's 2008 TC3 which led me to the animation ...
4 votes
1 answer
72 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 ...
2 votes
0 answers
47 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 ...
5 votes
0 answers
441 views

Why does titanium oxide around Betelgeuse produce this particular sawtooth-shaped absorption spectrum?

Betelgeuse Just Isn’t That Cool: Effective Temperature Alone Cannot Explain the Recent Dimming of Betelgeuse suggests that the recent dimming might be caused by increased optical absorption by dust ...
1 vote
0 answers
92 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 ...
1 vote
1 answer
55 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 ...
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 ...
4 votes
0 answers
47 views

How does SDSS estimate photometric redshift of sources?

I'm looking through the data in SDSS DR 12: ...
2 votes
1 answer
154 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 ...
7 votes
1 answer
386 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 ...
1 vote
0 answers
57 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: ...
1 vote
0 answers
91 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: ...
3 votes
0 answers
570 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 ...
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&...
4 votes
1 answer
174 views

Comprehensible full manual for bolometric light curve reconstruction

Could anyone point me to some manual or tutorial, possibly step-by-step one, on reconstruction of (quasi-) bolometric light curve using UBVRI-photometry data? Both links and names are fine for me.
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, ...
1 vote
0 answers
56 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 ...
4 votes
1 answer
109 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 ...
2 votes
2 answers
810 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 ...
3 votes
1 answer
74 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 ...
4 votes
2 answers
352 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 ...