<p>Whilst awaiting clarification, I'm going to assume that your question is asking about calibrating the reference frame within which the motions of stars at the Galactic centre are measured. </p>

<p>The process is described by <a href="https://arxiv.org/abs/0808.2870" rel="nofollow noreferrer">Ghez et al. (2008)</a>; does not involve observing quasars at infrared wavelengths; and the study does not suffer as a result.</p>

<p>The Galactic centre adaptive optics images are 5-10 arcseconds across, but contain thousands of stars. Only a fraction of these are close to the Galactic centre. The ensemble can be used, iteratively, to define a relative reference frame by minimising the displacements of thousands of stars, excluding those very close to Sgr A* and those with large motions.</p>

<p>This is really all that is required to do the orbital analyses of the black hole.</p>

<p>To put the coordinates in the International Celestial Reference System, a wider field is observed that contains a few infrared-bright giant stars, that are also bright maser sources at radio wavelengths. The positions of these are used to bootstrap the coordinates onto the ICRS, and this is important if, for example, we are interested in the relative motion of the Sun around the Galaxy.</p>

<p>The maser sources have ICRS positions by virtue of their <em>radio</em> source positions versus the <em>radio</em> coordinates of distant (but still bright and point-like) background quasars.The issue of background sources, extinction and confusion does not arise at radio wavelengths.</p>

CC BY-SA 3.0
2017-03-18T18:39:54.640Z
You correctly guessed the ultimate motivation, but the proximate question is an interesting exercise in and of itself, in my humble opinion. I am familiar with the use of water masers to set up a reference frame, but I still have a, "Wouldn't it be cool if we could use quasars in the same image to define the frame?" thought, and wonder what it would take to achieve that.
Sean Lake
CC BY-SA 3.0
2017-03-18T20:06:02.397Z
@SeanLake I don't understand what resolution has to do with it. You mean if the quasar is so faint that it merges with the general background of field stars?
ProfRob
CC BY-SA 3.0
2017-03-18T20:55:18.710Z
Bingo. Details added to question.
Sean Lake
CC BY-SA 3.0
2017-03-18T10:31:06.910Z
2017-03-18T10:31:06.910Z
ProfRob
<p>The center of the galaxy is densely packed with stars and obscured by a whole lot of dust between us and it. For those reasons, groups that study the motion of stars around the super-massive black-hole in the center of the Milky way need to use big telescopes at near infrared wavelengths. Questions of dust obscuration aside, what resolution would a telescope need to have to pick out a background quasar looking through the center of the Milky Way?</p>

<p>SDSS QSOs start at around 8th magnitude (Vega) in 2MASS photometry ($H$ and $K_s$ bands), and rise like a power law as flux goes down Here's a quick and dirty plot of SDSS QSO survey made from SDSS data release 10 made using <A HREF="http://www.star.bris.ac.uk/%7Embt/topcat/" rel="nofollow noreferrer">topcat</A>. I would convert the graph to counts per solid angle, but I'm too lazy to look up the survey area of the SDSS spectroscopic survey right now. 
<a href="https://i.sstatic.net/PXVvp.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/PXVvp.png" alt="Histogram of Quasar Fluxes"></a></p>

<p>There's a quick and dirty example of the effect I'm describing in Figure 2 of the <A HREF="https://arxiv.org/abs/0808.2870" rel="nofollow noreferrer">Ghez et al. (2008)</A> paper Rob Jeffries linked to below. The difference between the red points and blue points is both exposure time (more photons) and higher <A HREF="https://en.wikipedia.org/wiki/Strehl_ratio" rel="nofollow noreferrer">Strehl ratios</A> (a measure of how close to diffraction limited the image is and, hence, a proxy for resolution given the same optical system).</p>

<p>Generically, you could say that the question is: how do we determine the confusion limit in images with point sources? This is just specifically applied to the field of view on the sky with the highest density of resolvable point sources available. Concretely, say we wanted to resolve $22^{\mathrm{nd}}$ magnitude sources in views of the galactic center, what resolution is required to do so?</p>

<p>Getting to the full answer to the more concrete question "how deep would we have to look to see a background quasar" would require additional information: size of field of view, and dust extinction along the sight line. So, for simplicity, I'm assuming that if you're resolving $22^{\mathrm{nd}}$ magnitude sources in $K$ band, you're probably seeing a quasar. Thus we only need the projected density on the sky of sources brighter than that cutoff, roughly, and some description of how resolved sources need to be from the background brightness to detect them to answer the question.</p>

CC BY-SA 3.0
2017-03-18T07:55:00.380Z
I'm no expert, but I think it's not only a matter of resolution. If you look at trees in a forest, you simply can't see past the farthest trees. The signal of the pulsar on the other side just gets lost in the foreground signal.
agtoever
CC BY-SA 3.0
2017-03-18T08:09:11.317Z
@agtoever Doubtful - the stars are tiny compared with the space between them. I'm fairly sure that with sufficient resolution the amount of sky physically blocked by those stars is small.
Sean Lake
CC BY-SA 3.0
2017-03-18T08:23:04.570Z
"The trick is that half the Milky Way is obscured by gas and dust. So we don’t really know what structures are on the other side of the galactic disk. With more powerful infrared telescopes, we’ll eventually be able to see though the gas and dust and map out all the spiral arms." [https://www.google.nl/amp/www.universetoday.com/115203/what-part-of-the-milky-way-can-we-see/amp/]
agtoever
CC BY-SA 3.0
2017-03-18T09:56:14.570Z
Why would you want to observe a quasar whilst looking at the motion of stars at the galactic centre?  You are being very indirect.
ProfRob
<milky-way><angular-resolution><astrometry><quasars><galactic-center>
What resolution is needed to see a quasar through the galactic center?
Sean Lake
# What resolution is needed to see a quasar through the galactic center?

(Asked by: username_0 on 2017-03-18)

The center of the galaxy is densely packed with stars and obscured by a whole lot of dust between us and it. For those reasons, groups that study the motion of stars around the super-massive black-hole in the center of the Milky way need to use big telescopes at near infrared wavelengths. Questions of dust obscuration aside, what resolution would a telescope need to have to pick out a background quasar looking through the center of the Milky Way?

SDSS QSOs start at around 8th magnitude (Vega) in 2MASS photometry ($H$ and $K\_s$ bands), and rise like a power law as flux goes down Here's a quick and dirty plot of SDSS QSO survey made from SDSS data release 10 made using [topcat](http://www.star.bris.ac.uk/%7Embt/topcat/). I would convert the graph to counts per solid angle, but I'm too lazy to look up the survey area of the SDSS spectroscopic survey right now.
[![Histogram of Quasar Fluxes](https://i.sstatic.net/PXVvp.png)](https://i.sstatic.net/PXVvp.png)

There's a quick and dirty example of the effect I'm describing in Figure 2 of the [Ghez et al. (2008)](https://arxiv.org/abs/0808.2870) paper Rob Jeffries linked to below. The difference between the red points and blue points is both exposure time (more photons) and higher [Strehl ratios](https://en.wikipedia.org/wiki/Strehl_ratio) (a measure of how close to diffraction limited the image is and, hence, a proxy for resolution given the same optical system).

Generically, you could say that the question is: how do we determine the confusion limit in images with point sources? This is just specifically applied to the field of view on the sky with the highest density of resolvable point sources available. Concretely, say we wanted to resolve $22^{\mathrm{nd}}$ magnitude sources in views of the galactic center, what resolution is required to do so?

Getting to the full answer to the more concrete question "how deep would we have to look to see a background quasar" would require additional information: size of field of view, and dust extinction along the sight line. So, for simplicity, I'm assuming that if you're resolving $22^{\mathrm{nd}}$ magnitude sources in $K$ band, you're probably seeing a quasar. Thus we only need the projected density on the sky of sources brighter than that cutoff, roughly, and some description of how resolved sources need to be from the background brightness to detect them to answer the question.

**Tags:** milky-way, angular-resolution, astrometry, quasars, galactic-center
> **Comments:**
> **username_1**: I'm no expert, but I think it's not only a matter of resolution. If you look at trees in a forest, you simply can't see past the farthest trees. The signal of the pulsar on the other side just gets lost in the foreground signal.  
> **username_0**: @agtoever Doubtful - the stars are tiny compared with the space between them. I'm fairly sure that with sufficient resolution the amount of sky physically blocked by those stars is small.  
> **username_1**: "The trick is that half the Milky Way is obscured by gas and dust. So we don’t really know what structures are on the other side of the galactic disk. With more powerful infrared telescopes, we’ll eventually be able to see though the gas and dust and map out all the spiral arms." [https://www.google.nl/amp/www.universetoday.com/115203/what-part-of-the-milky-way-can-we-see/amp/]  
> **username_2**: Why would you want to observe a quasar whilst looking at the motion of stars at the galactic centre?  You are being very indirect.  

---

## 1 answers

### Answer by username_2 on 2017-03-18. Score: 1

Whilst awaiting clarification, I'm going to assume that your question is asking about calibrating the reference frame within which the motions of stars at the Galactic centre are measured.

The process is described by [Ghez et al. (2008)](https://arxiv.org/abs/0808.2870); does not involve observing quasars at infrared wavelengths; and the study does not suffer as a result.

The Galactic centre adaptive optics images are 5-10 arcseconds across, but contain thousands of stars. Only a fraction of these are close to the Galactic centre. The ensemble can be used, iteratively, to define a relative reference frame by minimising the displacements of thousands of stars, excluding those very close to Sgr A\* and those with large motions.

This is really all that is required to do the orbital analyses of the black hole.

To put the coordinates in the International Celestial Reference System, a wider field is observed that contains a few infrared-bright giant stars, that are also bright maser sources at radio wavelengths. The positions of these are used to bootstrap the coordinates onto the ICRS, and this is important if, for example, we are interested in the relative motion of the Sun around the Galaxy.

The maser sources have ICRS positions by virtue of their *radio* source positions versus the *radio* coordinates of distant (but still bright and point-like) background quasars.The issue of background sources, extinction and confusion does not arise at radio wavelengths.

> **Comments:**
> **username_0**: You correctly guessed the ultimate motivation, but the proximate question is an interesting exercise in and of itself, in my humble opinion. I am familiar with the use of water masers to set up a reference frame, but I still have a, "Wouldn't it be cool if we could use quasars in the same image to define the frame?" thought, and wonder what it would take to achieve that.  
> **username_2**: @SeanLake I don't understand what resolution has to do with it. You mean if the quasar is so faint that it merges with the general background of field stars?  
> **username_0**: Bingo. Details added to question.  

---

