<p>A smaller field of view would <em>not</em> necessarily help. The data transmitted back to Earth by Kepler was already limited by telemetry constraints and the portions of the CCD that were read out were already focused on small windows around (many) selected stars. Having a field of view that is at least 10 arcsecond across would be sufficient. What is more important is to make sure your star images are well-sampled. This is a bit of a problem for Kepler. The CCD pixels are quite big (4 arcseconds) to give a big field of view, but this is similar to the angular resolution of the telescope. This means all the light can fall in basically one pixel, which is not ideal for precise photometry. If you were focusing on one star you would make the field of view smaller but have lots of CCD pixels across your star image. This improves the precision of the photometry considerably.</p>

<p>If you were to construct a similar telescope designed to look at one star at a time, then in principle you could have a much higher telemetry rate for that star. Hence, if you had enough photons and a low CCD readout noise, you could take much shorter exposures and see finer detail in the light curve transits - maybe moons, rings, starspots etc. The standard Kepler cadence was 30 minutes for most stars and one minute for some of the brightest stars - see <a href="http://keplergo.arc.nasa.gov/PyKEprimerResources.shtml" rel="nofollow">this link</a>. A more rapid cadence would allow a much better characterisation of the host star through <a href="https://en.wikipedia.org/wiki/Asteroseismology" rel="nofollow">asteroseismology</a> (e.g. mass and radius), which in turn leads to better characterisation of its planets.</p>

<p>To take advantage of the higher telemetry rates, it would be advantageous to have a larger aperture telescope, simply to collect enough photons. The angular resolution of the telescope is not that important if investigating obviously single , bright sources. You would need an absolutely enormous telescope (100m+) to resolve the disks or planetary systems of even nearby stars, so this is not really feasible.</p>

<p>A larger telescope would also allow you to split the light by wavelength and investigate transits in a wavelength dependent way. This is really important for investigating atmospheric composition and chemistry. This is perhaps the only area where focusing on a single star might be useful - because you could design a slit spectrograph to monitor the transit spectrum. On the other hand, these days it is possible to design multi-object spectrographs using fibres or slit masks, so overall I can't see any real advantage to having a telescope that is capable of gathering data for only one star at a time.</p>

CC BY-SA 3.0
2016-09-13T18:00:39.390Z
How many photons per second hit Kepler from any one star on average? My fantasy here is focusing a Kepler telescope's all 5.4 million pixels on one single point source star with a known exoplanet at a time, from ingress to egress. Preferably with a unique wavelength spectroscopically captured by each of the 5.4 million pixels. Then move to the next star's transit event a few hours later.
LocalFluff
CC BY-SA 3.0
2016-09-13T17:24:06.530Z
2016-09-13T17:24:06.530Z
ProfRob
<p>If a Kepler telescope were built to focus on one point source star at a time, would it provide more information, or did Kepler capture virtually every photon from any given star anyway?</p>

<p>In order to study known transit events more closely one star at a time, to possibly detect moons or atmosphere or rings by resolving the transit light curve's shape better, is a larger aperture necessary or would a smaller field of view help? What field of view would be optimal for observing a single point source?</p>

CC BY-SA 3.0
2016-09-03T15:58:49.697Z
It is difficult to imagine any observational scenario where a larger aperture would not help, but in reality we are not going to be able to build big enough  single telescopes to resolve stellar discs any time soon (though I gather Hubble may have managed this for Betelgeuse): interferometry is required.
adrianmcmenamin
CC BY-SA 3.0
2016-09-06T09:58:05.387Z
I think the question was about resolving the light curve shape better, not resolving stellar disks. So interferometry isn't really relevant (but larger apertures are).
Peter Erwin
CC BY-SA 3.0
2016-09-06T10:21:47.960Z
@adrianmcmenamin Actually, about a dozen or more stars have had their disks resolved. This is [an incomplete list](https://en.wikipedia.org/wiki/List_of_stars_with_resolved_images). But Peter Erwin is right, I was thinking about resolving the point source light curve better in time and depth (and spectrum and polarization) to find out more about what is transiting it.
LocalFluff
<telescope><optics><planetary-transits>
Can a telescope be built to focus on a point source star?
LocalFluff
# Can a telescope be built to focus on a point source star?

(Asked by: username_0 on 2016-09-03)

If a Kepler telescope were built to focus on one point source star at a time, would it provide more information, or did Kepler capture virtually every photon from any given star anyway?

In order to study known transit events more closely one star at a time, to possibly detect moons or atmosphere or rings by resolving the transit light curve's shape better, is a larger aperture necessary or would a smaller field of view help? What field of view would be optimal for observing a single point source?

**Tags:** telescope, optics, planetary-transits

> **Comments:**
> **username_1**: It is difficult to imagine any observational scenario where a larger aperture would not help, but in reality we are not going to be able to build big enough  single telescopes to resolve stellar discs any time soon (though I gather Hubble may have managed this for Betelgeuse): interferometry is required.  
> **username_2**: I think the question was about resolving the light curve shape better, not resolving stellar disks. So interferometry isn't really relevant (but larger apertures are).  
> **username_0**: @username_1 Actually, about a dozen or more stars have had their disks resolved. This is [an incomplete list](https://en.wikipedia.org/wiki/List_of_stars_with_resolved_images). But username_2 is right, I was thinking about resolving the point source light curve better in time and depth (and spectrum and polarization) to find out more about what is transiting it.  

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## 1 answers

### Answer by username_3 on 2016-09-13. Score: 4

A smaller field of view would *not* necessarily help. The data transmitted back to Earth by Kepler was already limited by telemetry constraints and the portions of the CCD that were read out were already focused on small windows around (many) selected stars. Having a field of view that is at least 10 arcsecond across would be sufficient. What is more important is to make sure your star images are well-sampled. This is a bit of a problem for Kepler. The CCD pixels are quite big (4 arcseconds) to give a big field of view, but this is similar to the angular resolution of the telescope. This means all the light can fall in basically one pixel, which is not ideal for precise photometry. If you were focusing on one star you would make the field of view smaller but have lots of CCD pixels across your star image. This improves the precision of the photometry considerably.

If you were to construct a similar telescope designed to look at one star at a time, then in principle you could have a much higher telemetry rate for that star. Hence, if you had enough photons and a low CCD readout noise, you could take much shorter exposures and see finer detail in the light curve transits - maybe moons, rings, starspots etc. The standard Kepler cadence was 30 minutes for most stars and one minute for some of the brightest stars - see [this link](http://keplergo.arc.nasa.gov/PyKEprimerResources.shtml). A more rapid cadence would allow a much better characterisation of the host star through [asteroseismology](https://en.wikipedia.org/wiki/Asteroseismology) (e.g. mass and radius), which in turn leads to better characterisation of its planets.

To take advantage of the higher telemetry rates, it would be advantageous to have a larger aperture telescope, simply to collect enough photons. The angular resolution of the telescope is not that important if investigating obviously single , bright sources. You would need an absolutely enormous telescope (100m+) to resolve the disks or planetary systems of even nearby stars, so this is not really feasible.

A larger telescope would also allow you to split the light by wavelength and investigate transits in a wavelength dependent way. This is really important for investigating atmospheric composition and chemistry. This is perhaps the only area where focusing on a single star might be useful - because you could design a slit spectrograph to monitor the transit spectrum. On the other hand, these days it is possible to design multi-object spectrographs using fibres or slit masks, so overall I can't see any real advantage to having a telescope that is capable of gathering data for only one star at a time.

> **Comments:**
> **username_0**: How many photons per second hit Kepler from any one star on average? My fantasy here is focusing a Kepler telescope's all 5.4 million pixels on one single point source star with a known exoplanet at a time, from ingress to egress. Preferably with a unique wavelength spectroscopically captured by each of the 5.4 million pixels. Then move to the next star's transit event a few hours later.  

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