<p>CCDs are optimized for a certain wavelength range, and for a certain expected signal level. In astronomy, we tend to be short of light, so here we almost always want them to be as sensitive as possible (an exception may be observations of the Sun, which I don't know much about). But for instance, the Nordic Optical Telescope has a CCD which is optimized for blue wavelengths, but has quite a lot <a href="https://www.astro.uni-bonn.de/theli/gui/aboutbackground.html#fringing" rel="nofollow">fringing</a> in the near-infrared. And further out in the IR, CCDs aren't even used, instead using something which are just called "detectors".</p>

<p>However, whether the CCD is used for imaging (photometry and astrometry) or spectroscopy does not have anything to do with the CCD; it's just a matter of inserting a grism or not. I'm not really into the instruments of Gaia, but I assume that differences in the CCDs are due to different wavelength regions being probed. There may be a difference in how its sub-parts (it's actually an array of CCDs) are positioned (for instance, for spectroscopy in principle you don't need a large field of view, but can do with a long array rather than a more square one), but the design of the individual CCDs are the same.</p>

CC BY-SA 3.0
2015-10-06T15:52:27.513Z
More-or-less. But actually things like the plate scale matter too. For instance you might want smaller pixels to better characterise the centroid of a star for doing astrometry, but on the other hand you might want a bigger CCD with bigger pixels to give you a bigger wavelength range when doing spectroscopy. The Gaia spectra are in the infrared so you want a deep depletion CCD.
ProfRob
CC BY-SA 3.0
2015-10-07T06:37:46.367Z
@RobJeffries: But couldn't you just as well want smaller pixels to achieve a high spectral resolution, or a bigger CCD with larger pixels to get a large field of view for imaging?
pela
CC BY-SA 3.0
2015-10-07T13:44:21.680Z
Detectors are designed around high level "science goals". For example, the science goals for HST/COS: https://en.wikipedia.org/wiki/Cosmic_Origins_Spectrograph#Science_Goals
Donald.McLean
CC BY-SA 3.0
2015-10-07T16:33:50.197Z
There is a limit to the number of pixels so you have to make a choice, or have an array - but then data size might be a problem, especially on a spacecraft. @Donald.McLean is right, you would choose the CCD to match the highest level science goals. Actually, smaller pixels does not improve the spectral resolution unless it was undersampled to begin with. Centroiding astrometry is a different issue.
ProfRob
CC BY-SA 3.0
2015-10-06T13:23:53.557Z
2015-10-06T13:23:53.557Z
pela
<p>My knowledge of CCDs is that these are sensors which collect photoelectrons. That's about it. </p>

<p>What is a difference between CCDs used for astrometry, spectroscopy, and photometry? As an example, each type of CCD is currently used by Gaia. </p>

<p>Recommendations of where else to read about this at a detailed engineering level is appreciated. Thanks.</p>

CC BY-SA 3.0
2015-10-04T18:50:59.133Z
Doesn't answer the question, but I found an abstract in ADS that you may find interesting if you can view the entire article. 2012Ap&SS.341..31D
Jack R. Woods
CC BY-SA 3.0
2015-10-04T20:33:58.320Z
@JackR.Woods http://arxiv.org/abs/1201.3238
Thanks!
ShanZhengYang
<spectroscopy><astrometry><photometry>
Gaia: What is the difference between CCDs used for astrometry, photometry, and spectroscopy?
ShanZhengYang
# Gaia: What is the difference between CCDs used for astrometry, photometry, and spectroscopy?

(Asked by: username_0 on 2015-09-27)

My knowledge of CCDs is that these are sensors which collect photoelectrons. That's about it.

What is a difference between CCDs used for astrometry, spectroscopy, and photometry? As an example, each type of CCD is currently used by Gaia.

Recommendations of where else to read about this at a detailed engineering level is appreciated. Thanks.

**Tags:** spectroscopy, astrometry, photometry
> **Comments:**
> **username_1**: Doesn't answer the question, but I found an abstract in ADS that you may find interesting if you can view the entire article. 2012Ap&SS.341..31D  
> **username_0**: @JackR.Woods http://arxiv.org/abs/1201.3238 Thanks!  

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## 1 answers

### Answer by username_2 on 2015-10-06. Score: 1 | Accepted answer

CCDs are optimized for a certain wavelength range, and for a certain expected signal level. In astronomy, we tend to be short of light, so here we almost always want them to be as sensitive as possible (an exception may be observations of the Sun, which I don't know much about). But for instance, the Nordic Optical Telescope has a CCD which is optimized for blue wavelengths, but has quite a lot [fringing](https://www.astro.uni-bonn.de/theli/gui/aboutbackground.html#fringing) in the near-infrared. And further out in the IR, CCDs aren't even used, instead using something which are just called "detectors".

However, whether the CCD is used for imaging (photometry and astrometry) or spectroscopy does not have anything to do with the CCD; it's just a matter of inserting a grism or not. I'm not really into the instruments of Gaia, but I assume that differences in the CCDs are due to different wavelength regions being probed. There may be a difference in how its sub-parts (it's actually an array of CCDs) are positioned (for instance, for spectroscopy in principle you don't need a large field of view, but can do with a long array rather than a more square one), but the design of the individual CCDs are the same.

> **Comments:**
> **username_3**: More-or-less. But actually things like the plate scale matter too. For instance you might want smaller pixels to better characterise the centroid of a star for doing astrometry, but on the other hand you might want a bigger CCD with bigger pixels to give you a bigger wavelength range when doing spectroscopy. The Gaia spectra are in the infrared so you want a deep depletion CCD.  
> **username_2**: @RobJeffries: But couldn't you just as well want smaller pixels to achieve a high spectral resolution, or a bigger CCD with larger pixels to get a large field of view for imaging?  
> **username_4**: Detectors are designed around high level "science goals". For example, the science goals for HST/COS: https://en.wikipedia.org/wiki/Cosmic_Origins_Spectrograph#Science_Goals  
> **username_3**: There is a limit to the number of pixels so you have to make a choice, or have an array - but then data size might be a problem, especially on a spacecraft. @Donald.McLean is right, you would choose the CCD to match the highest level science goals. Actually, smaller pixels does not improve the spectral resolution unless it was undersampled to begin with. Centroiding astrometry is a different issue.  

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