<p>A reflection grating reflects its dispersed light away from the beam of the incoming light; a transmission grating or prism refracts it off at an angle to the incoming light as well. In both cases, you have to build the final part of the spectrograph (the imager with its sensor) at an angle to the optical axis of the instrument. This means that if you want to have both an imager and a spectrograph, they must have separate final optics and sensors, one (on-axis) for the imager and one (off-axis) for the spectrograph.</p>
<p>The special characteristic of a grism (usually a prism with a transmission diffraction grating carved into one face or a holographic transmission grating placed between two prisms) is that there is a &quot;central&quot; wavelength of light which passes <em>straight through the grism without deflection</em>; shorter and longer wavelengths are dispersed in opposite directions away from this. (This is achieved by the fact that the prism bends light in the same plane as the grating, but in the opposite direction, so that the diverging light from the grating is bent back into the original input direction; see figure below.)</p>
<p><a href="https://i.sstatic.net/4DxCW.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/4DxCW.jpg" alt="enter image description here" /></a></p>
<p>Sketch of grism behavior by Benjamin Weiner, from <a href="http://candels-collaboration.blogspot.com/2012/07/candels-spectroscopy-infrared-grism.html" rel="nofollow noreferrer">here</a></p>
<p>This means you can image your spectra onto a sensor located on the optical axis of the instrument.</p>
<p>So with a grism, you can economize by building <em>one</em> final imager (and sensor) for both direct imaging (grism kept out of the way) and for spectroscopy (grism placed in the beam). If the grism is compact enough, you can even mount it in the filter wheel, so that you just rotate the wheel to select either direct imaging with a filter or spectroscopy. Because the same camera is used for both imaging and spectroscopy, you can use an image of the field to properly match the spectra with the sources.</p>
<p>(This is not something unique to space telescopes; plenty of ground-based telescopes use grisms. The cost and space savings of an all-in-one imager-plus-spectrograph does make grisms especially suitable for space telescopes, though.)</p>
<p>(Also note that there is no &quot;cross-dispersion&quot; involved in a grism: it's all dispersion in the same plane.)</p>

CC BY-SA 4.0
2021-10-13T15:31:26.800Z
I keep looking at the image in the question and its description, and in this case it doesn't seem to match what you've explained. The grating is dispersing left-right and the prism is dispersing up-down. That's why I've asked about "cross dispersion". If they are crossed, there isn't a wavelength where the light passes straight through. The prism's dispersion is weak compared to that of the grating, but there's no wavelength with zero deflection for the case in the question at least.
uhoh
CC BY-SA 4.0
2021-10-13T18:43:27.637Z
Does this link help (e.g., the sketch of what the grism does)?
http://candels-collaboration.blogspot.com/2012/07/candels-spectroscopy-infrared-grism.html
Note that it’s not cross-dispersion: the grating and the prism are dispersing in the same plane.
Peter Erwin
CC BY-SA 4.0
2021-10-13T22:36:37.570Z
Sure enough that's a "space telescope GRISM" and those are parallel and counter-dispersive. Maybe the HST GRISM I chose for the example is peculiar... **update:** Figure 1 of [HST/WFC3 In-Orbit Grism Performance](https://www.researchgate.net/publication/252584964_HSTWFC3_in-orbit_grism_performance) shows images of all three (UVIS G280, G102, and G141) HST WFC3 GRISMs, and indeed the first one is different than the UVIS G280 (asked about) differs from the other two in that the grating and prism are cross-dispersing.
uhoh
CC BY-SA 4.0
2021-10-13T22:39:38.297Z
In the case of UVIS G280 it seems that a lot of work has gone into trying to recover information from multiple orders of the grating by reducing overlap via cross dispersion. cf [Into the UV: A precise transmission spectrum of HAT-P-41b using Hubble's WFC3/UVIS G280 grism](https://arxiv.org/abs/2003.00536) and [Trace and Wavelength Calibrations of the UVIS G280 +1/-1 Grism Orders](https://ui.adsabs.harvard.edu/abs/2017wfc..rept...20P/abstract) and [Updated Calibration of the UVIS G280 Grism](https://ui.adsabs.harvard.edu/abs/2020wfc..rept....9P/abstract)
uhoh
CC BY-SA 4.0
2021-10-13T22:41:02.317Z
and [Into the UV: A Precise Transmission Spectrum of HAT-P-41b Using Hubble's WFC3/UVIS G280 Grism](https://iopscience.iop.org/article/10.3847/1538-3881/ab7b78)
uhoh
CC BY-SA 4.0
2021-10-14T09:09:28.293Z
@uhoh None of those links describe a cross-dispersing spectrograph. I think you might be being confused by their use of "cross-dispersion" to mean "in the direction perpendicular to the dispersion direction" (as in extracting a profile from the recorded image in that direction, or, in the last link, moving *HST* in that direction).
Peter Erwin
CC BY-SA 4.0
2021-10-14T09:15:11.127Z
If you want an example of a grism used with cross-dispersion, there's the special GR700XD grism for *JWST*, which adds an extra (ZnS) prism in front of a standard grism. The extra prism is oriented in a perpendicular direction, to add (actual) cross-dispersion.

https://jwst-docs.stsci.edu/jwst-near-infrared-imager-and-slitless-spectrograph/niriss-instrumentation/niriss-gr700xd-grism
Peter Erwin
CC BY-SA 4.0
2021-10-14T10:58:34.990Z
My question is *already* "an example of a grism used with cross-dispersion"! It's described in great detail. Hopefully an answer will unravel why it's crossed.
uhoh
CC BY-SA 4.0
2021-10-14T12:10:54.737Z
@uhoh No, your question is *not* about "a grism used with cross-dispersion". There is no cross-dispersion in a grism.
Peter Erwin
CC BY-SA 4.0
2021-10-14T13:32:57.680Z
And yet *there most certianly is!* All the Hubble WFC3 literature calls the UVIS G280 a "grism" and it certainly has diffractive dispersion perpendicular to the refractive dispersion. It can be verified from the curved shapes for this one and not for the other two. If you want to challenge the use fo the word grism and say that the WFC3's designers and users misused a term, that would be a great answer! But the argument would be with all of them, not me.
uhoh
CC BY-SA 4.0
2021-10-14T15:37:30.313Z
@uhoh I'm sorry, but you do not understand what "cross-dispersion" means. To see what an actual cross-dispersed spectral image looks like, see [this slide set](https://www.ucolick.org/~bolte/AY257/ay257L7_2015.pdf), especially the ones labeled "X-SHOOTER", "IMACS", and "ESI". The cross-dispersion separates the individual orders (which can be curved -- but this itself is not a cross-dispersion phenomenon!) in the perpendicular direction. This is clearly *not* happening for UVIS G280: the individual orders sit on top of each other, because there was no cross-dispersion.
Peter Erwin
CC BY-SA 4.0
2021-10-14T21:41:10.893Z
Searching [user:7982 echelle](https://astronomy.stackexchange.com/search?q=user%3A7982+echelle) will show that I'm familiar with how crossed dispersion works in Echelle spectrographs. About a million years ago (undergraduate) I've even used one myself. Just because the prism in the UVIS G280 does a lousy/inadequate job of separating orders does not mean that its refractive dispersion is not perpendicular to the diffractive dispersion of the grating. It differs from the other two WFC3 "grisms" (their word, not mine) G102 and G141 in this regard, and that's why I've asked about UVIS G280 by name
uhoh
CC BY-SA 4.0
2021-10-14T21:51:10.093Z
So I'll post a new, specific question about this aspect in about 30 minutes, allowing for a better format for resolving this than SE comments. It will explore the "crossed-ness" and "grism-ness" of the Hubble WFC3 UVIS G280, both from an optical and a semantic point of view.
uhoh
CC BY-SA 4.0
2021-10-15T00:09:34.140Z
[Are the dispersion directions of the prism and the grating in Hubble WFC3 UVIS G280 perpendicular? Can we call this a "grism"? With cross-disparsion?](https://astronomy.stackexchange.com/q/47075/7982)
uhoh
CC BY-SA 4.0
2021-10-13T11:14:52.017Z
2021-10-14T10:01:16.150Z
2021-10-14T10:01:16.150Z
Peter Erwin
<p><a href="https://hst-docs.stsci.edu/wfc3ihb" rel="nofollow noreferrer">https://hst-docs.stsci.edu/wfc3ihb</a> lists the page <a href="https://hst-docs.stsci.edu/wfc3ihb/chapter-8-slitless-spectroscopy-with-wfc3/8-2-slitless-spectroscopy-with-the-uvis-g280-grism" rel="nofollow noreferrer">8.2 Slitless Spectroscopy with the UVIS G280 Grism</a> which contains details of one of the <a href="https://en.wikipedia.org/wiki/Grism" rel="nofollow noreferrer">GRISM</a>s of the Hubble Space Telescope (GRISM = Grating + Prism).</p>
<p><strong>Question:</strong> Why do space telescopes have GRISMS? Why use a grating AND a prism for cross-dispersion in slitless spectroscopy?</p>
<p>The explanation is detailed, but I don't understand the basic idea.</p>
<ol>
<li>What exactly is the purpose of a crossed grating+prism in slitless spectroscopy?</li>
<li>In what ways is the use of a GRSIM better than a grating or a prism alone?</li>
</ol>
<hr />
<blockquote>
<p>The G280 grism is a WF/PC1 spare. Figure 8.1 shows a spectrum of the wavelength calibration star WR14 observed as part of the Cycle 17 calibration program 11935. The circled spot shows the location of a direct image of the source obtained with a separate (undispersed) F300X filter exposure, but superposed on the grism image for illustrative purposes only. The prominent star-like feature near the center of the picture is the zeroth-order grism image, and the positive and negative higher orders extend towards the left and right of the zeroth order, respectively. The +1st order is defined to be the order with the higher throughput (due to the grating blaze), even though it falls at lower x-pixels than the position of the zeroth order. The +1st order extends to the left of the zeroth order a distance of about 1/4 of the image size. Further left there is heavy overlap with higher orders. Some prominent emission lines can be seen along the spectral trace.</p>
<p>Figure 8.1: Appearance of the G280 spectral orders on the detector.</p>
<p><a href="https://i.sstatic.net/XY8QL.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/XY8QL.png" alt="enter image description here" /></a></p>
<p>The circled source is the position of the direct image formed by summing an F300X image with the grism image. The stronger 1st order is to the left and the 0th order is in the center. Above the 1st orders, much weaker 2nd and 3rd orders are barely visible. The image shows the full extent of the detector in the x-axis and about 500 pixels in the y-axis.</p>
<p>There are several features of this grism that differ, for example, from the G800L grism on ACS. There is an offset of about 175 pixels in the y-direction between the direct image and the spectra, the zeroth-order is relatively bright due to a lower grating efficiency and clear substrate, and there is curvature of the spectra at the blue ends of the first orders (nearest the zeroth order). The amplitude of the curvature is about 30 pixels in the detector y-direction. Figure 8.2 shows a close up view of the first few positive orders of the WR14 spectrum, which illustrates the curvature at the short-wavelength end of each order.</p>
<p><a href="https://i.sstatic.net/IXdB1.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/IXdB1.png" alt="enter image description here" /></a></p>
</blockquote>
<hr />
<p>Hubble's not the only one!</p>
<p>From <a href="https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Kruk_WFIRST_APAC_April2018.pdf" rel="nofollow noreferrer">WFIRST Update; Jeffrey Kruk, WFIRST Project Scientist</a>  (<a href="https://web.archive.org/web/20211010211555/https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Kruk_WFIRST_APAC_April2018.pdf" rel="nofollow noreferrer">Archived</a>)</p>
<p><a href="https://i.sstatic.net/eof1P.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/eof1P.jpg" alt="from &quot;WFIRST Update; Jeffrey Kruk, WFIRST Project Scientist&quot; https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Kruk_WFIRST_APAC_April2018.pdf Archived https://web.archive.org/web/20211010211555/https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Kruk_WFIRST_APAC_April2018.pdf" /></a></p>

<spectroscopy><space-telescope><hubble-telescope><nancy-grace-roman-telescope>
Why do space telescopes have GRISMS? Why a grating AND a prism for cross-dispersion in slitless spectroscopy?
uhoh
# Why do space telescopes have GRISMS? Why a grating AND a prism for cross-dispersion in slitless spectroscopy?

(Asked by: username_0 on 2021-10-12)

<https://hst-docs.stsci.edu/wfc3ihb> lists the page [8.2 Slitless Spectroscopy with the UVIS G280 Grism](https://hst-docs.stsci.edu/wfc3ihb/chapter-8-slitless-spectroscopy-with-wfc3/8-2-slitless-spectroscopy-with-the-uvis-g280-grism) which contains details of one of the [GRISM](https://en.wikipedia.org/wiki/Grism)s of the Hubble Space Telescope (GRISM = Grating + Prism).

**Question:** Why do space telescopes have GRISMS? Why use a grating AND a prism for cross-dispersion in slitless spectroscopy?

The explanation is detailed, but I don't understand the basic idea.

1. What exactly is the purpose of a crossed grating+prism in slitless spectroscopy?
2. In what ways is the use of a GRSIM better than a grating or a prism alone?

---

> The G280 grism is a WF/PC1 spare. Figure 8.1 shows a spectrum of the wavelength calibration star WR14 observed as part of the Cycle 17 calibration program 11935. The circled spot shows the location of a direct image of the source obtained with a separate (undispersed) F300X filter exposure, but superposed on the grism image for illustrative purposes only. The prominent star-like feature near the center of the picture is the zeroth-order grism image, and the positive and negative higher orders extend towards the left and right of the zeroth order, respectively. The +1st order is defined to be the order with the higher throughput (due to the grating blaze), even though it falls at lower x-pixels than the position of the zeroth order. The +1st order extends to the left of the zeroth order a distance of about 1/4 of the image size. Further left there is heavy overlap with higher orders. Some prominent emission lines can be seen along the spectral trace.
>
> Figure 8.1: Appearance of the G280 spectral orders on the detector.
>
> [![enter image description here](https://i.sstatic.net/XY8QL.png)](https://i.sstatic.net/XY8QL.png)
>
> The circled source is the position of the direct image formed by summing an F300X image with the grism image. The stronger 1st order is to the left and the 0th order is in the center. Above the 1st orders, much weaker 2nd and 3rd orders are barely visible. The image shows the full extent of the detector in the x-axis and about 500 pixels in the y-axis.
>
> There are several features of this grism that differ, for example, from the G800L grism on ACS. There is an offset of about 175 pixels in the y-direction between the direct image and the spectra, the zeroth-order is relatively bright due to a lower grating efficiency and clear substrate, and there is curvature of the spectra at the blue ends of the first orders (nearest the zeroth order). The amplitude of the curvature is about 30 pixels in the detector y-direction. Figure 8.2 shows a close up view of the first few positive orders of the WR14 spectrum, which illustrates the curvature at the short-wavelength end of each order.
>
> [![enter image description here](https://i.sstatic.net/IXdB1.png)](https://i.sstatic.net/IXdB1.png)

---

Hubble's not the only one!

From [WFIRST Update; Jeffrey Kruk, WFIRST Project Scientist](https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Kruk_WFIRST_APAC_April2018.pdf) ([Archived](https://web.archive.org/web/20211010211555/https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Kruk_WFIRST_APAC_April2018.pdf))

[![from ](https://i.sstatic.net/eof1P.jpg)](https://i.sstatic.net/eof1P.jpg)

**Tags:** spectroscopy, space-telescope, hubble-telescope, nancy-grace-roman-telescope

---

## 1 answers

### Answer by username_1 on 2021-10-13. Score: 3 | Accepted answer

A reflection grating reflects its dispersed light away from the beam of the incoming light; a transmission grating or prism refracts it off at an angle to the incoming light as well. In both cases, you have to build the final part of the spectrograph (the imager with its sensor) at an angle to the optical axis of the instrument. This means that if you want to have both an imager and a spectrograph, they must have separate final optics and sensors, one (on-axis) for the imager and one (off-axis) for the spectrograph.

The special characteristic of a grism (usually a prism with a transmission diffraction grating carved into one face or a holographic transmission grating placed between two prisms) is that there is a "central" wavelength of light which passes *straight through the grism without deflection*; shorter and longer wavelengths are dispersed in opposite directions away from this. (This is achieved by the fact that the prism bends light in the same plane as the grating, but in the opposite direction, so that the diverging light from the grating is bent back into the original input direction; see figure below.)

[![enter image description here](https://i.sstatic.net/4DxCW.jpg)](https://i.sstatic.net/4DxCW.jpg)

Sketch of grism behavior by Benjamin Weiner, from [here](http://candels-collaboration.blogspot.com/2012/07/candels-spectroscopy-infrared-grism.html)

This means you can image your spectra onto a sensor located on the optical axis of the instrument.

So with a grism, you can economize by building *one* final imager (and sensor) for both direct imaging (grism kept out of the way) and for spectroscopy (grism placed in the beam). If the grism is compact enough, you can even mount it in the filter wheel, so that you just rotate the wheel to select either direct imaging with a filter or spectroscopy. Because the same camera is used for both imaging and spectroscopy, you can use an image of the field to properly match the spectra with the sources.

(This is not something unique to space telescopes; plenty of ground-based telescopes use grisms. The cost and space savings of an all-in-one imager-plus-spectrograph does make grisms especially suitable for space telescopes, though.)

(Also note that there is no "cross-dispersion" involved in a grism: it's all dispersion in the same plane.)

> **Comments:**
> **username_0**: I keep looking at the image in the question and its description, and in this case it doesn't seem to match what you've explained. The grating is dispersing left-right and the prism is dispersing up-down. That's why I've asked about "cross dispersion". If they are crossed, there isn't a wavelength where the light passes straight through. The prism's dispersion is weak compared to that of the grating, but there's no wavelength with zero deflection for the case in the question at least.  
> **username_1**: Does this link help (e.g., the sketch of what the grism does)? http://candels-collaboration.blogspot.com/2012/07/candels-spectroscopy-infrared-grism.html Note that it’s not cross-dispersion: the grating and the prism are dispersing in the same plane.  
> **username_0**: Sure enough that's a "space telescope GRISM" and those are parallel and counter-dispersive. Maybe the HST GRISM I chose for the example is peculiar... **update:** Figure 1 of [HST/WFC3 In-Orbit Grism Performance](https://www.researchgate.net/publication/252584964_HSTWFC3_in-orbit_grism_performance) shows images of all three (UVIS G280, G102, and G141) HST WFC3 GRISMs, and indeed the first one is different than the UVIS G280 (asked about) differs from the other two in that the grating and prism are cross-dispersing.  
> **username_0**: In the case of UVIS G280 it seems that a lot of work has gone into trying to recover information from multiple orders of the grating by reducing overlap via cross dispersion. cf [Into the UV: A precise transmission spectrum of HAT-P-41b using Hubble's WFC3/UVIS G280 grism](https://arxiv.org/abs/2003.00536) and [Trace and Wavelength Calibrations of the UVIS G280 +1/-1 Grism Orders](https://ui.adsabs.harvard.edu/abs/2017wfc..rept...20P/abstract) and [Updated Calibration of the UVIS G280 Grism](https://ui.adsabs.harvard.edu/abs/2020wfc..rept....9P/abstract)  
> **username_0**: and [Into the UV: A Precise Transmission Spectrum of HAT-P-41b Using Hubble's WFC3/UVIS G280 Grism](https://iopscience.iop.org/article/10.3847/1538-3881/ab7b78)  
> **username_1**: @uhoh None of those links describe a cross-dispersing spectrograph. I think you might be being confused by their use of "cross-dispersion" to mean "in the direction perpendicular to the dispersion direction" (as in extracting a profile from the recorded image in that direction, or, in the last link, moving *HST* in that direction).  
> **username_1**: If you want an example of a grism used with cross-dispersion, there's the special GR700XD grism for *JWST*, which adds an extra (ZnS) prism in front of a standard grism. The extra prism is oriented in a perpendicular direction, to add (actual) cross-dispersion.  https://jwst-docs.stsci.edu/jwst-near-infrared-imager-and-slitless-spectrograph/niriss-instrumentation/niriss-gr700xd-grism  
> **username_0**: My question is *already* "an example of a grism used with cross-dispersion"! It's described in great detail. Hopefully an answer will unravel why it's crossed.  
> **username_1**: @uhoh No, your question is *not* about "a grism used with cross-dispersion". There is no cross-dispersion in a grism.  
> **username_0**: And yet *there most certianly is!* All the Hubble WFC3 literature calls the UVIS G280 a "grism" and it certainly has diffractive dispersion perpendicular to the refractive dispersion. It can be verified from the curved shapes for this one and not for the other two. If you want to challenge the use fo the word grism and say that the WFC3's designers and users misused a term, that would be a great answer! But the argument would be with all of them, not me.  
> **username_1**: @uhoh I'm sorry, but you do not understand what "cross-dispersion" means. To see what an actual cross-dispersed spectral image looks like, see [this slide set](https://www.ucolick.org/~bolte/AY257/ay257L7_2015.pdf), especially the ones labeled "X-SHOOTER", "IMACS", and "ESI". The cross-dispersion separates the individual orders (which can be curved -- but this itself is not a cross-dispersion phenomenon!) in the perpendicular direction. This is clearly *not* happening for UVIS G280: the individual orders sit on top of each other, because there was no cross-dispersion.  
> **username_0**: Searching [user:7982 echelle](https://astronomy.stackexchange.com/search?q=user%3A7982+echelle) will show that I'm familiar with how crossed dispersion works in Echelle spectrographs. About a million years ago (undergraduate) I've even used one myself. Just because the prism in the UVIS G280 does a lousy/inadequate job of separating orders does not mean that its refractive dispersion is not perpendicular to the diffractive dispersion of the grating. It differs from the other two WFC3 "grisms" (their word, not mine) G102 and G141 in this regard, and that's why I've asked about UVIS G280 by name  
> **username_0**: So I'll post a new, specific question about this aspect in about 30 minutes, allowing for a better format for resolving this than SE comments. It will explore the "crossed-ness" and "grism-ness" of the Hubble WFC3 UVIS G280, both from an optical and a semantic point of view.  
> **username_0**: [Are the dispersion directions of the prism and the grating in Hubble WFC3 UVIS G280 perpendicular? Can we call this a "grism"? With cross-disparsion?](https://astronomy.stackexchange.com/q/47075/7982)  

---

