The NASA Goddard news item NASA to Demonstrate New Star-Watching Technology with Thousands of Tiny Shutters says:

The technology, called the Next-Generation Microshutter Array (NGMSA), will fly for the first time on the Far-ultraviolet Off Rowland-circle Telescope for Imaging and Spectroscopy, or FORTIS, mission on October 27. The array includes 8,125 tiny shutters, each about the width of a human hair, that open and close as needed to focus on specific celestial objects.

Question: What is an "Off Rowland-circle Telescope"? Are there "On Rowland-circle Telescope" as well?

And speaking of microshutters:


2 Answers 2


The term 'off-Rowland-circle telescope' is a bit awkward, I think normally we would call the arrangement of FORTIS an off-Rowland-circle spectrograph, and they probably just used the word 'telescope' to make their acronym sound better. If we're being pedantic, then I think everything before the primary focus would be the telescope, and everything after is the spectrograph.

There are certainly on-Rowland-circle spectrographs, it's the simplest type of reflective spectrograph, and the FORTIS paper linked above by @Keith McClary mentioned several: HUT, OREFUS and FUSE.

In terms of an off-Rowland-circle spectrograph, I think the footnote you mentioned on page 4 says it all:

In a Rowland circle spectrograph all the rays at wavelengths satisfying the grating equation entering a slit placed on a circle, with diameter is equal to the radius of the concave grating, are diffracted to a tangential focus on the same circle.

This is in contrast to FORTIS, which has magnification. In the figure below, I've taken the optical layout from the FORTIS paper and annotated it with an approximate Rowland circle in a bold black line. From this figure, we can see that the primary focus is inside the Rowland circle, and the secondary focus (at the detector) is outside the Rowland circle. For FORTIS to be a on-Rowland-circle spectrograph, both the primary and secondary focus need to be the same distance from the grating and on the Rowland circle.

FORTIS optical layout with annotated Rowland circle

  • $\begingroup$ Thanks! Can you also address "What is an 'Off Rowland-circle Telescope'"? Is it a Rowland-circle spectrometer duct-taped to a telescope, or is there some telescopic nature to the Rowland-circle optical design, or something else entirely? $\endgroup$
    – uhoh
    Sep 13, 2022 at 11:33
  • 2
    $\begingroup$ In my opinion, an off-Rowland-circle telescope is a bit of an awkward term that they probably used to make their acronym sound better. I haven't heard that term before, but I just took it to mean your former suggestion, a spectrometer duct taped to a telescope. $\endgroup$
    – Roy Smart
    Sep 13, 2022 at 12:25
  • 1
    $\begingroup$ @RoySmart That is correct :P And yes they have to make their acronyms sound nice! :D $\endgroup$
    – DialFrost
    Sep 13, 2022 at 12:35

@Roy_Smart has helpfully answered the second part of the question, so I'll answer the first :)


Basically NASA just pasted a Rowland circle on the telescope with adjustments and other equipment like microshutters to make FORTIS.

Longer answer:

First, let's have a look at the inside of the telescope:

FORTIS sounding rocket optical layout from Researchgate.net - FORTIS Pathfinder to the Lyman continuum:

enter image description here

Now this may sound confusing, but there is actually a Rowland circle, just not shown properly in the 2D layout:

Spectroscopic design in the far ultraviolet below Lyα demands a minimalist approach. The “two-bounce” prime focus designs of the HUT, OREFUS and FUSE observatories used relatively fast parabolic primary mirrors feeding the slit of a Rowland circle spectrograph. Although these two-bounce designs satisfy the requirement for efficiency they have poor off-axis spectral imaging performance. This is because fast focal ratios tend to increase the height of the astigmatic image, limiting the spatial resolution perpendicular to the dispersion. Astigmatism control is a major challenge in Rowland circle spectrographs and methods have been developed to eliminate it at select wavelengths, either by controlling the gratingfigure12or with holographic ruling methods.

From here, we can deduce a Rowland circle spectrograph is being used.

Noda first wrote out in detail the general aberration theory for holographic gratings, and since then techniques to minimize spectroscopic aberration shave become increasingly sophisticated. They reached a new level when solutions were found that made it possible to control not only astigmatism but coma at the high ruling densities required for the FUSE mission. The high resolution holographic gratings recorded for the Cosmic Origins Spectrograph (COS – slated to have been flown on the now canceled servicing mission 4 to HST) removed the spherical aberration with an aspherical grating figure along with a FUSE like aberration minimization solution. Recently developed numerically optimized solutions for two-bounce systems achieve excellent narrow band spectroscopic imaging with the grating used off the Rowland circle. An off-axis parabola design has a high enough spectral resolving power to cleanly separate the $O\; VI\; λλ1032 – 1037$ doublet over a 0.5° field of view and yield $4 – 9′′$ of spatial resolution.

Now according to the Researchgate.net - FORTIS Instrument Summary, there are definitely circles:

enter image description here

Further proven by the overall design of FORTIS:

enter image description here

Mathematical explanation of Rowland circle

From the previous research paper in a foonote:

In a Rowland circle spectrograph all the rays at wavelengths satisfying the grating equation entering a slit placed on a circle, with diameter is equal to the radius of the concave grating, are diffracted to a tangential focus on the same circle

For reference, here is the grating equation derivation from Newport.com - Diffraction grating physics and Newport.com - The Grating Equation:

From: enter image description here

Figure 2-2. Geometry of diffraction, showing planar wavefronts. Two parallel rays, labeled 1 and 2, are incident on the grating one groove spacing d apart and are in phase with each other at wavefront A. Upon diffraction, the principle of constructive interference implies that these rays are in phase at diffracted wavefront B if the difference in their path lengths, $d \sinα + d \sinβ$, is an integral number of wavelengths; this in turn leads to the grating equation. [Huygens wavelets not shown.]

We can deduce:

$$ m\lambda=d_G(\sin\alpha+\sin\beta_m) $$


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