Dark, narrow, straight lines move around within images from Interface Region Imaging Spectrograph (IRIS) in such a way that they appear intentional and systematic, and not just some imaging artifact as discussed in this SDO question and this SOHO question.

What causes this line?

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above: screen shots from YouTube videos of IRIS imagery.

below: The full YouTube videos of IRIS imagery.

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above: Photograph of IRIS from the detailed article about the IRIS' optical instrumentation at Spaceflight 101. Photo credit LMSAL.

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above: Details of IRIS' Spectrograph and Slit-Jaw Imager from the article about the IRIS' optical instrumentation at Spaceflight 101. Image credit NASA/LMSAL.

  • $\begingroup$ There is a lot of stuff contained in the optical path in the telescope: nasa.gov/mission_pages/iris/spacecraft/index.html I guess the line is part of a grid holding some component. It appears moving because the telescope is moving and the images have been realigned when making the video. Couldn't find a definitive answer... $\endgroup$
    – asdfex
    Commented Sep 20, 2016 at 11:13
  • $\begingroup$ @asdfex there is a lot more information in the Spaceflight 101 article! This is a fairly in-depth description of the complex optical system, but I still can't pinpoint what is really going on here. Remember, this dark line is in focus at the same time as the sun, so you can't just look for something just anywhere in the optical path, it is most likely happening at some intermediate focal plane. $\endgroup$
    – uhoh
    Commented Sep 20, 2016 at 11:38
  • $\begingroup$ Didn't find more hints in iris.lmsal.com/documents/iris_csr_instrument.pdf It's not a segmented CCD. It can be an artifact of the scanning procedure. The CCDs have 2 MPixel only, but the images seem to be higher resolution. Maybe it's just a gap between two scanning rows. Or even a broken row of the CCD itself $\endgroup$
    – asdfex
    Commented Sep 20, 2016 at 14:44
  • $\begingroup$ @asdfex look more closely - I think you found it! "E.6.1.3 UV Slit-jaw Imager Optical Design. The slit-jaw imager optics re-image the solar image reflected from the slit jaws onto a CCD detector." The slit jaws are mirrors!?! $\endgroup$
    – uhoh
    Commented Sep 20, 2016 at 15:04
  • 1
    $\begingroup$ You're right. The black area is the part of the image that is cut out to be sent to the spectrograph and therefore is missing in the CCD images. Section 1.4.2 of your "from here" link makes it clear. $\endgroup$
    – asdfex
    Commented Sep 20, 2016 at 15:22

1 Answer 1


@uhoh found the answer in one of the publications on the optical system of IRIS:

The IRIS telescope has two major measuring devices: First, an imaging device that can take normal photos of the surface of the sun in several different spectral bands. This is the source of the videos shown in the two videos.

The second device is the source of the black line in these pictures: A slit spectrometer. It consists of a slit jaw (two metal plates forming a narrow aperture) to cut out one line of the image of the sun and a prism that spreads the light depending on its wavelength. A usual CCD sensor is then used to measure the spectral distribution of the incoming light with high resolution.

The front of the slit jaw consists of mirrors to reflect the remaining light onto the imaging sensor. Naturally, this reflection misses the light that is used for the spectrometer and shows a black line as can be seen in the pictures. In order to gather spectral information across the whole surface of the Sun, the telescope turns slowly, selecting a different part for each successive image.

A rather extensive description of the optical system can be found in this publication

The scanning across the sun surface can be seen in this animation.

  • $\begingroup$ Thanks! I think the prism is called 'a pre-disperser', and the two spectrometers use highly dispersive diffraction gratings (3,600 lines/mm).. Also it looks like the fast scanning within the FOV is done with the active secondary mirror (sect. E.6.1.9 1st paragraph), but yep, the whole telescope does have to move to target different areas on the sun. $\endgroup$
    – uhoh
    Commented Sep 21, 2016 at 1:33

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