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I was looking on OPUS for New Horizons images from the Arrowkoth approach, and was surprised at the differences between back-to-back images taken with the same duration exposure. I'd expect them to be almost identical. Why are they often so different? Is there some contrast or sensitivity setting that changes, but isn't noted in the OPUS meta data?

For example, the images below (nh-lorri-lor_0408486979 and nh-lorri-lor_0408486989) are both 9.968 second exposures taken back-to-back at 2018-12-30T14:44:11.159 and 2018-12-30T14:44:21.159.

New Horizons image nh-lorri-lor_0408486979 New Horizons image nh-lorri-lor_0408486989

Why are there so many more objects visible on the right?

Another example are these two shorter duration exposures ((nh-lorri-lor_0408531907 and nh-lorri-lor_0408531908), also take back-to-back at 2018-12-31T03:13:08.527 and 2018-12-31T03:13:09.527.

New Horizons image nh-lorri-lor_0408531907 New Horizons image nh-lorri-lor_0408531908

These are shorter duration (0.601 s) exposures, so I am not surprised they are noisier, but why are their noise levels so different?

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    $\begingroup$ "sensitivity setting that changes" - very likely the amplification of the readout stage. But I can't find any hint either. $\endgroup$
    – asdfex
    Apr 14, 2023 at 9:15
  • $\begingroup$ Another hint: Pushing the contrast in the first image by a factor of 4 (i.e. using 0-63 as dynamic range instead of 0-255) gives almost precisely the second image plus obnoxious jpg artifacts. There doesn't seem to be a plain lossless image format available? $\endgroup$
    – asdfex
    Apr 15, 2023 at 14:57

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It turns out that the images on the OPUS website have been processed and calibrated to produce 8-bit images with the minimum intensity in each image set to 0 and the maximum intensity to 255. (The original raw Lorri images have 12-bit light level values which are converted to 32-bit double-precision numbers by a ground-based calibration.) Every web image may have a different dynamic range and scale.

The raw and calibrated images in Flexible Image Transport System (FITS) format can, however, be downloaded from the OPUS site. (Just click on the links above, add the image to your cart, then choose the formats you want to download.) It is not possible to upload .fits images here, so I have converted the top two raw images (lor_0408486979_0x633_eng.fit and lor_0408486989_0x633_eng.fit) above to png and show them here: Raw lor_0408486979 image Raw lor_0408486989 image

The two images are very similar, with the major difference being the short horizontal line in left image's upper right quadrant, that is almost certainly a cosmic ray. Raw LORRI images typically contain a few cosmic ray strikes which can appear as bright pixels or short streak. Such bright streaks can significantly change the light intensity scale, making dimmer stars invisible in the web images, which is likely why many fewer star are visible in left web image in the question. I would guess than even tiny changes in pointing the camera could also cause the scale in the web images to change as the brightest star may be spread over different numbers of pixels, changing the intensity of the brightest pixel.

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