Posted on May 30, 2019 May 30, 2019 by Evan Gough
Why do Some Hubble Images Have That Chunk Taken Out of the Corner?
Great question! The answer has to do with Hubble's history.
It's easy to forget that the Hubble has been in space since 1990. That's going on 30 years now. And during that time, it's been serviced and had its cameras upgraded.
The camera responsible for these "staircase" images is the Wide Field and Planetary Camera 2 (WFPC2). It was installed on the Hubble Space Telescope in 1993 during Servicing Mission 1 by the Space Shuttle Endeavour. WFPC2 was operational until 2009, when it was replaced by the Wide Field Camera 3 (WFC3).
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WFPC2 is senses light in different wavelengths, infrared, ultraviolet, and visible. It has four identical CCDs, each one 800 x 800 pixels. Three of them are arranged in an "L" pattern, and make up the Wide Field Camera. The fourth one has a narrower focus for capturing more detailed images of a smaller part of the WFPC's view.
A WFPC2 image of the Pillars of Creation in the Eagle Nebula with the telltale staircase feature. Image Credit: By Credit: NASA, Jeff Hester, and Paul Scowen (Arizona State University) – http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/34/image/a, Public Domain, https://commons.wikimedia.org/w/index.php?curid=129538
Typically, the view of all four CCDs is combined, and it's the fourth CCD with a narrower focus that fills in the small square in the "L" shape. That's because its higher magnification means the image form the fourth CCD needs to be shrunk in order to align with the other three.
This image of the Hubble Deep Field shows how the four separate sensors combine to create the telltale staircase image that WFPC2 is known for. Image Credit: By The original uploader was John Fader at English Wikipedia. – Transferred from en.wikipedia to Commons by Mike Peel using CommonsHelper., Public Domain, https://commons.wikimedia.org/w/index.php?curid=5576829
It's worth pointing out that most of these staircase images are just for us, interested members of the public. Scientists using the Hubble to study objects in space have access to the full-resolution images of the Planetary Camera portion of the image.
Spiral Galaxy M100 (NGC 4321) as imaged by the WFPC2 on the Hubble. Image Credit: By J. Trauger, JPL and NASA/ESA – http://www.spacetelescope.org/images/opo9449c/ (TIFF image), Public Domain, https://commons.wikimedia.org/w/index.php?curid=1824689
In 2009, the WFPC2 was replaced by the Wide Field Camera 3 (WFC3) during Hubble Servicing Mission 4 (STS-125, Space Shuttle Atlantis.) WFC3 also sees in optical, UV, and infrared, but doesn't have a CCD with a separate magnification. So it doesn't have that "annoying" staircase missing piece.
This image of the Pillars of Creation in the Eagle Nebula was captured by the newer WFC3. No staircase! It was taken as a tribute to the original Hubble WFPC2 image, but this one is much higher resolution thanks to the updated technology. Image Credit: By NASA, ESA, and the Hubble Heritage Team (STScI/AURA) – http://hubblesite.org/image/3471/news_release/2015-01, Public Domain, https://commons.wikimedia.org/w/index.php?curid=38165284
Since there are no more servicing missions scheduled for the Hubble, and there are no more space shuttles to perform them, WFC3 was designed to see the Hubble through to the end of its mission, whenever that sad day will be.
An infrared view of the Horsehead Nebula captured by the WFC3 on the Hubble. Note the fine resolution, and lack of staircase cut-out. Image Credit: By ESA/Hubble, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=25666692
NASA thinks that the Hubble could keep going through the 2020s, but at some point it will stop working. The original plan for the venerable Space Telescope was to retrieve it with a shuttle. But, obviously, that's not going to happen. (Who knows, maybe one of these billionaire space entrepreneurs will go up and get it. Imagine the cachet!)
A uncontrolled re-entry would probably not destroy the Space Telescope completely, and there's risk of human fatalities depending on where it comes down. But NASA was thinking ahead. As part of the final servicing mission in 2009, NASA installed the Soft Capture Mechanism (SCM) and the Relative Navigation System (RNS). Together, they're called the Soft Capture and Rendezvous System (SCRS). That system gives NASA some options to safely de-orbit the Hubble.
Until, then Hubble is still a busy, working science instrument, and one that gives us a steady diet of science eye-candy. Enjoy.
CategoriesAstronomy, Hubble, NASA TagsHubble Space Telescope, NASA, NASA Hubble Pictures, Wide Field Camera 3, Wide Field Planetary Camera 2
One Reply to "Why do Some Hubble Images Have That Chunk Taken Out of the Corner?"
Alkaid says:
Interesting but I'm not sure terminology is used correctly.
"Magnification" is a term which cannot be properly applied to astrophotography images – especially when displayed on digital equipment with variations in scale and such.
The WFPC2 had three sensors which each had an 800×800 array of pixels. These pixels yielded/imaged at 0.0996 arc-seconds per pixel (APP).
The fourth sensor still had an 800×800 but its array of much smaller pixels which gave an APP of 0.0455.
This meant that the fourth sensor with the smaller pixels was capable of greater potential resolution. "Image Scale" was thus different, not "magnification".
One other thing to consider? By today's amateur astrophotography standards the WFPC2 camera had relatively few pixels and lots of read noise (but FWC appears to be quite good). IIRC "Drizzling" was invented for the HST in order to give us better HST images.
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