What is the Triangular shaped Structure in STEREO Behind HI2?
Triangular shaped object in STEREO data explained
Some people have noticed a strange triangular or diamond-shape "object" entering the field-of-view of the HI2 telescope on STEREO Behind around December 26, 2011. You can see the feature in question in this movie moving from right-to-left, just above the trapezoidal occulter on the right side of the image, and more clearly in this close-up movie . What is this?
The answer lies on the exact opposite side of the image. At the same time as this strange-looking feature starts being visible, the very bright planet Venus enters the HI2-B field-of-view from the lower left. Notice that Venus and the feature stay in step almost exactly opposite each other across the middle of the detector. This is not a coincidence. The strange looking geometrical "object" is actually an internal reflection of the planet Venus within the telescope optics. This effect has been seen many times before. Here's a particularly striking example of internal reflections caused by the planet Earth as seen early in the STEREO mission, taken from our image artifacts pages.
Earth is also visible in the December 2011 movie, but is not as bright as Venus. It is the stationary object to the center left. As an added bonus, the tail of Comet Lovejoy can be seen entering from the left towards the end of the movie, just below Venus. Here is a diagram showing the location of the various features in the image:
Positions of STEREO A (Ahead) and B (Behind) on Dec 30 2011 10:15
Heliocentric distance (AU)
Carrington rotation number
Heliographic (HEEQ) longitude
Heliographic (HEEQ) latitude
Earth Ecliptic (HEE) longitude
Earth Ecliptic (HEE) latitude
Roll from ecliptic north
Roll from solar north
Light travel time to Earth (min)
Separation angle with Earth
Separation angle A with B
There are five imaging instruments on each STEREO spacecraft. They form STEREO's SECCHI imaging suite.
Together they image all the way from the Sun to the orbit of Earth - the first time this has been done with a single spacecraft. This is such a distance across the sky that it is difficult to depict on a flat screen.
The image above shows the fields of view (FOV) of the three sun centered imagers:
EUVI - Shows the Sun in extreme UV light out to 1.7 times the Sun's radius
Cor1 - Visible light coronagraph which observes from 1.5 to 4 solar radii
Cor2 - visible light coronagraph which observes out to 15 solar radii
In this depiction Earth's orbit is on the left and the Sun is on the right.
In addition to the data from three sun-centered imagers show above, here we also see data from the two Heliocentric Imagers, visible light imagers which are pointed away from the Sun:
HI1 - Observes visible light from the outer part of Cor2's field of view to about a third of the distance to Earth's orbit
HI2 - Completes the Sun-Earth view by extending the view to Earth's orbit
The illustration only shows the fields of view of the Ahead spacecraft. The Behind spacecraft observes the area between the Sun and Earth from the other direction, so that between them we see 180 degrees of sky.
Here's an image from the HI2-B telescope which shows a number of interesting internal reflections. Source
Link | Source
The brightest object in the field is our own planet Earth, which is responsible for all the reflection effects. Also visible is the Moon, and the planet Mars.
Internal reflections of the planet Earth, as seen in the HI2-B telescope on May 5, 2007.
Image artifacts - Beacon data
STEREO has two separate telemetry streams coming down from each spacecraft, the space weather beacon telemetry, and the science recorder playback telemetry. The beacon telemetry contains the most recent data and images, and is transmitted 24 hours per day. A volunteer network of antenna stations around the world collect as much as possible of this real-time data stream, and send it to the STEREO Science Center for processing. However, because the beacon telemetry rate is very low, the images need to be compressed by large factors, and are thus of much lower quality than the actual science data.
The science data collected by the STEREO spacecraft are written to the on-board recorder, which is then read out and transmitted to the ground during daily telemetry tracks using the NASA Deep Space Network. These data are of much higher quality than the beacon data, but take several days to reach the STEREO Science Center website. Thus, the most recent images on the STEREO Science Center browse tool will always be beacon images. These temporary beacon images are replaced with the full-quality versions as they become available, generally about 2-3 days later.
Beacon images can always be recognized by having the character " 7 " near the end of the filename, e.g. "n 7 euA", while the full resolution images will have the character " 4 " in that location.
Also check out the write-up on cosmic rays to see how the high compression factors used for the beacon data affect those artifacts.
Although the initial beacon JPEG images on the browse pages are replaced by the high resolution images as they become available, the original FITS versions of the beacon files are saved, and can be read using any of a large number of software packages , most of which are free.