PHOBOS-GRUNT BEFORE REENTRY
Any use of one of these images other than strictly private must be subject to prior authorization
This
video of Phobos-Grunt
was taken from Brunstatt (Alsace, France) on Decembre 23th 2011 at
5h56m28s UTC during a
passage at 56.9° of altitude direction NNE. Distance to observer: 310 km. Speed:
7.75 km/s.
On the video below, thanks to the specific orientation of the telescope mount (calculated with www.calsky.com), the movement of the satellite during the whole passage remains strictly horizontal, from left to right. Around culmination of the satellite, the Sun is on the right side and the trajectory of Phobos-Grunt is directed toward the Sun The images show that the Phobos-Grunt is moving backwards, with the solar panels deployed but not lightened by the Sun.
Instrument: Celestron
EdgeHD 8” Schmidt-Cassegrain telescope (focal length of 4000mm) on automatic
tracking system, as described on this
page. Camera:
Lumenera Skynyx L2-2 (12-bit files in fits format).
Downloadable DivX file (for private show only).
Notes about the reliability of small satellite images at the limits of telescopic resolution
At zenith, the maximum angular size of Phobos-Grunt, which is 5 times smaller
than the Space Shuttle, is only about 6 arcseconds. This size is to be compared
with the diameter of the Airy disc for a 14” telescope: 0.78 arcsec (1.1 arcsec
for a 10” telescope). This implies that, on the raw images, the satellite covers
a very small amount of pixels and that artifacts from many origins can appear
and can even outclass any real detail that would be recorded: atmospheric
(turbulence, dispersion….), instrumental (diffraction, various optical
aberrations such as chromatism, coma, astigmatism, shaking due to manual
tracking…), electronic (noise, image compression…).
To avoid to the maximum these risks and guarantee that all details are true, in
addition with the automatic tracking system, the following solutions were
chosen:
- a large aperture telescope (8” Schmidt-Cassegrain) with very good optics and a simple Barlow lens in front of the sensor.
- a
12-bit monochrome camera (uncompressed images in astronomical “fits” format)
with a green filter: in addition with the turbulence that is able to create any
arbitrary pattern on each raw image (as illustrated
on this page), the atmosphere also acts like a prism, spreading
colors along the vertical axis (blue towards zenith and red towards horizon). At
45° above the horizon, this dispersion of colors exceeds 1.5 arcsecond, to be
compared with the angular size of Phobos-Grunt (less that 4 arcseconds at 45°
above the horizon). This means that a color camera is useless for a so small
object since real color variations will be hidden by the atmospheric dispersion.
Moreover, the Bayer matrix of the color sensor introduces, on an object covering
a few pixels, artifacts that may subsist even if the image is converted to
black&white.
- application of a processing destined to improve the reliability of the images: all planetary imagers use images stacking since they know that one single raw image inevitably contains noise and that no, or very little processing, can be applied. Each image of the video above is a stack of 10 consecutive raw images, in order to improve the signal-to-noise ratio and to smooth the effects of turbulence.
-
the whole video sequence is presented, and not only one (or a few) arbitrarily
selected image, in order to show the consistency of the details recorded.
On the contrary, Ralf Vandebergh’s images of Phobos-Grunt taken on Nov 29th
2011 accumulate all handicaps. A standard JVC camcorder is used, delivering
8-bit compressed video files. The lens of the camcorder is placed behind an
eyepiece (on a 10” Newtonian), this represents a lot of glass and off-axis
aberrations and contributes to color artifacts due to the optics and the sensor,
as demonstrated on
this analysis of Nanosail images taken by the same author (this
analysis shows that the colors, as well as the shape visible on the image, do
not correspond to any real detail). Although manual tracking of the telescope on
an object moving at almost 2°/s and atmospheric turbulence (strong pressure
gradient and 50 km/h winds gusts over Netherlands at that moment, conditions
associated with significant turbulence) are able to create any distorted shape,
only one raw image is arbitrarily chosen and processed by heavy enlargement
(between 5 and 10 times) and by other processing able to make artifacts (such as
noise and compression effects) look like true details. Manual tracking implies
that the object wanders in the field of view of the camera and even goes in and
out of this field, the problem being that the off-axis images given by an
eyepiece and a camcorder lens suffer from off-axis aberrations, especially
astigmatism that may lead to extended and complex patterns. The details that are
supposed to be visible on these images are questionable also for the following
reasons:
-
considering the equipment used, the size of Phobos-Grunt on the raw video is
smaller than half the size of the satellite on my own images and these “details”
are smaller than the smallest details ever recorded by the same author on the
ISS,
-
the correspondence of these “details” with structures of Phobos-Grunt visible on
reference drawings and photos is vague: in the absence of any indication of
scale and of orientation of the satellite with regards to its trajectory and to
the Sun (and thanks to the ability of the human brain to find imaginary
correlations between groups of bright patches), many arbitrary and unverifiable
interpretations are possible. Picking up an image that "looks nice" and trying
all the possibilites of orientation of the satellite until a vague
correspondance is found, and then deducing that the image contains real details,
is a vicious circle reasoning.
The strong enlargement is performed at least in two steps, one with pixel resampling and one with pixel duplication. As a result, the processed image gives the illusion that the satellite covers much more pixels on the raw image that it actually does.
Unfortunately, the author does not accept to provide his raw video sequences of
this satellite or any other one, prohibiting any possibility of reliability and
consistency analysis of the raw data by peers.
(C) Emmanuel Rietsch 2012.