TRANSIT OF VENUS
08 JUNE 2004
TRACE WHITE LIGHT INGRESS/EGRESS IMAGING

Glenn Schneider, Steward Observatory, University of Arizona
gschneider@as.arizona.edu, http://nicmosis.as.arizona.edu:8000

in collaboration with Jay Pasachoff, Williams College
Jay.M.Pasachoff@williams.edu


BACKGROUND INFORMATION & PROGRAM OBJECTIVES

Motivated by historical accounts of previously observed transits of Venus (eg., Figure 1), and a desire to use the 8 June 2004 event as a surrogate to test observing methods, strategies and techniques which are being contemplated for future space missions to detect and characterize extrasolar terrestrial planets as they transit their host stars:

We used the Transition Region and Coronal Explorer (TRACE) Spacecraft to image the 8 June 2004 Transit of Venus in "White Light", and at high temporal cadence during (and flanking) the planet's crossing of the solar limb and while on the heavily limb darkened portion of the solar disk.

Our primary programmatic objectives are:

(a) to image the circum-Cytherian "aureola" (eg., Russell, 1874; Fig 1 top) - sunlight scattered by aerosols and refracted in the back-lit planetary atmosphere - with high spatial resolution and image stability and morphologically and photometrically study its "temporal evolution" (i.e., the spatially resolved surface brightness distribution as functions of phase angle ["impact distance"] and circum-planetary azimuth angles) prior to Contact II and after Contact III. See Tanga (Laboratiore Cassiopee - Observatoire de la Cote d'Azure) for an excellent description of these phenomena based upon historical observations.

(b) to confirm with space-based imagery that the conjecture that ground-based historical reports of "Black Drops" (eg., Bergman, 1761; Fig1 bottom) at inner planet transits arose from the convolution of the instrumental (telescopic/optical) plus atmospheric point spread function in conjunction with the limb-darkened photospheric illumination function (see Schneider, Pasachoff and Golub, 2004, Icarus, 168, 249), and investigate the detectability of circum-planetary refracted light during photospheric transit.


Fig 1. Historical observations of Venus atmospheric and "black drop" phenomena near transit contacts.


OBSERVATION DESCRIPTION

WAVELENGTH: All images (for our program) were obtained in TRACE's "White Light" (WL) configuration, which provides spectral sensitivity in the wavelength range from 1200 - 9600 Angstroms. The TRACE WL instrumental response to an un-limb darkened solar spectral energy distribution,  peaking 6000-8000 Angstroms, is shown HERE.

SPATIAL SCALE AND RESOLUTION: During the transit, Venus's angular diameter was app. 58.2 arcseconds (12,104 km at 0.289 AU), diametrically spanning 116 detector pixels.  The TRACE WL spatial resolution is Nyquist-limited by its 0.5" per pixel sampling of its CCD, not diffraction limited by the 30 cm. telescope.  With a WL pivot wavelength of 6200 Angstroms, Venus was resolved by app 60 band-integrated resolution elements. 

TEMPORAL CADENCE: With a desire for high inter-frame temporal cadence in highest fidelity data format (i.e., "lossless" or uncompressed), particularly during and near the limb contacts, we read out only a 512x512 sub-array of the 1024x1024 CCD detector due to both downlink bandwidth  and memory [image buffer] limitations.  Our observations were interleaved with observations taken for other programs (with other TRACE filters and data formats), thus the inter-frame temporal spacing was not uniform throughout our imaging sequences. During ingress we achieved a cadence of five frames per minute (uniformly spaced) across the limb contacts, but at a reduced rate of app two frames per minute at other times. At egress we obtained images at CIII +/- app 3 minutes at a (uniformly spaced) rate of 7 frames per minute, but at a slower rate of app one frame every 35s when the leading edge of Venus was further from the solar limb.  Some technical details of the as-planned observing program may be found here.


IMAGES, "MOVIES", AND STEREOGRAMS

We present here the first  photometrically rigorous results from the INGRESS and EGRESS imaging sequence taken in TRACE's WL pass band.  The TRACE WL optical channel, so we discovered, suffers from an app 1% intensity optical "ghosting."  Normally, this is not of concern for TRACE's solar observation programs, but as our interest rests primary in the "bottom" 1% of the (12 bit) dynamic sampling range, this presented a significant photometric "challenge."  With detailed characterization we found the "ghost" to be double, with different degrees of afocality and intensity, plus an additional diffuse (not afocally specular) component more difficult to analytically characterize. For each frame, we built and subtracted models of the instrumentally scattered light to remove these artifacts (examples of this will be added later). The images presented here have been post-processed in this manner, and while not "perfect" this very effectively mitigates the instrumentally ghosted/scattered light.  This procedure, however, cannot recover intensity information with sufficient photometric precision in an app 2 pixel  (1.0" - 1.5") wide region exterior to the photosphere where the solar limb is bifurcated by the Cytherian disk.  This is the reason for the "gap" you  will note in the image of Venus presented below as it transits the solar limb.

Image Orientation: North is "up" in all of the downloadable images and movies presented here.

Click HERE to view "3D" stereograms of the transit made from selected TRACE imaging frames.

The ingress sequence is comprised of 98 frames from 050553 UT to 054751 UT. The egress sequence is comprised of 84 frames 105231 UT to 112315 UT. Both sequences have been assembled into Quicktime movies (Fig 3), with a playback rate of 10 frames per second. To view/download the movies (21/24 Mbytes each, respectively) rendered as 8-bit (256 level grayscale) click on the title frames below.  Higher fidelity versions of the movie (42/48 Mbytes each) rendered as 16-bit (32K level grayscale) may be downloaded HERE.   (Need a QuickTime viewer?  Go HERE - even Windoze folks).

Click on Movie Title Frames (below) to View 8-bit Movies





Fig 3. Links to downloadable movies of the 8 June 2004 TRACE ingress and egress imaging - heliocentric registration
 
(Need a QuickTime viewer?  Go HERE - even Windoze folks).



VENUS CENTERED MOVIES

The evolution of the morphological and photometric characteristics of the light scattered and refracted by the Cytherian atmosphere is more readily visualized by holding the position of Venus "fixed" from frame to frame as it traverses the solar limb.  This was done for the relevent portion for th eingress and egress sequences and may be viewed in the QuickTime movie (appx 10 Mbyte each) below.

Click on Title Frame (below) to View  Movie



Fig 4. Links to downloadable movies of the 8 June 2004 TRACE ingress and egress imaging - Venus centered registration



Below, we have extracted and show several frames from the INGRESS and EGRESS movies, with explanation of the display format and comments on the images below.
 

SELECTED FRAMES FROM INGRESS IMAGING SEQUENCE

050628 UT - First Contact

052010 UT - Geometrical Mid Transit

052727 UT - Contiguous Atmospheric Scattered-Light Arc

053309 UT - Atmosphere Brightens with Decreasing Limb Distance

053706 UT - Approaching Second Contact

053850 UT - SECOND CONTACT - No Intrinsic "Black Drop"!

053850 UT - SECOND CONTACT -  PSF Broadening IS Seen

053924 UT - Clean Separation - The Trek Across the Photosphere Begins
Fig 4 - Ingress imaging (selected TRACE WL frames)

SELECTED FRAMES FROM EGRESS IMAGING SEQUENCE


105849UT - Intra-limb PSF convolution darkening seconds before CIII

105849 UT - (harder display stretch) - No Intrinsic "Black Drop"!
105949 UT (one minute later) - Venus atmosphere easily visible & asymmetric
110235 UT - Bi-lateral azimuthal asymmetry in atmospheric arc

110823 UT - Halfway through egress, half the arc remains

112239UT - Contact IV and a small piece of the arc is still visible (see movie)
Fig 5 - Egress imaging (selected TRACE WL frames)



IMAGE DISPLAY DETAILS

In the first six frames of the ingress sequence and in the "Cytherian Atmospheric Scattering" movies, a linear dynamic display range is used, from -15 (hard black)  to +55 (hard white) instrumental counts (DN = Data Numbers) per pixel.  (TRACE has a dynamic data range of 4096 DN.)  The background sky has been median normlized to zero DN, and the one-sigma noise in the background is appx 2 DN.  In the bottom two frames of the ingress sequence, and in the "No Intrinsic Black Drop" movies, a display stretch over a larger dynamic range, to better define the limb of Venus against the "noise" of the solar granulation.  The same two display stretches are used for the egress sequence images.

The solar photospheric radial limb darkening profile has been removed from these images. This "flattens" the intensity of the photosphere and collapses the dynamic display range, thus very well shows the photospheric granulation.  After "flattening" the intensity variation in the photospheric features is 20 times greater than the light seen, at its brightest, in the Cytherian atmospheric arc. 

To simultaneously visualize Venus's atmosphere against the sky, and Venus's limb against the photosphere, the solar disk (and with Venus in front of it) has been reduced in intensity by a factor of ten. The median level of the photosphere (after flattening by removing the radial limb darkening profile and subsequent reduction in intensity by a factor of 10) has been set to +25 DN (rather than zero, i.e., offset from the sky) to enable, very easily, seeing the solar limb against the sky background.  With this display stretch, the interior silhouetted disk of Venus appears hard black against the granular photosphere (i.e., similar to the contrast as it appears to the eye with the Sun appropriately filtered).  Simultaneously, the disk of Venus against the sky appears as the same intensity as the sky, which as Venus is back lit, is appropriate.

The photosphere, in the image frames from the "No Intrinsic Black Drop" movies have, has been attenuated only by an additional a factor of two, after removal of the solar limb darkening.  Venus obscures the portion of the solar limb between Contacts I & II and Contacts III & IV.  As a guide to the eye, during those time intervals, the location of the limb "behind" Venus is annotated with a thin arc at the photosphere/sky background interface.

COMMENTS: These images very clearly show the emergence of sunlight scattered (and refracted) by the Cytherian atmosphere as Venus approaches the line-of-sight to the solar disk.  The scattering, clearly, is asymmetric, as a small arc of atmosphere first becomes visible "below" the disk of Venus, not on a line perpendicular to the solar limb.  By "geometrical mid-ingress," when the solar limb is equally bifurcated by Venus, this arc of scattered light is readily apparent.  See the "movie" to see how early you can detect it.  Note that "geometric" mid-ingress is not coincident in time with temporal mid-transit due to the parallactic motion of Venus induced by TRACE's Sun synchronous orbit about the Earth.  (By the way, here are some other visualizations of the transit geometry).  About 12 minutes before second contact the atmospheric arc becomes "full" and contiguous, and grows brighter with decreasing angular distance to the solar limb. Upon egress the asymmetry in the brightness along the atmospheric arc is striking. Second (053850 UT image)  and third (115849 UT image) contacts are geometrically "sharp" except for "blurring" by the 1" FWHM TRACE WL PSF and residual instrumentally scattered light in the trace optical system, with no evidence of the classical/historical "black drop" effect (gone, now, the way of the Martian canals).

The above is intended to provide PRELIMINARY information in a timely manner.  This page will be updated frequently in the weeks (and months) ahead with additional information on the TRACE imaging program (and ground-based imaging from Thessaloniki, Greece), reflectance spectroscopy off the Moon from the Steward Observatory 2.3 meter Bok Telescope on Kitt Peak, and radiometric measures from the ACRIM III satellite.



ADDITIONAL INFORMATION AND ON-LINE DATA/IMAGES IN PUBLICATION

Please see our paper: Pasachoff, J. M., Schneider, G., Widemann, T., 2011, "High Resolution Satellite Imaging of the 2004 Transit of Venus and Asymmetries in the Cytherian Atmosphere", Astronomical Journal, 141, 112, for additional information, details, and data.

We have made our full set of instrumentally calibrated data images as FITS files, JPG displays, and animated movies available for public, non-profit, public use by other researchers and for Education and Public Outreach activities through the  on-line HTML enhanced version of the above Astronomical Journal paper hosted by IOP Science:  http://iopscience.iop.org/1538-3881/141/4/112/fulltext. We request appropriate accreditation/citation for use of these data/annimations/images.  Please direct any questions to Glenn Schneider (gschneider@as.arizona.edu).




ACKNOWLEDGMENTS

MANY thanks to Karel Schrijver, Ted Tarbell, and other members of the TRACE science and operations teams for their invaluable assistance and efforts in planning and implementing these observations (and tolerating all of my many times daily queries, comments, suggestions, etc. during that effort)! Kayla Gaydosh (Bryn Mawr College) is assisting with the image calibration and post-prrocessing under a summer research program supported by the Keck Northeast Astronomy Connsortium.  We also thank the Committee for Research and Exploration of the National Geographic Society for their support.



 More TRACE Venus Transit Imaging


The Transition Region and Coronal Explorer, TRACE, is a mission of the Stanford-Lockheed Institute for Space Research (a joint program of the Lockheed-Martin Advanced Technology Center's Solar and Astrophysics Laboratory and Stanford's Solar Observatories Group ), and part of the NASA Small Explorer program.


Venus Transit 2004 - Linking the Past to the Future
A Stepping Stone to Extrasolar Terrestrial Planetary Detection and Characterization
Some Pre-Transit Accounts in the Popular Press and on the Web


Inquires to: gschneider@as.arizona.edu (or gschneider@mac.com)

RETURN TO GLENN SCHNEIDER'S HOME PAGE