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Introduction
Summary of LASCO/EIT What is LASCO? What is EIT? (external link) FAQ Realtime Images RealTime Movies (SOHO Movie Theater) Download LASCO Data Data Products Image Gallery Movie Gallery Processing Levels FITS Header Keywords Data Policy Wavelet images/movies Coronal Mass Ejections Eclipse Observations LASCO Calibration LASCO C3 Planet transits (via Sungrazer) Team and Operations Resources LASCO/C1 at MPAe (Germany) LASCO at LAS (France) LASCO Handbook Technical Notes Detailed Documentation Acronyms Solwind Images and CMEs SOHO Home page SOHO and SOHO Instruments Other Solar Satellites and Observatories |
LASCO Movies and Images FAQ
1. Why is there a 16 hour gap in the real time movie ? 1. Why is there a 16 hour gap in the "real-time" movie ?SOHO is not in contact with the earth 24 hours a day. The Deep Space Network is a world wide network of antennas used to communicate with all deep space probes including SOHO, the Mars Rover, Voyager and Cassini. It is very busy supporting all of these missions. The Deep Space Network schedule varies from day to day and a spacecraft emergency for another spacecraft can affect the schedule. We are lucky to get a few hours a day of contact time. During the time when SOHO is out of contact the data are stored on a tape recorder (actually a solid state or RAM recorder). When the ground station establishes contact with SOHO, the solid state recorder containing the stored data is dumped and real time data starts. The hours of recorded data takes time to process so the 16 hour gap gets filled in over a few hours after contact and the tape recorder is dumped. When we are in contact the movie images get updated within minutes of the image being received. But remember that LASCO has a "1 hour" telemetry buffer (this actually varies according to the telemetry rate for a given day) so it may be an hour or more before an image taken aboard SOHO even starts being sent to the ground. All in all the daily gap varies from day to day and may extend up to 20 hours or more. See question number 6 below to see how to get our DSN contact schedule.
2. OK, I understand the data gap but why is the latest frame running 3 hours behind real-time ?This has almost the same answer as question 1. When we get archived data we start processing it even if there is real time data coming down. We get back to the realtime data as soon as we can. The forecasters really would like to know if something important has happened while out of contact. They get more warning time if we process the data in time order. Our MPG movies are updated on a roughly 3 hour basis and the JAVA movies on a 20 minute basis.
3. Why aren't there any LASCO data for the dates mm/dd/yy to mm/dd/yy ?SOHO uses its thrusters every few months to maintain its orbit and pointing. The flight operations team has to prepare very carefully for these operations since there are many checks and much data analysis required to make sure the thrusters fired properly and the spacecraft attitude and systems are operating correctly before and after the thruster firing. It is not lightly rescheduled since the orbit and thruster calculations would need to be redone. During that time LASCO and other SOHO instruments close their doors to prevent exhaust from the thrusters reaching their optics. We do calibration images (dark images , calibration lamp images or calibration opal ) or no image taking at all when our doors are closed. Thruster firings (somtimes noted as momentum management) are noted in the daily status reports and typically will take several days as the flight operations team analyzes the ranging data and does other measurements to make sure everything is okay. EIT does not close its door for thruster firing and usually continues taking images at a higher than normal cadence since LASCO is idle. Opening and closing doors for momentum management is done by many of the SOHO instruments and in a strict sequence. We have to wait our turn to reopen our doors.
4. Why aren't there any EIT data for the dates mm/dd/yy to mm/dd/yy ? What is a CCD bakeout ?The charge coupled device (CCD) is the part of the camera that is light sensitive. EIT does CCD bakeouts to improve the CCD performance every few months. There is a heater under the CCD that warms the CCD to drive off impurities and anneal the CCD. During this time EIT does not take images because a warm CCD doesn't have the high performance needed to see extreme ultraviolet light (EUV). The bakeouts typically last a day or two and involve warming the CCD from their normal operating temperature of -60 C to +25 C to drive off chemical contamination such as water vapor (ice) or gases from the spacecraft. There is also an annealing effect on the CCD which improves performance as defects due to cosmic rays are repaired. Ultraviolet instruments such as EIT are highly suspectible to contamination by chemicals because the UV radiation provides plenty of energy to change chemicals into a sticky goo that sticks to the cold surfaces of the instrument such as the CCD. Bakeouts are noted in the daily status reports. Over time the EIT instrument gets an image "burned in" to the CCD. Bright areas such as the latitude belt where active regions are common burn in faster. These areas will show as bright areas even though there is no real solar feature present. Rotating SOHO and doing off-points from sun center are done to help understand the degradation. The bake-out reduces the effect but it is a fact of life that the EIT CCD will degrade over time due to the EUV hitting the CCD. More EIT bakeout info
5. Aren't LASCO and EIT supposed to monitor the Sun all the time to forecast solar storms ?SOHO is a research satellite and LASCO and EIT are research instruments. There is a big difference between research instruments and instruments that are designed to meet an observational requirement such as constantly monitoring the Sun. We do observe the Sun for transient events, such as coronal mass ejections (CMEs) most of the time. But our mission is to do scientifically useful work. From time to time we may concentrate on taking images that are useful to someone's research such as taking EIT 304 angstrom images or take calibration images at the expense of our usual set of images. NOAA is the official agency in the U.S. charged by Congress to monitor the Sun and do solar forecasting. We provide data to NOAA and other agencies worldwide to help them do forecasting. NASA and other government agencies are specifically prohibited by Congress from making forecasts. See NOAA for more information. The images put on our Web site are for public education and outreach but our major mission is research not to provide images for the Web.
6. Are we seeing the images downloaded directly from the SOHO satellite ?No, SOHO, the satellite, is not on the Internet. NASA is working on Internet protocols for space, so perhaps someday you will be able to contact a spacecraft directly. In the meantime, telemetry from SOHO reaches the NASA Deep Space Network at one of many locations around the world (Goldstone, USA or Madrid, Spain or Canberra, Australia) and is sent to Goddard Space Flight Center in Maryland, USA and then to Naval Research Lab where the telemetry data are processed into images, converted to a movie format, and then put on the WWW. You can see the weekly DSN schedule of contacts with SOHO at the SOHO WWW site under Operations. The schedule uses day of the year and has start and end times in GMT for the contacts.
7. What can I see in a LASCO image ? What is that bright spot with a line through it ? Is it a planet ?This occurs for any object (whether a star or a planet) that is so bright that it is overexposed. The CCD detectors that we use "bleed" electrons along the columns of the CCD when the pixels cannot hold any more electrons. Chances are that the object is a planet or bright star. A look at the almanac and a star map will confirm that. We have had comets or dust particles bright enough to saturate our CCDs also. This saturation is similar to what you can sometimes see in TV or video cameras if they are overexposed. However new CCD cameras have special circuitry that minimizes the amount of "bleeding". But remember that in our case we are trying to cover a range of brightness that goes from the full brightness of the Sun to stars and a corona fainter than the naked eye can see. Many of the overexposed images are planets. A look at an almanac, astonomy magazine or one of the many sky almanac computer programs can help you identify objects you can see in our images. Nebula, star clusters, asteroids, planets and comets are visible in our images. Have a look at our Java annotated C3 image
8. Why can't I see the comet listed as being in the NW of the C3 frame ?An important note is that East is at the LEFT edge and West is at the RIGHT edge, North is at the top and South is at the bottom. So when the questioner has trouble finding the reported comet he or she is probably looking at the wrong side of the image. This follows the standard astronomical conventions of east and west as defined by the background stars. Most star maps also follow this convention (see for example Skyview Cafe) for a typical star map. Some of the comets are very small and difficult to see in single frames. You can check the Comet section of our WWW site for some examples if you are having trouble finding the comets in our images.
9. Why do I see cosmic ray streaks persisting from frame to frame in the movies ?Missing 32x32 pixel blocks are filled in with the corresponding block from the previous image. This was done for cosmetic reasons in the movies. It is not done in the raw data or archived data.
10. Does LASCO operate on 7/24 basis ?The LASCO and EIT instruments aboard SOHO are generally operating on a seven days a week, 24 hours per day basis. However the ground segments (commanding, data reduction, image processing and data archives) are manned on a normal work week (9-5 M-F Eastern time) following the Federal holiday schedule and with snow, hurricane and emergency days for Washington DC. That means that problems with processing (hardware, software, network, power) are not fixed until the next business day. Building maint. also tends to shut the power off once or twice a year on the weekends to work on air conditioning and heating.
11. What is that radial dark area in the C3 images at about the 7 o'clock position ?The 7 o'clock position is where the pylon that holds the occulting disk appears in the image. It has always been in the images. The C2 camera also has a pylon in the 7 o'clock position but it is less obvious than the C3 pylon due to a different optical design. The occulting disk and arm is similiar to what a person does when they want to look at something close to the sun; they hold up their hand and block the disk of the sun. Have a look at our Java annotated C3 image
12. What are those dark areas that I sometimes see in the images ?The dark areas are image processing artifacts and not a sensor problem. The images on the WWW page are processed by removing a background light model. The background light model is about 99% of the light received by the CCD and consists of instrumental stray light as well as coronal light. The amount of background light varys according to SOHO's orbit. When the model is wrong we subtract off too much light and we see a dark area. Correcting the model always lags the actual data. We are always using last month's model with today's data. So about two weeks to a month after today we will build the right model for today. The real-time images are a rough cut at the image processing and are intended for planning purposes for the next day's observations and for public outreach. The subtraction is particularly obvious during a spacecraft roll manuvuer when the spacecraft has rolled but our background light model has not rolled.
13. Why do I see a rectangular pattern on the EIT images ?The EIT telescope uses very thin filters (about 1000 angstroms which is about 0.00004 inches) that are so weak structurally that they need to be supported on a wire mesh. During launch, the instrument was evacuated so that the vibration of the air in the instrument would not break them. The mesh causes the dark lines in the EIT images.
14. What is that odd stuff I see in the single frame from yy/mm/dd hh:mm:ss ?It could be debris from the spacecraft. The C3 camera sees debris more often than the other telescopes due to its wide field of view. See Debris for some examples and explanation.
15. How do I report discovery of a comet in a LASCO image ?LASCO is the most successful comet discovery instrument ever built. Most of the hundreds of comets discovered by LASCO were found by amateur astronomers using images from our WWW site. Reports of the latest LASCO comets and information on how to report a comet discovery is at the main SOHO comets website.16. More Questions ?Additional questions and answers are available on the SOHO Comets FAQ page. Also, have a look at the Dr. SOHO FAQ and address a question to them. Tell them LASCO sent you! |
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