Starship Asterisk*

NGC3314 wrote: ...
You called it - I had not previously realized it, but found a graphic making it clear that the entire structure pivots to aim the telescope, sunshield and all (except, I would guess, solar arrays and antenna).

Nitpicker wrote:I'm confused too. I haven't seen anything that explicitly states that the mirror can move relative to the sunshield. My initial, poorly worded comment, was trying to suggest that the sunshield looked big enough to allow the whole structure to rotate a certain amount. I'm surprised (and pleased) it allows as much as 135 degrees from the Sun.

NGC3314 wrote:

Chris Peterson wrote:
I took this to mean they orient the entire structure using reaction wheels. That's the range over which they can tilt the sunshield and keep the instrument in shade and the solar panel and antenna pointed within spec. Looking at the tower the telescope is mounted on, I don't see any mechanism for tilt.

You called it - I had not previously realized it, but found a graphic making it clear that the entire structure pivots to aim the telescope, sunshield and all (except, I would guess, solar arrays and antenna).

https://directory.eoportal.org/web/eoportal/satellite-missions/j/jwst wrote:
<<The [JWST] is 3-axis stabilized. Two star trackers (+ 1 for redundancy) point the observatory toward the science target prior to guide star acquisition, and they provide roll stability about the telescope line of sight. Six reaction wheels (two are redundant) are mounted on isolators near the center of gravity of the bus to reduce disturbances to the observatory. These reaction wheels offload the fine steering control to maintain the fine steering mirror near its central position to limit differential distortion-induced blurring onto the target star.
...........................................................

Figure 71: FOR directions of the OTE in relation to the Sun,
Earth and Moon (red arrow), image credit: STScI

The FOR (Field of Regard) is the region of the sky in which observations can be conducted safely at a given time. For JWST, the FOR is a large annulus that moves with the position of the Sun and covers about 40% of the sky at any time. This coverage is lower than the ~80% that is accessible by Hubble. The FOR allows one to observe targets from 85º to 135º of the Sun. Most astronomical targets are observable for two periods separated by 6 months during each year. The length of the observing window varies with ecliptic latitude, and targets within 5º of the ecliptic poles are visible continuously, and provides 100% accessibility of the sky during a year period. The sunshield permits the observatory to pitch toward and away from the sun by approximately 68º, while still keeping the telescope in the shade (Figure 71). The continuous viewing zone is important for some science programs that involve monitoring throughout the year and will also be useful for calibration purposes. Outside the continuous viewing zone every area in the sky is observable for at least 100 days per year. The maximum time on target at a given orientation is 10 days.

The sunshield has dimensions of about 20 m x 14 m providing ample shielding from light of the sun and the Earth. The sunshield provides a 5 layer, "V" groove radiator design of lightweight reflecting material. It reduces the 300 kW of radiation it receives from the sun on its sunward side, to a mere 23 mW (milliwatt) at the back, sufficient to sustain a 300 K temperature drop from front to back. With a back sunshield temperature of ~ 90 K, the primary mirror, the optical truss, and the instrument payload can radiate their heat to space (at 2.7 K) and reach cryogenic temperatures of 30-50 K. These low temperatures and the total blocking of direct or reflected sunlight are crucial to the scientific success of JWST.>>

Chris Peterson wrote: I took this to mean they orient the entire structure using reaction wheels. That's the range over which they can tilt the sunshield and keep the instrument in shade and the solar panel and antenna pointed within spec. Looking at the tower the telescope is mounted on, I don't see any mechanism for tilt.

NGC3314 wrote:

Nitpicker wrote:Will the JWST be able to point opposite its sunshield and the Sun? Unless the mirror can change elevation angle with respect to the sunshield, it seems it might be difficult. I can't see any pics with the mirror pointing away from the sunshield. The sunshield appears to be designed to allow the mirror to rotate a certain amount whilst still being shaded, but not a huge amount.

This summary says that JWST can point between 85 and 135 degrees from the Sun at any time. It does change angle with respect to the sunshield structure to do that. I don't see right away what sets the 135 degree limit (except that it has to do with the sunshade design) - could be thermal, or needing control authority to keep the telescope pointing and guiding while one axis points at the Sun.

NGC3314 wrote:This summary says that JWST can point between 85 and 135 degrees from the Sun at any time. It does change angle with respect to the sunshield structure to do that. I don't see right away what sets the 135 degree limit (except that it has to do with the sunshade design) - could be thermal, or needing control authority to keep the telescope pointing and guiding while one axis points at the Sun.

Nitpicker wrote:Will the JWST be able to point opposite its sunshield and the Sun? Unless the mirror can change elevation angle with respect to the sunshield, it seems it might be difficult. I can't see any pics with the mirror pointing away from the sunshield. The sunshield appears to be designed to allow the mirror to rotate a certain amount whilst still being shaded, but not a huge amount.

Ann wrote:

Chris Peterson wrote:

geckzilla wrote:
JWST will very likely accidentally image many supernovas in distant galaxies, but knowing quickly enough that they're active supernovas is another matter entirely. Interesting, but probably a useless kind of interesting, like that random undiscovered supernova I found in Hubble's archive.

Some, I imagine. But SN are not bright in the IR. Say, one to two orders of magnitude dimmer than the visible with Webb's near-IR camera, and even fainter with its mid-range IR camera.

Which is one reason why I'm so, so sorry that more efforts are not put into ultraviolet astronomy. I so want to see ultraviolet imagery of galaxies and the whole sky. Ultraviolet imagery of the Sun doesn't interest me.

Are there any telescopes that monitor the whole sky, or observe galaxies?

https://en.wikipedia.org/wiki/GALEX wrote:
<<The Galaxy Evolution Explorer (GALEX) is an orbiting ultraviolet space telescope launched on April 28, 2003, and operated until early 2012. After its primary mission of 29 months, observation operations were extended to almost 9 years with NASA placing it into standby mode on 7 Feb 2012.

NASA cut off financial support for operations of GALEX in early February 2011 as it was ranked lower than other projects which were seeking a limited supply of funding. The mission's life-cycle cost to NASA was $150.6 million. The California Institute of Technology negotiated to transfer control of GALEX and its associated ground control equipment to the California Institute of Technology in keeping with the Stevenson-Wydler Technology Innovation Act. Under this Act, excess research equipment owned by the US government can be transferred to educational institutions and non-profit organizations. In May 2012, GALEX operations were transferred to Caltech.

A fund-raising effort called GALEX CAUSE is being run to try and complete its All-Sky UV Survey. Its unique ultraviolet observations shed new light on special studies of galaxies, black-holes, supernova, stars, and beyond.>>

Chris Peterson wrote:
How many missions are you aware of that were returning solid science and were shut down at the end of their funding cycle despite everything still working well? That's not common. Usually, missions that are successful are extended. That's a much smarter way of operating than simply saying the length of the mission will be dictated by how long the equipment lasts. Older missions need to justify their existence periodically. Even if the equipment works, it might not make sense to keep operating.

http://www.galex.caltech.edu/cause/ wrote:
GALEX CAUSE: Complete the All-sky UV Survey Extension

The Galaxy Evolution Explorer (GALEX) has begun a privately funded mission extension phase. A significant fraction of the Galactic Plane has not yet been surveyed, but is now accessible with the removal of all bright star limits. Our highest priority goal is to Complete the All-Sky UV Survey and pursue Milky Way science related to pre- and post-main-sequence stellar evolution, the cycling of matter between stars and the ISM, characterization of stellar variability, and a search for new classes of exoplanets. Other high priority science includes exploring the transient UV sky, searching for shock breakout supernova events, discovering tidal disruption of stars by massive black holes, and surveying the low mass universe.

We are now soliciting funding for continued operations from interested scientific or philanthropic institutions or individuals to pursue these or other novel and high-priority observations. All contributions are tax-deductable. We have been observing successfully in this fashion for the last four months in collaboration with several scientific consortia, individuals, and private support.

For reference, a month of operations costs approximately $120K. Roughly 300 single-orbit night-time pointings covering 300 square degrees with an average exposure time of 1400 seconds can be scheduled each month. Alternatively, up to 300 large angle scans could cover as much 4000 square degrees in a month. Cadenced observations are also possible exploring timescales from less than one second to months. Funding supports a skeleton operations team and no additional science analysis. Standard pipeline products are produced for all observations, which can be proprietary for up to one year.

We recognize at least three modes for interested astronomers to participate in this new venture:

Astronomers, private individuals, and philanthropic foundations who recognize the importance of completing the All-Sky Survey in the GALEX Near UV band and its legacy value to future astronomy may contribute at any level.
Astronomers wanting to form a consortium to perform a high-priority science investigation that interests a broader community may contact the GALEX project stating their desire to form a collaboration. The GALEX team will work with you to assemble consortia of groups and institutions with common observational goals.
Large survey projects that would benefit from GALEX Near UV imaging and/or grism spectroscopy observations can propose to contribute for one or more months of observations (a single month or spread over a number of calendar months).

Please contact Christopher Martin (cmartin@srl.caltech.edu or 626-826-7866) if you are interested in participating in any way.

Time is of the essence, as once funding is exhausted the satellite will be shut off forever. Please join us in our quest to complete high priority observations that will benefit future astronomers for decades. >>

http://en.wikipedia.org/wiki/Phidias wrote:
<<Phidias, or The Great Pheidias (in Ancient Greek, Φειδίας; c. 480 – 430 BC), was a Greek sculptor, painter & architect, who lived in the 5th century BC, and is commonly regarded as one of the greatest of all sculptors of Classical Greece: Phidias' Statue of Zeus at Olympia was one of the Seven Wonders of the Ancient World. The Golden Ratio has been represented by the Greek letter φ (phi), after Phidias, who is said to have employed it.

Phidias designed the statues of the goddess Athena on the Athenian Acropolis, namely the Athena Parthenos inside the Parthenon and the Athena Promachos, a colossal bronze statue of Athena.

Plutarch records that enemies of Pericles, who was a close friend of Pheidias, tried to attack Pericles through Pheidias who was accused of stealing gold intended for the statue of Athena in the Parthenon and of impiously portraying himself and Pericles on the shield of Athena's statue. The first charge was disproved but Pheidias was jailed for the second, where he died.>>

geckzilla wrote:
What a majestic, beautiful work of human ingenuity. I may be biased.

Chris Peterson wrote:

geckzilla wrote:JWST will very likely accidentally image many supernovas in distant galaxies, but knowing quickly enough that they're active supernovas is another matter entirely. Interesting, but probably a useless kind of interesting, like that random undiscovered supernova I found in Hubble's archive.

Some, I imagine. But SN are not bright in the IR. Say, one to two orders of magnitude dimmer than the visible with Webb's near-IR camera, and even fainter with its mid-range IR camera.

Chris Peterson wrote:That means that at any time, the possible targets lie on a circular region of the sky, and it requires one full orbit (a year) to access the entire sky.

geckzilla wrote:JWST will very likely accidentally image many supernovas in distant galaxies, but knowing quickly enough that they're active supernovas is another matter entirely. Interesting, but probably a useless kind of interesting, like that random undiscovered supernova I found in Hubble's archive.

Nitpicker wrote:Wouldn't it be a bummer if some fascinating event occurred in the sky, that was ideally suited to the JWST's capabilities, but it happened at the wrong time of year?

Nitpicker wrote:Will the JWST be able to point opposite its sunshield and the Sun? Unless the mirror can change elevation angle with respect to the sunshield, it seems it might be difficult. I can't see any pics with the mirror pointing away from the sunshield. The sunshield appears to be designed to allow the mirror to rotate a certain amount whilst still being shaded, but not a huge amount.