geckzilla wrote:The video I linked to along with numerous mentions about not even wanting to point the telescope at Earth (probably the effects of which were also misunderstood and/or miscommunicated) got me under the impression a supernova (within our galaxy) could potentially damage the instruments.
The HST can be, and has been pointed towards the Earth with its imaging CCDs active (for calibration purposes). Also, the Moon has been imaged (a tricky business that involved moving the spacecraft as the exposure was made).
I emailed the archive help desk recently about something unrelated but remembered this thread at the end and asked about eta Carinae on the side. I got a response from Dorothy Fraquelli and she indeed did not seem concerned that HST would become damaged by it but that is because of internal safe guards built into the telescope which shut it down. So unless those fail, the telescope is supposedly safe. She also said the CCDs can be overexposed but it takes a while for the excess charge to bleed off, which would be pretty annoying if your observations happen after such an event (and it has happened, iirc)...
The safeguards are presumably for the COS instrument, since the CCD cameras are not subject to damage from overexposure. Indeed, they are often deliberately overexposed in order to balance the excess charge trapped after normal exposures. This is called residual bulk image (RBI) and creates a ghost image that can last quite a while, messing up subsequent images if there was anything bright recently exposed. So there's a device that "flashes" the CCD, hitting it with bright light to erase the pattern in the RBI. The downside is that this slightly raises the noise level, but that's often preferably to some sort of pattern on the signal.
What would happen if Hubble looked at the sun?
Nearly instantaneous thermal damage. It would occur to internal elements other than the camera before the aiming was complete. That's why there is a strict protocol that forbids imaging anywhere near the Sun (I seem to remember that the safe zone is something like 45°, far more than it absolutely needs to be). Objects within that zone have to wait for a different time of year to be imaged.
[quote="geckzilla"]The video I linked to along with numerous mentions about not even wanting to point the telescope at Earth (probably the effects of which were also misunderstood and/or miscommunicated) got me under the impression a supernova (within our galaxy) could potentially damage the instruments.[/quote]
The HST can be, and has been pointed towards the Earth with its imaging CCDs active (for calibration purposes). Also, the Moon has been imaged (a tricky business that involved moving the spacecraft as the exposure was made).
[quote]I emailed the archive help desk recently about something unrelated but remembered this thread at the end and asked about eta Carinae on the side. I got a response from Dorothy Fraquelli and she indeed did not seem concerned that HST would become damaged by it but that is because of internal safe guards built into the telescope which shut it down. So unless those fail, the telescope is supposedly safe. She also said the CCDs can be overexposed but it takes a while for the excess charge to bleed off, which would be pretty annoying if your observations happen after such an event (and it has happened, iirc)...[/quote]
The safeguards are presumably for the COS instrument, since the CCD cameras are not subject to damage from overexposure. Indeed, they are often deliberately overexposed in order to balance the excess charge trapped after normal exposures. This is called residual bulk image (RBI) and creates a ghost image that can last quite a while, messing up subsequent images if there was anything bright recently exposed. So there's a device that "flashes" the CCD, hitting it with bright light to erase the pattern in the RBI. The downside is that this slightly raises the noise level, but that's often preferably to some sort of pattern on the signal.
[quote]What [i]would[/i] happen if Hubble looked at the sun?[/quote]
Nearly instantaneous thermal damage. It would occur to internal elements other than the camera before the aiming was complete. That's why there is a strict protocol that forbids imaging anywhere near the Sun (I seem to remember that the safe zone is something like 45°, far more than it absolutely needs to be). Objects within that zone have to wait for a different time of year to be imaged.