Polaris's star is on the rise...

[cid]

Apparently it's getting brighter...

[FZR1KG]

Can someone explain how the "brightness" of Polaris can damage a CCD imager or even is too powerful for a CCD imager?
The reason I ask is that CCD's aren't easily damaged and they can be clocked at varying rates so I have no clue how a faint star can be too bright for one.
I thought this article was an April 1st joke but it's February.

[Rommie]

In a word, over-saturation. For bright stuff like Polaris even the shortest CCD exposure on a medium-sized telescope (half a second) will over-saturate the CCD pixels (one can hold on the order of 100k photons, and if it's more than that it just spills over into the surrounding pixels). Thing is most people usually don't spend time looking at the bright naked eye stuff as they've already been studied in great detail, so this issue doesn't come up often.

I'm really amazed they let them point the HST at such a bright target, actually.

[FZR1KG]

I'm just curious why that is the case given that there is no real technical obstacle to reducing the exposure time.
For example, I'll assume that they are using full frame CCD technology (to maximize the resolution per unit area) so that requires a shutter during the clock out phase.
If the exposure time is at a minimum 0.5 seconds, what is the decision for that choice?
Shutter technology has been around a long time and they know the techniques for making shutter times down to less than 1/8000th of a second mechanically with MTBF figures near the million cycles mark. I can buy a camera for less than $2000 that has a shutter range from 1/8000th to open exposure, high quantum efficiency and the dynamic range to be able to image the horse head nebula to the Sun. The former requiring only a tracking device.

Basically, they seem to have limitations in the system which at least from an outside POV don't really need to be there and I'm curious, is the high minimum shutter speed a result of some other conflicting system requirement or do they just design to a budget or design to a range of objects they are interested in and not allow anything outside this range.

Seems like an unusual choice to me. Hence the curiosity.
Still not sure how a CCD can be damaged with exposure to anything that doesn't generate excess heat in the elements.

[Rommie]

To be clear, there's no permanent damage from over-exposure to a CCD that I know of to the chip, but in general no one really likes wasting their observing time and resources. Hence the surprised they pointed HST at it comment is that is a hugely over-subscribed telescope, so you'd have to really do your job to convince them you'd get good results from using it. (And I assure you in that article I think the author just said "breaking" the CCD instead of explaining over-exposure and all that, likely cause he doesn't understand the details that well.)

Shutter speed... I don't usually do this stuff so just asked a friend who does, he says the quickest his instrument he uses on the VLT in Chile is .1 seconds (so I guess .5 is more for amateur-level CCD cameras, as I know that's the limit for the one the students use in my practicum). The issue is you consistently always want your shutter to open/shut to an absurd degree most people don't care about outside of astronomy, and for speeds shorter than that the error starts to get significant. Don't forget, most of CCD imaging in astronomy isn't for pretty pictures, it's for taking spectra, so while it may sound odd to worry so much about this stuff in terms of imaging for spectra a few extra photons can really affect that.

I guess you could probably put some money and effort into trimming down that time, but honestly, no one is going to care 99% of the time because we're usually talking exposures of minutes or hours, not seconds. If you really want to study a bright star or whatever you can always just go to a smaller telescope that is better suited for it, with the added bonus of smaller ones are always in less demand. Telescope time is a precious resource so you always need to devote some time explaining why you need that particular one, so if you're wanting to observe something that pushes a telescope's limits on the short exposure side you'd probably just be told to go elsewhere unless your case is really compelling.

Caveat that I am not an optical astronomer, I just hang out with them, goes here.

[FZR1KG]

I think you just hit the nail on the head with the consistent open/close times.
I forgot that you guys need really precise and accurate brightness measurements.
So if the exposure times vary by just a bit then the error becomes too high to do a lot of research, like detecting planets by observing dips in brightness as an example. The accuracy aspect will be critical for long term measurements like comparing data decades older.

I knew there was a reason for it but just couldn't figure it out.
You're probably right about the reporter not understanding damaging very well.

Thanks. My curiosity is satisfied.