eMailing with NASA

Editorial: I recently had this eMail exchange with a Junior ESA/Hubble Public Information Officer from the NASA Education and Public Outreach Department.  This fine officer (‘OU’) has renewed my belief in spending tax dollars on space research if only to keep our infinitesimal brains slightly open to the notion that our dear earth is but a speck of dust floating across an infinite sea.  Go, NASA, go.
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Dear OU,

I just read and thoroughly enjoyed “A Snowstorm of Distant Galaxies” at this website: http://www.spacetelescope.org/images/potw1017a/

It says in the last line of the article that “the field of view of the image is about 3 arcminutes across”.  Can you give me some idea how wide or how narrow a field of view this might be when I’m looking up at the night sky?  For example, does this entire image occupy a tiny portion of the night sky?  To the naked eye does this snowstorm appear as many stars or perhaps less?  In short, I have no point of reference for what an arcminute might be.

Thank you,
CG
New York City
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Hi CG

Good question!

It’s pretty narrow – the full Moon is about 30 arcminutes across, so if you imagine a square about a tenth of the width and height of the Moon, that’s roughly what’s covered here. What you see in this picture is also very faint (hence the exposure times of 67 and 33 minutes through the different filters) – so it wouldn’t actually be visible as anything at all with the naked eye.

All the best,
OU
München, Germany
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Hello, OU,

Thank you so much for answering my question. It’s awesome to think that such a tiny piece of the sky can be so full and active and meaningful.

Can I pester you with one more question?  I’ve been looking at the image with my 6-year old daughter and we want to know how NASA even knows which tiny patch of blackness to study?  Meaning, if you are telling us that this little piece of the sky is too faint to see with the naked eye (such that it requires long exposure times), how do you know to point the cameras in that direction (versus all the other tiny black corners of the night sky)?  The size and scale seems overwhelming…

Thanks again,
CG
New York City
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Hi CG

To a large extent, it’s building on the work of other telescopes. The night sky is not a complete mystery – there are detailed maps of the sky that mean astronomers have a fair idea of what they’re likely to see when we point Hubble in any given direction. (Indeed – when astronomers apply for observing time, they have to say what they’re hoping to observe, and why).

There are a couple of special cases when Hubble has deliberately been pointed at the unknown, though. This picture, for example: http://www.spacetelescope.org/images/heic0406a/ (which is also 3 arcminutes across – it’s the standard size of image taken by Hubble’s main camera), was taken deliberately in a completely dark bit of sky with an extremely long exposure (about 280 hours), just to see what it would reveal. Because there was nothing bright there to outshine the very faint, distant galaxies, this spot has given us the deepest view of the cosmos ever.

You’re absolutely right about the sky being an overwhelming size – observing time on Hubble and other major observatories is hugely oversubscribed, and so has to be rationed as a result. There’s no way Hubble could ever study the whole sky in that level of detail.

All the best,
OU
München, Germany

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