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Comment by hn_throwaway_99 | Hacker News Reader

I have a sense that the article is presenting things in a more dramatic way than how it really occurred:

> So, with his job perhaps on the line, Williams went off, put together a small team of post-docs, and did exactly as he’d planned. For 100 hours, between Dec. 18 and 28, Hubble stared at a patch of sky near the Big Dipper’s handle that was only about 1/30th as wide as the full moon. In total, the telescope took 342 pictures of the region, each of which was exposed for between 25 and 45 minutes. The images were processed and combined, then colored, and 17 days later, released to the public.

I mean, each image was only exposed for 25-45 minutes. If the full hundred hours was such a huge risk, why not take, like, 5-10 photos first to see if you get much of anything? Seems like they would have been able to know pretty early on whether it was going to be worth it.

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> If the full hundred hours was such a huge risk, why not take, like, 5-10 photos first to see if you get much of anything? Seems like they would have been able to know pretty early on whether it was going to be worth it.

This has been answered already but just to pull some threads together - the suggestion to take a few exposures first and see what comes out is precisely backwards. They already knew, going into it, that what they were looking for was at the limits of sensitivity, so nothing would show up until they accumulated enough observing time.

In the absence of systematic effects, our signal to noise ratio improves as the square root of integration time. Think of a bucket accumulating photons - the signal accumulates in linear proportion to the integration time, while the RMS fluctuations increase as the square root of the integration time, so the signal to noise ratio improves as T/sqrt(T) = sqrt(T).

So the idea was to accumulate enough exposure that the faint galaxies would become visible at the telescope performance limits - can't get to something scientifically useful by imaging only the first few exposures. It might have made a pretty enough picture, sure, but not an informative one, compared to what we could already do from huge ground-based facilities.

I'm sure they did start stacking as soon as they got the images. From a statistical perspective, 100 hr is roughly 3x better than 10 hr.

If they found nothing in the first 5-10 images, what do you imagine they would do? Telescope scheduling is a very tricky business, not one that lets you change courses too quickly.

Schedule the 5-10 mins, then schedule again for later.

The amount of programming and orbital changes needed for this is not feasible.

For the hubble to point at something, I'd imagine it needed to station-keep and burn some hydrazine.

If you did infinite numbers of 5-10 min studies, you'd burn right out of the fuel store of a spacecraft before you'd get anything done.

From what I hear, hubble telescope times are scoped out, planned out a year or several years in advance - the telescopes and scientific instruments need to be programmed, the spacecraft needed to be moved/pivoted to aim at the right part of space, communications times needs to be booked with receivers/ground stations across the entire Deep Space Network so you can receive the data.

It's nontrivial - if you booked 5-10 min at a time you'd have wasted your most precious resource.

https://eyes.nasa.gov/dsn/dsn.html

Hubble points via gyroscopes, not thrusters! But they are slow, so that's part of why you need to schedule things well. YouTube video explaining how it points: https://www.youtube.com/watch?v=qEZI9DxIQss

Also, I don't think it needs to change orbital parameters at all for its operation. Station-keeping yea, but I don't think it needs to burn more fuel because of pointing at things.

The Hubble telescope is not a deep space probe being so close to earth and doesn't use the Deep Space Network to communicate.

This is a decent and sensible question IMO, barkingcat (downthread) provides a reasoned answer that seems to fit, but the question is good .. so why did the person feel the need to use a throwaway, and why is it downvoted. Unlike other questions it doesn't appear to be the sort of thing one ignorant of the answer could readily google for.

I know, we're not supposed to complain about downvotes but it just makes me sad that we - as a community - don't value honest questions.

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