Stars in background of deep-sky photos are in foreground

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offsky
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Stars in background of deep-sky photos are in foreground

Postby offsky » Thu Dec 27, 2018 6:26 pm UTC

I am interested in astronomy and I have a small telescope, but I wouldn't call myself an amateur astronomer yet. Anyway, I was recently looking through a magazine and saw a picture of a galaxy, like I've seen many times before, and I paused to look at the random background stars scattered around the picture. Then it suddenly occurred to me that the galaxy has billions of stars that are too small to see individually, so these random stars that I can see must actually be in the foreground between me and the galaxy. For some reason, I never really thought about it that way and I just assumed that the random stars were in the background, because stars are always in the background when you are sitting on earth. It seems so obvious in hindsight, but this was a 'wow' moment for me.

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ucim
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Re: Stars in background of deep-sky photos are in foreground

Postby ucim » Thu Dec 27, 2018 9:28 pm UTC

offsky wrote: I paused to look at the random background stars scattered around the picture.
Are you sure those are stars? If they are, they are in the foreground, yes. But could also be entire galaxies.

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Re: Stars in background of deep-sky photos are in foreground

Postby Heimhenge » Tue Jan 08, 2019 7:27 pm UTC

Unlikely distant galaxies would be visible through a beginner's scope. You need 8-10" aperture to even start seeing distant galaxy cores as fuzzy points of light ... and that's only under good seeing conditions. I'd feel safe to say ALL those other points of light are foreground stars.

There's several galaxies with bright foreground stars superimposed on their disc. Almost makes it look like some star in that galaxy is going nova. NGC 6946 is a fine example.

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Re: Stars in background of deep-sky photos are in foreground

Postby ucim » Tue Jan 08, 2019 7:56 pm UTC

Heimhenge wrote:Unlikely distant galaxies would be visible through a beginner's scope.
He wasn't looking through a beginner's scope. He was looking through a magazine. And while magazines often have stars, if you can afFord it, you can also find pictures of galaxies there.

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Re: Stars in background of deep-sky photos are in foreground

Postby Eebster the Great » Tue Jan 08, 2019 8:47 pm UTC

Yeah, I feel like using a telescope to read a magazine might be overkill.

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Re: Stars in background of deep-sky photos are in foreground

Postby pogrmman » Tue Jan 08, 2019 9:40 pm UTC

Heimhenge wrote:Unlikely distant galaxies would be visible through a beginner's scope. You need 8-10" aperture to even start seeing distant galaxy cores as fuzzy points of light ... and that's only under good seeing conditions. I'd feel safe to say ALL those other points of light are foreground stars.

There's several galaxies with bright foreground stars superimposed on their disc. Almost makes it look like some star in that galaxy is going nova. NGC 6946 is a fine example.


The most star-like galaxy I've seen through my 10" telescope was the quasar 3C 273 (it just looks like a faint star). Pretty much anything else within the range of a scope that size looks noticeably fuzzy (except for the handful of other quasars you might be able to spot from a dark place*). Even in the city, with lots of light pollution, where you can only see the cores of galaxies, they've got a fuzzy look to them.

*There really is only a handful them bright enough. I managed to get 3C 273 near its faintest at magnitude 13.1 from the suburbs (naked eye magnitude was very good for the location at around 5.3 on that night). A 10" has a magnitude gain of about 7.5-8, and I've seen down to about 7.2 (or maybe a tad lower) in very, very dark skies, meaning you could probably see down to between 14.5 and 15 (maybe a tad fainter) in perfect conditions. There are only about 17 quasars that are that bright.

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Re: Stars in background of deep-sky photos are in foreground

Postby offsky » Wed Jan 09, 2019 12:03 am UTC

Another "wow" thing that I heard recently was that if you are on the moon, the earth is always in the same place in the sky. It doesnt move. I cant remember where I read this, but since the moon is tidally locked to earth, this makes sense, although it does wobble, so I expect that the earth in the moon's sky will move slightly.

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Re: Stars in background of deep-sky photos are in foreground

Postby Heimhenge » Wed Jan 09, 2019 12:29 am UTC

offsky wrote:Another "wow" thing that I heard recently was that if you are on the moon, the earth is always in the same place in the sky. It doesnt move. I cant remember where I read this, but since the moon is tidally locked to earth, this makes sense, although it does wobble, so I expect that the earth in the moon's sky will move slightly.


Yes it would. The Moon's libration is like ± 6.5°, and the Earth's apparent diameter as seen from the Moon is about 2°, so if you lived in a lunar base at latitude 83.5° (or thereabouts) you could watch an Earthrise and Earthset cycle every month. That would be pretty cool. Lotsa other good reasons for putting a lunar base near the poles ... water ice and nearly continuous solar energy.

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Re: Stars in background of deep-sky photos are in foreground

Postby ijuin » Wed Jan 09, 2019 8:07 am UTC

Also, given Luna’s slow rotation rate, the primary reason for an equatorial launch/landing site (free discount on delta-v needed to land or launch) is not a significant factor.

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Re: Stars in background of deep-sky photos are in foreground

Postby Heimhenge » Wed Jan 09, 2019 5:37 pm UTC

ijuin wrote:Also, given Luna’s slow rotation rate, the primary reason for an equatorial launch/landing site (free discount on delta-v needed to land or launch) is not a significant factor.


I get that. But it seems like a polar landing would also be more difficult because of the trajectory change required. Equatorial off the Earth to equatorial around the Moon requires very little course correction. To change that to a polar orbit would cost energy. Unless they could do a "slingshot" thing with the Moon and use its gravity to make the course change? Anybody running Kerbal know if that's possible?

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Re: Stars in background of deep-sky photos are in foreground

Postby ucim » Wed Jan 09, 2019 8:17 pm UTC

Heimhenge wrote:Equatorial off the Earth to equatorial around the Moon requires very little course correction. To change that to a polar orbit would cost energy.
Well, not really. Once you've left the earth and are heading for the moon, you're not "equatorial" any more. You're just ballistic. If you happen to be aiming towards the south pole area, you will end up in a polar orbit. It takes very little change in aim to go from pole to equator to pole (especially at the outset).

Yes, if you are already in equatorial orbit around the moon, it takes a bit of delta v to change the orbit. But on initial launch from the earth? Nah.

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Re: Stars in background of deep-sky photos are in foreground

Postby Eebster the Great » Thu Jan 10, 2019 4:09 am UTC

I think his statement would be correct though if the distance to the moon were not so much greater than its diameter.

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Re: Stars in background of deep-sky photos are in foreground

Postby ucim » Thu Jan 10, 2019 5:34 am UTC

You mean, by using alternative facts? :)

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Re: Stars in background of deep-sky photos are in foreground

Postby PM 2Ring » Thu Jan 10, 2019 10:25 am UTC

Eebster the Great wrote:I think his statement would be correct though if the distance to the moon were not so much greater than its diameter.

And if the Moon's orbital plane were near Earth's equatorial plane rather than being a few degrees off the ecliptic plane.

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Re: Stars in background of deep-sky photos are in foreground

Postby speising » Thu Jan 10, 2019 11:22 am UTC

You could actually make a plane change from equatorial to polar Earth orbit via a lunar swing-by maneuvre for less delta-v than doing it directly. (~4 km/s vs. ~11 km/s)

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Re: Stars in background of deep-sky photos are in foreground

Postby Eebster the Great » Thu Jan 10, 2019 12:29 pm UTC

ucim wrote:You mean, by using alternative facts? :)

Yes. Also, if the Earth were an infinite flat plane, the gravitational field would not vary with altitude.

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Re: Stars in background of deep-sky photos are in foreground

Postby Nicias » Thu Jan 10, 2019 5:03 pm UTC

Also, you are probably not starting in a equatorial earth orbit anyway. The most energy efficient orbit to get into from a launch site is one with inclination equal to the latitude of the site. You can pick your LAN by the time of day you launch. Then picking the time of month, you can pick the location of the moon in its (inclined) orbit, so it seems reasonable that you can get a lunar flyby of almost any inclination with a prograde departure burn.

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Re: Stars in background of deep-sky photos are in foreground

Postby ijuin » Fri Jan 11, 2019 1:36 am UTC

It is worth mentioning that it is impossible to launch directly from a planetary (or other) surface into an orbit around that body that has a LOWER inclination than the latitude of the launch site. Essentially, the highest-latitude point of the orbit can not be less then the latitude of the launch site, because if it was not, then no part of the orbit would pass over the launch site. Reducing one’s orbital inclination below that of the launch site can only be accomplished either by gravitational slingshot around another body (e.g. the Moon), or by expending delta-v in order to accelerate equatorward.

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Re: Stars in background of deep-sky photos are in foreground

Postby ucim » Fri Jan 11, 2019 3:06 am UTC

ijuin wrote:It is worth mentioning that it is impossible to launch directly from a planetary (or other) surface into an orbit around that body that has a LOWER inclination than the latitude of the launch site. [It can] only be accomplished either by [...] or by expending delta-v in order to accelerate equatorward.
The requisite delta-v can be expended during the initial launch phase (launching in a curve). Would that not count as "directly"?

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Re: Stars in background of deep-sky photos are in foreground

Postby gmalivuk » Fri Jan 11, 2019 3:51 am UTC

Any delta-v you spend above a certain latitude isn't going to help you get into an orbit with a lower inclination than that.

Basically, acceleration can change the plane of your orbit, but it can't possibly change it to a plane that doesn't actually include your own position. It also has to include the center of Earth as that's what you're orbiting. A plane that includes those two points must be at least as inclined above the equatorial plane as your own latitude.
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Re: Stars in background of deep-sky photos are in foreground

Postby ucim » Fri Jan 11, 2019 4:55 am UTC

gmalivuk wrote:Basically, acceleration can change the plane of your orbit, but it can't possibly change it to a plane that doesn't actually include your own position.
Yes, so that last bit of acceleration must occur as you cross the equator (if you want an equatorial orbit). I suppose the launch itself doesn't thrust for long enough to get you there, so yes, you are right. After the first ten (or whatever) minutes of thrust, you are in (an) orbit, and if you haven't reached the equator yet, then subsequent delta-v is not part of the launch - it's changing an existing orbit. Even if it all happens on the first orbit. Which it theoretically could.

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Re: Stars in background of deep-sky photos are in foreground

Postby gmalivuk » Sat Jan 12, 2019 2:22 am UTC

It's most efficient as you cross the equator regardless of the final inclination you want, iirc
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