positional astronomy - SDSS: Inclination of a Field?

I am working with the Sloan Digital Sky Survey CasJob service.

I am trying to determine the ellipticity of a galaxy using the stokes parameters $U$ and $Q$ from which I can deduce $e$ and $phi$. But $phi$ is relative to the CCD chips. How can I get the real $phi$ (relative to the equator). If I am correct I would know the inclination if I knew what field the object is on and I knew the inclination of the field.

Each field belongs to a stripe. So I'm there if I know the inclination of the stripe? And How do I know the inclination of a stripe?

Is my problem realates with the column like phioffset_r described as Degrees to add to CCD-aligned angle to convert to E of N (I have no clue what this means)?

Note: I am not sure whether this is the right place for this question. Is there a better place?

Note 2: If this is the right place, then it would be nice if somebody could create the tag sloan-digital-sky-survey or SDSS. I could not figure out what existing tag would fit, so I did take positional-astronomy.

binary star - Peculiar orbit of circumbinary planet

I've created a Mathematica notebook file which should theoretically allow one to simulate any n amount of bodies. Whilst looking at a three-mass system where $m_1=m_2=2000m_3$, I noticed some peculiar orbital characteristics when the "planet" comes close to the "stars".
.

Pretty, but hardly what I expected. Are these kind of orbits viable, or is something going wrong here? Note in this case, the "stars" orbit each other in a near perfect circle.

For those with Mathematica, the notebook may be found here:

http://s000.tinyupload.com/index.php?file_id=07893360971974925836

astrophysics - Create heatmap from significance

I am working on my project in astrophysics and I would like to create my heatmap from the significance which is obtained by the gaussian fitting my histogram.

I get this map :

First picture : convolve heatmap at 2' - convolve completeness map at 2'
Second picture : convolve heatmap at 8' - convolve completeness map at 8'
Third picture : first one - second one

I plot the histogram and the gaussian which is fitting it :

And I obtain this values :

Mean value : -0.0138901955853

Variance value : 0.00824390390031

Sigma value : 0.0907959464971

I would like to get something like this :

==> Question :

How I can process in order to get the same significance map (Signal to Noise) ?

moonlanding - What instruments/science goals have the highest priorities for a combined orbiter and lander to Europa?

A NASA mission to Europa seems to have a friend in congress. What would be the most important instruments and science goals for such a mission? Seismometry, radar, surface chemistry, a drill/melter, hopper. What are the most important of the feasible kinds of instruments on a first mission to Europa that could be launched in maybe 5 years, say, for a Cassini kind of budget?

n body simulations - How to scale down solar system data to simulatable values

Okay, I am seriously ashamed for asking this especialy when I geniualy study on physics but there is something that bugs me with the simulation I am working on.

I am re-creating the solar system in an n-body simulation that i programmed before. And I've a problem with scaling down the solar data. So even if I use kg and km for the metric units, the values are far bigger than the variables can hold in programming. Also as some of you know, bigger the value is, bigger the floating point error it makes. (error noise in data) Also it makes it slower to process.

I decided to scale down the data with a reference point, and for that, I took the earth's radius as 1 unit. And scaled down every other distance and radius according to it. (So a unit is 6371 km just to be clear)

But I am not sure whether if I should scale down the mass or not. My common sense says that I should scale down the mass so density of each body should remain same. So I took the density, and calculated a new mass value for each body, with the new scaled down radius.
But I am somehow couldn't conviced myself about If it's true or not. So here I am, asking to you :) Should I also scale down the mass?

PS.1: I used using F = GMm/r^2 equation for the calculactions as usual. (Iterating it through each body pairs)

If there are other programmers like me interested in making a simulation like this, how did you accomplish this data size problem? Are there any better solutions than scaling down the values?

PS. I have created an excel file that does the scale conversion. So I am sharing the sheet in OneDrive. (http://1drv.ms/1NIekGo) If you can check my calculations and values, that I'd also be really helpful to me. Thanks for any help.

gravity - Why is it strange that outer stars are travelling with the same speed as inner while the total mass is also increasing?

This is accounted for in real models of the rotation curves of galaxies. The rotation curve of a galaxy is more complicated than the motion of planets in the solar system, for the reasons that you describe.

A trivial model would consider the galaxy as spherically symmetric and would use the shell theorem to estimate the centripetal acceleration of a star at radius $r$. Thus
$$ m r omega^2 = Gfrac{m M(r)}{r^2},$$
where $m$ is the mass of the star, $omega$ is the angular velocity and $M(r)$ would be the mass of all gravitating matter at radii $<r$. Note that this only applies to a spherically symmetric distribution of mass.

To make further progress demands that you know the density distribution of the gravitating matter. Let's just assume that density $rho$ is constant for the moment. Then
$$ r^3 omega^2 = G int rho 4pi r^2 dr = frac{4pi}{3} G rho r^3$$
$$ omega = sqrt{frac{4pi G rho}{3}}$$

Thus the angular velocity would be constant with radius and the rotation speed, $v = omega r$ would increase with radius. This I think is the situation that your question supposes and so yes, if there was a constant (or perhaps slowly declining) density of material in the Galaxy then this would produce a rotation curve that increased (or was flat).

The trouble is that the density of material in our Galaxy inferred from the matter that we can see is not constant with radius. It declines rapidly and exponentially, such that there is very little visible matter beyond a radius of about 15 kpc. If we take a situation where we go to radii beyond the gravitating matter, then a similar treatment to the above suggests that
$$ m r omega^{2} = Gfrac{mM}{r^2},$$
where $M$ is now the total mass of the (visible) Galaxy. In this case
$$ omega = sqrt{frac{GM}{r^{3}}}$$
and the rotation velocity $v = omega r$ should decline as $1/sqrt{r}$ (like it does in the solar system).

It is the fact that the rotation velocity of stars and gas at large radii ($>15$ kpc) continues to be flat or even increase that leads to the conclusion that the mass that we can see is not all that there is. i.e That we need "dark matter" to explain the high rotation speeds at radii where there is very little visible matter.

Realistic models of the galaxy do not make the assumption that the visible matter is spherically symmetric (it isn't). But the conclusions I qualitatively set out above hold in the same way.

What would the night sky look like if the interstellar medium didn't exist to absorb or block light?

The main effect would be that the Milky Way would become much more prominent and asymmetric.

At the moment, our view into the Galactic plane is limited to around 1000-3000 parsecs by dust. If you look at the Galactic latitude distribution of naked eye (Aren't there more naked-eye-visible stars in the Milky Way plane? ) you see that most naked eye stars are much closer than this and there is only a modest concentration towards the Galactic plane.

What I think this means is that there would not be a big increase in the numbers of resolved naked eye stars. It is the sensitivity of our eyes, rather than dust that limits how far we can see them. However, the numbers of unresolved stars in the Milky Way would be greatly increased. The number of stars in the resolution element in your eye would increase as distance squared, but the light received decreases as distance squared. This means each "shell" you add contributes equally to the observed brightness.

The Sun is in a 30000pc diameter Galactic disc, about 8000pc from the centre. Thus I estimate that going from being able to see stars to 2000pc, seeing them to anywhere from around 7000pc (away from the Galactic centre) to 23000pc (through the Galactic centre) will increase the brightness of the Milky Way by factors of 3-10, strongly concentrated towards the Galactic plane (more so than now), depending on which way you look. This asymmetry will be increased by the increasing stellar density towards the Galactic centre.

In addition, the "bulge" would become more prominent. This is a pseudo-spherical region of diameter 4000pc, with a greater density of stars, centred on the Galactic centre. The bulge is mainly hidden by dust now, but without dust we would see a brighter, roughly circular blob towards the Galactic centre (Saggitarius), with an angular diameter of 30 degrees.