21cm band. Refurbishing the dish with a new Cantenna.

[SteveBz]

10 hours ago, ZiHao said:

We always correct to LSR

Both radial velocities and frequencies or just RVs?

There are corrections you can only apply if you haven't transformed to LSR.

[ZiHao]

35 minutes ago, SteveBz said:

Both radial velocities and frequencies or just RVs?

There are corrections you can only apply if you haven't transformed to LSR.

You can also convert the corrected rvs back to frequencies, but I don't recall seeing that in observations reports, maybe there are other reasons for doing that🧐

[ZiHao]

9 minutes ago, ZiHao said:

You can also convert the corrected rvs back to frequencies, but I don't recall seeing that in observations reports, maybe there are other reasons for doing that🧐

And like what you have mentioned in the previous posts, working on the raw frequency spectrums will be easier for baseline subtraction, noise etc. It should be the final spectrum that we are using to convert to LSR.

[SteveBz]

4 minutes ago, ZiHao said:

It should be the final spectrum that we are using to convert to LSR.

Exactly.  So maybe we can't share until all the calibrations have been done.

[ZiHao]

27 minutes ago, SteveBz said:

Exactly.  So maybe we can't share until all the calibrations have been done.

👍It would be a very exciting project when we can assemble data collected from different parts of the world to create a rather complete hydrogen intensity map of the sky! 

[SteveBz]

57 minutes ago, ZiHao said:

👍It would be a very exciting project when we can assemble data collected from different parts of the world to create a rather complete hydrogen intensity map of the sky! 

It would be great.

[Victor Boesen]

On 22/04/2022 at 23:50, Victor Boesen said:

I think I may have found another, perhaps simpler, way of correcting the radial velocity with respect to the Local Standard of Rest. I followed this page, which nicely explains how SpectralCoord works and etc.

https://docs.astropy.org/en/stable/coordinates/spectralcoord.html

I've just run the following code with my observations from ealier today, and all of them seem to line up better with the H-line survey simulations.

from astropy import units as u
from astropy.time import Time
from astropy.coordinates import SkyCoord, SpectralCoord, EarthLocation, AltAz, ICRS, LSR, Galactic

def radialVelocityCorrection(self, az, alt, radial_vel):
    horizontal_coord = AltAz(alt = alt*u.degree, az = az*u.degree, pressure = 0*u.bar, obstime = self.TIME,location=self.QTH)
    eq_coord = SkyCoord(horizontal_coord.transform_to(ICRS()), radial_velocity = radial_vel*u.km/u.s,frame = 'icrs',distance = 10*u.kpc)

    # Define the observed radial velocity
    observer_loc = self.QTH.get_itrs(self.TIME)
    spectral_coord = SpectralCoord(ANALYSIS.freqFromRadialVel(radial_vel)*u.Hz,observer=observer_loc,target=eq_coord)

    freq_wrt_lsrk = spectral_coord.with_observer_stationary_relative_to("lsr")

Where the QTH is an EarthLocation describing the observer's location and TIME is the current time

self.QTH = EarthLocation(lat = lat*u.degree, lon=lon*u.degree,height=0*u.m)
self.TIME = Time.now()

Still need to test it further, but as of now it seems to lign up!

Victor

I have just realized I'm making a, rather significant, mistake here. I'm handling the observed radial velocity as the radial velocity with respect to the barycentric frame of reference (ICRS). It's a quick but important fix!

[SteveBz]

9 hours ago, Victor Boesen said:

I'm handling the observed radial velocity as the radial velocity with respect to the barycentric frame of reference (ICRS).

I'm not sure what that means? Is that the mistake? You mean you haven't added the earth round the sun?

[Victor Boesen]

23 minutes ago, SteveBz said:

I'm not sure what that means? Is that the mistake? You mean you haven't added the earth round the sun?

Yes, that's the mistake. It basically means the "raw" uncorrected radial velocity is used as the radial velocity as it would be from the barycenter.

Which, like shown in the beautiful drawing above, is not the case!

[SteveBz]

18 hours ago, Victor Boesen said:

Yes, that's the mistake. It basically means the "raw" uncorrected radial velocity is used as the radial velocity as it would be from the barycenter.

Which, like shown in the beautiful drawing above, is not the case!

Yes, that's right.  The only issue I'm having is that the calculations seem to return numbers with different sign conventions, or so it seems.  To get the right answer you seem to have to (and I may have got this wrong), but you seem to have to add the one and subtract the other 🥴

[SteveBz]

Another thing is that the approximation we make around differential rotation (ie V0=V) also seems to be wrong.  The actual answer for Q1 & Q2 is V=V0, but the answer for Q3 & Q4 seems to be V = -V0, because it's roating towards us.  That's why @ZiHao got the answer he did.

Furthermore, in areas where V0*Sin(l) ~ Vr, then the denominator has a very high error rate and is unreliable, especially, it seems to me, where 'l is near 180.  Sin(l) becomes very small and the R calculation diverges.  What do you think?

[ZiHao]

1 hour ago, SteveBz said:

Another thing is that the approximation we make around differential rotation (ie V0=V) also seems to be wrong.  The actual answer for Q1 & Q2 is V=V0, but the answer for Q3 & Q4 seems to be V = -V0, because it's roating towards us.  That's why @ZiHao got the answer he did.

Furthermore, in areas where V0*Sin(l) ~ Vr, then the denominator has a very high error rate and is unreliable, especially, it seems to me, where 'l is near 180.  Sin(l) becomes very small and the R calculation diverges.  What do you think?

I don't think you should worry about the sign in this case. If you want to substitute V=-V0 into the equation(the one that differs with a negative sign) for Q3 and Q4, we will still get the same equation. You can also try playing with the signs in the radial velocity formula and the patterns of receding, approaching, etc in the quadrants will match with what we expected.

At locations for l=0,90,180,270, I believe there will be a peak centered more or less at 0km/s because the velocity of component is parallel or perpendicular to us so radial velocity is zero. The R at l=0,180 is not easy to obtain with this method. I am not sure about what are the other methods to obtain the particular R at those locations, but perhaps the other peaks around the 0km/s peak should give a good enough indication of the vicinity of the region that you are observing.

[SteveBz]

2 minutes ago, ZiHao said:

I don't think you should worry about the sign in this case. If you want to substitute V=-V0 into the equation(the one that differs with a negative sign) for Q3 and Q4, we will still get the same equation. You can also try playing with the signs in the radial velocity formula and the patterns of receding, approaching, etc in the quadrants will match with what we expected.

At locations for l=0,90,180,270, I believe there will be a peak centered more or less at 0km/s because the velocity of component is parallel or perpendicular to us so radial velocity is zero. The R at l=0,180 is not easy to obtain with this method. I am not sure about what are the other methods to obtain the particular R at those locations, but perhaps the other peaks around the 0km/s peak should give a good enough indication of the vicinity of the region that you are observing.

My numbers near 180 on both sides are very strange! Sometimes R is positive and sometimes negative, which doesn't really work. 😅

[SteveBz]

Here I've replaced the marker with the quadrant number for debugging purposes.  You can see that some of it fine and some is very strange.

 

[ZiHao]

10 minutes ago, SteveBz said:

Here I've replaced the marker with the quadrant number for debugging purposes.  You can see that some of it fine and some is very strange.

 

Hmm...Are the LSR corrections correct? I try substituting the values from the LAB survey, they seems to give positive R. If the corrections are correct, then I guess you have to reject the values obtained near l=180, from the earlier pdf paper you shared, it seems like there is no data points around that area also.

Edited April 25, 2022 by ZiHao

[SteveBz]

3 minutes ago, ZiHao said:

Hmm...Are the LSR corrections correct? I try substituting the values from the LAB survey, they seems to give positive R. If the corrections are correct, then I guess you have to reject the values obtained near l=180, from the earlier pdf paper you shared, it seems like there is no data points around that area also.

It's possible, isn't it.  I'll look at that.

Tx.

[SteveBz]

On 25/04/2022 at 15:07, SteveBz said:

It's possible, isn't it.  I'll look at that.

Tx.

Hi ZiHao,

I think you're right, it is the LSR.  However, I can't see how it's wrong.  Here's a plot:

My values are in the red box.  rv = 11 km/s and R is negative.  Obviously that is not going to work out well.  l = 190 deg is quadrant 3 and should have positive X and negative Y. The opposite is true.

The corrected LAB value is 6 km/s.  The LSR and planetary compound correction value is -41 km/s, quite large in comparison.  The orange curve is the Gaussian-fitted curve.  If anything it's skewed to lower values.

I started by calculating the corrections independently (-29 km/s and -12 km/s) and then togther by feeding the planetary correction onto the LSR calculation.  Both give -41 km/s.

Noty sure what to do next.

Steve.

[SteveBz]

34 minutes ago, SteveBz said:

Hi ZiHao,

I think you're right, it is the LSR.  However, I can't see how it's wrong.  Here's a plot:

My values are in the red box.  rv = 11 km/s and R is negative.  Obviously that is not going to work out well.  l = 190 deg is quadrant 3 and should have positive X and negative Y. The opposite is true.

The corrected LAB value is 6 km/s.  The LSR and planetary compound correction value is -41 km/s, quite large in comparison.  The orange curve is the Gaussian-fitted curve.  If anything it's skewed to lower values.

I started by calculating the corrections independently (-29 km/s and -12 km/s) and then togther by feeding the planetary correction onto the LSR calculation.  Both give -41 km/s.

Noty sure what to do next.

Steve.

Actually 6 or 11, makes little differnce. They both come out R<0.  I must have got the formula wrong.

[SteveBz]

I think I said the labels were the quadrants, yes? So the 1's look right.  Some of the 2's look right.  You can see that's it's gone asymptotic at 180 degrees for somew of the 2's and all the 3's.

There are two 3's south of the GC.  These have angles very near 180. Then there are a load of 2's in quadrant 4.  I don't know what is happening there.

 

[ZiHao]

7 hours ago, SteveBz said:

Hi ZiHao,

I think you're right, it is the LSR.  However, I can't see how it's wrong.  Here's a plot:

My values are in the red box.  rv = 11 km/s and R is negative.  Obviously that is not going to work out well.  l = 190 deg is quadrant 3 and should have positive X and negative Y. The opposite is true.

The corrected LAB value is 6 km/s.  The LSR and planetary compound correction value is -41 km/s, quite large in comparison.  The orange curve is the Gaussian-fitted curve.  If anything it's skewed to lower values.

I started by calculating the corrections independently (-29 km/s and -12 km/s) and then togther by feeding the planetary correction onto the LSR calculation.  Both give -41 km/s.

Noty sure what to do next.

Steve.

I think there's some mistake with the formula that you are using, I tried plugging in rv=6.18, as well as rv=11, I get positive R. You can refer to the notes I attached earlier or the equation you corrected in the pdf. Is it possible for you to send the raw data for this spectrum, l=190? I will try to correct it to LSR later and see if it matches with LAB survey.

[SteveBz]

5 hours ago, ZiHao said:

I think there's some mistake with the formula that you are using, I tried plugging in rv=6.18, as well as rv=11, I get positive R. You can refer to the notes I attached earlier or the equation you corrected in the pdf. Is it possible for you to send the raw data for this spectrum, l=190? I will try to correct it to LSR later and see if it matches with LAB survey.

Yes it's here.  Thanks for looking.

Steve.

debug(ra=94.1,dec=21.1).json

[ZiHao]

3 hours ago, SteveBz said:

Yes it's here.  Thanks for looking.

Steve.

debug(ra=94.1,dec=21.1).json 131.77 kB · 1 download

Alright, I tried plotting the results just now and I have the same results, rv=11. This is interesting, are your rv for spectrum of other locations off by few kilometers as well? If yes, then I think it will be good to check with @Victor Boesen LSR correction and compare the results.

[SteveBz]

2 hours ago, ZiHao said:

Alright, I tried plotting the results just now and I have the same results, rv=11. This is interesting, are your rv for spectrum of other locations off by few kilometers as well? If yes, then I think it will be good to check with @Victor Boesen LSR correction and compare the results.

And what do you get for R, X & Y?

[SteveBz]

2 hours ago, ZiHao said:

Alright, I tried plotting the results just now and I have the same results, rv=11. This is interesting, are your rv for spectrum of other locations off by few kilometers as well? If yes, then I think it will be good to check with @Victor Boesen LSR correction and compare the results.

I don't use @Victor Boesen's corrections yet.  These are the ones we discussed a few pages ago.  These are just the raw frequencies, uncorrected.

[ZiHao]

12 hours ago, SteveBz said:

And what do you get for R, X & Y?

Using rv=11,
R=11.9, r=3.47, X=-0.603, Y=3.42
X=rsinl  Y=-rcosl
Add 8.5 to Y if you want the galactic centre to be (0,0).

 

 

Edited April 28, 2022 by ZiHao