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

[SteveBz]

7 minutes ago, Victor Boesen said:

Yes, it is. You can only use the radial method, I believe it's in one of the papers you shared, for galactic longitude between 0-90 degrees (in reality more like 20-75 because of the galactic bulge and etc).

Radial method above.

Victor

And what can we use for the other quadrants?  In fact the paper I shared already has at least one fatal error in it's equations.  Maybe I need to go through them again.

Edited April 16, 2022 by SteveBz

[Victor Boesen]

9 minutes ago, SteveBz said:

And what can we use for the other quadrants?  In fact the paper I shared already has at least one fatal error in it's equations.  Maybe I need to go through them again.

I'm not so sure actually. Shouldn't be too much different for the fourth quadrant (270-360deg longitude). However, the outer quadrants are a little tricky, hence the huge error bars you see on this image for areas further away than the sun.

[SteveBz]

13 minutes ago, Victor Boesen said:

I'm not so sure actually. Shouldn't be too much different for the fourth quadrant (270-360deg longitude). However, the outer quadrants are a little tricky, hence the huge error bars you see on this image for areas further away than the sun.

Hi Victor,

I'll try to plot the errors afterwards.  I haven't even begun to think of that 😅

In the meantime I have 4 nice curves that are all in the wrong place!!! They should be centred on the galactic bulge, by they really aren't.  They don't even have a shared centre of curvature and I don't understand, either, why my coordinate does not seem to be centred on the Sun, when that is how it is defined.  There is even another level of points, that I haven't been able to extract yet because of the effect of electrical noise in my vicinity.

  

 

[Victor Boesen]

18 minutes ago, SteveBz said:

Hi Victor,

I'll try to plot the errors afterwards.  I haven't even begun to think of that 😅

In the meantime I have 4 nice curves that are all in the wrong place!!! They should be centred on the galactic bulge, by they really aren't.  They don't even have a shared centre of curvature and I don't understand, either, why my coordinate does not seem to be centred on the Sun, when that is how it is defined.  There is even another level of points, that I haven't been able to extract yet because of the effect of electrical noise in my vicinity.

  

 

It looks cool though!! Definitely see a lot of potential in your idea. Could make a very interesting map!!

[Ian McCallum]

The word Cantenna always reminds me of this...🤣

Edited April 16, 2022 by Ian McCallum

[SteveBz]

1 hour ago, SteveBz said:

Hi Victor,

I'll try to plot the errors afterwards.  I haven't even begun to think of that 😅

In the meantime I have 4 nice curves that are all in the wrong place!!! They should be centred on the galactic bulge, by they really aren't.  They don't even have a shared centre of curvature and I don't understand, either, why my coordinate does not seem to be centred on the Sun, when that is how it is defined.  There is even another level of points, that I haven't been able to extract yet because of the effect of electrical noise in my vicinity.

  

 

So it seems to me that the yellow line is about right.  The red and orange line is incorrect because R>R0 and I can't see how to fix that.  It's going faster, so it should be outside the yellow line.  Yes? No? The blue line and the short yellow lines are incorrect because they are in the wrong quadrants, also I don't see how to fix that.  Maybe I can just change the sign of the sin(l) for R>R0.

[ZiHao]

3 hours ago, SteveBz said:

Hi Victor,

I'll try to plot the errors afterwards.  I haven't even begun to think of that 😅

In the meantime I have 4 nice curves that are all in the wrong place!!! They should be centred on the galactic bulge, by they really aren't.  They don't even have a shared centre of curvature and I don't understand, either, why my coordinate does not seem to be centred on the Sun, when that is how it is defined.  There is even another level of points, that I haven't been able to extract yet because of the effect of electrical noise in my vicinity.

  

 

Very interesting idea, especially the color that shows the amount of redshift. I think the equations used to plot the cartesian view of the milky way applies for all cases, you will get two positive r due to the two intersection points along the line of sight for quadrant I and IV. For quadrant II and III, I believe you will get one negative r and one positive r, I might be wrong. For the two positive values of r,  seems like we have to reject the data as shown in your attached pdf, I wonder how can we further validate the values. 

It might be a good idea to cross check your spectrum with the ones from LAB survey, you can download the data in the link below. I have attached a general illustration of roughly how the arrangement of colored dots should look like, the picture is from DARA https://www.youtube.com/watch?v=RQiLMxQHxr8&t=2946

Edited April 16, 2022 by ZiHao

[SteveBz]

On 16/04/2022 at 14:00, ZiHao said:

Very interesting idea, especially the color that shows the amount of redshift. I think the equations used to plot the cartesian view of the milky way applies for all cases, you will get two positive r due to the two intersection points along the line of sight for quadrant I and IV. For quadrant II and III, I believe you will get one negative r and one positive r, I might be wrong. For the two positive values of r,  seems like we have to reject the data as shown in your attached pdf, I wonder how can we further validate the values. 

Yes I agree about the PDF.  The data and the formulae are very suspicious.

I also agree about validating the data from scratch.  I'm working through it manually and indeed I'm finding a lot of errors.

Here's the baseline.  Before I took all the samples within 5 degrees of the galactic equator and that's why I had so many apparently nice points.  Now I've gone through the whole set of data again manually and I've just taken the single point from each scan with the highest signal intensity.  I've only examined samples from Quadrant I, so I have about six data points in total.

Here is the base scan for the first point:

Then I adjusted it for Local System of Rest and flipped the axis to get radial velocity and Gaussian fitted it.  Here it is. The blue line is the original line. The green and pink lines are the result of subtracting the standard Gaussian curves. 

Then I looked at the University of Bonn result:

If you look at the Bonn result there are 2 clear peaks at about -55 km/s and about 10 km/s which correspond to the values on my chart of -38 km/s and 27 km/s.  Ie my results are about 20 km/s too large.  Maybe I didn't get the LSR adjustment right.

I feel I have to get this bit right first.

Any thoughts welcome.

Kind regards

Steve.

[SteveBz]

22 minutes ago, SteveBz said:

Yes I agree about the PDF.  The data and the formulae are very suspicious.

I also agree about validating the data from scratch.  I'm working through it manually and indeed I'm finding a lot of errors.

Here's the baseline.  Before I took all the samples within 5 degrees of the galactic equator and that's why I had so many apparently nice points.  Now I've gone through the whole set of data again manually and I've just taken the single point from each scan with the highest signal intensity.  I've only examined samples from Quadrant I, so I have about six data points in total.

Here is the base scan for the first point:

Then I adjusted it for Local System of Rest and flipped the axis to get radial velocity and Gaussian fitted it.  Here it is. The blue line is the original line. The green and pink lines are the result of subtracting the standard Gaussian curves. 

Then I looked at the University of Bonn result:

If you look at the Bonn result there are 2 clear peaks at about -55 km/s and about 10 km/s which correspond to the values on my chart of -38 km/s and 27 km/s.  Ie my results are about 20 km/s too large.  Maybe I didn't get the LSR adjustment right.

I feel I have to get this bit right first.

Any thoughts welcome.

Kind regards

Steve.

My LSR correction looks to be about 40-50 km/s and that seems quite large.  I would have thought a value nearer 30 or less would be better.  In fact, if I ignore the inflection at + 30 or 40 km/s, which is hard to evaluate precisely, the difference between the baseline result and the Bonn result seems to be about 30+/-5 km/s.

Edited April 17, 2022 by SteveBz

[ZiHao]

c=299792.458
rest_freq=1420.40575e6
#freq=frequencyarray
velocity=c*(1-(freq/rest_freq))
corrected_vel=velocity*u.km/u.s

rv=c*((rest_freq/rest_freq)-1)*u.km/u.s

t = Time(, format='mjd')

loc = EarthLocation(lon=,lat=,height=)
ra=*u.deg
dec=*u.deg
sc = SkyCoord(ra,dec)

vcorr = sc.radial_velocity_correction(kind='barycentric', obstime=t, location=loc)  
rv = rv + vcorr

my_observation = ICRS(ra=*u.deg, dec=*u.deg, \
        pm_ra_cosdec=0*u.mas/u.yr, pm_dec=0*u.mas/u.yr, \
        radial_velocity=rv, distance = 1*u.pc)

new_rv = my_observation.transform_to(LSR()).radial_velocity

corrected_vel=corrected_vel+new_rv

Attached above is my attempt on LSR correction, I chose rv as zero so it is easier to add the final rv (new_rv) to the frequency array as I find it takes quite long to compute for individual frequency point. I don't think there will be significant difference in the final result using this method, I will check this again later. You can try the code above, just enter the corresponding info required. I have checked this with the LAB survey with my limited data at hand. The main peak in my data is around +9km/s, which agreed with the survey. 

[ZiHao]

58 minutes ago, SteveBz said:

Yes I agree about the PDF.  The data and the formulae are very suspicious.

The formulae should be correct, attached is the derivation.

[SteveBz]

3 hours ago, ZiHao said:

c=299792.458
rest_freq=1420.40575e6
#freq=frequencyarray
velocity=c*(1-(freq/rest_freq))
corrected_vel=velocity*u.km/u.s

rv=c*((rest_freq/rest_freq)-1)*u.km/u.s

t = Time(, format='mjd')

loc = EarthLocation(lon=,lat=,height=)
ra=*u.deg
dec=*u.deg
sc = SkyCoord(ra,dec)

vcorr = sc.radial_velocity_correction(kind='barycentric', obstime=t, location=loc)  
rv = rv + vcorr

my_observation = ICRS(ra=*u.deg, dec=*u.deg, \
        pm_ra_cosdec=0*u.mas/u.yr, pm_dec=0*u.mas/u.yr, \
        radial_velocity=rv, distance = 1*u.pc)

new_rv = my_observation.transform_to(LSR()).radial_velocity

corrected_vel=corrected_vel+new_rv

Attached above is my attempt on LSR correction, I chose rv as zero so it is easier to add the final rv (new_rv) to the frequency array as I find it takes quite long to compute for individual frequency point. I don't think there will be significant difference in the final result using this method, I will check this again later. You can try the code above, just enter the corresponding info required. I have checked this with the LAB survey with my limited data at hand. The main peak in my data is around +9km/s, which agreed with the survey. 

OK, very nice. My code only has the last three statements of yours. I'll double check it.

Tx

Steve.

[SteveBz]

3 hours ago, ZiHao said:

The formulae should be correct, attached is the derivation.

Perfect. That's what I got too.

What I don't have is the derivations for the other quadrants or outside the solar circle. Do you think they are different?

[SteveBz]

He gets this bit wrong.

No brackets and wrong V.

Edited April 17, 2022 by SteveBz

[Victor Boesen]

5 hours ago, ZiHao said:

c=299792.458
rest_freq=1420.40575e6
#freq=frequencyarray
velocity=c*(1-(freq/rest_freq))
corrected_vel=velocity*u.km/u.s

rv=c*((rest_freq/rest_freq)-1)*u.km/u.s

t = Time(, format='mjd')

loc = EarthLocation(lon=,lat=,height=)
ra=*u.deg
dec=*u.deg
sc = SkyCoord(ra,dec)

vcorr = sc.radial_velocity_correction(kind='barycentric', obstime=t, location=loc)  
rv = rv + vcorr

my_observation = ICRS(ra=*u.deg, dec=*u.deg, \
        pm_ra_cosdec=0*u.mas/u.yr, pm_dec=0*u.mas/u.yr, \
        radial_velocity=rv, distance = 1*u.pc)

new_rv = my_observation.transform_to(LSR()).radial_velocity

corrected_vel=corrected_vel+new_rv

Attached above is my attempt on LSR correction, I chose rv as zero so it is easier to add the final rv (new_rv) to the frequency array as I find it takes quite long to compute for individual frequency point. I don't think there will be significant difference in the final result using this method, I will check this again later. You can try the code above, just enter the corresponding info required. I have checked this with the LAB survey with my limited data at hand. The main peak in my data is around +9km/s, which agreed with the survey. 

Very interesting stuff ZiHao!! Is that astropy? It looks excellent for this kind of stuff. I might have a look at astropy instead of PyEphem. It's my impression that it's better suited for astronomical data processing.

[ZiHao]

7 hours ago, SteveBz said:

Perfect. That's what I got too.

What I don't have is the derivations for the other quadrants or outside the solar circle. Do you think they are different?

I am quite sure that the equations will be the same for other quadrants. I will try to derive it later..

 

5 hours ago, Victor Boesen said:

Very interesting stuff ZiHao!! Is that astropy? It looks excellent for this kind of stuff. I might have a look at astropy instead of PyEphem. It's my impression that it's better suited for astronomical data processing.

Hi Victor. Yes, it is astropy. I tried looking for procedures for lsr correction but didn't manage to get any, and it sounds like a complicated task to me, so luckily astropy has all these built in functions!

[SteveBz]

On 16/04/2022 at 14:00, ZiHao said:

I have attached a general illustration of roughly how the arrangement of colored dots should look like, the picture is from DARA https://www.youtube.com/watch?v=RQiLMxQHxr8&t=2946

This was really useful, although I thought the guy was quite patronising to his his audience. 'Work hard, write up your results' etc.

[SteveBz]

16 hours ago, ZiHao said:

The formulae should be correct, attached is the derivation.

I should have said, it's very nice to see this all written out by hand!  Your notes are much neater and more organised than mine! 🥴

Edited April 18, 2022 by SteveBz

[SteveBz]

16 hours ago, ZiHao said:

c=299792.458
rest_freq=1420.40575e6
#freq=frequencyarray
velocity=c*(1-(freq/rest_freq))
corrected_vel=velocity*u.km/u.s

rv=c*((rest_freq/rest_freq)-1)*u.km/u.s

t = Time(, format='mjd')

loc = EarthLocation(lon=,lat=,height=)
ra=*u.deg
dec=*u.deg
sc = SkyCoord(ra,dec)

vcorr = sc.radial_velocity_correction(kind='barycentric', obstime=t, location=loc)  
rv = rv + vcorr

my_observation = ICRS(ra=*u.deg, dec=*u.deg, \
        pm_ra_cosdec=0*u.mas/u.yr, pm_dec=0*u.mas/u.yr, \
        radial_velocity=rv, distance = 1*u.pc)

new_rv = my_observation.transform_to(LSR()).radial_velocity

corrected_vel=corrected_vel+new_rv

Attached above is my attempt on LSR correction, I chose rv as zero so it is easier to add the final rv (new_rv) to the frequency array as I find it takes quite long to compute for individual frequency point. I don't think there will be significant difference in the final result using this method, I will check this again later. You can try the code above, just enter the corresponding info required. I have checked this with the LAB survey with my limited data at hand. The main peak in my data is around +9km/s, which agreed with the survey. 

OK, I just used your code.  It improved my code by about 7 km/s in the right direction, but I think I'm still about 10 km/sec too fast.  See here (the blue is corrected and the orange uncorrected).

Tx

Steve.

[SteveBz]

I wonder @Victor Boesen, should I have calibrated my rtl-sdr?  The guy in the video suggested by @ZiHao talked about calibration.  What do you think? ZiHo, did you calibrate yours?

Steve

[SteveBz]

In fact here (https://www.reddit.com/r/RTLSDR/comments/2pkue7/best_way_to_calibrate_with_a_known_signal_source/) they say:

'I purchased a Nooelec dongle and have been very pleased with it. Right out of the box, I am picking up air traffic as far as Philadelphia from Baltimore. I noticed, however, that when using gqrx in Linux that some stations appear to be off by 50khz or so.'

I think that's about 10 km/s, so about the same error as I'm getting.  I don't know how close the Nooelec is to the RTL-SDR Blog v3, however.

Steve.

[Victor Boesen]

46 minutes ago, SteveBz said:

I wonder @Victor Boesen, should I have calibrated my rtl-sdr?  The guy in the video suggested by @ZiHao talked about calibration.  What do you think? ZiHo, did you calibrate yours?

Steve

I have two RTL SDR's and both of them are accurate within 2PPM (probably even lower). However, you can definitely try to calibrate in on a known signal anyways.

[SteveBz]

Actually, it seems to be nearly linear in Dec.  

[ZiHao]

1 hour ago, SteveBz said:

I wonder @Victor Boesen, should I have calibrated my rtl-sdr?  The guy in the video suggested by @ZiHao talked about calibration.  What do you think? ZiHo, did you calibrate yours?

Steve

Nope, I didn't calibrate mine, like what Victor had mentioned, 1-2ppm shouldn't cause that much of a difference.

46 minutes ago, Victor Boesen said:

I have two RTL SDR's and both of them are accurate within 2PPM (probably even lower). However, you can definitely try to calibrate in on a known signal anyways.

Btw Victor, for the pyrtlsdr wrapper, in sdr.freq_correction, it seems that a value has to be entered, how did you deal with this?

[ZiHao]

1 hour ago, SteveBz said:

OK, I just used your code.  It improved my code by about 7 km/s in the right direction, but I think I'm still about 10 km/sec too fast.  See here (the blue is corrected and the orange uncorrected).

Tx

Steve.

Could it be pointing error in the mount? Maybe you could try pointing in the zenith, I think it's much easier.