[daip] Velocity - Frequency conversion in AIPS

[Toby Brown]

Hi Eric,

Thanks a lot, very useful.

So just to confirm, if we trust the alternate reference in the original cube, the AIPS converted velocity cube would have the correct axis and standard reference key words?

Cheers
Toby

-- 
Research Officer
Herzberg Astronomy & Astrophysics Research Centre
National Research Council of Canada
drtobybrown.com <https://drtobybrown.com/>
@drtobybrown
 <https://twitter.com/DrTobyBrown>Pronouns: He/him/his

> On 10 Feb 2021, at 11:27, egreisen <egreisen at nrao.edu> wrote:
> 
> On 2021-02-10 10:32, Toby Brown via Daip wrote:
> 
> AIPS uses the values stored in keywords ALTRVAL and ALTRPIX to convert between
> frequency and velocity.  Looking in the history recorded in the FITS file I
> see that at one point the reference pixel in the combined data set for
> frequency was changed from 1 to 32.  It is now back at 1 for some reason.
> Each channel is about 10 km/sec so the error is about 300 km/sec.  It just
> a question of the reference pixel being in error or perhaps the reference
> frequency.  The 2 observations that were combined had frequencies different
> by 1.66 MHz due to being observed many months apart.  They were 1410.82 and
> 1412.48.  The header frequency converted to pixel 32 is 1410.83 which
> corresponds to one of the 2 observation dates.  The old VLA did the Doppler
> correction properly (and tracked properly) so I would trust the alternate
> reference pixel description.  The 300 km/sec is a question of the date of
> observation which may have gotten confused when combining the 2 data sets
> to make the images.  BTW - nice image.
> 
> Eric Greisen
> 
>> Hi,
>> I’m experiencing an issue converting between frequency and velocity
>> with an AIPS produced cube I was wondering if you could help me with.
>> I should caveat this with the fact I have zero experience with AIPS
>> but plenty playing with radio cubes using python and casa etc.
>> We have a VLA 21 cm cube produced in AIPS with a frequency axis. When
>> converted to velocity in python, CASA, CARTA, and by hand following
>> either the optical or radio conventions, the cube is at a
>> systematically higher velocity than the true system velocity of the
>> galaxy. However, when the conversion from frequency to velocity is
>> done using "ALTSWTCH" in AIPS, the result has the correct velocity.
>> The VLA cube can be downloaded here [1].
>> Here I show the VLA 21 cm spectrum (orange) and ALMA CO (grey)
>> spectrum comparison. The ALMA spectrum matches published
>> multi-wavelength velocities, while the VLA 21 cm cube is offset
>> approximated +300 km/s.
>> Now I’m showing the spectrum from the VLA cube *after* converting to
>> velocity with ALTSWITCH (orange) and the same ALMA CO (grey) spectrum.
>> You can see that the offset goes away.
>> Do you know why AIPS is able to make the correct conversion between
>> velocity and frequency, while the standard conventions do not work?
>> Cheers,
>> Toby
>> --
>> Research Officer
>> Herzberg Astronomy & Astrophysics Research Centre
>> National Research Council of Canada
>> drtobybrown.com <http://drtobybrown.com/> [2]
>> @drtobybrown
>> Pronouns: He/him/his
>> Links:
>> ------
>> [1] https://www.dropbox.com/s/2yow86gqxoqjroc/NGC4383.cube.fits?dl=0 <https://www.dropbox.com/s/2yow86gqxoqjroc/NGC4383.cube.fits?dl=0>
>> [2] https://drtobybrown.com <https://drtobybrown.com/>
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