Mailman 3 February 2016 - AstroPy

GSoC'16 reg
by Lakshmanan Meiyappan Feb. 29, 2016

Feb. 29, 2016

Hi, I'm Lakshmanan Meiyappan, 2nd year Engineering student from India, and I wish to contribute to AstroPy in this GSoC, can anyone guide me. Thank you, Lakshmanan Meiyappan

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Question re. astropy.table
by Brian York Feb. 27, 2016

Feb. 27, 2016

Greetings, I have a number of situations where I'm reading data from a (rather large) table in order to add that data into a separate numpy array (sample code below): t = Table.read(name, format) x_locations = t['X'] y_locations = t['Y'] fluxes = t['FLUX'] image_data[y_locations, x_locations] += fluxes Now, this works well for relatively short tables (~100,000 rows), but sometimes I end up with considerably longer tables (~5-10 million rows), and there tends to be a fairly high memory overhead in loading the table in those circumstances (especially given that the table has more than just the three columns -- the above code is an example, not exactly what I'm doing). Is there any way to load a table N rows at a time? Or some other way to reduce the memory footprint? Thank you, -Brian York

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Cosmology Recession Velocity
by Michael Seifert Feb. 24, 2016

Feb. 24, 2016

Hi, today I tried to determine cosmological recession velocities based on the paper of Davis, Lineweaver, 2001 (http://arxiv.org/pdf/astro-ph/0011070v2.pdf) with astropy.cosmology. But I'm not sure what I'm doing wrong (or if I just misunderstood their reasoning). But they said the recession velocity is just "v_rec = da(z)/dt * comoving distance(z)" and "da/dt = H(z)*a(z)" so the recession velocity should be with "cosmo = astropy.cosmology.WMAP9" and redshift "z=1000": v_rec = cosmo.H(z) * cosmo.scale_factor(z) * cosmo.comoving_distance(z) but this gives me 62 times the speed of light where it should only be between 2 and 4 (based on the paper). But just using v_rec = cosmo.H0 * cosmo.comoving_distance(z) gives me 3.25 * c which seems to fit their calculations. Is my formula or reasoning wrong or is there something strange in the comoving distance calculation of WMAP9? I hope this question is not too basic but I'm not very deep into cosmology and it just seemed strange to me. Many thanks in advance, Michael

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Re: [AstroPy] AstroPy Digest, Vol 113, Issue 12
by Paul Ray Feb. 22, 2016

Feb. 22, 2016

> On Feb 22, 2016, at 7:00 AM, <astropy-request(a)scipy.org> <astropy-request(a)scipy.org> wrote: > > An alternate way to do this: > > import astropy.time > import astropy.units as u > t = astropy.time.Time.now() > tnew = t - 1*u.d > print tnew.isot > > Not a full solution, but does what you need. Thanks for the reply, but that doesn’t do what I need. the isot format uses YYYY-MM-DD, not YYYY-DOY. And, my example was just to be able to ask the general question about making my own custom string output formats for Time objects, like you can do with strftime in unix. Thanks, — Paul

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Time custom formats
by Paul Ray Feb. 22, 2016

Feb. 22, 2016

Hi, I've been trying to figure out how to print an astropy.Time with a custom string format and have not been able to. In [2]: t = astropy.time.Time.now() In [3]: t.yday Out[3]: '2016:050:13:45:21.260' Now, what if I want the string '2016-050T13:45:21' instead? What I'd like is basically the astropy equivalent of the C library strftime(). I see there is a way to make a new subclass of TimeFormat, but I don't really want a whole new format, just a way to customize the string representation (possibly for input as well as output). Maybe all I need is an example of how to subclass TimeFormat to accomplish that. I had some trouble getting it to work in the little experimenting I did. Thanks for any tips, -- Paul

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Re: [AstroPy] Time custom formats
by Varun Bhalerao Feb. 22, 2016

Feb. 22, 2016

An alternate way to do this: import astropy.time import astropy.units as u t = astropy.time.Time.now() tnew = t - 1*u.d print tnew.isot Not a full solution, but does what you need. -- Varun ____________________ - Varun Bhalerao - Office: A 214, IUCAA Ph: +91 20 2560 4214 www.iucaa.in/~varunb > On 20-Feb-2016, at 5:30 pm, astropy-request(a)scipy.org wrote: > > Send AstroPy mailing list submissions to > astropy(a)scipy.org > > To subscribe or unsubscribe via the World Wide Web, visit > https://mail.scipy.org/mailman/listinfo/astropy > or, via email, send a message with subject or body 'help' to > astropy-request(a)scipy.org > > You can reach the person managing the list at > astropy-owner(a)scipy.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of AstroPy digest..." > > > Today's Topics: > > 1. Time custom formats (Paul Ray) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 19 Feb 2016 08:51:46 -0500 > From: Paul Ray <paul.ray(a)nrl.navy.mil> > To: astropy(a)scipy.org > Subject: [AstroPy] Time custom formats > Message-ID: <699E05F6-DE20-4D9E-92D9-54C7B6BE6F0E(a)nrl.navy.mil> > Content-Type: text/plain; charset=us-ascii > > Hi, > > I've been trying to figure out how to print an astropy.Time with a custom string format and have not been able to. > > In [2]: t = astropy.time.Time.now() > > In [3]: t.yday > Out[3]: '2016:050:13:45:21.260' > > Now, what if I want the string '2016-050T13:45:21' instead? > What I'd like is basically the astropy equivalent of the C library strftime(). > > I see there is a way to make a new subclass of TimeFormat, but I don't really want a whole new format, just a way to customize the string representation (possibly for input as well as output). Maybe all I need is an example of how to subclass TimeFormat to accomplish that. I had some trouble getting it to work in the little experimenting I did. > > Thanks for any tips, > > -- Paul > > > > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > AstroPy mailing list > AstroPy(a)scipy.org > https://mail.scipy.org/mailman/listinfo/astropy > > > ------------------------------ > > End of AstroPy Digest, Vol 113, Issue 11 > ****************************************

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installation error
by ihab riad Feb. 18, 2016

Feb. 18, 2016

Hi, I am trying to install astropy and I am getting the following error. ImportError: cannot import name version_info I found some information saying that I need to del ~/.astropy but I went looking for it and I couldn't locate this directory. Please help. Cheers Ihab

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[Astropy] python install prb.
by Xavier Couvelard Feb. 15, 2016

Feb. 15, 2016

Hello, I went through the tutorial https://python4astronomers.github.io/installation/python_install.html to install python, it went well, but at the end i have problem with matplotlib. In [7]: print(matplotlib.__version__) --------------------------------------------------------------------------- NameError Traceback (most recent call last) <ipython-input-7-dc737d9da186> in <module>() ----> 1 print(matplotlib.__version__) NameError: name 'matplotlib' is not defined If i follow anyway: In [9]: plt.plot(x, sin(x)) --------------------------------------------------------------------------- NameError Traceback (most recent call last) <ipython-input-9-298a676ae7cb> in <module>() ----> 1 plt.plot(x, sin(x)) Does any one can help ? Thanks Xavier

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Equivalent expression for SXPAR from IDL in python
by Roberts, Michael Feb. 15, 2016

Feb. 15, 2016

I'm hoping this question is relatively straightforward for those proficient with IDL and python and fits file handling (I'm using the astropy.fits module) I'm looking to find an equivalent to SXPAR (whose function is to obtain the value of a parameter in a FITS header) from IDL into python... http://idlastro.gsfc.nasa.gov/ftp/pro/fits/sxpar.pro "sxpar.pro" - idlastro.gsfc.nasa.gov<http://idlastro.gsfc.nasa.gov/ftp/pro/fits/sxpar.pro> idlastro.gsfc.nasa.gov Apostrophes are stripped ; from strings. If the parameter is logical, 1b is ; returned for T, and 0b is returned for F. ; If Name was of ... This is the original code in IDL: nxpix_uw1_ext1 = sxpar(hima_sk_uw1_ext1,'NAXIS1') nypix_uw1_ext1 = sxpar(hima_sk_uw1_ext1,'NAXIS2') This is my conversion attempt into python using astropy.fits: from astropy.io import fits hima_sk_um2 = fits.open('') #Will contain the directory extension to the fits/img file I wish to open # Compute the size of the images (you can also do this manually rather than calling these keywords from the header): nxpix_um2_ext1 = hima_sk_um2[1].header['NAXIS1'] nypix_um2_ext1 = hima_sk_um2[1].header['NAXIS2'] Firstly, is this correct? And secondly, is this the best way to handle an extension value? Any advise would be warmly received. Many thanks, Michael

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Creating a new Image file in astropy with specified dimensions of another image file and then applying corrections
by Roberts, Michael Feb. 14, 2016

Feb. 14, 2016

I'm having a problem wit fits file manipulation in the astropy package, and I'm in need of some help. I'm trying to convert an old IDL script into python. I essentially want to take an image I have in fits file format, and create a new file I need to start inputing correction factors to and a new image which can then be used with the correction factors and the original image to produce a correction image. Each of these will have the same dimensions. This is where I am at thus far: import numpy as numpy from astropy.io import fits #URL: to M33_sum_epoch1_um2_norm.img : https://www.dropbox.com/s/q7e14uosaj5u9en/M33_sum_epoch1_um2_norm.img?dl=0 ima_sk_um2 = fits.open('/Users/UCL_Astronomy/Documents/UCL/PHASG199/M33_UVOT_sum/UVOTIMSUM/M33_sum_epoch1_um2_norm.img') # Compute the size of the images (you can also do this manually rather than calling these keywords from the header): nxpix_um2_ext1 = ima_sk_um2[1].data['NAXIS1'] # IDL: nxpix_uw1_ext1 = sxpar(hima_sk_uw1_ext1,'NAXIS1') nypix_um2_ext1 = ima_sk_um2[1].data['NAXIS2'] # IDL: nypix_uw1_ext1 = sxpar(hima_sk_uw1_ext1,'NAXIS2') # Make a new image file to save the correction factors: corrfact_um2_ext1 = numpy.array(nxpix_um2_ext1,nypix_um2_ext1) hdu_corrfact = ImageHDU(corrfact_um2_ext1) HDUList([hdu_corrfact]).writeto('/Users/UCL_Astronomy/Documents/UCL/PHASG199/M33_UVOT_sum/UVOTIMSUM/M33_sum_epoch1_um2_corrfact.img') # Make a new image file to save the corrected image: ima_sk_coincorr_um2_ext1 = numpy.array(nxpix_um2_ext1,nypix_um2_ext1) hdu_coincorr = ImageHDU(ima_sk_coincorr_um2_ext1) HDUList([hdu_coincorr]).writeto('/Users/UCL_Astronomy/Documents/UCL/PHASG199/M33_UVOT_sum/UVOTIMSUM/M33_sum_epoch1_um2_coincorr.img') # Define the variables from Poole et al. (2008) "Photometric calibration of the Swift ultraviolet/optical telescope": alpha = 0.9842000 ft = 0.0110329 a1 = 0.0658568 a2 = -0.0907142 a3 = 0.0285951 a4 = 0.0308063 for i in range(nxpix_um2_ext1-1): #do begin for j in range(nypix_um2_ext1-1): #do begin if (numpy.less_equal(i, 4) | numpy.greater_equal(i, nxpix_um2_ext1-4) | numpy.less_equal(j, 4) | numpy.greater_equal(j, nxpix_um2_ext1-4)): #then begin #UVM2 corrfact_um2_ext1[i,j] == 0 ima_sk_coincorr_um2_ext1[i,j] == 0 else: xpixmin = i-4 xpixmax = i+4 ypixmin = j-4 ypixmax = j+4 #UVM2 ima_UVM2sum = total(ima_sk_um2[xpixmin:xpixmax,ypixmin:ypixmax]) xvec_UVM2 = ft*ima_UVM2sum fxvec_UVM2 = 1 + (a1*xvec_UVM2) + (a2*xvec_UVM2*xvec_UVM2) + (a3*xvec_UVM2*xvec_UVM2*xvec_UVM2) + (a4*xvec_UVM2*xvec_UVM2*xvec_UVM2*xvec_UVM2) Ctheory_UVM2 = - alog(1-(alpha*ima_UVM2sum*ft))/(alpha*ft) corrfact_um2_ext1[i,j] = Ctheory_UVM2*(fxvec_UVM2/ima_UVM2sum) ima_sk_coincorr_um2_ext1[i,j] = corrfact_um2_ext1[i,j]*ima_sk_um2[i,j] This is essentially a lot of muddling between IDL, python and astropy. In essence I want to have a new .img file that has the same dimensions as the original file but with the correction factors calculated in the loop as the new image array, and then the coincidence correction as the two multiplied together, so the correction factor times the image input. A dropbox link to the image file is in the script, so hopefully that should be self-containing. Hopefully there is someone here that knows how to guide me on this. Many thanks, Michael Roberts

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