# Plotting with .get\_transform? **URL:** \ **Category:** Astropy\ **Tags:** question\ **Created:** [March 12, 2024, 4:22am UTC](https://community.openastronomy.org/t/plotting-with-get-transform/937 "2024-03-12T04:22:21Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![sepiagiraffe](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/sepiagiraffe/32/517_2.png) [@sepiagiraffe](https://community.openastronomy.org/u/sepiagiraffe)\ **Post date:** [March 12, 2024, 4:22am UTC](https://community.openastronomy.org/t/plotting-with-get-transform/937/1 "2024-03-12T04:22:21Z")
Hello! I’m having issues with plotting things on top of images (using imshow, and SkyCoord). When I try plotting on top of an image with a SkyCoord object, I have to use ax.get\_transform(‘fk5’) to get it to line up, but I don’t understand why if the coordinates are in ICRS and the image should be in ICRS. I have a point I want to plot as a SkyCoord object (In this case it’s the known center of the source): center = SkyCoord('18:57:25.59555586 ', ‘+16:24:55.8404388’, unit=(un.hourangle, un.deg)); When I go to plot it, I try: ax.plot(center.ra.degree, center.dec.degree, markersize=15, marker=‘\*’, color=‘white’); Now, using the ax.get\_transform, with different arguments passed I get a variety of different plots I don’t understand. Using ax.get\_transform(‘icrs’), which means it would transform the passed coordinates (which are in ICRS as it’s the SkyCoord default, right?) that are in ICRS, to the WCS (or the image’s coordinates), right? However, it seems that doesn’t work exactly… As I get the orange star. Now, if I try ax.get\_transform(‘fk5’), it works (albeit some error)! Which is the black star (under the white dot). However, this would mean the coordinates should be in FK5 (they’re not), and/or the image would be in FK5 (which I find hard to believe). Also, just to try it I did ax.get\_transform(‘world’), which also works?! This is the white dot (which is on top of the black star). I tried looking around in the documentation, and what I came up with was: in ax.get\_transform(‘frame’) the frame and the frame of the coordinates should be the same, even the documentation uses ax.get\_transform(‘fk5’), but if ICRS is the standard why would the example use FK5. ![all](https://europe1.discourse-cdn.com/flex005/uploads/openastronomy/original/1X/34dff58b03139f353674ff97a94239fc712fdd0b.png) ---
**Author:** ![ayshih](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/ayshih/32/84_2.png) [@ayshih](https://community.openastronomy.org/u/ayshih)\ **Post date:** [March 12, 2024, 6:47pm UTC](https://community.openastronomy.org/t/plotting-with-get-transform/937/2 "2024-03-12T18:47:40Z")
Everything you have described indicates that your WCS is in FK5, even if that is not what is intended. Can you print out `ax.wcs.wcs`? ---
**Author:** ![sepiagiraffe](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/sepiagiraffe/32/517_2.png) [@sepiagiraffe](https://community.openastronomy.org/u/sepiagiraffe)\ **Post date:** [March 12, 2024, 7:39pm UTC](https://community.openastronomy.org/t/plotting-with-get-transform/937/3 "2024-03-12T19:39:59Z")
Ah! It seems you are right! flag: 137 naxis: 2 crpix: 0x7fe937a483d0 256.00 257.00 pc: 0x7fe937a4f4c0 pc[0]: 1.0000 0.0000 pc[1]: -0.0000 1.0000 cdelt: 0x7fe937a5a3d0 -4.1667e-08 4.1667e-08 crval: 0x7fe937a4c490 284.36 16.416 cunit: 0x7fe937a58450 “deg” “deg” ctype: 0x7fe937a56790 “RA—SIN” “DEC–SIN” lonpole: 180.000000 latpole: 16.415511 restfrq: 0.000000 restwav: 0.000000 npv: 0 npvmax: 64 pv: 0x7fe9985d4000 nps: 0 npsmax: 8 ps: 0x7fe937a512b0 cd: 0x7fe937a5b6e0 cd[0]: 0.0000 0.0000 cd[1]: 0.0000 0.0000 crota: 0x7fe937a5c0f0 0.0000 0.0000 altlin: 4 velref: 3 alt: ’ ’ colnum: 0 colax: 0x7fe937a57180 0 0 cname: 0x7fe937a4ecf0 UNDEFINED UNDEFINED crder: 0x7fe937a4fd20 UNDEFINED UNDEFINED csyer: 0x7fe937a506b0 UNDEFINED UNDEFINED czphs: 0x7fe937a4ec70 UNDEFINED UNDEFINED cperi: 0x7fe937a4f740 UNDEFINED UNDEFINED wcsname: UNDEFINED timesys: UNDEFINED trefpos: UNDEFINED trefdir: UNDEFINED plephem: UNDEFINED timeunit: UNDEFINED dateref: UNDEFINED mjdref: 0.000000000 0.000000000 timeoffs: UNDEFINED dateobs: “2021-07-02” datebeg: UNDEFINED dateavg: UNDEFINED dateend: UNDEFINED mjdobs: 59397.000000000 mjdbeg: UNDEFINED mjdavg: UNDEFINED mjdend: UNDEFINED jepoch: UNDEFINED bepoch: UNDEFINED tstart: UNDEFINED tstop: UNDEFINED xposure: UNDEFINED telapse: UNDEFINED timsyer: UNDEFINED timrder: UNDEFINED timedel: UNDEFINED timepixr: UNDEFINED obsgeo: UNDEFINED UNDEFINED UNDEFINED UNDEFINED UNDEFINED UNDEFINED obsorbit: UNDEFINED radesys: “FK5” equinox: 2000.000000000 specsys: “TOPOCENT” ssysobs: UNDEFINED velosys: UNDEFINED zsource: UNDEFINED ssyssrc: UNDEFINED velangl: UNDEFINED aux: 0x0 ntab: 0 tab: 0x0 nwtb: 0 wtb: 0x0 types: 0x7fe937a4ca30 2200 2201 lngtyp: “RA” lattyp: “DEC” lng: 0 lat: 1 spec: -1 time: -1 cubeface: -1 err: 0x0 lin: (see below) cel: (see below) spc: (see below) m\_flag: 137 m\_naxis: 2 m\_crpix: 0x7fe937a483d0 (= crpix) m\_pc: 0x7fe937a4f4c0 (= pc) m\_cdelt: 0x7fe937a5a3d0 (= cdelt) m\_crval: 0x7fe937a4c490 (= crval) m\_cunit: 0x7fe937a58450 (= cunit) m\_ctype: 0x7fe937a56790 (= ctype) m\_pv: 0x7fe9985d4000 (= pv) m\_ps: 0x7fe937a512b0 (= ps) m\_cd: 0x7fe937a5b6e0 (= cd) m\_crota: 0x7fe937a5c0f0 (= crota) ``` m_colax: 0x7fe937a57180 (= colax) m_cname: 0x7fe937a4ecf0 (= cname) m_crder: 0x7fe937a4fd20 (= crder) m_csyer: 0x7fe937a506b0 (= csyer) m_czphs: 0x7fe937a4ec70 (= czphs) m_cperi: 0x7fe937a4f740 (= cperi) m_aux: 0x0 (= aux) m_tab: 0x0 (= tab) m_wtb: 0x0 (= wtb) ``` lin.\* flag: 137 naxis: 2 crpix: 0x7fe937a483d0 256.00 257.00 pc: 0x7fe937a4f4c0 pc[0]: 1.0000 0.0000 pc[1]: -0.0000 1.0000 cdelt: 0x7fe937a5a3d0 -4.1667e-08 4.1667e-08 dispre: 0x0 disseq: 0x0 piximg: (nil) imgpix: (nil) i\_naxis: 0 unity: 1 affine: 1 simple: 1 err: 0x0 tmpcrd: 0x7fe937a585f0 m\_flag: 0 m\_naxis: 0 m\_crpix: 0x0 m\_pc: 0x0 m\_cdelt: 0x0 m\_dispre: 0x0 m\_disseq: 0x0 cel.\* flag: 137 offset: 0 phi0: 0.000000 theta0: 90.000000 ref: 284.36 16.416 180.00 16.416 prj: (see below) euler: 284.36 73.584 180.00 0.28260 0.95924 latpreq: 0 (not required) isolat: 0 err: 0x0 prj.\* flag: 105 code: “SIN” r0: 57.295780 pv: (0) 0.0000 0.0000 phi0: 0.000000 theta0: 90.000000 bounds: 7 ``` name: "orthographic/synthesis" ``` category: 1 (zenithal) pvrange: 102 simplezen: 1 equiareal: 0 conformal: 0 global: 0 divergent: 0 x0: 0.000000 y0: 0.000000 err: 0x0 w: 0.017453 0.0000 1.0000 -1.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 m: 0 n: 0 prjx2s: 0x0133eeda90 prjs2x: 0x0133eee320 spc.\* flag: 0 type: " " code: " " crval: UNDEFINED restfrq: 0.000000 restwav: 0.000000 pv: (not used) w: 0.0000 0.0000 0.0000 (remainder unused) isGrism: 0 err: 0x0 spxX2P: 0x0 spxP2S: 0x0 spxS2P: 0x0 spxP2X: 0x0 Would astropy default to FK5, or does it choose it based on the original file? I looked in the FITS header and it never said FK5 or ICRS. ---
**Author:** ![ayshih](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/ayshih/32/84_2.png) [@ayshih](https://community.openastronomy.org/u/ayshih)\ **Post date:** [March 13, 2024, 5:00pm UTC](https://community.openastronomy.org/t/plotting-with-get-transform/937/4 "2024-03-13T17:00:25Z")
The two FITS keywords to look for are `RADESYS` and `EQUINOX`. It sounds like `RADESYS` is not defined in your FITS header, so I’m guessing that `EQUINOX` is explicitly defined in your header to be `2000`. The FITS standard says: ```auto RADESYSa – [string; default: ’FK4’, ’FK5’, or ’ICRS’: see below]. Name of the reference frame of equatorial or eclip- tic coordinates, whose value must be one of those speci- fied in Table 24. The default value is ’FK4’ if the value of EQUINOXa < 1984.0, ’FK5’ if ’EQUINOX’a ≥ 1984.0, or ’ICRS’ if ’EQUINOX’a is not given. ``` That is, you’re getting FK5 rather than ICRS because `EQUINOX` is defined. ---
**Author:** ![sepiagiraffe](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/sepiagiraffe/32/517_2.png) [@sepiagiraffe](https://community.openastronomy.org/u/sepiagiraffe)\ **Post date:** [March 25, 2024, 6:46pm UTC](https://community.openastronomy.org/t/plotting-with-get-transform/937/5 "2024-03-25T18:46:06Z")
Got it! Thank you so much!!