questions about output files, thank you!!

[Yi Yang]

Dear all,

Sorry to bother you! First, I would like to say thanks to the astrometry.net team for all of your
great work and efforts on this software! Actually I don't want to take your precious time since you have a lot of important things
to do, so I tried to solve them by myself for several days, and still cannot figure them out. My advisor urged me to finish this
part of work asap and begin write the paper, so I have to turned to you for help, sorry about that and thank you very much for your
patience!!

 

I'm a first year grad student at the department of physics and astronomy, Texas A&M University, currently working on a
tiny project which putting a 1-cm Nikon fish eye camera at Dome A, Antarctica during the winter season to monitor the variance of 
atmosphere transparency, cloud cover, and do photometry for about 2000 bright stars. The interesting part of this project is the extremely 
large FOV (about 90 degree across the field), the unique site and more than 100-day multi-band light curves for more than 2000 stars. 
However, such a large FOV involves dramatic distortion, especially near the edge of the FOV.

Now I have finished almost all the works in this project except for astrometry. After that I will begin to write the paper. My
advisor recommended me your software and then I begin learning to use it. I installed it on my Ubuntu 11.10 and actually solved
some images, it was fantastic! However, after I solved the image, I still have two questions, would you please to help me?

 

1. I divided my  FOV into 25 pieces of 400 times 400 pixel fits frame since it is really hard to solve such a distorted field at
once. For example, the central part solved like:

Field center: (RA,Dec) = (163.4, -87.25) deg.
Field center: (RA H:M:S, Dec D:M:S) = (10:53:24.362, -87:14:43.754).
Field size: 16.3783 x 16.3843 degrees

However, your software may fail to solve the pieces near the corner of my FOV, since the distortion is so dramatic from a
1-centimeter fish eye lens. I just wonder if there exist a possible way to deal with that? It doesn't matter so much to the project
I am working on, but I still want to have a try. Thank you very much!

 

2. After I solve a piece of my field, I obtained a series of output files like

<base>-ngc.png
an annotation of the image.
<base>.wcs
a FITS WCS header for the solution.
<base>.new
a new FITS file containing the WCS header.
<base>-objs.png
a plot of the sources (stars) we extracted from the image.
<base>-indx.png
sources (red), plus stars from the index (green), plus the skymark (“quad”) used to solve the image.
<base>-indx.xyls
a FITS BINTABLE with the pixel locations of stars from the index.
<base>.rdls
a FITS BINTABLE with the RA,Dec of sources we extracted from the image.
<base>.axy
a FITS BINTABLE of the sources we extracted, plus headers that describe the job (how the image is going to be solved).
<base>.solved
exists and contains (binary) 1 if the field solved.
<base>.match
a FITS BINTABLE describing the quad match that solved the image.
<base>.kmz

(optional) KMZ file for Google Sky-in-Earth. You need to have “wcs2kml” in your PATH. See


However, I cannot find a way to open those FITS BINTABLEs. I just want to extract the coordinate, RA and DEC, as well as the
magnitude from both the image and index files to get a ASCII file, such as .dat or .txt, but I cannot find a way to do that. Would
you please to help me?

 

Besides, when I tried to use:

wcs-xy2rd -w wcs-file -i xy-list -o radec-list
To convert a list of pixel coordinates to RA,Dec coordinates, it says that: 

anwcs.c:957:anwcs_open_wcslib: Wcslib support was not compiled in
anwcs.c:979:anwcs_open_wcstools: WCStools support was not compiled in
wcs-xy2rd.c:57:wcs_xy2rd: Failed to read WCS file "wcs-file", extension 0
wcs-xy2rd-main.c:168:main: wcs-xy2rd failed

I installed the Wcslib and WCStools, but it still doesn't work. Is this necessary for me since I just want to extract the xy
position, RA/DEC and magnitude to see which one is which one.

I also attached an .rar file to include my results on one piece of image, it is just 1/25 of my entire FOV, would you please to
help me to take a look at that?

 

Sorry to take up your time and many, many, many thanks to your help and concern!!

 

Best wishes

Yi

[Dustin Lang]

Hi,

Cool data set!

A couple of ideas for the distortion:

1.  You could try increasing the matching distance: say, "solve-field -c 0.02" or 0.03 or whatever, to let the shape-matching be more accepting.

2.  You could try taking the WCS from the central region (which solves) and using it as an initial guess for the whole image.  The "tweak" process then might be able to find matches in the middle part and "grow" the matching region out to the edges.  The only complication doing that is that when you plug the solved WCS back into the whole image, you will need to adjust the "CRPIX" values to compensate for the offset of the subimage.  That is, if your central subimage starts at pixel 1000,1000, then you want to add 1000 to CRPIX1 and CRPIX2 in the WCS FITS header before plugging it back into the whole image; you might want to use the "new-wcs" program to plug the WCS header into (a copy of) the original image.

3.  More work: You could try to try to figure out the functional form of the distortion and remove it.  With a fish-eye lens, the distortion is probably all radial (you would hope!).  Try grabbing the largest sub-image that still solves and produces a good-looking "-indx.png" plot (red circles and green circles aligned).  In the ".corr" file you have a list of the X,Y,RA,Dec positions of stars in your image, and reference stars, that we think matched.  Plot the sqrt((field_x - XC)**2 + (field_y - YC)**2) vs sqrt((index_x - XC)**2 + (index_y - YC)**2), where XC,YC is the image center.  That is, plot the radial difference in the image stars and index stars.

The catch is that by default we try to correct the distortion with SIP coefficients, so either turn that off with "solve-field --no-tweak", or try using wcs-rd2xy with the "-t" flag to ignore the SIP terms, to project the RA,Dec coordinates in the .corr file into (distorted) X,Y coordinates, and then make the plot as above.

> However, I cannot find a way to open those FITS BINTABLEs. I just want to extract the coordinate, RA and DEC, as well as the magnitude from both the image and index files to get a ASCII file, such as .dat or .txt, but I cannot find a way to do that. Would you please to help me?

Try the "tablist" program, eg     "tablist out.rdls"

> Besides, when I tried to use: wcs-xy2rd -w wcs-file -i xy-list -o radec-list To convert a list of pixel coordinates to RA,Dec coordinates, it says that:

anwcs.c:957:anwcs_open_wcslib: Wcslib support was not compiled in anwcs.c:979:anwcs_open_wcstools: WCStools support was not compiled in wcs-xy2rd.c:57:wcs_xy2rd: Failed to read WCS file "wcs-file", extension 0 wcs-xy2rd-main.c:168:main: wcs-xy2rd failed

You shouldn't need wcstools or wcslib.  Probably the "wcs-file" you told it to use doesn't exist.  What is the exact command-line you are using to run wcs-xy2rd?



What exactly are you asking us to do with the attached data?



cheers,
--dstn

[Yi Yang]

Hi,

Thank you very much for your quick and clear reply to a rookie! That helps a lot!! Now I can focus on how to deal with the distortion. Just ignore the attached file!

I met several small questions when I tried to follow your suggestion NO.2 for the distortion, would you please to help me?

1. Is it possible to specify a series of star which I'd like to let the software using them to match the index? I tried to input the coordinate file but it seems that ASCII files cannot be recognized, so what should I do to convert the ASCII.dat files to .xy files as an input?

2. Would you please to tell me how to  increase the number of field-object to match the index?
Now it exceeded to 200 and I would like to use more.

Thank you again for your patience and help!!

YY

[Dustin Lang]

1. There is a script, text2fits.py , that will convert ASCII to FITS binary tables.

The first line tells it what the columns should be called, eg:

```
> cat xy.txt
# x y
12 34
56 78

> text2fits.py xy.txt xy.fits

> tablist xy.fits

                       x                    y
   1                   12                   34
   2                   56                   78
```

2.  I'm not sure what you mean... you want to ask it to find more matches?

One thing you can try is: run solve-field as usual; this produces an X.wcs file.  Then run solve-field again, with "solve-field --verify X.wcs" plus your usual arguments.  It will try to "verify" the X.wcs solution against ALL the index files, and it should find more matches in the denser index files.

cheers,
dustin