Rfits task question
Gabriel wrote on Jul 08, 2009
Hi,
this is a very simple question. I have a *.fits file about 63Mb. After I run rfits on that file (with 'datatype = ushort') I get apparently exactly the same image file, except this new file is exactly half the size of the original one.
I've read the help in IRAF about 'rfits', but it doesn't say much (or maybe I'm not understanding everything)
Anyway, I would really appreciate if someone explained to me what this task does, and why is it reducing the size of my file.
Thanks!
Gabriel
this is a very simple question. I have a *.fits file about 63Mb. After I run rfits on that file (with 'datatype = ushort') I get apparently exactly the same image file, except this new file is exactly half the size of the original one.
I've read the help in IRAF about 'rfits', but it doesn't say much (or maybe I'm not understanding everything)
Anyway, I would really appreciate if someone explained to me what this task does, and why is it reducing the size of my file.
Thanks!
Gabriel
Mike Fitzpatrick wrote on Jul 08, 2009
What is the BITPIX value in the original FITS image? If it is 32 or -32 then then data are stored as 4-byte integer or as real respectively, doing the RFITS conversion to type ushort writes the image out (possibly with overflow of the values) as 2-byte values in the iraf image. Does an IMSTAT on the input and output image show the same results?
Note that RFITS is used to convert .fits files to .imh files, however if your 'imtype' environment variable is set the 'fits' (as it is in v2.14) and you don't explicitly provide a .imh extension on the output filename, the default output image type is also FITS. You can use FITS images directly in the system without converting them.
-Mike
Note that RFITS is used to convert .fits files to .imh files, however if your 'imtype' environment variable is set the 'fits' (as it is in v2.14) and you don't explicitly provide a .imh extension on the output filename, the default output image type is also FITS. You can use FITS images directly in the system without converting them.
-Mike
Gabriel wrote on Jul 08, 2009
Hi Mike,
IMSTAT on the original (63 Mb) file reports:
NPIX MEAN STDDEV MIN MAX
164 -1438. 1550. -8311. 9.309
and this is the output of the 'rfits' on the original (63 Mb) file:
size=4064x4064 bitpix=-32 scaling=none pixtype=ushort
WARNING: FITS extensions may be present
This way I get a new (31.5 Mb) file, and IMSTAT on this file gives:
NPIX MEAN STDDEV MIN MAX
2 5.5 4.95 2. 9.
Does this 4-byte to 2-byte conversion affect the image in any way (if this is what's actually happening)?
I mean: will this 'rfits' process affect the data I obtain from the image frame (after the CCD photometry is done)?
This of course is critical, and I'm not sure that this conversion doesn't affect it (and if it doesn't affect the results: why use a larger file in the first place?, why not convert all files to 'ushort' type?)
Thanks a lot for your time and help,
Gabriel
IMSTAT on the original (63 Mb) file reports:
NPIX MEAN STDDEV MIN MAX
164 -1438. 1550. -8311. 9.309
and this is the output of the 'rfits' on the original (63 Mb) file:
size=4064x4064 bitpix=-32 scaling=none pixtype=ushort
WARNING: FITS extensions may be present
This way I get a new (31.5 Mb) file, and IMSTAT on this file gives:
NPIX MEAN STDDEV MIN MAX
2 5.5 4.95 2. 9.
Does this 4-byte to 2-byte conversion affect the image in any way (if this is what's actually happening)?
I mean: will this 'rfits' process affect the data I obtain from the image frame (after the CCD photometry is done)?
This of course is critical, and I'm not sure that this conversion doesn't affect it (and if it doesn't affect the results: why use a larger file in the first place?, why not convert all files to 'ushort' type?)
Thanks a lot for your time and help,
Gabriel
Mike Fitzpatrick wrote on Jul 08, 2009
The RFITS output shows bitpix=-32 meaning the data are stored as real (4-byte) and so converting to ushort not only stores 2-byte pixels but is truncating the floating-point values to unsigned integer values. Of course there is a difference between floating-point and ushort data and they are NOT interchangeable for analysis,
If you don't explicitly request a change in data type the values will be preserved, but as I said, you don't need to do the conversion to use the file.
-Mike
If you don't explicitly request a change in data type the values will be preserved, but as I said, you don't need to do the conversion to use the file.
-Mike
Last post on Jul 08, 2009