#1
|
|||
|
|||
[fitsbits] CDELTn
Francois,
Well, my question about CDELTn=0 was not related to WCS -- a simple multidimensional hypercube which has several NAXISn set to 1 -- a current practice in FITS. And I'm surprised by Phil's remark: Regardless of the value of NAXISn, computing the coordinate value at a pixel would involve dividing by CDELTn. For example: CRVALn + (1 - CRPIXn) / CDELTn. So it's not as harmless as it may seem. ... I always understood that the coordinate value is CRVALn + (i-CRPIXn)*CDELTn which simply means that the value of the coordinate along this axis is constant and equal to CRVALn, which seems to me perfectly correct (a 1-point dimension is by definition a constant). The formula is not reversible, but it does not be, as far as I know. You're right, of course (except that CRPIXn doesn't have to be 1). I made a dumb mistake, a sign error (in the exponent), and didn't spot it until after sending the message. Since David Berry already pointed out that the problem with CDELTn being zero is transforming from pixel to world coordinate, I thought everyone already saw that I should have multiplied by CDELTn rather than dividing by it. But I thought your question was related to WCS because it was about CDELTn, which is a WCS keyword. Phil |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
[fitsbits] CDELTn | Francois Ochsenbein | FITS | 0 | September 19th 07 09:30 AM |
[fitsbits] CDELTn | Mark Calabretta | FITS | 0 | September 19th 07 01:00 AM |
[fitsbits] CDELTn | Steve Allen | FITS | 0 | September 18th 07 06:14 PM |
[fitsbits] CDELTn | David Berry | FITS | 1 | September 18th 07 04:43 PM |
[fitsbits] CDELTn | Francois Ochsenbein | FITS | 0 | September 18th 07 12:17 PM |