erik replied to the topic 'FITS Keywords' in the forum. 4 years ago

han.k wrote:
The FITS standard 4.0 specifies this:
[±C]CCYY-MM-DD[Thh:mm:ss[.s...]]

So four digits, accuracy 0.1ms.


To me, the 4.0 standard doesn't read as if it was restricted to 3 or 4 decimal places. I'm aware of the fact that 6 decimal places might be far from reality, but for instance would a INumberProperty be sufficient to state the number of decimal places that are written to the FITS header?

Concerning the actual definition of the timestamp, whether it is the starting point or somewhere in the middle of the exposure, I guess you have to know what the program wrote when making the FITS. Am I right when assuming, that INDI generally states that to be the beginning?

And how accurate should you be on mid exposure of a long exposure?

Paul


Well, of course, generally the longer the exposure the less relevant is the accuracy of the timestamp, but I want to image satellites and measure their orbit. Thus using exposure times of approx. 0.1 seconds. Assuming an average speed of 7 km/sec, I'd like to have the uncertainty as low as possible.

As Han pointed out 0.1 ms would be already quite good, but is yet not implemented in INDI indiccd missing one decimal place. If this collides with other programs, would it be possible to use MJD-BEG, MJD-AVG or MJD-END? I'd be happy to do the implementation and make the PR.

Same goes for the altitude, might be not too interesting when imaging stars and planets, but for me, knowing I can later read all the information I need from the FITS header makes it somewhat simpler to lay out the over all schedule of measurering satellite orbits.

Cheers
Erik

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