ISIS - effect of tilt on wavelength calibration

[Robin Leadbeater]

In IRIS the tilt algorithm uses a projection onto the X axis rather than a rotation algorithm. This gives fewer artifacts but changes the dispersion by cos(tilt) Is the same algorithm used in ISIS?

Thanks
Robin

[Christian Buil]

Yes Robin, ISIS use X-projection for correct the tilt, like IRIS. But note an important point : The calibration emission lines are also rotated in a similar way, so "cos" calibration deviation is not a problem (of course, it is necessary to have the same geometry for science images and calibration images - the basic for good calibration).

Christian

[Robin Leadbeater]

Thanks Christian,

It is important for Star Analyser users to understand that if they use ISIS, they should not use a dispersion equation calculated for a different angle. They will not expect this if they normally use the rotation function in VSpec or RSpec where the dispersion does not change with angle.

Cheers
Robin