Self-correcting objects

Image tubes are not always tremendously stable; it may not be reasonable to assume that the details of the distortion do not change with time. This means that if there is only one multi-hole calibration image, probably taken at the start of the night, a distortion analysis based on this single early image may become progressively less correct as images from later and later in the night are processed. And, of course, there is always the possibility that no such image was taken at all.

In these cases, it is always possible to produce a distortion analysis from a spectrum of a single point object. An image of such an object can be used to calibrate itself, from a distortion point of view. The disadvantage of this over the use of a multi-hole exposure is that the correction based on a single object will be exactly correct only at the position of that object in the slit-the sky nearby will not be quite so well corrected.

So the choice of using objects to calibrate themselves, against the use of a multi-hole calibration image, is a trade-off of one source of error against another. The choice has to be yours.