[MLParkinson (Murray)]

Here is an image of the Andromeda galaxy recorded at Wiruna, Central Tablelands, NSW. This spectacular object is always low on the horizon when observed from the latitude of the Central Tablelands (33 South).

In order to pull-off an LRGB image of M31, one must record the sub-frames as the object rises and sets through a large cross section of atmosphere which is highly refractive at low elevation angles. I recorded the red frames first, then the green, blue and then finally the luminosity frames when the object was close to the meridian passage – a standard strategy.

The second image shows a close-up of the top-right hand corner of the image which was located closest to the horizon. I use Nebulosity to align and stack the sub-frames. The transformations used in this program are all linear – shift, rotate and equal scaling in orthogonal directions. It is impossible to bring all the stars into alignment across the entire image without using non-linear transformations which compensate for the effects of strong atmospheric refraction.

What software do you use to align sub-frames and will it cope with this demanding situation? Does it employ non-linear transformations to align the sub-frames?

My hunch is that if the sub-frames can be scaled independently in orthogonal directions then I will succeed in aligning the sub-frames through several re-alignment cycles.

Murray