lamp calibration errors

Robin Leadbeater

Ok the reflector position looks ok. ( I was concerned that the reflector might not be rotating to the correct position. It can be adjusted by operating  the switches in a particular sequence) Not sure what any offset in the lamp might do (The asymmetry in the lamp internally is normal) and I cant see mine at the moment as it is mounted on the scope but is probably best not to change anything until you have checked the calibration with  lamp spectra taken at the same time as the star spectrum.

I have been having a look at some of my past calibrations and I am also seeing  wavelength errors between lamp and star (I  use just the lamp without the star Balmer lines) In my case H alpha is spot on but there is an increasing error towards the blue  to around 2-3A by 3900A where the lamp lines end, so errors of this order might be typical. What are other other ALPY users seeing?

We might be seeing a fundamental limitation in the calibration accuracy using the internal lamp.  At this level (~1/4 the slit width) effects such as the position of the star on the slit and subtle differences in line shape due to optical aberrations come into play.