Star Analyser Focus

[Andrew Smith]

I have been having some difficulty in being sure I had good focus with my SA. Robin had given good advice but I still found it quite hard. To this end I did a set of exposures to try to characterise my setup. I initially focused on the zero order using @Focus2 and then did a series of exposures from +100 to -100 steps of my focuser which are about 4.45 microns each. I then measured the depth of the Hbeta line of the A0V star I was using in RSpec and also recorded the maximum it used in the calculation. I then plotted the ratio of the depth of the line to the maximum against focus position. I did two extra sets of data on zero order focus (Ch1 & Ch2) to ensure there were no significant drifts over time. I also did the measurement in a random focuser position order & then took a set of three exposures at each setting.

I attach the results. The scatter shows why I have some difficulty focusing which in part I put down to the seeing. The SAs resolution is seeing limited.

I plotted various trend lines and solved for the maximum in one case and the intersect in the other and got 11 and 9 as the optimal position. This is a relief as I have calculated this size of offset using both OSLO to model setup and a rough estimate based on the distance to the CCD from the SA and the known curvature and plate scale.

I can recommend this approach if you have a motorised and repeatable focuser. In future I will use @Focus the offset by 10 steps. In case you are worrying about the sign of the move my focuser moves in when I push the OUT button - I could change it but...

Regards Andrew