Testing ALPY - the effect of telescope focus

[Robin Leadbeater]

My Alpy is now assembled in its final configuration with the IR cut filter removed and the guiding (with 23um slit) and calibration modules installed.
I have made some tests, first measuring a star from the Miles database to be used as a reference and then measuring three other Miles stars at similar elevation, correcting these for instrument response and atmospheric extinction using the differential method. These were then compared with the spectra of the same stars in the Miles database.
The results were broadly as expected but I thought I would share them as it shows the effect of changing the focus between the reference and target measurements. The setup is a 280mm SCT (C11) with a 0.63 reducer and the change is most likely caused by a change in the wavelength dependent sampling through the slit due to the achromatic focal reducer. This is similar to the effect seen by Christian here.
http://www.astrosurf.com/buil/dispersion/atmo.htm
After a new reference star measurement was made at the new focus setting, the agreement between the measured spectrum and the spectrum from the Miles database was once more good.

The results are in the attached pdf which also includes for interest a comparison between the Miles spectra for each star and the generic Pickles spectra for the same spectral type. There are some differences, confirming that it is better to use actual rather than generic spectra as references where possible.

Cheers
Robin