No problem, Steve, this is the objective (or one of them) of the forum to help new observers (and to improve the results)
A (or B) type stars are used as reference to compute the response (atmospheric+instrumental) for two reasons :
- the continuum of the objects we observe are not kwown, complex or variables
- A/late B stars show only a few number of lines (at least, less than hotter or coller stars)
The reference star must be, approximatively at the same altitude than the target (say the difference must be under 5° ; it depends of the altitude)
You have to take a reference for each target ( exception : the altitude is high, the exposure time is low, all the targets are in the same zone)
You have to take reference star each night because atmospheric conditions may change
I always take the same reference for one target.
During collective survey, we use the same refernce star
To choose a refrence star, here's a spreadsheet
http://www.astronomie-amateur.fr/Docume ... inder.xlsm
Try to choose a reference whose interstellar extinction is low
Also, here's an abacus showing the acceptable difference of altitude between target and reference
http://www.astronomie-amateur.fr/Docume ... ef_013.pdf
Example :
Your acquire a spectrum of the target at a mean altitude of 50°(vertical scale)
You accpet on error of 3% (choose the curve), you get : the difference between target and refrence must be under 3 ° (under mean atmospheric conditions)
The see than under an altitude of 30°, the goal is challenging !
All the best,
François