How to measure slit efficiency ?

[Christian Buil]

Une méthode simple pour mesurer la transmisson réelle de la fente d'un spectrographe (LISA, LHIRES, ...). La technique consiste à
comparer du signal d'une même étoile avec et sans fente. Le résultat va dépendre du seeing, de la qualité du suivi, de la focalisation de l'étoile... :

Here a simple method for evaluate real slit transmission for the conditions of observation (seeing, tracking, focusing, ...) - comparison of star signal with and without slit :



Full version:

http://www.astrosurf.com/buil/forum/pos ... ciency.png

Une transmission de fente comprise entre 0,3 et 0,7 peu être considérée comme typique et normale.

Slit transmission between 0.3 and 0.7 is typical.

Christuan Buil

[Olivier Thizy]

the method is very simple:

1/ take a spectrum of a star with your spectrograph with the slit
2/ remove the slit, make sure no light foes through the new opening and record another spectrum

In ISIS, use the statistics and measure the "counts" within the same window/area for both images. Divide the count numbers and you get the slit efficiency. This inclused 92% of glass transmission you can remove by dividing your result with that figure.


For exemple:
image with slit: count C1 = 110500
Image without slit: count C2 = 182800

Slit transmission is C1/C2 = 0.60 (60%)

92% is due to glass and 0.60/0.92=65% is due to the geometry of the slit.


Slit transmission between 0.3 and 0.7 is typical.