Hello there, I am still a new learner to Zemax.

I have an optical system of 7 lenses focusing collected light from a telescope onto an image plane. My goal is to image 1 arcsecond portion of the sky and add a 1 arcsecond slit. Given the F#, diameter, and EFFL of the telescope, I calculated a slit width of 0.0097 mm. 

I am running into a problem inserting a slit of this width inside the system. The field lines that pass through it have to be so small,  (0.0001, 0) (-0.0001, 0) (0,0). Is this normal to block out that much light? 

Here is a screenshot of my series of lenses. Aperture slit location is sized up and highlighted in orange. 

Here is a zoom in of the slit, only allowing (0.0001, 0) (-0.0001, 0) (0,0) field lines through.