2010 April 29

Several days after installing NIRI on the telescope the beam splitter wheel began to have problems getting into position. The beam-splitters are flat mirrors which send light to the science detector and allow the surrounding field to go to the OIWFS. Each mirror is sized to match the field of view of the associated camera. The f/32 beam splitter mirror is the smallest, only sending a 22 arcsecond field through the instrument. The f/14 mirror sends a 51 arcsecond field, and the f/6 mirror is the largest, sending a 2 arcminute field. Since the OIWFS is currently not being used it is possible to use the largest beam splitter (f/6) to direct light to all three cameras. The obvious disadvantage to using an oversized beam splitter is that extra light is directed into the f/14 and f/32 cameras, which will result in higher background and possibly internal reflections. We have compared GCAL flats taken with the normal beam splitter and the oversized f/6 beam splitter and the inrease in background is minimal.

F/14 Camera
Using the f/14 camera with the f/6 beam splitter we see a median background increase of ~2% in H, 4% in K, and ~5% in Y and J. However, the increase in background is not uniform over the field as shown by this ratio image and this cut through the center of the ratio image.

F/32 Camera
Using the f/32 camera with the f/6 beam splitter we see a median background increase of ~3% in Y, H, and K, and ~5% in J. The structure of the background increase is shown by this ratio image and this cut through the top of the ratio image.

Vignetting
The f/14 camera has always shown vignetting when used in combination with the Altair field lens. Using the f/6 beam splitter changes the vignetting slightly, thus care should be used when comparing or combining previous data using the f/14 beam splitter with newer data using the f/6 beam splitter.

Reflections
While internal reflections may be expected using an oversized beam, we have taken several observations of bright stars and seen no significant additional reflections.