May 2007: First light observations of MAX38 at the Higashi-Hiroshima Astronomical Observatory

The mid-infrared camera MAX38 successfully saw the first N-band (10 micron) light on the Kanata telescope at the Higashi-Hiroshima Astronomical Observatory. On-sky performances were evaluated based on the astronomical data. A newly developed cold chopping system installed in MAX38 worked very well for the astronomical

# Members : Miyata, Sako, Nakamura, and Doi
# Date: June 2-7, 2007

fig1: MAX38 attached on the Cassegrain focus of the Kanata 1.5m telescope

1. On-sky performances of MAX38

We carried out PV observations to evaluate the on-sky performance of MAX38.

• Optical performance
  • Point source image with the FWHM of 1.5arcsec at 8.9 micron was successfully obtained. This corresponds to the diffraction limited performance of the 1.5m telescope.
  • No distortion and no vignetting were confirmed.
  • No degradation of the image quality with tilting the instrument were confirmed.
  
  
• Sensitivity
  • The sensitivity of 1sig1sec at 8.9 micron was estimated to be 4Jy. This is almost consistent with the designed value.
  • Inhomogeneity of the sensitivity within the field of view was sufficiently small.
  
  
• Vacuum and Cooling
  • High vacuum （10^-6torr以下）・and very low temperature （7K以下） was stably kept during observations.
  • Vibration of a GM cooler was not affected on the image quality.
  
  
• Mechanics and Telescope I/F
  • A rotation mechanism of the filter wheel in the cold optics had some minor problems.
  • Attachment of MAX38 to the Kanata telescope works well.
  
  
• Cold Chopping System
  • Observations with using the cold chopping system were successfully carried out for the first time.
  • Closed loop control for stabilizing stellar images during the chopping cycles worked well.
  • Time variations of the background were accurately subtracted with the cold chopper even in the worse weather condition in Japan.
  • Much larger throw of the chopping is needed for observing extended objects.
  • The control loop sometimes became unstable at some tilting angle of the instrument.

2. First light images

This figure is a typical MAX38 data obtained in this observing run. Both high speed chopping (∼ 5Hz) with the cold chopper and slow (every a few 10sec) nodding of the telescope were conducted. Therefore a reduced data shows four images, two are positive and two are negative.

fig2: (Left) A mid infrared image of a nova Sco 2007a at 8.9 micron obtained by MAX38. Chop-and-nodding technique was conducted. (Right) An enlarged view of a pair of the chopping. There is an interval of 4.3 arcsec between two images.

The maximum throw of the current chopper is 5 arcsec. This is a strong limit for observing extended object larger than 5 arcsec. Improvements of the chopping throw are essentially needed. In this observing run extended objects were observed without chopping (nod- only). We note that it caused serious degradation of the sensitivity.

fig3: A sample of the extended object obtained by nod-only observations of MAX38. This is an image of a planetary nebula NGC7027 at 8.9 micron. Warm dust heated by the central star was clearly seen.