Direct Imaging Camera at OAGH

Overview

The direct imaging camera or Cámara Directa is mounted at the f/12 Cassegrain focus of the telescope, and is equipped with a filter-wheel capable of holding 11 filters at a time. At the image scale of 8.185''/mm, this amounts to approximately 0.20''/pix for the TK1024 CCD as the detector. Considering that the seeing is never better than 1'' at the telescope it is advisable to use the 2x2 or 3x3 or 4x4 binning option available in the PMIS software. This will not only reduce the image size in kbytes (without decreasing the field of view), but also will increase the S/N ratio (readout noise will be less) for the same exposure time.

CCD

A typical quantun efficiency curve for the TK1024 chip is shown below This CCD is also used with the Boller & Chivens Spectrograph.

Data acquisition and storage

Filters available

Observing Strategies

Calibration Frames

• Bias frames: You may wish to take 10-15 of these at some convenient time when the dome is dark; they can be combined into a master bias frame by using zerocombine. In reality, none of the two chips have much bias structure (the hot columns cannot be effectively removed via biases), and so this is fairly pro forma.

• Dark frames: You may wish to take three frames of the same length of time as your longest exposure to evaluate how much dark current is contributing if you are attempting to do long-slit spectrophotometry without local sky subtraction. In practice, the dark current on all the two chips is fairly minimal, and hence dark frames are not taken. If you are taking, be sure to make these exposures with the dome dark.

• Flat-field calibration: Flat-field calibration is needed to remove pixel-to-pixel sensitivity variations on the chip.

  Any line-free source will do to remove the first of these: either exposures of the large white spots on the side of the dome, or exposures with the twilight sky. For narrow bands, dome flats take too much time to get decent frames.

• Transformation: Calibration to flux can be readily achieved by observing one or more photometric standards during the night. The average extinction curve for Cananea is available. But considering its proximity to Kitt Peak, such a curve for Kitt Peak can give equally good result.

Guiding Cameras

Off-set Autoguider

Standard stars

M67 Dipper Asterism

Calibration and Efficiency

V - v₀ = a_v + b_v(B-V)
U - u₀ = a_u + b_u(U-B)
B - b₀ = a_b + b_b(B-V)
R - r₀  = a_r + b_r(V-R)
I  - i₀  = a_i + b_i(V-I)

where UBVRI are standard magnitudes of Chevalier, C & Ilovaisky, S.A. 1991, (A& AS, 90, 225) (U from ...). v₀, u₀, b₀, r₀  and i₀ are extinction corrected instrumental magnitudes defined as

v₀ = -2.5log10(counts/sec) - k_vX_v,

where k_v and X_v are extinction coefficient and airmass for the V-band image.

a and b are the "zeropoint" and "color coefficient" respectively. Zeropoint is defined as the magnitude that gives 1 count/sec (or equivalently 1.85 e^-/sec) in the system for a star of zero color (A0). The derived values of the coefficients are given in the following table: