The 0.9-Meter Telescope



From Feb 1 2003 the 0.9-m is one of the CTIO small telescopes being operated by the SMARTS CONSORTIUM. In semesters 2006A and 200r6 NOAO users are entitled to approximately one quarter of the time on the small telescopes.

The 0.9-meter telescope is a Cassegrain telescope mounted on an off-axis asymmetrical mounting and used on the east side of the pier. This telescope is at present only used for direct imaging with a dedicated 2048x2046 CCD detector. A CCD-based autoguider, which looks at a small off-axis field, is permanently installed.


 
Detector Parameters (f/13.5)
Pixels 2048 x 2046
Pixel Scale 0.396"/pixel
Field Size 13.5'


The observer can control and change several CCD parameters. These are: the CCD readout format, the binning, and the gain. To reduce the readout time, the CCD can be read out through multiple amplifiers (DUAL or QUAD mode). A single region-of-interest (ROI) can be read out, positioned at an arbitrary place on the CCD. Observers should think carefully whether they need all the field (if not, read a ROI) or the resolution (if not, bin 2x2). Even though the CCDs with QUAD readout have short read times by big-CCD standards, substantial gains in efficiency are possible by reducing the format.




*** Table of gain values ***
Index Read Noise 1/Gain Read Noise Quad Read
  LL LR UL UR LL LR UL UR LL LR UL UR Time
  (ADU) (e-/ADU) (e-) (sec)
1: 1.8 1.7 1.5 1.5 3.0 3.0 3.1 3.0 5.4 5.1 4.7 4.6 25
2: 2.3 2.6 2.3 2.3 1.5 1.5 1.6 1.5 3.5 3.8 3.7 3.3 32
3: 3.3 3.4 3.0 3.0 1.0 1.0 1.0 1.0 3.4 3.4 3.2 3.0 39
4: 5.1 5.0 4.4 4.5 0.6 0.6 0.6 0.7 3.1 3.0 2.7 2.7 53


Saturation of upper amps occurs at 65535 ADU at all gains. Saturation of lower amps begins at 60000 ADU for gain 1 and 65535 ADU for gain 2-4. Read times are quoted for full frame quad readout with no binning. Linearity: gain increases by <0.3% over signal range. Patterns have been minimized for full frame readout at gain 1 and will be more apparent though still a minor source of noise at higher gains or when regions of interest and/or binning are used.

A raw quad-amplifier picture looks a little unusual. Each quadrant has a slightly different electrical offset (ie overscan level), and the four overscan segments are in the center of the picture. The real time display automatically removes the overscan and applies an offset to each quadrant to normalize the DC level. In addition, the real time display colors any saturated (ie 65535) pixel in red.

QUAD CCD pictures have to be trimmed and overscan-subtracted in a separate pre-processing IRAF task called quadproc (in the quad package). Note that even after the pictures are trimmed and overscan subtracted the sky levels in each quadrant will not be identical (unless the sky level is zero). This is because each of the four CCD amplifiers has a slightly different gain. After the trimming and overscan correction with quadproc, the images can be processed (bias subtraction, flatfielding) in the standard manner.

The standard filter size for the 0.9m CFCCD is 3x3 inch. The filters are mounted in two filter wheels which each can hold up to 8 filters. Usually, 2 positions in each wheel are taken up by a color balance filter (for dome flats) and the "clear" position. We also have a single wheel which can hold up to five 4x4 inch filters. This filter wheel can be installed in place of one of the 8-position wheels.

See the Filter list for more information on the available filters.

Please refer to the CFCCD User Manual.

Important Tip:

The data acquisition computer (ctioa4) occasionally has trouble handling simultaneous processes, resulting in ARCON crashing. It is recommended that you do not do anything (display another image, reduce data, write tapes, etc.) that uses ctioa4's CPU or accesses ctioa4's disks from another computer while an image is reading out. It is perfectly safe to do these things while the image is EXPOSING. If you want to work on your data, it is best to imcopy the raw images over to ctio36 (but not during image readout!) and do all the work there.

A night assistant is not regularly provided on the 0.9-meter telescope. A TCS software manual is available in the control room.

Domeflat exposure times (gain option #2) to reach a level of about 30,000 counts.
 
 

Filter Exp Time (sec)
B 90 sec
V 50 sec
R 60 sec
I 65 sec


A sample "master" Bias frame (quad readout) after overscan correction and trimming with quadproc (click on the image for an expanded view). The frame was created by averaging 20 individual frames (rejection option "minmax").


A sample "master" Flatfield frame (quad readout, V filter) after overscan correction and trimming with quadproc (click on the image for an expanded view). The frame was created by median combining 10 individual frames (rejection option "avsigclip").



For projects that require accurate timing information, here is a quick overview on how the time ends up in the image headers:

There are two different header entries relating to time, UT and UTSHUT. UT is read from the TCS. While there is a GPS receiver that displays accurate Universal Time in each telescope's console room, this information is currently not communicated to the TCS automatically, but instead has to be manually entered at the beginning of each night. The absolute accuracy is therefore at best ~1 sec. In addition, the TCS clock often drifts by a considerable amount during the night and is therefore not reliable for both absolute and relative timing.

UTSHUT is taken from the ARCON controller's clock, which is synchronized against the SUN data acquisition computer's clock every time ARCON is started up. After that it runs on the controller's internal clock. The UTSHUT timestamp is the time when the shutter opens and should provide the most accurate timing information currently available. The SUN computer's clock is synchronized with the time reported by the US Naval Observatory and other internet sites (the details of this process are given here). Occasionally, e.g. after a power outage or network problems, this synchronization process fails and it is important to check the accuracy of the SUN clock at various times during a run by visually comparing it to the GPS time display.

To check for the correct UT time on the web, try this link to the USNO Master Clock .







  • CROSSTALK

    When using multiple-amplifier readout, signal from one amplifier can "leak" into the signal of another. Crosstalk is an additive effect and is proportional to the signal strength in a given amplifier. In the sample frame shown below, this effect shows up as negative (white) images of bright stars in the lower left and upper right quadrants. The white images in the upper right quadrant correspond to the (black) star images of the lower left quadrant (flip the upper right quadrant left-right and up-down). We are currently working on software to correct for this effect.
    A reduced 2048x2048 frame (quad readout) showing examples of "crosstalk" (click on the image for an expanded view).


    Please direct any questions regarding the CTIO 0.9-m telescope to Dr. John Subasavage (jsubasavage@ctio.noao.edu).

    This page was last modified on 27 Jan 2011