The Bigger Picture: DECam's (Unoffical) Blog: September 2012

Sunday, September 30, 2012

First DECam Guiding

In a previous blog post (The DECam guider), I wrote about the first stages of testing the new DECam guide system. Progress was made quickly at first, but then slowed down. Eventually, the guide system expert, Santiago from Spain, hopped on an airplane, and 50 hours later (nightmare travel), he arrived on Tololo. But Santiago didn't sleep when he arrived on Tololo--he went right to work with the rest of the commissioning team. Here are two of the more obscure issues that were encountered, and luckily, eventually fixed.

PROBLEM: In "self" mode (when the guider automatically is supposed to select the guide star), no stars were identified.

SOLUTION: It was discovered that there was a desynchronization between the guider readout and the CCD shutter. A guider exposure of 300 ms was taken, but the CCD shutter would open only after this time was done. With the CCD integrating during the guider readout, it is no surprise that no guide stars showed up! The guide exposure time was increased to 1000 ms, and now, finally guide stars were seen.

PROBLEM: Although guide stars are selected, the guider was still not working.

SOLUTION: The TCSINTERFACE command relays information to the telescope control system (TCS). This value needs to be a float value, with no special characters. Unfortunately, a RA=%5.2f in the code was adding a space! This was a quick fix -- but only once identified.

This image shows the interface of the DECam guider--here one region of interest of one (of the four) guide CCDs is shown. The "best" guide star is marked in green, and second guide star is also in the region of interest, to further help with accurate guiding.

The guider is not working 100% yet, but it is exciting that the first guided images with DECam have been achieved.

Thursday, September 27, 2012

Linear DECam CCDs

Dear DECamers,

Today Jiangang H. (Fermilab) and I have been running CCD linearity and full-well tests for the DECam ccds with beautiful results (graphics and statistics courtesy of work by Jiangang Hao - Fermilab).

Linearity and full-well analysis for the left amplifier of DECam CCD N4 , showing inverse gain and full-well depth.

Linearity and full well analysis for the right amplifier of DECam CCD N4, showing inverse gain and full-well depth.

Analysis for the remaining CCDs is on-going,

a la belle etoile,

David James/Jiangang Hao

CTIO/Fermilab

DECam Flat Fields

Dear DECamers,

After a very busy, very eventful 5-nights on the mountain this week, with Andrea's expert help for our non-NOAO visitors, we've made a lot of progress this week; not least among our successes, are the psuedo-final settings for the flat-field projector (shown below) settings.

DECam will be calibrated by the DECal system. Designed and constructed by our friends at Texas A&M University (Darren Depoy, Jennifer Marshall, Jean-Philippe Rheault and formerly Jason Wise), and with me acting as CTIO's local "expert", the system consists of three main components:

An LED projector cone
A photodiode (and thermocouple)
A fibre projection unit

One of DECal calibration units, which total four in number, prior to its installation

of the top ring of the Blanco 4m telescope. The large black cone houses the seven LED

lamps that shine onto the flat-field screen inside the dome. The medium-sized black-coloured

box houses a photodiode and thermocouple assembly, while the small black unit (on the right) is

the fibre projection unit.

Each of the four LED projection units consists of seven LEDs, which for are designed for flat-field and internal calibration purposes, are powered contemporaneously. The emitting light level of each LED can be adjusted independently, allowing us to set the flux level of flat-field data in each filter per unit exposure time. The constraining parameters for our "ideal" exposures are for a mean flux of about 15k ADUs above BIAS across the array in a minimum exposure time of 10-seconds.

Through empirical experimentation, afternoon dome flat-flats are now routinely obtained using the following parameters:

LED# Peak LED
Wavelength Power

LED1: 6500 Angstroms 5%
LED2: 3650 Angstroms 100%
LED3: 7800 Angstroms 5%
LED4: 9700 Angstroms 8%
LED5: 10300 Angstroms 75%
LED6: 9050 Angstroms 8%
LED7: 4700 Angstroms 15%

Filter: Exp ADUs

Time above BIAS

u' 15-secs ~ 13k
g' 30-secs ~ 14k
r' 10-secs ~ 17k
i' 22-secs ~ 17k
z' 10-secs ~ 19k
Y 10-secs ~ 17k

For those of you with access to the archives, a good set of flat-field calibration data are to be found in files #136529 -- 136584.

a la belle etoile,

David James,

djj@ctio.noao.edu

Dark Energy Camera Calibration Team [DECal],

CTIO Staff Astronomer.

Guider working on-going tonight

Wednesday, September 26, 2012

BCAM testing

BCAMs are devices for measuring small angular displacements. We use them to monitor lateral alignment and tip/tilt of DECam with respect to the primary mirror. Then we can really be sure that DECam is perfectly aligned so that the best possible image quality can be achieved. Each BCAM (there are four on the Blanco) has one CCD and two red laser diodes. By imaging pairs of BCAMs, displacements can be inferred.

We performed a test to see if the BCAMs were working. We recorded the BCAM values as a function of hour angle and constant declination. If the BCAM values at each positive and negative hour angle are the same, this gives us some confidence that the BCAM values are correct. Here is a photo of part of the team working on recording the BCAM values. From front to back is Klaus, Marcelle, David and Claudio.

Now this plot (courtesy optical engineer Roberto Tighe) shows the results.

The x-axis shows the hour angle (in degrees) and the y-axis shows the various BCAM values for the x-, y- and z- angles (which can then be turned into alignment corrections). At a first glance, the behavior looks normal and gives us confidence that the BCAMs are working. A more detailed analysis is underway.

We did notice that every once in a while, some anomalous BCAM values were produced. These unexplainable and unphysical values were discarded from the above figure. Tonight we will try to isolate instances when the BCAM values are anomalous and try to diagnose this problem.

Tuesday, September 25, 2012

Tololo Traffic

CTIO staff member Arturo Gomez took this beautiful photo illustrating how busy it is on Tololo at night at the moment! Note that this photo was taken when there were clouds overhead. We usually don't shine so many lights around on a dark clear night.

Monday, September 24, 2012

Klaus Honscheid checks out DECam from the dome catwalk.

From our CTIO Director: Chris Smith

As the CTIO Director, I am not always available to be at ground zero, observing at the Blanco 4m on Cerro Tololo. This weekend, however, is different. I packed my bags and headed up to the telescope to help with the DECam commissioning. It is not only exciting to learn to drive this impressive optical imager (I'll be joining the instrument support team as we move into operations), it is also exciting for me to see the progress made since we had first light 1.5 weeks ago. The first light images were surprisingly good, but since then we've achieved sub-arcsecond images quite often and we are testing many of systems that will be part of the nightly operations, such as calibration sequences, focus scans, the guider system, astrometric information, "donut" analysis for focus, and others. The commissioning team (a combination of CTIO, Fermilab, and other DES colleagues) is working well together here in Chile with support from sites in US, Spain, England, Germany, and Switzerland (hopefully I didn't forget anyone!) to tune the many supporting sub-systems into a smoothly running instrument+telescope system. We've still got quite a bit of work in front of us, but the progress is excellent.

Friday, September 21, 2012

The DECam guider

The main task of the guide software is to analyze the images taken with four Guide CCDs, compute the offsets of the telescope pointing with respect to the required position on the sky and relay this information to the telescope control system (TCS) to ensure precision tracking. The past few nights the commissioning team has begun testing the guide software. Out of the box, the guiding software correctly identified stars in the guide CCDs and was able to place the best ones (stars that are relatively isolated, not too bright, not too faint, far from the edge of the CCD) in a region of interest. This image shows the four guide CCD region of interests with the guide stars selected.

The guider GUIs do not work yet, and this is being investigated. We also still need to continue testing the guider in the various modes it is designed to operate in. But slowly, slowly, it is all coming together.

The guider software has been developed by our colleagues in Spain with the aim of creating a stable
guider with the most useful features together with a user-friendly interface. It has about 3200 lines
of code written in Pytho and Tkinter (Tcl/Tk).

Thursday, September 20, 2012

DECam in the News

First light images from the DECam imager are causing a lot of excitement for us at CTIO! But it is not just CTIO that is excited. Many news outlets are reporting on this achievement as well. See here for a list of articles to read:

****NEW (since Sep 27)*****