The ALMA Optical Pointing Systems

Transcription

1 The ALMA Optical Pointing Systems Design Considerations, Prototype Description, and Production Model Suggestions Jeff Mangum (NRAO) DRAFT June 20, 2005

2 Contents 1 Introduction 1 2 Design Considerations Optical Telescope Sensitivity Centroid Determination Probability Resolution and Field-of-View Mounting CCD Frame Grabber Control PC Star Catalogs Data Acquisition and Analysis System Summary Design Requirements 7 4 Prototype Design Characteristics OPT Monitor and Control OPT Image Viewing OPT Design Summary Current Limitations 11 A Peak Finding Algorithm 16 1

3 Abstract This document describes the optical pointing telescope systems for the ALMA prototype antennas, including detailed descriptions of the telescope specification, design, and control. These optical pointing systems are used to assess the stability and accuracy of the ALMA antenna tracking system and the form and stability of the ALMA telescope pointing behavior. They served as fundamental tools for the diagnostic tests conducted on the ALMA prototype antennas, and those tests to be conducted on the ALMA production antennas. We also list suggested production model design changes.

4 1 Introduction Optical pointing systems have become standard equipment on millimeter and submillimeter telescopes. There are benefits and limitations associated with the reliance on optical pointing systems on radio telescopes: 1. Benefits: (a) There are many more bright stars which can be used as optical pointing sources than radio pointing sources. (b) With a CCD camera, optical data acquisition can be done on time scales as short as a few tenths of a second, as opposed to the tens of seconds time scales necessary for radio pointing measurements. (c) Positions of optical stars can be derived to sub-arcsecond accuracy, which is a much higher precision than that obtainable through single antenna radio measurements. 2. Limitations: (a) Differences between the position and behavior of the optical and radio signal paths can make the derivation of the radio pointing coefficients from optical pointing measurements difficult. (b) Optical pointing is not possible during overcast weather conditions. (c) Optical pointing cannot be used in the daytime, unless the system has been designed with near-infrared sensitivity. Optical pointing systems find application in such areas as: Pointing and tracking diagnosis Pointing coefficient derivation and monitoring Auto-guiding 2 Design Considerations An optical pointing system for the applications described above can be constructed from off-the-shelf commercial components. The main components of the optical pointing system are: Optical telescope Commercial CCD camera (with near-infrared sensitivity if daytime observations are desired) PC-based or self-contained frame grabber 1

5 Control PC Data acquisition and analysis system In the following, we discuss the design considerations for each of these components. 2.1 Optical Telescope Sensitivity The optical telescope system should be sensitive enough to detect large numbers of astrometric stars (which represent a small subset of the existing stars at any given magnitude) with exposures of no more than a few seconds. The magnitude sensitivity of an optical telescope and camera system can be derived from the magnitude relation: [ (Da ) ] 2 m a m b = log 10 (1) where m a and m b are the limiting magnitudes and D a and D b are the diameters of two optical telescope systems. The optical pointing system that was used on the 12 Meter Telescope had a (8.255 cm) refracting telescope and Cohu 4810 uncooled CCD camera, which resulted in an overall limiting magnitude of about 7. For example, assuming that m b = 9.5 in Equation 1, if we use the same CCD camera we need an optical telescope with a diameter 3.1 times as big as the one in 12 Meter Telescope optical pointing system, or D b = ( cm). Equivalently, magnitude sensitivity can be gained by using a more sensitive CCD camera. The sensitivity requirements for an optical pointing telescope differ from those for an optical guiding system. If one can construct a system which is sensitive to a limiting visual magnitude of about 10, then in any given field it is likely that there will be an astrometric star which can be used for pointing or tracking. For pointing alone, a limiting visual magnitude of 7 is sufficient to provide several hundred astrometric stars which can be used to derive pointing characteristics Centroid Determination Probability The reliability of our determination of the centroid of our point source measurements is one of the limitations of this optical pointing system. In order to characterize this centroiding sensitivity, we have run a series of simulations which derive the centroid of a point source. The point source is assumed to be located in the center of the image and to have a FWHM of 1 pixel. Synthetic images with random noise and varying peak point source intensities were constructed and fit with gaussian models. For each peak signal-to-noise ratio a series of 10,000 fits were performed. If a gaussian fit derived the known centroid position in both axes to within at least n pixel, the fit was deemed a success. Since we generally 10 have a pretty good guess as to where the pointing source in an OPT image should be, the gaussian fit routine in these simulations was given this position as an estimate to the fit. Two values for n were assumed in the simulations: 1 and 3. Figure 1 shows the results from these fits. From these simulations we find that: D b 2

6 Figure 1: Gaussian fit simulation results. A signal-to-noise ratio of 12 is required to derive the centroid of a gaussian point source with 50% confidence for centroid determination to at least 0.1 pixel. A signal-to-noise ratio of 7 is required to derive the centroid of a gaussian point source with 50% confidence for centroid determination to at least 0.3 pixel Resolution and Field-of-View The detector will be a CCD placed in the focal plane. Atmospheric seeing will limit the actual resolution to about 1 5. Since the ability to find the centroid of a star is proportional to the actual resolution divided by the signal-to-noise of the measurement, the pointing precision is much less than the actual resolution. A resolution of about 1 per CCD pixel should be sufficient. The plate scale and diffraction limit of the optical telescope system is given by: ( ) 1 P S = arcsec/µm (2) Mf 3

7 Figure 2: Optics layout with Barlow lens. ( ) λ θ diffract = 1.22 D radians ) = ( λ D arcseconds (3) where f is the focal length of the telescope in millimeters and M is the magnification factor by any additional optics in the system, such as a Barlow lens. Note that the magnification of a Barlow lens is given by the following: M = d Barlow f Barlow + 1 (4) where d Barlow is the distance from the detector (CCD) to the Barlow lens. The optics layout when a Barlow lens is included is shown in Figure 2. By combining the plate scale with the length (l CCD, in µm), width (w CCD, in µm), and number of samples in the horizontal (N l ) and vertical (N w ) dimensions, we can derive the 4

8 field-of-view (FOV) and effective plate scale (EPS) of the optical pointing system: F OV = P S l CCD w CCD arcsec arcsec (5) ( ) lccd w CCD EP S = P S arcsec arcsec (6) N l N w Note that N l and N w are the smaller of the number of CCD pixels in each dimension and the sampling produced by the frame grabber Mounting The mounting of the optical pointing system should meet the following requirements: It should be rigidly attached to the structure of the telescope so that it mimics the motion and flexure of the radio telescope structure. It should have a remotely-controlled focus translation stage which does not adversely affect the optics stability. It should be accessible. Two traditional locations for optical pointing systems are the prime focus and the backup structure (where an access hole in a panel must be made). 2.2 CCD The requirements for the CCD detector are driven by the need for sub-arcsecond spatial resolution. Most commercial CCD detectors have a sufficient number of pixels and sensitivity to meet these requirements. For daytime operation, extended near-infrared sensitivity is a feature that can be found on many commercial CCD detectors. 2.3 Frame Grabber The frame grabber should conform to the type of control computer (in this case, a PCI-bus based PC). It must be able to process at least the same number of pixels as exist in the CCD and have sufficient memory and frame transfer capacity to process images which are a few seconds in duration. The dynamic range of the frame grabber should be comparable or greater than that of the CCD camera. 2.4 Control PC The requirements for the control PC are not very stringent. Any PC with a > 200 MHz processor will be sufficient. The operating system used is determined by the requirements for the frame grabber driver, data acquisition, and analysis systems. 5

9 2.5 Star Catalogs Two derivatives of the full Tycho-2 catalog are currently available within the ATF monitor and control system: TYCHO: The full Tycho-2 catalog. TYCHO NDS: A filtered version of the Tycho-2 catalog which has been filtered for double stars. The algorithm used to eliminate neighbours puts stars into a grid of cells whose size is RA is scaled by cos(dec) to account for shortening at Dec = ±90. After the stars have been placed in these cells, then each cell is inspected for neighbours. The distance to all stars in neighboring cells is measured. If the absolute distance is < 10, the star is deleted from the catalog. Also, if there is more than one star in the current cell, then all of those other stars are deleted from the catalog. 2.6 Data Acquisition and Analysis System The data acquisition and analysis system for the ALMA optical pointing system operates as follows. Items displayed in italics are options which might be implemented in the future: 1. The telescope control computer sends commands to the optical pointing computer (OPC) for setup and initialization: (a) Turn on power to camera. (b) Open shutter. (c) Open solar filter. (d) Select filter position. 2. Using the tycho graphical user interface (GUI) on the antenna control computer, a subset of available stars based on defined selection criteria are selected from a master catalog of astrometric stars. 3. For the chosen list of astrometric stars, the control computer and OPC execute the following for each target star: (a) The telescope control system does some OPT hardware initialization, which includes the measurement of a dark frame taken with the OPT shutter closed. (b) The telescope control computer sends the J2000 source coordinates to the radio telescope positioning system, and the radio telescope slews to the star. Once the star is acquired, frame integration begins. (c) The telescope control computer calculates the required integration time based on the magnitude of the target star and commands the OPC to set the integration time to this value. 6

10 (d) The frame grabber acquires images of the star at a rate of 8 10 frames per second. The PC reads the CCD array from the frame grabber buffer and integrates the signal for a user-specified integration time (0.5 second is a typical value) (or for the integration time sent down from the control computer). (e) The integrated image is acquired. (f) The dark frame is removed from the image. (g) The centroid of the target star is computed by the control computer as follows: i. Measure peak pixel location. ii. Calculate the RMS noise floor near the peak. There are two cases for this calculation: A. If the peak is within the inner half of the CCD frame, calculate the RMS over a pixel area around the peak, excluding the pixel area centered on the peak. B. If the peak is outside the inner half of the CCD frame, calculate the RMS over a the entire CCD frame, excluding the pixel area centered on the peak. The advantage of doing the RMS calculation in this way is that the calculation is not influenced by the non-uniform background produced by the CCD (during daylight observations, mainly). iii. Measure the exact centroid of the peak using a center-of-mass peak detection algorithm described in Appendix A (h) Calculate the signal-to-noise from the peak and RMS calculations done above. (i) If the signal-to-noise of the measurement meets the signal-to-noise limit set by the observer, the results of the centroid calculation are written to a file for later processing. 4. Once all of the available stars are measured, analysis of the centroiding results are done off-line. 5. If we are done using the optical pointing system, the control computer sends some shutdown commands to the OPC, such as: (a) Turn off power to camera (b) Close shutter (c) Close solar filter. 3 Summary Design Requirements In the following we list some considerations for each component in the OPT design. A summary of these considerations is given in Table 1. Primary: 7

11 The prototype OPTs had a limiting visual magnitude of about 9 for an integration time of a second or two. This indicates that a primary diameter of 4 inches is sufficient for both the primary (pointing) and secondary (tracking) needs. As NAOJ has found that the Meade 102ED lens cell used in the prototypes may be the source of the measured focus change in these devices, the lens cell glass should be made of low-expansion coefficient material. Mount: Low-expansion material is crucial. The rolled Invar used in the prototypes worked very well. CFRP would be an acceptable (and lower-weight) alternative. Camera: Other Optics: The Cohu 4920 cameras used in the prototypes worked quite well. There noise, pixel size, and dynamic range properties should be followed for the production models. One option that is more trouble than it is worth is the extended infrared sensitivity. The gain of a few hundred nanometers in spectral response was not worth the effort. IR filtering is enough for daytime measurements. If digital camera technology has advanced sufficiently, a digital camera might be an attractive option. This could potentially avoid the hassle of having to use a video camera with a frame grabber. Nonuniformity in the CCD response was a slight problem for daytime observations (confuses the peak finding algorithm). Would be useful to have a more uniform CCD response. Barlow Lens. Yes. Improves plate scale without significant loss of sensitivity. Focuser. Yes. The design added to the prototypes works quite well and is stable. Frame Grabber: If necessary. The 8-bit frame grabber used in the prototypes was a limitation. A 16-bit frame grabber would be much better. Control PC: If necessary (digital camera might provide onboard frame transfer over ethernet). Any modern laptop running linux will do. Control Software: The ATF design worked just fine, and will require only minor improvements. 8

12 Table 1: Summary ALMA Optical Telescope Requirements Telescope Additional Lenses Filters Camera Frame Grabber Control PC Control Software D 4 refractor f/9 (920mm focal length) or longer Low-expansion material mount (rolled Invar or CFRP) Low-expansion lens material X2 Barlow IR continuum (λ > 700nm) and clear focus correction > 56 db dynamic range pixels 8 µm pixel size Response uniformity > array capability > sampling (RS-170) >2MB VRAM >4ms frame transfer > 96 db (16 bit) dynamic range Any modern laptop ATF design 9

13 4 Prototype Design Characteristics 4.1 OPT Monitor and Control Monitor and control of the optical pointing telescope is done by entering commands via a python interface. In the following, it is assumed that opt is defined as the optical telescope on antenna number 1 (OPT1): Monitor Points Shutter Status: opt.shutter() Solar Filter Status: opt.solarfilter() Filter Status: opt.irfilter() Photodetector Status: opt.photodetector Telescope Tube Temperature: opt.tubetemperature CCD Temperature: opt.ccdtemperature Electronics Temperature #1: opt.electronicstemp1 Electronics Temperature #2: opt.electronicstemp2 Control Points Camera Control Enable Motor Power: opt.enablemotorpower() Disable Motor Power: opt.disablemotorpower() Shutter Control Open Shutter: opt.openshutter() Close Shutter: opt.closeshutter() Solar Filter Control Open Solar Filter: opt.opensolarfilter() Close Solar Filter: opt.closesolarfilter() Filter Control Open Filter: opt.openirfilter() Close Filter: opt.closeirfilter() Frame Grabber Commands Start Exposure: opt.startexposure( exposuretime ) Wait Exposure: opt.waitforexposure() Abort Exposure: opt.abortexposure() Write FITS Image: opt.writefits( filename ) 10

14 Query Exposure Time: opt.exposuretime Query Image X Size: opt.sizex Query Image Y Size: opt.sizey Query Exposure Time Remaining: opt.timeremaining 4.2 OPT Image Viewing Image frames can be previewed using a separate application, but to write FITS files of images, the command line interface must be used. To run the image viewing application, execute the command optview. The window that pops up has one entry box, two buttons, and an image area. The entry box is used to set the integration time in seconds. Be aware that the integration time is not calibrated, and there are significant delays in transmitting the image via CORBA over ethernet, so don t expect an image right after the time you entered has elapsed. However, the integration time is a multiple of the value entered in the box, so it is useful to adjust this number. Finally, the image pixel values are scaled to eight bits before being displayed, so the effects of changing the integration time may not be visually obvious. Images are grabbed with the grab image button. One can then follow this with a writefits command via the python interface. 4.3 OPT Design Summary 1. Figure 3 shows a picture of the OPT1 system. 2. Table 2 lists the design properties for the prototype ALMA optical pointing systems. 3. Table 3 lists the prototype OPT component properties. 4. Figure 4 shows a plot of the spectral sensitivity of the Cohu 4920 CCD camera with extended IR sensitivity ( EXview HAD CCD ). 5. Figure 5 shows a plot of a representative measurement of the OPT1 interior and exterior temperatures, along with the meteorological conditions, for a typical day. 6. Figure 6 defines the axes orientations for OPT1 and OPT2. 5 Current Limitations Focus Drift: Both OPTs experience a slow focus drift over a period of several hours. The drift is about 500 µm in magnitude, and its source is not understood, although tests by NAOJ indicate that the primary lens material may be the source of the problem. Re-focusing with the focus translation stage solves this problem. 11

15 Figure 3: The ALMA OPT1 optical pointing telescope. 12

16 Figure 4: Spectral sensitivity characteristics for ICX248AL CCD in Cohu 4920 camera Figure 5: Recent measurement of the OPT1 interior and exterior temperatures, along with the meteorological conditions, for a representative day in October

17 Table 2: Prototype ALMA Optical Telescope Specifications Telescope Additional Lenses Filters Camera Frame Grabber Control PC Control Software Meade 4 model 102ED Apochromatic Refractor f/9 (920mm focal length) NRAO-design rolled Invar tube and mount X2 Barlow IR continuum (λ > 700nm) and clear focus correction Cohu camera Extended IR sensitivity to 900nm 56 db dynamic range pixels µm pixel size mm image area Imaging Technologies PC-Vision array capability sampling (RS-170) 2MB VRAM 4ms frame transfer 48 db dynamic range 200 MHz Pentium NRAO design Diffraction Limit 1. 2 (for λ = 500 nm) Plate Scale arcsec/µm Effective Plate Scale per detector pixel Field of view 12 9 Sensitivity 7 th magnitude in 5 second exposure Mount location Backup structure (requires access hole in panel) Mount Three-point Invar; accessible 14

18 Table 3: OPT1 Component Properties Primary Model Clear Aperture Focal Length Diameter Near Focus Meade 102ED Apochromatic Refractor 4 inches (102 mm) 920mm (f9) 110mm 50 feet (15.24m) Barlow Lens Model Antares Magnification 2 Focal Length 42mm Filters IR Filter IR Filter Bandpass Focus Corrector Murnaghan IR-C-Cell % λ > 700nm Murnaghan FC-Cell-1.25 CCD Camera Model Cohu Frequency Range (see Figure 4) Active Pixels Pixel Size µm Image Area mm Video Output 1.0V p-p, 75Ω, unbalanced Signal-to-Noise 56 gamma=1, gain=0 db Cooling Normal (5 ± 2C); T amb = 5 25 C Hot Ambient (10 ± 2C); T amb = C Maximum ( 22C below ambient), T amb = C Ambient (no cooling); T amb = C 15

19 (0,0) OPT1 OPT2 Elevation (0,0) OPT2 OPT2 Y Pixel OPT2 Azimuth OPT2 X Pixel OPT1 Y Pixel OPT1 Elevation OPT1 X Pixel OPT1 Azimuth Figure 6: Orientation of the (Az,El) and (X,Y) axes on the ALMA optical pointing telescope CCD detectors. IR Focus Difference: Even though the focal point when the IR filter is inserted should be the same as the focus when the focus corrector is inserted, it isn t. Re-focusing with the focus translation stage solves this problem. Frame Grabber Dynamic Range: The frame grabber dynamic range (48 db) is less than that of the CCD (56 db). The production OPTs should use a frame grabber with higher dynamic range (something like 16 bit would be good). Peak Finding Algorithm: The peak finding algorithm currently suffers from a nonstatic 1 pixel quantization under high intensity (either bright stars or daytime) conditions. A Peak Finding Algorithm The following centre-of-mass algorithm was suggested by Ralph Snel. 1. Select a sub-image, such as pixels centered on the brightest pixel. 2. Let x = [ ] etc., replicated in the y direction, and y the transpose of x. Then the center of light of the subimage has coordinates: (x sub image) (sub image) 16

20 (y sub image) (sub image) 3. The biggest practical problem with the OPT is finding out whether or not there is any saturation, in particular during daytime measurements. Here is a sample bit of C-code to describe this algorithm: sum_x=0; sum_y=0; sum_image=0; for (j=-lim;j<=lim;j++) { for (i=-lim;i<=lim;i++) { sum_x+=(sub_image[i,j]-avg)*i; sum_y+=(sub_image[i,j]-avg)*j; sum_image+=(sub_image[i][j]-avg); } } x_centroid=sum_x/sum_image; y_centroid=sum_y/sum_image; 17