PSF-fitting techniques for crowded field 3D spectroscopy

From a historical point of view, it was only through the advent of the CCD as a linear, high dynamic range panoramic detector that it became possible to overcome the source confusion problem for stellar photometry, e.g., in star clusters or nearby galaxies. The ability of accurately sampling the point-spread-function (PSF) in two dimensions and to use it as a template for fitting severely overlapping stellar images is of fundamental importance for crowded-field photometry, and has thus become the foundation for the determination of accurate color-magnitude diagrams of globular clusters and the study of resolved stellar populations in nearby galaxies. Analogous to CCDs, the introduction of integral field spectrographs has opened a new avenue for crowded-field 3D spectroscopy, which benefits in the same way from PSF-fitting techniques as CCD photometry does. This paper presents first experience with sampling the PSF in 3D spectroscopy, reviews the effects of atmospheric refraction, discusses background subtraction problems, and presents several science applications as obtained from observations with the PMAS instrument at Calar Alto Observatory.     

HST WFPC2天体测量和测光方法最新进展

对于哈勃空间望远镜(HST)来说，其广角行星照相机(WFPC2)的CCD性能的数据处理校正对提高天体测量和测光结果的精度有着重要意义。最近几年由于WFPC2对精度要求越来越高，其数据处理方法和校正方法包括点扩散函数、行误差和畸变修正等各个方面的研究都有了很大改进和进展。对近年来上述方面的最新进展作了介绍，并评价了其优缺点，提出了一些有待解决的问题。     

Front‐ vs. back‐illuminated CCD cameras for photometric surveys: a noise budget analysis

Exoplanetary transit and stellar oscillation surveys require a very high precision photometry. The instrumental noise has therefore to be minimized. First, we perform a semi‐analytical model of different noise sources. We show that the noise due the CCD electrodes can be overcome using a Gaussian PSF (Point Spread Function) of full width half maximum larger than 1.6 pixels. We also find that for a PSF size of a few pixels, the photometric aperture has to be at least 2.5 times larger than the PSF full width half maximum. Then, we compare a front‐ with a back‐illuminated CCD through a Monte‐Carlo simulation. Both cameras give the same results for a PSF full width half maximum larger than 1.5 pixels. All these simulations are applied to the A STEP (Antarctica Search for Transiting Extrasolar Planets) project. As a result, we choose a front‐illuminated camera for A STEP because of its better resolution and lower price, and we will use a PSF larger than 1.6 pixels. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optimising optimal image subtraction

Difference imaging is a technique for obtaining precise relative photometry of variable sources in crowded stellar fields and, as such, constitutes a crucial part of the data reduction pipeline in surveys for microlensing events or transiting extrasolar planets. The Optimal Image Subtraction (OIS) algorithm of Alard & Lupton (1998) permits the accurate differencing of images by determining convolution kernels which, when applied to reference images with particularly good seeing and signal‐to‐noise (S/N), provide excellent matches to the point‐spread functions (PSF) in other images of the time series to be analysed. The convolution kernels are built as linear combinations of a set of basis functions, conventionally bivariate Gaussians modulated by polynomials. The kernel parameters, mainly the widths and maximal degrees of the basis function model, must be supplied by the user. Ideally, the parameters should be matched to the PSF, pixel‐sampling, and S/N of the data set or individual images to be analysed. We have studied the dependence of the reduction outcome as a function of the kernel parameters using our new implementation of OIS within the IDL‐based TRIPP package. From the analysis of noise‐free PSF simulations of both single objects and crowded fields, as well as the test images in the ISIS OIS software package, we derive qualitative and quantitative relations between the kernel parameters and the success of the subtraction as a function of the PSF widths and sampling in reference and data images and compare the results to those of other implementations found in the literature. On the basis of these simulations, we provide recommended parameters for data sets with different S/N and sampling. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

基于CSST多色测光的时序测光精度分析

中国巡天空间望远镜(Chinese Survey Space Telescope, CSST)是中国的首个大型空间光学望远镜, 将对包括系外行星探测在内的诸多科学目标开展研究, 有望取得前沿科学进展. 时序测光精度是CSST重要的性能指标, 受到物理噪声和仪器噪声的影响, 需要通过数值模拟对其分析和评估. 模拟基于目前公布的CSST主要技术参数, 建立了时序的恒星信号和噪声模型, 以CSST的i波段为例, 分析凝视观测模式下的测光精度. 通过数值仿真, 展示了孔径测光中各项噪声的贡献, 特别是由指向抖动和像素响应不均匀性导致的抖动噪声. 模拟结果还给出了测光孔径的推荐范围. 为了获得更高的信噪比, 可以减小仪器抖动振幅和像素不均匀性, 或者采用参考星较差测光的方式. 结果为CSST后续的时序测光精度与不同指标参数的相关性分析、系外行星探测能力评估以及测光数据处理提供了模拟数据的支撑.     

Correcting CCD photometry of stars for seeing effects

Systematic variability in stellar magnitudes, as derived from profile fitting to CCD images, may in some instances be due to variable seeing. It is suggested that this happens in cases where the stars are unresolved pairs, typically with sub-arcsecond separation between the components. It is shown that the fitting of suitable Generalised Additive Models to time series photometry can disentangle intrinsic stellar variability and seeing-induced brightness changes. It is possible that there will be a fixed seeing response associated with a given star which exhibits the effect: estimation of this response from several long photometric runs is demonstrated.     

Photometry from online Digitized Sky Survey plates

Online Digitized Sky Survey (DSS) material is often used to obtain information on newly discovered variable stars for older epochs (e.g. Nova progenitors, flare stars, etc.). We present here the results of an investigation of photometry on online DSS material in small fields calibrated by CCD sequences. We compared different source extraction mechanisms and found that even down near to the sensitivity limit, despite the H-compression used for the online material, photometry with an accuracy better than 0.1 mag rms is possible on DSS-II. Our investigation shows that the accuracy depends strongly on the source extraction method. The SuperCOSMOS scans, although retrieved with a higher spatial resolution, do not give us better results. The methods and parameters presented here allow the user to obtain good plate photometry in small fields down to the Schmidt plate survey limits with a few bright CCD calibrators, which may be calibrated with amateur-size telescopes. Especially for the events mentioned above, new field photometry for calibration purposes mostly exists, but the progenitors were not measured photometrically before. Also, the follow-up whether stellar concentrations are newly detected clusters or similar work may be done without using mid-size telescopes. The calibration presented here is a 'local' one for small fields. We show that the method presented here gives higher accuracies than 'global' calibrations of surveys (e.g. Guide Star Catalogue-II (GSC-II), SuperCOSMOS and the US Naval Observatory Astrometry Catalog B).     

An optimal extraction algorithm for imaging photometry

This paper is primarily an investigation of whether the 'optimal extraction' techniques used in CCD spectroscopy can be applied to imaging photometry. It is found that using such techniques provides a gain of around 10 per cent in signal-to-noise ratio over normal aperture photometry. Formally, it is shown to be equivalent to profile fitting, but offers advantages of robust error estimation, freedom from bias introduced by mis-estimating the point spread function, and convenience. In addition some other techniques are presented, which can be applied to profile fitting, aperture photometry and the 'optimal' photometry. Code implementing these algorithms is available at http://www.astro.keele.ac.uk/~timn/.     

Optimizing point-source parameters for scanning satellite surveys

We describe a method for deriving the position and flux of point and compact sources observed by a scanning survey mission. Results from data simulated to test our method are presented, which demonstrate that at least a 10-fold improvement is achievable over that of extracting the image parameters, position and flux, from the equivalent data in the form of pixel maps. Our method achieves this improvement by analysing the original scan data and performing a combined, iterative solution for the image parameters. This approach allows for a full and detailed account of the point-spread function (PSF), or beam profile, of the instrument. Additionally, the positional information from different frequency channels may be combined to provide the flux-detection accuracy at each frequency for the same sky position. Ultimately, a final check and correction of the geometric calibration of the instrument may also be included. The Planck mission was used as the basis for our simulations, but our method will be beneficial for most scanning satellite missions, especially those with non-circularly symmetric PSFs.     

Relative astrometry and near‐infrared differential photometry of nearby southern orbital binaries with adaptive optics

We present relative astrometry and differential photometry measurements for a sample of nearby southern orbital binaries making use of the technique of Adaptive Optics. The observations were made in December 2000, with the ADONIS camera mounted at the 3.6‐m ESO telescope from La Silla Observatory, equipped with the broad‐band near‐infrared filters (J ‐, H ‐, K ‐passbands). Our sample contains stars which do not fit very well the empirical mean mass‐luminosity relation (according to our previous study), but for which accurate parallaxes (determined by the Hipparcos satellite) and high‐quality orbits were available thanks to many previous efforts. We derived accurate positions and J, H, K magnitudes of the individual components of those binaries. The individual stellar components have near‐infrared colour indices well grouped in those plots and are comparable to standard single stars. The data reduction procedure used for deriving those results is described in detail. It is based on a least‐squares fit of Moffat‐Lorentz profiles in direct imaging for well‐resolved systems and on Fourier analysis for very close pairs. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stellar CCD Photometry: New Approach Principles and Application

A new approach is proposed and developed to handle pre-processed CCD frames in order to identify stellar images and derive their relevant parameters. The present method relies on: 1) identifying stellar images and assigning approximate positions of their centres using artificial intelligence techniques, 2) accurate determination of the centre coordinates applying an elementary statistical concept and 3) estimating the image peak intensity as a stellar magnitude measure employing a simple numerical analysis approach. The method has been coded for personal computer users. A CCD frame of the star cluster M67 was adopted as a test case. The results obtained are discussed in comparison with the DAOPHOTII ones and the corresponding published data. Exact coincidence has been found between both results except very few cases. These exceptions have been discussed in view of both methods' bases and the cluster plates. It has been realized that the method suggested exhibits very simple, extremely fast and high precise approach in stellar CCD photometry domain. Moreover, it is more capable for handling blended and distorted stellar images than the DAOPHOTII. These characteristics show the usefulness of the present method in some astronomical applications such as auto-focusing and auto-guiding sensing approaches beside the main purpose, viz. stellar photometry. This revised version was published online in July 2006 with corrections to the Cover Date.     

依巴谷卫星和哈勃空间望远镜的天体测量观测结果

综述了以下几个方面的工作：（１）依巴谷卫星３０个月观测资料的初步处理结果；（２）空间望远镜精密导星传感器的性能测试；（３）依巴谷卫星和空间望远镜近期在观测和仪器改进上的进展；（４）我们开展空间天体测量工作的概况。     

A new algorithm for differential photometry: computing anoptimum artificial comparison star

Our new algorithm for differential photometry solves the problem of identifying proper comparison stars without a prior detailed study of the field of view. The comparison stars' variability is determined in a self‐consistent way, and their weighted average is used as a reference level. The maximum error in differential photometry using objects and reference stars of different spectral types is estimated. The results from these calculations show that the photometric band chosen greatly determines the level of accuracy achieved. Finally, an important application of high‐precision differential photometry are planetary transits. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

电荷守恒与CCD孔径测光

Lick天文台的CCD控制器是一类新型控制器,它能够在同一CCD系统内实现MPP(Multi—pinned phase)和通常工作模式间的转换．自2002年起中国至少有4架 CCD照相机使用这类控制器．此控制器的弱点是溢出发生在非线性前,而且远在饱和以前．虽然这对亮的面光源观测不利,并且它妨碍了使用亮星构建点扩散函数(psf)．但是由于电荷守恒原理,对亮的点源(恒星)的孔径测光依然可行．观测证明了此结论．     

The metal homogeneity of the globular star cluster M5

This work is concerned with the precision measurement of the magnitudes of the stars across a large field of the galactic globular cluster M5. The colours of the red giants are subjected to a thorough statistical analysis in order to quantify the extent of any star-to-star variations in metal abundance within the cluster. Use is made of an extensive optical/near-infrared observational data set acquired with a large-array CCD detector and the Johnson V and I filters. The results of this photometry, which is accurate to within a few millimagnitudes for the brighter stars, are used to compile a colour–magnitude diagram.
The analysis finds that in a metal-rich M5 ([Fe/H]=−1.2) the extent of any metal inhomogeneity, Δ[Fe/H], is less than ±0.1 dex, and in a metal-poor M5 ([Fe/H]=−1.5) then     . (The literature contains a range of derived values of metallicity for M5.)     

Spanish Monitoring of Comets: Making Sense of Amateur Photometric Data

Amateur astronomers have always represented an important observing group in cometary astronomy. Much of our knowledge of cometary light curves has come from amateur data, initially in the form of total visual magnitude estimates and now increasingly in the form of CCD observations. The increasingly widespread use by amateur astronomers of CCD cameras of excellent sensitivity and good cosmetic quality has revolutionised astrometry, allowing far more intensive astrometric monitoring of comets to be carried out down to magnitude 18 and fainter, with a corresponding enormous increase in the quality of calculated orbits. Although amateur CCD photometry is extensively available in the Internet, its use has been less widespread. The reason is the lack of standardisation in the way that this data is taken that leads to amateur CCD light curves having enormous dispersion. All amateur CCD photometry is aperture photometry, but it is seen that neither does it represent well the equivalent of m1 (total visual magnitude), even with a large aperture, nor is it close to the definition of m₂ (the nucleus magnitude). The problem is examined using data from the Spanish Comet Observers Group archives to show that by careful standardisation of data acquisition amateur CCD data can produce high quality, well-sampled and physically meaningful light curves. Examples are presented of the results for recent comets including 19P/Borrelly, 51P/Harrington, C/2001 TU80 (LINEAR-NEAT), C/2000 WM1 (LINEAR) & C/2001 A2 (LINEAR).     

K-band transit and secondary eclipse photometry of exoplanet OGLE-TR-113b

We present high-precision K -band photometry of the transit and secondary eclipse of extrasolar planet OGLE-TR-113, using the SOFI near-infrared instrument on European Southern Observatory's New Technology Telescope. Data were taken in 5-s exposures over two periods of 3–4 h, using random jitter position offsets. In this way, a relative photometric precision of ∼1 per cent per frame was achieved, avoiding systematic effects that seem to become dominant at precisions exceeding this level, and resulting in an overall accuracy of 0.1 per cent per ∼10 min. The observations of the transit show a flat-bottomed light-curve indicative of a significantly lower stellar limb darkening at near-infrared than at optical wavelengths. The observations of the secondary eclipse result in a 3σ detection of emission from the exoplanet at 0.17 ± 0.05 per cent. However, residual systematic errors make this detection rather tentative.     

Faint UBVRI CCD sequences for wide-field surveys – II. UBVR sequences at δ=−30°

We present results from a continuing campaign to secure deep multi-colour CCD sequences for photometric calibration in UK Schmidt fields with galactic latitudes | b |>50°. In this paper we present UBVR photometry in 12 fields and BR photometry in a further 14 fields observed within UK Schmidt survey fields centred at δ =−30°. Photometric errors are at the 0.05 level at 20.2, 21, 20.5 and 20 mag for UBVR sequences respectively. Our observations were carried out with the 0.9-m telescope at the Cerro-Tololo Inter-American Observatory. These data are not intended for use as highly accurate individual photometric standards, but rather for use as sequences, using a large number of stars to calibrate wide-area data such as photographic plates. The data are available electronically at http://icstar5.ph.ic.ac.uk/~scroom/phot/photom.html .     

Operational testing of the SBIG ST-6 CCD camera

The SBIG Model ST-6 CCD Imaging Camera was tested and found suitable for various kinds of astronomical observations on telescopes with apertures from 6 mm to 0.6 m. We discuss different noise sources, linearity, sensitivity, and other detector characteristics. Limits, which they set to the observations and accuracy of data evaluation process, are also discussed. Detection, accurate astrometry, and VR photometry limiting magnitudes are about 21, 20, and 17.5, respectively, with the 0.6-m telescope and 10-min integration time. Detection of 5th magnitude 1-second optical transients is possible with the 6-mm wide-field lens.