Rugate filters for OH-suppressed imaging at near-infrared wavelengths

Ground-based observations at near-infrared wavelengths are severely affected by atmospheric OH bands. Many authors have recognized the potential gains in sensitivity from suppressing these features. Dispersive instruments show some promise but are both expensive and complicated to build. OH suppression filters using single or periodic notches have the advantage of simplicity but significant gains have not yet been realized.   Rugate filters (with graded index inhomogeneous coatings) offer key advantages for astronomical imaging. It is possible to produce a transmission profile comprising a series of irregular and sharply defined bandpasses. We demonstrate through numerical simulation of rugate filters that it should be possible to achieve 95 per cent suppression of the OH features in the J photometric band, while retaining roughly half of the spectral coverage. This would lead to extraordinary gains in sensitivity even for observations of continuum sources. In addition, these filters allow longer exposures before the detector saturates on the sky background. I - and z -band filters can also be envisaged.   In 1-arcsec seeing, a J -band rugate filter used in conjunction with a 4-m telescope would detect a J  = 23 continuum source at 5.0σ in a single 10-min exposure. In comparison, a conventional J filter requires multiple exposures for a 10-min integration time and achieves only a 2.5σ detection. For emission-line sources, the rugate filter has an even bigger advantage over conventional filters, with a fourfold increase in signal-to-noise ratio possible in certain instances.   Astrophysical studies that could benefit from rugate filters are searches for very low-mass stars and galaxy evolution out to z  = 3.     

Transmission profile of the Dutch Open Telescope Hα Lyot filter

Accurate knowledge of the spectral transmission profile of a Lyot filter is important, in particular in comparing observations with simulated data. The paper summarizes available facts about the transmission profile of the Dutch Open Telescope (DOT) Hα Lyot filter pointing to a discrepancy between sidelobe‐free Gaussian‐like profile measured spectroscopically and signatures of possible leakage of parasitic continuum light in DOT Hα images. We compute wing‐to‐center intensity ratios resulting from convolutions of Gaussian and square of the sinc function with the Hα atlas profile and compare them with the ratios derived from observations of the quiet Sun chromosphere at disk center. We interpret discrepancies between the anticipated and observed ratios and the sharp limb visible in the DOT Hα image as an indication of possible leakage of parasitic continuum light. A method suggested here can be applied also to indirect testing of transmission profiles of other Lyot filters. We suggest two theoretical transmission profiles of the DOT Hα Lyot filter which should be considered as the best available approximations. Conclusive answer can only be given by spectroscopic re‐measurement of the filter. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A versatile birefringent filter

The theory of the olc birefringent filter is complicated and the general formulae describing its optical properties do not invite to any simple physical picture. Due to these formal difficulties, many of its inherent possibilities have not been fully appreciated. In this paper we point out some new possibilities for filters based on this general design.The detailed shape of the transmission profile is a function of the angles of the crystal plate optic axes. It is shown how unwanted transmission sidelobes can be suppressed to any desired level by altering the distribution of plate angles. By the same means, the transmission band can be split into two symmetrically placed replicas, and the distance between the two bands can be varied. In this way the filter can easily be tuned over half the free spectral range.Some of the error sources that are important to filter performance are discussed. An expression is given for the amount of parasitic light introduced by errors in the orientation of the plate optic axes and it is found that errors as large as 0°.5 can be tolerated. A laboratory experiment with 16 birefringent plates has shown that accuracies an order of magnitude better than this figure can easily be achieved. Manufacturing errors in the thickness of the plates can be compensated for by assembling the pile of plates in a particular way. The very strict tolerances usually quoted can therefore be considerably relaxed.     

The build-up of haloes within Press–Schechter theory

Modelling the build-up of haloes is important for linking the formation of galaxies with cosmological models. A simple model of halo growth is provided by Press–Schechter (PS) theory, where the initial field of density fluctuations is smoothed using spherically symmetric filters centred on a given position to obtain information about the likelihood of later collapse on varying scales. In this paper the predicted halo mass growth is compared for three filter shapes: Gaussian, top-hat and sharp k -space. Preliminary work is also presented analysing the build-up of haloes within numerical simulations using a friends-of-friends group finder. The best-fit to the simulation mass function was obtained using PS theory with a top-hat filter. By comparing both the backwards conditional mass function, which gives the distribution of halo progenitors, and the distribution of halo mergers in time, the build-up of haloes in the simulations is shown to be better fitted by PS theory with a sharp k -space filter. This strengthens previous work, which also found the build-up of haloes in simulations to be well matched to PS theory with a sharp k -space filter by providing a direct comparison of different filters and by extending the statistical tools used to analyse halo mass growth. The usefulness of this work is illustrated by showing that the cosmological evolution in the proportion of haloes that have undergone recent merger is predicted to be independent of mass and power spectrum and to only depend upon cosmology. Recent results from observations of field galaxies are shown to match the evolution expected, but are not sufficiently accurate to distinguish usefully between cosmological parameters.     

Investigating the efficiency of the Beijing Faint Object Spectrograph and Camera(BFOSC) of the Xinglong 2.16-m reflector

The Beijing Faint Object Spectrograph and Camera(BFOSC) is one of the most important instruments operating in conjunction with the 2.16-m telescope at Xinglong Observatory. Every year there are~20 SCI-papers published based on observational data acquired with this telescope. In this work,we have systemically measured the total efficiency of the BFOSC that operates as part of the 2.16-m reflector, based on observations of two ESO flux standard stars. We have obtained the total efficiencies of the BFOSC instrument of different grisms with various slit widths in almost all ranges, and analyzed factors which effect the efficiency of this telescope and spectrograph. For astronomical observers, the result will be useful for them to select a suitable slit width, depending on their scientific goals and weather conditions during observations. For technicians, the result will help them to systemically identify the real efficiency of the telescope and spectrograph, and to further improve the total efficiency and observing capacity of the telescope technically.     

Quick Detection System for Planck satellite

Q uick D etection S ystem or qds is a software package that has been developed for detecting point sources in the Planck satellite data as soon as the data become available, a few days after transmission to the Earth. Point sources are detected by filtering the data with a filter defined by the Mexican hat wavelet. An alert is generated on those detections that are found to be interesting, such as prominent flaring, according to the criteria specified to the software. The goal is to detect spectral or flux variability in active galactic nuclei so that instant multifrequency follow-up observations with other instruments could be arranged to study the interesting behaviour.     

Effects of Interference and Oxidation on the UV/Visible Rejection Properties of Filters for Soft X-ray Detectors

We report on UV/Visible transmission measurements of aluminum coated Lexan filters designed as UV blocking filters for soft x-ray detectors. Transmission of the filters in the 2300-8000 Å wavelength range is significantly higher than expected. It cannot be accounted for applying a simple slab model of the transmission and adopting material properties reported in the literature. We show that this is due to interference effects which are strongly dependent on the filter geometry, and to oxidation of exposed aluminum surfaces and/or chemical interaction with the plastic support. The results of this work have led to the redesign of the Advanced X-ray Astrophysics Facility High Resolution Camera UV blocking filters.     

The thin and medium filters of the EPIC camera on-board XMM-Newton: measured performance after more than 15 years of operation

After more than 15 years of operation of the EPIC camera on board the XMM-Newton X-ray observatory, we have reviewed the status of its Thin and Medium filters. We have selected a set of Thin and Medium back-up filters among those still available in the EPIC consortium and have started a program to investigate their status by different laboratory measurements including: UV/VIS transmission, Raman scattering, X-Ray Photoelectron Spectroscopy, and Atomic Force Microscopy. Furthermore, we have investigated the status of the EPIC flight filters by performing an analysis of the optical loading in the PN offset maps to gauge variations in the optical and UV transmission. We both investigated repeated observations of single optically bright targets and performed a statistical analysis of the extent of loading versus visual magnitude at different epochs. We report the results of the measurements conducted up to now. Most notably, we find no evidence for change in the UV/VIS transmission of the back-up filters in ground tests spanning a 2 year period and we find no evidence for change in the optical transmission of the thin filter of the EPIC-pn camera from 2002 to 2012. We point out some lessons learned for the development and calibration programs of filters for X-ray detectors in future Astronomy missions.     

A novel multifrequency technique for the detection of point sources in cosmic microwave background maps

In this work we address the problem of simultaneous multifrequency detection of extragalactic point sources in the maps of the cosmic microwave background. We apply a new linear filtering technique, the 'matched matrix filters', that incorporates full spatial information, including the cross-correlation among channels, without making any a priori assumption about the spectral behaviour of the sources. A substantial reduction of the background is achieved thanks to the optimal combination of filtered maps. We describe the new technique in detail and apply it to the detection of radio sources and estimation of their parameters in realistic all-sky Planck simulations at 30, 44, 70 and 100 GHz. Then, we compare the results with the single-frequency approach based on the standard matched filter, in terms of reliability, completeness and flux accuracy of the resulting point source catalogues. The new filters outperform the standard matched filters for all these indexes at 30, 44 and 70 GHz, whereas at 100 GHz both kinds of filters have a similar performance. We find a notable increment of the number of true detections for a fixed reliability level. In particular, for a 95 per cent reliability we practically double the number of detections at 30, 44 and 70 GHz.     

Detection/estimation of the modulus of a vector. Application to point-source detection in polarization data

Given a set of images, whose pixel values can be considered as the components of a vector, it is interesting to estimate the modulus of such a vector in some localized areas corresponding to a compact signal. For instance, the detection/estimation of a polarized signal in compact sources immersed in a background is relevant in some fields like astrophysics. We develop two different techniques, one based on the Neyman–Pearson lemma, the Neyman–Pearson filter (NPF), and another based on pre-filtering before fusion, the filtered fusion (FF), to deal with the problem of detection of the source and estimation of the polarization given two or three images corresponding to the different components of polarization (two for linear polarization, three including circular polarization). For the case of linear polarization, we have performed numerical simulations on two-dimensional patches to test these filters following two different approaches (a blind and a non-blind detection), considering extragalactic point sources immersed in cosmic microwave background (CMB) and non-stationary noise with the conditions of the 70 GHz Planck channel. The FF outperforms the NPF, especially for low fluxes. We can detect with the FF extragalactic sources in a high noise zone with fluxes      Jy for (blind/non-blind) detection and in a low noise zone with fluxes      Jy for (blind/non-blind) detection with low errors in the estimated flux and position.     

中国出口双折射滤光器

中国科学院南京天文仪器研制中心在20世纪80年代研制35cm太阳磁场望远镜滤光器取得巨大的成功，其在双折射滤光器领域内的水平，已被国际太阳物理界公认。1989年以来，已向日本和韩国出口了11台复杂而昂贵的Lyot滤光器，并已在有关国立天文台和天文馆投入常规观测。最近，还为国外客户修复3台Lyot滤光器，它们都是德国和法国在30年前生产的。     

The selection of filters for reduction of optical contamination in astronomical CCD X-ray images

The European Photon Imaging Camera(EPIC), is the X-ray imaging and medium spectroscopy instrument for theESA X-ray Multi Mirror telescope(XMM) mission. TheCCD detectors to be used in the three focal plane cameras will provide images in the energy band from 0.1 to 10 keV. However, spectral studies may be compromised by low energy, optical photon contamination. In order to reduce this effect, a number of filters will be incorporated onto a rotating mechanism in the camera head. The filters will be chosen to provide a significant reduction in the optical contamination from a source whilst minimising the attenuation of the X-ray flux. Four commercial filters are described here and their effects on calculated typical source fluxes evaluated. In addition, two alternative filter designs are described and their effects on a simulated source spectra are debated. In both cases, particular attention is given to the problem of maintaining high sensitivity at soft X-ray energies (less than 2 keV).     

A rigorous demonstration of the conservation of integrated transmission of the tunable Lyot filter

In this paper I have derived the primitive function of the transmission function of the Lyot filter, hence a formula for calculating its integrated transmission. I have then derived the limit, as the filter order tends to infinity of the ratio between the transmissions through the main band and the whole filter. I have given a rigorous demonstration of the conservation of the integrated transmission of the tunable Lyot birefringent filter, discovered previously from numerical calculation by HU and AI, [1]. Lastly, I have proposed a method of measuring the total amount of light outside the main band of filters.     

A Minimal Spanning Tree algorithm for source detection in γ-ray images

We developed a source detection algorithm based on the Minimal Spanning Tree (MST), that is a graph-theoretical method useful for finding clusters in a given set of points. This algorithm is applied to γ-ray bi-dimensional images where the points correspond to the arrival direction of photons, and the possible sources are associated with the regions where they clusterize. Some filters to select these clusters and to reduce the spurious detections are introduced. An empirical study of the statistical properties of MST on random fields is carried out in order to derive some criteria to estimate the best filter values. We also introduce two parameters useful to verify the goodness of candidate sources. To show how the MST algorithm works in practice, we present an application to an EGRET observation of the Virgo field, at high Galactic latitude and with a low and rather uniform background, in which several sources are detected.     

Simulation of old open clusters for UVIT on ASTROSAT

The Ultra Violet Imaging Telescope(UVIT) is one of the payloads on the first Indian multiwavelength satellite ASTROSAT,which is expected to be launched by the Indian Space Research Organisation(ISRO) in the year 2015.We have performed simulations of UV studies of old open clusters for the UVIT.The colour magnitude diagrams(CMDs) and spatial appearances have been created using 10 filters associated with the FUV channel(130-180 nm) and NUV channel(200-300 nm)that are available for observations on the UVIT,for the three old open clusters M67,NGC 188 and NGC 6791.The CMDs are simulated for different filter combinations,and they are used to identify the loci of various evolutionary sequences,white dwarfs,blue stragglers,red giants,subgiants,turn off stars and the main sequence of the clusters.The present work helps in identifying a potential area of study in the case of these three old open clusters by considering the availability of filters and the detection limits of the instrument.We also recommend filter combinations,which can be used to detect and study the above mentioned evolutionary stages.The simulations and the results presented here are essential for the optimal use of the UVIT for studies of old open clusters.     

Photometric redshifts for weak lensing tomography from space: the role of optical and near infrared photometry

We study in detail the photometric redshift requirements needed for tomographic weak gravitational lensing in order to measure accurately the dark energy equation of state. In particular, we examine how ground-based photometry  ( u , g , r , i , z , y )  can be complemented by space-based near-infrared (near-IR) photometry  ( J , H )  , e.g. onboard the planned DUNE satellite. Using realistic photometric redshift simulations and an artificial neural network photo- z method we evaluate the figure of merit for the dark energy parameters  ( w ₀, w _a )  . We consider a DUNE -like broad optical filter supplemented with ground-based multiband optical data from surveys like the Dark Energy Survey, Pan-STARRS and LSST. We show that the dark energy figure of merit would be improved by a factor of 1.3–1.7 if IR filters are added onboard DUNE . Furthermore we show that with IR data catastrophic photo- z outliers can be removed effectively. There is an interplay between the choice of filters, the magnitude limits and the removal of outliers. We draw attention to the dependence of the results on the galaxy formation scenarios encoded into the mock galaxies, e.g. the galaxy reddening. For example, very deep u -band data could be as effective as the IR. We also find that about  10⁵–10⁶  spectroscopic redshifts are needed for calibration of the full survey.     

Near-infrared polarimetry of the Red Rectangle

Imaging polarimetry through J and H broad-band filters and a 3.4 μm narrow-band filter is used to highlight the regions of scattered light in the Red Rectangle. We find that the scattered light identifies the circumbinary dust component of the molecular disc seen in CO emission. This region also appears to be the origin of the recently discovered Blue Luminescence. We find that the degrees of polarization are consistent with the amorphous carbon dust model invoked by Men'shchikov. Spectropolarimetry from 1.4 to 2.5 μm confirms that the degree of polarization in the central arcsecond region is very low. This suggests that the central bicone seen in the near-infrared is predominantly due to emission from hot dust and/or from stochastically heated nanoparticles, rather than due to scattering by large grains.     

一种空间目标光电跟踪新方法

针对目前空间目标光电观测的自动跟踪过程中,容易受到恒星干扰等影响丢帧后失跟的问题,从概率观点分析了光电跟踪的两个环节:状态外推和实时采样.根据估计理论,建立了一种新的光电跟踪方法.方法利用卡尔曼滤波数据融合的本质提高外推的稳健性,通过均值漂移方法实现跟踪窗口的调整.仿真试验表明,新方法能够有效的降低跟踪中干扰的影响,提高跟踪的稳健性.     

The magneto-optical filter

Transmission curves and theoretical calculi of the magneto-optical filter, designed and built by the authors, are shown together with some observed transmission spectra. At present the filter has a total halfwidth of ≈80mÅ; and the maximum transmission is 25%. From the analysis of the theoretical curves and from the observed spectra, we deduced the possibility of making up a filter with a very narrow passband (less than 30mÅ) and a very high transmission (up to nearly 100% apart from losses arising from the glass cell, lenses and polarizers). The article concludes with a selection and discussion of photographs obtained with the filter.