The Space Stellar Survey “Lyra” and Establishing a Large Grid of Photometric Standards A. Mironov A. Zakharov M. Prokhorov Sternberg Astronomical Institute Moscow, Russia

The space experiment “Lyra” is now being prepared in Russia

The main goal of the experiment is to perform multicolor photometry for all V<16 mag stars.

The telescope will be mounted on the Russian segment of the International Space Station.

“Lyra” – the Russian segment of ISS

We think that it will look so:

We are planning to make multicolor photometric catalogs for 100–400 million objects of V<16 mag; to scan repeatedly all sky during 3–5 years (usually 20 times per year); to use 10 bands from 190 to 1000 nm; to achieve high precision and high accuracy photometry for stars 3 m – 16 m ; to create an extensive uniform system of photometric standards; to measure coordinates of all objects of V<15 with uncertainties within 1 mas and of all other objects with uncertainties within 10 mas.

”Lyra”: the schedule Design and manufacturing of equipment.2008 – 2015 Launch2015 Performing the experiment. Preprocessing. Preliminary catalog – 2020 Finishing reduction. Creating of the final catalog – 2023

Scanning mode of observations Time delay and integration (TDI, drift) mode 10-band photometry Full coverage of the sky The main principles of the experiment

Ritchey-Chretien telescope with afocal lens corrector Primary mirror diameter: 500 mm Focal length: 3 m Mirror material: SiC Mass of the primary mirror: 2.5 kg Mass of the secondary mirror: 0.5 kg “Lyra”: the telescope

“Lyra”: sky scanning i = 51.6º P pr ≈ 70 d Orbital period: 92 min. One of ISS axis always directed to Earth. Fixed telescope is fastened on the board of ISS. The vizier axis is directed away from the Earth. During every revolution around the Earth, the field of view covers a ring strip on the sky. Precession shifts the strip by a quarter of a degree in every pass of the orbit. Therefore each star will be observed on 4-5 successive revolutions.

“Lyra”: covering the sky E If the vizier line lies in orbit plane, then, for one precession period, we cover a large band in the sky along the equator between –52° and +52° (equator mode). To reach polar regions, we must incline the axes of sight by the angle supplementary to orbit inclination (polar mode). A choice between the two modes will depend on the position of the Sun. Equator mode Polar mode

The angular size of the corrected field of view will be about 2.0 degree in the sky. 22 CCDs connected in pairs will be installed in the focal plane. The first pair (CCDs 1st and 2nd) will be covered only by anti-reflective coating and realize an panchromatic passband. The remaining 10 pairs of CCDs will be covered by interference coating so as to create 10 different photometric bands in the 200–1000 nm spectral range. During scanning, stars will pass sequentially through all the CCDs and will be recorded in the TDI mode. “Lyra”: Focal plane Main photometer 22 CCDs 2250×300 pxl, pixel size: 12×12 μ m Scan and readout direction Good field of view 2.0º 6 guide CCDs 512×512 pxl pixel size: 16×16 μ m 1 st CCD (panchrom) 20 th filter coated CCD

Detection of ISS micro-tilts corrected field of view CCDs of image stabilization device Vibrations of the station smear star images. To measure the micro-tilts the six auxiliary CCDs will be located around the field of the main photometer.

Compensation for ISS tilts effect piezo stepper linear actuators HEXAPOD moves and rotates the focal plane assembly with an accuracy of about 1 micrometer

“Lyra”: Passbands We plan to perform a multicolor all-sky survey in 10 passbands over the wavelength range from 200 to 1000 nm. Photometric system will be implemented by 10 back illumination CCDs. In addition, another CCD will be set to implement a panchromatic band. Only one of the bands 4a, 7a and 10 will be retained based on simulations which are carried out now.

0 (nm) Δ (nm) Brightest (overflow) faintest for 5 years observation  = m  = 0.01 m (W) (P) (B) (V) (R) panchrom Filter summary: limiting V magnitudes for A0V stars

One of the most important tasks of the "Lyra" experiment is the creation of a large grid of high-precision photometric standards.

An important question is: for what purpose we need a reference system of standard stars? The obvious one is: to set the zero point and the scale of the observed magnitudes.

What do we want from a system of reference photometric standards? With this system, we should be able: to calculate the atmospheric parameters for accurately take into account atmospheric extinction; to derive transformations between different photometric systems; to reduce the results of measurements obtained in the current instrumental system, whose spectral parameters are slowly drifting, into the common photometric system.

Here are the requirements that standard star catalog must meet :  the stars should span a wide range of magnitudes; stars should be selected from a representative sample of spectral types and luminosity classes; catalog stars should tightly and evenly cover all the sky.

The stars included into catalog of photometric reference standards should have the following properties: Stars should not have variations exceeding the error of the apparatus. Stars should not have close neighbors that would interfere with the measurements. Stars should be measured with a small error as far as possible depending on the magnitude, spectral type and photometric band. Stars should have the spectral energy distributions which allow accurate reduction to the photometric bands different from the passbands specified in the catalog.

In addition, the catalog of photometric reference standards MUST be supplemented by:  A set of response curves of catalog’s photometric bands.  A set of typical spectral energy distributions for stars of different spectral types in the spectral range corresponding to the reaction curves of catalog passbands; this set should include stars of different temperatures, pressures on the photosphere, elemental abundances, interstellar reddening, etc.  A description of procedures and data for the calculation of the average atmospheric transmission curves under various ground conditions, and a description of procedures to derive observation based corrections to the average atmospheric transmission.

How can the catalog of magnitudes and color indices for “Lyra” standards in the "Lyra" passbands be useful for the actual observer? Precise and accurate catalog of standards enables the wide use of the synthetic photometry technique. If you have a catalog with the properties listed above, you can easily create a catalog of standards in your photometric system. To do this you will not need to make additional observations!

Creation of user’s standard magnitudes Lyra passbands User passbands Average empirical spectra Reddening law Creation of a set of spectra Set of spectra Synthetic Color-Indices Derivation of transformation formulae Lyra catalog User standards

The Lyra experiment should produce just such a catalog of standard stars supplemented by a set of necessary data and description of procedures.

Can the “Lyra” standard star catalog meet all the above requirements? 1)How many stars will the “Lyra” telescope “see” in different passbands? Our simulations based on an extrapolation of Hipparcos and Catalog of Nearby Stars (CNS3) data showed that:

The expected average number of stars in a square degree, measured with an error of less than 0 m.01 In the ultraviolet bands we will measure practically only stars of spectral types B and A. We will have from about a dozen to one hundred stars (mostly the main sequence stars) per square degree. Only in the band "350" we will have an appreciable number of F, G, and K stars. ● (b lue) – MS ● (r ed) – giants ● ( black) – total Spectral type

The expected average number of stars in a square degree, measured with an error of less than 0 m.01 The maximum number of stars will be observed in the band "440": more than 1,000 objects of various spectral types per square degree. The number of K-giants exceeds the number of red dwarfs. ● (b lue) – MS ● (r ed) – giants ● ( black) – total Spectral type

The expected average number of stars in a square degree, measured with an error of less than 0 m.01 Note that K-giants are dominant both in the visible and infrared bands ● (b lue) – MS ● (r ed) – giants ● ( black) – total Spectral type

2)A large number of stars to be measured in the “Lyra” experiment will certainly allow us to find a few million of standards uniformly distributed over the sky.

3)Simultaneous measurements of magnitudes in 10 bands will make it possible to apply the correlation method for discovering variable stars. (Mironov A.V., Zakharov A.I., Nikolaev F.N. On the New Technique for Discovering Variable Stars. Baltic Astronomy, vol.12, , 2003)  The method is based on searching for correlation between magnitude variations in different filters.  The method allows detection of variability with amplitudes as small as 0.3σ – 1.0σ..

4)The magnitudes of the “Lyra” standard star catalog will be accurately transformable into other photometric (especially broadband) systems because:  “Lyra” bands cover the operating spectral range without gaps.  There are enough bands to take into account the great variety of stellar spectra and interstellar extinction.  Stars can be tested for suitability as standards. “Lyra” magnitudes of standard stars in different passbands should be mutually transformable.

“Lyra” calibration. Ground based: 1.Careful relative measurements of equipment properties (spectral, photometric, polarimetric etc.) 2.Absolute black body calibration. During the mission: A. Instrumental calibration. 1.Regular flat field calibrations. 2.Regular dark current calibration. 3.Regular spectral calibration of total optical path. B. Observation calibration. Extra calibration based on observations of standard stars. If an opportunity arises then the focal plane assembly will be returned to the Earth for a posteriori laboratory calibration.

The above methods, procedures are now developed at the Sternberg Astronomical Institute. We are confident that the “Lyra” experiment will produce a universal catalog of standard stars to be used in a wide range of astronomical tasks. The building of Sternberg astronomical institute in Moscow

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