This is a rough draft and far from being finished. Feel free to add comments.
DISCLAIMER
This experiment is based on "Project 1: Colour-Maginitude Diagrams of Star Clusters" of The Open University and adapted to the setup as provided by the Campus Observatory Garching (COG). ![/!\](/moin_static1911/modernized/img/alert.png "/!\"){kind=small}
Is this enough?
General Introduction
This section should contain a general overview of astronomy (Stars, HRD, Star Clusters) and instrumentation (CCD, Mounting, Optics, Filters).
Errors and limitations (Seeing, Air mass, CCD-related errors,)
Section 1.3 Background, which covers the following topics: star clusters, astronomical magnitudes and colours.
Question: What are the two types of star clusters that are present in the galaxy and how do they differ from each other?
Answer: Open clusters are loose collections of typically a few hundred stars, in the galactic disk. These clusters are generally young, with ages of a few million to a billion years old. Their stars belong to Population I. Globular clusters are tightly bound collections of typically a few hundred thousand to a million stars in the Galactic halo. These clusters are generally old with ages of a few billion years. Their stars belong to Population II.
As noted above, in this project you will obtain colour-magnitude diagrams (CMDs) of two/one open clusters and one globular cluster. Your aim will be to investigate the differences between the CMDs of these three/two clusters and explain these differences in terms of cluster ages.
You will also estimate the distances to the open clusters using the technique of main-sequence fitting.
>>> HRD Figure 1.1 <<<
A CMD plots the apparent magnitude of stars against their colour index, where an astronomical colour index is simply the difference between two magnitudes obtained through different filters. A CMD of a star is the observational analogue of a Hertzsprung-Russel diagram (HRD). On a HRD, absolute magnitude or luminosity is plotted increasing up the vertical axis (i.e. most luminous stars near the top) and spectral type or temperature increasing to the left is plotted on the horizontal axis (i.e. hottest stars on the left), as shown in Figure 1.1.
Question: Why is it possible to replace absolute luminosity with apparent magnitude when studying a star cluster?
Answer: All the stars are at roughly the same distance and suffer from the same amount of stellar extinction. Therefore the apparent magnitude, m, of all the stars is related to their absolute magnitude, M, by a simple offset, according to: M = m + (5 - 5 log10(d/pc) - A) where d is the distance to the cluster and A is the interstellar extinction of the cluster in question. The absolute magnitude of each star in the cluster is proportional to the logarithm of its luminosity.
Question: Why is it possible to replace temperature with a colour index such as (B-V) when studying a star cluster?
Answer: Magnitudes obtained through different filters, such as B and V, sample different parts of the stars black-body like continuum spectrum. A difference between two apparent magnitudes is proportional to the ratio of two fluxes. Since all the stars are at the same distance, this is also proportional to the ratio of two luminosities in different parts of the spectrum. Since stars with different temperatures will have different spectra, they will also have different colours.
Hertzsprung-Russel diagrams and colour-magnitude diagrams are vital tools for understanding the evolution of stars and their composition. HRDs produced from theoretical calculations can be tested against observational HRDs to judge the accuracy of a particular theory. As you will see later, CMDs of star clusters can be used for distance determination purposes and also indicate the ages of different stellar populations.
Goals
The aim of this "advanced lab course" is to construct colour-magnitude diagrams (CMDs) for star clusters (open and globular clusters). The targets for the observation should be planned ahead of the observation. The analysis of the stellar clusters should reveal properties of the stellar population of the observed cluster. The analysis is conducted using a widely used, standard tool for image preocessing in astronomy, IRAF .
The objectives are: ( Maybe add some objectives)
To prepare a schedule for the observations you want to do during the night. What calibration frames do you need? Choose observable star clusters (at least one globular and one/two
open cluster) for this night. Obtain CCD images during the night for your objectes through at least two different broad-band filters and write a night log.
Analyse your images by first creating a general correction frame for each filter (combined bias frames, dark frames and flat fields)
Perform apeture photometry on the stars in each cluster and obtain magnitudes (/!\ reative magnitude or calibrated (standard field) absolute magnitude?
Produce a CMD for each cluster and discuss it (ages and distances).
anything else?.
Preperation
Before starting with the above objectives you should familiarise yourself with the important technical terms. You should be able to describe the telescope and its components, also the basics of data reduction should be known.
This project is concerned with stellar photometry - that is measuring the apparent magnitudes of stars through one or more astronomical filters. The targets of observation are star clusters, which means that many individual stars of interest (typically tens or hundreds of stars) will be contained within each target image that you obtain. As well as obtaining images of the clusters themselves you will need to obtain appropriate calibration frames to correct for the bias signal and dark current, and to carry out a flat-field-correction.
original version goes on as follows: However, in this project you will not be asked to measure standard stars in order to determine the extinction coefficient epsilon and zero-point offset chi to calibrate magnitudes. Instead you will use a reference star of known magnitude in each target image, and determine the magnitudes of other stars relative to this reference point.
In order to determine extinction coefficient epsilon and zero-point offset chi to calibrate magnitudes, you will also observe a standard field. A standard field is a set of stars with known/tabulated absolute magnitudes.
The quickest part of this project is likely to be actually taking the images of the star clusters. Planning the observations, including which targets to observe when, which calibration frames to take, etc. and analysing the data, are each likely to take far longer. It is therefore vital to plan how your group will carry out thins project, including who will do what and when.
1.4 Preparing for observations: Changed, because the students should find targets on their own. What program to use? What catalogues can be used with the telescope (Messier, NGC)?
As a first task you have to find suitable objects for the night you are going to observe. Keep in mind that the night sky changes with season and also during a night, objects rise and set again. The following criteria should be met by the objects you choose:
- at least one globular cluster and one open cluster
- the objects should be visible between dusk and midnight, which should be the time frame for the observations
minimal airmass, meaning a high declination (
correct?)
You should prepare a time schedule for your observations including taking calibration frames. At this point you should talk to your tutor and discuss your schedule.
Observation
1.5 Data taking: Changed, because the setup is different to the one used in The Open University description
Getting started with the telescope (
there is a wikipage for that already? Link is missing) - Taking Flats with ACP?
- Taking Darks and Bias frames
- Adjust focus
- Observe an object and check maximum count, avoid saturation
- Start observational sequence for this object
- Continue with other objects including standard field
Following is taken form 1.5 Data taking
For each cluster you will need to obtain two images, or sets of images: one through the B-Filter and one through the V-Filter ( We don't use Johnson filter). The integration times you use will be long enough that faint stars are detected, but not so long that your reference stars saturate the detector ( We won't use reference stars, but a standard field calibration?), or the field drifts significantly during the exposure ( not relevant for our telescope, but can be checked by looking at the raw images). You may want to experiment with exposures 5 s, 10 s and 30 s for instance. You can also average multiple images of the same field to obtain longer effective exposure ( Sentence should be adopted to our setup). Don't worry if 3 or 4 of the brightest stars in the image are saturated - you can look up the magnitudes of these bright stars at CDS - it is far more important that the majority of the stars are adequately exposed and that your 9th and 10th magnitude reference stars are not saturated ( Same with this sentence). To make the data reduction easier, it is simplest if all your images through the same filter have the same exposure time ( True for IRAF?).
It is good practice to obtain bias frames and dark frames before and after your target observations, or interspersed between them. Also, it is most convenient if your dark frames have the same exposure times as your target frames, as this saves having to scale them later.
Remember that you will need to take flat fields through each of the filters during dusk.
As you obtain each target image or calibration frame, verify that it is adequate for its purpose by quickly examining it on screen. If the image drifts during exposure (which should never happen with our telescope), if the stars you are interested saturate, if they are underexposed, or the image is otherwise unsuitable, then repeat the observation until you obtain one that is satisfactory. When you have a suitable image make sure you save the file and note down the details in your observing log. ( This is true if we use MaximeDL, but might be different with ACP. A decision should be made.)
Analysis
Log on to your account on the Linux machine via VNC. The tutor should have copied the raw images to your working directory. As a first step it is useful to produce a local backup in case you need to start over.