C.A.R.A. PROCEDURE

 

INSTRUMENTATION

 

To make a correct photometry you should prefer telescopes corrected for all three colour bands, therefore all reflector types are a good choice. A particular care must be taken in using lens type telescopes, actually you should consider only apochromatic ones. Acromatic ones could be used only either with R or I or interferential filters. These filters reduce, but do not eliminate the chromatism of this kind of telescopes.

It is recommended to use a CCD camera without antiblooming because its chip is linear. ABG camera can also be used if you know the sensitivity curve of your sensor therefore limiting the pixel saturation within the linear range of the curve (AAVSO CCD Observing manual suggests to limit pixel saturation to 50% of the well depth of the sensor).

It is highly recommended the use of at least one photometric filter either Cousin R or Cousin I ranges. Additionally an interferential type filter in the range of 647 to 650 nm (with at least 10 nm FWHM) can be used to isolate better the dust component if the comet is sufficiently bright.

 

SOFTWARE

 

It is necessary to have at least the following software packages installed:

 

PREPARING AN OBSERVING SESSION

 

Preparing an observing session is extremely important so you can take some decisions allowing standardization in procedures.

1.                  if the night is good (i.e. without passing clouds) and the comet is sufficientely high over the horizon (at least 20°), in general you shall select an Hipparcos reference star with a colour index B-V between 0.5 and 0.7, possibly in the main sequence (V class). Considering that it could be difficult to find such a star in the same field as the comet, it will be necessary to take a separate frame of the reference star with an exposure time as long as to avoid saturation . This star shall be selected among those  not farther than 3°    from the comet and possibly at its same elevation. If it is not possible to use an Hipparcos star or there is the suspect of thin clouds, you can use a Tycho star with equivalent characteristics. It should be better to use multiple reference star whenever possible.

2.                  If the comet and the star are at different elevations from the horizon and are below 20° above the horizon itself, it is mandatory to take into account the atmospheric extinction (see item 5 for details). However if the night is not stable it is possible to follow the procedure as below outlined (item 3).

3.                  If the night conditions are not good, it is better to try to “point” the comet in such a way as it is possible to get either the comet and a reference star on the same frame. In this case it is possible to select even a Tycho cat star whose data shall be taken from Vizier site available from the internet. Also in this case the colour index shall be in the range 0.5 to 0.7 (Winafrho software can connect directly with Vizier site).

4.                  It is necessary to verify comet proper motion with the software to determine its proper motion in arcsec/min. Therefore knowing the resolution of your telescope + CCD system, it is possible to find the exposure time corresponding to a maximum of 1 pixel displacement.

5.                  The atmospheric extinction is mainly caused by the airmass you are looking into. Therefore is responsible for the difference in magnitude, respect to the catalogue, of two objects placed at different elevations above the horizon. Since we are dealing with differential photometry of two object not far one from the other (less than 3° as per item 2) on the sky, it is sufficient to determine the effect of the extinction with respect of only one of the two. Let’s have a star at an elevation greater than the comet, then the star shall appear brighter (less atmospheric extinction). Therefore we shall have:

Dm = (secZ1-secZ2)*kx

Where we indicated with:

Dm = magnitude difference

Z1 and Z2 = zenith distances of the two objects

kx = constant characteristic of each photometric range (it should be determined each observing night).

However considering that the zenith distances of the objects are not so different (within a few degrees), it is possible to use average standard values without introducing big errors. The indicative values for each separate photometric range (V, R , I) are as follows:

§         KV = 0.22

§         KR = 0.14

§         KI = 0.10

If the star is higher above the horizon with respect to the comet, then Dm shall be subtracted from catalogue magnitude. On the contrary if the stat is lower it shall be added up. The value of the magnitude corrected for extinction shall be used within Winafhro instead of the catalogue one.

 

OBSERVING SESSION

 

Once decided either the exposure time and the comparison star, it is possible to go ahead in our observing session. If we decide to take several frames of the comet, it would be better to do sequences separated by frames shot on the comparison star (e.g. comet_01, comet_02, …, comp_01, comet_11, comet_12, …,comp_11,..). In any case, also to verify that the sky condition are not varying, it is better to take comp star frames at least twice, before and after the comet.

It is necessary to take a sufficient number of frames such as obtaining a “good” S/N ratio, avoiding a too low signal that could give false readings either in measures and in image processing.

 

DATA REDUCTION

 

Once saved the images on your HD, each image shall be corrected either for dark frame and flat field. Then you shall average combine (not median combine) the images so that at the end of the process you get one image of the comet and one image of the comp star for each sequence. Open “Winafrho” software, select the observed comet, input the date, the time of exposure and filter used. In the filter menu there are several options to be selected:

§         “Vu”     shall be used for observations done without filter with Sony chips

§         “Ru”    shall be used for observations done without filter with Kodak type chips

§         “Bc”    blue continuum interferential filter (not yet well calibrated)

§         “V”       Johnson V filter (usually not indicated for comet observations)

§         “R”      Cousins R filter

§         “I”        Cousins I filter

§         “S”       Vilnius S range filter to be selected when using a narrow band red continuum filter (647 – 650 nm)

 

However it is recommended, as already written, the systematic use of at least one band filter.

 

Press the “automatic window size” button. In the program window you will find indicated the dimension in pixel of the measuring square annulus. The program default indicates only three values. If the observed comet is very active, it should be better using at least five different values. In this case it is possible to add manually intermediate values with respect to those given by the software. A good rule is to use an odd number of pixels as aperture window.

The length of the exposure of either the comet and the comp star shall be written in the relevant dialog boxes.

 

Then it is time to select the reference (comp) star. If this star is included in the Hipparcos catalogue then it will be possible to use directly the data base enclosed with the software. On the contrary, if it is a Tycho star, before using the data base enclosed, it is better to check Vizier site for additional data. If the B-V colour index of the star is included between 0.5 and 0.7, then press “compute colour index DWARF”. “GIANTS” button shall be used only if we are certain that the selected star is a giant. However even if you select “dwarf” the maximum error that could be introduced is much less than 1%.

 

Open the comp star file. First of all select an area without any star or in any case very poor of them, so that the program can calculate the background flux (in ADU). Then select the photometric centre of the reference star and indicate the aperture window to measure the star. It shall be at least three times the FWHM of the star itself. The program then calculates the flux of the reference star. Go back to the main window.

 

The same procedure will be used for the comet file with the only difference that the software defines the apertures to be used directly. If you have more than three measures, you shall take a note of the values of the background and of the coordinates of the comet used to calculate the additional values.

After obtaining the flux (in ADU) of the comet and coming back to the main window, we can press “afrho compute”. The software reduces the input data and visualizes the result that can be saved either appending to an old .dat file or creating a completely new one.