Cometary Archive for Afrho (CARA) method of astrometry and photometry

 

Updated 2017 January 17

 

Contents

 

Introduction

 

The main purpose of the project is to derive a value for Af(rho), or Afρ, which is a measurement used to study dust in cometary comae. Potential participants should read the following before further involvement;

 

- Welcome to CARA Project website at http://cara.uai.it/home

- Policy at http://cara.uai.it/policy

 

The two main points to note in the Policy are items 3 and 7 i.e.;

 

3.         Observations must be produced according to our adopted method in order to grant a high degree of standardization and an internal consistence of the database.

7.         Data concerning specific observing campaigns are reserved until the results are published.

 

There is a CARA mailing list at http://it.groups.yahoo.com/group/COMETS-CARA/

 

Joining the project

 

To participate use the form on the CARA website at http://cara.uai.it/contact

 

If you are considering joining the project then I strongly recommend reading the various web pages listed under ‘About us’, Science’ and ‘Observing comets’ at http://cara.uai.it/home

 

The Wafrho software and is available to observers once they have joined the CARA project.

 

The CARA method

 

See also http://cara.uai.it/cara-photometry

 

I would like to thank Erik Bryssinck for his help in guiding me through my first attempts at using this method.

 

1) Filters

 

Common practice is to use a Cousins/Bessel R filter but note that exposure times using such a  filter may be 3x as long as those taken unfiltered to obtain a similar SNR. I and narrow band filters can also be used. See http://cara.uai.it/filters for more information on this subject.

 

2) Targets

 

Observations of all comets are accepted by  CARA but specific comets are the subject of  observing campaigns. Such comets will usually be mag 12 or brighter as af(rho) only becomes important at that brightness. Target comets should normally be 30 degrees or more above the horizon.

 

3) Reference stars

 

Comet magnitudes are calculated by comparison to reference stars adjacent to the subject comet. Check for suitable reference stars using Wafrho before setting up observations.

 

The choice of a good reference star is VERY important for producing good afrho-measurements. Such a star meets the following requirements;

- the stars (B – V) color index must be between 0.5 and 0.7 (or 0.4 and 0.8 if none in that range are available) 

- magnitude of the reference star versus error in magnitude: about 9 and 10 but the magnitude error must as low as possible.

- magnitude error of ref. Stars must be < 0.05 but if no reference star, with a margin of error smaller than 0.05 magnitude can be found this parameter can be 

  increased to 0.1

 

4) Stacking images

 

Comet images can be stacked using Astrometrica to improve SNR.

- use average when stacking and the exposure time of a single image in the stack (e.g. if 3x60sec exposure images are stacked, 60sec will be used in Wafrho)

or

- use add when stacking and the sum of the exposure times of the images (e.g.  if 3x60sec exposure images are stacked 180sec will be used in Wafrho)

 

Using the Wafrho software

 

This example uses an image obtained on 2014 April 28 using the SSON robotic telescope. 6 x 60 sec images were obtained through a John-Cousins R filter. This image, 1_211807.fts, can be downloaded via the images page

 

2012K1_20140428_0728_dymock

 

1) Defining your Observatory Card

 

In the Main dialog window above choose Option/Set Default Value which brings up the window below. This has been filled in with data relating to the SSON robotic telescope. Data for several observatories can be entered by clicking on the down arrow next to the Select Observatory and the selecting Observatory data 01 or 02 or 03, etc. Once all the data has been entered click on Save Default Option and EXIT. The relevant observatory can then be selected by choosing the required number under CARA code in the Main dialog window.

 

 

2) Completing the Main dialog window

 

Select the correct observatory under CARA code (01)

Enter;

- Date/Time

- Binning (x2 adjacent to Res. X/Y arcsec/pixel boxes)

- Photometric band (R)

- Comet (C/2012 K1 (PanSTARRS)

- Seconds of exposure (60 under Values for comet)

 

 

3) Selecting a suitable reference star

 

In the Main dialog window select NetRequest, Hipparcos/Tycho/APASS@Vizier and then UCAC4. this brings up a list of stars from which the higlighted star was selected. This meets the necessary criteria i.e. a (B – V) color index between 0.5 and 0.7 (or 0.4 and 0.8 if none in that range are available) and a magnitude error of  < 0.05.

 

It is good practice to repeat the process at least twice and preferable four times using different reference stars to confirm results. All results can be included in a single file and sent to CARA as explained below.

 

 

 

The selected reference star is shown on the Megastar chart below.

 

 

Select the star then Append selected Stars to Checklist and EXIT

 

Bring up the checklist by clicking on ViewSelected/View CheckList in the Main dialog window and double click on the selected star to transfer data to Main dialog window.

 

 

In the Main dialog window enter the Seconds of exposure for the star  (60 secs) under Values for reference star

 

4) Calculating the ADU of the reference star

 

- select ViewFit Images/ViewReferenceStar and open the reference star image – screen shot below

- adjust the image brightness, etc by using the sliders in the bottom left hand corner of the window

- to select background area. draw a small rectangle in an area of the image devoid of stars with the mouse and click on Background.

- select photometric centre. Click on Obj. Center and Selection and locate the centre of the reference star using the small window to the left of the aforesaid box. A

  circle appears around the cursor when it is correctly positioned

- enter a window size (12 pixels)

- click on Compute ADU  (if rectangular window does not enclose whole of reference star enter a larger number in the From box and recompute

- click on Return values to update Main dialog window.

 

 

To calculate ADU for the comet

- select ViewFit Images/Viewcomet and open the comet image (same as ref star image)

- adjust the image brightness, etc by using the sliders in the bottom left hand corner of the window

- select background area

- select photometric centre

- select pixel window size to give Eq. Radius (km) equal to approx. 100000k (From 210 To 10 step 10)

- click on Compute ADU Values and then Return values to update the Main dialog window.

 

 

In the Main dialog window click on Press for af(rho) values

 

 

Plot the af(rho) dataset by clicking on Plot current af(rho) dataset

 

 

Save data file as 20140428DYM01CK12K01.CSV

 

Data file imported into Excel and edited to remove blank cols and add col headers

 

Comet

Julian Date

 Earth distance (AU)

Sun distance (AU)

Phase angle

Filter

Mag

Eq radius

Af(rho)

Error

Cat

CARA code

Ref star

Date, CARA code, Comet

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'9.760'

'0101617'

'00001645'

'00082'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'9.792'

'0096778'

'00001677'

'00084'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'9.833'

'0091939'

'00001699'

'00085'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'9.872'

'0087100'

'00001731'

'00087'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'9.907'

'0082261'

'00001774'

'00089'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'9.945'

'0077423'

'00001820'

'00091'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'9.986'

'0072584'

'00001870'

'00094'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'10.03'

'0067745'

'00001924'

'00096'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'10.08'

'0062906'

'00001979'

'00099'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'10.13'

'0058067'

'00002042'

'00102'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'10.19'

'0053228'

'00002111'

'00106'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'10.26'

'0048389'

'00002182'

'00109'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'10.34'

'0043550'

'00002259'

'00113'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'10.43'

'0038711'

'00002339'

'00117'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'10.54'

'0033872'

'00002415'

'00121'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'10.67'

'0029033'

'00002490'

'00125'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'10.84'

'0024195'

'00002560'

'00128'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'11.05'

'0019356'

'00002629'

'00132'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'11.34'

'0014517'

'00002693'

'00135'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'11.76'

'0009678'

'00002734'

'00137'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

'CK12K01'

'2456775.81262'

'1.483'

'2.141'

'24.70'

'R '

'12.57'

'0004839'

'00002594'

'00130'

UC4

'DYM01 '

'UC4 692-052106  '

'20140428DYM01CK12K01'

 

Submitting results

 

The CSV files should be uploaded to the files section of the CARA mailing list at http://it.groups.yahoo.com/group/COMETS-CARA/ If a folder for the comet you wish to contribute data to exists then use that one - if not create a new one. An Add file screen shot is shown below. Select your file using the ‘Choose File’ button and add a description e.g. ‘Af(rho) – R Johnson-Cousins – 20120609DYM010029P’. Ticking the ‘Send a message…’ box will post a message to the group letting everyone know that a new file has been updated.

 

 

Other features

 

Ephemeris

 

Select a comet using the drop down box under Values for comet in Main dialog page.

Select Orbital elements/Ephemeris generator and input; from date, step (Days) and items and then click on COMPUTE. Example below.

 

 

Orbit data

 

Select a comet using the drop down box under Values for comet in Main dialog page.

Select Orbital elements/Orbit data editor and update as required. Example below.

 

 

Orbit diagram

 

Select a comet from the drop down list in the Main dialog window.

Select CometModel/View orbit. The viewing angle, objects included and other parameters can then  be selected. Example below.

 

 

Comet data file

 

The latest comet data can be downloaded from the MPC (recommended). Select NetRequest/Comet data file @ MPC, click on CONNECT  in the resulting window  (below) to obtain the Soft00Cmt.txt file from the MPC. Click on SAVE as file and save in the Wafrho folder as Soft00Cmt.TXT.

 

 

 

Contents