CCD Astrometry and Photometry

 

Updated 2014 January 15

 

Contents

 

Introduction

 

 

Important note

 

Recent reports show that this procedure does not give correct visual equivalent magnitudes for the brighter comets. I therefore recommend that if your FoCAs multibox reports are approaching mag 10 for the larger box sizes that you do not process the results with kphot and do not report the resulting total magnitude.

 

While this is a somewhat lengthy process it is not difficult to follow through. By doing so, observers will be able to extract a significant amount of useful data from CCD images.

 

The objectives are  to;

- measure the position and magnitude of the comet (using Astrometrica and FoCAs)

- to obtain a visual equivalent magnitude using Kphot

- measure the coma diameter using AIP4WIN

- to measure tail length and position angle using Astrometrica or Aladin

- to report the relevant measurements to the; MPC, Cometas Group, COBS database and BAA Comet Section and the ICQ

 

Generating visually equivalent magnitudes and comae diameters from CCD images will enable;

- CCD observations to be integrated with visual observations

- lightcurves for a greater part of a comet’s orbit to be produced

 

Obtaining CCD images of comets is covered in a separate tutorial. Images must have been obtained using a clear or no filter and calibrated (application of dark frames, bias frames and flat fields).

 

Processing of red or narrow band filtered CCD images, the CARA process, will be covered in a separate tutorial

 

This tutorial will cover

- processing unfiltered CCD images with Astrometrica, FoCAs and Kphot

- reporting results to the Minor Planet Center, BAA Comet Section, Cometas Group, COBS database and ICQ

 

The images (1_118702 – 6) used in this tutorial can be found at Images. If you process them as described here then you can be satisfied that your results are correct (or that we have both made the same mistakes !!!) Please note that some of the screen shots have been changed to reflect new versions of the software used so may not relate to the images mentioned above.

 

The procedures described here are applicable to both known and newly discovered comets. If you are the one making the discovery then accurate astrometry and photometry, obtaining confirmation of your discovery and reporting it in a timely manner is vitally important but all of that will be the subject of a separate tutorial.

 

The flowchart below shows the overall process. Please note that Astrometrica, FoCAs and Kphot must be used in that order. The dotted arrows connecting Kphot, AIP4WIN and Astrometrica or Aladdin indicate a suggested sequence but any applications that will achieve the same ends will suffice.

 

Observers are requested to follow the procedure through all its steps but may of course not wish to do so. The alternatives are;

1) Process images with Astrometrica and generate MPC reports from Astrometrica

2) Process images with Astrometrica and FoCAs and generate MPC and multibox reports from FoCAs

3) As in step 2 and also process images using Kphot plus the other applications shown in the flowchart below to produce a complete ICQ report

 

Checklists may help you through the procedure and to record the necessary data for input to the Comet Observation Database (COBS). Three versions are available;

- a blank which you can use to record your own data

- a preformatted example of mine which includes the information common to all observations I make using one of the Sierra Stars Observatory Network’s  telescopes

- a completed example with data listed in the order it is input to COBS

 

 

CCD imaging

 

MPC Observatory Code

 

You will need an MPC Observatory Code prior to reporting CCD astrometry and photometry to the Minor Planet Center. It is not actually required to report the same to the BAA or ICQ however obtaining one would give others greater confidence in your reports. How to do this is described in detail in the MPC’s ‘Guide to Minor Body Astrometry’ in particular paragraphs 15 and 16. It is also covered in my book ‘Asteroids and Dwarf Planets and How to Observe Them’, Chapter 10.

 

Astrometry and Photometry

 

Please note that this procedure describes how to process the images using Astrometrica so that they can be further processed using FoCAs and Kphot.

 

The above flowchart shows MPC reports emanating from both Astrometrica and FoCAs. Such reports need only be sent from Astrometrica if  no further processing is to be carried out. Under normal circumstances MPC and multibox reports will be sent from FoCAs.

 

Astrometrica

 

The Astrometrica homepage is at http://www.astrometrica.at/ There are links on this page to Downloads and Registration which costs Euro 25 (very well worth the price). Tutorials are included in the downloaded software under Help.

 

It is advisable to keep your comet (and asteroid) files up to date.

- to download the latest comet elements start up Astrometrica and go to Internet/Update MPCCorb and select Comets

- to download the latest asteroid elements start up Astrometrica and go to Internet/Download MPCorb

 

Catalogues

 

Images must be processed twice with Astrometrica. Use UCAC 4 (the preferred MPC catalogue) for astrometry first (USNO-B.1 was used here as I did not have access to UCAC-4 at the time) and CMC-14 or USNO-A2.0 for photometry second. The lower magnitude limit for CMC-14 reference stars can be adjusted from 14 to 18 to increase the number of ref stars. Only uses USNO-A.2 if not enough reference stars are available using CMC-14 even with the lowered mag limit. If you get a message ‘No reference stars read’ when using the CMC-14 catalogue it means that the catalogue does not cover the area of sky where the comet is located. In such cases use the USNO-A.2 catalogue.

 

This tutorial was constructed using the USNO-B.1 catalogue but, as mentioned above, the UCAC 4 catalogue should be used for astrometry.

 

Configuration

 

The screen shots below are my settings for using the Sierra Stars Observatory Network telescope and USNO-B1 catalogue. The CMC-14 settings are fairly similar so I won’t repeat the screen shots here. Example configurations for the SSON USNO B-1 and SSON CMC-14. These can be saved in your Astrometrica folder and then modified to reflect your own situation.

 

 

If changing the observing location in Astrometrica then make change and reload Astrometrica or the MPC observatory code in FoCAs reports will be incorrect

 

 

 

Aperture radius should be set to approximately 2 x FWHM which is given in the Object Verification window below. The aperture circle does not need to encompass all of the head of comet.

 

The lower (fainter) magnitude limit is set to 14, for greater accuracy, when using the CMC-14 catalogue. FoCAs requires that at least ten reference stars are used so, if  Astrometrica uses ten or less when using the CMC-14 catalogue, then adjust the lower limit to include fainter stars. You will have to restart Astrometrica for this change (and any other) to take effect.

 

Under Program/Reference Star Matching and Image Alignment select a large number of stars to avoid incorrect alignment e.g. at least 50. A clue that all is not well is if the images jump around the screen when blinking them.

 

 

Note that some catalogues are available on-line while others have to be stored on your PC.

 

 

 

MPC reports must include the magnitude as measured by Astrometrica which is the nuclear or m2 magnitude. Do not report magnitudes as generated by kphot which is the total, visual equivalent or m1 magnitude.

 

Image processing

 

If results are to be further processed using FoCAs do not stack images as FoCAs cannot handle these.

 

The first run is carried out using the USNO-B 1.0 catalogue (UCAC-4 should now be used) to produce the astrometric results. Open the required images and then click on the Astrometry button. The screen shot below shows the Data Reduction Results so obtained.

 

 

Place the cross hairs over the head of the comet (do not use the Control key when doing this) and click the left mouse button. Click the button under Object Designation and select the comet from the list. If  there is a large error (dRA and/or dDe grater than say .02’) then you may have the wrong object (or your MPCORB catalogue is out of date). If other nearby asteroids or comets are listed then these can also be measured (if visible on the images). It is always worth blinking the images to see if any ‘unknowns’ turn up. In addition the Known Objects Overlay under Tools will, as its name suggests, mark known minor bodies.

 

You can process the images by first blinking, stopping blinking and then stepping through them using Control/F10. When all images have been processed resume blinking and you will see the comet, tagged by it’s ID, moving across the images.

 

Ways of checking images for asteroids and comets will be covered in the Discovery tutorial.

 

 

Clicking Accept will add the comet ID to the image and generate an MPC report. After processing all the images (five in this case) the completed MPC report can be viewed by selecting File/View MPC Report File – see below.

 

 

Now change to the CMC-14 catalogue (or USNO-A2.0) and repeat the above process to generate the photometric results. The conversion formula applied by Astrometrica to derive the Cousins R magnitude from CMC 14 r’ magnitude is: R = r' - 0.22

 

In this particular instance there were not enough CMC-14 reference stars so USNO-A2 had to be used.

 

The completed MPC Report now looks like (N indicates a nuclear magnitude – m2);

 

 

If you do not wish to go further into this process (and we hope you will) then combine the photometry from the CMC-14 (or USNO-A2.0) lines with the astrometry from the USNO-B 1.0 (UCAC-4 should now be used) lines, see example below, and submit the report to the MPC and The Astronomer (for onward transmission to the BAA Comet Section).

 

    CK11F010  C2012 07 05.21193 14 03 13.69 +42 40 18.6       14.7    G68

 

Astrometry USNO-B1.0 input,

Photometry USNO-A2.0 input

 

The report can then be submitted to the Minor Planet Center using the Internet/Send MPC Report option.

 

FoCAs

 

Recent reports show that this procedure does not give correct visual equivalent magnitudes for the brighter comets. I therefore recommend that if your FoCAs multibox reports are approaching mag 10 for the larger box sizes that you do not process the results with kphot and do not report the resulting total magnitude.

 

The Cometas homepage is at http://www.astrosurf.com/cometas-obs/. There you will find links to a description of  FoCAs and the multiaperture method of photometry. A FoCAs manual, including the installation procedure, can be found at http://www.astrosurf.com/orodeno/focas/  These pages are in Spanish but a Google translation works reasonably well (sometimes it does and sometimes it doesn’t !!!). A description of the mult-aperture method, in English, can be found here. In simple terms FoCAs uses Astrometrica data to calculate magnitudes for various size circular apertures as listed in the multibox report below.

 

Having completed the Astrometrica processing start up FoCAs. If asked delete any previous MPC and mutlibox data. Clicking on Images brings up the following screen which confirms that five images have been processed in Astrometrica with both catalogues.

 

 

Click on Select valid images which highlights all five images and then select Process and the result is;

 

 

The two graphics on the right of the window show position (top) and magnitude (bottom). These should both be more or less straight lines as the images were taken over a short (2 hour) period. Any major deviation is likely to indicate a measurement problem. If any measurement is out-of-line then I suggest you reprocess your images using both Astrometrica and FoCAs omitting the troublesome ones. This is because the mutlibox report is calculated from the average of all the images and not just the ones you select for the MPC report.

 

Select Object class/Comet in Graphical analysis window.

 

Highlight all the results and click on Save report. Check the MPC report (below) to ensure the headers are correct (in particular the observatory code) and that any COM field is correct. In this case the COM field was used to indicate that the catalogue used for photometry was different from that shown on the ‘NET’ line.

 

 

View and save MPC and Multibox reports under object name, date and mpc or multibox e.g.;

 

C2013 E2 2013 Aug 13 mpc.txt

 

 

C2013 E2 2013 Aug 13 multi.txt

 

 

The upper aection of the above screen shot shows; magnitudes and errors calculated using various size apertures are shown together with Signal to Noise ratio, MPC code, catalogue used.

 

The lower part displays; distance of comet from the Earth and the Sun, (to be completed when BOX explained)

 

Select Internet (MPC and Multibox) to send your report to the previously set up email addresses;

 

 

It is advisable to check results against previous measurements in the Cometas database at  http://www.astrosurf.com/cometas-obs/ Of course it is possible that you may have, for example, been lucky enough to record an outburst but significant deviations from previous measurements or their trend should be reexamined (BUT NEVER ALTERED MERELY TO CONFORM).

 

 

Lightcurve for C/2011 F1 (LINEAR) from the Cometas website.

 

Kphot 

 

The purpose of Kphot is to generate visual equivalent magnitudes so that CCD observations can be used for analysis together with visual observations. It also allows the comet to be monitored over a larger part of its orbit when it would be too faint for visual observations.

 

Kphot was written by Uwe Pilz and it is with his help that I have written this description of how to use it. Kphot can be downloaded from

http://kometen.fg-vds.de/kphotsoftware.zip Create a folder named Kphot and unzip the downloaded file to that folder.

 

In addition;

- .NET software needs to be installed and can be obtained from http://www.microsoft.com/net

- a comets.dat file must be downloaded from http://www.minorplanetcenter.net/iau/Ephemerides/Comets/Soft02Cmt.txt, saved as comets.dat and subsequently referred to in the Kphot command line – see below. There are a number of comet files in different formats on the MPC website so make sure you download the correct one. If the comet for which you are processing data is not in the dat file you will get a (calculated) message as shown in the example below. Updating the dat file and rerunning kphot resulted in the correct output (the Afρ value need not concern us here).

 

COD I81

OBS Denis Buczynski

CATALOG: USNO A2.0 / CMC-14 - BAND: R

 

                                   10x10  20x20  30x30  40x40  50x50  60x60   SNR   SB   COD

OBJECT        DATE       TIME        +/-    +/-    +/-    +/-    +/-    +/-     N  FWHM  CAT

------------  ---------- --------  -----  -----  -----  -----  -----  -----  ----  ----  ---

C/2013 R1     26/09/2013 02:42:52  13.97  13.12  12.66  12.33  12.05  11.82  19.5  17.1  I81

C/2013 R1     26/09/2013 02:42:52*  0.01   0.03   0.03   0.04   0.05   0.05     5   4.4  CMC

 

FoCAs 3.36

www.astrosurf.com/cometas-obs

es.groups.yahoo.com/group/Cometas_Obs

   2013R1  2013 09 26.11  Z 10.8 MC 30.5T 6      04.88                     HAE             (calculated)

   2013R1  2013 09 26.11  Z 10.8 MC 30.5T 6      04.88                     HAE         Afρ=507 cm

 

Kphot uses FoCAs output to generate total magnitude, m1,  (which is what a visual observer would see and measure) the starting point thus being the multibox report which lists the magnitude of the nucleus, m2 i.e.;

 

COD G68

OBS R.Dymock

CATALOG: USNO A2.0 / CMC-14 - BAND: R

 

                                   10x10  20x20  30x30  40x40  50x50  60x60   SNR   SB   COD

OBJECT        DATE       TIME        +/-    +/-    +/-    +/-    +/-    +/-     N  FWHM  CAT

------------  ---------- --------  -----  -----  -----  -----  -----  -----  ----  ----  ---

C/2011 F1     05/07/2012 06:05:03  14.72  13.78  13.34  13.06  12.91  12.82  25.7  18.1  G68

C/2011 F1     05/07/2012 06:05:03*  0.04   0.06   0.08   0.08   0.09   0.09     5   3.6  CMC

 

The FoCAs output above must be saved as kp.txt in the Kphot folder.

 

Open a DOS/Command Prompt window and change to the Kphot folder. Usually something like cd c:\kphot

 

In the command line below;

-m 8 indicates the 8th line of input (above) is to be used

-n 6 is the number of multibox results (10x10 to 60x60 in this case)

-T MC 61.0C 10 is; T for telescope type, MC for CMC-14 catalogue, 61.0C for SSON 61 cm Cassegrain telescope, 10 is f no.

-B DYM is observer code (1st 3 letters of Dymock)

-g c:\kphot\comets.dat is location of  the comet.dat file

 

The command line must start with the full path to kphot.exe i.e. c:\folder name\sub-folder name\kphot\kphot etc. Mine just happens to be c:\kphot\kphot etc. Similarly the path to the comets.dat file must be input in full.

 

 

The output is added to the input file kp.txt in the Kphot folder;

 

COD G68

OBS R.Dymock

CATALOG: USNO A2.0 / CMC-14 - BAND: R

 

                                   10x10  20x20  30x30  40x40  50x50  60x60   SNR   SB   COD

OBJECT        DATE       TIME        +/-    +/-    +/-    +/-    +/-    +/-     N  FWHM  CAT

------------  ---------- --------  -----  -----  -----  -----  -----  -----  ----  ----  ---

C/2011 F1     05/07/2012 06:05:03  14.72  13.78  13.34  13.06  12.91  12.82  25.7  18.1  G68

C/2011 F1     05/07/2012 06:05:03*  0.04   0.06   0.08   0.08   0.09   0.09     5   3.6  CMC

    2011F1  2012 07 05.25  Z 11.8 MC 61.0C 10      03.64                     DYM         Afρ=1354 cm

 

2011F1                        Comet designation

2012 07 05.25             Date and time

Z                                  Magnitude method key (Z = visual equivalent brightness calculated from CCD images)

11.8                             Total magnitude

MC                              Magnitude reference key (or catalogue used) – Carlsberg Meridian Catalogue (CMC-14)

61.0C                          61.0 for SSON 61 cm telescope and C for Cassegrain

03.64                           Coma diameter
DYM                           ICQ observer code 

Afρ=1354cm                Measurement of dust in the coma – see here and here

 

The results generated by Kphot can be seen at http://kometen.fg-vds.de/obsaktinh.htm#kphot It is not our intention however to go down this route. Kphot will be used to generate only the visual equivalent magnitude which will be used to input to COBS and generate ICQ reports.

 

AIP4WIN

 

Using the Profile tool you can measure the diameter of the coma. Open an image of the comet, select Measure/Profile tool and draw a line through the comet. Selecting Auto Min/Max in the Line Profile Tool window produces the trace shown. Save the profile by clicking on Save Profile in Data Log.

 

Note. Measure what you see and do not be swayed by what others have reported. Longer exposures, varying apertures and focal ratio combinations will give varying diameters from system to system. That is why it is important that accurate focal ratios, apertures, exposure times and pixel size be reported.

 

 

The data can then be imported into Excel (or any other spreadsheet software) and the profile generated using pixels for the x-axis and ADUs for the y-axis. Measure the diameter of the coma between the points where the curve merges with the background as indicated by the arrows in the diagram below.

 

The coma diameter  is then calculated by multiplying the number of pixels by the plate scale in arc secs/pixel. In this case;

 

Coma diameter = (130 x 0.8)/60 = 1.7 arc mins.

 

If the line is drawn at an angle to the horizontal then this must be factored into the measurement. For example if the line were to be drawn at 45º then the multiplier would be 0.8/sin 45º =  1.1 arc secs/pixel

 

 

Tails

 

On 2013 February 4 I imaged comet P/2012 B1 (PanSTARRS) with the SSON telescope. This comet had developed a small tail as shown in the (stack of five) image below 

 

 

There are a number of ways of measuring tail length and position angle but, since we have used Astrometrica in this tutorial we will use that to do so. You can print the image and measure with ruler and protractor or use the distance measuring ruler in the Aladin Sky Atlas at  http://aladin.u-strasbg.fr/  Aladin can be used on-line or downloaded as indicated on the aforesaid page.

 

Astrometrica (and some maths)

 

Using Astrometrica position the cursor over the head and then the end of the tail and note the coordinates which in this case are;

- head;             RA12h 23m 06.32s. Dec +04º 31’ 57.5”

- end of tail;      RA 12h 22m 56.29s, Dec +04º 33’ 11.9”

 

Now a little geometry is needed.

- difference between the two RA values = 10 mins = 150 arc secs

- actual difference = 150 x cos(dec) = 150 x 0.9969 = 149.5 (rounded to 150 so makes no difference in this case)

- difference between the two dec values = 74 arc secs

 

 

Length of tail =  = 167 arc secs = 2.8 arc mins

 

Position angle (measured from north through east) = 270 + tan -1 (74/150) = 270 + 26 = 296º

 

These values can then be entered into the COBS database.

 

Astrometrica (without maths – thanks to Pieter-Jan Dekelver for this tip)

 

Load an image and astrometrically calibrate it (depending on your imaging software the image may already be in that condition – if RA and Dec values are shown at the bottom of the screen as you move your mouse across the image). While keeping Shift pressed move the mouse from the head of the comet to the end of the tail and the length and position angle will be displayed below the image.

 

 

These values can then be entered into the COBS database.

 

Aladin

 

On the page at http://aladin.u-strasbg.fr/java/nph-aladin.pl open your image by clicking on the Open file icon and selecting File in the resulting window (below).

 

 

Browse to the location of the file and the image will appear in the main window. Choose Overlay/dist – Draw a distance vector. Position the cursor on the head of the comet and, holding down the left mouse button, drag it to the end of the tail and release. The results appear momentarily under the image but can be recalled by positioning the cursor over the vector. In this case the position angle was given as 295º and the tail length as 2.7 arc mins.

 

 

If your image is not astrometrically calibrated (the Location box indicating this) you will need to carry out this procedure. This is explained in the Aladin manual accessed via Help/Other documentation/Aladin Manual and one method, matching stars, is described below.

 

Load an image and orientate it north up, east left using Image/Symmetry/ then select Top/Bottom and/or Left Right as required.

 

Load a comparison image, in this instance a Digital Sky Survey image using File/Open/Server Selector.

 

Load a chart from a catalogue using File/Open/Server selector/All Vizier and entering the centre coords of your image, catalog (UCAC4) and radius (14’)

 

Enter the centre coordinates of your image, click on Submit and close the Server Selector window.

 

Display the two images and chart by selecting multiview.

 

Select Image/Astrometrical calibration and ‘by matching stars’ in the Astrometrical calibration window.

 

Alternatively click on stars in your image and the related stars in the DSS image – at least four as shown in the window below.

 

Click on Create and close the window.

 

 

Your image and that downloaded from the DSS are now aligned and overlaid with the UCAC4 chart

 

 

The Location box will now indicate the RA and Dec as you move the mouse across your image

 

The tail length and PA can now be measured as previously described and input to COBS (this tail data was not input to COBS (below) as this feature is new to this tutorial)..

 

COBS

 

The Comet Observation Database (COBS) is located at http://www.observatorij.org/cobs/

 

The first step is to obtain a login username and password.

 

To enter observations;

Select the Observations tab and CCD observations below the tab and complete the form as directed. Screen shots of the input are shown below. While this might seem a little tedious, once the various ICQ codes have been negotiated and a set of observations for your telescope/CCD combination has been entered, adding further observations is much simpler. Drop down boxes for instrument, camera and chip types have recently been added which should make input easier. Please note that it is the visual equivalent magnitude calculated by kphot which must be input to COBS and not the nuclear magnitude as generated by Astrometrica.

 

The process for measuring the coma had not been developed when this part of the procedure was written and therefore the measurement is not included in the screen shots below but is shown in the following ICQ report.

 

ICQ codes can be found at http://www.icq.eps.harvard.edu/ICQKeys.html and http://www.icq.eps.harvard.edu/ICQRec.html

 

 

 

 

 

 

 

Selecting the ‘Analysis’ tab allows a lightcurve, coma diameter and data (not shown ) for a selected comet to be displayed. By selecting Visual observations and CCD observations using the Control key both types are displayed and identified with different markers as in the chart below. This illustrates how CCD and visual observations can be successfully merged and how the former ‘stretch’ the lightcurve beyond the visual range.

 

 

 

 

You can view your observations by selecting the ’My OBS’ tab and save them to your PC by clicking on save. If no comet is selected all observations are saved in a dat file. Wordpad, screenshot below, can be used to view/edit this file rather than Notepad which does not properly format the lines of observations.

 

 

Observation can then be submitted to the BAA Comet Section Director.

 

Reporting results

 

This is covered in the various sections above but is summarised here.

 

Minor Planet Center

 

MPC reports - send to [email protected]

Website is at http://minorplanetcenter.net/

 

Cometas Group

 

MPC and Multibox reports - send to [email protected]

Website is at http://www.astrosurf.com/cometas-obs/

 

COBS database

 

ICQ reports – input via website is at http://www.observatorij.org/cobs/

 

BAA Comet Section

 

MPC Reports  - send to The Astronomer editor at [email protected] for onward transmission to the BAA Comet Section Director.

ICQ reports – send to the BAA Comet Section Director.

Comet Section website is at http://britastro.org/baa/index.php?option=com_content&view=article&id=32&Itemid=70

and http://www.ast.cam.ac.uk/~jds/

 

Contents