CCD Astrometry and Photometry of faint comets


Originated 2017 January 16




My thanks to Richard Miles for his help with this tutorial.




FoCAs, as described in CCD Astrometry and Photometry, cannot handle stacked images which imposes a limitation on the measurement of the Visual Equivalent or Total magnitude (m1) of faint comets. The procedure described here uses Astrometrica and Iris to measure such magnitudes - the method being similar to that used for bright comets as described in the aforementioned document. It is assumed that the reader is familiar with that document as it includes more detail than mentioned here.




So what is a ‘faint comet’ as far as this procedure is concerned? I would suggest it is one that is barely or not visible in a single image but shows up quite clearly in a stack of images. I can’t define an actual magnitude as that depends on your telescope and camera combination and atmospheric conditions.


For this tutorial I used four images of comet 10P/Tempel, obtained on 2015 June 12 using the Sierra Stars Observatory Network robotic telescope located in California, USA, which can be downloaded from here


Faint comets process flow


Figure 1. Process flow


Image processing




You may find it helpful to read Astrometrica Tutorial III which can be accessed via Help/Track and Stack/Tutorial III


As shown in the flowchart the objective of running Astrometrica at this point in the process is to produce an image stacked on the motion of the comet to enable the diameter of the coma to be measured using Iris.


Start up Astrometrica;

- select a configuration file using the UCAC4 catalogue

- Initially set the Aperture Radius, in Pixels, to the equivalent of 5.5 arcsecs (or as near as your configuration will allow) – Figure 2 (SSON plate scale is 0.8 arcsecs/pixel and therefore 5.5 arcsecs = 7 pixels)


Figure 2. Initial Aperture Radius setting


- complete the various boxes under the Internet tab – Figure 3 (see Astrometrica/Help/Settings)


Figure 3. Internet tab settings


-.using the Stack option choose the required images by clicking on the Add button – Figure 4


Figure 4. Selecting images to stack


- images can be previewed and any of poor quality deleted removed by using the relevant buttons.

- click OK on the Select Images window and on each date and time window

- in the Coordinates, Tracking and Stacking window, Figure 4, select Stacking/Add and enter the comet’s ID in the Object box or click on the button to the right of the box to select the comet from a list

- note that some of the data in this window may be obtained automatically from the FITS header and via the Object Browser as described below, if not ensure that the Right Ascension, Declination, Speed

  and PA boxes are completed

- if the second option enter the comet’s ID in the Filters window of the Object Browser, Figure 5, and click on the magnifying glass icon.

- from the list select the required object and click on OK


Figure 5. Relevant comets listed in the Object Browser


- the comet is now shown in the Object box of the Coordinates, Tracking and Stacking window, Figure 6


Figure 6. Completed Coordinates, Tracking and Stacking window


- click on OK to stack the images and generate the data reduction results – Figure 7


Figure 7. Stacked image and data reduction results


- note the time and date by accessing Images/Edit Image Parameters – 2015 06 12, 05h 21m 00s

- save the image as 1_416302+3_comet.fts


An MPC report can be generated at this time;

- left mouse click on the comet.

- in the Object Verification window - Figure 8

            - if necessary centre the aperture over the comet using the Centroid facility

            - Click on the Object Designation button

            - select the comet (normally the first object on the list in the Object Identification window – Figure 9) ensuring that the dRA and dDE are zero (or very

               close to zero)

            - click OK and the comet designation will appear in Object Verification window

            - click Accept


Figure 8. Object Verification window


Figure 9. Object Identification window


To view the MPC report, select File/View MPC Report File. Please note that at least three positions must be reported and therefore the example below, listing only a single position would be rejected. Also note that the nuclear magnitude (m2) is included in this report and not the total magnitude (m1).


To send the report select Internet/Send MPC report and click on Send – Figure 10


Figure 10. MPC report




Iris is now used to measure the coma diameter.

- open the image 1_416302+3_comet.fts

- adjust the brightness of the image using Threshold/Auto (reposition the sliders if required)

- orient it to match the Astrometrica image (makes it easier to find the comet if it is very faint)

- draw a box round the comet

- right click in the box and select Growth curve. The result is shown in Figure 11.


Figure 11. Iris Growth curve


The axis can be adjusted to enable the coma diameter to be more accurately measured – Figure 12. From the Growth curve the radius was estimated to be 14 pixels.


Figure 12. Adjusted Growth curve.


This image will be used to calculate the total magnitude of the comet and therefore it can be left open in Iris at this time.




The zero point can now be determined. Under the Program tab in the Astrometrica config file set the Aperture radius to 14 – Figure 13.


Figure 13. Astrometrica config file set to radius determined using Iris


- stack the images on the stars using a value of zero for both Speed and P.A. in the Coordinates, Tracking and Stacking window – Figures 14 and 15


Figure 14. Settings for stacking on stars


Figure 15. Images stacked on stars


Note that the zero points for only the first two images in the stack are shown. To obtain the zero point of the stack;

- save the stacked images as 1_416302+3_stars.fts

- close the image and reopen it

- perform an astrometric reduction and note the zero point, 29.55 – Figure 16


Figure 16. Zero point determination




a) Preferred method


Now Iris can be used to determine the total magnitude of the comet:

- select Analysis/Aperture photometry

- select Circle number 3 and Median background

- input the radius of the coma, 14, as Radius 1

- Radius 2 is Radius 1 + 5 (19) and Radius 3 is Radius 1 +10 (24)

- input the Magnitude constant – 29.55

All inputs are shown in Figure 17


Figure 17. Inputs to determine the total magnitude of the comet


- click on OK

- place the ‘gun sight’ over the comet and left click the mouse

- the results appear in the Output widow - Figure 18 (the measurements can be taken several times)

- the total magnitude of the comet was determined to be 17.4


Figure 18. Total magnitude measurement


b) Alternative method


If field stars are present in the inner aperture or outer annulus then this alternative method can be used;

- select Analysis/Aperture photometry

- select Circle number 1 and Median background

- input the radius of the coma, 14, as Radius 1

- input the Magnitude constant – 29.55

All inputs are shown in Figure 19


Figure 19. Inputs to determine total magnitude – alternative method


- click on OK

- place the ‘gun sight’ over the comet and left click the mouse

- place the ‘gun sight’ over an area of the image devoid of stars and left click the mouse

- several measurements can be taken and an average of the results calculated

- the results appear in the Output widow - Figure 20


Figure 18. Total magnitude measurement – alternative method


The total magnitude is calculated as follows;

- comet + skybackground intensity = 263841

- sky background intensity (average) = 194327

- instrumental magnitude = -2.5 x log(263841 – 194327) = -12.11

- zero point/magnitude constant from Astrometrica = 29.55

- comet magnitude = 29.55 – 12.11 = 17.44


Comet Observation Database input


Together with observer name, image details, telescope configuration and location you now have all the data necessary to add your observations to the COBS database.


COBS lightcurve for 10P

Figure 19. Lightcurve from the COBS database – The result of this exercise is marked with an X