New tool for CCD photometry - tutorial

June 27, 2017
CCD photometry of comet still remains as a very difficult work, especially for most wanted total coma magnitudes, compatible with visual observations. Also the usual problem is creating of proper ICQ coded line of observation. To help observers with this, I programmed an application - tool for comet observers, which is now in testing - beta phase.

New application is called KOPR, its actual version is 0.53 and it is in beta stage.

You can download the application for:

Tutorial for CCD observers:

1. Preparation of images

Right now, the software works well with output images stacked in Astrometrica. Images should be corrected for both dark frame and flat field. If comet own movement is small and stars are not too long trails, you can use one image for measure both comet and reference stars. Else we need to stack images first time on comet and second time on stars. However KOPR is not able to compare images with different exposure times in beta phase. So both images for stars and comet must be made from same images and same type of stacking (usually Sum).

2. Measure of comet

Now with stacked images, we can start measure in KOPR (3rd icon on toolbox). First we have to write comet short destignation (For example 103P for numbered periodic comet 103P/Hartley, or C/2015 V2 for long periodic comet C/2015 V2 (Johnson)). Than after that we can load the image via button. After image is loaded, the mean background intensity and maximum value on image is measured, also date and time is filled if image FITS header contains it (please always review if the date and time is correct for UT). Sky background and maximum values are visible in fields sky / max. The maximum value is there to set limit for saturation, if you are sure that the value is lover than saturation level, please manually edit the max value to higher number (else it can cause you difficulties with measure stars or comet with brightest pixels inside).

Do not forgot to put measure radius in pixel first, please note, that there are two modes of measure:

- Default mode starts to measure from 1 px radius and gradually increase the measure untill the value you set. If the brightening level will decrease under 0.3 mag on 1 degree radius or the brightness growth will starts to gradually increase (bright star in aperture), the procedure will stop and measure will be set for the actual aperture which can be smallet that the aperture you originally set.

- Fixed aperture more always measure only the aperture size you write in field. You cannot perform Afrho measure with this mode.

Also do not forget to put correct filter settings for images.

Now with button "Measure" the image is open and by right click on comet center the photometric measure will perform.

3. Measure of tail

Click on the "Measure button" in Tail section line. Image of comet will open and by two click on comet center and tail tip end, the tail length and position angle will be automatically derived from image, if the astrometry is correct. If you click wrongly on the tip of the tail, you can keep clicking untill you will be satisfied with tail line.

4. Measure of reference stars

If you use same image for comet and stars, you simply click on "Measure" button near "Load star image" section. Do not forget to write first the pixel radius for star aperture. If you use different image for stars, simply load the star image with "Load star image" button.

After clicking on measure a dialog is open to write a magnitude limit, after you put limit, KOPR will automatically connect Vizier and plot all stars brighter than your limit on map. You can check astrometry quality, if stars are plotted outside of real stars on image, you will have to stack images again. The stars from Vizier are plot as color squares, the blue and red squares are for stars with B-V color index smaller than 0.5 or larger than 0.8 mag. Orange squares are for stars with similar color as our Sun, we should use only this stars as reference stars for comet.

With right click you simply click on all stars that you want to use as reference stars, after each click, the star is measured and data are downloaded from Vizier online. Only APASS magnitudes are used. If star got some pixel value on image maximum, red cross will plot over star and it will not be measured. If KOPR will not be able to find a star near clicked position a error dialog will open, the value of star will be saved, but you have to manually add star magnitude later.

5. Filtering of reference stars

When you close the image, stars are saved and you can check and edit them via "Reference stars" button. You can see list of measured stars and their deviation from mean value. Stars with large deviations should be excluded from further calculations, you can recalculate deviation for new amount of stars by buton "Update deviation". If star was only measured, but data from Vizier are missing (usually generate extremely large deviation in tens of magnitudes), you can write star magnitude manually or download from Vizier use "Query vizier button" and entering star UCAC4 catalog id.

6. Final processing and sending of observation.

After saving your stars, make sure, all settings are correct and do not forget to write number of frames used for stacked image in field "Nr. of  frames". You can also fill a note into comment line. Process measure will calculate the comet magnitude and build ICQ format and CometObs format.

You can also see the resulting comet magnitude in commentary after "Calculated mags:" text. If the measure is good and only small magnitude scatter (max 0.3 mag) is between measures, you can proceed to publish observation.

The ICQ line can be simply copy pasted and uploaded to COBS database here.