CCD Photometry with KOPR – Tutorial & FAQ

KOPR is freeware program for comet observers, actually the program is in beta phase. Latest release is 0.58: Download Windows | Download Linux

Before start, please check Youtube tutorial video about how to use program, do not forgot to turn on subtitles! Note that video was done in previous version, some parts of GUI and measure did change since that time.


Frequently asked questions:

When should I stack images on both comet and stars?

If comet own movement is relatively small and stars are just little elongated, there is no need to stack images on stars as well. Only in cases when stars are long it is necessary to restack images.

Left side, stars are elongated but can be used. Right, stars are too long and mixed.

How to select correct aperture to measure comet?

To see how large aperture size is necessary to measure total coma magnitude, is required to stretch images to see maximal extension of coma.

Two vies of one image, first image look to have good aperture to measure comet brightness. However after image is stretched, we can see that coma extends much far away and correct aperture size is almost 4x greater!

There is star in coma, what should I do?

Sometime when we stretch image and see the real extension of coma, we notice that there are stars inside aperture. Mostly the stars doesn’t cause any issues, because they cover very small portion of aperture and they do not increase total coma brightness more than 0.1 mag.

We can find if star causing problem or not in final magnitude profile of coma (or Afrho plot).

Alternatively we can use “Aperture” using “ACF Filter” settings which can fix problem with some bright stars. See more about ACF Filter.

When we select correct aperture, we can find a stars inside aperture.
Some stars that looks bright doesn’t cause real problems.
We can see with resulting plot, if the star causing a real problem, like in this cause.

There are stars in background sky square subsection, is that a problem?

No! Background sky intensity is calculated from 60% faintest pixels in square subsection, so unless stars covering more than 40% of this field, there is no problem with that.

Program doesn’t find any stars on my image, what should I do?

First it is necessary to check star limit to plot, try set higher magnitude limit first. If there are still no stars on image, there are apparently precise APASS data missing for selected region. This cannot be unfortunately fixed, because there is not much different useful magnitude sources, because most of magnitude catalogues contains data which are totally improper for photometry use.

Which stars should I use for reference?

Optionally we should compare comet brightness with stars that have similar colour index as Sun (B-V is between 0.5-0.8 mag). We can find that stars marked with orange squares. Other stars we can use only as last resort.

Only stars in orange squares should be used as reference stars.

When to use “Fix Astrometry” on image?

Sometimes, the astrometry of image is shifted. There is function “Astrometry fix”, that allows to correct this shift of stars. When you click to measure star, program is looking for most close star with known magnitude info (they are plotted as coloured squares), if square is shifted away from star, but there is no other squares that is closer, “Astrometry fix” is not necessary to use. Only in cases when squares are too far away, that program cannot identify star (error dialog appears) or if square of other stars are closer to star on image than square for this star.

Astrometry on this image is wrong, but squares are not too far away for program to identify correct stars with measured stars from image. “Astrometry fix” should not be used.

How can I know if my measure is ok?

To determine if our measure is correct we have to check magnitude plot against radius in km around nucleus. Usually the cometary coma radius extends more or less over 50 000 km, if our measure shows very small coma (for example radius 15 000 km) we definitively did wrong measure and coma must be measured with larger aperture. Also if it is apparent that curve of growth is still increasing fast at end of plot, it is apparent that coma extends far away from our selected radius. Usually in this case program gives a warning “Increase aperture”.

Also if the curve of growth is not smooth, there are some stars interference with measure, and therefore the image cannot be used for photometry.

Apparently wrong measure. Radius diameter is too small (15000 km) and the curve of growth is too steep at the end!
Correct measure, aperture is large enough and there is almost no magnitude increase at the end of curve.

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