Averaging circles filter

A new feature was added to KOPR software, first it is possible to use it since version 0.57 (it was added in version 0.56 but didn’t work properly).

Averaging circles filter is doing pixel intensity measure in circle slices around coma photometric center and finding a median value for pixels in circle selection. By limit set (100% by default) a pixels with value larger than median * (1+limit) are excluded from average calculation, than total number of pixels are multiplied by resulting average value.

Comet with bright star inside aperture radius.
Bright star causing a large error in comet magnitude, causing it to be ~1 mag larger.
Same measure with use of ACF filter. star influence was removed from result.

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.

C/2017 E4 (Lovejoy) – time of death

In a previous post I note a high risk of disintegration of this comet, in fact, when the post was made, the nucleus disintegration was already done. The images of Jean-François Soulier show a great loss of gas coma between 8. and 9. April. It seems to be the time, when solid nucleus of this comet cease to exist.

Date of disintegration is independently confirmed by Terry Lovejoy with astrometric measures, as after 8. April, the residuals starting to systematically grows.

Residuals of comet C/2017 E4 plotted by Terry Lovejoy

So what exactly happened with this comet? I already suggest that nucleus size of this comet must be very small with diameter between 400 – 600 meters. Also the light curve shows a extremely chaotic behaviour as comet shows strong brightening and two small outbursts. There can be hardly established any stable parameters for this comet brightening. This behaviour is very common for small fragments of larger comets, for example, see comparison with comet C/1988 A1 and its smaller fragment designated as C/1996 Q1 here. We can see, that behaviour of comet Lovejoy is extremely similar to that of comet Tabur (C/1996 Q1).

So what happened to the nucleus? We already know that small fragments are very fragile, for example fragments of comet 73P/Schwassmann-Wachmann never exist longer than 1-2 returns. Also the fragments are very small. The size and fragility causing the photometric curve instability as the brightness more depends on a very random events (when a part of nucleus is detached) than distance from Sun. We can imagine that volatiles and ices are something as a “glue” for large boulders, but small sized nucleus loosing them relatively fast. In time when nucleus loose all the glue, there is not much more gases to refill gaseous coma and the brightness will start to decline. In same time, the absence of glue cause boulders to separate into inactive swarm of debris. Many of them collapse into thin dust, creating a large cloud dust. One emission of different sized dust grain in same time is knows as a synchronic feature – a line of dust that is aligned in one direction from original nucleus, where most heavy dust remains closely behind, while most thin dust moving far away on the line. Synchronic feature can be observerd a long time after nucleus cease to exist.

Brightness evolution from COBS data plotting magnitude data corrected for geocentric distance (real activity level) against log of heliocentric distance. It is apparent that after initial fast brightening, the comet calmed down and then a nucleus disruption occurred around distance 0.59 AU from Sun

Comet C/2017 E4 (Lovejoy) prospect

Comet C/2017 E4 (Lovejoy) is definitively a returning, long-periodic comet, actual MPC orbit points to original orbit with period 11603 years and semimajor axis 512 AU. The photometric behavior also points to a returning comet. When it was discovered it shows a very low activity and absolute magnitude below the Bortle survival limit. However it seems that comet was brightening fast and the activity level increased very fast. There was 2 very small outbursts recorded by observers and after second one, comet stopped fast brightening phase and entered calm phase instead.

However quick analysis of photometric parameters shows that even with smaller rate, the activity of comet is still increasing as comet approaching closer to Sun with pretty normal rate for most of comets. Therefore comet looks healthy now and also actual photometric parameters placing comet above Bortle survival limit:

H0 =12.69 mag, n =12.61 [ -40 ,-20.5 ]
H0 =9.04 mag, n =3.34 [ -20.5 ,-13 ]

Actual astrometry of comet shows good result for both orbits with non-gravitational parameters and without. Orbit with them giving A1 = 2.32E-07, A2 = -1.82E-07. For actual activity level of comet, this corresponding to nucleus with maximal mass 1E+07 to 5E+07 tons. With average comet nucleus density this means spherical nucleus with diameter between 400 – 600 meters.

Small activity level, non calm photometric behavior and possible non gravitational forces definitively points to a small comet with sub-km sized nucleus. Because it is returning comet, it doesn’t  necessarily means, the comet can totally disintegrate, even the risk is very high. If it will disintegrate, there is a high possibility, this comet can be a smaller fragment of different comet, which may return in a different time.

C/2013 V5 (Oukaimeden) in disintegration risk

Comet C/2013 V5 (Oukaimeden) belongs to group of dynamically new comets as its original 1/a was calculated by MPC to 0.00001097 . Absolute magnitude of this comet (8.3 mag) is nearly 2.4 mag above its survival limit (10.7 mag) calculated using formula developed by J. Bortle. However in past time, there were observed many comets which also seems to e on the right side of border and still disintegrated. Bortle calculated this formula specially for comets with small perihelion diistance, however in past year there were disintegrating comets observed in larger heliocentric distance. As the formula seems to works perfectly for comets bellow this line (100% success) there are some comets which exceed their survival limit and still failed to survive its perihelion.

However it is necessary to keep in mind that the Bortle formula was calculated more then 20 years ago and today we are observing many fainter comets which could be in previous years undetectable. Such small comets undergoing large brightnes enhancement during destructive errosion of their nucleus therefore they can seems to be “more active” comets then they really are.

Comet C/2013 V5 (Oukaimeden) is on the plot with companion of another dynamically new comets which failed to survive perihelion passage as C/1999 S4 (LINEAR), C/2010 X1 (Elenin) and C/2002 O4 (Hoenig) and few others. There seems to be real chance that this comet can also fail to survive while it approach on 0.62 AU to Sun.

The light curve analysis and comparison with other comets shows that its activity lies near lever similar to other comets. On plot you can see C/1999 S4, C/2002 O4, C/2010 X1 and in addition C/2012 S1 (ISON) as red dots and lines and C/2003 T4 (LINEAR) which survived its perihelion but probably heavy damaged as green. Actuall data for comet Oukaimeden are black.

Observing of comet Oukaimeden this days is very highly valuable as day to day follow up can help catch the disintegration process it if occurs. The end of comet is usually precceded by fast brightening or outbursts, later followed by unexpected fading and morphology changing in coma – lose of central condensation and cigar shaped inner coma.

Comet C/2003 T4 on a disintegration border

During analyzing probability of disintegration for recent comets, I have found an interesting comet on “border-line”. Comet C/2004 T4 (LINEAR) should have only twice mass as previously disintegrated comet ISON. Known non-gravitational forces allows us to calculate limit for mass of this comets and previously disintegrated C/1999 S4 (LINEAR). Known water production rates tell us how much water mass they totally produced.

It seems that comet C/1999 S4 (LINEAR) which disintegrated very far away from Sun was extremely water poor, only having 7% of its mass in water. On other hand comet ISON with similar mass survived much farther and disintegrated very close to Sun and having nearly three times larger water fraction of its nucleus.

The “nearly” disintegrated comet C/2003 T4 have at least twice larger fraction of water in its nucleus mass then C/1999 S4, which was apparently sufficient for its surviving, even the commet suffered a heavy damage to its nucleus.

Water can play major role in comet surviving/disintegrating. Sublimating of water molecules can tranfer away heat form nucleus and protect fragile mineral content of nucleus from heat stress.

Whole study “Unexpected fading of comet C/2003 T4 (LINEAR) and disintegration of C/2012 S1 (ISON)” can be found on:  http://arxiv.org/abs/1408.3860

Comet C/2012 X1 (LINEAR) anaysis

Comet C/2012 X1 (LINEAR), long-periodic comet with initial period 1740 years, is one of most interesting comet at all. My light curve of this comet require 7 intervals with different photometric parameters.

Originally this comet probably started to brighten before its discovery in distance 5 AU from Sun, first month it seems that its brightness was increasing fast. Shortly after discovery, its brightening continue calmly following usual comet brightening slope.

After conjunction with Sun it was recovered nearly 6 mag (250 times) brighter then expected in distance 2.5 AU from Sun, brightening continued only for few weeks.

Then comet started to fade, apparently stepping back from its absolute magnitude peak. In distance 1.9 AU, brightening started again with usual comet brightening slope. Its activity was still almost 4 mag (40x) larger then pre-outburst values.

After perihelion passage, it fade very slowly for nearly 70 days its activity was not stepping back as expected. In distance 1.9 AU brightness started to decrease fast again until now.

Comet C/2012 X1 (LINEAR) is one of small portion comets with calculated non-gravitational parameters ( http://www.minorplanetcenter.net/db_search/show_object?utf8=%E2%9C%93&object_id=C%2F2012+X1 ), in this case they are very strong, suggesting that huge activity over large portion of surface was pushing relatively small nucleus against Sun direction.

Presence of NG elements allows us to estimate upper limit for comet nucleus mass which is 2.9 * 10^12 kg, assuming density 0.5 g/cm^3 it means a spherical nucleus with 2.2 km in diameter.

However estimated water production needs much more surface then spherical 2.2 km sized nucleus can provide, before major outburst, such nucleus could have active <5% surface, after-outburst activity needs much more then 100% of such surface.

This pointing to possibility, that after outburst, nucleus shape is extremely irregular and out-gassing from icy grains in coma is present. A fragmentation can also explain this, however no fragments was discovered so far.

Swan song of comet ISON

By its past evolution I’m actually convinced that what we are now observing in case of comet ISON is its “swan song” last performance at end of its long cometary life. I would like to introduce my opinion about what is happening now with it.

Originally when it was discovered, this comet seems to be active enough near level of many other dynamically new comets and it was hard to imagine that this comet may not survive its perihelion passage. Few months later its activity become to leveling off and in spring of this year, photometric parameters of this comet pointed it near its surviving limit, defined by John Bortle. Based on his study ~70% of comet under this line end their life near perihelion passage. His study was based on 84 comets where 16 totally disintegrated. Reliability of his conclusion could be tested in some past comets, and out of 4 comets bellow this line, all totally disintegrated too. When comet ISON passed 2 AU distance from Sun its activity pointed it still deeper and deeper under this level.

First I tried to make a quantitative study of this comet compared to comets that demise and others which survived their perihelion passage. Comet ISON seems to not fit well both groups, in part of its orbit its activity pointing to larger nucleus then disintegrating comets usually have, but in other hand ISON is going much closer to Sun then any of compared comets and many of these which survived would probably not make the approach as ISON going to do.

I have found that ISON was with its photometric behavior and absolute activity very close to few of them. First one was comet C/2000 WM1 (LINEAR) which survived, but undergone a large outburst with 3 mag amplitude shortly after it passed its perihelion. I suspect that comet ISON may also show such behaviour, because both comets seems to be very comparable bodies. And this thing exactly happens in similar distance from Sun (ISON ~0.65 AU, LINEAR ~0.55 AU), but while comet LINEAR was already moving away from Sun its nucleus was no more stressed by increasing heat flow from Sun. Maybe it this comet was continue move to very proximity of Sun its entire nucleus will erode and disintegrate.

Also very similar object was another comet LINEAR – C/1999 S4, one of best described case of comet which entirely disintegrated. When it reached its perihelion (0.77 AU from Sun), the glue of nucleus – frozen water and volatiles, was exhausted and whole nucleus turned to dust cloud and several larger fragments with diameter from 20 to 50 meters, which could be something as original “cometesimals” – basic building blocks of cometary nuclei. They were found by HST 12 days after terminal break-up of nucleus, and some of them may remain intact and fly away from Sun. It was only case when they were found, but simply that can be just because this was only one demised comet which were examined by large telescopes. No other disintegrated comets was researched by instruments that may allows to reveal such small objects.

Nucleus size itself is crucial for surviving prospect of comet, fast rotational speed, local structural instabilities and irregular shape may cause disruption of larger nucleus, but simply if nucleus is very small it will fast erode and most of it mass will sublimate, when it gets too small, heat will reach very deep inside and gas pressure may finish their fragmentation. In other hand, larger nuclei will have its interior intact and any destructive forces will affect only their surface.

Now what happened to comet ISON and what we may expect? It seems to be truth that nucleus of ISON is larger then nuclei of past disintegrated comets and simply it ISON would move in their orbits, it will loose large portion of its mass, but definitively it will survive its passage around Sun relative intact. But because it is approaching much closer then any of them, it would need even larger nucleus for us to be sure it will survive intact.

The main difference between small (sub-kilometer) and large (kilometer sized) nuclei is in stability of their photometric behavior. While brightening of larger comets is very stable in long-term, the smaller nuclei showing large fluctuations in their brightness.

Reason is obvious, see and examples:
2 km diameter nucleus have 12.6 square km surface, if a 50 meter fragment will split from it, total surface will increase by 0.01 square km. We will not even notice such increase which is smaller then 0.1%.
200 meters diameter nucleus have 0.13 square km surface, it it split to near 200 (lets simplify it) and 50 meters fragments, their total surface will grow by almost 8% (and so its activity), such change is already noticeable as small outburst.

The increase in brightness is of course higher, because there is released dust and fresh surface rich in volatiles and water. While for larger comet the change will be smaller then 0.1 mag to total magnitude, same change for smaller nucleus can cause a magnitude outburst.

Beautiful example was two comets originated from one spitted comets. Large fragment C/1988 A1 (Liller) showing very stable and calm brightening, while smaller fragment C/1996 Q1 (Tabur) show very disturbed light curve with at least 3 outburst followed by 3 declines and after last one it totally disintegrated.

Another very nice example is fragmented comet 73P/Schwassmann-Wachmann. Main fragment C of original comet show very calm and stable light curve, while smaller fragment B during its 2006 return exhibited at least 4 peaks followed by fast fading. Smaller fragments also tends to continue the disintegration process very fast.

Now back to ISON, what is going on with this comet? As its said before, this comet seems to be definitively larger, then usual disintegrating comets, but maybe to small to be a surviving sungrazer. Mass of its nucleus is somewhere near limit to survive one perihelion passage, most probable smaller.

N. Biver estimated that until first outburst, comet could release mass similar to sphere with 200m diameter. Assuming density 0.5 g/cm3 that means 5*10^5 tons mass.

13. November outburst lasted 2.5 days and had amplitude almost 3 mag. It could be caused by splitting of small nucleus to two or more fragments but generally leads only to increase out-gassing and natural growth of dust production. Unfortunately there was no or poor evidence of possible fragments, maybe because poor resolution. Non-gravitational forces reached extreme high level, large reaction force of jets was pushing nucleus on its orbit against gravity of Sun.

19. November another very slow outburst occurred with amplitude 1 mag. 6 days later gas emissions of comet dropped by factor 20. Part of nucleus or maybe fragment/s completely disintegrate and later, during 26. Nov a resulting synchronic feature appeared. By out-gassing and dust production comet may lost totally another 5*10^5 tons of material. The mass of dust in synchronic feature is unknown for me. But if its same as occurred in case of comet Lovejoy it can be 2*10^6 tons (assuming density of dust 2g/cm3).

22. November, dramatic decrease of comet activity occurred due effects named before.

24.-26. November activity of comet has stabilized. If we assume that this is normal non-outburst activity level of this comet and we will compare it with stable activity of another two comets in similar distance from Sun – C/2006 P1 (McNaught) and C/2011 L4 (Panstarrs), and also assume that all comets have spherical nucleus and similar activity per square meter. Another assumption (I know it seems to be too much, but this are comets) that both of that comets had nucleus in diameter 1 to 4 km giving us result, that ISONs activity if there is one intact nucleus corresponds to diameter between 100 and 630 meters. That corresponds to total mass between 1.5*10^5 to 3.7*10^7 so the comet may already lost from 95% to 8% of its original mass.

27. November another outburst happened with amplitude at least 4 mag so far. Comet will definitively loose another large partition of its mass, lets assume 5*10^5 tons again. In case of minimal mass this means final disintegration, but apparently remaining mass is at least slightly larger then 1.5*10^5 tons.

28. November ISON is passing its perihelion. In work of Z. Sekanina about comet Lovejoy, he used for calculations this evolution of its diameter caused by erosion – 400 meter 0.6 days before perihelion, 280 meters in diameter at perihelion, and 150 meters in diameter 1.6 days after perihelion. This means 9*10^6 tons of mass lost. In case of maximal mass for ISON estimated before deducted by last outburst and erosion during approach to Sun, the comet would totally loose at least 25% of its entire original mass (therefore small chance for surviving, if it wont fragmenting). To survive and disintegrate as comet Lovejoy it should by now have at least similar diameter (400 meters).

Even comet ISON seems to be larger then regular disintegrating comets, it is still very small comet. The last outburst may be also the last one for this comet, if not, simply erosion of nucleus may consume all of its mass within hours during close approach. It seems that there exist small chance for remaining nucleus disintegrating after perihelion, but chance to survive entire revolution is extremely small. However its “swan song” can be still very impressive to see.

And at the end, here is nice example of comet, which actually fragmented, but its nucleus was large enough to remain intact, fragmentation caused permanent increase of comet C/2001 A2 (LINEAR) activity, and not only transient as we can see for ISON.