COMET MAGNITUDE ESTIMATES via CCD ImagesAssuring consistency and accuracy with saturated vs. unsaturated images
P. Clay Sherrod
Above: Comet c/2013 US on Feb. 3, 2016 taken from Arkansas Sky Observatories
Although this comet appears very bright and total brightness (m1) was estimated at mag. 7.1
on this date, CCD measurements of the exact nuclear magnitude (m2) revealed that the
actual core brightness was 11.2 via CCD estimates .
Although there are visual observers in the ranks that do very quality work on comets
the magnitude estimates for publication (for MPC or other agencies who serve as repositories):
are subject to very high variability and inaccuracies when comparing estimates for
identical comets made under similar conditions.
It is very difficult, if not impossible to get comparable and reliably research quality estimates
from a visual observer next to one obtain via CCD.
Several parameters come into play:
1) the experience of the visual observer;
2) the size of the telescope and type of instrument used visually
3) the length exposure (saturation) with the CCD observer
4) the quantum efficiency of the CCD camera/sensor being used for the estimate.
But there should be consistency and there CAN be in today's modern equipment
between observers using CCD for photometric measurements.
I do believe and I follow a variable aperture technique for all comets,
including many that appear to be stellar; many years nearly 70% of my comet
analysis is on comets which exhibit no nebulosity (faint comets) and thus
reduction of magnitude many times can be similar to that of measuring stellar,
provided that a reference index star is always used for proper adjustment and
calibration (this is done here at H45).
However on comets with an extended coma the aperture used to do the actual
densitometry reduction MUST be used consistently in proportion to the overall
coma size of the comet.
Put simply: the larger the comet, the larger the CCD "aperture frame" must be
to accept to overall diameter of the comet's coma.
Using the old method of assigning a measuring aperture of "11" or 13" is
simply not practical.
There is a huge difference in result, for example, in the measurement of a tiny
comet at threshold (say, mag. 19.1 which clearly shows NO coma nor nebulosity)
and one that extends a huge coma with considerable scattering. For the faint comet,
and aperture of "13" is far too large, allowing sky brightness to fill the field being
photometrically measured, whereas the "13" size aperture setting for the photo above
would lead to eliminating more than 50% of its overall size and outer brightness.
A very small aperture (I believe that "7" is appropriate for stellar comets, while a variable
aperture for large comets depending on overall coma size) is needed for faint stellar comets
and an appropriately larger aperture for large comets.
By "aperture" I am not referring to the telescope aperture - rather the aperture of the computer
program IRIS being used to measure brightness; all measuring CCD programs have this and the
size of aperture is variable and can be changed from 3" to larger than 27" .
Above: From a screen shot of the same comet on the same date, here is the CCDops Crosshairs program
for precise measuring of the comet's magnitude. Note the small white box just to the right of the Crosshair window;
that is set to aperture "25 x 25" (see selection just below red crosshairs); that is the IRIS setting for the box and
you can clearly see that this aperture is still too small for the overall Coma size of the bright comet to its right.
My measurements are made using the same box, with size to FIT the brighter stars that I might pick to the left
and their magnitudes adjusted to coincide with the UCAC-4 U-Mass star catalog; once that calibration is done,
the box is moved to the COMET and size adjusted (IRIS) to fit the comet's coma. The magnitude is provided
via the program once that size is selected. CCDops allows the box to move with the user's cursor.
The assignment of aperture is relatively subjective at this point,
based on the overall witnessed extent of the coma from a resulting CCD image of
good saturation. I strive to include the "major extent" of a coma rather than
the "total coma" since on many, many comets (28p, Pons-Gambart, etc.) you have
actually two gradients of coma: outer and inner. If the inner coma is
distinctly segregated from a large and very veiled outer coma, that inner coma
ONLY is used and the aperture on the densitometer (digital) is adjusted for that
size, NOT the overall larger extended coma.
Keep in mind one primary rule about selecting IRIS diameter on your measuring program:
- For TOTAL (m1) the IRIS setting must be large enough to cover the entire saturated image of the COMA of
the comet, but not so large as to let the SKY BACKGROUND be an influence on the final
reading. Hence, for a tiny stellar 19th magnitude comet, the IRIS setting should be near
minimum unless some sign of nebulosity is noted.
- For NUCLEAR (m2) magnitudes, you want to isolate the relative brightness of just the
actual hard component, nucleus, of the comet; hence stopping down the IRIS setting to a
standard "13" is recommended, unless the comet has no nebulosity whatsoever....in that case
the aperture should be minimal.
The reason for this is that if we use the entire extended outer coma, we are
beginning to change our relative measurements of sky background vs coma
intensity and the results become muddy. Perhaps years of practice of doing this
(first by hand and now by computer) have resulted in me being able to quickly
assess exactly where the aperture of the densitometer needs to be set for each
Once that aperture is set, I use that size to measure a base value of my
background sky, the move the aperture to a known stable star on the UCAC-4
catalog and calibrate the given magnitude accordingly; once set, the aperture is
then moved and centered over the extended inner coma (or just coma if it is
condensed and very well delineated) and my value is obtained accordingly.
This method has proved incredibly consistent and accurate; I simply do not know
what all the fuss and error is about among comet observers. The method for CCD
should be quite simple and consistent if everyone would measure this way. In
fact, one of the most common CCD camera operating systems, CCDops by SBIG,
provides somewhat this very concept on their "Crosshairs" measuring device. It
is a well kept secret for some reason, but their program is amazingly quick and
quite accurate when comparing with my custom programs that I use.
The trick is to use the system consistently for all comets and for all
observations. I have found in my 50 years in astronomy that there are many
"wolf-criers" out there who like to see their names in print and in the Internet
circles. Many of these are professional astronomers and many of them are
amateurs held in high regard. Some of these folks will semi-intentionally put
out values that get our attention on a comet, simply so that their observations
will cause a knee-jerk reaction. Such is the case among the horribly scattered
data from which assessments such as c2013 US 10 (above) were being made.