[Aavso-photometry] Toads Campaign nd HV Vir

[Wolfgang Renz]

Hi George

> I attached an image.

The single FITS V band image you sent me was taken with MaxIm
DL V4.58, is dated '2008-05-24T04:20:16' UTC and has an expo-
sure time of 90 sec. The peak value of the brighetst star in the FOV,
the 13.0 mag V comp, is just 482 ADU above the mean image back-
ground of 108 ADU with a StdDev of ~ 9.0 ADU (which rises to 32.5
ADU for the full frame with stars, cosmics and hot pixel). I get with
Mira a brightness of 14.87 +/- 0.03 mag for the object using the 4
brightest comps wit mmag accuracy.

The HISTORY entry in the FITS header says that the darks were just
60 sec long and a BDF (bias+dark+flats ?) was applied with MaxIm
DL V4.61 (that I don't own and therefore cann't say much about).

Did you use the scaled darks or the matching darks option ?

I ask, because the image shows some hot pixel although being fully
calibrated and I found accidentially the hotest pixel with 35826 ADU
at position (797,378) that spoiled the centroiding for the radial profile
of a close star.
There are in total about 2 dozen hot pixel with ADU vales above ~
800 ADU that didn't calibrate out ! This should not be normal for a
calibrated light frame!
So its a hint that probably something went wrong with the calibration.

How old were the used (1?) bias and (4?) dark frames ?
How many bias and dark frames did you actually use ?
Usually one uses a median or min-max/sigma/SDM-rejected master
of at least 10 of them that were taken close in time. Taking more of
them is much better. I usually use ~ 60 SDM-rejected frames to cre-
ate my bias and dark masters. I usually take 10 of them before I start
imaging and 10 afterwards and combine the data of 3 obs night for
the master. This ensures that I've always a sufficient number of cali-
bration frames even if something significantly changes or if I have to
stop imaging for some reason.

If you have hot pixel in the non-linear range or even saturated, its
probably better to use time-matching darks. If you don't use too
many different exposure times, its not to much additional work, I
take dayly bias frames too, but just to check for changes in the
bias structure and to be save if I forget to take darks at an expo-
sure time. I usually use skaled darks just for calibrating my twilight
flats (as I try to keep the max ADUs in the flats at a high, about
constant level, I adapt the exposure time every or every second
image) or if I use too many different exposure times.

The V band image was taken at -20.4°C when set to -20.0°C and the
FITS header says that the temperature of the bias+dark+flats were
-20°C. So this shouldn't be the isse.

> > > More confusing is the fact that on two nights running i find on my
> > > images a mag 14.6 star where the chart (080520) says HV Vir
> > > would be. After the first night I didn't report anything because I
> > > feel am in test mode.  When the same star appeared after last
> > > nights run I assumed I would find something in recent observations.
> > > But CMP for May 24 shows a mag less than 17.1 in V (the same
> > > filter I was shooting thru)  Can someone help me understand what
> > > my mistake(s) are? I can supply the images if that helps.
> > > I am using a C14 and an ST8 (NABG)
> > 
> > Was your first run in begin of March ?
> > At that time it was at ~ 14.5 mag:
> > http://www.aavso.org/cgi-bin/newql.pl?name=HV%20Vir&output=html
> 
> First image was two nights ago

OK, then it must be something else

> > If its not this and HV Vir (or a background star) doesn't show an
> > outburst, it might be one of those:
> > 
> > How exacly is it the same position ?
> > Are you sure its not the 14.1 mag comp star ?
> 
> I'm rather positive it's not comp star 141

Yup, in that image it really looks like that its right at the position of HV Vir.

> > Have you already checked at the MPC if it might be a minor planet ?
> 
> The object did not move in the 24 hour period. I blinked them to make sure

That just says that, if its a moving object, its very very slow, or heads
right at us. But thats is both probably very improbable. Its even less
probable that two different objects are at the same position in your
two runs.

Well, the MPC says:
"The following objects, brighter than V = 30.0 mag, were found in the
10' region around R.A. = 13 21 03.10, Decl. = +01 53 30.0 (J2000.0)
on 2008 05 24.18 UT:
Object designation        R.A.        Decl.     V     Offsets '     Motion "/hr  Orbit  Further observations?
                          h  m  s     °  '  "   mag   R.A.   Decl.  Mot.  PA °      #   Comment (Elong/Decl/V at date 1)
         2002 CP28       13 21 03.8 +01 54 14  20.9   0.2E   0.7N   12.2 252.7     3o   None needed at this opposition.
 (13956) Banks           13 20 31.0 +01 50 55  16.7   8.0W   2.6S    5.4 265.3    11o   None needed at this time."
So both too far off.

> > Have you checked if it might be a hot pixel/cluster or a cosmic ?
> 
> Not either.

Yup, its too large for them.

> > Did you make more than one image per night ?
> 
> Each night I did a VBRI series. It appears in all.

OK, then it shouldn't be a cosmic, if it appears at the same position
in 8 images.

> > Does the bias/dark/flat frames show something at that position ?
> 
> This was my first thought, but there are no such artifacts on the flats,
> bias or darks. It also appeared on the un-processed image.

Did you check them with sufficient histogram streching ?
The ADU range betwenn min and max should not be more than 50
ADU.

> > Does it look star-like ?
> 
> Yes. Perhaps a bit fuzzy
> 
> > Is the FWHM and the radial profile similar to a close star of nearly
> > equal birghtness ?
> 
> Its FWHM is 11.7.  Ref star 141 is 8.01

Compared with the other stars in the ~ 6'x8' FOV, its indeed fuzzy.
The object looks about round and has a FWHM of 13.5 pixel with a
peak value of ~ 41 ADU above the background.
The stars with a peak values below that of the object have one of 5.9
to 9.4 pixel and stars with peak values above that of the object have
one of 7.0 to 8.7. So its consistent constant with the median at ~ 8.1
pixel.

So it looks like as its definitly not a star that was imaged through the
optics like the other star in the image. But if its an extended object
of that size, it must be pretty close if it changes its brightness so
dramatically within 2 hours.

> > Is it at the same position of the CCD ?
> 
> Not sure of your meaning, but the images on the two nights are on
> different parts of the ccd chip.

Thats what I meant.
If it was located in different parts of the CCD, its very probably not a
reflection.


So, now I run out of ideas.
Oh, ..., wait, I've an other one.

Did you image or focus on a real bright star unfiltered or with the Ic filter
just before the two runs ?

Then it might be a RBI ghost (which doesn't explain why its exactly
at the same position in the two runs).


Sombody else with other ideas ?


Clear skies
Wolfgang

--
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO