[Aavso-photometry] Toads Campaign nd HV Vir

Tue May 27 06:11:41 EDT 2008

Hello George and Wolfgang!

I would bet it is a residual charge as Wolfgang suspected. They behave
just like that. I had one similar case not long ago when there was a
bright star on previous target. The same imaging script was run each
night so the 'bit fuzzy' star (a supernova suspect in fact) was
repeated very similar on different runs.

Maybe George was using a bright star to check the pointing of the
telescope just before observing HV Vir?

The residual charges are quite common with Kodak chips and fade away
quite slowly. You have to avoid or at least check bright stars on the
previous frames when doing photometry. Using V filter helps a lot as
the residual charge is mostly from IR light.

br,

arto

2008/5/27 Wolfgang Renz <w_renz at onlinehome.de>:
> 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
>
>
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-- 
Arto Oksanen
arto.oksanen at jklsirius.fi
Muurame, Finland