PGC55

pgc55 RA: 0.15583 Dec: 33.60094 Ra = 0.67 (arcmin) Rb = 0.20 (arcmin) PA = 13.12 (degrees)

# position angle (PA) [Degrees: Up=0, Left=90]"

run_sex <root> <ra> <dec>
./run_sex.csh pgc55_g 0.15583 33.60094

SExtractor first outputs

filter	RA	DEC	MAG	Re	Ra	Rb	b/a	PA
	deg	deg	mag	arcmin	arcmin	arcmin		deg

pgc55_u	0.1555	33.6014	17.03	0.17	0.75	0.20	0.27	9.30
pgc55_g	0.1555	33.6013	16.10	0.15	0.67	0.20	0.30	13.12
pgc55_r	0.1555	33.6012	15.74	0.15	0.63	0.20	0.31	13.20
pgc55_i	0.1555	33.6012	15.53	0.14	0.65	0.20	0.31	13.53
pgc55_z	0.1555	33.6012	15.61	0.12	0.50	0.14	0.28	12.21

GLGA-qa

minskywid = 27. / as_pix	 ; 27 arcsec is minimum sky width
factor = 1.5                     ;  1.5 default
skyinner = factor*semimajor
skyouter = semimajor*sqrt(1+factor^2) > (skyinner+minskywid)  


Sigma multiplier for thresshold (default =  2.5): 2.0
Magnitude faint limit (default =  24.0): 25

I increased the aperture size, until the difference between APR and ASY magnitudes are less than 0.05 mag.

SKY

• u

  final sky/px, skyerr/px     [mJy/px]:    0.00068215   0.00010009  ds/s = ~15%
  final musky, muskyerr  [AB mag/as^2]:     22.304     0.159
  

• g

  final sky/px, skyerr/px     [mJy/px]:    0.00083100   0.00003426  ds/s = ~4%
  final musky, muskyerr  [AB mag/as^2]:     22.090     0.045
  

• r

  final sky/px, skyerr/px     [mJy/px]:    0.00194121   0.00005404  ds/s = ~3%
  final musky, muskyerr  [AB mag/as^2]:     21.168     0.030
  

• i

  final sky/px, skyerr/px     [mJy/px]:    0.00371400   0.00009490  ds/s = ~3%
  final musky, muskyerr  [AB mag/as^2]:     20.464     0.028
  

• z

  final sky/px, skyerr/px     [mJy/px]:    0.01061923   0.00031765  ds/s = ~3%
  final musky, muskyerr  [AB mag/as^2]:     19.323     0.032
  

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Changing the sky aperture

Now, I keep everything the same but the size of the inner aperture

minskywid = 27. / as_pix	 ; 27 arcsec is minimum sky width
factor = 2.0                     ;  1.5 default
skyinner = factor*semimajor
skyouter = semimajor*sqrt(1+factor^2) > (skyinner+minskywid)  

SKY

• u

  final sky/px, skyerr/px     [mJy/px]:    0.00068317   0.00010508  ds/s = ~15%
  final musky, muskyerr  [AB mag/as^2]:     22.302     0.167
  

• g

  final sky/px, skyerr/px     [mJy/px]:    0.00083044   0.00003543  ds/s = ~4%
  final musky, muskyerr  [AB mag/as^2]:     22.090     0.046
  

• r

  final sky/px, skyerr/px     [mJy/px]:    0.00193863   0.00005487  ds/s = ~3%
  final musky, muskyerr  [AB mag/as^2]:     21.170     0.031
  

• i

  final sky/px, skyerr/px     [mJy/px]:    0.00371275   0.00009834  ds/s = ~3%
  final musky, muskyerr  [AB mag/as^2]:     20.464     0.029
  

• z

  final sky/px, skyerr/px     [mJy/px]:    0.01062065   0.00033289  ds/s = ~3%
  final musky, muskyerr  [AB mag/as^2]:     19.323     0.034
  

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• left : factor = 2.0 i.e. expand by 100%
• right: factor = 1.5 i.e. expand by 50% (original)

---

Knowing the fact that estimated error for the background level is <~5%:

• left: Adding 1% to the estimated sky value

  IDL code: sdss_radprof.pro
  skyint = 1.01*skyint     ; ehsan-edit
  

• right: original

Left panel shows that, the sensitivity to the sky background is too high and probably the estimated precession for the sky background level is wrong.

Final conclusion up to this point (for pgc55): If the profile looks good, we can claim that we've got to the precession of 5% in magnitude (e.g. 0.05 mag).