Results from on-sky SensitiveAngleTracking engineering (Feb18,2008, 40min)
contact: Jorg-Uwe Pott
Limiting magnitudes- Lband dichroics
Measured KAT.intensity counts:
HD77948 (J/H/K = 6.5/6.1/6.0),
data from UT: 11:19:08-11:22:07 (AT1..2,FT1 tracking)
AT-intensity plots look relatively stable, so we just give the mean/stddev:
Primary side
Secondary side
AT1: 8.7k/0.8k
AT3: noise: 0.3k/0.2k
AT2: 8.8K/0.7k
AT4: noise: 0.3k/0.1k
FT0tracking: 3.6k/0.6l
HD78715 (4.5/4.1/4.1)
data from UT: 11:33:50.00-11:35:0.00 (AT1..4,FT1..2 tracking)
AT-intensity plots look relatively stable, so we just give the mean/stddev:
Primary side
Secondary side
AT1: 75k/3.6k AT3: 1.8k/0.1k
AT2: 75K/4.7k
AT4: 1.6k/0.1k
FT0tracking: 25k/5.6k (Nph00)
FT1tracking: 38k/5.1k (Nph00) (note: this is the standard second FT in the prim.
FTcamera, we are in Nuller mode)
Results / Questions:
*) We talk on average about 44x(4.1mag) more flux on the primary side due to using the
Lband dichroics and K/N transmission, in Nuller configuration (not SPR), this is weird,
since at daytime we measured 25x..30x more flux thru the Lbands, and the primary side
having the K/Ns in transmission should get intrinsically less flux due the the imbalance in
T/R of the K/Ns, this cannot be a K1 vs K2 problem: Is there another H-band insensitive
optic in the S-path? – A possible reason for higher nighttime (starlight) P/S flux ratios
than daytime (white light source) test numbers is that the starlight has a broad NIR
continuum, while the white light is peaked. (MMC: “At H-band, the FATCAT internal
source + coupler has most of its output at 1650 +/- 35 nm, with much less at the band
edges.”)
*) The 2MASS derived intrinsic flux-ratio of (6.3+/-1.0)x is slightly below a mean KAT
count ratio of 8.5x for the two test stars, which might be due to wrong mags, or a slight
non-linearity of KAT intensity towards fainter sources. – Also a different transmission
profile of the 2MASS and KAT-H-band filters can contribute, since the stellar spectral
types are slightly different (HD78715:G5III and HD77948: K0III).
*) Since KAT can track on the level of 750 counts (a factor of two lower than seen at this
experiment), the primary side can be pushed down to ~8.6mag (assuming correct 2mass
mags) in Nuller mode, or down to ~9.4mag in V^2-SPR (including 50% flux gain due to
the missing pupil-split) or down to 10.6-11.1mag in V^2 (assuming 0.2..0.3 transmission
of the K/Ns), which is consistent with the expectations. Although apparently the K/N’s Jtransmission / reflection ratio is closer to 1 (MMC).
*) Is it normal that AT*.Intensity and AT*.CentroidIntensity show EXACTLY the same?
Yes, it is in normal KAT centroiding, without applying the new algorithm.
*) Apparently fringe-tracker-status TM_LOCK is lost when Nph00 falls below 2e4
counts (e.g. with HD78715 at about UT11:28:27), is that a normal limit? No, it’s not.
MMC: The fringe tracker can go much fainter than that. It depends on source V2, but it
can go less than 500 dN WL+Spec (from older testing, at 150 dN on WL only, we were at
the fringe-tracking limit; with the spec coadded in, this was probably a total effective WL
flux of 350 to 400.)
K-band FATCAT dichroics – comparison measurement
ATTENTION: these KAT numbers are J-band
Star: HD93391 (J/H/K: 4.3/3.5/ 3.2)
data from UT: 14:27:58-14:31:51 (AT1..4,FT0..1 tracking)
AT-intensity plots look relatively stable, so we just give the mean/stddev:
Primary side
Secondary side
AT1: 37k/3k
AT3: 67k/6k
AT2: 24K/4k AT4: 45k/7k
P/S=0.55x
(fluxratio)
FT0tracking: 42k/10k
FT1tracking: 58k/20k (note: this is the second FTcamera, we are in SPR mode)
data from UT 14:54:24-15:00:38 (AT1..4,FT0..1 tracking)
AT-intensity plots look relatively stable, so we just give the mean/stddev:
Primary side
Secondary side
AT1: 32k/3k
AT3: 58k/6k
AT2: 18K/3k AT4: 33k/6k
P/S=0.55x
(fluxratio)
FT0tracking: 32k/8k
FT1tracking: 45k/17k (note: this is the second FTcamera, we are in SPR mode)
data from UT 16:18:03-16:25:27 (AT1..4,FT0..1 tracking)
AT-intensity plots look relatively stable, so we just give the mean/stddev:
Primary side
Secondary side
AT1: 51k/8k
AT3: 57k/9k
AT2: 38K/5k AT4: 47k/7k
P/S=0.85x
(fluxratio, why so high?)
FT0tracking: 38k/9k
FT1tracking: 17k/7k (note: this is the second FTcamera, we are in SPR mode)
Results: A mean flux increase by a factor 1.8x from PrimATs 1&2 to SecATs 3&4
intensity is observed, and reflects the known NIR-transmission deficiency of the K/N's
(0.2..0.3 transmission vs. 0.5 reflection). However the numbers from the last part a little
inconsistent, only 1.2x flux-ratio in favor of secondary channel, and in both last part
significantly higher K1/K2-ratio than normal. Why?
Correlation between primary and secondary beamtrain tip-tilt correction
*) J-band-KAT with ordinary K-band dichroics:
The following plots of J-band angle-tracking on HD93391 (J=4.3mag) show ‘Y-vs-Y’
plots of the TTM-telemetry items fastX and fastY.
They demonstrate that a strong correlation exists for each telescope between the P- and
S-beamtrain (elongated diagonal feature in the Y-vs-Y plot), and that no significant
correlation exists between the telescopes.
Keck2-P-vs-S, fastx
Keck1-P-vs-S, fastx
Keck1-Keck2-P-vs-P,fastX
*) In the SAT-H-angle-tracking data on HD78715 (H=4.1mag) such a correlation is only
barely seen. The reason is that we are not in SPR-mode, so the complete secondary LDL
beamtrain is seen on AT3&4, and in addition there is low flux on the secondary side, so
less SNR. On the second (brighter) star of the SAT experiment fringes have been tracked
for some time. If we pick out those times only there appears some correlation in the
YvsY plots, where when the Nph00 flux drops, and the FT-loop breaks, both P- and Sside appear to be uncorrelated (circular Gaussian Y-vs-Y-plot).
K2PfastX vs K2SfastX, FT1&2 tracking before FT-loop-break
K2PfastX vs K2SfastX, FT1&2 not tracking
K2PfastX vs K2SfastX, FT1&2 tracking again, after FT-loop-break
*) Results: The implementation of a Primary-toSeconday angle-tracking feed forward
appears worthwhile in particular for the SPR case, as long as on one side there is much
more flux, that is a higher AngleTracking SNR, as for the SAT case. This would be a
prerequisite for the combination of SAT and SPR.
K2PfastX vs K2SfastX, FT1&FT2 tracking
K1P vs K1S, fastX-fastX
K2P vs K2S, fastY-fastY
Since it can be assumed that the tip-tilt correlation seen in J, doesn’t break down in H,
this here apparent non-correlation between P- and S-beamtrain is probably due to the low
flux in the seconday beam train, that is angle-tracking feedforward might have been