GROND Baffle Report

Prepared by F. Selman, 2005-12-15



The following reports the tests performed with the WFI and the new Baffle for GROND.

Several tests were performed:

(1) pinhole imaging of the telescope with the new and old baffles;

  1. dome and sky flat fields with the old and new baffles;

  2. sky flat fields at two very different zenith distances to determine baffle vignetting if any.

  3. Photometric zero point measurements

The results of the tests are shown below and summarized here: the new baffle appears to do a

better job at screening unwanted light than the old one. It does this without vignetting. There is

no significant difference in the dome flats taken with the new and old baffle: the ratio of dome

flats taken in different nights with the old baffle show similar variations. The ratio of the sky flats

are different and show a variation of ±2.5%, much of this difference is due to the use evening

sky flats for the old baffle and morning sky flats for the new one. Doing the ratio of morning and

evening sky flats with the old baffle show typical variations of ±1-2%. The photometric zero

points measured with the new baffle are, to within 1% for BVRI and to within 3% for U, the

same as those for the old baffle further supporting the no vignetting hypothesis. Given the many

sources of uncertainty in WFI photometry the author of this report does not believe that the new

baffle will degrade the performance of WFI as a photometric instrument.



Pinhole image with new GROND baffle:

The general aspect of the light distribution is similar to that

of the old baffle but with fewer bright regions. It appears that

this baffle does a better job at screening straight light than the

old one.


Pinhole image with OLD baffle:

We can see two bright spots above the center which were

not previously present. This should be investigated as it

points out to a missing ring or to a change in the FEROS

adapter.






Ratio of Dome Flat Field GROND baffle over OLD baffle (Rc filter):

The image below has the cuts adjusted to 0.98,1.02. Thus, the darker

spots are 2% lower and the brighter one are 2% higher than average. Over most

of the field the variation is below 1%.








Ratio of Sky Flat Field GROND baffle over OLD baffle (Rc filter):

The image below is the ratio of Sky flat fields obtained with the GROND baffle over the

sky flat field obtained with the new baffle. Regrettably, the GROND baffle flat field was

obtained during morning twilight flat while the OLD baffle flat field was obtained during

evening twilight. The peak-to-valley variations are on the order of 5%.




Ratio of Sky Flat Field GROND baffle High ZD (15 degrees) over Low ZD (60degrees) (Rc filter):

We have found in the past that if the baffle is vignetting the edges of the field, then

the ratio of flat fields obtained at different inclinations show a characteristic circular

pattern produce as a consequence of slight differences of vignetting product of flexions.

The image below shows the ratio of such flat fields obtained through the Rc filter

during morning twilight. The cuts have been adjusted to 0.98,1.02. No effects of

vignetting are apparent.




Below is a cut through the above image between the highest variation

region (upper left dark area to upper right bright area):






Photometric zero points:

They were determined measuring the Landolt standard stars in Tphe, placed

in chip #51. The zero points were measured using the standard procedure

for WFI with tmag on the raw frames (no bias subtracted and/or flat fielded):



Filter

Reference zero pt.

Measured zero pt.

Airmass

Delta (%)

U/50

22.22

22.19

1.13

3%

B/123

25.10

25.13

1.13

3%

V/89

24.23

24.23

1.14

1%

Rc/162

24.55

24.55

1.15

1%

I/203

23.54

23.54

1.16

1%