Science Operations

Looking at the Data: Image Gallery

Contents

  1. Overscan and trim regions
    1. Discontinuities and gradients in the LBCB overscan region
  2. Electronic Noise
    1. LBCB Chip 2 Noise
    2. Low-level “horizontal” banding:
      1. LBCB
      2. LBCR
  3. LBCR “Chevron”

On this page, several features and artifacts of LBC images are collected and discussed. Also, examples of pupil images which are good, characteristic of good seeing and collimation, and poor, showing aberrations which the focus/collimation routine may have trouble handling, are provided.
This page is under construction. Please check back for updates.

Overscan and trim regions

Each LBC chip has a 50-column prescan and a 206-column overscan region. The science data cover 2048 x 4608 pixels and this is recorded in the headers as trimsec = [51:2098, 1:4608].
The overscan region runs from 2099:2304 and in the headers, this is recorded as biassec = [2099:2304, 1:4608].  Note, however, that there is a discontinuity and both vertical and horizontal gradients in the LBCB overscan regions. These must be dealt with in the reduction. We hope that this feature will go away when the current controller is replaced. The LBCR overscan regions do not show similar structure and appear flat.

Discontinuities and gradients in the LBCB overscan region

For LBCB, there is a discontinuity in the overscan level that occurs 50 columns into the overscan.  For this reason, it is recommended to adopt an overscan region that extends across all rows, but avoids at least the first 50 columns of the overscan section, biassec =  [2115:2300,1:4608].
It is always a good idea to examine the overscan of one or two images before removing it. Also, because of the horizontal banding issues described below, it is recommended to remove the overscan row-by-row rather than make a smooth fit to the combined columns as a function of row.
The plots below illustrate the discontinuities in the overscan region and also the fact that the prescan and overscan levels differ.

The LBCB overscan regions also show vertical and horizontal gradients. These are illustrated below:

The figure above shows the first 1000 rows for the 1st bias (lbcb.20210212.145619.fits) and the 18th one (lbcb.20210212.150600.fits), in a series of 25 biases. The 50 columns of prescan and the 206 columns of overscan are included in both cases. The discontinuity and the gradients are easily visible in the first bias while in the 18th, all that rremains is a horizontal gradient in the data section.

In the figure above, the first 1000 rows of the overscan region of series of 3, 3 and 5 twilight sky flats are displayed. The discontinuity and gradient appear strongest in the first image of the series, and both gradually diminish throughout.

Electronic Noise

LBCB Chip 2 Noise

Since Dec 2017, we have occasionally seen noise in chip2 of LBCB. The noise shows up only in biases and low-background images, and it seems to appear only in colder conditions (less than ~0 C).
In late September, 2020, the controller (#6) was swapped out and another one (controller #0) was swapped in to see whether this would make a difference. As of late October, we have not seen any “LBCB chip 2 noise”, however it has not been that cold yet. It is not clear that the controller swap will resolve this issue, and we are discussing it with the LBC team and Skytech.

The copointing image above illustrates the LBCB Chip 2 noise, and the histogram of counts for chips 2, 3 and 4, below, shows the clustering of counts in chip 2, but not in chips 3 or 4, which is suggestive of a stuck bit.

Low-level “horizontal” banding:

LBCB

Low-level “horizontal” banding was seen occasionally in October 2020, when controller #0 was being used for LBCB Science. Eventually, it was swapped out and controller #6 is being used. This banding has not recurred.

At the end of September 2020, the repaired controller (0) was received from Skytech and swapped in for LBCB Science controller (6) in a pro-active measure to try to prevent the “LBCB Chip 2 noise”. The initial set of biases looked good, however on October 01 we noticed noise in the biases. The noise pattern is not exactly aligned with the rows and therefore the overscan does not perfectly remove it. It is low-level (6 ADU from peak to valley) and it does not contribute significantly to the read noise — the effective read noise (measured by the standard deviation of counts in a subregion of the chip) is only 1 e- higher when the noise is seen than when it is not seen.

Even though the noise is low-level, the pattern causes it to stand out on biases and on low-background exposures, such as the copointing images. Below is an example of a copointing image which illustrates the noise on the raw image (left) and also the effect of overscan subtraction (right).

This noise was present on Oct 01, but did not reappear until Oct 16, and now it seems to be a constant feature. We are discussing this with the LBC team and Skytech.

LBCR

Since September 2019, we have occasionally seen low-level banding on LBCR biases and low-background exposures. This is actually aligned with the rows and comes out pretty well in the overscan subtraction.

LBCR “Chevron”

At the top of LBCR chips 2 and 4, there is a ‘chevron’-shaped artifact that does not come out through flat fielding.
The image below shows the ‘chevron’ at the top of LBCR chip 2.

The zoom below shows unrelated features which are typically seen on all chips of LBCR images: horizontal lines at multiples of 512 (perhaps a clock or timing issue) and horizontal ripples.