HLA WFPC2 Source List Photometric Quality Checks

Version: August 25, 2008

Brad Whitmore

1. Introduction

2. Comparison with Ground-based Stetson Photometry

3. Comparison with Repeat HLA WFPC2

4. Comparison between WF and PC

5. Comparison with HLA ACS Source List

6. Summary

1. Introduction

While HLA WFPC2 source lists are not included as part of Data Release 2 (DR2), we have performed a number of checks to verify the integrity of the images and to quantify the rough level of accuracy that can be expected.

Some of these test are included in this packet. In addition, a demo source list is available via the HLA FAQ titled “What do I need to know about photometric zeropoints for the ACS and WFPC2 images and source lists”. A PASP article is being drafted.

The single most important point is that the HLA images are in units of ELECTRONS/SEC (like the ACS), not DATA NUMBER (DN).

2. Comparison with Ground-Based Stetson Photometry( available at: http://www3.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/community/STETSON/standards/ )

• The comparison is complicated by the fact that this is a very crowded region so ground-based observations often have companions in the aperture.

• “Best” sources on the right (solid circles) are handpicked to not have companions and not be saturated (bright stars on the left part of the figures).

CONCLUSIONS

• Zeropoints match ground based to ~0.03 mag (NOTE: IMAGES ARE IN ELECTRONS/SEC NOT DN).

• WFPC2 photometry is at least good to RMS ~ 0.05 (e.g., a better test of internal accuracy in the next viewgraph)

• Transformations between Johnson and HST filters are an important uncertainty.

This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency.

3. Comparison between repeat HLA WFPC2 Source Lists

Comparison with no shift in position (hence no CTE issues) shows:

• RMS < 0.01 mag for bright objects

• RMS ~ 0.05 mag for fainter objects.

This demonstrates that the HLA WFPC2 data has the potential to be roughly as accurate (but generally not as deep) as ACS for relative photometry.

• The bottom plot has large shifts and rotation in the two images, hence CTE should degrade the comparison.

• RMS increases from:

• 0.01 to 0.02 (bright)

• 0.05 to 0.07 (fainter )

4. Comparison between WF and PC

An important complication of working with the HLA WFPC2 images is that they include data from both the Wide Field (WF) and Planetary Camera (PC).

While nice for continuity, this complicates the job of doing photometry

The PC chip has been resampled to match the scale of the WF, but it is impossible to match all the characteristics.

An important question is how similar is the photometry on the WF and PC.

• The upper right plot shows that repeat PC observations are similar, but slightly worse quality than on the WF due to the resampling.

• The bottom three plots show significant offsets (~0.2 to 0.4 mag) between the WF and PC, due to greater CTE loss on the PC.

• HLA source lists, when available, will correct for this to the degree possible.

4. Comparison Between WF and PC - continued

As shown on the previous viewgraph, caution is required when doing photometry with HLA WFPC2 images, due to the increased Charge Transfer Efficiency (CTE) loss on the PC relative to the WF chips.

In some cases the effect may not be this severe (e.g., when using efficient filters which result in higher backgrounds: see the WFPC2 Website and Instrument Handbook for details about CTE loss; and the next viewgraph).

However, in many cases it may be necessary to either attempt to correct for CTE loss, or to remove the objects on the PC from your sample.

See the HLA FAQ titled “An IRAF script for determining the original chip number, and the original X and Y positions on the chip for WFPC2 images”.

Also note that if you are only interested in the objects on the PC, you should use the WFPC2-PC images available under the Advanced Search since they have the full spatial resolution, rather than the combined WFPC2 images.

4. Comparison Between WF and PC - continued

This figure (from Whitmore,

Heyer, & Casertano, 1999,

PASP, 111, 1559) shows the

primary dependencies of CTE

loss:

1. Target brightness

2. Background level

3. Time since launch

The primary reason for the

relatively large CTE loss (~ 30

%) in our example is the low

background (i.e. since using

F439W and on the PC which

has smaller pixels, hence

lower background).

5. Comparison with HLA ACS Source

List A comparison between an HLA

ACS and WFPC2 source list.

While generally satisfactory

(especially for brighter sources),

the RMS for fainter sources is

rather large. This is due to two

related factors:

1. This is a very crowded region

2. The apertures have not been matched (i.e., 3 pixels in both cases hence 0.15” for ACS and 0.30” for WFPC2).

Summary

The HLA WFPC2 source lists are still under development and will not

be ready for circulation until the Fall or Winter 2008.

An important point is that the HLA images are in units of ELECTRONS/SEC (like the ACS), not DATA NUMBER (DN).

The prototype catalogs indicate that good photometric accuracy can be obtained from the mutidrizzled chip-combined HLA images (e.g., zeropoints and RMS generally match to a few hundreds of a mag for bright sources.)

However, caution is required when including data from both the WF and PC at the same time (primarily due to different levels of CTE loss).