CCD parameters

Postflash

Postflash has been available for use with the WFC3/UVIS channel since Cycle 20, and the ACS/WFC since Cycle 11. Its use is recommended for observers concerned about effects of CTE losses on their data, especially for faint sources and/or low backgrounds. The WFC3 and ACS Exposure Time Calculators (ETCs) enable users to calculate signal to noise (SNR) and exposure times for imaging data when adding postflash electrons. The ETC treats postflash in a similar manner to readnoise in that postflash noise is time-independent. A non-zero postflash adds additional dark-current time to each exposure, in the amount of 1 second per 14 postflash electrons for ACS, and 0.4 seconds per postflash electron for WFC3 (or about 5 seconds for 12 electrons/pixel). This small additional dark-time is currently not factored into the ETC calculations, but has negligible contribution to the overall noise budget.

Post-flash electrons are entered into the appropriately labeled box under “Specify Additional CCD Parameters”. The default value is 0 electrons. For WFC3/UVIS and ACS/WFC, users can enter integer numbers between 0 and 40 (WFC3) and 5733 (ACS) electrons per pixel. We recommend first running calculations without postflash (i.e. the ETC default value). If the background level per pixel is less 20 electrons/pixel, the ETC issues a warning message along with the estimate of the background. Note that the ETC predicted backgrounds will vary depending upon zodiacal light, earthshine, and airglow, effects specified in Section 5 (“Specify the expected background levels”) of the ETC.

For advice in choosing the optimum flash level for WFC3/UVIS, users can consult WFC3 ISR 2020-08: Strategies for Mitigation of CTE Losses in WFC3/UVIS. For advice in choosing the optimum flash level for ACS/WFC, users can consult ACS ISRs 2014-01 and 2018-02. For both instruments, the choice of post-flash level should be made taking source structure and peak flux level of the target into consideration, as discussed in the ISRs.

Gain

All CCD calculations assume Gain = 1. CCD Gain is used only for readnoise and saturation values.

ACS CCD Parameters

Subsequent to Servicing Mission 4 (SM4), ACS/WFC supports only GAIN=2 imaging. This critically samples the WFC readnoise and avoids digital saturation at the CCD full-well capacity of ~84000 electrons. Prior to SM4, GAIN=1 had also been supported for WFC. The ACS/WFC ETC still allows GAIN=1 for historical purposes, but does not currently check for digital saturation (when electrons per pixel exceeds 65335).

The High Resolution Channel of ACS was not recovered during SM4, so it remains unavailable for any science or calibration uses.

The user can specify the number of distinct frames (exposures) composing an observation to mitigate the deleterious effects of cosmic-ray hits and bad pixels. Cosmic-ray hits in long exposures can be remedied with the “CR-SPLIT” parameter, which allows (1) fewer numbers of detected cosmic rays per exposure, and (2) the identification and removal of cosmic-rays during data reduction. Dithering is recommended over the use of CR-SPLIT because dithering mitigates the effects of both cosmic-ray hits and bad pixels and enables greater sampling of the point-spread function. Within the ETC, users should be aware that the term ‘CR-SPLIT’ refers to the sum total of independent exposures, be they dithers and/or actual CR-SPLITs. Also note that the ETC default is CR-SPLIT=2, whereas the default Phase 2 APT configuration for ACS imaging is ‘CR-SPLIT=NO’ and un-dithered. If the ETC user wishes to run the calculation for the case of a single exposure, the value 1 should be entered into the CR-SPLIT field.

Quantum Yield correction for CCD detection of UV photons

For CCD detectors in the optical, each detected photon usually generates a single electron (i.e., photons absorbed × the gain correspond to the total number of electrons). However, in the near UV, shortward of ~3200 Å, there is a finite probability of creating more than one electron per detected UV photon (see Christensen, O., 1976, J. App. Phys., 47, 689). The throughput curves adopted in pysynphot correctly predict the number of electrons generated per incident photon and implicitly include this UV quantum yield correction. However, since multiple electrons are generated from a single photon, the actual number of photons detected, and therefore the S/N obtained, is less than the number of electrons detected would suggest.

To take this into account, the ETC corrects the number of electrons calculated by pysynphot by dividing the results of the pysynphot calculation by Q, the mean number of electrons generated per photon. For imaging mode calculations, this correction is calculated by applying the correction appropriate for photons at a wavelength equal to the “effective wavelength” determined for the pysynphot calculation. For spectroscopic CCD observations, Q is calculated correctly for each wavelength bin. The “source count” rate reported by the ETC for CCD observations is actually this corrected count rate rather than the true number of electrons predicted by pysynphot. However, the true uncorrected number of electrons is used for comparison with the CCD saturation limits and for the “Brightest Pixel (single exposure)” quantities.

STIS CCD Parameters

For the STIS CCD, the default gain value of 1 offers the lowest read noise, but the analog-to-digital converter then limits the maximum signal that can be detected without saturation to about 33,000 e^- per pixel. Using a CCDGAIN setting of 4 allows the full well of the CCD to be used. The ETC assumes a full-well value of 120,000 e^-, corresponding to the full-well near the edges of the CCD. The full-well near the center of the CCD is actually higher, up to 144,000 e^-. The CCDGAIN=4 setting has the disadvantage that an additional large scale pattern noise is imposed on the image. It has the advantage that the CCD response when using CCDGAIN=4 remains linear even beyond the full well limit if one integrates over the pixels bled into; for specialized observations needing extremely high S/N, this property may be useful.

The STIS CCD also allows pixels to be binned by factors 1, 2, or 4 during the readout. This will reduce the read noise at the expense of spatial information. For spectroscopic observations, separate binning factors can be specified for the spatial and dispersion directions; for imaging observations, the same binning is used on both axes.

For the STIS CCD, CR-SPLIT=2 is the default.

STIS ACQ modes always use the CCD with CCDGAIN=4 CR-SPLIT=NONE and no binning, so the STIS ACQ mode ETC does not include any user adjustable additional CCD parameters.

Dark rates of [low, medium, high] correspond to the [top (E1 position), middle, bottom] of the detector.

For calculations using the STIS MAMA modes, the settings specified for the additional CCD parameters are ignored.

WFC3 CCD Parameters

Postflash

The WFC3 CTE webpages are here.

For comparison to ETC-predicted backgrounds, the table below summarizes for each filter the approximate exposure time (in seconds) required to reach 12 e^-/pix background (based on archival data evaluated in Baggett & Anderson, ISR 2012-12, WFC3/UVIS Sky Backgrounds.

Filter	Exptime	Filter	Exptime	Filter	Exptime
F200LP	150.9	F438W	1290.7	F645N	4056.3
F218W	14534.2	F467M	3126.6	F656N	3689.8
F225W	12000.0	F469N	6829.3	F657N	2400.0
F275W	12000.0	F475W	628.9	F658N	3557.3
F280N	7819.1	F475X	412.5	F665N	5131.6
F300X	8188.2	F487N	3384.6	F673N	1954.7
F336W	2635.2	F502N	4876.2	F680N	2797.5
F343N	8188.2	F547M	940.6	F689M	894.5
F350LP	133.8	F555W	342.9	F763M	1409.1
F373N	5859.0	F600LP	316.7	F775W	557.4
F390M	5131.6	F606W	316.7	F814W	517.8
F390W	1290.7	F621M	868.0	F850LP	1025.6
F395N	4056.3	F625W	412.5	F953N	5131.6
F410M	3126.6

# Frames

Here the user can specify the number of distinct frames (exposures) comprising their observation.

This is analogous to the “CR-SPLIT” parameter that was popular in the early days of HST, but today has been supplanted by distinct exposures taken in tandem with dithering, allowing not only the rejection of cosmic rays but also the masking of detector artifacts and a resampling of the point spread function.

Note for WFC3 IR: This item does not specify the number of non-destructive reads taken within a single exposure, but rather specifies the number of independent exposures (e.g. different dither pointings). The readnoise for each independent exposure (“frame”) assumes the maximum allowed number of non-destructive readouts and full frame.

Detector Chip

The WFC3 detector consists of two halves, similar to the ACS/WFC. Chip 2 is more sensitive in the UV than chip 1.

CCD Saturation Limits

Detector	Saturation	Threshold	Subthreshold
	(electrons)	(DN)
IR	70,000		0.7
UVIS1	64,000	41,290 [1]	0.9
UVIS2	67,000	43,226	0.9

[1]

WFC3 UVIS channels Gain 1.5 corresponds to 1.55 e^-/DN