STIS Modes

Selecting a Slit/Filter

All of the supported slits for this Cycle are available on the ETCs. The pull- down menu gives the supported slits and filters for the chosen grating/prism. Since the filters and the slits are in the same wheel, you can choose either a slit or a filter. In case of the neutral density filters, the pull-down menu gives the density of the filter in “log10” units, i.e. ND=1 corresponds to an attenuation factor of 10, ND=2 corresponds to an attenuation factor of 100, etc. In the case of the slits, the pull-down menu gives the slit height and slit width in arcsec. To choose the best slit width, you may need to know the number of detector pixels corresponding to the slit-width. The table below gives the plate scales for different gratings.

Plate Scales for Different Gratings

Grating Plate_Scale_(arcsec/pixel)
Along_Dispersion Along_Slit
First Order CCD 0.05 0.05
G140L, G230L 0.0244 0.0244
G140M, G230M 0.0290 0.0290
E230M 0.0290 0.0290
E140M 0.0290 0.0290
E230H 0.0290 0.0290
E140H 0.0290 0.0290

Detector Countrate Restriction

The ETC uses the current MAMA countrate restrictions, which are taken from the STIS Instrument Handbook. If the observation will exceed any of the countrate restrictions, a warning message will be given in the output page. This is particularly useful to check if the observations will exceed the bright-object protection (BOP) limit of the MAMA detector. The countrate restrictions for the detectors are as follows:

CCD Saturation Limits

Gain Saturation Level Full Well
(electrons) (ADU) (electrons) (ADU)
1 33,000 30,000 120,000 [1] 120,000 [1]
4 120,000 [1] 30,000 120,000 [1] 30,000

[1](1, 2, 3, 4) The fullwell limit for gain=4 is 144,000 near the center of the chip and only 120,000 near the edges.

MAMA Countrate Restrictions for Different Modes

Detector Target Local Limit Global Limit
(First-order + Prism Modes) (Echelle Modes) (Imaging Mode)
(counts/sec/pixel) (counts/sec) (counts/sec) (counts/sec)
NUV-MAMA Non-variable 100 30,000 200,000 200,000
FUV-MAMA Non-variable 75 30,000 200,000
NUV-MAMA Irregularly Variable 75 12,000 80,000
FUV-MAMA Irregularly Variable 75 12,000 80,000

In the case of the echelle modes, there is some extra noise because of the scattered light which runs across the orders. The updated version of the ETC takes this extra noise into account (to a first approximation) in the calculation of the S/N ratio. The global countrate estimates take the scattering into account.

For imaging mode the peak countrates mentioned in the output need some explanation. The peak countrates are used only to check for the MAMA bright object protection issues. Since the health of the detector sometimes relies on our ability to predict the peak countrate in a given observation, we have been a bit conservative in our estimates of peak countrates, particularly for the MAMAs. At present, the calculation of peak count rates for the point sources assume that the encircled energy in the central pixel is 30% in case of the CCD, and 25% in case of the MAMAs. This can be sometimes over conservative and can be slightly larger than a factor of 2 for some MAMA modes. A more accurate algorithm will be added in the future, which will take the spectral shape of a given source and the filter combination into account to calculate the appropriate percentage of enclosed energy in the central pixel.

Detector Binning

The CCD has 3 binning factors: x1, x2 and x4. In Imaging mode the binning factors must be the same in each detector dimension, but for spectroscopy the factors can be different. Since the readnoise of the CCD applies only to a ‘binned’ pixel, using a binning factor greater than one can reduce the overall noise in some cases. However, this comes at the price of degraded spatial (or spectral) resolution. The readnoise and saturation characteristics of the CCD are given in the Table below. (Note that the MAMA detectors have no readnoise, so the binning option is ignored in these cases.)

Detector Background

STIS ACQ and ACQ/PEAK observations are always done using an unbinned subarray and a CCD gain setting of 4. For these parameters, the expected dark current (for low, medium (default), and high values) and read noise are similar to the CCD (Gain=4) line in the table below.

Detector Dark Read_Noise
CCD (Gain=1) [0.015, 0.018, 0.022] electron/pixel/s 6.2 e-
CCD (Gain=4) [0.015, 0.018, 0.022] electron/pixel/s 8.2 e-
NUV-MAMA 1.5 x 10-3 count/pixel/s
FUV-MAMA 1.5 x 10-4 count/pixel/s [2]
[2]Assuming a “low” STIS FUV-MAMA glow region value of 1\times10^{-4} counts/pixel/s contributing to the total dark rate.

Setting the CCD gain to 4 is useful if you are expecting a large number of counts. For faint sources, where the counts are expected to be lower, gain=1 may be preferable since the readnoise in that case is lower.

To avoid an excessive number of cosmic ray detections with the CCD, longer exposures should be split (CR-split) in order to:

  • keep the number of detected cosmic rays low
  • be able to remove the cosmic rays during data reduction

Using CR-split increases the effective readnoise, which is taken into account in the exposure time calculations. The default CR-split is 2.

Selecting a Checkbox Size for STIS Target Acquisitions

For extended source acquisitions, the user sets CHECKBOX=n, where n must be an odd number between 3 and 105: the checkbox will then have dimension n x n pixels. CHECKBOX should be set to the minimum size which ensures that the brightest checkbox will be the one centered on the region of interest (i.e., if your object is peaked within a region of 1 arcsecond, set CHECKBOX=21 [= (1 arcsecond) / (0.05 arcsecond pixel-1) + 1]. The maximum checkbox is 105 pixels on a side, or ~5X5 arcseconds. The subarray used for a diffuse-source acquisition target image is CHECKBOX+101 pixels on a side. The STIS Target Acquisition Simulator can be used to determine the optimal CHECKBOX size.