Grism Dispersion

PROCEDURE : -

TITLE : Grism Dispersion

CATEGORY : Calibration

PURPOSE :

The main goal is to measure the wavelength dispersion of the G800L grism and its spatial dependence over the HRC and WFC fields. Information on the order separation and ratios will also be obtained.

FREQUENCY :

Execute once.

DETECTOR :

Flight build detectors HRC#1 (amp C) and WFC#4 (amps ABCD).

PREREQUISITES/BACKGROUND :

The nominal dispersion of the G800L grism is 40 Ang/pixel for the WFC and 28.6 Ang/pixel for the HRC and varies by ~12% and ~3% over the WFC and HRC FOVs, respectively.

HARDWARE REQUIREMENTS :

The pinhole array is placed at the RAS focal plane (like the Ronchi ruling in Item 9: Geometric Distortions). The micrometer stage is adjusted until all the dispersed spectra are well positioned in the WFC FOV. The illumination is provided by Hg and Ar emission-line lamps which are mounted directly behind the pinhole array and collimated with a lens on the array. If possible, both lamps will simultaneously illuminate the array, otherwise images will be acquired with one lamp, and then with the other lamp. The combination of lamps should provide emission lines over the range of the grism, 5800-11000 Ang.

A labeled diagram of the pinhole array is shown in Fig. 1 and an auto-cad image in Fig. 2. It consists of 13 pinholes separated by 0.35" on a side. The array is imbedded in a 2.5"x2.5" plate so it can easily slide in the holder at the RAS focal plane. The holes have a diameter of 25 um and project to ~4 pixels on the WFC field. The resultant images will be similar to the dispersed RAS/HOMS fiber spots at 6328 Ang, examples of which are shown in Figs 4-5 of Item 1 : Image Quality vs Field Position.

• Figure 1 : Diagram of the pinhole array
• Figure 2 : Image of the pinhole array

SOFTWARE REQUIREMENTS :

IRAF and IDL will be used to view and analyze the images and the SLIM package for simulations and modeling.

COMMAND MODE : Real time.

PROGRAM/EXPOSURES :

Once the pinhole array and lamp illumination have been configured correctly, only 2 exposures/lamp/detector are necessary for a successful execution of this calibration item. A "direct" image of the pinholes is first acquired without the G800L grism. This exposure is immediately followed by an exposure with the grism commanded in the light path (FW1). The pinhole spots will then be dispersed over several orders at the wavelengths of the most prominent emission lines of the lamp spectrum, simultaneously providing spatially-dependent information on the wavelength dispersion and the orders. This 2-exposure sequence is repeated for each line lamp and each detector.

VARIANTS:

None.

TOTAL EXPOSURES:

Both the WFC and HRC will be read as full frames. The minimum number of exposures is four per lamp (two each for the WFC and HRC). Ideally, several lamps would be used to provide the most complete coverage from 5800 to 11000 Ang.

TOTAL TIME :

Assuming 8 and 2 min for read-out and dump of a WFC and HRC frame, respectively, then the minimum execution time is ~20 min for one lamp. But a total allocation of ~2 hours is probably more realistic to account for instrument configuration, overhead for test exposures (to determine the correct integration times and location of the spots, for example), and any contingencies.

ANALYSIS :

For a given order and pinhole, measure the separation between the spots of each wavelength and determine the non-linear wavelength dispersion. Repeat for all 13 pinholes and derive the spatial dependence over the HRC and WFC FOV. Also, for a given wavelength, measure the order separation and flux ratio of the orders. The results will be compared and modeled with SLIM.