Before you get too far into the process of preparing a proposal, check to see if your target has already been observed. If it has, you may want to consider an archive proposal. Or, if you don't know the target you want to observe, check if a similar proposal has already been approved. ChaSeR allows you to search the database of all observations. Lists of approved programs, which include archive and theory proposals, are also available.
Once you have selected a target, check to be sure that the object is visible to Chandra during the proposal cycle and that there are no bright sources in the field-of-view that you will need to avoid. You can check target visibility over time using the tool ProVis. The ObsVis software will allow you to inspect sky images with overlaid instrument fields-of-view and facilitates detailed manipulation of Chandra pointing and instrument parameters.
Once you know the target is visible to Chandra and has not previously been observed, decide which instrument configuration would work best for your science goals. Detailed information about Chandra instruments is given in the Proposers' Observatory Guide. Additional information can be found in the Instruments and Calibration pages.
The ACIS has two arrays of CCDs, one (ACIS-I) optimized for imaging wide fields (16x16 arc minutes) the other (ACIS-S) optimized as a readout for the HETG transmission grating. One chip of the ACIS-S (S3) can also be used for on-axis (8x8 arc minutes) imaging and offers the best energy resolution of the ACIS system.
For detailed information about ACIS, please refer to Chapter 6 of the POG.
ACIS Operating Modes
A timed exposure refers to the mode of
operation wherein a CCD collects data (integrates) for a
preselected amount of time - the Frame Time. Once this time
interval has passed, the charge from the active region is
quickly transferred to the frame store region and
subsequently read out.
Within timed exposure mode, you have the option to impose a shorter-than-nominal frame time and you may opt to restrict the region of the CCD(s) in which data will be taken by defining a subarray.
With alternating exposure times, all CCDs are clocked in unison, but have two exposure times. In some instances, it is desirable to have both long and short frame times. If the exposure time is made very short, pile-up may be reduced, but the efficiency of the observation is greatly reduced by the need to wait for the full time for the frame-store array processing. The short exposures are used to reduce photon pile-up, and the long exposures are useful for studying the fainter objects in the field of view.
In the continuous clocking mode, data are continuously clocked through the CCD and frame store. This allows 3 msec timing at the expense of one dimension of spatial resolution. Details as to the spatial distribution in the columns are lost - other than that the event originated in the sky along the line determined by the length of the column.
Faint format provides the event position in detector coordinates, an arrival time, an event amplitude, and the amplitude of the signal in each pixel in the 3×3 event island that determines the event grade. The bias map is telemetered separately. Note that certain grades may be not be included in the data stream
Graded format provides event position in detector coordinates, an event amplitude, the arrival time, and the event grade. Note that certain grades may be not be included in the data stream
Very Faint format provides the event position in detector coordinates, the event amplitude, an arrival time, and the pixel values in a 5×5 island. As noted in Table 6.7 of the POG, this format is only available with the Timed Exposure mode. Events are still graded by the contents of the central 3×3 island. Note that certain grades may be not be included in the data stream. This format offers the advantage of reduced background after ground processing but only for sources with low counting rates that avoid both telemetry saturation and pulse pile-up.
Selecting ACIS chips
There are two types of CCD chips. ACIS-I is comprised of front-illuminated (FI) CCDs. ACIS-S is comprised of 4 FI and 2 back-illuminated (BI) CCDs. The BI S3 chip is at the best focus position and is normally used for ACIS-S imaging observations. ACIS-I is better when wider field (16'x16') and/or higher energy response is needed; ACIS-S imaging is better when low energy response is preferred and a smaller (8'x8') field of view is sufficient. For more information about ACIS chips, please refer to the POG, in particular, section 6.22.1 discusses the choice of CCDs.
The HRC comprises two micro-channel plate imaging detectors, and offers the highest spatial (<0.5 arc second) and temporal (16 msec) resolutions. The HRC-I has the largest field-of-view available on Chandra (31x31 arc minutes). The HRC-S is most commonly used to read out the dispersed spectrum from the LETG. For more information about the HRC, please refer to Chapter 7 of the POG.
The HETG is optimized for high-resolution spectroscopy of bright sources over the energy band 0.4-10 keV. It is most commonly used with ACIS-S. The HETG is discussed in detail in Chapter 8 of the POG.
Density-sensitive emission lines in a T Tauri star is an example that uses a real proposal to explain how to estimate the required exposure time for a proposal looking at emission lines with HETG.
The LETG provides the highest spectral resolving power (E/ΔE > 1000) on Chandra at low energies (0.07 - 0.2 keV). The LETG/HRC-S combination is used extensively for high resolution spectroscopy of bright, soft sources such as stellar coronae, white dwarf atmospheres and cataclysmic variables. The LETG is discussed in detail in Chapter 9 of the POG.
After choosing an instrument, you should simulate the observation to determine the ideal instrument settings, observing time, constraints, etc. A variety of tools are available to help with simulating Chandra data. These include such software as PIMMS, Sherpa, WebSpec and the rest of the Proposal Toolkit.
One factor that will affect the exposure time needed to detect a point source is the amount of intervening Hydrogen column density in the direction of the source. COLDEN will give you this information with a limited amount of required input. COLDEN help file
You will also need to determine the predicted Chandra count rate. PIMMS is appropriate to use for calculating count rates for sources with simple spectra. It will also return information about pile-up percentage and the background count rate.
Pileup mitigation is discussed in detail in the POG
Software such as WebSpec and Sherpa can be used to simulate an ACIS spectrum. They can also help you determine whether or not pileup is likely to affect your results.
You can simulate HETG spectra with software like Sherpa or ISIS.
When simulating a Chandra grating spectrum, you will likely need to use the Grating RMFs/ARFs for the proposal cycle.
A general method to simulate the spectra would be to first read in the response files, define a model for the spectrum, create a fake Chandra dataset, then assess the output.
Use the Chandra Proposal Software (CPS) to submit a proposal. For help with this, please see the CPS guide page.