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Send all questions to kfkaplan@astro.as.utexas.edu
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Q: How do I observe stars or point source objects?
A: IGRINS is optimized for observing stars, and they are the simplest types of objects to observe. Stars are placed in either position A or position B along the slit, and dithered between these two positions between exposures. Typically we take exposures in a sequence called a "quad" where you start at position A, take one exposure, move to position B, take two exposures, and then back to position A for one final exposure, resulting in an ABBA quad. The B positions are subtracted from the A positions such that B acts as the sky, bias, and dark subtraction for A and vice-versa. This way all exposures get signal from your science target, while simultaneously acting . The data reduction pipeline then fits the combined of A-B frames to optimally extract the signal for your science target.
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Q: How do I observe extended source objects
A: Similar to piont point sources (see question above), but you must dither your object ON and OFF the slit. A quad would look like ON-OFF-OFF-ON, similar to an ABBA quad but with ~1/√2 times the signal. The pipeline will subtract the OFF frames from the ON frames to account for sky, dark, and bias.
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Q: I have a star or object of magnitude X and want to get a signal-to-noise of XX. How do I calculate my exposure times?
A: You can look at this chart and equation here for A0 type stars
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. It is also a good first order estimate for stars of any typical spectral type. You can also calculate your own signal-to-noise estimates with the Exposure Time Calculator (ETC) which can be downloaded here. Note that Park et al. (2014; SPIE Proceedings) found that the S/N estimates from the ETC should be multiplied from the ETC by 0.83. For ABBA quads, set your number of exposures to 4. For ON-OFF-OFF-ON quads, set the number of exposures to 2.