Ask a question, get an answer! This will eventually be a repository for posting outstanding IGRINS issues and questions, and getting answers.
Send all questions to kfkaplan@astro.as.utexas.edu
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. The data reduction pipeline then fits the combined of A-B frames to optimally extract the signal for your science target.
Q: How do I observe extended source objects
A: Similar to 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 S/B since you are not getting signal during OFF exposures. The pipeline will subtract the OFF frames from the ON frames to subtract the sky, dark, and bias.
Q: I have a star or object of magnitude X and want to get a S/N of XX. How do I calculate my exposure times?
A: You can look at this chart and equation here for A0 type stars. It is also a good first order estimate for stars of any typical spectral type. You can also calculate your own S/N 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.
Q: What S/N and exposure times should I get for an A0 star telluric standard?
The simple answer is you want your telluric standard to have a higher S/N than your science target. Why? When you divide your science target spectrum by the telluric standard, the object with the lowest S/N will dominate the final S/N after applying your telluric correction. The telluric corrected S/N (S/N)corr can be calculated from the following equation (see this website for detailed derivation):
(S/N)corr = [ (S/N)sci-2 + (S/N)std-2]-1/2
where (S/N)sci is the S/N for your science target and (S/N)std is the S/N for your A0 telluric standard. It is clear that the smallest S/N will dominate the sum of (S/N)sci-2 + (S/N)std-2, thus would dominate (S/N)corr.