• Observing strategies
• Calibrations
• Recommended Dome Flat Exposure Times
• Fringing
• Telescope Defocusing

Spectrophotometric Observing Strategies

The strategies presented here are based on feedback from observing teams with special thanks to Drs. Ian Crossfield and Avi Mandell. Please see Dr. Crossfield's Appendix A in Warm Ice Giant GJ 3470b for more details concerning some recomendations.

1. Obtain dome flats for a mask immediately before or after the mask's science exposures.
   • It is Important not to move the CSU bars between calibrations and science exposures.
   • CSU bar positions are repeatable to 0.02" on the sky (McLean et al 2012) or roughly 3% of a typical 0.7" wide slit.
   • A two-mask observing example:
     1. Acquire arcs and flats with the first mask in the afternoon
     2. Focus using autofoc (see below)
     3. Acquire science observations with the first mask
     4. Configure CSU for the second target
     5. Acquire science observation with the second mask
     6. In the morning, acquire arcs and flats with the second mask
2. Use Autofoc instead of MIRA to focus the telescope
   • Tell the OA and SA! It is important to clearly communicate the need to run autofoc instead of MIRA to your OA and SA on duty; otherwise, mira is used by default
   • Running MIRA will move the CSU bars because we image the field to measure telescope focus using MOSFIRE. You want to avoid moving the CSU bars
   • Autofoc will put the telescope close to the nominal focus using the guide camera. This method is good enough for spectrophotometric work with MOSFIRE.
3. Avoid placing the target at the border of two CSU bars.
   • You can do this by simply centering an object between the two circular regions markers overlayed on the imaged displayed in ds9. The two green circles identify the middle edges of the slits.
   • Use the command-line script nbfilt to image the field so that you don't cause persistence on the detector due to over-illumination.
4. Nod the telescope during your spectroscopic observing sequence to reduce fringing effects.
   • Nod the telescope by a few arcsecs using the standard observing pattern "mask-nod"
   • Employing pair subtraction of the images acquired at the two nod positions will minimize fringing effects.
   • Please see below for more details on fringing.
5. De-focus the telescope
   • Set the telescope focus to +200 microns from the optimal focus
   • Set the guider focus to 13.8 (nominal is 14.0)
   • This should improve exposure efficiencies by 2/3 as it will keep the star from saturating in short exposures. Please see telescope focusing below.

Calibrations

The mask with narrow slits is used to obtain arclamps so that a rough wavelength solution may be obtained for your program. These calibrations are completed like normal masks and your SA can walk you through the process.

Your wide slitmask requires that you acquire flats manually. Please see the exposure recomendations for wide slits for details on setup and a procedure for acquisition.

Fringing

First identified by Dr Ian Crossfield and his team( Please see Appendix A of Warm Ice Giant GJ 3470b ), observers detect MOSFIRE fringing in spectroscopic mode of night sky lines when slits are relatively large (i.e. greater than 2.0 arcsec ). Dr. Crossfield determined that in K-band, the fringes reach a maximum (slit-width-dependent) amplitude of roughly 4 ADU/s/arcsec. They observed that the amplitude, spatial frequency in the slit direction, orientation, and phase of the fringes all vary both spatially and temporally. Fringing with MOSFIRE is problematic if the fixed position or "stare" mode is used ( a common strategy employed by exoplanet observers). Instead, Keck Observatory nodding precision is very accurate and repeatable, and thus nodding the telescope during observing can minimize fringing effects.

In the image below, there are four slits visible. Each of the four slits has a set of night sky lines and because the width of the slit was 15 arcsec, the night sky lines are parallelograms. The fringing is the sinusoidal pattern evident when looking from top to bottom on a night sky line.

Below is a line plot of prominent fringing along one of the brighter night sky lines from the top to the bottom. Monochromatic emission from telluric OH lines may be causing fringing on the order of 50 ADUs peak-to-valley. The plot below shows relatively larger fringing with an amplitude of 250 ADUs.

Below is an image of the difference of a nod pair which shows that the fringing is well subtracted. Spectra of a couple of objects are visible in the differenced pair of images.

Telescope defocusing

CONCLUSION: we can defocus the telescope by +200 microns and improve the on-sky efficiency of the observations without impacting the high precision spectrophotometric observations.

With the telescope de-focused, we also determined the best focus for the guide camera, which needs to have a good focus to maintain guiding. Guider focus should be set to the following values for different telescope focus offsets. Values listed in bold font are offsets that work well.

Tel Foc offset (microns)	Guider Focus (steps)
Optimal	14.00
+100	13.90
+200	13.80
+300	13.64
+400	13.54
+500	13.46