Spectrophotometric Observing Strategies

Observers attempting high-precision spectrophotometry with MOSFIRE should understand that there are a couple of limiting factors: the CSU bar may be re-positions to an accuracy of 0.02 arcsec on the sky, and there is fringing present in the night sky lines. However, observers can achieve higher precision on masks (typically two a night) by using the strategies described below. These strategies were developed for observers acquiring spectrophotometric data of relatively bright stars, 6-10 magnitudes at K band. The strategies described below not only address mitigation of CSU motions and fringing, but also how to observe relatively bright targets. If you have questions please contact your support astronomer.

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.
  2. Use Autofoc instead of MIRA to focus the telescope
  3. Avoid placing the target at the border of two CSU bars.
  4. Nod the telescope during your spectroscopic observing sequence to reduce fringing effects.
  5. De-focus the telescope


Acquiring calibrations for wide slits (+2.0") is a manual procedure. It is recommended that you have two versions of your slitmask. One is the science mask with 10-15 arcsec wide slits. A second mask has the same design, but the slits are narrow (0.7").

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.


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

On 31 August 2013, we acquired images of a PSF star with the telescope focused between the optimal focus (0.64) and a de-focused value of 1.14 mm. Dr. Avi Mandell analyzed the MOSFIRE PSF out-of-focus images and found that the drop in the peak flux is 2/3 and the PSF broadens by about a factor of 2.5 when the telescope is de-focused by 200 microns. The image below displays the in and out-of-focus images and horizontal cuts through the diameter of the psf. Three is slight doughnut shape at +200 microns, but the PSF exhibits a mostly flat topped profile (red) relative to the in-focus image (black).

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
Guider Focus
Optimal 14.00
+100 13.90
+200 13.80
+300 13.64
+400 13.54
+500 13.46