UIST: Engineering – Weird arcs with IJ/JH Grisms (and open filter wheel)

UIST: Engineering – Weird arcs with IJ/JH Grisms (and open filter wheel)

June 2005: Installation of new IJ/JH grisms

With the IJ and JH grisms we see a curious crescent at the long-wavelength side of the array (see below). The crescent ONLY appears when the grisms are used in conjunction with the two filter wheels set to open. If we set to ANY filter in EITHER wheel the crescent vanishes.

The crescent is seen with SOME of the other KRS-5 grisms, though it is no where near as pronounced. We’re guessing that the re-direction of the beam as it passes through the grism results in a mis-alignment of the beam somewhere along the optical axis. A similar thing may happen with the other grisms, though because of their different wedge angles the effect is less severe.

In any case, we’re a long way from understanding what we’re seeing…

Is the crescent the result of vignetting?
Why is the crescent bright rather than dark?
Does moving the filter wheel from OPEN to PK50 stop down the beam, thereby blocking the vignetted portion of the beam?

JH flat + open

JH flat + pk50

JH arc + open

JH arc + pk50

Flats and Arcs through the JH grism, with the filter wheels either set to open (left) or pk50 (right). Hi and Low limits are the same in each image (though the spectra from the cuts [horizontal white line] are auto-scaled). We essentially get the same effect with the IJ grism. Wavelength increases to RIGHT.

Results from daytime tests(3 June ’05):

  • Luckily we have pk50 installed in both filter wheels. Setting filter-1 to open and filter-2 to pk50 gives the same flats and arcs as filter-1 to pk50 and filter-2 to open. This suggests that the steps for the two filter wheels are correct and that the filters aren’t loosing steps (datuming has no effect).
  • The crescent seems to be ADDITIVE. However, there is probably some extra diffuse stuff on the left too. Note: with Pk50 in the beam, the arc line fluxes across the array reduce by about 10% (compare bottom two images in the above table). However, ON THE RIGHT/LONG-WAVELENGTH SIDE OF THE ARRAY the continuum between the lines decreases by a factor of ~35; ON THE LEFT/SHORT-WAVELENGTH SIDE OF THE ARRAY the continuum between the lines decreases by a factor of ~5.
  • Had a look at arcs for each grism to see which had/hadn’t any crescent-like features. Results in table below…

Pupil Imaging – and affect of nudging wheels on Crescent…

With the JH grism, 4-pix slit, and the pupil imager, tried looking at the BB (5.0mm) lamp with and without Pk50 in the beam. The former (filter wheels both open) gives a curious ripple in the left-hand edge of the circular pupil image (bottom-right)…

Tried nudging wheels by a few hundred/thousand counts to see if this had any effect; BOTTOM LINE – it doesn’t, because the left-hand edge of the pupil corresponds with the outer (inner?) edge of each wheel…

GRISM WHEEL 2 (contains JH)

Notes: High/Low stretch on all images same; displaying same field-of-view (full array) in all images; Wavelength increases to LEFT.