Note that there is a separate Web page which shows a table of means over the frame of all 400 combinations of inner and outer filter wheel combinations. This may be useful in diagnosing filter wheel problems.

Diagnosing the problem

If you want to know where the filter wheels are, without relying on the counter (i.e. assuming there may have been some slippage), there is a specific procedure to follow. The procedure below assumes that, while there has been some slippage in the past, you can currently drive the filter wheel reliably.

1. Rotate to a position that helps the wheels make contact.

   Move the rotator to physical p.a. -240 degrees. If the instrument is in the telescope, move to a relatively low elevation. The horizon is fine.

2. Assume that the wheels are fine and move to the LM blocker with the gr60 grism. The command is "filter 17 9". This moves the inner filter wheel to position 17 and the outer to position 9.

3. Insert a slit.

   Type "slt1 180" to insert the narrow slit at column 180, the nominal spectroscopic position.

4. Take an image: "tint 0.5 ; coadd 1 ; goiv".

   If the filter wheels are at the correct position, you will see the thermal IR at the right side of the detector. This will be hugely bright, so you can take a short, 0.5 second exposure. If you see nothing (other than the bias level, which is around 23,000 DN for a single coadd) then you either do not have the right grism in or you do not have a filter which transmits at 3-5 microns in.

5. Check the inner filter wheel.

   At this stage assume that the outer wheel is OK. In that case you have already ruled out inner filter wheel positions 12-19, which should transmit thermal IR. In other words, if the inner filter wheel is offset by some value between 0 and 19 positions, you have ruled out offsets of 0-2 and 15-19.

   Move to what is supposed to be the ch4 position, by typing fwi_ 5. Repeat the exposure and see if you get any significant flux. If you do not, then you know that the filters from Ls to the open position are not in, hence you have ruled out offsets of 7-14.

   Move to what is supposed to be the hk position, by typing fwi_9. Repeat the exposure. If you again get no flux, you have ruled out offsets of 3-10. In other words, it is likely that the outer filter wheel is not correct.

6. Check the outer filter wheel.

   At this stage assume that the inner wheel is OK. The goal now is to identify one of the grisms in the lower wheel, so put the HK blocking filter in the inner wheel by typing fwi_ 9.

   The outer wheel contains gr60 in position 9, gr120 in position 8, and gr150 in position 7. With the hk filter in the inner wheel gr120 will produce spectra centered more or less on the detector, and gr150 will produce spectra more on the right side.

   Type fwo_ 0 and take an image. Follow with fwo_ 2, fwo_ 6, etc. until you get to fwo_ 18. Once you see a spectrum, check the outer wheel positions on either side to determine which grism you have found.

   If you do not find a grism using this technique either (a) one or both wheels are not moving reliably, or (b) both wheels are in unknown positions. To test (a) you may have to shine a light into the dewar to watch the wheels as you try to turn them. However, use a red filter over the lens; otherwise you will destroy the sensitivity of the detector. This does not apply if the detector is at room temperature.

   If you suspect that both wheels are in unknown positions, you will have to repeat the tests as follows: at every other outer wheel position you should set the inner wheel to positions 17, 5, and 9 and look for flux. This will be very tedious and you may want to consider this option carefully.

7. Determine whether the offsets are an integral number of positions.

   Once you think you have determined the offsets for the inner and/or outer wheels, double check to make sure that the filters are reasonably well centered. On the sky, put in pmfm in the primary mirror and take images in what you think is the K filter. Look for missing segments in the pattern. The filters are near a pupil, so any misregistration of the filters may vignette the pupil and block one or more segment image.

   If the outer filter wheel is suspect, line up a star on the slit in imaging mode and take a spectrum using the hk blocking filter in the inner wheel and gr120 in the outer wheel. The spectrum should lie accurately along rows on the detector. If it does not you can change the raw value of the outer wheel to align it. First determine the current raw value by typing "show -s nirc fworaw". Use "modify -s nirc fworaw = xxx" and find a value of "xxx" which aligns the spectrum. Note that the filter positions are 2000 raw counts apart, so you should move in steps of less than this.

Hopefully you will be able to find the current positions of the wheels using these directions. The main goal, of course, is to never be in a situation in which the wheel positions are unknown, but this does happen from time to time.