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GMOS-N B600 grating unavailable
Original GMOS-N B600 grating no longer available.
Replacement grating will be ordered.
The B600 grating is one of the original gratings delivered with GMOS-N in 2001. This grating has proved to be one of our most popular gratings. However, this grating has been observed to visually degrade with time, accumulating dust that has not been removable with standard forced air grating maintenance tactics. This grating also shows the highest level of scattered light among the GMOS-N gratings.
Recently we have been investigating more effective means of cleaning the GMOS-N gratings, employing a compound whose manufacturer recommended its use for these types of gratings. Initial tests employed on spare and little used gratings gave good results with no harmful effects observed on the gratings being cleaned. Unfortunately however, the same procedures have resulted in a loss of throughput and an increase of scattered light for the B600 grating as some of the coating was removed and/or damaged during the process.
We are currently assessing data taken using the GCAL quartz halogen lamp to estimate the impact on throughput and scattered light. Initial estimates are that scattered light has increased by a factor of ~ 2-4 with some spatial dependence, and the effective throughput is lower by ~ 15-20%. Given the current low demand from programs in the GMOS-N queue using the B600 grating, we have elected to remove the B600 grating from science use until a replacement grating is commissioned. For this reason further efforts to more accurately quantify the original B600 grating throughput and scattered light properties using standard stars will not be pursued. A replacement grating will be ordered. Once we have more information on the delivery schedule we will assess the impact on programs in the 2009A queue and also provide an update on the web.
For programs in semester 2009A (as well as existing roll-over programs not yet completed) that originally planned to use the B600 grating we offer the following advice for PIs, NGOs and contact scientists: A replacement B600 grating is expected to be commissioned for GMOS-N in mid-April. Therefore programs with targets that are no longer visible at the end of May must switch to one of the other available gratings. The B1200 will deliver comparable throughput but only half the spectral range. The R150 offers the next best throughput in the blue, but at significantly lower resolution. The R831 is another option which may provide acceptable throughput and spectral coverage. The R400 and R600 gratings are also available but they provide the lowest blue sensitivity. Any changes to grating configuration due to these circumstances are implicitly approved and there is no need to request approval for them from the Gemini North Head of Science Operations. Please see the gratings webpage for more information to help you select which grating to use:
Those programs with targets having at least one hour of visibility at the end of May may opt to observe with the replacement B600 gratin and therefore may define their phase II observations accordingly. However, PIs should use their best judgement if their targets are already setting by evening twilight at the end of May, especially if their programs require very long total exposure times or if their programs are in Band 3. PIs who elect to define their observations using the B600 grating may be contacted and requested to change grating at a later date depending on queue demand or grating delivery schedule developments. Alternatively, PIs may elect to substitute targets that are visible later in the semester for their early targets if this allows their science goals to still be addressed as this may allow them to use t replacement B600 grating. Remember that any target changes to queue programs, even in this instance, must be approved by the Gemini North Head of Science Operations:
We regret any inconvenience this may cause you.