CFHT, Instruments, Spectroscopy, Gecko: The Coudé f/4 Spectrograph


Six weeks before runs start, CFHT requires PIs to fill in the Observer Preparation Form.
How can I figure out what configuration to ask for?

In the Observer Preparation Form, the Instrument to be used should be set to GECKO/CAFE, and the Detector can be EEV2 or MIT2. In addition, in the Gecko/CAFE section, you have to indicate which wavelengths you want, which order, and which setup (grism of filter).

Here are a few guidelines:

  1. Pick the wavelength(s) you want to observe.
    Remember that the CCD will cover about 150Å at 9500Å, but only 60Å at 3800Å (see this table for more numbers). If you have 2 wavelengths close together but not close enough that they will both be seen comfortably at the same time on the chip, ask for 2 different central wavelengths.

    If you want to cover some wavelength range, plan to have some overlap too. For example, to go from 6000Å to 6550Å, you would ask for wavelengths 6040, 6130, 6220, 6310, 6400 and 6490Å.

  2. Determine which Gecko to use, RED or UV.
    The coating on the collimator and the camera mirrors have been optimized for different wavelength ranges. From about 3800Å to the IR, Gecko RED can be used; if your wavelengths are all below about 4100Å, you can use Gecko UV instead, which will give you a slightly higher throughput than Gecko RED.

    Note that we cannot switch from Gecko RED to UV (or vice versa) in the middle of the night, and one configuration (RED or UV) has to be setup for a minimum of 4 nights in a row.

    You can put your choice of RED or UV in the Special Request box of the Observer Preparation Form.

  3. Determine if you want to use a grism or a filter to do the order sorting.
    Since Gecko is optimized to record only ONE order at a time (the other, adjacent orders being out of focus), you have to chose how the extra orders or overlapping orders will be "removed".

    For blue wavelengths, a grism (to separate overlapping orders) is recommended, whereas for red wavelengths, a filter (to filter out unwanted orders) will be more appropriate because a grism cannot completely separate overlapping orders in some cases. You may want to consult this table, but here is a better guide, which is based on actual setups:

    To chose a filter, please consult this database.

    Note that if you are not sure if you should use a filter or a grism, or even which one, the SA can check if something similar has been done before, or we can even do the setup with 2 configurations, and check with a flat field lamp if the orders are well separated (in the case of grisms) and which configuration gives the highest throughput.

  4. Determine which order to use for each wavelength.
    Use this figure and this table to determine which order will give you access to your wavelength. Pick the order which will give you the highest response. If your wavelength falls between 2 orders and you are not such which one to ask, the SA may have suggestions, or we can do the setup with both orders and check during setup time which configuration gives the best throughput and resolution.

    You may also want to pick the order based on the resolution (dispersion) you prefer; see this table to get the dispersion for each order.

Frequently Asked Questions

  1. How easy and how fast is it to switch from one wavelength to another?
    It's pretty easy and can be very fast.

    To change wavelength, the user only needs to issue one command. A script will take care of setting the mosaic grating at the right angle (to get the right wavelength) and putting the right filter or grism. The user does not need to do anything else.

    Going from one wavelength to another takes the longest time when the configuration goes from one grism to another (for example, from BLUE to RED). If it's with the same filter or the same grism, it can take only 30 seconds.

  2. Can I ask for an additional wavelength when I get to the telescope?
    NO. The setup is done days before a run starts. For each wavelength, quite a few parameters have to be set, and this takes time. Therefore, a last minute change or addition can not usually be accommodated.

  3. Can I do more than one wavelength (or order or filter or grism) per night?
    Yes. You can do more than one wavelength, or order, or filter or grism every night. The only thing you cannot do is switch between Gecko RED and UV during a night.
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