Spectroscopy, Gecko: The Coudé f/4
Six weeks before runs start, CFHT requires PIs
to fill in the Observer
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:
- 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.
- 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
instead, which will give you a slightly higher throughput than Gecko
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.
- Determine if you want to use a grism or a filter to do the order
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
table, but here is a better guide, which is based on actual setups:
- 3790Å to 4340Å: UV grism
- 4455Å to 5110Å: BLUE grism
- above about 5000Å: a filter or a grism (BLUE or RED)
To chose a filter, please consult
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.
- 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
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
- How easy and how fast is it to switch from one wavelength to
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
- Can I ask for an additional wavelength when I get to the
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
- Can I do more than one wavelength (or order or filter or grism)
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.
Send comments to manset -=AT=- cfht.hawaii.edu.
Copyright © 2004. CFHT Corporation. All rights reserved.