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Filters

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A list of the Michelle broad and medium-band filters is presented here. Since the filter wheel is common to both the imaging and the spectroscopic light paths the filters are available in both modes. The longpass filters are used for lowN, medN1 and MedN2 spectroscopy, while the medium-band filters are used with the echelle (all spectroscopy filter selection is done internally by Michelle and need not be specified by the PI). Prior to 2008A, spectroscopy with the medN2 grating also used the medium-band filters. Transmission data are available for some filters, as indicated. There is room for a total of ~25 filters in the Michelle dewar, so more filters will be added in the future.

Note that imaging is not available through the broad band N and Q filters, because the sky+telescope background saturates the detector in less than the shortest possible exposure time. The Q' filter may be useful under exceptional conditions - please contact Gemini staff for advice if you would like to use this filter. The N', Qa and medium-band filters are available for general imaging use.

For spectroscopy the LP-7 filter outperforms the N filter at wavelengths shorter than 8.0um and longer than 13.0um. The N filter has slightly higher transmittance than LP-7 over most of the interval between 8.0um and 13.0um.



Current Michelle Filter Complement

Filter Name
Name in Headers a
Central Wavelength (microns)
Bandwidth (microns)
Approximate 50% power points (microns)
Transmission Data (text)
Transmission Curves (jpeg)
Currently
Available
Mediumband and Broadband Filters
N'
I112B21
11.2
 2.4
10.1 - 12.5
Yes
Qa
I185B9
18.1
1.9
17.13  -  19.06
Yes
Q'
I198B27
19.8
 5.4
17.1  -  22.5
-
-
Yes b
N
I105B53
10.5
5.3
7.7 - 13.0
Yes b
Q
I209B42
 20.9 8.8
16.5 - 25.3
Yes b
Silicate Filters
Si-1
I79B10
7.7
0.7
7.39  -  8.08
Yes c
Si-2
I88B10
8.8
0.9
8.35  -  9.25
Yes
Si-3 d
I97B10
9.7
1.0
9.2  -  10.2
Yes
Si-4 d
I103B10
10.3
1.0
9.8  -  10.8
Yes
Si-5
I116B9
11.6
1.1
11.15  -  12.25
Yes
Si-6
I125B9
12.5
1.2
11.9  -  13.1
Yes
Longpass Filters for 10um and 20um Spectroscopy
LP-7 Nblock - - 6.8 - >14 - -
Yes
LP-16 Qblock - - 16.1 - >25 - -
Yes
Narrowband Filters
- Ne IIe 12.86 0.18 12.78-12.96 0.56 at peak -
Yes

a Value of "FILTER" keyword in Michelle file headers. Indicates approximate central wavelength and bandwith (e.g., I116B9 is centred at 11.6 microns with a 9% bandwidth)
b Unavailable for imaging due to array saturation on sky + telescope background
c The transmission data for Si-1 are for the TReCS filter but are also applicable to Michelle (the Michelle and T-ReCS filters are from the same manufacturer and operate at approximately the same temperature)
d These filters used to have a small long-wavelength leak that was fixed during semester 2005B by inserting BaF2 blocking elements (which were already in place in the other silicate filters). The effect on photometry of sources with T>140K (N-Q<-4) was negligible.
e Transmission 40% at Ne II rest wavelength (12.81 microns). Not currently available with ITC. To estimate required integration time on a pure continuum source (either with the echelle or the filter alone) use the Si-6 filter and multiply by 6 the time required to achieve the required s/n in that filter, or use the sensitivity table. For line emission use the sensitivity table.

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