The spectrograph doublet is based on a standard “crown in front” design. The crown glass (higher Abe number) is IR grade calcium fluoride, the flint glass is IR grade fused silica.

Glass	n1 @ 1.25μm	n2 @ 1.65μm	n3 @ 2.20μm	ν (Abe number)	P (Partial dispersion)
IR F-Silica	1.44748	1.44280	1.43501	35.5110	0.6246
IR CaF2	1.42746	1.42556	1.42280	91.4696	0.5907

With: and .

Figure 1Top and side view of a sequential zemax model of the spectrograph.

Spot diagrams: Doublet only; field: 0-2.5degrees; wavelengths: 1.25, 1.65, 2.2microns.

Spot diagrams: sequential model of the spectrograph; small dispersion; wavelengths: 1.5, 1.65, 1.8, 2.0 2.2, 2.4microns from bottom to top.

Spot diagrams: sequential model of the spectrograph; large dispersion; wavelengths: 1.5, 1.65, 1.8, 2.0 2.2, 2.4microns from bottom to top.

Effective focal length as a function of wavelength and environment parameters:

λ / μm	EFFL1, t = 20°C, P = 1atm	EFFL2, t = -196°C, P = 1e-6atm	EFFL2/EFFL1
1.25	100.607021	99.494684	0.9889
1.50	100.690099	99.580348	0.9890
1.65	100.720604	99.612232	0.9890
1.80	100.740452	99.633756	0.9890
2.00	100.753048	99.649047	0.9890
2.20	100.751078	99.650318	0.9891
2.40	100.734387	99.637458	0.9891

Spot diagrams, and effective focal length as a function of the air space between the lenses @ 1.65μm – default air space is 4mm, “Quick focus” spot size X only. T = 77K, p=1e-6atm

Air space / mm	2.5	3	3.5	4	4.5	5
EFFL	101.08	100.9	100.10	99.61	99.13	98.66
Spot geom radius / μm field 0deg	15.101	10.179	5.385	0.714	3.835	8.264
Field 1deg	13.102	8.207	3.554	5.753	10.967	16.034
Field 1.5deg	10.594	5.709	6.968	12.369	17.911	23.290

Effective focal length as a function of the air space between the lenses @ 1.65μm – default air space is 4mm, “Quick focus” spot size X only.

Air space / mm	3	3.5	4	4.5	5
EFFL	101.71	101.21	100.72	100.23	99.75
Spot geom radius / μm field 0deg	11.660	6.847	2.158	2.409	6.857
Field 1deg	9.656	4.950	4.146	9.387	14.480
Field 1.5deg	7.140	6.525	10.708	16.280	21.689

Ray fan aberrations, air space is 4mm.

Ray fan aberrations, air space is 3.5mm, , T = 77K, p=1e-6atm

2005-04-18T10:55:49