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Mid-Infrared Background: The Effect of Painting a Black Spot on NGST's Secondary Mirror
STScI-NGST-R-0016 A

Space Telescope Science Institute Next Generation Space Telescope Mission

Mid-Infrared Background: The Effect of Painting a Black Spot on NGST's Secondary Mirror

28 October 2002 Issue A


Mid-Infrared Background: The Effect of Painting a Black Spot on NGST's Secondary Mirror
REVISION HISTORY ISSUE A Initial release DESCRIPTION DATE 28 Oct. 2002


Next Generation Space Telescope Mission

Mid-Infrared Background: The Effect of Painting a Black Spot on NGST's Secondary Mirror October 28, 2002

PREPARED BY:

Bernard J. Rauscher _______________________________ NAME _______________________________ SIGNATURE _______________________________ NAME _______________________________ SIGNATURE

__________ ORG. 8 Oct. 2002 __________ DATE __________ ORG. __________ DATE

APPROVED BY:

_______________________________ SIGNATURE Peter Stockman _______________________________ NAME JWST Division Head/Project Scientis _______________________________t TITLE

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Abstract
This document models the Next Generation Space Telescope's (NGST's) mid-infrared ( = 5 - 2 8 µ m) background under the assumption that a small black spot has been painted onto the secondary mirror. This might be done for example to suppress reflections from calibration lamps in the Integrated Science Instruments Module (ISIM). We find that so long as the black spot is colder than about T 40 K elvin, N G S T' s mid- inf r ar ed s ens itivity w ill not be compr omis ed f or the mid-infrared compatible architecture (see NGST Monograph 3). These results imply that if a black spot were to be painted on the secondary, there would have to be an additional requirement placed on the optical telescope element (OTE) that the temperature of the spot be T40 Kelvin so as to not compromise the mid-infrared instrument's (MIRI) sensitivity.

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Introduction

For a variety of reasons, it may be desirable to include calibration lamps in the ISIM. Examples include flat field lamps in all instruments and wavelength calibrators in spectrographs. One concern regarding calibration lamps is that they might be visible to other instruments via direct reflections off the secondary mirror. These reflections could be moderated by painting the portion of the secondary mirror that falls within the secondary obscuration black. If this were done, the spot would have to be sufficiently cold that thermal emission from the spot would not compromise MIRI's sensitivity at long wavelengths. To ascertain how cold the spot would have to be, we considered the "mid-infrared compatible" NGST architecture discussed in NGST Monograph 3. Strictly speaking, NGST Monograph 3 considered an 8-m OTE. However, the Monograph 3 backgrounds should not change in going to the current 6-m NGST baseline on account of the larger sky-projected pixel size offsetting the smaller primary mirror. The Monograph 3 background (see Figure 1) is dominated by zodiacal light at w avelengths s hor ter than =12 µ m , a n d b y b a c k s c a t t e r e d e m i s s i o n from the sunshade at longer wavelengths. So long as the black spot is colder than about T=40 Kelvin, we find that these components will continue to dominate the background. At warmer spot temperatures however, we find that sensitivity at long wavelengths will become increasingly dominated by thermal emission from the spot. The remainder of this document is structured as follows. In section 2, we discuss the model and results. In section 3 we summarize the results and present our recommendations. Appendix A lists the Mathematica Notebook that was used for the modeling.

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Discussion

We modeled thermal emission from the black spot using Mathematica (see Appendix A for the Mathematica Notebook). All other background components, with the exception of the detector, are taken directly from NGST Monograph 3, Figure 2.1.4). The analysis assumes that the black spot is the only element that acts to block rays coming from within the area of the secondary obscuration. We note that the scientific instruments may incorporate internal cold baffles to block much of the emis s ion coming f r om this por tion of the s econdar y. O ur analys is cons ider s the w or s t case, which is an instrument that does not block any rays using internal cold baffles. For the detector, Monograph 3 assumed an excessively noisy idark=10 e-/s/pixel detector. For this report, we have assumed a detector having idark=0.18 e-/s/pixel. Such a detector would meet NGST's detector requirements (NGST Document 641) with about half of the noise arising from shot noise on dark current. The reason for making the change is mostly to ensure that up-todate performance figures are carried forward. For the cases modeled, the detector was never the dominant noise component.

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Mid-Infrared Background: The Effect of Painting a Black Spot on NGST's Secondary Mirror

Figure 1. NGST background components for the mid-IR compatible NGST architecture scaled to 6-m aperture (see NGST Monograph 3). The purple lines show thermal emission from a hypothetical perfectly black (emissivity =100%) spot painted on the secondary mirror to absorb radiation from e.g. calibration lamps in the ISIM. The black spot is assumed to fill 10% of the secondary mirror's area (=32% of secondary radius). We find that so long as the temperature of the spot is less than T=40 Kelvin, the total background will be substantially unaffected by the spot. In other words, for spot temperatures T40 K, the background at wavelengths 12 µm will be determined by zodiacal light while at longer wavelengths it will be set by backscattered emission from the sunshade. The amount of thermal emission from the spot depends on the spot's size, its emissivity, and its temperature. The spot was assumed to be perfectly black with emissivity =100%. In practice, real coatings will be a few percent reflective. For calculating the amount of thermal emission from the spot, assuming that it is perfectly black is reasonable since the amount of emission would be reduced by only a few percent if an imperfect black coating were used. To bound the spot's size, we assumed that the spot filled the secondary obscuration (=10% by area =32% by radius; TBR). Our analysis did not consider scattering of sunshield radiation from the painted area since this does not occur with a perfectly black spot. Figure 1 shows the result of the modeling. We modeled a range of likely spot temperatures including T=60, 50, 40, and 30 Kelvin. For spots colder than about T=40 Kelvin, the backgrounds are essentially the same as those for the "mid-IR compatible NGST" discussed in NGST Monograph 3.

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Mid-Infrared Background: The Effect of Painting a Black Spot on NGST's Secondary Mirror Summary and Recommendations

For sufficiently cold spot temperatures, Tspot 40 Kelvin, we find that painting a portion of the secondary mirror black may be a reasonable approach to blocking light from calibration lamps and other sources in the ISIM.1 At these temperatures, the mid-infrared background will be essentially that of the mid-infrared compatible NGST architecture (see NGST Monograph 3) which is dominated by zodiacal light at wavelengths shorter than about 12 µ m, and by backscattered sunshade emission at longer wavelengths. That said, it is not clear that paining a black spot on the secondary mirror is a particularly good approach to follow. Even though the spot itself may not glow, the problem of adequately suppressing emission from the ISIM will remain. The best approach to ensuring that the instruments do not interfere with one another is to require that the instruments do not emit light while any other instrument is in use (either for calibration or science observations). We believe that this can be achieved using properly designed shutters and blank-offs. For example, at least one blank off will be needed to take dark exposures in any case.

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We note that by the same arguments, the temperature average across the central baffle obscuration should be less than 40 K unless one uses interior pupil baffles.

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Appendix A: Mathematica Notebook used for Modeling


Black_NGST_Secondary2.nb

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In[24]:=

Preliminaries According the the "MIR Compatible" Yardstick NGST model monograph 3 backscattered sunshade emission becomes dominanant at 12.4 m with a flux at the detector 297.8 e s pixel. We take this as the crossover point for now. One conclusions see end is that the secondary needs to be colder than about T 58 K for this to work. In other words, at warmer temperatures, the background at wavelengths longer than 12.4 microns is dominated by emission from the black spot on the secondary rather than by backscattered sunshade emission.

In[25]:=

OTE Parameters SecondaryObscuration 0.1; 10 % by area, see L2 requirements OTEFocalRatio 20; See MIRI IRD v. 3 OTEDiameter 6; See MIRI IRD v. 3 Spot 1.00; Emissivity of black spot TSpot 20; K, Temperature of secondary. For now, I change this by hand to calculate the background at each temperature. MIRI Parameters MIRIThroughput 0.5; R 5; Spectral resolution. 4.625 yields 8 filters Nyquist 8 10 6 ; Wavelength for critical sampling PixelScale 206265 1.22 Nyquist OTEDiameter 2; Calculate pixel size at Tel. focal plane PixelSize OTEDiameter OTEFocalRatio PixelScale 206265; Calculate pixel area PixelArea PixelSize2 ; Pixel area in microns2 4;

In[26]:=

In[27]:=

In[28]:=

In[29]:=

Calculate OTE area OTEArea 3.14159 OTEDiameter2

In[30]:=

Prepare to work with Planck Functions Needs "Miscellaneous`PhysicalConstants`" The Planck Function, photons s 1 m 2 m PlanckWl _, T_ : Module c, h, k , c h k SpeedOfLight Second Meter; PlanckConstant Joule Second; BoltzmannConstant Kelvin Joule; hc
1 1

ster

1

2 h c2 Exp h c

5 1
1

kT

The Planck Function, photons s PlanckNu _, T_ : Module c, , c 2 c
In[33]:=

m

2

Hz

1

ster

1

SpeedOfLight Second Meter; c ; PlanckWl , T

Calculate solid angle subtended by secondary OTE 1 OTEFocalRatio 2 ;


Black_NGST_Secondary2.nb

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In[34]:=

Calculate solid angle subtended by spot Spot SecondaryObscuration OTE; Define a function for flux at constant T. Wavelength is now in microns. Flux wl1_, wl2_ : MIRIThroughput NIntegrate PlanckWl , TSpot , , wl1 10 6 , wl2 10 6 PixelArea Spot; Make a table of fluxes R 4.625; DeltaLogWl 1 R; wl1 0.6; i 0; While wl1 50, wlc Exp Log wl1 DeltaLogWl 2 ; wl2 Exp Log wl1 DeltaLogWl ; xi wlc; yi Flux wl1, wl2 ; wl1 wl2; i; Make a table of the background over the wavelength range 0.6 30 microns strm OpenWrite "blackspot.txt" i, 1 , " " , PlotData i, 2 , i, 1, Length PlotData

In[35]:=

In[36]:=

In[37]:=

In[38]:= Do Write strm, PlotData

In[39]:=

Close the file Close strm