Stray Light Correction of the XRT Temperature Response Functions

(7, September, 2015 by Aki Takeda)
(11 Sep,2015 added ratio curves & estimation of open fraction.)
(16 Dec,2019 figures of telescope.)
(23 Feb,2026 added derivation method of open fraction.)

1. Introduction.

What is the XRT temperature response functions?

2. XRT's effective area function.

[Effective Area] = [aperture] * [prefilter] * [mirror1] * [mirror2] * [filter1] * [filter2] * [ccdqe] * [contamination]
aperture=2.28 cm^2
filter1=1.0 (FW1=OPEN)
filter2=1.0 (FW2=OPEN)

Effective Area modified partially opened prefilter (without X-ray filters).

3. Modified response functions.

After multiplying the FW1 and FW2 responses, take convolution with the model solar spectra calculated based on Chianti version 7.1.3.
Here we assume the Open Fraction having the value of 0.45 and 0.95 (see Section 5 for derivation).

Open/Al-meshOpen/Ti-poly
Al-poly/OpenThin-Be/Open
Open/thick-AlOpen/thick-Be

4. Modification of the Filter Ratio.

Al-poly/Al-meshTi-poly/Al-mesh
thin-Be/Al-polythick-Be/thick-Al

5. Estimation of the "Open Fraction" of the prefilter.

G-band images taken with VLS(visible light shutter) Open or Close can be used to estimate the amount of stray light coming through the open area of the prefilter.

VLI transmission from the Blue Book section 2.5.1.

The VLI transmission should not be the peak value (0.026) but the averaged value (0.020) over the band width (since Nov-2025). In addition, the effective area of G-band optics was initially assumed as 25mm*25mm*3.14, from the size of G-band lens (having 50mm diameter). However, the actual value should be smaller because of the way it is mounted in a long housing tube. The new value of VLI effective area is 20mm*20mm*3.14 (since May-2026).

G-band normal image: I_vls_open = Is * (25*25*3.14) * 0.026 * 0.4932
  ⇒[correction]⇒  G-band normal image: I_vls_open = Is * (20*20*3.14) * 0.020 * 0.4932
Stray light component: I_vls_close = Is * (open area) * 0.4932

VLS_open image : [normal G-band image] overlapped with [stray light image]
VLS_close image : [stray light image] only.

[Total intensity of VLS_open image] = 1.0820e+11 (DN/sec)
[Total intensity of VLS_close image] = 4.8259e+10 (DN/sec)
[Total intensity of normal G-band image] = 5.9941e+10 (DN/sec)

Ratio of total image intensity :

[VLS_close]/[VLS_open] = 4.8259e+10/1.0820e+11 = 0.446

[stray light comp.]/[norm. G-band]
= 4.8259/5.9941 = 0.8051

[open area ratio] = [open area (mm^2)]/[XRT effective area (228 mm^2)]
[open area ratio] = [4.826e10 * 0.00026 * 1963.5] / [228 * 5.994e10] = 0.001802
[open area ratio] ⇒ [4.826e10 * 0.00020 * 1963.5] / [228 * 5.994e10] = 0.001386 (applied since 10-Nov-2025)
[open area ratio] ⇒ [4.826e10 * 0.00020 * 1256.6] / [228 * 5.994e10] = 0.0008875 (applied since 31-May-2026)

Ratio of total image intensity :

[VLS_close]/[VLS_open] = 1.0767e12/1.1295e12 = 0.953..

[stray light comp.]/[norm. G-band] = 1.0767e12/5.2812e10 = 20.4

[open area ratio] = [1.077e12 * 0.00026 * 1963.5] / [228 * 5.281e10] = 0.04566
[open area ratio] ⇒ [1.077e12 * 0.00020 * 1963.5] / [228 * 5.281e10] = 0.03513 (applied since 10-Nov-2025)
[open area ratio] ⇒ [1.077e12 * 0.00020 * 1256.6] / [228 * 5.281e10] = 0.02248 (applied since 31-May-2026)

6. Software to use the modified response functions.

Under development....