Investigation of shadows and light emissions on HSC images
around the edge of the ﬁeld-of-view
October 2022
 It has been found that arc-shaped shadows occasionally appeared on the images of HSC’s CCDs
which are located on the edge of the ﬁeld-of-view. In addition, local light emissions from CCDedge also occurred around some of the shadow positions.
 These shadows are likely to be caused by fragments of cable ties which seem to be caught between
CCDs and the edge of the dewar. We actually conﬁrmed two pieces of cable tie fragments byvisual inspection from the dewar window.
 As a result of our investigation of HSC archival data, we found 23 cases of shadows and/or lightemissions probably due to cable ties on 17 CCDs. This fact indicates that a plurality of cabletie fragments are widely scattered in the inside of the dewar.
 The shadow/light emission cases started from 2017 and have rapidly increased since 2021. Manyof the cases occurred in the area around CCD 100 (0 31).
 Direction of such shadows often changes at a timing when the telescope moves largely. Most ofthe light emissions disappeared within a short time, while some cases continued intermittently
for several days.
 There seems to be no signiﬁcant inﬂuence on science data from this issue in most cases becausesuch shadows and light emissions occur around the edge of the ﬁeld-of-view where the sensitivity
is comparatively low due to vignetting.

Summary of the known cases of shadows and light emissions probably due to cable tie fragments.
Case ID

Date (UT)

CCD

Channel

Event

Case01
Case02
Case03
Case04
Case052017/06/29
2017/12/24
2018/01/18-02/12
2018/11/03
2018/12/08-09
2018/12/11-14
2018/12/30
2019/03/29
2020/03/03-19
2020/03/19
2020/05/24-25
2020/10/20-2021/01/07
2020/10/20-23
2020/11/11-13
2020/11/11-13
2021/01/09-18
2021/01/15-18
2021/01/18
2021/02/06
2021/02/10
2021/02/10-11
2021/06/09-10
2021/06/10
2021/10/28
2022/05/26-27
2022/05/26-27
2022/06/05-07
2022/06/28-07/06
2022/06/28
2022/06/28-07/01
2022/06/28-07/01
2022/06/29-30
2022/06/29-07/04

104 (1 48)
103 (1 31)
061 (1 19)
022 (0 27)
022 (0 27)
022 (0 27)
102 (0 35)
070 (0 03)
004 (1 42)004 (1 42)004 (1 42)004 (1 42)
100 (0 31)
029 (1 03)
101 (1 35)
062 (0 07)
095 (0 42)
077 (1 27)
053 (1 15)
104 (1 48)
104 (1 48)
106 (1 52)
090 (0 47)
104 (1 48)000 (1 53)
106 (1 52)
095 (0 42)
095 (0 42)
103 (1 31)
022 (0 27)
100 (0 31)
000 (1 53)
106 (1 52)

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ch4
ch1,2ch1ch1,2ch1ch1ch1ch1ch1ch1ch1ch1,2ch4
ch1ch3,4ch1,2ch1ch2,3ch1,4ch4ch4
ch2,3ch1,2ch4ch4
ch1ch1,2ch1ch1,2ch1,2
ch4
ch4

Shadow and light emission
Shadow
Shadow
Light emission
Shadow
Shadow
Shadow
Shadow
Shadow
Light emission
Shadow
Shadow
Shadow and light emission
Shadow
Shadow
Light emission
Shadow
Shadow
Shadow
Shadow
Shadow and light emission
Shadow and light emission
Light emission
Shadow and light emission
Shadow
Shadow and light emission
Shadow
Shadow
Shadow
Shadow
Shadow
Shadow
Shadow and light emission

Case06
Case07
Case08Case09Case10
Case11
Case12
Case13
Case14
Case15
Case16
Case17
Case18
Case19Case20
Case21

Case22
Case23

Positions of shadows and light emissions probably due to cable tie fragments.

Time series plot of the known cases of shadows and light emissions.
( “Area #100” means the region around CCD 100 (0 31) )Case 01:

Case 02:

Case 03:

Case 04:

Case 05:

Case 06:

Case 07:

Case 08:

Case 09:

Case 12:Case 10:

Case 11:

Case 13:

Case 14:Case 15:

Case 16:

Case 19:

Case 17:Case 20:

Case 21:

Case 22:

Case 23: