David's Astronomy - Notes 2021-10-22 (S926)

Session Aims & Highlights

- Observing Result
- Night Summary Plot
- Session Event Log

Operational Issues

- Critical Issues (0), Major Issues (3), Minor Issues (2), Small Defects (0), Continuous Improvement (5)

Images from 2021-10-22 >> [ Local Files >> ]

2021-10-23

Daytime Tests - Examination of IR Leakage from Observatory WebCam

2021-10-23

ObsCam
ObsCam is an observatory web cam which operates 24x7 and provides a realtime view of the interior of the observatory and of the telescope in particular. In low light conditions (when observatory is shut or at night time) the webcam automatically switches on its IR LED Lights to provide sufficient light for b/w night vision.

Potential IR leakage during Imaging
During an observing session the ObsCam view of the telescope is useful for recording & diagnosing any issues with the telescope movements or resolving an ambiguities over the telescope position and operations. The IR lights that are used to illuminate the interior of the observatory for the webcam viewing however has the potential of straying on the sensitive CCD chip in the SBIG ST-10XME camera that is attached to the telescope. This could occur through reflection into the front of the telescope or via any leakage path at the back of the telescope or back of SBIG camera. Issues with stray light have seen and previous tests showed that the IR leakage was large via the TCF-S focuser (see ObsCam - IR leakage onto CCD Images (tests and solution, 2021-04-15). Entry of reflected IR light through front of telescope, and entry of direct IR light through the back of the camera were ruled out. The IR leakage problem was mitigated by wrapping a jumper around the focuser and the tube attachment to the camera.

This mitigation has generally been successful in cutting down on stray IR light however it has been perceived that there are certain telescope positions where some stray IR light was still getting onto CCD Images. This was highlighted by a set of 180s exposures from Target 'NGC 514 w/SN' in session S921 (2021-10-04) taken at Az 166.7° / Alt 45.1° where was a distinct stray light artifact along the top of raw frames (internal reflection from guide chip mirror ?) . See picture below.

CCD Image (50% size, linear scale)
5 x 180s exposure (average combine), 3x3 binning, C Filter
2021-10-05 00:11 UT (#921139-43)
12" LX200R (at f/10.4) + ST-10XME
Auto-guided using TS 80mm APO, ZWO ASI178MC & PHD2

AllSky Measurements & AllSky pictures show clear very dark skies at the target location with no indication of any bright light source shinning towards the observatory, whilst ObsCam pictures show the back of the CCD Camera lit up by the IR light. So it would seem that the stray light is IR light from WebCam coming into the camera via a leak path at the back of camera.

The following photo was accidently taken in the observatory on another occasion. The camera was moving for part of the exposure, but it makes quite an interesting shot. Looking like a jellyfish the IR Led lights associated with the ObsCam (not really visible with naked eye) are nicely highlighted in the photo. It is the light from these IR Leds that is straying onto CCD Images.

Daytime Tests (2021-10-23)
Telescope was turned on, then slewed to Az 166.7° / Alt 45.1° which was the position that NGC 514 target suffered a stray light artifact in session S921. Telescope tracking was turned off to keep the scope located on this specific position for Test 1a and 1b.

Test 1a (IR Lights On)
With the front of telescope covered and with the WebCam's IR lights turned on, a 120s exposure (#926250) was taken with the telescope SBIG ST-10XME camera.
Dark Subtraction was made using a Master Dark Frame of the same exposure and CCD Temp (-10 degC)
An obvious light artifact is present at a similar position to that in the earlier image of 'NGC 514 w/SN'
Test 1b (IR Lights Off)
With the front of telescope still covered but with the WebCam's IR lights turned off, a further 120s exposure (#926251) was taken.
Dark Subtraction was again made using a Master Dark Frame of the same exposure and CCD Temp
Image doesn't show the light artifact that was seen when IR lights where on

This proves that light artifact in NGC 514 w/SN images in session S921 was due to IR lights

Test 2 (IR Lights On)
With the front of telescope covered and with the WebCam's IR lights turned on, a series of 120s exposure (#926252--270) were taken with the SBIG ST-10XME camera with the telescope positioned at a number of locations based around Az 165° Alt 45° position. The results are tabulated below :

Image Num	Az (°)	Alt (°)	Background (ADU average)	Excess Light compared with Master Dark (background 224 ADU)	Excess Light compared with #926253 (background 181 ADU)	Excess Light ADU/s
926250	165	45	424	200	243	2.02

926253	110	45	181	-43	0	0.00
926252	130	45	186	-38	5	0.04
926254	145	45	185	-39	4	0.03
926255	155	45	189	-35	8	0.07
926256	160	45	213	-11	32	0.27
926257	165	45	355	131	174	1.45
926258	170	45	576	352	395	3.29
926267	175	45	928	704	747	6.22
926259	175	45	968	744	787	6.55
926260	180	45	1241	1017	1060	8.83
926262	185	45	386	162	205	1.71
926261	190	45	209	-15	28	0.23
926263	210	45	187	-37	6	0.05

926264	175	35	184	-40	3	0.02
926265	175	40	199	-25	18	0.15
926269	175	43	299	75	118	0.98
926259	175	45	968	744	787	6.55
926267	175	45	928	704	747	6.22
926268	175	47	268	44	87	0.73
926266	175	50	183	-41	2	0.01

926270	180	43	325	101	144	1.20
926260	180	45	1241	1017	1060	8.83

The results show that peak leakage is occuring at Alt 45° and Az 175-180°. See picture below :

The region affected by some IR leakage appears to be a rectangular region 30° wide and 8° high (Az 160° - 190°, Alt 40° - 47°)

The Observatory has now been programmed to automatically turn off the ObsCam whilst recording images from each target. (2021-10-23) which will prevent the possibility of any stray IR light reaching the CCD Camera in future.

Next Steps

Build facility into the AstroObsCam program to turn ObsCam On/Off using new Pegasus Astro USB Control Hub and ASCOM Switch.
Already done 2021-10-16 (AstroObsCam 1.6)
Build facility into AstroMain & AstroObsCam programs to allow AstroMain Observatory Control program to remotely turn ObsCam On/Off using TCP/IP messaging
Already done 2021-10-18 (AstroObsCam 1.6.1 & AstroMain 3.44.3)
Incorporate turning ObsCam On/Off into job queue execution in order to make the webcam's IR lights be turned off during target imaging. But be turned on in-between for general telescope/observatory monitoring (ie whilst waiting, slews, centering, focusing etc)
Done 2021-10-23 (AstroMain 3.45)

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Notes - Session 926 (2021-10-22)

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2024-03-23

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David Richards

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Notes - Session 926 (2021-10-22)

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2021-10-23

Daytime Tests - Examination of IR Leakage from Observatory WebCam

David's Astronomy Pages Notes - Session 926 (2021-10-22)

Session Aims & Highlights (2021-10-22)

Operational Issues (2021-10-22, S926)

2021-10-23

Daytime Tests - Examination of IR Leakage from Observatory WebCam

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Notes - Session 926 (2021-10-22)