1. Timing in the Optical Domain
Aart Olsen
IOTA annual meeting
Carson City, Nevada
9 September 2017

2. Optical timing
Time is inserted into the optical path, in the same image as the objects being recorded
The time you see is the actual time – no need to correct for delays or other processing
Can work for any type of camera
But in practice may not be so straightforward

3. Optical timing
Early methods used a blinking LED based on the pulse-per-second (PPS) output of some GPS receivers
Very precise time, but not in every frame
Only seconds were blinked
The problem of “which minute?”
Unfocused blinks raised the background noise floor and degraded the video signal-to-noise

4. Goals Time displayed in every field DIY capable
Cheap enough for multiple deployment
Accurate, reliable, convenient, etc.

5. Techniques discussed here
Based on Arduino microcontroller board
Inexpensive, easy to program, use 5-12VDC, USB
Does one thing at a time, so quite fast
Easy interface to GPS and displays

6. Components Arduino – standard Uno model or similar, or anything close
GPS receiver – 5VDC, Gnd, RX, TX, PPS
Display – 7 segment digits, coarse LED matrix, fine pitch OLED display, single LED, piezo beeper

11. What the software does Reads NMEA sentences from GPS receiver
Parses into H, M, S variables
At the PPS, display the contents
Counts subseconds using onboard timer
Use Arduino Libraries
TinyGPS++ to interpret NMEA sentences
LedControl to send data to LEDs or SPI to send data to LEDs
TimerOne to program the internal timer
U8X8lib to send data to OLED

12. 7 segment digits 8 digit 7 segment LED display HHMMSSss
Watec 120N+ at High 1, no integration
Note the difference between optical and VTI times: you need to subtract a .033s correction from the Kiwi time, so seconds are – = 57.82
Subsecond digits are somewhat overlapped but probably readable

14. 7 segment digits Notice what happens when we integrate.
Watec 120N+ at 4 frames integration.
The subsecond segments are overlapped and everything looks like 88

16. 128 X 64 OLED matrix
The solution for overlapping digits is to use separate LEDs for the subsecond times and light them sequentially
OLED displays have tiny LEDs so easy to shrink the display
Following is two successive fields recorded by a PC164EX2 (no VTI correction needed).
64 subseconds are sequenced by individual LEDs just below the HH:MM:SS digits

18. 64 LED matrix This uses coarse LED display
The left three columns display the hours, minutes and seconds in BCD format (top four LEDs show the 10s, bottom four LEDs show the units).
The right five columns have the 40 remaining LEDs; each of these is sequentially lighted for 1/40 of a second.
This recording uses a PC164EX2, so no correction needed.

21. Optically insertion of time into the camera
One way is to use a telescope in reverse – a collimator – in front of the telescope. It projects an image of the display so the telescope can focus on it at the same time it sees the sky object you want to record.
Like a Telrad finder – the Telrad is the collimator and your eye is the camera
The size of the image formed by the collimator + telescope in the camera is:
telescope F.L.
image size = display size X
collimator F.L.
The camera sensor is small and the image needs to be even smaller. So either the display size should be really tiny or the collimator focal length should be very long.

23. OLED and collimator
Now with a collimator and 90mm refractor, using a Canon mirrorless camera

27. 128 X 64 OLED matrix
The collimator approach has some potential on a large, open-tube telescope, but is clumsy.
Maybe an off-axis guider could be used backwards, to image the time display (at the side) into the camera, using short F.L. lens

29. Use a single LED
Instead of showing the time in a array, we can show the same information with a single LED blinking a sequence of BCD codes.
Put the LED behind a pinhole and you have a very small display, so you don’t need a long F.L. collimator to fit the image on the camera sensor.
The camera images the pinhole as a blinking artificial star

31. Use a single LED
Here is a 3-minuteTangra light curve of the blinking artificial star

34. Use a single LED
Works with integration too. Here is a Tangra light curve from a Wated 120N+ operating at 4 frame integration

36. Back to an unfocused blinky LED
But focusing a pinhole on the camera sensor is difficult
Let’s go back to a unfocused LED in front of the telescope
Flash LED to light up the background; use a BCD code to show the time

37. Back to an unfocused blinky LED
But what about all that noise in the background?
Flash the BCD hours and minutes only on demand – i.e. before and after the occultation
Blink only the start of each minute

44. UFO event time
01:44: /- 0.02

45. Or Beeps You can replace the LED with a piezo beeper
Works well with cameras that have microphones
Doesn’t add any noise to the video background, so you could beep the BCD time every minute and also have PPS beeps.