Hydra Dark Current I: SITe 400mm camera (2001)

Status at 2001 Jun 29

Roger Smith

(action items in yellow)

Requirements:

I have invested considerable effort in control of extraneous light sources. Read noise in the SITe 2Kx4K CCD in the Hydra Camera is low enough (3.4 e-) that it can easily be overwhelmed dark current, spurious charge, phosphorescence or light leaks. Recall that

Noise_Floor = SQRT{ read_noise² + Xbin*Ybin*(Spurious_charge + Dark_Current*time) }

Read Noise (in the overscan) is 3.0 e- for the high gain setting. If the longest exposure time is half an hour, and highest binning factor is 2x2, then we require:

and,

(All figures below are quoted for unbinned pixels.....)

Detector performance

• Spurious charge = ~ 0.25 e- / pixel on the high gain setting.
• When gain is changed, parallel clock voltages are manipulated to achieve this low spurious charge at the cost of well capacity. Note however that the well capacity for high gain, 15000 e- / (0.84e-/ADU) = 18000 ADU, when binned 2x2 exceeds the ADC range by about 10% so the loss in well capacity will not ofetn be noticed.
• Dark current < ~0.7 e-/pix/hr, when detector protected from bright lights.

Thus...

Spectrograph

• The lens cap and camera baffle/cover are necessary to avoid phosphorescence or image remnance effects. Values of ~5e-/pix/hr from the detector (window?) are easily achieved by exposing the detector to a flashlight briefly. I have not tested this aggressively since I didn't want to compromise the observer.
• The detector cap plus baffle are sufficient ot make it safe to adjust the spectrograph using flashlight for illumination without risk of compromising the observing. The curtain gives an extra margin of safety for refills.
• However the Schmidt corrector (or surrounding cable?) is mildly phosphorescent. If hit with a flashlight will increase dark current to 3-10 e-/pix/hr decaying over an hour or so to negligible levels. The plan is to divide the camera baffle into two overalpping parts, one serving to protect the corrector from accidental illumination.
• The test has not been performed to see if doing flats is enough to cause significant phosphorescence bt the corrector. I suspect this wont be a problem given based on my flashlight test: a series of images alternating between darks and flats should be done to test for this.
• The LEDs in the filter "windmill" which illuminate the slit were shown to turn off fully now that a relay had been installed.
• A conventional dark frame is an exposure with the shutter closed. Since the shutter is immediately in front of the spectrograph camera, the dark frame does not reveal light leaking into the optical path. A cap is needed over the slit to permit measurement of dark current plus light leaks. (Even when the dome lights are off, a significant amount of light comes down the fibers.)

The room

• Detector limited dark current is achieved whether or not the dome lights are on provided that the baffle is installed and provided that shutter is closed. (Detector uncapped and schmidt corrector not exposused to light for hours).
• If the slit is wrapped in Aluminum foil and black cloth, one can achieve detector limited dark current even with the shutter open, but only with done lights off. When dome lights are on the "dark current" increases about 10 fold. Hence there is a leak into the spectrograph room.
• Removing the baffle over the spectrograph camera increases "dark current". This test was done before I had determined that light leaked into the room from the dome so it is not clear whether the light blocked by the baffle was coming from the dome or the Arcon.
• The black tape which seals up the doors and various feedthroughs from the dome is actually translucent. The room needs to be imaged with Arcon plus camera lens needs to be used to image the room to see whether the black tape needs to be replaced or augmented.

Arcon

• The communication fibers produce a huge amount of light (~850nm) where they enter the Arcon front panel. A rough metal box with holes for the cables to pass though was made, but this requires large amounts of black tape and black cloth to seal it. A more precisely made cover is needed to provide an adequate seal while being easily demounted to provide access for maintenance.
• There are 6 LEDs inside the detector cap, controlled by the preflash parameter in the setdetector menu. These allow one to verify CCD performance. Since it is also useful to be able to check dark current with the cap on, the fact that there is leakage current through the LEDs when off is a problem. A transistor needs to be added to the LED drive circuit in the Arcon power supply to shunt the current past the LEDs when off. This is preferable to a relay since the LED pulse must be very short.
• The white plastic diffusor in front of these LEDs was removed since it was found to phosphoresce (tens of e-/pix/hr) with seemingly infinite decay time. This effect is clearly distinct to the LED leakage current problem as it iss present when the LED cable was unplugged.
• The dark current is seen to rise after the Nitrogen is refilled. This is because the addition of liquid Nitrogen to the pumped (solid) Nitrogen raises the temperature of the heat sink for the detector, for an hour or so, until the pumping (and consequent accelerated boil off) cools and re-solidified the Nitrogen. It should be noted that dark current is so critical in this application that the CCD is being operated at the lowest temperature achievable (about 150K): CCD temperature is not actively stabilized. The extra 16 K or so of cooling proved by pumped Nitrogen, is possibly due to poor thermal contact between the CCD and its supporting plate due to differential contraction between the Al plate and the stainless steel bolts which pass through it. Spring washers may be difficult ot retrofit due to the cramped space, and may be insufficient. Since the pumping and refill process are working well it has been decided to continue with this solution. Therefore, vacuum lines need to be routed to the large mechanical pump located in the corridor behind the telescope, instead of using the small pump currently located on the cass elevator platform directly under the telescope.
• Not related to darks but important: I need to figure out why the combination of gain=1 and Xbin=2 causes a strong pattern in the images.