|MITLL2 failed in January 2000. It has since been revived, but now uses the other readout amplifier on the chip, and is known as MITLL2A. See the MITLL2A page for information on this CCD post Mar 2000. The information presented here is maintained for historical purposes.|
|Ratio of QE (compared to TEK at 200K*) integrated
over Imaging Bandpasses
|* Recall that the TEK is used at 200K for imaging applications to obtain higher QE at the cost of higher dark current. It is used at 170K for spectroscopy.|
A few words about the QE measurements are in order. The 'before coating' numbers come from Gerry Luppino at the University of Hawaii. They were not made with the CCD in the same operating conditions at which it is now being used. The 'after coating' measures were made by John Barton at the AAO. John had to struggle somewhat in order to derive sensible number from three different photo-diodes with out of date calibrations. The QE curves for the 'after coating' device are therefore uncertain by as much as +-5% in the extreme blue and extreme red.
Having said this, a QE of 35% short of 4500A is exactly what one would expect from the lumogen coating and the 'before coating' curve, so we beleive these results are robust at the +-5% level. No 'before coating' QE measurements are available in the blue for this device. However, based on a comparison with similar MIT/LL devices, the before coating QE at 4000A was ~ 25% and at 3500A was ~ 1%. So while the coated device is worse in the blue than the TEK, it is much better than it was.
The main conclusion to draw is that longward of 5000A the coated MITLL2 device has QE as good as, or better than, the TEK 1K.
Comparison of these numbers with the expected count rates of the TEK at prime (which requires some correction for extra optical surfaces, reflections and filter throughuts), shows that the LL device is about a factor of two down on the TEK (170K) at B - in line with expectations.
There is a 'brick-wall' pattern present on all these LL devices, caused by a flaw in the laser annealing process. The brick wall is wavalength dependent, being much stronger in the blue than in the red. Prior to lumogen coating it produced variations in the device QE of about 20% at U and 7% at B and even less at redder wavelengths. After lumogen coating this was reduced to about 4% at U. This is not surprising since the photons the CCD detects at U are actually being re-radiated by the lumogen at longer wavelengths (about 5500A), where the brickwall is much smaller.
For an example of this pattern see http://gardiner.ucolick.org:80/~ccdev/lincoln/w66c2/w66c2.html which shows some test results carried out at Lick for other LL devices. Because this is a QE variation, and in the chip it flat-fields out. There are fewer cosmic rays than our Tek1K, - probably due to a known problem in TEK manufacture which is not present in these LL devices.
In dispersed light the fringing pattern appears as a very regular series of bands with peaks appearing at a wavelength spacing of about 30A.
Broad-band images obtained with Taurus seem to show no evidence for fringing.
The lumogen coating seems to have had no effect on fringing - again as expected.
It also sits approx 200um behind the location of the TEK focal plane in its dewar, and so best focus for each instrument will be different to that appropriate for the TEK.
NONASTRO speed can only be used with the optical fibre serial link between the CDD controller and the Large external memory. If your set-up has been done with the 'copper' serial link, NONASTRO won't work. In the near future the optical fibre links will become the standard.
Full well seems to be about 140Ke-, but this limit is only reached in the FAST and NONASTRO modes - all other modes are limited by the 16-bit A-D at 65535 counts. The next generation of AAO-2 controllers will decrease these read times for large format devices significantly.
To remind you of the TEK performance see Chapter 2 of the CCD imaging manual from which the following table comes
|Readout Speed (TEK)|
|Readout time (s)||394||120||75||52||33|
|Readout noise (e-)||2.3||3.6||4.8||7.2||11|
|Alpha x 1.e-6
As of 23 Dec 1997 only cosmetic tests of the flat-fields of the coated device have been carried out. A thorough report can be found here. However, to summarise -- while the coating applied was not of the ideal uniformity we would have liked, this non-uniformity only affects the performance at wavelengths longer than about 8000A, and then at a non-serious level.
The lumogen coating was successful, however not as uniform as one would like. The coated CCD has a pastel green appearance which is notably darker in the central rectanglar region of the image area extending to within a few mm of the edges. This "rectangle" is slightly wider towards the readout end of the CCD and narrowes slightly towards the far end of the CCD where it shows quite rounded corners. Surrounding the central rectangular region are three ring patterns, the outer one close to the edges of the image area.
The coated CCD looks clean, the original spots and deposits on the surface were still evident and the only new features were about 8 smudgy spots where the coating appeared to be thinner.
The CCD was illuminated by various lamps with two 35mm diameter diffusers interposed about 50 and 80mm above the lamp. In between the lamp and the diffusers various filters could be placed. The lamp and diffuser assembly was held about 300mm away from the CCD so that it was equivalent to about an f/10 beam. This eliminated the effect of dust particles and defects on the window and provided a fairly even illumination over the 60x30mm CCD image area.
In all flats except for two, the brick wall ("BW") structure could be seen and in these profiles the peak-to-peak amplitude was estimated as a % of the flat-field intensity.
Curiously, only the I band seems to be affected by the uneven coat with the central rectangular region being clearly more visible in the flats. The fringing above 700nm (not seen with the I band filter on QH lamps, but seen only with strong emission line sources) probably has nothing to do with the coating. The flats taken below 600nm down to about 300nm are very flat, the brick wall pattern dominating in all of these.
A critical examination of the brick wall pattern in the QH + B filter flat showed that at worst the peak-to-peak pattern was about 9%. Overall, a histogram of this flat showed that 99% of all pixels fell within an intensity range that spanned the average +/- 6% and this includes the fall-off in illumination at the edges and the corners of the CCD.
The MIT/LLs are excellent devices from the point of view of sampling.
|Field Sampling with Taurus II|
For countrate calculations, it may help to know that the LDSS B filter + MITLL2 on Taurus produces count rates of 11 photon/s for a B=22.5 star, and the KPNO R + LL on Taurus gives count rates of 52 ph for a R=22.5 star.
There is little reason to prefer the TEK over the LL device for TTF use.
For Taurus use in the blue, however, the TEK device may be preferable.
The MITLL2 device has been commissioned on UCLES. UCLES users will get smaller pixels (15um) improving spectral and spatial resolution, a somewhat increased wavelength coverage, and complete sampling (with no inter-order gaps) much further into the red.
The fringing numbers found with the RGO ( a regular pattern with an amplitude of 3% peak-to-peak at 7000A, 6.5% p-p at 8000A, 10% p-p at 9000A, and 10% p-p at 10000A) can be expected to produce similar effects with UCLES.
When tested at the AAO's Epping laboratories the MITLL2's dark current was small - a 4000 sec dark showed it was less than 0.3 e-/pix/4000sec. Tests carried out in the coude room at the AAT, however, have showed significantly higher dark currents, which decrease with a time constant of about a day. This can have a significant impact on UCLES observations of very faint targets. UCLES observers are strongly urged to read the following report.
An important point to note is that the current UCLES camera optics cannot illuminate the entire area of the LL detector (which is 60 x 30mm in size). In fact the unvignetted region which can be observed is more like 38.5 x 18.8 mm (for less than 10% vignetting). The region covered at 50% vignetting is 60 x 34 mm, which is approximately the entire LL chip, however unless you are working in the very red, the echellogram will not put any light on much of the chip.
The following sample GIF images from ECHWIND the region of the LL chip illuminated and the effects of vignetting.
Suitable windows should therefore enable the read time to be cut by a factor of 2 over the 'full chip' times given above. If binning in the spatial direction is used, a further factor of about 2 should be obtained, which should make use of the XTRASLOW speed tractable (4 minutes).
(If the fact that we can't actually illuminate the whole chip seems insane to you, then I suggest you contact your ACIAAT representative and start lobbying for the UCLES Camera upgrade as soon as possible!)
Note of course that the 'boxes' shown are for one wavelength set-up. You can move the echellogram anywhere on this field - but the relative locations of the boxes will stay the same. As with Taurus, the main reason for preferring the TEK over the LL device is its superior QE in the blue.
The MITLL2 device has been used on UHRF.
When tested at the AAO's Epping laboratories the MITLL2's dark current was small - a 4000 sec dark showed it was less than 0.3 e-/pix/4000sec. Tests carried out in the coude room at the AAT, however, have showed significantly higher dark currents, which decrease with a time constant of about a day. This can have a significant impact on UHRF observers, where data is often binned over up to 1200 pixels. UCLES and UHRF observers are strongly urged to read the following report.
UHRF users get smaller pixels (15um vs 24um), and a significantly increased wavelength coverage - though not the full 4096 pixels, as the shutter currently limits the clear range to ~3300 pixels. Resoutions of 900,000 have been obtained with the MITLL2. The main reason for preferring the TEK over the LL device is its superior QE in the blue.
The MITLL2 device has been used on RGO. RGO users get smaller pixels (15um vs 24um), together with a much increased wavelength coverage. Observers can either use smaller slits, or get better sampling of sky lines. Once again vignetting stops the whole chip from being used, with only about 3000 pixels being illuminated by the RGO - which is still more than double the wavelength range covered by the TEK1K. You can play with the possible options using the WWW version of RGOANG (with user a specified detector of 3000 x 15um pixels).
Once again only observers looking at wavelengths longer than 5000A should consider using the LL device, while those observing in the UV may prefer the TEK. Observers of single objects with the LL can achieve quite short exposure times by windowing out the largely useless spatial direction.