12K x 8K Mosaic for the Oschin Schmidt on Palomar Caltech Optical Observatories Ernest Cromer (Tech) Richard Dekany (Lead) Anna Moore (Optics)Hal Petrie (ME) Gustavo Rahmer (EE) Roger Smith (EE) Palomar Transient Factory meeting

12K*8K Mosaic for PTF 2 of 37 Telescope layout Clear Aperture Diameter = 1.26 m Focal Length = 3.06 m f ratio = 2.5 Plate scale = 15 μm/arcsec Latitude = 33  21 ’ N Longitude = 116  51 ’ W Elevation = 1706 m Spherical primary is larger than beam to accommodate the wide field (diagram at top right) Focal surface radius = focal length of primary = 3m. Field flattener design for QUEST: 4th order polynomial on outer surface, thicker at center and edge. Flat on side facing primary. Placed at radius of curvature of primary. Spherical Focus at half radius of curvature of primary

K*8K Mosaic for PTF 3 of 37 Telescope Field of View Active area: QUEST: 112* 600*1200pix * 0.87”/pix = 10.4 deg 2 PTF: 12* 2048*4096pixels * 1“/pixel = 7.77 deg 2 30% smaller, better quality CCDs, smaller gaps 20% vignetting at 430mm, QUEST image (includes gaps) 10% vignetting at 380mm Unvignetted to 307mm diameter 2.86 o radius Since beam moves off primary at large field angles 30% vignetting at 480mm PTF 12K * 8K …only 500um gaps

K*8K Mosaic for PTF 4 of 37 Requirements  Implications Maximize survey speed, given fixed budget  use 12K*8K mosaic of MITLL CCDs purchased from CHFT  re-use as much of this system as possible: eg focal plane assy; front section of dewar, dewar wiring, ~half the electronics, data acquisition software (ccdcom) and instremental calibration s/w (ELIXIR). Minimize telescope modifications (expensive)  Replace large LN2 cooled dewar with compact dewar with closed cycle cooling so it mounts on Schmidt focusing hub (only 6” from hub to focal surface); goal is to load fully assembled instrument through side of telescope without removing Schmidt corrector. Filter mechanism is NOT REQUIRED … right ?  Manual filter insertion is cheaper and causes no vignetting s exposure times  reduce read time from 60s to ~15e-/min in B. Need greater analog BW, faster CCD waveforms and faster driver for DMA card.

K*8K Mosaic for PTF 5 of 37 Requirements  Implications Accurate photometry using standard stars in science fields  This and ability to use short (5s) exposures for flats, sets shutter timing accuracy requirements. For super-flats just need stability. Minimize obscuration of beam by instrument  Mount electronics outside telescope; longer cable probably requires warm video preamps at hermetic connectors. High reliability with minimal maintenance  Custom low impact shutter (timing requirements imply dual blade); purchase from experienced vendor. Image quality better than 2” (goal 1.7”) with minimal interruption to observing for focus & tracking adjustment.  We will attempt to derive focus (and tracking?) corrections from science images; using temperature sensors to predict focus is expected to be problematic on this telescope at least until data has been accumulated over a long period.  Can we do better? Tracking is possibly the limit. Best image quality during photographic survey was 1.1” but with frequent mirror support tweaks. Seeing is worse than this about half the time.

K*8K Mosaic for PTF 6 of 37 While one can write requirements for productivity in terms of down time and throughput, a judgment call is required to decide how much to invest now to possibly “save later”. Invest now to save later ? CCD SELF TEST Need daytime flats with stable long term intensity... Temperature regulated LEDs on back of instrument shine back to screen inside corrector cover. Tie together lab scripts to measure and log: offset stability, noise in image and overscan; dark current; charge injection, e-/ADU, Shutter timing error vs field position, serial and parallel CTE. QE stability in current filter, linearity, well capacity. Include images in archive along with calibrations. DATA LOGGING Collect time tagged diagnostic information to diagnose problems with image quality or reliability. Data collected may include: pointing, measured image quality, pointing tracking & focus settings for each image, telescope and environmental temperatures, dewar pressure & temperatures, results of CCD self test, system up time and exposure duty cycle. There is some motion towards this under way on Palomar already.

Repackaging Concept

K*8K Mosaic for PTF 8 of 37 The current CFH12K Move the electronics boxes outside the beam Only the blue bits fit on the Schmidt instrument support hub.

K*8K Mosaic for PTF 9 of 37 Manually installed filter slide. Use filters from CFH12K New dual blade servo controlled shutter from U.Bonn Use existing CFH12K front plate, but change window to serve as field flattener. Tweak QUEST optical design for different filter position & thickness, and different (?) distance from window to CCD. Approximate Layout (cartoon, not to scale) Vacuum housing. Re- use front section of CFH12K dewar. Connector sub-plates: cables are hidden behind shutter overhang Access hatch for bolted thermal links? This surface attaches to telescope’s instrument focus hub. Attachment must allow for tilt adjustment. 25W Cryotiger,  =114mm (no moving parts). T head = 135K, T CCD =153K

K*8K Mosaic for PTF 10 of 37 Mosaic footprint (looking towards primary from corrector, approximate scale) Shutter housing Central obscuration will be dominated by shutter blades. Barn door shutter for quest avoids this but timing error is greater and variable across field. Connectors and cables hide behind shutter overhang. 60 mm 338 mm 60 mm Dewar photo with window removed facing wrong way and only 4 CCDs installed Support vanes Instrument mounting hub (focus stage) On-axis beam Hide Cryocooler head in corner? Simpler installation than QUEST since cooling system does not span hub and vanes.

K*8K Mosaic for PTF 11 of 37 Solid modeling of space allocation Focus mechanism Spider Shutter Filter holder Cryocooler (not in final position)

K*8K Mosaic for PTF 12 of 37 Other views Cryocooler might fit behind CCDs, coming in from side. Tricky to fit cryocooler here Rear view

K*8K Mosaic for PTF 13 of 37 Manual filter holder Existing CFH filter frame: no problem with size in faster beam Filter slide, for manual insertion Retention device.

K*8K Mosaic for PTF 14 of 37 CCD Performance

K*8K Mosaic for PTF 15 of 37 From …. CFH12K characteristics MITLL CCDs from foundry run 15um pixel  1”/pixel One amplifier per CCD 9 standard, 3 deep depletion (better red response, less fringing) MASK IS NOW BLACK TO REDUCE REFELECTIONS

K*8K Mosaic for PTF 16 of 37 QE patterns Blue flats have quilting due to laser annealing of boron implant which overcomes surface potential which would otherwise trap photogenerated charge. This effect is strongest in B band and still presnt in V. Red flats show fringing, except in 3 thick CCDs. AR coating differentiates others. Fringing in Z band is twice R band. Blue extreme (10%)Red extreme (5%)

K*8K Mosaic for PTF 17 of 37 Cosmetics, etc. Only 0.4% pixels are bad, including 200 bad columns, mostly in lower right CCD. Compare to gaps between CCDs which are ~10% of area. Bad pixel/column clusters are all narrower than CCD gaps so dither that fills the gaps also fixes bad columns. CFH have produced many beautiful images like this one by dithering to fill in gaps. Full well ~150,000e-; Saturation leaves ~4e-/min image persistence

K*8K Mosaic for PTF 18 of 37 Initial tests at Caltech Cabled together, pumped and cooled, initially in shipping container with window cover used for shipping. The light leak is minor. A sealed light box has been built and internal light source is under construction. Photo here Small vacuum leaks were found at the hermetic connectors (6mT/hr when warm). This was absorbed by the getter at 77K, and hold time was still 24 hours after several days of operation. Since we will be more sensitive to leaks when the activated carbon getter is only cooled to 133K, these leaks (which have probably been present for a long time) will have to be fixed.

K*8K Mosaic for PTF 19 of 37 Recent Bias Caltech

K*8K Mosaic for PTF 20 of 37 Recent 1200s Dark Caltech 5-10e- in 2 min at -85C (warm)

K*8K Mosaic for PTF 21 of 37 Recent Caltech Red LED; gradient is illumination Hot columns on this CCD are still sensitive

K*8K Mosaic for PTF 22 of nm flat Lower left CCD in previous image

K*8K Mosaic for PTF 23 of 37 Noise These are very low noise CCDs No sign of electrical interference in spite of low random noise floor (see slide background) Only moderate noise degradation is expected at higher pixel rate.

Design issues

K*8K Mosaic for PTF 25 of 37 Vacuum envelope & mounting There is 6.4” from mounting hub to focal surface. The existing dewar front end is 7” high with CCDs recessed 1” behind the window exterior and thus fits without modification. A new back plate will replace the LN2 dewar and provide attachment to focus hub. It may hold the Cryotiger. Flexure TBD. The attachment to the focus hub must support static adjustment of tilt to <20um across 236mm diagonal of image area. See focus budget allocation. Screws or shims? A thermally isolated (passively cooled) radiation shield with minimal gaps is highly desirable to reduce load on the cryocooler. New/modified heat spreaders are required within dewar to bring CCDs closer to cold head temperature.

K*8K Mosaic for PTF 26 of 37 Mounting to focus hub and tilt A stiff instrument support plate (orange) is attached to the focus hub. The back plate of the dewar (green) overhangs so that it can be bolted to the instrument support plate at the edges. This back plate can be “thin” since bow in center doesn’t matter. It is stiffened by the dewar wall at the edges where flexure counts. contact surfaceThe contact surface between the two plates will be adjusted (machined or shimmed) to achieve correct tilt. Mosaic dewar base: just fits ! Telescope spider Focus hub

K*8K Mosaic for PTF 27 of 37 New cooling system Polycold Closed Cycle Cooler (Cryotiger) with no moving parts in cold head. Rough thermal budget –11 W radiation to CCDs assuming 90% emissivity for CCD and window –5 W radiation to shield (3% emissivity product) –3 W conduction by wiring –2W conduction in supports –5 W CCD heater (servo); T CCD =-163K  50% safety margin. CCD heater power goes to zero if load increases from 21W to 33W.  Expect ~135 C at cold head and getter. It looks like one cold head will work and can be made to fit (with some effort). Note: IMACS 8Kx8K mosaic reaches -85C with one Cryotiger and no radiation shield. We could run this warm if necessary but we aim for more margin.

K*8K Mosaic for PTF 28 of 37 U.Bonn Precision 2 Blade Shutter No new technology: our requirements are well within U.Bonn experience range ~ $US 25K for size required. Estimated by Klaus Reif, U.Bonn. Image area = 200 * 126 mm Opening = ~215 * 141 mm Footprint = ~335 * 493 mm 1 ms Uniform, low exposure error Precision servo matches velocity profile for open and close. Low acceleration for low telescope vibration and long life: tested to 10 7 exposures !  14% vignetting (>11% just for dewar)

K*8K Mosaic for PTF 29 of 37 Shutter footprint calculation Given: ImageX = 6*2048* *GapX = mm (GapX is TBC) ImageY = 2*4096* GapY = mm (gapY is TBC) D = distance from shutter to focal plane = 30 mm...? Allowing for beam expansion OpeningX = ImageX + D/2.5 OpeningY = ImageY + D/2.5 Then, according to U.Bonn, the shutter footprint, including clearances from the minimum opening to the actual shutter aperture: ShutterX = OpeningX +120mm = mm ShutterY = 3 * (OpeningX + 10) +10 = mm (The shutter moves along the CCD columns) This gives 58mm of shutter overhang beyond the 13.5"=337.5mm dewar footprint in the direction of travel and none in the X direction. This could not be reduced much without seeing connectors and cables. The X dimension can be reduced by perhaps 10mm on each side, says Klaus Reif. This would allow for the filter holder to attach to the dewar flange directly so it bridged the shutter rather than attached to it.Note: I have guessed the CCD gaps. We should calculate them from the CCD center to center distances in the drawings.

K*8K Mosaic for PTF 30 of 37 Optical analysis tasks Re-optimize QUEST field flattener prescription for new filter position and distance to CCD. Allocate image quality budget: –Aberrations (various sources) –Seeing –Residual tilt (static) –Flexure –Residual CCD height variations –Focus motion error (resolution, hysteresis) –Focus measurement error –Tracking error & wind shake. –Differential track rate due to atmospheric refraction

K*8K Mosaic for PTF 31 of 37 Current focal plane flatness is not good enough Degradation in image size from 0.53” to 0.7” implies 0.45” defocus or 144 um CCD height variation. On P48 this would produce 3.8” images ! We have been offered use of the Carnegie Observatory’s optical profilometer to scan the full focal surface to 0.5um accuracy. Once this height variation has been verified, the focal plane will be disassembled to adjust CCDs to be closer to coplanar. CFHTP48 Blur circle0.45”3.8” Arcsec / 15um pixel f-ratio Height change144um Image size vs position on CFHT

K*8K Mosaic for PTF 32 of 37 Automate focus Find focus: Read one band per CCD, each ~20% of area, so read time per 10s exposure at each focus position is only 6s. Automatic 7 point focus sequence takes just 2 minutes. [ We don’t need multiple exposure with charge shift since read time is < exposure time. We don’t want it because source confusion makes automatic analysis complicated. ] Then follow focus: Automatically analyze science frames to measure image size and astigmatism versus CCD height which will be known very accurately from profilometry. (CCDs cannot be made perfectly coplanar) Tracking and focus error will be distinguishable, since tracking error is invariant with CCD height. By using all the stars in the frame (computing the major and minor axes of the 2D autocorrelation function) one should be able to resolve small focus and tracking errors relative to seeing and static aberrations. It may be possible to trim track rate as well as focus !

K*8K Mosaic for PTF 33 of 37 Electronics: Move electronics box outside telescope tube to reduce obscuration and heat in beam: –New cables –Install a preamp at hermetic connectors (in air) to drive longer cable length. Reduce pixel time from 6.5us to 3us (readout from 58s to 27s). –Same video card: video BW limits pixel rate 1MHz Datel ADS937 ADC converter is not limiting. –Shorter dwell times (definitely more noise) –Faster charge dump (less settling time may degrade linearity) –Overlap serial clocks with entire pixel (maybe more noise) –AD conversion must dodge transients to avoid code drop outs. –Existing two fiber links run at 50Mb/s limit pixel time to >2.4us We don’t need to upgrade fiber links and DSP code as first thought.

K*8K Mosaic for PTF 34 of 37 Software by camera team Photons to FITS: Move CFHT’s CCD control program, ccdcom, to a modern computer and Linux version. –A command line interface will be supplied for for manual operation: no GUI’s. ccdcom provides an interface for camera control by scripts or the Observatory Control Software package Install new driver to support higher data rate (10 MB/s) Setup VNC for remote access and IRAF for image analysis in lab and when commissioning. Augment existing lab scripts to support automatic self test. [.. ] = if we decide to do this

K*8K Mosaic for PTF 35 of 37 Tasks for science team(s) Scheduling and Observation Control software. –command telescope and dome –initiate exposures –Work form existing solutions: QUEST or P60? Near real time image analysis for focus and tracking adjustments Collect and append information to image headers (eg pointing, UT) What time accuracy is required? Data logging and analysis for ongoing performance improvement. This has much in common with image header creation. Note that “data logging” allows for collection of information in one place which, if only stored in headers has to be mined from the image archive. It also allows for recording of information which is collected asynchronously to exposures or collected when no exposures are being taken. Manage local data storage and transfer –Tell camera where to store data; check when disks fill. –Temporary backup to second spindle, when space available –Image transfer to IPAC –Image deletion after transfer. Data archiving, reduction, and community access.

K*8K Mosaic for PTF 36 of 37 Palomar Roles ….TBC Removal of QUEST [ not part of this project ] Contract with Vertex-RSI to complete the telescope control system installation by calibrating pointing and tuning tracking. ($20-30K?) Advise observation control software authors regarding TCS interfacing. Assist with instrument installation, alignment and acceptance test. As budget permits (to support image quality improvement): Add telescope temperature sensors to existing system for data logging. Add primary mirror positions sensors and logging.

K*8K Mosaic for PTF 37 of 37 Decisions needed (summary) Filters: When changed? Manual change ok? (Can we get by without a filter change mechanism?) Image quality –How hard do we work ? [Science team] –Note that primary mirror support adjustments by Bob Thicksten, during the photographic surveys, based purely on mirror position measurements were successful in making image quality better than the 1.8” limit seen for QUEST. ACTION [Palomar]: (Later, as budget permits?) install electronic sensors to measure primary mirror position, log it and flag when adjustment is needed. ACTION [COO] : Decide who will have overall responsibility for image quality. ACTION: Allocate image quality budget and assign responsibilities for each element. ACTION: assign responsibilities for software tasks. Slides ACTION: Allocate photometric accuracy budget (3%, goal=2%). E.g: Shutter timing accuracy wont be an issue. Scattered light probably will be.

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