UNIVERSITY OF CALIFORNIA OBSERVATORIES / LICK OBSERVATORY
PHOTOGRAPHIC PLATE SENSITIZATION: BAKING IN FORMING GAS|
|The photographic plate sensitization system used at Lick Observatory was set up by Gene Harlan starting in 1982. A baking oven (Blue-M) was purchased and several baking boxes were made by Harlan. These baking boxes accomodate several sizes of plates used in direct and spectrographic modes at various telescopes. The system, based on nitrogen-gas purge and baking in Forming Gas, follows that established at Kitt Peak National Observatory. See Harlan's manual for original description (some parts out-of-date).|
|1. PLATE STORAGE (current status)|
|Eight deep freezers, located in the basement of the Recreation Hall (beside the swimming pool) were used until recent years. Several are now out of service or used for other storage.|
FREEZER 1: Small (8x10). Types 103aF, O, D, G. Dated 1982-1988.|
FREEZER 2: (empty)
FREEZER 3: Various (8x10, 4x5, 4x6,5x7, 8x10, 10x10, 14x14,
2.25x3.25). Types 103aG, O, D, E. Types IIaO, D. Also Ektochrome (film), Ortho (plate), Portrait 52, Kodak 33 (from ARK thesis work brfore 1961), 098407. Dates 1961-1962 and 1980-1991.
1 FREEZER 4: (empty)
FREEZER 5 and: Shane telescope building, storing other things.
FREEZER 7: Large plates (mostly 17x17-in) for 20-inch astrograph.
Older Hamburg plates 103aG (8x10-in) and TechPan (9.4 x 9.4-in). Others. Baked plates in their bake boxes.
FREEZER 8: Smaller plates for 36-in refractor and 20-in astrograph.
Newer Hamburg Technical Pan plates (9.4 x 9.4-in)- received from Hamburg 1999 Spring (mfg 1994). Also 103aG (mfg 1988-1990?) recevied from Hamburg 2000 Feb. This freezer has the BEST remaining photographic plates.
|2. SENSITIZATION: BAKING IN FORMING GAS|
Refer to diagram for nitrogen and forming gas plumbing.
|2.1 Preliminary Steps|
PLATE WARMUP: Take needed plate box from freezer to table in the plate
loading/cutting room (basement of Rec Hall). Let them warm to nearly
room temperature- in about 3 hours. The plates are still in their Kodak
boxes with room light on. After the plates reach ambient temperature,
go to one of the two next steps. THIS IS DONE IN THE LOADING ROOM.
PLATE CUTTING (if needed): After the plates have warmed, the plates may be cut to desired size with room lights off. See instructions for cutting plates. The cut plates are loaded next into the bake box (see below).
LOADING BAKE BOX: There are several bake boxes, some with fixed racks and others with removable racks. Pick the appropriate set-
The large, round bake box and also one of the two larger flat-sided boxes contain fixed racks for Hamburg 9.4 x 9.4-in plates. Other racks and bake box combinations are used for smaller plates or film. Place the bake box on the left side of the table with cover removed. Use left side because the nitrogen gas tube (blue-tinted) sticking out of the south wall is low down and quite short. It has to reach the inflow fitting on left side of the bake box.
With room lights off, open the Kodak plate box (unless plate was just cut in the dark) and insert the plates in the appropriate rack. Put the lid on the bake box. For the large, round bake box, the bolts simply flip up around the periphery of the lid and are tightened by hand. For the other bake boxes insert the (4) bolts and tighten with the special wrench- it is the thick, round "silver-dollar"-sized aluminum piece with opening at center on one face that fits over each bolt in turn. Turn room lights on after the cover in firmly down.
|2.2 Pre-Baking Nitrogen Purge (Plate in Bake Box)||
First, Atttach the blue-tinted hose from the outlet on south wall to
the proper fitting (inflow) of the bake box. (Notice there are two fittings:
the inflow fitting and the outflow fitting (=bypass valve), so that gas
may flow through the bake box. The inflow fitting is long and thin- do
not push the hose too far into it- it is very hard to pull off later on.
See that both valves are open (turned 4-5 turns CCW = ``open'').
Next, step into the freezer room. Against the wall to the right of the Baking Oven are the dry nitrogen (right) and Forming-Gas tanks (left), chained to the wall. We use Forming-Gas (2% hydrogen, 98% nitrogen). N98H2
Each tank has associated with it a series of three valves, levers, knobs. Each MUST be opened/closed in proper order!! If uncertain, do not try. Damage to meters or serious personal harm may result from improper release of high-pressure gas!!!!!!!!!!!! (More below). From right to left-
NITROGEN-PURGE SYSTEM: Opening the tank valve at the top (turn CCW) lets dry nitrogen gas flow through the Pressure Regulator (T-handle), and then through the Flow-Rate meter (black knob) and finally to the bake box (with plates) placed on the table in the plate loading/cutting room next door.
PURGING STEPS: First, turn the Pressure Regulator control (T-lever) to CCW until it ``feels free''. Then, turn the black knob directly below the Flow-Rate meter to CCW by several turns (4-5). This ensures an open channel to the bake box (and out).
Next, turn (top) tank valve CCW about one-half turn. Notice that the right-hand Pressure Regulator guage now shows some pressure. Turn the Pressure Regulator control (T=handle), usually CW, until the guage reads 10 lbs pressure (left guage of close pair). NEVER turn on tank pressure (top valve) unless the Pressure Regulator control (T-handle) is fully open (CCW). Damage WILL OCCUR if excess pressure is put against the Pressure Regulator when it is partially closed. Next, adjust the Flow-Rate meter (black knob) to 2.0 units (units are cubic feet per hour). Look for a small "spherical bubble" as indicator. Let the dry nitrogen flow through the bake box for a sufficient time: usually 15 minutes (larger boxes) or 5 minutes (smaller boxes).
When done, go to the plate loading room. First, close the outflow (exhaust) valve (turn CW) of the bake box. Then, close the inflow valve (turn CW). We DO NOT want ambient are to slip bake box when it is taken to bake oven. Disconnect the blue-tinted hose from the bake box. Carry the bake box to the bake oven in the freezer room.
Close the top valve (CW) of the nitrogen tank. We will not need it again until the baking is done many hours later.
|2.3 Bake Oven (+Forming Gas)||
With the plate box placed in the bake oven, connect the
short hose it inside oven to left to the inflow valve on the
left side of the bake box.
We now use the Forming-Gas tank located left of the nitrigen tank. First, turn the Pressure Regulator control (large black knob) to CCW until it ``feels free''. Then, turn the small black knob directly left of the large black knob to CCW by several turns (4-5). This ensures an open channel to the bake box (outflow still closed).
Next, turn top tank valve CCW about one-half turn (or bit less). Notice that the right-hand Pressure Regulator guage now shows some pressure. Turn the Pressure Regulator control (large black knob), usually CW, until the guage reads 6 lbs pressure (left guage of close pair). NEVER turn on tank pressure (top valve) unless the Pressure Regulator control (large black knob) is fully open (CCW). Damage WILL OCCUR if excess pressure is put against the Pressure Regulator when it is partially closed. Next, adjust the Flow-Rate meter (small black knob) to 2.0 units (units: cubic feet per hour). Look for a small "spherical bubble" as indicator.
Go to the bake box and open both valves by several turns (4-5 turns CCW), so that Forming Gas flows through the bake box and into the oven (and out). Close the doors of the bake box. We are ready to start baking.
PLATE BAKING OVEN: Note that the lower panel of the bake oven, below the doors, contains several things. To upper left is the Temperature Guage (deg Celsius). At upper center is a ON/OFF lever for the oven.
Below are are adjustment knobs/dials used to change the desired temperature, etc. DO NOT touch any of these, as the calibration will be lost- unless written instructions are carefully followed. If change in temperature adjustment is necessary, try to get current responsible person (if any) to do this. If not available, read carefully the instruction manual taped to the oven in the manila envelope. If done incorrectly, the calibration will be lost!
TO START: First, pull down the Upstairs Oven Disconnect lever (up when not used for freezer room oven). The lever is on the wall just above-right of the nitrogen tank. Then, on the bake oven panel turn on the bake oven (pull lever UP). Monitor the temperature guage. It should reach between 60 deg and 65 deg Celsius in a few minutes. After about one hour check again (about 63-64 deg when stable).
DURATION OF BAKING: Assume that Forming-Gas (2% hydrogen, 98% nitrogen) is being used. Other ratios may be availble (like 5% and 8% hydrogen), so that generally shorter baking times may be used.
KODAK Technical Pan (glass-back): bake time = 24 hrs (Lars Winter).
Others find bake time = 18 hrs (to 12-14 hrs?) with lower background density.
For Kodak O, G plates Harlan finds a baking time of 2.5 to 2.75 hours, plus one hour general warm-up time to ambient temperature.
END OF BAKING: Turn OFF the bake oven (lever on top-middle of lower panel of oven). Then pull UP the Upstairs Oven Disconnect lever (UP = not used for freezer room oven). Open the bake oven doors. Close the two valves (CW) on the bake box (prevent inflow/outflow). Pull off the hose from the bake box.
Next, turn off nitrogen supply by closing the top valve of the Forming-Gas tank. This must be done first, so pressure will flow out of system. The bake box will be too hot to handle without gloves. It takes over 30 minutes to cool down. Carry the bake box to the plate loading room table for the post-bake purge in order to remove the Forming Gas..
|2.4 Post-Baking Nitrogen Purge||
Post-baking purge removes the active Forming Gas inside the bake box and
replaces it with inert nitrogen gas- necessary step before allowing plates
to be exposed later to ambient air.
With the bake box on the table, connect the blue hose to the proper (left side) inflow fitting. The inflow and outflow valves are both still closed to prevent air from entering. REPEAT THE STEPS FOR PRE-BAKING NITROGEN PURGE (see above). Let the nitrogen flow at least 15 min (30 min better), to be sure all of the forming gas has been driven out of the bake box.
After the purging time, close only the outflow (exhaust) valve of the bake box. Wait and watch the Pressure Guage on the bake box until it reached pressure = 3 lbs (NEVER over 5 lbs!!!). Takes only 2-3 min, or so. Then close the inflow valve to the bake box. The plates now reside in an inert nitrogren atmosphere until needed at the telescope. NOTE: the bake box is still quite hot and needs to cool down more- for hour or more.
Go immediately to the freezer room and close the top valve (CW) of the nitrogen tank (this MUST done first before any other valves). Verify that top valves of both tanks are closed to avoid unnecessary waste of gas.
|2.5 Storage (or Use) of Baked Plates||
From about 1982 to mid-1990's there were nitrogen-purging systems located
also at other telescopes (3-m Shane Coude darkroom, 20-in astrograph).
These external sites no longer exist (mid-1999), so that nitrogen purging
can be done now only in the freezer room/plate loading room below the
The bake box can be taken to the telescope (if plates are needed soon) or put in a freezer (if needed on a future night). The bake boxes are quite heavy (larger ones, at least). Alternativly, the plates may be transferred to Kodak boxes in the plate loading room for easy transport to any telescope. The plateholders are then loaded the usual way in a local plate loading room. Unused plates should be returned to a deep freeze as soon as possible (refrigerator at telescope or the freezer room). They should keep for 30 days in air.
The plates stored in a bake box in nitrogen atmosphere are said to last months without significant loss of sensitivity.
COMMENT: If the bake box is opened for removal of some plates, the bake box must be re-purged and re-pressurized if any plates remain in it (see above). Do NOT keep plates at room temperature for extended periods of time. This causes undue speed loss.
|3. SUMMARY STEPS||
Based on summary by Lars Winter (1996 Oct).
1. FREEZER: remove plates from freezer.
2. WARM: let plates warm up to ambient temperature (3 hr).
3. LOAD: plates into plate (hypering) box in plate loading room.
4. PURGE: flow of dry nitrogen (2 cuft/hr), 20 deg C, 30 min.
5. BAKE: plate box in oven- T hrs (see table below), (2 cuft/hr), 63 to 65 deg C (see table), in Forming-Gas.
6. PURGE: flow of dry nitrogen (2 cuft/hr), 20 deg C, 30 min. If plates need to be rapidly cooled, increase flow of nitrogen.
7. COOL: let plates cool down (1-2H).
8. STORE: in plate box at 4 deg C (deepfreeze), in nitrogen, 0.15 bar.
COMMENT: Lars Winter says that there is no need to add HARDENER
to the HYPO (F-5) for TechPan and Tmax plates. This is becaus
they are of different manufacture- the surfaces are already
hardened upon completion of processing. However, adding
hardener does no harm- nor does it help the hardening process.
|| 4. FINAL NOTES||
This manual is a new, expanded version (1999 November) of the plate
sensitization manual, in part, based on author's exposure to the system
in the 1980's (t20sens1.tex). This is done in anticipation of the
continuation of the Lick star cluster astrometry program using Hamburg
Technical Pan plates starting late 1999 with the 36-in refractor.
Early versions were written by E.H Harlan in 1983 January and reissued in 1992 February in hand-typed form. The later Harlan version was retyped by A.R. Klemola, so as to be computer-readable (1997 Sept), with minor changes- mainly adding paragraph headings and added words for clarification (t20sens.tex). All computer versions are based on AAS LATEX language.