Kitt Peak Flat Field Calibration lamps
All the telescopes have sets of flat field calibration lamps attached to them. There are four projectors for the high intensity lamps and four for the low lamps. Both sets of lamps are 12 volt. The setup is the same for all telescopes.
These projectors are special because they hold the MR16 lamps and have a place for two or more 2X2-inch filters. The company that made the projectors is no longer in business. I know of no source for this type of projector. So be careful with them. The projector top opens up by pulling up on the cover over the filters first. Pay careful attention to the position of the projector on the telescope. They maybe upside down, so when you open the first lid the filters will fall out on the floor. Have your hand in place to catch the filters so they can be safely removed. Carefully remove the filters if present. The high intensity lamps have no filters. Next, you can open the back part of the projector by lifting the lid from the front. This will allow one to have access to the lamp. The lamp is captive and will not fall out. (WARNING: IF THE LAMPS HAVE BEEN ON THEY WILL BE EXTREMELY HOT) Be careful. To remove the lamp, on the left side of the lamp there is a heavy wire that you push down that will lift the lamp out of the socket. Remove the lamp and install a new one by pushing it all the way down in the socket until the release wire comes forward all the way. (WARNING: DO NOT HANDLE THE INSIDE OF THE REFLECTOR OR THE LIGHT BULB). Oils from your hands will ruin the lamp when it is turned on.
The High Lamps:
These lamps are special, they are 12Volt 50 watt, 40-degree beam, lamps. They are a GE type EXN no. 20833, Q50MR16C/FL40. These lamps must have their reflectors aluminized. When the lamps are purchased they must be given to the NOAO coatings lab to have the reflectors coated. Make sure to allow enough time to have this done as it does take some time to do. If you do not have the coating done the UV will drop by nearly 50% or more, making calibration for the U filters nearly impossible. It is very hard to tell if the lamp is coated unless you have an uncoated lamp to compare with it. So be careful if you make a lamp change. These lamps have no filters of any kind in front of them.
The Low Lamps:
These lamps are 12Volt 20 watt, 40 degree beam lamps. They are a GE type BAB no. 20814, Q20MR16C/FL40. These lamps are not aluminized. The projectors for these lamps have two filters installed in the slots under the first lid. These filters are NOT held in by anything other than the lid. So if you open the lid, make sure they will not fall out.
The filters are two 2mm. thick 2X2 inch BG 34 filters and a 2mm thick no.1 ND filters. You can use combinations of different thickness filters to make come out with the same package if needed. You must have this combination in each projector. I would always place the ND filter in front of the lamp and then the BG 34s.
All the projectors should be cleaned and checked each summer. The lamps should be replaced. They do have a long life time 4000 hours, but bugs will get burned by the lamps and ruin the reflectors of the high lamps. These can not be cleaned without damage to the aluminized surface. When cleaning the filters, check the ND filters against a white background. When in use they tend to get the center burned away, leaving a hot spot on the flat field screen. During the year it is best to look at the flats screen from time to time and check the low lamps. If you see two or more hot spots on the screen from the lamps the ND filters must be replaced.
Currently there are replacement lamps in the TA cabinet in the Adm. Building on the Mt. The spare filters are in the filter cabinet in the support office.
The Flat Field Screens:
This material is given so everyone will know the construction and why the screen are coated, in case one ever needs to be redone.
Ideally, the screens should have a flat spectral response and a flat spatial response. Both are difficult, but the spectral character turned out to be the most difficult. Most white paints, for example, are really red paint. If your eye were sensitive down to 3200 A, you would find that the familiar white domes (titanium oxide paint) are very red.
We adopted paint first proposed for coating the insides of spot sensitometers used, coincidentally, for calibrating photographic plates. This is documented by Wu et al. (1972, AAS Photobulletin 1, 9) and is based on a barium sulfate mixture. The recipe for the paint is reproduced below for those who do not have access to old issues of this journal.
In order for this paint to adhere to a surface for a long period, the surface must be cleaned very well, and then primed with a white oil-based primer. The screen is masked so that the diameter of interest (several inches larger than the telescope aperture) is obstructed. The region exterior to the mask is then sprayed with flat black paint. After the black section dries, it is masked, and the central circle sprayed with the white paint mixture. You must use a spray gun capable of spraying very coarse paint. Apply several thin coats, allowing each coat to dry out before the next. If you don't wait the paint will run off the surface,2/8/02 removing the coatings. It takes 3 or 4 pints to cover a 4-foot diameter spot. It takes some practice to coat this material, as it will not behave like any regular paints. This paint is easily damaged and will scratch off or flake off easily if touched.
The screen must be made of a stiff material that does not flex easily; otherwise the white paint will flake off. Thus we found that plywood and fabrics do not work well. We have adopted a material called "Blue Seal Sandwich board". If you need this material it was last available from a company called Hexcel Corp, 7711 Center Ave, Suite 305, Huntington Beach California 92647. This board is a special lightweight aluminum material honeycombed between two thin aluminum sheets. We used a 4-foot by 8-foot 3/4inch sheet cut down to size for the .9meter and two sheet carefully joined for use at the 2.1meter telescope. Special clips are available from Hexcel, but the aluminum sheets are light enough that normal mounting screws will hold them to the dome surface.
Here is the recipe for the high reflectance white paint, as taken from the paper by Wu et al. (1972, AAS Photobulletin 1, 9).
"To make one pint, heat 50ml of distilled water to 52-66 C (125-150 F) and slowly add 2.25 grams of elvanol polyvinyl alcohol (Grade 72-60), mixing until all the alcohol dissolves. To this mixture add 150 ml of distilled water and 200 ml of 200 proof anhydrous ethyl alcohol. Then slowly add (stirring continuously) 227 grams of USP grade barium sulfate to the mixture.
In practice it seems best to mix this paint just before use. A coating of Krylon 1502 flat white paint was sprayed as a primer.