The beginning of this document gives step-by-step instructions for setting up the telescope for flat fields. This section will deal with the procurement of these flats. Having installed the white spot screen and the filters, begin both the telescope control computer and the camera control computer. You may wish to place an object into your star catalogs which is named "Calibrat." The advantage is that the master dark and bias frames would be placed under a subdirectory by this name, if these are taken along with the flat fields. As of this writing the telescope will move to the coordinates of whatever object is selected, but I plan to incorporate a change so that an automatic move to the flat field position is made. For now you may wish to abort the telescope move by turning off the wall switch labeled telescope, turning it on again, and restarting the tracking from the Main Menu. Move the telescope manually to the white spot. When the automatic move is incorporated, it will place the telescope at an hour angle of zero and a declination centered on the white spot. When you reach the desired position, put the proper filter in place and turn off the tracking using the Main Menu option. The filter wheel will not turn if the telescope is not tracking, so you must restart tracking before any filter change. Also do not stop the tracking using the wall switch since that will send unwanted characters to the telescope control computer.
Under normal operation we have found the following exposure times work well for flat fields:
|Filter||Light banks||Exposure time (s)
|Blue||Both||15 - 20|
|Visual||Low||10 - 15|
|Red||Low||8 - 10|
I have not tested the other filters as yet, but if you do, please report the result. The aim is to achieve an intensity of 3000 or greater on the average pixel. You can use the camera control mouse, click on any pixel in the image and see the intensity at the bottom of the display.
Use the CCD menu option on the camera control computer to take the flat frames. I have been using about 20 flats per color, but experience will be our guide. Ideally, you probably want to leave the dome during the flat field procedure, since reflected light from you may be transmitted to the white spot and recorded. I also think its a good idea to turn on the lamps about 15 minutes before you begin recording flats in order to give the current through the filament a chance to stabilize. Following the flats you may wish to record master dark frames. When you are finished remove the white spot and carefully store it below the floor.
As you can see the calibration procedures for CCD photometry can take some time. It is probably wise to arrive at the observatory near sunset so that you are ready to begin observing by the end of twilight.
Flat Field Lamps
Low Bank Filters
Specifications for the "White Spot."
The white spot has two coats of different types of paint. The undercoat is Krylon 1502 flat white paint applied with a Pace air brush and a number 2 head. The surface coat is a special blend that can be made as follows:
To make one pint, heat 50 ml of distilled water to 55-60 C and slowly add 2.25 grams 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.
Reference - NOAO Newsletter, December 1991, page 26.