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Figure 4. Example of a pinhole exposure at the coarsest plate scale. At higher scales, one would "zoom" in towards the center of the field.
These are
not user tests, in general, but are here for diagnostic completeness.
Dark
frame: (also known as cold plate image) Take a 2-3
second exposure of the cold plate (i.e. choose cold plate from filter
selections). Display the image using nproc.
You should see a bright line at the top of each quadrant. This is sometimes called the “brightness
effect”, and subtracts out.
Flat-Field: These are useful to verify that the detector sees light, that the dewar window is not fogged, and that the detector is not vignetted.
1. Change the filter to J-band.
2. Focus a uniform light onto the aperture, e.g. the dome screen or sky.
3. Select the appropriate square aperture for the chosen plate scale.
4. Turn off any room lights and/or put a dark blanket over the path from the projector to the dewar window.
5. Expose for at least a few seconds. Check the exposure immediately to see if the exposure was long enough. If you see four bright plumes of light on the top and bottom middle parts of the array, it was not long enough. Counts should be about 20,000 ADU on average.
6. Investigate possible vignetting by looking at the image and at crosscuts through the image. Look for radial gradients from the center to the sides and corners.
Pinhole array: This is another good test for detector vignetting and centering.
1. Use J-band filter.
2. Focus a uniform projector light on the aperture. (see flat field on how to do this).
3. Select position 3 on the aperture wheel; this should be the pinhole array. Check to be sure. This pinhole array was made to test the MLO optics (used at WIYN) but it can be used for the UNISIS (Mt. Wilson) optics as well. It is three concentric squares with smaller pinholes towards the center.
4. Take an image with a several second exposure.
5. Display the image in IRAF. The square patterns are pretty well centered in the aperture, so they should be pretty well centered in your image. If they are not, there may be a misalignment of the optics. Also, the values of the peaks of the pinholes in the same pinhole square should have nearly the same counts. Note that because the pinholes increase in size in the outer squares, the outer pinholes will have more light than the inner pinholes. If pinholes of the same square do not have the same counts, it may be a vignetting effect.
Figure 4. Example of a pinhole
exposure at the coarsest plate scale. At higher scales, one would
"zoom" in towards the center of the field.