An instrument in which the variations in a fluctuating electrical quantity appear temporarily as a visible wave form on the fluorescent screen of a cathode-ray tube


Something used to adjust your digistar projector.

Here is a photo of the front of the Tektronix 465 oscilloscope.

These 4 knobs are important.

Use the horizontal and vertical adjustment knobs to get the picture where it needs to be.

Then set the volts/division to 20 (2 with the 10x probe).

Set the time/division or sweep to 50.

If all goes well, you should see the stars as spikes on the scope with sirius as the highest spike.

Follow the instructions in the Digistar Technicians Guide {4.5Mb PDF file} to properly adjust your digistar.

Here is a snipet from the guide:

You must adjust the projector CRT's brightness carefully in order to obtain the most accurate star field possible. To achieve this, you must satisfy these objectives:
* Stars with one percent brightness should not be visible on the dome.
* There should be no "hot dots" (false stars) visible at the zenith.
* The star-field density should appear to be realistic; it should look the way the real sky does when you view it on a clear, moonless night, away from city lights.

There are three potentiometers on the correction card (E&S part number 240561-100) that affect brightness:

The INTENSITY potentiometer (R115) controls video gain. INTENSITY controls the amount of buffered video drive to be applied to the video card (part number 207590-100). Video is typically observed with an oscilloscope connected to the video card test point (E32 or E33).

CAUTION: The intensity potentiometer should never be increased beyond the levels recommended in this section.

The video-dc threshold level potentiometer, THRESHOLD (R107), controls the level at which the CRT just begins to conduct. THRESHOLD directly affects the richness of the star field and is typically set for about 130 V dc as observed on the video-card test point (E32 or E33).

The screen voltage potentiometer, G2 ADJ (R82), is adjusted to set the one percent brightness level on the projector. It is not easy to measure G2, but it is usually set around 800 to 1,000 V dc.

Each potentiometer increases brightness when turned clockwise and decreases brightness when turned counterclockwise. All brightness-adjustment potentiometers are multi-turn potentiometers.

CAUTION: You must follow the brightness-adjustment procedure carefully to avoid damaging the projector's CRT. If you set the brightness controls incorrectly, you might permanently burn the phosphor of the CRT. Always be sure that the video-card ground strap and the high-voltage power-supply return strap are attached to the chassis when you operate the projector. If any of these steps do not appear to work correctly, recheck all of the previous connections and alignment steps. If you are in doubt about the nature of the problem, you should always misadjust the static-focus knob. Defocusing the image should help prevent phosphor burns.

There is a wide degree of variation in domes and CRTs, so each projector may be set up slightly different. To adjust the projector's brightness controls, follow these steps:

1. Erase any pictures that are being displayed by Digistar II. Power the projector down while leaving the graphics processor booted.

2. Connect an oscilloscope to VIDEO OUTPUT test point E33 or E32 on the video card (part number 207590-100). Set the scope for 20 V dc per division with 0 V dc at the bottom of the screen. Turn all of the brightness potentiometers (R115, R107, and R82) to their minimum setting (maximum counterclockwise). Purposely defocus the projector by turning the STATIC FOCUS knob to its maximum counterclockwise or clockwise position.

3. Disable the BEAM switch (U1) on the correction card and power up the projector. Allow the projector to warm up for at least 30 minutes.

4. Observe the waveform. You should observe a dc level of approximately +140 to +150 V dc with no pictures being drawn by the graphics processor. (Ignore the small XMAX/YMAX pulses.) This dc level represents the blanking level for the CRT.

5. With the beam still disabled, enter these commands in the Commands window of the GUI:

declare global stars
attitude inertial 6 0 0
draw stellar stars

Remove the video input cable from J3 on the correction card and adjust the oscilloscope until the waveform is in sync. Adjust R107, which adjusts the threshold level for the CRT, until the bottom of the waveform measures approximately +130 V dc. Reconnect the video input cable.

CAUTION: Increasing the INTENSITY control beyond the following setting results in oversized stars and the possibility of burning the phosphor of the CRT.

6. Do not enable the beam at this point. Adjust the INTENSITY control (R115) clockwise until the bottom peak of Sirius (the tallest dot) is located approximately 60 V above ground (0 V). This setting applies about 70 V of video (minus UNBLANK) to the CRT. The preferred brightness setting is at 70 V of video (60 V above ground). Although the picture will be somewhat brighter if you increase the INTENSITY control, the spot size will also be larger and the risk of burning the CRT will be greater. During adjustment of the INTENSITY control, you may have to adjust the oscilloscope to lock onto the tallest dot. Figure 6-13 shows a simplified version of what you are looking at on the oscilloscope.


7. Enter this command to return the star-field brightness to one percent:

adjust stars brightness 1

Remove the oscilloscope probe from the video-card test point. Darken the theater and wait until your eyesight is fully adapted to the dark. Enable the BEAM at the correction card and, if required, at the console. You should not be able to observe light in the projector lens.

NOTE: When you apply anode voltage above 30 kV to a CRT, you may see a very small amount of light in the tube under fully dark-adapted conditions.

8. Slowly increase G2 (R82) until you see a dim image in the lens. Adjust the STATIC FOCUS knob until the lens image is sharp. Continue increasing G2 until you can barely detect a faint image on the dome. Now back off G2 until the image completely disappears. This sets the one percent brightness level. If you cannot get an image, even at the maximum setting of G2, your CRT has a slightly different conduction point. Readjust the threshold setting (step 5) for +125 V instead of +130 V and repeat steps 6, 7, and 8.

NOTE: You can only set proper one percent brightness levels for the star field. You may be able to detect a faint object with a large number of vectors even though its brightness is set for one percent.

9. The projector is now capable of displaying pictures at full brightness. Disable the beam and use the STATIC FOCUS knob to defocus the projector.

10. Enable the BEAM switch. You should see very little light, if any, from the projector lens. Enter this command to adjust the brightness of the star field to 70 percent:

adjust stars brightness 70

If the projector's light output appears to be normal, adjust the STATIC FOCUS knob until an image is focused on the dome.

11. Enter this command to adjust the brightness of the star field to 100 percent:

adjust stars brightness 100

Observe the star field under darkened conditions.

a. If the star field appears to be too rich, turn THRESHOLD (R107) down, counterclockwise, until the star field appears to be correct.

b. If the star field appears to be too sparse, increase the THRESHOLD setting.

c. Recheck the zero brightness setting (step 8).

You will need to adjust the brightness settings periodically. You should also adjust the projector's brightness whenever you replace these parts:

* Video card (E&S part number 207590-100)
* Correction card (E&S part number 240561-XXX)
* CRT (E&S part number 801188-502)
* VG card (E&S part number 240110-100)