Installed during February of 1997 as a facility instrument, the 4-color bolometer has center wavelengths at 1.3 mm, 0.87 mm, 0.45 mm, and 0.35 mm and approximately 50 GHz bandwidth. It was developed and built by E. Kreysa, and replaced the single channel bolometers used previously. The cryostat has four independent bolometer radiometers, which share the same He-4 cryostat and the same He-3 cooler. Each bolometer system is close to single moded (i.e. diffraction limited) for it's operating wavelength. The horns were designed to give the same illumination on the subreflector. This has been verified by beam measurements at the subreflector. Recent observations suggest that we might be under illuminating the beam somewhat. The combined holdtime of the He-3 and He-4 stages is in excess of 48 hours. (The Liquid N2 hold time of the outer can is 24 hours). The refills are done by the SMTO staff (usually the site manager). During operation the He-4 is at 1.5 K and the He-3 is at 300 mK. In addition to a high gain AC-coupled preamp, each bolometer is also connected to a low gain DC-coupled preamp, acting as a total power channel. In the future, the total power channel may be used for skydips and monitoring sky emission continuously. All eight channels are continuously active and connected to DRUMBEAT via multichannel preamps and VFC's (voltage to frequency converter), although only the bolometer that is placed above the offaxis mirror is coupled to the telescope. This design allows a quick change of wavelength according to weather conditions and, at the same time, to optimise the performance of each wavelength channel without having to compromise.
Setup for the 4-color bolometer is straightforward: There are three basic areas that you have to deal with: computer commands, tertiary mirror setup, and bolometer configuration.
In OBST (the computer control program, see Chapter 6 for details and see Chapter 2.4.1 for examples), the observer types the following commands: (The ! marks the start of a comment and OBST> is the OBST prompt.)
OBST> SOURCE MARS ! Set the first source OBST> SET BOLO 1 ! Set the bologain ! to the appropiate level OBST> RX B130C4 ! 4-color bolometer: 1300 microns or OBST> RX B087C4 ! 4-color bolometer: 870 microns or OBST> RX B045C4 ! 4-color bolometer: 450 microns or OBST> RX B035C4 ! 4-color bolometer: 350 microns
The RX command will set a number of parameters in the computer so that you will be ready to proceed with continuum observations with that bolometer setup.
The 4-color bolometer is mounted in the right hand cabin (as you enter from the stairs). At this point, you are ready to change the tertiary mirror so that it directs the radiation to the correct receiver!
Following the instructions in Section 3.1.1. Make sure that you have the correct orientation (facing the right cabin for the 4-color receiver normally!). In daylight, you can see the secondary out through the optics from the right hand cabin!
You are now ready to make the necessary adjustments to the bolometer itself. There are three steps. Go up to the right hand cabin and face the bolometer. It is a gold-colored dewar that sets in the back corner. (The HEBOL has claimed pride of place - directly looking at the tertiary).
If the HEBOL is present, you will need to move the bolometer mirror into position. You will find the correct mirror position marked on the flange. You can find the mirror motor in the rack beside the HEBOL - near the bottom. First, make sure that the mirror assembly is lowered to direct the radiation from the tertiary into the bolometer.
Examine the bolometer. You will see a set of micrometers underneath the bolometer (labeled x and f), and set into the center of the bottom of the bolometer support plate, there is a single pin. Further forward is the entrance path for the radiation into the appropiate bolometer element. On the top of the bolometer is the preamplifier box where you do the bias adjustment for the bolometer electronics. Alas-due to the presence of the HEBOL, it is more difficult to access the bias adjustment than in years past.
Be very careful not to damage the HEBOL receiver!
You have to adjust the bolometer's physical position, the x and f micrometer readings, and the bolometer bias to make the bolometer ready for use at the wavelength you desire.
Adjust the bolometer location so that the appropiate detector is in the optical path. At the base of the bolometer, there are small arrows (and numbers above them - like 1300, 800, etc). The arrow on the baseplate is on the near-right hand side as you look towards the tertiary mirror. It is just a little pencil mark - nothing fancy!
You must first loosen a bolt underneath the bolometer to enable you to rotate the bolometer structure. It is located at the very center of the bolometer (to the right of the micrometers). The allen wrench that fits should be just by the bolometer. Turn clockwise, probably about half a turn or so. The bolometer should rotate easily at this point on its axis. You do not have to lift the bolometer up! Line up the small arrow (with the desired wavelength) with the arrow on the base. Be sure you do this carefully or your beam alignment to the secondary will be poor!
Retighten the bolt (counterclockwise) to the tightness you found it. Do not overtighten-you don't want to strip the bolt. Be careful not to pull out any wires or tubes while doing this!
It is probably best to ask a Friend of a Telescope or Site Manager to help you through this the first time!
Adjust the micrometer readings according to the following table 3.1. This will set up the horizontal and vertical settings for the mirror optics below the entrance hole. Make sure you adjust the appropiate micrometer (x or f).
Adjust the bias (mV). This is done by plugging a voltmeter into the bias plug on the pre-amplifier box, located on the top of the bolometer. You read the bias in mV and adjust using the bias adjust knob. The appropiate settings are in Table 3.1. Afterwards, remember to disconnect the micrometer to reduce noise in the system.
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Note: these values are somewhat different from previous years, due to an adjustment in the tertiary mirror location.
At this point you are ready to go to a point source and check your pointing! Planets or bright young stellar objects in the pointing catalog are best. Check the BOLOGAIN to make sure it is set to the correct value for observing planets (typically 5 for the bright planets versus other sources where the bologran is typically 1). If your counts go over 400,000 in ONLINE CHEF, your signal is saturated and you should increase the bologain.
See Chapter 2.4.1, Chapter 6, Appendix A and Appendix B for the gory details from this point on!
We recommend that all observers observe calibration sources, such as the Sandell list of submm calibrators. This will allow the observer to convert counts to Jy very easily - using the ONOFF program. Our numbers for the 1.3mm bolometer suggest that you can expect something like 1100-1300 counts per Jy.
In addition, it is wise to do at least one SKYDIP at each of the wavelengths you observe. We have preliminary estimates for the conversion of tau(225 GHz) to bolometer tau that we would be happy to share with you, but they are only preliminary numbers.
Most observers use the bolometer at 1.3mm. If you plan to use the bolometer at other wavelengths, you will want to verify the pointing offsets. The pointing model for the bolometer is derived from 1.3mm observations.
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