Last update: 04/05/2005 MF

The McDonald White Guider is an offset guider similar to the McDonald
photometry offset guiders. Here are some basic points for its use:

 1) Be sure the offsets are near 0,0 on the vernier by the eyepiece for
    pointing to an object (if you intend to look through the eyepiece).

 2) Focus values for the CCD and WHT guider is ~1500 to 1950 at the 30"
    depending on the temperature. Anywhere else, a good rule of thumb is to
    focus the telescope to the eyepiece. It is usually in or near focus.

 3) If the mirror knob (right side of guider) is in, the light from the
    telescope comes out the eyepiece. To move it out, turn the knob
    counterclockwise until it stops to take it out of locked position,
    pull the knob away from the guider as far as it will go, then lock it 
    again by turning the knob slightly clockwise. In this position, the light
    all goes down through the filters to the CCD. This guider has an unused
    second channel option. If you see no light at all in the eyepiece
    (other than the reticle), flip the knurled knob at the left of the
    eyepiece at its base. This flip mirror allows the light to be sent
    down the non-existent second channel as well as the eyepiece.

 4) The filter wheel is reached by removing the four knurled screws on the
    right side of the guider below the mirror plunger. You'll have to
    disconnect the filter motor cable. The filter wheel is attached to the
    plate you've just loosened. Gently pull the filter wheel out. It is a
    tight fit. The filter wheel that is normally left in the guider has
    eight 1x1" holes. The filters normally resident in it are B, V, R, I
    and BVR (a thin clear filter useful for finding faint objects). Other
    filter wheels are available; some have two 1x1" holes and two 2x2" holes.
    Adapters are available for other sizes and shapes. There is an allen
    screw holding the filter wheel to its holder. Loosen this to change the
    wheel (it is a captive screw). Make sure the wheel is tight before you
    put it back into the guider. It is possible for telescope motion to
    loosen it. Reconnect the filter motor cable. With the filter motor off,
    set the filter wheel to position 1. Be sure the wheel is solidly in the
    detent. Turn the filter motor on.  ICE assumes that you start in filter
    position 1. Be sure to home the filter wheel in ICE any time you restart
    ICE or IRAF or if you have hit CTRL-C or if you have had the filter motors
    turned off. Check from time to time as you observe that the filter
    locations seem to be correct, i.e, ON the filter number and not between
    numbers. The motor occasionally gets out of synch. If you notice
    non-repeatable flat fields, you may wish to operate without the filter
    motor and turn the wheel by hand.

 5) To offset guide, first set up on your object, positioning it as desired
    on the chip. Then loosen the two black knurled knobs on the front left
    of the guider. This will give full motion of the eyepiece around your
    object. Move the eyepiece physically around the field, being careful
    not to shove the telescope off of your object. When you find a guide star,
    lock down the black knurled knobs with the object near the center of
    the crosshair. You can then use the brass knurled knobs at upper and
    lower left to adjust the fine positioning of your object on the crosshair.
    If you run out of travel on the fine motion, turn the knob several
    full turns the other way, then move the guider using the coarse
    adjustments (not the telescope) back to center your object. Then do the
    fine adjustment with the knobs. Once you've centered on your guide star,
    doublecheck to make sure the black knurled knobs are tight. This will
    prevent any slippage of the guider while the telescope is tracking.

Some Suggestions:

 1) If you don't want to have to focus separately with each filter, do your
    focus on the thickest filter you'll be using. The focus for the rest
    will probably be pretty good. If you focus using a thin one, the
    reverse does not always hold true.

 2) To help save time, focus the telescope to the CCD, then focus the
    eyepiece to your eye. Then, if the focus changes during a long
    integration, you can do a rough focus by eye. The focus WILL change
    with change in temperature, particularly at the 82" and the 30". When
    doing a focus at the 36", try to pick stars close to the the center of
    the field. Objects away from the center will show a lot of coma.

 3) If you want to make a quick test integration to find and center your
    object, use the BVR filter. It is a clear-looking thin filter supplied
    with each guider. If the filters you are using are very thick, doing a
    test integration with no filter can produce a very out of focus image.
    BVR lets most of the light through, but the focus will be better. This
    will save you time if you are trying to locate faint objects to be
    observed with thick or narrow-band filters.