Finding Guide Stars for Long Integrations

When imaging with tne Andor camera on the HBO telescope, long exposures (greater than 60s) will either have to be broken up and stacked so that guiding drift does not streak the images, or else a long exposure will have to be guided dynamically. There is a guiding camera in the base of the telescope that intercepts a 20x20 arcminute field offset 30 arcminutes north of the center of the imaging camera. It can guide effectively on stars down to about 13th magnitude -- obviously brighter is generally better, though stars brighter than fifth magnitude will streak as the camera has no shutter.

The ideal way to find whether your field will permit easy guiding -- and to determine in advance of the observations what the requisite parameters for the guider will be -- is to use the Aladin tool to preview the field and consult catalogs of appropriate guide stars.

Starting Aladin and identifying your target

• Click the "Aladin Virtual Observatory Tool" icon on the desktop to start the Aladin tool
• Type the name of your target into the Location prompt and hope that it resolves. If not, you will have to try alternative catalog designations or even look up your target with another tool and enter coordinates directly. If you are forced to use manual coordinates, make sure that you select the coordinate equinox through the pulldown menu to the right of the Location bar.
• Click the File pulldown and select "Load Instrument FOV" (FOV stands for "Field of View"). You can now select "Offset Guider" for the "Hartung Boothroyd Telescope". There are also fields of view for the Andor imaging field itself and for the 100 micron width slit of the longslit spectrograph.
• Click File, Open, and select the Surveys tab on the right column of the selector. The Tycho-2 survey has about the same limiting magnitude as the HBO guider, so select it. Specify a radius of 40' so that you will in fact look up stars that extend as far out as the guider field of view, then select Submit.
• Zoom out by scrolling the mouse wheel while looking at the image frame so that you see the target field and the guider FOV window (hopefully populated with a few candidate guide stars). Click on these stars in turn and look at their properties in the data window at the bottom to select one of the appropriate magnitude for guiding.
• The applet below lets you copy coordinates for your science target (or another target object visible in your same imaging field of view) and your desired guidestar in the offset area. It will then calculate the appropriate settings for the N/S and E/W dials to position the offset guider steering mirror appropriately.
• To copy coordinates from aladin, first select a target by either clicking it or by drawing a window around it. A list of potential objects will appear in the data window below the viewport. Select one of these lines by right clicking on it, and from the menu that appears, navigate to "copy" and then to "copy object coordinates". Place the mouse in the appropriate area of the form below and use the middle mouse button to paste (you can also press the right mouse button and select "paste" from the menu).
  For reference, the offsets are given by the equations:

    E/W # = 325 + (17 * RA Offset West)  
    N/S # = 123 - (8 * (Dec Offset North - 20))   
            [19 Sep 2009 calibration]

• Check for sanity that the E/W dial offset is between 150 and 450, and that the N/S dial offset is between 975 and 150. The applet does this for you and will display "bad" if you the offsets are inappropriate. Then carefully rotate the guider offset dials on the telescope to move the guider appropriately.

Applet for finding guidestar dial positions

E/W Settings (for manual use)

N/S Settings (for manual use)

Nota Bene: Attached are the postscript files for the above plots.

-- DonBarry - 2009-08-18