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Applicability
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This tool is intended for users of broad spectrum spectrographs, such as the Alpy 600 to determine the magnitude of flux loss at the blue and red ends of the spectrum for the slit in any of three typical orientations. Although users of narrow band spectrographs, such as the LHIRES or StarEx are generally not affected by effects of atmospheris dispersion, this tool can provide confirmation of this.
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Slit Orientations
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This tool providse the user with the ability to calculate loss of flux through a spectroscopic slit for the slit oriented at one of three typical orientations:
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1. Slit Perpendicular to the Horizon - This is the ideal slit orientation for minimal to no flux loss across the spectrum. For an alt/az telescope mount, this orientation can be set with no need for further adjustment. For an equatorial mount, this orientation requires the ability to rotate the slit to this desired orientation for each object observed.
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2. Slit Perpendicular to the Celestial Equator (CE) - This orientation is achieved by setting the slit to be perpendicular to the horizon when pointing along the meridian. This slit orientation will result in minimal or small flux loss at the blue and red ends of the spectrum when observing near the meridian. Blue/red flux loss will increase with this slit orientation for objects observed farther from the meridian.
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3. Slit Parallel to the Celestial Equator - This orientation is achieved by setting the slit to be parallel to the horizon when pointing along the meridian. This slit orientation can result in significant flux loss at the blue and red ends of the spectrum when observing near the meridia but with blue/red flux loss decreasing with this slit orientation for objects observed farther from the meridian.
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Note on Rotating the Slit
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The ideal situation would be to have a motorized slit rotator than can change its orientation continuously during the integration time to maintain the slit perpendicular to the horizon. One complication to this is that the autoguider used to maintain the star on the slit is typicaly calibrated for a particular slit orientation. As such, it is impractical for the amateur spectroscopist to rotate the slit in a continuous manner. The best possible alternative is to maintain the slit in a fixed orientation throughout the total integration time but rotated to the optimal orientation at teh integration mid time. Is is expected that the loss of flux at at teh blue and red ends of the spectrum will be minimal and can be verified by selecting the "Slit Perpendicular to the Horizon" option.
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Instructions
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This tool consists of two worksheets: The Slit Loss Calculator worksheet and the Data worksheet. The user of this tool will interact with it entirely throug the Slit Loss Calculator worksheet. Ther is no need to interact or change anything in the Data worksheet.
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The Slit Loss Calculator worksheet is organized such that user inputs are entered in the area defined by columns B through D and results are provided in the the area defined by columns F through J. User inputs are the yellow shaded cells in the input section and the slit loss results are shown in blue shaded cells in the Results section.
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1. Select Slit Orientation
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Click on the merged cell C4:D4 to show the drop down list in cell E4, if not already shown. From the drop down list select one of the three available slit orientations.
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2. Enter Object Parameters
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Enter the object name. This is just for further reference and not used in the calculations
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Enter the object RA in the J2000 coordinate reference frame. Cell D8 shows the expected format for RA (hh:mm:ss).
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Enter the object Dec in the J2000 coordinate reference frame. Cell D9 shows the expected format for Dec (+ /-dd:mm:ss). Hovering your mouse over cell C9 will bring up three examples for declination as a reference.
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3. Enter Exposure Parameters
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Enter the UTC date of the observation in dd/mm/yyyy format.
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Enter the UTC time of the observation in hh:mm format.
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Enter the exposure total duration in seconds. This is the product of the individual frame exposuretime and the number of frames.
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4. Seeing and Atmosphere Conditions
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Enter the atmospheric seeing (arcsec) for this observation
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Enter the atmospheric pressure in mm Hg if known. Otherwise use the default value.
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Enter the atmospheric temperature in degrees C if known, otherwise use the default value.
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Enter the atmospheric water vapor pressure in mmHd if known. Otherwise use default value
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5. Observing Location
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Enter the observation site longitude in degrees east
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Enter the observing site latitude in +/- degrees
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6. Spectrograph Configuration
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Enter the telescope effective focal length in mm. This focal length includes the effects of any focal reducers or extenders
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Enter the spectrograph slit width in microns
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Enter the slit length in microns. If not known, enter a value at least 10 times the slit width.
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7. Wavelength Range
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Enter the shortest (blueward) wavelength produced by the spectrograph for this observation
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Enter the longest (redward) wavelength produced by the spectrograph for this observation
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Note: for a broad wavelength spectrograph, such as the Alpy 600, this may be something like 380 nm and 780 nm. For a high resolution spectrograph, such as the LHIRES centerd on the H alpha line, the wavelength range may be something like 648 nm and 664 nm.
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Results
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The Results section on the Slit Loss Calculator worksheet shows the results for the inputs entered.
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Object ID - The Ob ject ID for this observation
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RA - The RA entered in the inputs section
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Dec - The Dec entered in the inputs section
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UTC Date - The UTC date of the observation entered in the inputs section
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Reference Wavelength - A calculated value. It is the center wavelength between the shortest and longest wavelengths entered in the inputs section. This center wavelength is the reference poing for which flux throughput is set to a value of unity.
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Slit Orientation - The slit orientation selected in the inputs section
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Time - The times of the exposure start, exposure mid time, and exposure end.
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Azimuth - Object azimuth at the exposure start time, exposure mid time, and exposure end time.
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Elevation (unrefracted) - Object elevation at the exposure start time, exposure mid time, and exposure end time.
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Zenith Angle - Object zenith angle at the exposure start time, exposure mid time, and exposure end time.
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Airmass - Object airmass at the exposure start time, exposure mid time, and exposure end time.
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Local Mean Sidereal Time - LMST at the exposure start time, exposure mid time, and exposure end time.
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Hour Angle - Objecthour angle at the exposure start time, exposure mid time, and exposure end time.
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Parallactic Angle - Object parralactic angle at the exposure start time, exposure mid time, and exposure end time.
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Intensity Loss at (lower wavelength rage) nm at the exposure start time, exposure mid time, and exposure end time.
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Intensity Loss at (higer wavelength rage) nm at the exposure start time, exposure mid time, and exposure end time.
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