Selected Instrument

Red Blue Channel Diagram

Central Wavelength

The central wavelength determines the mid-point of the signal to observation. You should try and make this the same as you would for actually observations to get the most meaningful exposure time or signal to noise estimate.

Choose Aperture

The following slits widths are available for Red and Blue Channel. As a rule of thubm, the median seeing at the MMT is on the order of 0.85".

Slit Width (arcsec) Blue FWHM (pixels) Red FWHM (pixels)
0.75 2.5 2.6
1.00 2.8 3.2
1.25 3.1 3.8
1.50 3.6 4.1
2.0 4.5 5.3
3.5 7.9 10.0
5.0 11.5 16.0


The f/9 topbox has the following filters avaialble. If other filters are needed, significant advanced planning is required with MMTO staff.

Blue Blocking Filters

Red Blocking Filters

Neutral Density Filters

Enter Object AB magnitude

Currently, calculations are performed assuming the target is flat if Fν (i.e. has a constant AB magnitude). These calculations assume a point source . This may impact the calculation if you are studying an extended source or a source wher the majority of its flux is arising due to emission lines rather than continuum.


The desired binning on the detector is required in order to properly treat the noise in the detectors itself (outside of shot noise). Typically, both Red and Blue Channel are operated using no binning (1) in the spectral direction and binning by 2 in the spatial direction. Unbinned pixels have a scale of 0.3" / pixel. Median seeing at the MMT is on the order of 0.85".s

Lunar Phase

Enter the lunar phase between -14 and 14 (lunar age from Observer's almanac). Recall that the new moon is at phase = 0, and the full moon is at +/- 14.


Clearly, the airmass at which an obseration is completed will have a significiant impact on the final signal to noise reached. Enter the mean airmass for your observations. A sample extinction curve is shown below.


The size of the seeing disk determines the fraction of the incoming source light that is lost by not making it through the slit. You need to specify the seeing you hope to achieve for your observations to estimate those slit losses. (Also, these calculations assume a point source . Extended sources may have higher losses).

Calculation Output

Would you like to calculation the Integration Time (given a desired signal-to-noise ratio) or the Signal-to-Noise Ratio for an observation of a given exposure time?

Integration Time is the optimal observation time for an exposure based on the parameters you provide in the next step.

Signal-to-Noise Ratio is the quality the spectrum will obtain based on the parameters you provie and a fixed exposure time.


Enter the desired signal-to-noise value that you wish your exposure to reach. The convention of this calculator is the signal-to-noise per extracted pixel (summed across the spatial direction for a point source).

Red Channel Gratings:

Grating (lines/mm) Order Blaze Wavelength (Å) R (@ Blaze, 1" slit) Resolution (Å) Dispersion (Å) Coverage (Å)
150 1st 4800 230 21.0 6.37 6574
270 1st 7300 640 11.4 3.59 3705
300 1st 4800 460 10.3 3.21 3313
600 1st 4800 960 5.0 1.63 1682
600 1st 6310 1290 4.90 1.64 1692
1200 1st 7700 3930 1.96 0.80 826
1200 1st 9000 5097 1.77 0.78 805

Blue Channel Gratings:

Grating (lines/mm) Order Blaze Wavelength (Å) R (@ Blaze, 1" slit) Resolution (Å) Dispersion (Å) Coverage (Å)
300 1st 4800 740 6.47 1.96 5268
500 1st 5410 1430 3.79 1.19 3198
800 1st 4050 1730 2.34 0.75 2016
600 1st 9630 3330 2.9 1.0 2688
2nd 4800 3310 1.45 0.50 1344
832 1st 7790 3830 2.04 0.72 1935
2nd 3900 3830 1.02 0.36 968
1200 1st 4830 3340 1.45 0.50 1344