Guiding with near infrared (NIR)

The ONAG uses a dichroic beam splitter (BS). The near infrared (NIR) light goes through the BS and can be used by your guider.
Guiding in NIR is a common practice among professional astronomers.

Most stars radiate a lot of energy in NIR.  The starlight spectrum (power density versus wavelength) is given by the black body radiation theory. In short the spectrum of a black body, like a star, can be totally defined from its temperature (at equilibrium).

More than 75% of the main sequence stars have surface temperatures lower than 3700°K (class M). Click here to lunch a nice Java applet on black body radiation (credit university of north-Iowa).
The applet allows to plot the black body power density spectrum for a given temperature, try 3700°K to see how a class M star spectrum looks like.

Below the main sequence chart and the stellar classification table:

   

 

Now to understand how NIR works for guiding we need to take in account the guider sensor quantum efficiency (QE), and ONAG's optical transfer function as well.

Below a plot from 400nm to 1000nm (X axis) of the classical B&W ICX429AL Exview Sony chip QE used in many guider cameras, such as the Loadstar from Starlight Xpress, and a class M star spectrum to illustrate the concept.


      

In blue the chip quantum efficiency (1=100%) and, in red the class M star spectrum, the green dotted line is the ONAG cut-off transition (750nm).

The overall system efficiency*, namely the ONAG associated with the ICX429AL chip, can be found by the integration of the product between the chip QE and star spectrum for a given surface temperature, over the ONAG bandwidth (750nm to 1000nm in this example),

The next plot shows the ONAG+ICX429AL efficiency in % relative to the same chip (ICX429AL) using the full spectrum (no ONAG, integration bandwidth from 400nm to 1000nm in this example) versus the star surface temperature (X axis in °K).

           

 
 
To place this in perspective with the star absolute magnitudes and luminosities the plot below shows the Hertzsprung-Russell diagram with the ONAG+ICX429AL chip system efficiency (left orange axis) and the star main sequence relative occurrence in % (right white axis):
 
 
      
 

One can see that at 3700°K (stellar class M), the ONAG reduces the signal level in about a half (versus using the all spectrum, visible+NIR).

However this should be put in perceptive with the ONAG field of view (fov), versus an OAG for instance, and the fact that the ONAG shares the scope F-number (filter wheel does not limit the ONAG either).

OAG are limited to an off-axis fov using a pick up prism leading most of the time to lower F-numbers.

The sketch below shows a typical OAG and ONAG fov (in yellow) for a large APS-C ship (such as a KAF6303 ship uses in the SBIG STL6303) and a 2" scope aperture. On the left an OAG, on the right the ONAG:

 

             

With the ONAG you have access to the all scope fov and therefore you are much more likely to find a suitable guide star. For SCT scopes using an on-axis guide star is interesting because the typically edge effect of those scopes may lead to large off-axis optical aberration (coma).

The last plot is the percentage of main sequence stars with surface temperature below or equal to a given value (X axis, in °K). That means there are many stars to choose for guiding in NIR with the ONAG technology. 


     

* Atmospheric extinction is neglected here, since it is not a major concern for this range of wavelengths.