ONAG at work:
Guiding with a long focal
The images below are 1 minute (bin 1x1) unprocessed
luminance frames taken with an
(5.4x5.4 microns) at prime focus of a
Hyperion (f =2.54m @
Both have been cropped the same way.
Field of view: 224x224 arcsec near NGC 2683.
There are 38 minutes apart.
credit: Frank Colosimo
Blue Mtn Vista Observatory
Mount: Paramount ME
Guider: ONAG+SBIG ST402
The reference star is marked with a cross (not the guide star).
Initial image (t=0'), reference star:
Centroid X=80.71 Y=171.83 pixel
Star 3D profile (Maxim DL)
2nd image (t=38'), reference star:
Centroid X=81.38 Y=171.40 pixel
The reference star is offset by 0.79 pixels.
With the U8300 pixels (0.43x0.43 arcsec) this translates to 0.34 arcsec.
This is a total offset including all sources of error.
Below both images have been combined, without any registration and alignment, to provide an easy estimation and visualization of the total tracking performance.
Average image, reference star:
Centroid X=81.08 Y=171.54 pixel
Star 3D profile (Maxim DL)
The result shows no visible guiding effect.
Heavy duty focuser:
Full body compressing ring
The ONAG features a low profile 1.25" guider focuser (GF) associated with a
T-thread (M42 x0.75mm) and integrated with the ONAG's X/Y stage.
It has been designed to remove any possible flexure even with a heavy camera. The focuser uses a full length compressing ring mechanism made of high grade 6061 aluminum alloy. It applies a considerable pressure (on 360 degrees) all the long the focuser drawtube insuring a constant and efficient grip.
As a matter of fact when the focuser screw is hand tighten the all system is
as rigid as an unique solid piece of aluminum.
The next image shows the full body compression ring element and its screw.
Unlike thumb screws used with low cost systems, the stain less steel focuser screw compresses the all focuser body against the drawtube. The mechanism is lubricated with an extended temperature range anti-seize grease. It is designed with 2 groves and set screws to insure the drawtube will not leave the focuser by accident.
Should you want, or need, to use any 1.25" nosepiece just remove the set screws and replace the focuser drawtube by your piece of equipment. A very handy solution if your guider camera nosepiece does not come off.
Below the compressing ring with the focuser drawtube in place.
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.
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
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
NIR guiding reduces seeing effects
The seeing will eventually limit the guiding quality. However NIR offers a unique opportunity to improve the tracking accuracy. Longer wavelengths are less sensitive to seeing and its effect decreases as the six fifth power of the wavelength.
Let’s use 550nm for the average visible wavelength (classical guiding) and 850nm for the NIR one (ONAG guiding), this leads to an improvement of:
[1 – (850/500)^(6/5) ] 100 = 68.6%
The back and white pictures below of M83 were taken with Dr. M. Motta f/6 – 32″ relay telescope at 14 degrees above the horizon to test poor seeing effect while guiding in visible or NIR (>750nm) only. The comparison shows significant improvement on FWHM and overall image quality using the NIR guiding strategy with an ONAG XT, since the tracking was much smoother in NIR. This is an extreme situation, imaging that low above the horizon is obviously not recommended, but here the goal was to test NIR auto-guiding improvement using NIR, in a context of poor seeing.
M83 using the visible range (350nm to 660nm) and an OAG for guiding (f/6 – 32″)
M83 using NIR (>750nm) with ONAG® XT for guiding (f/6 – 32″)
* Atmospheric extinction is neglected here, since it is not a major concern for this range of wavelengths.