The new ONAG® XT

Introducing the new full frame ONAG® XT (up to 50mm in diagonal).
Most anticipated larger version of the ONAG®.

The ONAG® XT features:

- Rigid 59mm dovetail system for scope and image ports
- Tilt/tip user adjustable dichroic mirror (laser aligned at factory)
- Integrated corrective optics for the guider port providing seeing limited guide stars
- 68mm imager back focus (compare with 66mm for the ONAG®)
- Compatible with AO units, including the AO-L from SBIG
- Same X/Y stage as the ONAG® with a T-thread M42 x 0.75mm connection as well as 1.25" nosepiece compatible


"I  feel this is the ultimate design in guide star tracking and imaging. I am a firm believer in the system and plan on solely using this type of tracking from now on. My only problem now is... I need to re-image the entire sky to upgrade my stock of sky images!"

Owner of a f/6  - 32" relay telescope

Dr. Mario Motta

Read more at our user feedback page.

 Mario Motta's f/6 32" scope   

Dr. M. Motta f/6 32" relay telescope

ONAG XT on Mario Motta's scope + AO-L

ONAG® XT on f/6 - 32" scope with a SBIG AO-L unit

ONAG® at work:

Guiding with a long focal

The images below are 1 minute (bin 1x1) unprocessed luminance frames taken with an Apogee U8300 (5.4x5.4 microns) at prime focus of a Hyperion (f =2.54m @ f/8).
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
Seeing: Average

The reference star is marked with a cross (not the guide star).


Initial image (t=0'), reference star:
FWHM=2.44 arcsec
Centroid X=80.71 Y=171.83 pixel


Star 3D profile (Maxim DL



2nd image (t=38'), reference star:
FWHM=2.36 arcsec
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:
FWHM=2.43 arcsec
Centroid X=81.08 Y=171.54 pixel

Star 3D profile (Maxim DL

The result shows no visible guiding effect.

To know more about guiding error
and near infrared (NIR) visit:

How much guiding error is too much?

Guiding with NIR

"Although I generally use multiple stars are registration points when I stack my deep-sky frames, I could often dispense with this step when stacking images made with the ONAG®."

Dennis di Cicco, Sky & Telescope December 2012, pages 60-63


Heavy duty focuser:

Full body compressing ring

"The ONAG® has a very rigid connection, and it's especially noteworthy because the device has an adjustable X-Y mounting for the guide camera , which helps in the search for suitable guide stars."

Dennis di Cicco, Sky & Telescope December 2012, pages 60-63

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.

You want to see more? Just download our 3D ONAG® eDrawings®







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.
NIR guiding reduces seeing effects for tracking (see below for further information)

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:

Main sequence    Star classes


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 of the QE, from 400nm to 1000nm (X axis), for a classical B&W Exview high efficiency Sony chip and a class M star spectrum to illustrate the concept.
Those chips are used in the security camera market with high NIR QE for night vision. They are also commonly found in guider cameras, such as the Moravian G0 and G1 series recommended for the ONAG®.

       ICX429AL chip and class M star

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).

           Efficiency ratio equation

NIR ONAG efficiency

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):
       Hertzsprung Russell Diagram

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 aperture D (filter wheel does not limit the ONAG® either).
Stars are point sources and unlike extended objects, such as galaxies or nebulae, the amount of energy received (signal) is only a function of the square of the scope aperture D, not the focal length nor the F number (scope "speed"). The start apparent size on the sensor is only related to the seeing, or at best the diffraction limited optics.
This means that a 8" scope will gather [(8*25)/80]^2 = 6.25x more signal than a typical 80mm guide scope. This is a gain of 2 magnitudes, and a 11" scope will results to almost 3 magnitudes.
This more than offsets the efficiency reduction from using NIR for guiding.

OAG are limited to an off-axis fov using a pick up prism which may lead in starlight energy loss.

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®:


              OAG versus ONAG FOV

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. 

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 (in 2D) as the two fifth power of the wavelength, this is a well know result, for more information and basic equations see:

"Optical Resolution Through a Randomly Inhomogeneous Medium for Very Long and Very Short Exposure", by D.L. Fried, Journal of the Optical Society of America, Volume 56, Number 10, 1966.

Let's use 500nm for the average visible wavelength (classical guiding) and 850nm for the NIR one (ONAG guiding), this leads to an improvement of:

                                                         [1 -  (850/500)2/5 ] 100 = 23%

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 on 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.

M83 guiding in visible

  M83 using the visible range (350nm to 660nm) and an OAG for guiding (f/6 - 32")


M83 guiding with ONAG XT

   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.



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