Frequently Asked Questions



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


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.

GF compressing ring alone

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.

GF compressing ring and drawtube





  1. Frequently Asked Questions

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    1. Why could I see through the ONAG?

    2. From the ONAG imager port the reflected light seems colorized, is this normal?

    3. What is the best procedure to focus a guide star?

    4. How should I clean the ONAG dichroic beam splitter (mirror)?

    5. How to minimize flexure?

    6. I see dim "donut" shape like ghost images near bright stars. what is this and what to do to eliminate it?

    7. Can I use a focal reducer with my ONAG?

    8. I have an adaptive optic module. Will it work with my ONAG?

    9. How could I guide using near infrared (NIR)?

    10. I run my equipment in automation what should I consider when using the ONAG?

    11. How much guiding error is too much?

    12. How to use a light pollution reduction filter with the ONAG?



    Why could I see through the ONAG?

    Answer #1


    Although the dichroic beam splitter reflects 95% or more of the visible light, the 5% left allows to see through when looking at a bright source, like day light.
    This is because the human eye is a very sensitive receiver.

    From the ONAG imager port the reflected light seems colorized, is this normal?

    In astronomical applications light beams have a very little divergence, 1 to 2 degrees at most. Under those conditions the DBS acts like any normal mirror would.

    However when you look at the ONAG alone, "naked eye", on the open you are very far from this nominal condition. Diffused illuminations, such as day light scenes, and absence of any optical element, will result in a wide reflection angles leading to noticeable color variations.

    What is the best procedure to focus a guide star?

    Answer #3


    The ONAG guider port has a built-in focuser, with an effective travel of 9mm (about 1/3"). Its allows you to fine focus the guide star without disturbing your imager focus point, or involving your scope focuser mechanism.

    First be sure you have selected the right extension tube combination for your set-up, by considering both imager and guider back-focus (including any other element, such as a filter wheel). Please refer to the ONAG user manual.


    When you move the ONAG guider focuser drawtube all the way, the guide star should change form from a vertical ellipsoidal shape to a horizontal ellipsoidal shape, or the opposite in function of your CCD reference frame position. The optimal focus point is achieved when both ellipsoid collapse becoming a spot, or a little cross. This is normal and not a source of concern. This feature becomes handy when manually seeking for best focus.
    Since most scopes and optical components are not optimized for the near infrared (NIR) there is maybe small distortion involved.
    Autoguider algorithms are mainly based on centroid algorithms and are not sensitive to this. They average pixels from the all guide star area, so the maximum pixel value or FWHM are not much relevant in this case, unlike for imaging.
    If you use computer assisted focusing software, such as Maxim DL, the right figure of merit should be the half flux diameter (HFD), or ½ FD.
    The half-flux diameter is the diameter in pixels that contains half the energy in a star image. In other words, if you add up the pixel values (less the background) inside the diameter, and outside the diameter, you will get the same number. This measurement gives a very similar answer to FWHM, but it is much more robust in the presence of seeing, noise, and can handle non circular distorted images, even out-of-focus like "donuts". The HFD varies linearly with focus position making it reliable to locate the best focus regardless the star shape.
    If you use the PHD guiding software watch the SNR value, you should seek for its maximum. If you do not use any software, the best focus will be achieved when the guide star cross like shape is minimized and symmetric.

    The two images below show the same guide star seen from the imager (IP) on the left or from the guider (GP) on the right at best focus, same camera, set-up, and cropping.

    Guide star seen from IP                              Guide star seen from GP

                     Guide star seen from IP             Guide star seen from GP
                  HFD=6.0px, FWHM=3.4px         HFD=6.2px, FWHM=5.2px

    The cross like shape of the guide star viewed from the GP is clearly visible. Yet as far as the energy budget is concerned both cases have almost the same HFD. The GP larger FWHM is due to the star non-circular shape.

    When you reached the desired focus, hand tighten  the ONAG focuser screw (stainless steel) to avoid any motion or flexure.

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    How should I clean the ONAG dichroic beam splitter (mirror)?


    Answer #4


    The ONAG DBS is a high quality multi-coating optical element and must be handle with care. Should you need to clean it, first remove any dust, or other large particles, using either optical grade compressed air, or an optical grade brush.
    In any case do it very gently. As a general rule, touching the DBS surface as little as possible. If you have to do it use an optical multi-coating cleaning solution (available from many sources), never apply it directly to the surface, use a lens tissue.
    We do recommend to wear disposable latex glove for the task. Always apply the minimum amount of pressure and force when cleaning the DBS surface.

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    How to minimize flexure?


    Answer #5


    The ONAG is designed to avoid flexure, using the same scope and optical train than your imager.
    Be sure that the guider focuser screw and the eight (2 x 4) X/Y stage nylon screws are hand tightened before starting any astrophotography session.

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    I see dim "donut" shape like ghost images near bright stars. what is this and what to do to eliminate it?


    Answer #6


    The ONAG dichroic beam splitter (DBS) is fully multi-coated, including a wide range anti-reflection (AR) coating on its back. However If you well over expose stars, a very small fraction of the near infrared (NIR) light reflected back could show up as very dim "donut" like shape few millimeters away from their actual location on your imager CCD.

    This is no different than other stray reflections you may experience in any optical systems, even with proper AR coating, if you work with light levels way in excess to your sensor saturation (clipping) point.

    Chance is you may have a CCD blooming problem before you would ever see such reflections.

    Should you experience the problem anyway, be sure your imager has a NIR blocking filter.
    Most , if not all, one shop color cameras and DSLR do. If you work with a filter wheel and a monochrome camera, this should be taken care by your LRGB filter set.
    However be aware there are out there RGB filters without NIR blocking feature, and even some luminance filters do not cut the NIR too. Do not confuse clear filters with luminance filters, a common mistake.
    In such case you may have to add a UV-NIR blocking filter, or use a LRGB filter set cutting UV-NIR (should cut off below 370nm, and above 700nm).
    Also it is recommended to avoid clipping any star as a general good practice rule. Just doing this should avoid any ghost images.

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    Can I use a focal reducer with my ONAG?


    Answer #7


    Yes you can.

    Focal reducers (FR) should be at, or near, its specific distance DFR to the imager focal plane to fulfill their specifications.
    However popular 0.63x reducer-correctors, such as the Meade/Celestron FR, will work fine in a range of typically +/- one inch (25.4mm) of their DFR. In such conditions the FR optical correction remains essentially untouched.
    Yet deviation from the DFR will change the FR reduction factor h.

    In first approximation h is given by: 

    h = 1-q/f

    Where q is the actual distance between the FR and the imager focal plane, and f the FR's focal length.

    We also have:


    For instance:

    Celestron 0.63x focal length = 235mm
    DFR = 235*(1-0.63)=87mm

    If we place the reducer one inch further to the imager focal plane this leads to:

    q = 87+25.4 =112.4mm and h = 1-112.4/235=0.52x

    A 17% lower reduction factor h, meaning more reduction.

    There are two options for placing the FR with the ONAG:

    The FR can be located on front of the ONAG at its scope port (SP). In this case you need to consider the 66mm (2.6") ONAG back focus in the calculation of q.

    Alternatively the FR can be inserted between the ONAG imager port (IP) and your imager (or filter wheel if any).
    In this configuration the ONAG back focus does not play any role in the calculation of the distance q.

    Yet be aware that the ONAG user manual differential back focus table will not provide the correct selection for the extension tube set up anymore.
    Most likely the guider camera should be moved further away from the ONAG guider port (GP).

    In order to avoid large guider extension paths IF offers a adjustable focal reducer (AFR) to be placed in front of your guider. This one has been optimized for near infrared (NIR) using aspheric optics. The AFR works with most FRs, including the Celestron, Meade 0.63x, 0.33x, Starizona SCT corrector, Optec, ... See the ONAG user manual for further information.

    The image below shows the ONAG with a C8 "orange" tube.
    A Meade 0.33x FR is placed before the imager SBIG ST4000XCM.
    Here the IF's adjustable NIR focal reducer (AFR) has been used with a Meade DSI camera for guiding.

    ONAG with Meade 0.33x focal reducer and AFR
              ONAG with a Meade 0.33x focal reducer and a AFR

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    I have an adaptive optic module. Will it work with my ONAG?


    Answer #8


    Yes it will.

    Stand alone adaptive optic (AO) products, such as the ORION SteadyStar (TM), or the SXV-AO from Startlight XPress can be placed anywhere between your scope optical back and the ONAG scope port.
    If you have a SBIG AO-8 adaptive optic module, IF provides an AO-8 adapter plate to interface it with your ONAG.

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    How could I guide using the near infrared (NIR)?


    Answer #9


    Unfiltered CCD/CMOS sensors are sensitive in the near infrared range (NIR), above 750nm. This is why for imaging you need IR blocking filters. Most one shot color cameras have built-in UV-IR filters, while monochrome cameras do not. They rely on external filters, such as LRGB, or narrow band filters.

    The amount of NIR energy received from a given star is a function of its temperature. The star's spectrum can be found from its surface temperature using the black body radiation theory.

    Let's define the light visible range from 350nm to 750nm and the NIR range from 750nm to 900nm.
    Spectral integration shows that there is 70% or more as much energy in the NIR range than in the visible one (as defined above).
    This energy can be used for NIR guiding.

    The stellar classes M, L, T are related to surface temperatures 3700K (red).
    Stars from those classes radiate a large amount of energy in infrared. A lower star surface temperature means more NIR energy.

    More than 76% of the main sequence stars belong to class M, which makes them the most common stars across the universe.
    Because this, and also since the ONAG integrated X/Y stage gives access to a large FOV, there is a very high probability to find a usable guide star using NIR with the ONAG.

    To learn more about NIR guiding click here.

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    I run my equipment in automation what should I consider when using the ONAG?


    Answer #10


    The ONAG can be used in fully automated remote observatory situations.
    If you use a rotator be sure you can freely rotate 360 degrees without bumping to anything, this is not different than with any other equipment, such as filter wheel and cameras.

    You also want to have a good cable management strategy. Avoid any guider cable traction during tracking and rotation. Beside you could lose a connection, of break something, excessive force may result in flexure of the guider assembly, such as extension tubes.

    For guiding and associated software set-ups you need to consider that, unlike for an off-axis guider (OAG), the ONAG does not have any mirror (reflection) involved in the guider light path.
    Therefore you should select the option "Self-Guiding", or an equivalent  one, when available.
    Failure to do so will most likely lead to erratic behavior and impossibility to guide.

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    How much guiding error is too much?


    Answer #11

    This is a very good and important question, and its answer is fundamental for good quality images.

    For further information on this topic please click here.


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    How to use a light pollution reduction filter with the ONAG?


    Answer #12

    Light pollution reduction filters selectively reject narrow bands associated with wavelengths from common urban skyglow, such as sodium vapor street lights.

    Skyglow imaging filter transmission plot

    (credit Orion Telescopes & Binocluars)

                     Light pollution reduction filter plot

    Most of them will also block UV and NIR lights (>700nm). Therefore such filter can not be placed in front of the ONAG (SP) otherwise the guider camera will not receive enough, if any, NIR light (>750nm).

    The filter should be located at the ONAG's imager port (IP) in front of the camera, usualy an one shot color camera, or DSLR.

    This can be achieved with a filter wheel (FW), such as the manual 2" ORION FW, which offers T-threaded connections.

    Below an image of the ONAG associated with a 2" ORION FW and a SBIG ST2000XCM one shot color camera. The FW is used for the light pollution reduction filter, when not needed a clear (empty) position of the FW can be selected instead.

    ONAG + FW

    ONAG with a 2" ORION filter wheel for a skyglow imaging filter

    An alternate option is available for some DSLRs. There are clip style filters which can be placed inside the DLSR body, before the camera mirror. This avoids the need for a FW and minimize the back focus.

    For further information please visit:

     Astronomik clip filter

    There is yet another solution to place the filter between the ONAG's IP and the imager camera without any filter wheel.

    This approach will reduce the extra back focus needed for the filter to a minimum.

    Our partner PreciseParts provides ONAG custom adapters for most scopes, imagers, accessories, and specific set-ups.

    You can order:


    A generic 2" (48mm) filter to your imager camera specific connection (such as a female T-thread) adapter.



    An IF ONAG imaging port to generic 2" (48mm) filter adapter.

    In both cases you would ask for a minimum adapter back focus of 5.6mm, or 0.22" to insure clearance for the filter.

    In this configuration the filter is placed in between the two adapters. This leads to a very compact and low profile solution.

    Please visit:

    ONAG adapter solutions or our product page under custom ONAG adapters.


    There are two possible choices for the filters (48mm).
    Either generic, which should be used for any 2" filter with a thread M48 x 0.75mm  (excepted Celestron, Meade, and Orion brands).
    Otherwise select Meade/Celestron/Orion 48mm for those suppliers, or for any 2" filter with a M48 x 0.6mm thread.

    For further information contact us

    +1-215-885-3330, FAX: +1-215-884-0418, or email

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