Astrodon MegaMOAG 3″ to 4″ Off Axis Guider



MOAG means Manual Off-Axis Guider. It allows you to guide your telescope to obtain sharp, round stars by guiding in front of your filters using unfiltered light in the same optical beam as your imaging camera.

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MegaMOAG  3″ to 4″ Off-Axis Guider

The MegaMOAG takes the design of the MonsterMOAG and adapts it for telescope systems with larger optical openings.  It comes with the same camera-side adapter as the MonsterMOAG but it has a 4″ opening on the scope side and comes with a 4 1/16″ – 24 male adapter to connect to larger extension tubes found on the back of many larger telescopes.  The body is larger and the pick-off prism is further from center.  It has the same 1.25″ body thickness as the MonsterMOAG.  It comes with a single guide port.   As with the MonsterMOAG, a black metal plate will cover the unused port for the MeMOAG.  A second port assembly can be added at a later time with the MonsterMOAG Extra Port Assembly (MMOAG-PORT) to make a two port MeMOAG as shown in the picture.


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What’s in the box?

MegaMOAG Off Axis Guider

MMOAG Optical Configuration

You will need to balance the following distances for your optical system. These constraints are interconnected. PLEASE attempt to make these calculations prior to contacting Astrodon or any of our resellers. Of course we are available to help you finalize your choices, but making the calculations in advance will expedite the process and increase your confidence that the overall system will work for you.

Balancing Distances
Focal plane distance of the imaging CCD to the back of your scope, such as from field correctors or reducers. This can be adjusted with scope-side adapters or extension tubes.
Distance from the MMOAG prism to the focal plane of the imaging CCD (distance B below)
Distance from the center of the prism to the focal plane of the guider CCD (distance A below)
A must equal B to reach focus for the guide camera


Select MMOAG camera side adapter (backfocus/thickness = Ad)
Determine B = 0.625″ + CBF + Ad
Match A to B by using the Helical Focuser, as supplied with the MMOAG or replacing it with the provided shorter 1.25″ manual nose cup
For the Helical Focuser, A = 3″ + GBF, or
For the short 1.25″ cup, A = 2″ + GBF

Camera-Side Adapters from Astrodon to Determine Ad

SBIG STL (dovetail to 2.156″ male)

MMOAG-STL-S (Ad = 0.1″), MMOAG-STL-L (0.75″)
SBIG ST (dovetail to t-threads – 42 mm male)

MMOAG-ST-S (0.1″), MMOAG-ST-L (0.75″)
SBIG STX, Apogee U16M/9000, FLI Proline (dovetail to 3″ male)

MMOAG-CFW (included, 0.1″), MMOAG-CFW-A (0.2″), MMOAG-3-L (0.75″)

Some Camera + Filter Wheel Backfocus CBF (please verify with your vendor)
SBIG STL/FW8 1.95″
SBIG ST/FW 1.65″
SBIG STXL/CFW with Guider 2.44″
STT/FWG8 2.16″
Apogee U16M/FW-7S/9R 2.15″
Apogee U16M/FW-10S 2.25″
Apogee U16M/FW-7S/9R (deep cooling) 2.53″
Apogee U16M/FW-10S (deep cooling) 2.63″
FLI Proline/CFW 1.73″
FLI Proline/CenterlineFW 1.98″

Some Guide Camera Backfocus GBF
SBIG RGH, ST-402 0.69″
SBIG ST-i -0.5″ (guider inserted into helical focuser)*
Starlight XPress Lodestar -0.5″ (guider inserted into helical focuser)*

* This means if the ST-i or Lodestar is inserted down into the helical focuser by 0.5, the focal plane will be at the top of the helical focuser. So, decrease 3″ by the insertion depth of 0.5″ = 2.5″ and then ADD back the 0.5″ recess of the detector 2.5″ + 0.5″ = 3″, for example.

An Example Calculation – Apogee U16M/7-PositionFW and SBIG ST-402 Guider

We will use the MMOAG-CFW-A (0.2″). CBF for the Apogee system with the 10-slot FW is 2.25″. Therefore, B becomes 0.625″ + 0.2″ + 2.25″ = 3.075″.

With the helical focuser (3″) and ST-402, A becomes 3″ + 0.7″ = 3.7″. Therefore the guider focal plane will be 3.7 – 3.075″ = 0.675″ above the focus determined from B.

So, we can either ADD 0.75″ between the MMOAG and U16M with the MMOAG-3-L. Or, if we want to keep the distance between the MMOAG and imaging focal plane as short as possible (my choice), we can replace the helical focuser with the included short, manual nose cup. In this case, A becomes 2″ + 0.7″ = 2.7″. All you need to do is slide the ST-402 up 0.3″ using its 1.25″ nose piece and it will be close to focus. You can fine tune the focus with star images. This process takes only a few minutes and once locked down, the focus does not change. It is a little less convenient than the helical focuser, but provides this important option for configuring your system.

Please note that camera manufactures often specify the MECHANICAL back focus of their products. 3 mm thick filters will ADD 1 mm (0.04″) to that mechanical backfocus as measured from the telescope or from the MonsterMOAG prism. The camera window, often 3 mm thick will add another 1 mm (0.04″). So, You may have to add 2 mm (0.09″) to the mechanical backfocus.
There may be some confusion as camera manufacturers measure backfocus from the focal plane of the CCD to the outer surface of the camera. When they account for the thickness of the filters, the SUBTRACT the 0.04″, which is correct as measured from the CCD. However, most people measure backfocus from the back of their scope of from a corrector, and then add/subtract spacers to arrive at the correct backfocus. In this case, as measured from the scope, the 0.04″ must be ADDED. A subtle point, but does get people in trouble from time to time.
Also, please note that different filter wheels from the same manufacturer may have different backfocus, as indicated in the list above.

SBIG ST-i or SX Lodestar

These guiders are 1.25″ cylinders with the focal plane recessed about 0.5″ up from the end. So for the example considered above where B = 3.075″, you can use the helical focuser. So, by inserting either guider ~0.5″ down into the top of the helical focuser brings the focal plane of the guider to the TOP of the helical focuser. A = 3″ – 0.5″ insertion distance + 0.5″ internal backfocus = 3″. The +/-0.2″ travel of the helical focuser allows you to reach optimum focus.

MMOAG Scope-Side Connections

The MonsterMOAG comes equipped with a dovetail to 2.7″-24 males threads. This works for most RC (e.g., RCOS) and Astro-Physics scopes with 2.7″ extension tubes, and adapters that go from 2.7″ to larger (3.5″ and 4″) systems. This is where you make adjustments to bring the focal plane of the imaging camera at the specified distance from your scope, focal reducers or field correctors.

There are dozens of different connection possibilities at the back of your scope, such as reduces, field correctors, focusers, rotators, etc. We cannot make adapters for all possibilities, so we just provide the male 2.7″ adapter described above. In many cases you will need a custom adapter. We suggest you utilize They provide a “configurator” where you specify the threads on both sides of the part and the thickness of the part. It will provide a drawing and a quotation immediately. It has the MonsterMOAG dovetail specifications, so on one side you will specify the Astrodon MonsterMOAG.

For Takahashi FSQ and TOA owners, you can add the CAA-APF (0.4″) adapter along with a Spacer27. This converts the back of the 100 mm camera-angle-rotator (CAA) from 92 mm to 2.7″-24 female threads. The MonsterMOAG then screws into this. You will also need our Spacer27, a 0.2″ thick 2.7″ washer/spacer.

For those with Meade or Celestron SCTs with 3.25″ visual backs, you can purchase a 3.25″ female to 2.7″ female adapter. This screws onto the visual back of your SCT and accepts the 2.7″ male threads from the MonsterMOAG. In this way you can enjoy the benefits of off-axis guiding in the main beam without worrying about differential flexure with guide scopes or possible primary mirror movement as the scope tracks across the sky.