Articles & FAQ

Articles & FAQ


MonsterMOAG Thin Off-Axis Guider

The MonsterMOAG™ is the latest innovation from Astrodon in off-axis guiding (OAG) equipment. We listened to your comments regarding our ROAG and MOAG OAGs and developed the MonsterMOAG. 

There is only one MonsterMOAG that has been tested on telescopes/ CCD camera systems under full weight load.   Our MonsterMOAG comes with Astrodon quality and support, and is not a knockoff with unstable mechanical parts. 

Please see our system chart below for our full line of Astrodon quality MonsterMOAG adaptors.

  • Smallest backfocus 1.25"
  • Solid body for no flexure
  • Large central opening of 2.55"
  • Use with  large CCD cameras (KAF16803, KAF9000)
  • Helical focuser for easy focus
  • Interchangeable short 1.25" nose piece holder included if helical focuser is too tall to come to focus
  • Spacers to place pick-off optic further into the light beam
  • Dovetail connectors - rotate and lock  - to align guide camera with imaging camera and telescope
  • M/F AP 2.7"-24TPI dovetail connectors provided
  • 3" male connector optional for direct connection to new, large filter wheels.
  • All parts CNC machined with low reflectivity inner surfaces

Off-axis guiding (OAG) has several advantages:

  1. Uses the same light beam for guiding as seen by the imaging detector
  2. Avoids potential differential flexure in using a separate guide scope
  3. Guides your system with unfiltered light in FRONT of your filters
  4. Use a variety of guide cameras with different detectors, resolutions and sizes

As a result your guiding becomes easier:

  1. Find more guide stars
  2. Do not worry about different guide exposure times among different filters
  3. Allow you to follow any mirror wobble in SCTs compared to a separate guide scope
  4. Eliminates a separate guide scope and its mounting/flexure issues

The image  below on the left is a side view on the MonsterMOAG with its helical focuser , showing the male and female dovetail connectors in place and the width of 1.25". The tapped holes in the side accept strong 1/4-20 bolts that can secure steel lanyards to your camera and prevent it from falling to the ground.

The image on the right shows the MonsterMOAG on the new Takahashi FSQ-ED with an SBIG STL11000XM camera and an SBIG STL-remote guider atop the MonsterMOAG. 



Most importantly, the MonsterMOAG was designed for the new, large imaging detectors (Kodak KAF16803 and KAF9000) with their 36 x 36 mm square size and a 52 mm diagonal.  The image to the right shows an Astrodon 50mm square blue filter and a scale drawing of these detectors. This image shows that there is sufficient room even with faster optics to use the MonsterMOAG with common AP 2.7" threaded connectors and extension tubes without significant vignetting the imaging detector.



For those with smaller detectors, such as the 24 x 36mm KAI11000 interline CCD, the pick-off optic can be placed further into the light cone with extra spacers supplied with the MonsterMOAG, as shown left.






It is important to realize that the placement of the MonsterMOAG, or any OAG, in the optical train must be carefully considered in advance.  The distance between the pick-off optic and the focal plane of the guding camera (A) in the above schematic must be equal to the distance between the pick-off optic and the focal plane of the imaging camera (B).  Place the MonsterMOAG too close to the imaging camera and the focal point may move from above the focuser down into the body of the focuser.  In this case we can offer several solutions.  The helical focuser can be removed (unscrewed) and replaced with a shorter part shown above (provided).  This brings the guiding camera up to about  0.9" closer to the pick-off optic.  Focus adjustment is manual in this case, but it may be the only option.

Another option is what we use for the MOAG-A sandwiched between the SBIG AO-L and STL camera. The MOAG-A is so close to the STL's focal plane that focus occurs just inside the top of the helical focuser.  Thus, we supplied a slow plano-concave lens (-250 mm focal length)  with the MOAG to act like a Barlow lens to bring the focal plane above the focuser. This is placed in the top of the body of the MOAG-A.  The same lens (optional) can be placed in a shallow counterbore on the top of the MonsterMOAG at the bottom of the helical focuser.

Lastly, an easy solution is to put a spacer, such as a 0.75" AP (Astro-Physics) extension tube (or longer) between the MonsterMOAG and the CFW/CCD. For this reason, you will need to know the backfocus distance of your CFW/CCD system and plan in advance to purchase other parts if needed.

The standard connectors shown here are included with the MonsterMOAG.  The male 2.7" connector is placed on the telescope side.  The other part faces the camera.  It accepts 2.7" male threads on the ID but has 3"-24 threads on the outer diameter.  You can see the dovetail that fits into the MonsterMOAG body in these parts.  In this case, you must supply a connector from your CFW supplier with male AP 2.7" threads.

The four 8-32 set screws on each raised flange coming out of the MonsterMOAG press into the dovetail and pull the connector flat into the body of the MonsterMOAG to eliminate and tilting of your camera system.  This also allows you to rotate the part screwed into the connector relative to the MonsterMOAG body and then securely lock it in place.

The new, large filter wheels housing Astrodon 50mm square filters for the KAF16803 and KAF9000 CCDs all have 3"-24 threaded openings on the telescope side, including:

  1. Apogee Instruments  AI-FW50-7s 
  2. Yankee Robotics Cyclops
  3. Finger Lakes Instruments CFW-4-5 or - 7

Of course, the large opening of the MonsterMOAG is also useful for the KAF6303 and KAI11000, as found in the SBIG STL6303 and STL11000

Astrodon optionally supplies a dovetail to 3" male connnector that is 0.75" long for direct connection between the MonsterMOAG and the above CFWs, which have 3"-24 female threads on the telescope side. 

This helps to equalize the distance between the prism and the the guider and imaging cameras.  The negative lens can be added to the MonsterMOAG if focus needs to be moved up further.

These new, large CFWs have 3"-24 female threads facing the telescope. There may be instances where longer distances between the MonsterMOAG and CFW are required to come to focus at the guide camera.  As a result, Astrodon optionally offers a M/M connector that screws into these CFWs and has male AP 2.7"-24 threads on the telescope side.  As a result, you can place any of the popular Astro-Physics extension tubes between the CFW and the MonsterMOAG.

An optional adaptor is offered to connect the MonsterMOAG to the 3.25" visual back of a Meade or Celestron SCT telescope.  It accepts the male 2.7"-24 adaptor that comes with the MonsterMOAG allowing you to off-axis guide these SCTs without the problems of differential flexture of a separate guide scope.

Lastly, 4"-24 extension tubes are now becoming popular on some larger telescopes, Astrodon provides a MonsterMOAG dovetail-to-APmale MonsterMOAG adaptor that takes up 3/4" of backfocus.

An adaptor chart showing the various options is presented below:

Astrodon understands that there are hundreds of possible system configurations and connections.  Our goal is to provide the most common ones.  For different configurations, we recommend that you contact PreciseParts for manufacturing of unique connectors with a rapid turnaround.  PreciseParts knows the MonsterMOAG connector threads and dimensions.