Optec - Sagitta Off-Axis Guider

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Optec engineers designed the Sagitta OAG to provide a solution for remote imagers who require the ability to adjust guide camera focus independently of the imaging camera focus.

Optec’s Sagitta OAG can be purchased with the guider motor attached (#19340) or without the motor for manual guider focus (#19342). The motorized version requires the FocusLynx control hub and is an ideal use of the Focuser 2 port for owners of FocusLynx or FocusBoss II controllers.

SKU: OPT-19340
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Product Description

Optec Sagitta Off-Axis Guider

Optec engineers designed the Sagitta OAG to provide a solution for remote imagers who require the ability to adjust guide camera focus independently of the imaging camera focus.

Optec’s Sagitta OAG can be purchased with the guider motor attached (#19340) or without the motor for manual guider focus (#19342). The motorized version requires the FocusLynx control hub and is an ideal use of the Focuser 2 port for owners of FocusLynx or FocusBoss II controllers.

Remote Focus for Guide Camera

Featuring a full 3-inch (76.2mm) clear aperture, Sagitta includes a high-quality fully multi-coated pick-off prism with 12.5mm clear aperture to the guide camera. Sagitta features a large brass split-clamp for attachment of any 1-1/4” format guide camera with plenty of range for setting the guide camera parfocal to the imaging camera. Adjusting the prism stalk height can be accomplished with a single tool and provides prism engagement adjustment of 6.4mm. Overall, guide camera adjustments exceed 36mm while the OAG body thickness is held at only 31.75mm or 1-1/4”.

Designed as a robust alternative for the OAG market, Sagitta can be installed and configured for years of unattended use. A new, shorter 3-inch dovetail standard, the OPTEC-3000-S, rounds out the package with a variety of mating adapters. Any required adapter not available from Optec’s expanding inventory can be ordered through Precise Parts (www.preciseparts.com) to complete the camera side or telescope side connections to your imaging gear.

Package Contents

Confirm the contents of your Sagitta OAG off-axis guider package. The package should contain the following:

Sagitta OAG assembly with attached motor (not included with manual version),

  • Cat-5e or Cat-6 Ethernet cable,
  • Male Astro-Physics 2.7-inch threaded adapter,
  • Female Astro-Physics 2.7-inch (AP2.7) threaded adapter,
  • Tool kit including 3/32” Ball driver and 5/64” T-handle tool

The included Cat-5e or Cat-6 Ethernet cable is intended to be attached between the FocusLynx hub Focuser 1 or Focuser 2 port and the Sagitta motor. This cable can be replaced with any suitable length Cat-5e or Cat-6 cable.

IMPORTANT NOTE: Optec’s FocusLynx hub and driver software are required for motorized control of the Sagitta OAG focuser.



The overall thickness of the Sagitta OAG is 1.25-inches with adapters. This is the same thickness as most guide cameras on the market today. Therefore, the camera holder is slightly wider at 1.4” or 35.5mm. Both telescope-side and camera-side interfaces are 3-inch bores with three oval-point stainless screws to secure our new line of OPTEC-3000-S short 3-inch dovetail adapters. Each adapter will typically add 1/10” (2.5mm) for an overall working optical thickness of 1.45” or 36.8mm. The outside diameter of the main body is 3.8” ( 96.5mm). With a focal travel distance for the guide camera of 0.5-inch, the overall length to the top of the guide clamp varies from 150mm to 172.7mm.

Sagitta’s focus motor is neatly nestled along one side and engages directly with the manual focus knob which doubles as an 80-tooth gear when the motor is attached.

Sagitta Focus Motor

The Sagitta focus motor includes a high torque bi-polar NPM stepper motor in a 25mm diameter body and 2:25 gearhead. The integrated circuit board includes a temperature sensor similar to the QuickSync and DirectSync motors offered by Optec. The motor can be detached for manual guider focus by removing four 4-40 socket cap screws using the 3/32” ball driver tool.

Like the TCF series focusers, Sagitta’s motor is a true absolute homing focuser. As such, it will home upon initial power up and return to the last parked position. Home-On-Start can optionally be disabled in a future release of FocusLynx Commander. Sagitta’s focuser mechanism is very robust and will maintain absolute position between power cycles.

FocusLynx Controller

Optec’s FocusLynx controller (stock #19690) running firmware version V2.3.2 or later is required for operation of the Sagitta motor.

The FocusLynx standard package includes our 12VDC power supply with wall cord, flat Cat-5e or Cat-6 Ethernet cable (interchangeable with Sagitta’s motor cable), and Optec’s USB/Serial communications cable.

A second stepper board can also be ordered separately (stock #19533) for owners of the FocusLynx/FocusBoss II hub.

FocusLynx is fully compatible with all ASCOM and ASCOM Alpaca clients as well as TheSky X and INDI.


A little advance planning will make installing the Sagitta OAG much easier in the field. We want to set the guide camera and the main imaging camera at the same telescope focal point. The Sagitta OAG motor has a full 12.7mm of travel but we still need the two focal points to be close. In practice, we’ll want the guide camera focus and imaging camera focus to be within at least 10mm of each other initially.

Optical Path Length

Our goal is to set the auto-guider and imaging sensor Optical Path Length to the same distance. We call this a parfocal adjustment or simply “parfocalizing” the system. In practice, it is far easier to parfocalize your system than the long explanation below suggests.

The Optical Path Length or OPL is the optical distance that light must travel to reach the camera sensors. The OPL is typically the mechanical distance from a common reference surface plus approximately 1/3rd the thickness of any intervening glass such as filters or CCD cover glass. We need to add this extra optical distance due to the refraction of light. A full explanation is beyond the scope of this document, but for simplicity sake, we can add 1/3rd the thickness of any optical filters to the OPL when estimating back-focus distances. Most astronomical filters are approximately 3mm thick and are only placed behind the OAG so we can add 1mm to the mechanical distance to the imaging sensor. Thus, a camera with a mechanical flange back-focus distance of 17mm plus a 20mm thick filter wheel attached directly to the camera face will have an OPL of 38mm (17mm + 20mm + 1mm for the filters). Ideally, we want our auto-guider OPL to match this distance.

To clarify and simplify, we should choose a common point-of-reference for both auto-guider and imaging sensor. For the Sagitta OAG, we can use the front (telescope-side) of the OAG body as our reference surface. The diagram above shows the reference surface.

OAG Adjustments

The diagram below shows the available adjustments that can be made with the Sagitta OAG. For example, the pick-off prism stalk can be adjusted by at least 6.4mm (see the Set Prism Depth section below). The diagram indicates a range of 13.7mm to 20.0mm as measured from the center of the telescope’s optical axis. The telescope optical axis corresponds to the center of the imaging sensor.

Additionally, the Sagitta focus mechanism provides another 12.7mm of adjustment. Measured from the center of the pick-off prism, this distance ranges from 43.8mm to 56.5mm when the pick-off prism stalk is all the way up (20mm from the telescope optical axis). To calculate the OPL from the reference point, we will add 15.9mm (distance to the center of the pick-off prism) plus any extension of the pick-off stalk.

As an example, let’s assume the pick-off stalk is fully extended and the focus mechanism is at the midpoint of travel. Then our guide camera OPL equates to 15.9mm plus 6.4mm plus 50.1mm PLUS the sensor depth for the guide camera. The Lodestar cameras are typical with a 12.5mm depth to sensor. Thus, in this example, our guide camera OPL totals 84.9mm or around 85mm. We want to match the imaging camera OPL to this value plus or minus about 6.3mm (half the focuser travel).

Yet one more adjustment is available for the Lodestar type cameras that slip into the 1-1/4” receiver of the Sagitta OAG. These camera types are designed to fit directly into a 1-1/4” receiver and are often referred to as “lipstick” cameras because of their shape. Lipstick cameras have an additional 17.5mm of outward adjustment with the Sagitta’s unique clamping mechanism.

In total, the Sagitta has a wide range of adjustment from slightly less than 60mm to over 96mm.

Once the guide camera OPL range has been qualified we can mount the imaging camera and then bring the guide camera to match the imager’s OPL. In nearly all cases, the Sagitta focus mechanism will be more than adequate to bring the auto-guider perfectly parfocal with the imaging camera. Advance planning for extensions and adapters will ensure success.

Camera Adapters

Off-axis guiders are most effective when placed directly in front of the camera’s filter wheel. This allows unfiltered light to enter the prism and on to the guide camera. Full spectrum light results in brighter stars available for auto-guiding. For monochrome work or one-shot color (OSC) cameras, the camera will usually be mounted directly behind the OAG. When attaching your camera, be sure the pick-off prism is visible on the opposite (telescope) side.

Sagitta will work well with a wide range of large format sensors such as the KAF-50100 without vignetting.

Adapter Options

Sagitta includes a male Astro-Physics 2.7-inch (AP2.7) adapter [#19871] on the telescope side and the female AP2.7 adapter [#19872] on the camera side. AP2.7 spacers and adapters for various camera and telescope configurations are available from Optec and many other vendors. If additional spacing for the imaging camera is required to match the guide camera OPL range, adding AP2.7 spacers is a cost effective solution. See table below for camera and filter wheel adapters for the AP2.7 thread standard. AP2.7 spacers can be ordered directly from Optec – see this Spacer Table for a convenient guide to calculate any additional back-focus distance from 1/2” to 6-3/4”

Our AP2.7 adapters can also be removed and replaced with any of a variety of OPTEC-3000-S adapters for direct connection. The table below lists many of the common filter wheel and camera interfaces for most astronomical imaging camera systems.

Determine the mechanical interface for your filter wheel or camera. Common interfaces include Tthread (M42), M54 x 0.75mm, and 2” SCT threads. Larger cameras often use an even larger mechanical interface. To avoid any chance of vignetting, use the largest interface available for your equipment. Obtain a suitable adapter from Optec, Precise Parts, or your favorite dealer.

In our example above, our target imaging camera OPL is 85mm plus or minus 6.3mm. Starting with the front surface reference point we can calculate 15.9mm x 2 (OAG thickness is 31.8mm) plus 2.5mm for the #19950 adapter PLUS 55mm for a Canon DSLR with T2-ring. The total imager OPL works out to 89.3mm which is within our target range. We also have the advantage of clamping the Lodestar camera some 4 to 5mm higher which will move our default focus closer to the center of the Sagitta travel.


Name Optec - Sagitta Off-Axis Guider
Manufacturer Optec
Model OPT-19340


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