Perfect tracking, within a few microns, is needed for long exposure astrophotography. It is essential to correct for even small discrepancies such as imperfect polar alignment, optical or mechanical flexure, imperfections in gears, and other factors. The mount therefore needs to be “guided” to ensure we are tracking the sky perfectly.
There are several methods used for guiding. Here we will concentrate on the Off Axis Guider (OAG). Having tested about a dozen OAGs, I’ll be reviewing my preferred one, the Optec Sagitta. The Optec Sagitta OAG is the only one I have found that meets all my requirements.
The Off Axis Guider – OAG
Guiding usually involves a guiding mechanism, a small guide camera, and software. Having tested both using a guide scope and “unguided” mount modelling, I noticed my tracking was often best with the OAG. It is very robust, accurate, light, and relatively simple to use.
The OAG is simply a tiny prism that directs part of the image at the edge of the field (outside the image frame of the imaging camera) to an independent guide camera. The guide camera then constantly monitors any star in its view and makes tiny corrections to the mount when needed to make sure the system is “locked on” (think of a periscope used to steer a submarine towards its target). The OAG is securely connected adjacent to the main imaging camera so it will see the exact same errors as the imaging camera.
The Optec Sagitta
The Sagitta is Optec’s large aperture OAG with several unique features.
The Mechanics – One of the main advantages of the Sagitta is that is has a very large clear aperture of 76.3mm (3.0 inches). This means you can use even the largest sensors without the prism getting in the way of the imaging sensor, even with very fast telescopes. It can work with a 62mm diagonal sensor with ease.
I was a bit concerned about its weight given its size, but the body is lightened by CNC machining and weighs only 245 grams (0.5 lbs). It is also designed to carry very large and heavy camera systems with the help of its solid construction, dovetail and set screw connections (it came with male and female Astrophysics 2.7-inch adapters). The body is 31.7mm thick (1.25 inches), which may be more than some telescopes will allow in terms of back focus.
Prism – Another advantage of the Sagitta is its relatively large prism. Typical sizes are 8x8mm or 10x10mm, and these are often further reduced by the clear opening of the prism stalk.
The Sagitta’s 12.5×12.5mm prism offers more than twice the area in which to find a guide star. This is very helpful when using longer focal length telescopes where you don’t have as many stars to pick from (because of the slower telescope’s smaller field of view). I usually don’t have issues finding a guide star using a 2500-mm focal length telescope.
Adjustment – A unique feature that I like is the adjustable prism stalk. It has a geared mechanism that allows you to move the prism in and out of the field with millimeter precision by inserting and turning the provided hex (Allen) key. This is very helpful in allowing you to place the prism just out of the field of view of the main camera but not so far out that the telescope won’t be able to provide guide stars. It would have been great if this could be done with the OAG in place, but you do have to remove it from the telescope to access the mechanism.
Focuser – While the body of some smaller OAGs are 10m to 20mm thick, their focusing mechanism is often about 50mm thick, which mean you can’t really place it in between certain parts at certain angles.
The Sagitta’s focuser is designed to keep it tucked within its body thickness, so it can be placed and orientated at any angle. I also like the very secure brass clamp mechanism. I have noticed the star image shifts a bit when the focuser is moved in and out, though.
Motor – You can add a motor to the focuser for $325 later or buy the OAG with the motor installed for $895 as I did. It’s compact and only weighs 150 grams. This is needed to change focus if you are using filters with different thicknesses, especially using a remote setup. The motor will need an external controller ($325); I use a Pegasus Ultimate Power box that can run the motor. Note that with the motor installed, you can’t focus it manually.
I have always achieved more consistent results with off-axis guiding than with other methods, especially after taking a bit of time to optimize the guiding software settings (I use PHD2, which has a helpful assistant for this).
With my guiding showing a very low total error of 0.38 arcseconds (see the PHD2 tracking graph above), I’m able to get pinpoint, round stars at a focal length of 1700mm with 500-second sub exposures. Note the very low eccentricity value of 0.31 of the image below, indicating excellent tracking.
While the Sagitta does cost more than typical OAGs, I’d say the main disadvantage is that it uses up a considerable amount of back focus. This is not an issue for some telescopes, but some might find that they won’t be able to use it.
However, if you are able to incorporate it, you’ll notice that it is very solid, the massive aperture can work with any camera sensor, finding guide stars is easier with its large prism, and the motorized focuser is a lifesaver for remote and semi-remote setups like mine.
Overall, I’m very happy with the Optec Sagitta and can confidently say it is an excellent choice for those looking for a solid, large OAG, with or without a focusing motor.