OVL Off Axis Guider Review

Monday, 29 April 2013  |  Steve

The new Slim Off Axis Guider from OVL shows promise so we sent one to Steve Richards for an in depth review.

Steve Richards is the author of ‘Making Every Photon Count’ and writes for The Sunday Times, BBC Sky at Night Magazine and BBC Focus Magazine.


Introduction

Short exposures up to perhaps 2 minutes maximum can be taken with many GoTo Equatorial mounts using short focal length telescopes but it is likely that a percentage of the sub-frames will still have to be discarded as they will suffer from unwanted star trailing. Star trailing is caused by a failure of the mount to accurately track the movement of the sky over time and is a result of Periodic Error (PE) and spurious unwanted movements of the mount. Although PE can be improved by the use of the Periodic Error Correction (PEC) feature built into many mounts, this only results in an average improvement in tracking accuracy and takes no account of weight shift, mirror flop, wind shear, atmospheric effects and random mechanical glitches in the mount’s drive system.

OVL Off Axis Guider Image 1

UNGUIDED                                                                               AUTOGUIDED

Unless you have a very expensive mount - and even then tracking improvements can still be made – the long exposures required for imaging deep sky objects will require some kind of guiding system to be implemented. Historically, this has been done by aligning a ‘guide star’ in a crosshair eyepiece attached to a second telescope attached to the same mount and correcting any movement of that star by use of the direction buttons on the handset. You could still do this but having tried it myself, I don’t recommend it as it was one of the most nerve-wracking and unpleasant experiences that I can remember! A far more civilised way is to have a second camera (the ‘guide camera’) controlled by autoguiding software that checks for movement of a test star then automatically sends a guiding instruction to the mount to correct any unwanted movement. As both cameras are attached to the same mount, it can be seen that if the guide camera is accurately tracking the sky then so to will the imaging camera.

A simple way of setting up a guiding system is to attach a second telescope to the mount for the sole use of the guide camera. Unfortunately, this can lead to flexure between the two telescopes which results in the imaging telescope not quite matching the movement of the guide telescope. The solution to this is to use an Off Axis Guider (OAG). An OAG comprises a mounting ring or tube that attaches to the focuser on one side and the imaging camera on the other side. Inside is a small prism that picks off a sample of light from just outside the light cone falling on the imaging sensor and diverts it through a tunnel to the guide camera. This system reduces overall system weight, takes account of mirror flop and makes use of the imaging telescope for guiding as well.

OAGs are not without their problems though. They can be tricky to set up to ensure that both the imaging and guide cameras are in focus at the same time and you have to ensure that the pick off prism doesn’t obstruct any of the light cone falling on the imaging sensor. If you are using a typical focal reducer or coma corrector it can be challenging to get the sensor to corrector spacing right unless the OAG is very slim.

In this review, we will be looking at a new, slim OAG recently announced by Optical Vision Limited (OVL).

Specification of the review sample

Price: £129.00
Weight: OAG 172gm, Nosepiece 39gm
Depth: Body 13.0mm (15.0mm overall)
Diameter: 72.0mm
Connections: 48.0mm thread (female) to telescope, ‘T’ thread (male) to both cameras
Supplier: First Light Optics

What's in the box?

OVL Off Axis Guider Image 2

The box contained the fully assembled OAG and a 48mm to 2” nosepiece adaptor with a tapered waist to help with securing it tightly in the focuser’s eyepiece holder.

The OAG comprises a substantial outer casing with a female 48.0mm internal thread on the telescope side, a prism retained in an adjustable turret, an adjustable table for the turret for mounting your guide camera and a rotatable male ‘T’ threaded adaptor at the rear for attachment of your imaging camera.

Description

All the components of the OAG were machined to a very high standard and the unit had a good, solid feel to it which is important in ensuring that there is no flexure in the system as this would lead to tracking errors despite apparently good guiding. The four main components had a smooth, matt anodised finish with chromed steel adjustment knobs and there were no visible internal reflective surfaces to rob the light path of contrast. Tests in bright light conditions also showed that no external light could intrude into the casing.

The 90° prism was of high quality and no aberrations were visible when examining a daylight scene through it. The prism is held firmly in its socket at the end of the adjustable turret by a single tiny grub screw. There is no tilt adjustment for the prism but the 45mm long turret allows for plenty of adjustment in and out of the casing and this is secured in position by a single knurled bolt on its short side. I would have preferred to have had a second bolt working at right angles to the first to stop any chance of the turret ‘rocking’ in its channel but this is just in my personal wish-list as none of the commercially available OAGs have such a second bolt. I made up a tiny shim using kitchen foil to act as a buffer on one side of the turret and this ensured that there was no unwanted movement of the turret in any direction. The light port through the turret is 5.6mm in diameter which suited my 4.9mm x 3.7mm sized guide camera well but would cause vignetting with larger sensors – not a disaster but a shame as it could rob larger sensors of an increased guide star choice.

On top of the turret there is a circular, height-adjustable table through which the turret passes and this is machined with a male ‘T’ thread around its circumference for attachment of the guide camera.

OVL Off Axis Guider Image 3

                                               FRONT                                                                                   REAR

Depending on the design of your guide camera, there are various options for installation, some of which will require additional components. For example, an Atik Titan (although a little on the large size) will fit directly to the ‘T’ thread and so too will a Baader LVI Smartguider-2, Sky-Watcher SynGuider, Orion StarShoot or QHYCCD QHY5 autoguider once you have removed the 1.25” nosepiece. If necessary, you can then space these further from the table using standard ‘T’ extenders or variably using a Baader Varilock 29 lockable T-2 extension tube.

OVL Off Axis Guider Image 4

Guide cameras from Starlight Xpress including the LodeStar, SuperStar, CoStar and SXV guide head can be installed in one of two ways. All four have an internal ‘C’ thread so you can use a ‘T’ thread to ‘C’ thread adaptor attached to the table or make use of the fact that they have 1.25” diameter which is the same as a standard eyepiece so you can use a ‘T’ thread to 1.25” eyepiece adaptor. Again, you can add extension with standard ‘T’ thread extenders or use a Baader T- thread to 1.25" Helical Focuser for variable focus.

For my tests with a Starlight Xpress SXV guide head, I used a Baader 2” to 1.25” reducer which just happens to have a ‘T’ thread in its base and I focused the guider by sliding it in and out of the brass compression ring in the 1.25” holder. However, the Baader Helical Focuser mentioned above would also have been an excellent solution.

The imaging CCD camera attaches to the rear of the OAG via the supplied male ‘T’ thread adaptor and spacing to the correct distance can be achieved by using the same solutions as for the guide camera. The important thing to remember here is that both imaging and guide sensors must be spaced such that the light falling on them travels the same distance from the front of the OAG to ensure that both are in perfect focus at the same time.

If you are not using a focal reducer, field flattener or coma corrector, you can make use of the supplied 2” nosepiece to attach the OAG to your focus tube and you are ready to go. However, if you are using one of these reducers/correctors, you will have to bear in mind the spacing requirement of these devices,most of which use 55.0mm to 56.0mm with a tolerance of ± 1.0mm. This spacing has become a ‘standard’ based on the nominal sensor-to-camera face of a DSLR camera being 45.0mm and the depth of a standard ‘T’ to camera bayonet adaptor being 10.0mm.

The OAG has a 48.0mm internal thread at the front so it is a good match for the Sky-Watcher range of reducers designed for the 80 ED, 100 ED and 120 ED which will attach directly to it. Other reducers/correctors have a male ‘T’ thread so a Baader Adaptor T2 female to M48 male will be required. This adaptor has a shoulder depth of 3.5mm which could well be sufficient to throw out the critical spacing requirement so a shoulderless adaptor like the TS T2-M48 Adaptor – Zero Profile might have to be used instead. If the shoulderless adaptor is used, it is imperative that it is attached to the reducer/corrector first to ensure that the thread cannot penetrate the OAG far enough to touch the turret and damage the prism’s surface.

The adaptor at the rear of the OAG has a male ‘T’ thread for attachment of your imaging camera. The adaptor is waisted and held in place by three knurled chrome bolts that allow approximately 15° of rotation to help with alignment of the imaging camera’s sensor with regard to the prism. In addition to this small amount of rotation, three cut-outs machined to fit around the turret allow the whole adaptor to be placed in any of three positions 120° apart giving a reasonable amount of choice in the rotational position of the camera.

Tip: The best way to set up an OAG is in the daylight with your telescope aimed at a distant, wide, solid surface like a neighbor’s house wall or roof. Calculate the required spacing for your imaging and guide cameras and assemble the various components onto the focus tube. Start taking a continuous series of short exposures (very short as it is daytime) with the imaging camera and adjust the telescope’s focuser until the distant object is in focus. Now swap to the guide camera and continue the series of test images, adjusting the vertical position of the guide camera manually until this too is in focus. Tighten up any clamps and the job is done.

OVL Off Axis Guider Image 5

I was unable to achieve focus on both cameras at the same time at first as I couldn’t achieve sufficient inwards travel of the guide camera. To resolve this, I had to obtain a 3.0mm ‘T’ extension ring to move the imaging camera further out which, of course, gave me an additional 3.0mm of focus travel for the guide head. This worked perfectly but upset my calculated spacing to my focal reducer by 3.0mm so I also obtained a shoulderless adaptor as discussed above and this gave me back 3.5mm which resolved the issue.

I was very impressed with the guide star shapes (below) that were produced after refraction by the prism. This can be a weak-point with some OAGs but not this one! Although guiding software works on a star’s centroid so it will accommodate a certain amount of star distortion, a good guide star shape to start off with is always a bonus. There was also no shortage of guide stars to choose from and calibration using MaxIM DL was painless and quick allowing me to take a series of guided 5 minute exposures with excellent tracking and no lost subframes.

OVL Off Axis Guider Image 6

Conclusion

The OVL OAG is very well built indeed resulting in a flexure-free guiding solution. A wide range of accessories is available from third party manufacturers to suit most guide and imaging camera combinations.

However, if you are using a focal reducer, field flattener or coma corrector that requires the standard spacing of 55.0mm to 56.0mm you will need to carefully calculate the depth of the various adaptors required to obtain that all important focus on both cameras yet maintain the correct sensor to corrector distance. You will almost certainly have to allow the turret to protrude inside the throat of you guide camera as well to close up that spacing.

To make this job easier, the turret could do with a length reduction of a few millimetres but because of the design of the table, this too could be reduced in height a little. These changes would increase the amount of inwards travel of the guide camera but could also result in the prism intruding further into the light cone. This intrusion would be fine for a small imaging sensor like the Atik 314L+ or Starlight Xpress SXVR-H9 but could be an issue with larger sensors like those with an APS ‘C’ sized sensor which would include DSLR cameras.

It is possible to overcome these spacing issues without any modification even using an APS ‘C’ size imaging sensor as I have demonstrated, it just takes a little preparation and patience.

Having used the OAG on the night sky (see image of M3 below) I have to commend OVL on producing such a well-engineered solution at such a reasonable price and I would certainly recommend it to any deep sky imager wishing to improve their mount’s guiding accuracy.

I will not be returning this one to First Light Optics now that I have completed this review!

OVL Off Axis Guider Image 7

M3 Test Image guided with OVL OAG


Steve Richards is the author of ‘Making Every Photon Count’ and writes for The Sunday Times, BBC Sky at Night Magazine and BBC Focus Magazine.