Baader Planetarium flip mirror II Star Diagonal

May 12, 2023

Since the dawn of the age of astrophotography, there has been the problem of imaging at the same time as monitoring the camera's target. A solution was seized upon that used a telescope accessory that was already in existence, the star diagonal. A right-angled tube containing a 45-degree plane mirror, it redirects the telescope's emerging beam to an observer-friendly angle. It has saved legions of astronomers the pain of a cricked neck.

Coming to the rescue of astro-imagers, its single 90-degree exit port was left in place, but the mirror was given a pivot so that it could swing out of the optical axis, allowing light to proceed to a new rear port that would receive the now undeflected light. The resulting device, called a flip-mirror diagonal, has rightly become a standard astronomical accessory. With Baader's introduction of their FlipMirror II, this evolution has taken a fascinating step further.

The first thing that strikes you is how well engineered and concise the FlipMirror II is. Its minimal dimensions are designed to offer the least consumption of back focus – an important consideration, bearing in mind how much equipment it may eventually have hanging off it. It comes with a sprinkling of pre-installed adaptors, ready to help mediate between the telescope and assorted visual and imaging equipment ‘downwind’. Mindful of adaptor ring fittings, Baader have saved that scramble for slotted and grub-screw utensils by supplying their own FlipMirror toolkit. This consists of no less than four Allen keys, a flathead screwdriver and an M48/T2 pin wrench.

Fresh out of its box, the first thing to review the action of the flip-mirror deployment. The mechanical performance of this is vital. Thoughtfully, the knob is heavily castellated to allow non-slip operation by either bare or gloved fingers. The flip-mirror knob is spring-loaded, imparting a satisfyingly positive movement between its two configurations. The action of this powered leverage meets its stop with a firm touch, and without an unsavoury harsh metallic clink too.

For my review, I employed the services of my 355mm (14-inch) LX200, a telescope already well broken in with focal point accessorising.

My first task was to assess the flip mirror's reflectance and alignment, so I arranged adaptors to hold matched eyepieces at each of the conventional flip-mirror apertures – the right-angle and rear ports. I had a pair of identical (binoviewer) eyepieces at the ready, to confirm the equivalence of reflected and direct images. High in the southern sky, a first quarter Moon offered itself as a willing test object.

I found that the lunar surface, reflected from the right-angled mirror, appeared equally bright to that through the rear port, which confirmed the quality of the multicoated aluminium reflecting surface.

To verify that the two ports and the mirror were perfectly orthogonal, I turned my telescope's gaze towards the edge of Mare Crisium and carefully centred the crater Proclus in one eyepiece field. Upon flipping the mirror I was gratified to see that I had exactly the same view in the other eyepiece – excellent proof of the fine engineering inside the diagonal. I should add that even if the views had been misaligned, it is possible, via access screws placed on the surface of the flip-mirror's cage, for all the light paths to be tweaked with precision and then locked into place using the apex tool of the supplied pin wrench.

Observing a star at higher magnification, I could see no evidence of flexure, nor could I induce any. The device's claim that it can handle heavy loads is undoubtedly well founded.

I then moved on to configuring it conventionally with a CCD camera and reticuled eyepiece. The pre-installed Baader adaptor rings were a great facility for achieving this, but as I dealt with mating these to my own equipment, not for the first time (and through no fault of Baader's) I yearned for greater uniformity in attaching accessories to one another, for at the dawn of the age of astrophotography, the collision of photographic, microscopical and astronomical equipment spawned a mismatch of interacting standards that we still endure. Some accessory manufacturers even inexplicably create their own bespoke connector threads. Fortunately, Baader provide a plethora of interfacing rings to assist. As described in Tolkien's The Lord of the Rings, I do long, in the darkness, for "one ring to rule them all". Perhaps one day a multithreaded variable-diameter adaptor may appear.

Employing my Baader Polaris I guiding eyepiece at the primary reflected port, I was easily able to govern the view of my CCD camera at the linear (rear) port, and took some lunar images for the sake of record. I also noted, as previously confirmed, that the centre of the field of view was the same for each axis.

Baader's FlipMirror II has another string to its bow, namely a tertiary (auxiliary) port in the base. Among other things, it offers the simultaneous mounting of an off-axis guider, snicking off a piece of the incoming beam regardless of the orientation of the flip mirror. This aperture can receive either Baader's own off-axis guider or the older version of Celestron's Radial Guider (#94176). Not to miss an inventive opportunity, this third port has an additional trick up its sleeve. On the back of the flip mirror, an additional zone of reflection allows the user to position a calibration lamp during spectroscopic imaging.

I already have Celestron's Radial Guider, so I used it during the review. Fitting the off-axis guider nosepiece requires the cooperation of very tiny grub screws in the base-port assembly. Some might find this fiddly, and even during my own reassembly one of the grub screws made a failed bid for freedom. However, thanks to the supplied kit I did at least have the required tools for the job – and to be fair, a user would probably do this delicate installation only once.

Fielding three axes of additional equipment, the Baader FlipMirror II deals with the load with no qualms whatsoever. All adaptor rings are held tight with no mechanical movement at all. Rigidity of the cage under stress is astonishing – each port is clearly able to grip heavy DSLRs, binoviewers and other supplementary equipment with alacrity. It hardly needs saying that if the accessories require rotational reorientation, then the FlipMirror II can do this too.

Baader have even gone to the trouble of squeezing in two screws on one flank of the housing to offer the opportunity to fix additional equipment, or even cable ties. That is as good an indication of Baader's faith in the strength of the FlipMirror II's cage as you can get. Speaking of additional facilities, Baader have thoughtfully equipped the neck of the flip-mirror knob with cog teeth to permit the use of an optional motorised belt for remote operation.

As with any accessory deployed at the focus, attention needs to be paid to balance – particularly with a device such as this that can structurally field any number of additional appliances. This is more problematic with telescopes mounted within a yoke, such as fork-mounted Schmidt–Cassegrain telescopes, where the centre of gravity cannot be changed by moving it along the optical axis. The good news here is that, despite its inherent strength, the Baader FlipMirror II gives you a good start by weighing (without adaptor rings) only 195 grams.

This is a very versatile beast, capable of adapting itself to various individual needs. Apart from having to temporarily cannibalise some of my existing equipment for additional adaptors and collars for the purpose of this review, the FlipMirror II was a joy to play with. In addition to functioning as a high-quality conventional star diagonal and flip mirror, the additional port in its base provides it with supplementary functionality in guiding and spectroscopy. Indeed, I think it is entitled to call itself an optical hub.

The frame's offering of multiple configurations is backed to the hilt by Baader's own extensive range of adaptor rings and collars (including, of course, Baader's Universal Filter Changer). This means that as your needs and requirements change, the Baader FlipMirror II will steadfastly support you come what may.

At a glance

Available ports: 3

Weight (without adaptors): 195g Collimatable mirror, made from multi-layer aluminium with dielectric protective overcoat, with 94 per cent reflectance.

Included accessories: Laterally adjustable T-2a top ring, Two M48i/S52 dovetail rings, Two reducing rings, M48a/T-2i and M48a/T-2a Inverter ring M48a/M48a, Tool kit, with 3mm screwdriver, four hex Allen keys, pin wrench

Steve Ringwood is a regular contributor to Astronomy Now.