The HoTech advanced CT laser collimator fills a niche in the collimating arsenal that is normally dominated by equipment for Newtonian telescopes – being designed with Cassegrain telescopes in mind.
Any telescope mirror can suffer from slight slippage, where the optical axis fall out of alignment, known as miscollimation. Compared
to Newtonians, this is less of a problem in compound telescopes, but nonetheless the optics can occasionally become misaligned, leading
to degraded performance. This is where this laser collimator comes into its own.
At first glance, this accessory doesn't look like your everyday laser collimator – it doesn't slot into the focuser like the ones designed for use with Newtonians for a start. Instead it is made up of a laser target display that sits in front of a compound telescope, a fine adjustment adaptor to attach this target to a tripod, and a 1.25- or 2-inch reflector mirror that slots into the focuser at the back of the telescope to be collimated. Also included are a soft carry case and a 3V CR123 lithium battery to power the laser.
In principle, the collimator is simple to operate. Indeed, once set up it does make collimating a compound telescope relatively easy. However, the devil is in the detail, and great care does need to be taken in the initial setting up of the target display in relation to the telescope's optical axis. This really came to light when we performed our initial collimation on a borrowed early model 8-inch Schmidt-Cassegrain.
We deliberately threw out the scope's collimation and then used the collimator to realign the mirror. It looked to be successful, but when we performed a star test we found all of the stars to be comet shaped. The solution was two-fold: first we needed to co-align the target and telescope. The second consideration was that if you primarily image with your setup, then you need to collimate with the rear reflector in the focuser, but if you mainly use it for visual observing you need to collimate with your star diagonal attached and the rear mirror installed in that instead. The quality of your star diagonal will also affect the accuracy of the collimation, so it's worth investing in a good one.
We set up the target display again and spent at least 45 minutes lining it up as accurately as we could with the telescope's optical axis. We did this by adjusting the tripod’s height and position carefully. We have to emphasise that this is the most critical step – if the target display is not aligned properly with the telescope then it can affect the result of the collimation, as we discovered.
We diligently followed the instructions supplied and, once happy that everything was set up and the target display aligned, we once again made small adjustments to the secondary mirror. When we were happy with the position of the lasers back on the target we again took the telescope outside for another star test. This time it was spot on, the focused stars being sharp points of light, while the intra and outer focus cones of light were concentric.
The collimator is designed for use with compound telescopes that have apertures of at least 7.5 inches, but we also used it successfully with our 7-inch Maksutov. So long as the three laser beams can pass into and back out of the front of the telescope without being impeded, it should work, though we did feel our 7-inch Maksutov was probably close to the smallest scope it could collimate. When everything is followed carefully to the letter and particular attention is paid to the initial setup stage, this accessory does its job brilliantly.
An effective process
The Advanced CT Laser Collimator works by creating three parallel beams of light that simulate the path of light from a distant star. This means you can collimate your scope indoors or in daylight – in other words, in the absence of a real star – without cloud or atmospheric distortion affecting the process.
Not only that, but the target can be placed only a few feet away. Usually the length of the telescope tube will work, but a little farther does help to improve the accuracy. Starlight passes through the scope, reflects off the back of the reflector mirror at the focuser end, and returns to project the three beams onto the target display. The target has concentric circles as guides and the aim is to adjust the collimation until the three returning beams all fall on the same concentric circle, ensuring the optical axis is truly aligned.
Being able perform this collimation of your scope indoors is a great advantage. Once we got the knack of setting it up we found it a breeze to collimate our instruments.
Target display - The target display is machined from a solid block of aerospace-grade aluminium, hardened and anodised to give a rigid unit. At the front is a series of concentric circles with a graded scale, allowing you to position the returning laser beams accurately.
Mode selector - The mode selector gives a range of options: Mode 0 is off, Mode 1 turns the crosshair laser on, Mode 2 turns the crosshair laser and alignment lasers on, while Mode 3 turns both of those and the target backlight on. This final mode is for night usage.
Rear Schmidt-Cassegrain 1.25- or 2-inch reflector mirror - When purchasing the collimator you specify which rear reflector mirror you require for your setup. This mirror slots into the focuser and reflects the three laser beams back to the target display. The outer surface also has a reticule on it and allows you to see the laser positions.
Fine adjustment tripod adaptor - The fine adjustment adaptor allows much closer control of aiming the target lasers squarely at the telescope, so that they fall back on the target display. Sideways and horizontal positioning with the tripod is performed first, then fine adjustments can be made using the adaptor.
Target display lasers - The lasers provide the three parallel beams (simulating the light path of a distant star), a crosshair and a diffuse cone of light. The crosshair and light cone help you to obtain co-alignment of the target and telescope, which is vital for accurate collimation.
This review originally appeared in the November 2014 issue of BBC Sky at Night Magazine. All prices correct at time of going to press.