Borg 89ED f/4.3 astrograph
A self-assembly telescope aimed at intermediate imagers, The objective lens was finely multicoated to increase light transmission and free of blemisheshes
Taking delivery of a Borg refractor is a most unusual and surprising experience. There’s no long box: rather, the telescope is supplied in component form in several small boxes ready for final assembly by the end user. This modular approach has advantages for both portability and upgradeability.
Assembly was very straightforward, with each component mating accurately with the next. The parts supplied for this review form a very wide-field astrograph; in other words, a telescope specifically designed with deep-sky imaging in mind. Looking at the optics, the objective lens was finely multicoated to increase light transmission and free of blemishes.
We were originally caught out by a set of silver rings that had no internal or external threads in them. However, a little research revealed these to be simple spacers, one of which (the 4mm one) was vital to making sure that the camera attachment couldn’t foul the protruding lens element at the rear of the two-part reducer-flattener.
In common with many other focal reducer-field flatteners, the spacing between the camera’s sensor and the mounting face of the reducer is set at 55mm. This measurement makes it compatible with a typical DSLR camera when using a Borg-supplied and camera-specific bayonet adaptor. Unfortunately, when not using a DSLR two adaptors must be used in the image train, eating into this spacing allowance by 13mm.
Although this leaves sufficient back focus for a typical CCD camera and filter wheel, there is insufficient allowance for an off-axis guider as well, so autoguiding must be carried out using a separate guide telescope. We mounted the review telescope on our own side-by-side mounting bar and used a 4-inch refractor as our guide telescope.
With its short focal length, this scope captures a field of view 3° 20 arcminutes wide by 2° 13 arcminutes deep when used with a camera with an APS-C sized sensor. To put this into some kind of sky perspective, this field of view would capture the whole of NGC 7000, the North America Nebula. Many other deep-sky objects are also fairly large, so a wide field of view like this is a very attractive proposition.
Short focal length telescopes have a very narrow focus ‘sweet spot’, but with the precision FeatherTouch Crayford focuser supplied, achieving focus was a simple task. We also noted that there was no image shift when racking the focuser in and out and it had no difficulty supporting the weight of our CCD camera.
We used a Starlight Xpress SXVF-M25C one-shot-colour CCD camera with an APS-C sized sensor for our imaging tests. To combat the effects of light pollution, we inserted a 2-inch Hutech IDAS light pollution filter into the focal reducer’s built-in filter drawer. This very useful feature, combined with additional filter holders available from Borg, could remove the need for a filter wheel when using a mono CCD camera to produce colour composite images.
Among the deep-sky delights we imaged were the Pleiades and the more elusive Horsehead Nebula. These two objects and their surrounding areas encompass a star cluster, reflection nebulosity, dark nebulosity and emission nebulosity, giving us the opportunity to assess the telescope’s performance over a range of different objects and brightness levels. Star shapes were good over the majority of the field of view with just the tiniest amount of field curvature making its presence known in the far corners. We were impressed with its performance and would recommend it to intermediate imagers looking for a wide-field telescope.
The Borg 89ED is part of a highly modular range of telescopes designed very much with portability in mind. This modular approach allows you to upgrade components incrementally as your requirements change. It also makes transporting equipment very simple indeed: Borg telescopes are not only supplied in component form, but are designed to be dismantled again and taken overseas. It is, therefore, possible to carry just the critical components – such as the objective lens – in airline hand luggage while the rest of the instrument goes in the hold.
Tube diameters are reduced in size to exactly match the light cone of the objective lens, and the other components are similarly matched to keep the telescopes as compact and lightweight as possible. The engineering of the various modules is very fine indeed, and when it came to assembly we found that each part screwed smoothly into the next. Once assembled, the telescope felt solid but surprisingly
light, with no obvious sign of its modularity.
Camera rotator - It is important to be able to rotate the imaging camera to accurately frame your chosen object. The built-in camera rotator has three knurled thumb screws, which loosen a collar so you can spin the rear section through 360°. Re-tightening the collar after rotation resulted in zero image or focus shift.
FeatherTouch focuser - The telescope is supplied with a FeatherTouch 2-inch dual-speed 10:1 Crayford focuser. This fine focuser installs seamlessly into the telescope tube. The action is perfect, very smooth yet able to comfortably support the weight of our CCD camera. There was no focus shift when using the locking bolt.
Dew shield - The retractable dew shield has an aluminium screw-fit dust cap and removable plastic centre disc, which forms an 81mm aperture mask. The dew shield extends by 81mm, offering the front element excellent protection from stray light and dew, but we would have liked to have seen a locking bolt.
Focal reducer-field flattener - The 0.65x focal reducer-field flattener is designed to provide a wide and flat field of view. It is comprised of four elements, one of which is made from Ohara ED (extra-low dispersion) glass. Star shapes were generally excellent, with just a hint of elongation at the corners of the field of view.
Filter drawer - The filter drawer unit is installed just before the rear section of the focal reducer-field flattener. The holder is easy to remove and accommodates 52mm or 48mm filters. To minimise disturbance, a standard mechanical shutter release cable with an M3.2 tapered thread can be used to eject the filter holder.
This review originally appeared in the March 2014 issue of Sky at Night Magazine. Prices correct at time of going to press.