Telescope Review

Out of the Box

Though the secondary mirror was slightly off, the first impression was highly positive. A solid tube and a rigid, smooth dual-speed focuser. The bundled 28 mm Kellner eyepiece delivers clear views, however, the secondary mirror shadow is projected in the center, faint but notable on bright objects. No big issue, but it tells that eyepieces longer than 20 mm are not really useful. A 20 mm 1.25" Plössl eyepiece and shorter do not show the central shadow.

Sliding the Focuser

The Crayford focuser is very smooth throughout its travel and the fine 10:1 adjustment is a great touch and it is not wacky. Made of shining aluminum, the focuser tube could cause internal reflections. Likewise, the secondary mirror has shiny glass edges. Image quality could benefit from blackening shiny parts inside the tube.

Bundled Eyepiece

Surprisingly, the low-cost eyepiece is not bad at all and a welcome gesture. Stars look crisp in the center region and a bit stretched outwards which is due to reflector-inherent coma. With an apparent FOV of 52° it is not exactly a 'wide' eyepiece, but with its 2-inch barrel diameter the FOV is 'comfortably wide' (probably subjective). Its simple Kellner design with only three elements does have the advantage of less air-to-glass interfaces.

Typically there are two reasons to go with 2" eyepieces. First, to get a wider field of view (FOV). Second, to impress your friends. The widest field eyepiece possible with a 1.25" barrel is 24 mm with an apparent field of view (AFOV) of 68°. For wider views, you must use a 2" diagonal and eyepiece. A 2" barrel eyepiece with say, 72° AFOV can be fully used in a 2" sleeve only. The AFOV would shrink to about 58° in a 1.25" sleeve.

Connecting Imagers

Laudably, this Newtonian has been designed with imaging in mind. Its backfocus is 86mm. A DSLR reaches prime focus without optional parts, however no means of integrating a 2-inch filter. Naturally, extensions are required for working with barlows. The lens part of a 2-inch, 2x barlow screwed into a T-ring requires a 2-inch extension with 30 to 50 mm optical path with which focuser travel can be kept short alongside the advantage of having a 2-inch filter thread. So sandwiched, the focal length increases to FL=1250 mm (FR=8.3). A CMOS camera with a 1.25" nose adapter reaches focus with or without a 2x barlow. The bundled 47 mm long 2-inch extension tube is threaded for Ø48 mm filters and dimensioned for bringing eyepieces into focus. It can of course be universally used for other telescopes with 2-inch focuser sleeves.

With this being confirmed, this Newtonian is nearly perfect for various deepsky and planetary imaging tasks.

The 'Orion Nebula', M42, in Orion.

BKP150, Nikon D5500, no filter, 70 x 30 seconds each at ISO400, FOV = 1.35 x 0.76°

The 'Eagle Nebula' M16 in Serpens.

BKP150, Nikon D5300, no filter, 38 x 120 seconds each at ISO1600, FOV = 1.68 x 0.94°

The 'Double Cluster' in Perseus.

BKP150, Nikon D5500, no filter, 100 x 60 seconds each at ISO1600, FOV = 1.74 x 0.98°

Dumbbell nebula, M27, in Vulpecula.

BKP150, Nikon D5300, no filter, 25 x 30 seconds each at ISO3200, FOV = 34.5 x 19.41'

'Lagoon Nebula', M8, in Sagittarius.

BKP150, Nikon D5300, no filter, 120 x 30 seconds each at ISO1600, FOV = 1.68 x 0.94°

Messier 35 in Auriga.

BKP150, Nikon D5300, no filter, 65 x 60 seconds each at ISO1600, FOV = 1.72 x 0.9°

Mare Frigoris

BKP150, ASI462MC, IR-Cut filter, 2x barlow, 2.5ms at gain 200, 138fps, 600 frames, FOV = 12.95 x 7.28'

Crater Janssen at dawn.

BKP150, ASI462MC, IR-Cut filter, 2x barlow, 3ms at gain 200, 138fps, 400 frames, FOV = 13.56 x 7.12'

Craters Aristarchus and Kepler

BKP150, ASI462MC, IR-Cut filter, 2x barlow, 5ms at gain 200, 138fps, 600 frames, FOV = 12.75 x 7.12'

Centered on Crater Bürg

BKP150, ASI290MC, IR742nm filter, 2x barlow, 10ms at gain 200, 100fps, 600 frames, FOV = 12.84 x 7.30'

Craters Deslandres, Tycho, Clavius

BKP150, ASI290MC, IR640nm filter, 2x barlow, 4ms at gain 200, 170fps, 600 frames, FOV = 13.03 x 7.34'

Six Days Old Moon

BKP150, ASI290MC, IR742nm filter, no barlow, 4ms at gain 120, 170fps, 200 frames, 4-pane mosaic, FOV = 32.78 x 22.25'

A Newtonian is free of chromatic aberration as light is reflected, not refracted. It produces diffraction spikes around bright stars which are not astronomical but beautiful. The only concern may raise about fast-reflector-inherent coma at the edges of the view. However, if you use a DSLR with a crop sensor the worst of the coma will be off of the edges in that a coma corrector may not be worth spending a little fortune for. You may as well wish to crop a, say, 6000x4000 pixels image by 20% to 30% thus cutting off comatic (elngated) stars.

However, if coma and its removal by image cropping is not your preference, an optional coma corrector will do the trick. Sky-Watcher offers a dedicated corrector threaded for Ø48 mm T-rings for its fast F5 Newtonians, while Baader Planetarium offers its own version compatible with F4/F5/F6 focal ratios, threaded for both, Ø42 mm and Ø48 mm T-rings. Note that coma correctors typically require 55 mm distance to the image sensor in order to work as advertised. Since the flange back of CMOS cameras are short (7.5 to 12.5 mm) an extension tube between the coma corrector thread and the camera thread is required. A coma corrector is usually directly plugged into the 2" eyepiece sleeve of the BKP150.

Helix-shaped reflections that form on stars other than diffraction spikes are commonly found with newtonian reflectors. The issue can be reduced or eliminated by attaching an aperture mask entirely covering the brackets (clips) that hold the outer edge of the primary mirror. It should be preinstalled by the telescope manufacturer. The only minor drawback is loss of mirror area, reducing the effective focal ratio by a practically unnoticeable amount.

Pros

• Convincing optical performance for both, visual and photographic use.
• Coma-free image circle large enough for planetary CMOS cameras.
• Stunning views of nebulae and star clusters under a dark sky.
• Rigid, smooth dual-fit 1.25"/2", 10:1 focuser with lock screw.
• Thin spider vanes, donut ring on primary mirror to aid collimation.
• Both tube rings threaded for accessories.
• Bundled 2" Kellner eyepiece performs better than expected.
• Bundles all adapters required for imaging and visual use.

Cons

• Silver shiny surfaces inside the tube (focuser).
• Needs a costly coma corrector for serious deepsky imaging.
• Extensions sleeves use screws instead of modern clamp rings.

Recommendation

• The 6x30 finder scope could be replaced with a simple red dot finder, also for saving a bit weight.
• Preinstallation of an aperture mask. Blacken all silver surfaces.

Verdict

The fast optics and mirror positioning render this scope an astrograph, but it can as well be used for visual observation (to impress your friends). A coma corrector will provide a flattened field with sharp round stars in the image edges. This telescope is a regret-free purchase.