Skywatcher BKP250 on NEQ6 - Review

This is a "first-light" review of a Skywatcher BKP250 f/4.8 OTAW Newtonian, and a NEQ6 pro mount. I've aquired this telescope mainly for observing deep sky objects and planetary imaging. Despite the relatively cheap price, BKP250 is a moderately good all around telescope. However a some improvements are necessary in order to fully utilize capabilities of its 10" aperture.

What I did like:

  • Cheap, and yet acceptable quality (optical and mechanical). Probably best quality to price ratio for both the mount and the OTA of this category.
  • Mobility and relative ease of setup, despite its monstorous looks. Takes me 15 minutes to put everything together, and the OTA fits easily into my car's backseat.
  • Ergonomics - In my opinion Skywatcher's solutions are better than GSO for example.
  • Focuser quality, considering the price. It is quite rigid (with ~1kg load on it), absolutely zero image shift and smooth. Might not be sufficient for a heavy CCD setup though.
  • Mirror cell and spider assembly design. Both the secondary and the primary mirrors are fairly easy to collimate and hold collimation well.
  • Pyrex primary mirror.

What I did not like:

  • Vignetting - 58mm secondary is too small and cuts aperture even in the middle of FOV.
  • Mount noise is quite annoying, especially at high speed slewing.
  • Back focus is too long - 135mm. It is a result of Skywatcher's attempt to find a compromise between visual and photographic setup.
  • During cold temperatures the focuser tends to slip.
  • Not enough counterweights (required if you use a Paracorr and a heavy eyepiece)
  • Mount head is quite bulky and heavy (cast aluminium).
  • Some backlashes in the mount which harm the stability.
  • The supplied dovetail plate is too long and thin for such a heavy OTA.
  • The OTA tends to hit tripod's legs. It can be fixed by adding a column extension for the EQ6, or using a column instead of tripod. Though I got used to it pretty quickly.
  • There are no clutches and encoders, which makes manual slewing a pain.
  • There optical quality of the main mirror isn't perfect (slight spherical aberration and probably a slight turned edge as well) however quite acceptable for the given price, since a premium 10" mirror would cost 2-3 times more than this whole OTA.
  • The secondary mirror had slight astigmatism
  • Stability and dumping time (see dovetail and backlash issues).

Improvements and tweaks:

  • Dovetail: I've replaced the flimsy "vixen" dovetail with a Losmandy 15" plate from ADM - and it improved overall stability and dramatically reduced the dumping time.
  • Releasing the tightened primary mirror holders is a must to prevent pinched optics.
  • I've increased the secondary mirror (described in this article) since the stock one was too small and vignetting occured even at the center of FOV. For visual use a 63mm is sufficient and for deep sky photography 75mm is a better choice (see graphs below). This also took care of astigmatism which the stock diagonal introduced.
  • Mount tuning: The least is to replace its grease, center the worm gears (if required), and adjust backlashes (which also affects mount stability).
  • Cooling: As in any large Newtonian a cooling fan is a must for the main mirror. I've added a large 120mm fan on the back of the OTA
  • Dew control. Secondary mirror can dew up quite easily under humid conditions, which requires attaching a dew sheild of using a heater.
  • Quality extension tubes with a compression ring (the stock ones are rubbish).
  • EQMOD cable, which enables ASCOM control over the mount by PC (i.e. Stellarium software). It's a must-have accessory if you are serious into astrophotography.
  • A velcro tape to hold the SynScan controller on the mount is a big help.
  • A padded wheeled bag or box for the heavy equatorial head.
  • Reflex finder (such as "Telrad") as an addition to the supplied 8x50 scope.
  • Flocking the OTA and the focuser tube to minimize internal reflections.

Photos:

bkp 250 newtonian package
Unpacking the 10" OTA
ew6 mount packing
Unpacking the NEQ6 Pro mount head
bkp250
Supplied accessories and adaptors

The OTA has a nice black finish with sparkling "stars". A good surprize for me was its length: only 1.12 meters, which is fairly short for a 10" f/4.8 instrument. This is because the OTAW version is advertised as "astrophotography" instrument with a longer back-focus and shorter tube. OTA diameter is 29 cm, and it weights approximately 14kg. The tube is made from a thin rolled steel. It's relatively light and perfectly rigid for visual or planetary photography. However its deformations are noticeable under heavier loads (over 1 kg at focuser).

bkp-250-newtonian-ota
The OTA in its sparkling black finish

The NEQ6 mount has a heavy and bulky head (~16kg), two counterweights (which is enough if supplied shaft extender is used), and a relatively heavy and rigid steel tripod. Both losmandy and vixen style dovetails can be mounted. There is a small bubble level, which is pretty useless since it's not accurate. Synscan hand controller and a few cables complete the set.

eq6
NEQ6 mount assembled

As you can imagine - the assembled scope is quite big. It's a good idea to buy or build a high observing chair for comfortable observing.

skywatcher 250mm newtonian on eq6
The whole setup assembled
battery for eq6
Powering the EQ6 mount from a portable 12V Li-Ion 10Ah battery

In order to deal with stability issues - I've replaced flimsy vixen-style dovetail with a 15" losmandy-style wide plate from "ADM accessories". It definitely helped with vibrations, and improved rigidity of the whole setup. Note the bolts at two sides of the plate, which are pushing into the rings.

ADM losmandy dovetail skywatcher newtonian
A replacement "Losmandy" dovetail plate to reduce the vibrations

The Skywatcher dual speed focuser turned out to be quite good for my purposes. Obviously it's not a "Feathertouch", but still quite pleasing to use, rigid, and has no image shift. Main drawback is its small travel. Also it lacks brass compression rings.

skywatcher dual speed focuser
The stock dual speed crayford focuser is smooth and shift-free

Spider: it's quite thin and holds collimation of the secondary well.

skywatcher spider assembly
The "spider" and the secondary mirror holder
primary mirror of bkp250
10" Pyrex primary mirror held on 3 back points and 6 side points
mirror cell
Primary mirror collimation adjustment and set screws
skywatcher transport
Scope packed and ready for transportation

Travel "box" for EQ6 mount head and its counterweights. This In my opinion is a much cheaper, lighter, and easier to transport solution than hard Pelican-style cases. (I haven't yet added a foam padding to the bag)

case for eq6 mount head
Temporary transportation solution for the mount head

Optics

Optical quality of this telescope is moderate, as was expected from Synta's mass production parabolic mirror. It's great for deep sky observing and probably deep sky photography. However for serious planetary observing and imaging I would consider replacing the primary mirror (though a premium 10" mirror costs more than this whole telescope).

I was able to observe hints of Saturn's Encke division and saw quite a lot of detail in its belts, as you can see in atached planetary photos. However the quality isn't perfect - there is a small amount of spherical aberration and probably a slightly turned edge. The star testing was performed with a small secondary and an artificial star, which might have affected results.

skywatcher bkp250 newtonian star test
A star test

To give another perspective: These are a photos of Mars and Saturn which I've taken taken through this telescope (with a 5X powermate).

mars and saturn with 10" newtonian
Photos of Saturn and Mars taken with the BKP250 Newtonian

Vignetting

Skywatcher tried to make "astrophotography" version so they increased the back focus to 135mm, which also enabled them to shorten the OTA. Both of these parameters are generally considered good. However the back focus is too long, considering the tube flimsiness. Also it introduces additional difficulties when balancing the mount.

Another issue: Increasing the back focus usually requires a larger secondary mirror. However this Newtonian got only got 58mm secondary which clearly isn't enough. There is a slight vignetting (darkening) even in middle of FOV.

Using ISAAC program I produced vignetting graphs for 63mm and 75mm secondary mirrors, while the original is 58mm (graphs are for this specific OTA only). As we can see - for visual use 63mm (which I've already installed) is sufficient. For wide field photography a 75mm is a better choice to ensure illumination of a larger chip.

newtonian secondary mirror vignetting 63mm
63mm diagonal mirror vignetting graph
newtonian secondary mirror vignetting 75mm
75mm diagonal mirror vignetting graph