Review – Explore Scientific AR152 Achromatic Refractor

I have wanted to own a large refractor for a long time, but I just never could justify purchasing one. I certainly can’t fit a large apo into my budget and reflectors seem to be an overall better value.  However, I eventually decided to do some research on 6″ achromats and took the plunge with an Explore Scientific AR152, a 6″ (152mm) f/6.5 achromatic refractor. The AR152 has received several good reviews since it came out, along with all of Explore Scientific’s products, and is offered at the unbelievable price of $750. I also considered purchasing the Celestron C6R or the Celestron Omni XLT 150R.  However, I was afraid the C6R, with its longer OTA at f/8,  would be too unwieldy for the CG-5 I planned to mount it on and the Omni XLT 150R was a little faster than I wanted at f/5. Also, the AR152 purportedly had a better build quality and better accessories than the Celestron offerings. I have now owned the Explore Scientific AR152 for 5 months, which is plenty of time to have experienced all of the good and bad things about this telescope. Let’s start from the beginning:

I ordered my AR152 from Astronomics, http://www.astronomics.com, in late September 2012 for $750. My order also included a Celestron CG-5 ASGT mount for the AR152 which was an additional $700. As usual, Astronomics was very quick in shipping my order.  I placed my order on Wednesday night and had the scope and mount by Saturday.

The telescope arrived in a single box which included the OTA, cradle with handle, dovetail mounting bar, 2″ dielectric diagonal, and an 8×50 straight through finder. There were no instructions of any kind included with the scope. All of the components have a solid high quality feel that exceeded my expectations given that this is a budget oriented achromat. I expected a few more cut corners for a large refractor at this price point.

Unboxed AR152

Instead of using the 8×50 straight through finder that came with the AR152 I decided to install a GSO 8×50 right angle correct image finder. I had to remove my GSO right angle finder from its stock bracket and install it in the finder rings that came with the AR152 since the GSO finder bracket has a Vixen style dovetail which isn’t compatible with the AR152. The AR152 finder rings have a spring loaded screw on the top of each ring and two nylon screws for adjusting the alignment of the finder. Once aligned, the finder holds alignment pretty well between observing sessions.

GSO 8×50 RACI in Stock AR152 Finder Rings

The AR152 is a large scope, but it wasn’t the beast I expected it to be.  I find the OTA to be quite easy to manage thanks to the included handle attached to the scope’s cradle — a very nice touch. The scope balances close to center on the CG-5 in declination, but one thing became clear very quickly — I should have ordered an extra 11lb. counterweight for the CG-5.  The CG-5 comes with one 11lb. counterweight, which isn’t near enough to balance the 23.5lb. AR152. Luckily, my CG-4 mount had a 7lb. counterweight that fit the CG-5 and was just enough to balance the AR152 in RA. I promptly ordered an extra 11lb. counterweight for the CG-5.  In practice the CG-5 has proven to easily handle the AR152 for visual use up to magnifications of about 250x with the tripod legs partially extended.

AR152 on CG-5 (left) – Celestron Omni XLT 102 on CG-4 (right)

 

AR152 (foreground) ES 127 ED Apo (background)

 

The front lens of the AR152 is gorgeous. The lens is a deep green color and has 3 sets of 2 push-pull type collimation screws for adjusting the collimation. The inside of the tube is nicely baffled and painted a very flat black and produces no reflection when shining a light down the tube.

AR152 Objective Lens (Black background is the inside of the dew shield.)

 

AR152 lens (left) – Omni XLT 102 lens (right)

 

The AR152 sports a large dew shield that is held on by friction provided by three strips of felt attached to the lens cell. To remove the dew shield you just simply pull it forward. The dew shield is also quite effective — I’ve never had the front lens dew up even in farily heavy dewing conditions.

The focuser on the AR152  is a 10:1 dual speed focuser with a  thumb screw for locking the focus and another thumb screw to adjust the tension. The focuser on the AR152 I received was pretty terrible. It would grab/jerk at certain points along the focuser’s travel.  After racking the focuser in and out a half dozen times hoping  it would smooth out, 2 lines/scratches began to appear on the focuser tube where the paint was being rubbed off. I called Explore Scientific and they promptly sent me a replacement focuser. The new focuser arrived about 3 days later with a prepaid shipping label for sending the original focuser back. The new focuser was very easy to install (there are only 2 set screws holding the focuser to the OTA) and was very smooth with no problems. The focuser holds the diagonal and all of my heavier 2″ eyepieces without any slippage near zenith.

Optical Performance

Collimation

My AR152 either arrived very slightly out of collimation or installing the new focuser caused a slight misalignment. I used a cheshire collimation eyepiece designed for collimating refractors to tweak the collimation which only took about 10 minutes. I just tweaked the push pull collimation screws one set at a time until I discovered the one that needed adjustment. The adjustment needed was very slight.

Chromatic Aberration

Chromatic Aberration (CA), the inability to bring all wavelengths of light to focus at the same point, is inherent in all achromatic refractors, especially ones with fast focal ratios like the f/6.5 AR152. This optical aberration manifests itself as a violet halo around the brightest objects and also causes a slight loss of contrast due to unfocused wavelengths of light. If you tend to be bothered by CA, or if you’ve never owned an achromat before, this is not the telescope for you. There is plenty of CA on objects of 2nd magnitude and brighter and it gets worse on nights with poor atmospheric conditions.  On nights of poor seeing, I’ve seen CA on objects as dim as 3.5 magnitude. On nights of good seeing, the CA is actually pretty well controlled and not all that distracting. The seeing conditions on my first night out with the AR152 were pretty bad. While trying to split E. Lyra (the double double)  I was able to get a dirty split; however, all 4 components exhibited a greenish color. A few nights later I tried E. Lyra again with better conditions and got a very clean split with all 4 components showing a normal white color. The only object that the CA has really bothered me on is the double star Alberio. At magnifications of less than 50x the blue and gold color of the stars is visible, but anything over that causes the striking colors to be lost to CA.

Chromatic Aberration on the Moon. (This is a low quality image taken by holding a cell phone camera to the eyepiece. Eyepiece is an 8.8mm Meade 5000 UWA giving 112x.)

 

I’ve tried using a 2″ Baader Semi-Apo filter to help reduce some of the chromatic aberration, but found that I actually preferred the unfiltered view. For me the filter dims the view too much and reduces contrast. Other reports have indicated that the Baader semi-apo increases contrast, but that was not my experience.

More recently I acquired  a 2″ Baader Fringe Killer filter which has a higher light throughput than the semi-apo filter. I’ve found the Fringe Killer to be a much better filter overall than the semi-apo. The Fringe Killer reduces some of the violet fringing around bright objects and increases contrast without dimming the view like the semi-apo filter.  The Fringe Killer does impart a slight yellowish tint to some objects, but I find this a lot less objectionable than the dimmer view provided by the semi-apo filter. I plan on leaving the Fringe Killer in the diagonal pretty much all the time.

Moon and Planets

To the apo only crowd, viewing the Moon and planets with a large fast achro is like committing an unpardonable sin. These folks will have you believing that an achromat will only show planets as a  fuzzy blob of false color and it might even turn your hair and face permanently purple if you look at Jupiter through one. However, even with the CA, I find that the AR152 gives nice sharp views of the Moon and planets.  The planetary views are definitely not as nice as you would get with an apo, but they’re not darned bad either. With that said, if the Moon and planets are your primary interest and you want every last possible detail, then you should be looking at other telescopes.  But don’t be fooled into thinking that an achromat cannot give a decent view of these objects with plenty of high contrast detail.

On nights of good seeing I’ve had very good views of Jupiter up to 300x and regularly use a 4.5mm Meade 5000 HD-60 on the giant planet giving 220x.  The GRS is an easy target for the AR152 as are larger barges and festoons.  The CA on Jupiter is obviously about as bad as it gets.  The planet exhibits a bluish halo around it that extends out from the planet about 1/2 the planet’s radius. The belts and zones on the planet also appear more of a grayish color instead of their natural reddish brown.  However, the belts and features on the planet are very crisp and sharp with loads of detail during moments of good seeing.

Open Clusters

Open clusters are definitely the AR152’s strong point. I would even go as far as to say that the AR152 has given me a new appreciation of these celestial wonders.

The double cluster in Perseus is especially beautiful in the AR152.  Using a 38mm Orion Q70 eyepiece both clusters are framed beautifully and show sharp pinpoint stars from field stop to field stop.  At a club observing event back in November 2012 the owner of a Meade 12″ SCT even commented that the double cluster looked much better in the AR152 than in his 12″ SCT.

The Wild Ducks cluster (M11), M46, M35, and M7 are all wonders to behold in the AR152. M46 looks like fine grains of sand scattered across a velvety black background with the small planetary nebula NGC 2438 easily visible; even from my light polluted backyard.

The bright stars in M45, the Pleiades, do show a slight amount of CA in the AR152, but it’s not objectionable.

Globular Clusters

The AR152 provides fairly good views of globular clusters. Six inches of aperture seems to be kind of the magic aperture where globular clusters really start to resolve.

M13 and M15 show many resolved stars in the AR152, whereas my 4″ achro shows these clusters as fuzzy patches with a few stars here and there.  My 10″ Dob; however, shows many more stars than the AR152. The dark propeller feature in M13 is detectable in the AR152 at around 140x, but it takes some work — this is normally an easy feature in my 10″ Dob.

Nebulae

M42 is spectacular in the AR152; as it is in just about any telescope. Six inches of clear aperture is enough to start showing actual structure in the nebula.  The E and F stars in the trapezium are fairly easy at around 90x on nights of good seeing.  Using an Orion 38mm Q70 or an ES 82 degree 24mm eyepiece gives a very beautiful high contrast low power view of the nebula.  The view has an amazing 3-D like effect.

M17, the Swan Nebula, is also a nice target in the AR152.  The “number 2” shape of the nebula is very prominent and stands out well in the AR152.

From a dark site NGC 2024, the Flame nebula, is an easy target for this scope showing the prominent dark lanes often referred to as the “tank tracks”.

M57, the Ring Nebula, easily shows its oblong ring shape with a few glimpses of structure visible. It doesn’t have quite the same “pop” as it does in my 10″ Dob, but it’s still nice.

Galaxies

To be honest I’m not that much of a galaxy hunter. If I plan on hunting down dim galaxies I typically pull out my 10″ Dob.  The only galaxies that I’ve viewed with the AR152 are M31 along with its companion galaxies and M33.  M31 honestly doesn’t look much different than it does in my 10″ Dob from my backyard.  M32 and M110 are both visible in the same field of view as M31 and nicely framed using a 38mm Q70 eyepiece. M33 was just a smudge of light and would have been difficult to detect had I not know what I was looking for.  One of the spiral arms of M33 is discernible from my backyard using my 10″ dob, but no such structure is visible with the AR152.

Update: 4/01/2013

I recently took a look at the M81/M82 galaxy pair with the AR152 from my backyard and must say the scope performed very well on these bright galaxies. Both galaxies fit nicely in the field of view of an ES 24mm 82 degree eyepiece.  The oblong round shape of M81 and the ragged cigar shape of  M82 were readily apparent. Increasing the magnification doesn’t really reveal much more detail on M81, but on M82 the ragged edges of the galaxy show some interesting detail. I was able to increase the magnification to 147x using an ES 6.7mm 82 degree eyepiece and still have plenty of light.

Conclusion

If you are looking for a well made large refractor but don’t want to take out a second mortgage on your house, then the AR152 may be the telescope for you. It’s certainly not a premium grade refractor, but at its price point I don’t think you’ll find a better built 6″ refractor. It really is an amazing package considering what you get for your money. For $750 you get a large refractor that is ready to go right out of the box without a lot of upgrades such as a better diagonal and focuser. The only component I felt the need to upgrade was the finder, but that’s personal preference — I’m not a fan of straight through finders.

With 6″ of clear aperture the AR152 is an excellent deep sky performer, especially on open clusters, and performs pretty well on planets too if CA doesn’t bother you too much. If CA is an issue for you, then you should steer well away from this telescope.

Overall I’m very happy with the AR152 — there’s just something magical about a large refractor.