Explore Scientific’s ED Triplet Apochromatic Refractor Telescope Excels Visually and For Astro Imaging

By James R. Dire, Ph.D.

James Dire has an M.S. degree in physics from the University of Central Florida and M.A.and Ph.D. degrees from The Johns Hopkins University, both in planetary science. He has been a professor of physics astronomy at several colleges and universities. Currently he is the Vice Chancellor for Academic Affairs at Kauai Community College in Hawaii. He has played a key role in several observatory projects including the Powell Observatory in Louisburg, KS,which houses a 30-inch (0.75-m) Newtonian; the Naval Academy observatory with an 8-inch (0.20-m) Alvin Clark refractor; and he built the Coast Guard Academy Astronomical Observatory in Stonington, CT, which houses a 20 inch (0.51-m) Ritchey Chrétien Cassegrain.

Image 1: The authors set up the Explore Scientific Essential Series 80-mm Apo to visually test the instrument’s capabilities.

Explore Scientific is known for refracting telescopes, quality eyepieces, the David H. Levy Comet Hunter , a Maksutov-Newtonian telescope and Twilight I and II alt-azimuth mounts. I own a Twilight II mount and enjoy using it with various small refractors and my 6-inch f/4 Newtonian reflector. However, prior to beginning this project, I had never used an Explore Scientific telescope.

Explore Scientific carries three series of refracting telescopes: the AR Doublet Series (102-mm, 127-mm and 152-mm doublets), the ED Triplet Carbon Fiber Series (80-mm, 102-mm, 127-mm and 152-mm Apos) and the ED Triplet Essential Series (80-mm, 102-mm and 12-7mm Apos). In this article, I am focusing (pardon the pun) on the 80-mm apochromatic refractor (hereafter called the ES80), the smallest telescope in the ED Triplet Essential Series (see Image 1).

The telescope arrived packed in a double cardboard box (Image 2). Impressively, the optical tube

Image 2: The ES80 is superbly packed for carefree shipping.

assembly (OTA) was housed in a third rugged cardboard box with Styrofoam padding. The diagonal was contained in another padded box with cool star charts on the outside and inside, also packed in Styrofoam. Image 3 shows everything unpacked.

The telescope comes with a quality 2-inch carbon-fiber diagonal, a 2-inch-to-1.25-inch adapter, a Vixen-

style dovetail plate (see Image 1), two focus extension tubes required for straight through photography, a two-speed focuser with locking nut, and a finder scope mounting bracket. Fortunately, I have a 9×50 right-angle erect-image finder with a mounting rings that fit the bracket perfectly.

If you look at this model on the Explore Scientific website, you notice slight differences in the ES80

Image 4: Graduations on the focuser shaft allow quick return to focusing when changing eyepieces. The right-hand focusing knob has two speeds for adjusting the focus.

model shipped to me compared to the version pictured there. Mine came with a larger diameter dew shield with the logo on the dew shield. The version pictured on the website has a smaller diameter dew shield with the logo on the tube. The dew shield slides in and out making the telescope more compact for transport. The OTA cap fits over the objective lens inside of the dew shield and not on the dew shield itself, like other-make refractors. So, taking off or replacing the cover requires reaching inside the dew shield or sliding the dew shield all the way back.

The ES80 contains an air-spaced triplet combining genuine HOYA extra-low-dispersion (ED) glass and proprietary EMD enhanced multi-layer coatings on all optical surfaces. This apochromatic lens virtually eliminates all chromatic aberration. The f/6 focal ratio yields a 480-mm focal length. Image 4 shows the telescope with the focuser fully extended. Note the graduated focuser shaft, useful for recording focal points for various eyepieces or cameras.

Image 5: The author installed the ES80 in his observatory for CCD imaging.

The views through this telescope were quite nice. My 22-mm Tele Vue Nagler eyepiece yielded 21x. I scanned the summer Milky Way from Scorpius to Aquila, stopping at numerous star clusters, bright nebulae and dark nebulae. The color and contrast were great, and visually, the stars looked sharp throughout the field of view. My 5-mm Nagler gave 96x, sufficient to view Saturn and its splendid rings. Titan and two minor Saturnian moons were clearly visible. During moments of exceptional seeing, the Cassini gap in Saturn’s rings was visible.

I always like to test the imaging capabilities of any telescope I review. Therefore, I mounted the telescope piggyback to the main scope in my observatory (Image 5). Note in the image I attached a 9×50 finder scope, the two focuser extension tubes and an SBIG ST-8300C CCD camera. In this configuration, the ES80 could serve as a guide scope for imaging with the main telescope. But for the purposes of testing, I guided with a 10-inch Newtonian astrograph, and imaged with the ES80.

Image 6 displays a 30-minute exposure taken of the Dumbbell Nebula (M27). The stars and the nebula show good color and are sharp through the center portion of the image. Unfortunately, all triplet Apos suffer from field curvature, the effect being greater with smaller focal ratios. Fortunately, I happen to have a third party field flattener made for CCD imaging similar to the one sold by Explore Scientific. Image 7 shows the Dumbell Nebula again

Image 5: The author installed the ES80 in his observatory for CCD imaging.

exposed for 30 minutes, but this time with the field flattener in use. In this image, the stars are pinpoint throughout the entire field of view!

To show the benefit of using a field flattener, I have enlarged the upper right hand corner of both Image 6 and Image 7 and placed them side-by-side in Image 8. On the left side of Image 8 (without the field flattener), the stars are elongated radially outward from the center of the image and blurry looking. However, with the field flattener (right side of Image 8) the stars are all round and sharp down to the faintest star!

For my final test image with the ES80, I took a 30-minute exposure of the Wild Duck Cluster, M11, in the constellation Scutum (Image 9). The image spans 2.2 degrees east to west. The brightest star in the field is the

Image 6: A 30-minute exposure of the Dumbbell Nebula (M27).

Overall the Explore Scientific ED Triplet Essential Series 80-mm refractor is a great grab-and-go telescope that will provide sharp, color-corrected views of celestial objects. The telescope can also double as a guide scope piggybacked on larger refractors or reflectors. With a field flattener, the telescope has excellent potential for wide field-of-view CCD or DSLR imaging.

 

 

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