The SharpStar 61mm refractor is ideal for anyone looking for an ultra-compact travel scope for both visual and photo use.
Small refractors have become very popular in recent years, primarily for use in deep-sky photography as wide-field “astrographs” — dedicated photo scopes. The 61mm is the smallest in Sharpstar’s line of EDPH-series instruments, which also includes a 76mm (reviewed here at AGT) and a 94mm.
All are triplet apochromatic refractors employing ED glass and with a relatively fast focal ratio of f/5.5. All also have matching 0.8x Reducer/Field Flatteners that take each scope down to f/4.5 for imaging. The reducer for the Sharpstar 61mm is unique to that telescope, as it is smaller than the units made for the 76mm and 94mm. The 61EDPH is called the model II as it employs a better grade of lens than the first model. However, I tested the 61EDPH II first with the lens it came with, then with a replacement lens and cell supplied by Sharpstar for free to refit units that had been shipped in error with a sub-standard lens. More about that below.
Tube Fittings
The little 61mm is not much larger than a modest telephoto lens, allowing it to fit easily into a camera bag. Good thing, as no case is provided. The dew cap extends another 1.5 inches (3.7 cm). It doesn’t lock but stays securely in place when aimed up. To suppress glare, the tube interior is well blackened and serrated with ridges, but with no baffles other than the focuser’s drawtube.
Without the Reducer but with all the tube fittings attached, the tube weighs 4 lbs 6 oz (2 kg), heftier than a telephoto lens, but more versatile for attaching accessories and setting the camera angle.
Like the larger EDPH models, the 61mm includes a slotted handle to which a guidescope or an accessory such as a wireless controller can be bolted. There’s also a Synta-standard shoe that can accept a small guidescope, finder scope or red dot finder.
The clamshell-style tube ring fastens with a single hand bolt (no tools are required). The ring is equipped with a short 4-inch long Vixen dovetail bar. While its position did make the scope back heavy with the Reducer and a camera attached, the slight imbalance wasn’t an issue with the mounts I used it with.
However, users who plan to attach a long and weighty train of imaging accessories will need to purchase a longer dovetail bar to allow the scope to be mounted closer to the center of mass for proper mount balance in declination. Unless it is placed on riser blocks, a long bar will then restrict the rotation range of the focuser.
The focuser is a solid 2.5-inch rack-and-pinion with a 10:1 fine-focus knob, and 4.8 cm of travel. All eyepieces I tested (both 2-inch and 1.25-inch models) reached focus. The focuser has a tension knob and a separate locking screw, and bolt holes to accept a third-party auto-focuser unit such as from ZWO. When manually focusing, I found the focuser precise yet secure when holding my mirrorless Canon cameras.
The entire focuser can rotate, and the Reducer has its own camera angle adjustment as well. Both have slotted screws that needed initial tweaking to get the rotation tension just right.
I did find these screws could loosen with use, allowing the camera to tip relative to the focal plane, creating soft stars on one side of the frame, as shown above. If you see misfocus across a frame like this, loosen the tension adjustment screws, push the focuser or camera angle adjuster back flush with the tube, then re-tighten the screws enough to allow rotation when required.
Visual Performance
While 61mm might seem small for a visual telescope, with optics as good as the Sharpstar’s it can serve a user very well, particularly for air travel when packing a larger scope is not practical.
A critical star test showed nearly perfect optical correction, with no evidence of astigmatism and the merest touch of spherical aberration. Star images at high power (with a 3.5mm eyepiece) looked cleanly defined and nearly textbook perfect.
Racking through focus showed very little false color on bright stars. That was with the new replacement lens, described below. With the original lens there was indeed noticeable chromatic aberration, with bright stars showing a magenta rim outside of focus, and a cyan rim inside focus. But even that level of false color cleaned up with the new lens.
Where the 61mm excels is in wide-field views. A 36mm Baader Hyperion eyepiece provided a whopping 7.5° field at 9 power and with a 6.7mm exit pupil, for an impressive binocular-like field of view. This would be a great scope to take to the Southern Hemisphere, or on a solar eclipse trip.
Photo Performance
With the three-element Reducer in place (necessary to keep stars sharp to the corners of a full-frame camera), the photographic field at f/4.5 is 7.5° by 5°, fabulous for framing Milky Way star fields, large deep-sky objects in context, and nebula complexes. A cropped-frame APS sensor would provide a 5° by 3° field, still impressively wide.
Star images proved to be very sharp to the corners of a full-frame sensor, showing only slight astigmatism at the extreme corners. I have (or had) apo refractors costing much more that do not work as well as this little Sharpstar for edge-of-field sharpness.
While some reviewers claim the Sharpstar 61 isn’t suitable for full-frame cameras, that’s a conclusion I find puzzling and demonstrably wrong. I found the little 61 works very well with a 36 x 24mm sensor. Light fall-off is slight, darkening the corners by only one-third of an f-stop.
A flaw noted by other reviewers is the 61’s chromatic aberration. This was quite noticeable in the unit I purchased in October 2021. It showed itself in images as blue halos around many stars. While the retrofit program was never officially announced, word circulated that Sharpstar was offering to replace the lens of 61’s with excessive false color. Apparently, some units had been shipped with poor lenses, a mark against Sharpstar’s quality control.
I contacted Sharpstar and sent them a test image which they deemed proof that I indeed had a bad lens. I received the new lens in March 2022, and shot comparison pairs of images of the same field.
While the new lens does not totally eliminate false color, it does greatly reduce it, especially toward the corners where stars were exhibiting colored tails. With the refit, I deem the Sharpstar 61EDPH II to now be quite free of imaging aberrations. I can only assume that any 61EDPH II you buy now will have the proper lens.
Recommendations
The 61mm is a fun little scope for framing fields along the Milky Way. With a focal length of only 275mm, it might be possible to get away without auto-guiding with the Sharpstar on a portable tracker, for a minimal travel kit.
However, I’d suggest a small equatorial GoTo mount, such as the Sky-Watcher EQM-35 I reviewed here at AGT or the new Sky-Watcher Star Adventurer GTi, as a better match for this scope when assembling a portable and budget astrophoto rig.
In all, the Sharpstar 61EDPH II offers superb performance for the price. I can recommend it to an experienced astrophotographer looking for a wide-field astrograph for travel, or to a beginning astrophotographer wanting a quality instrument without committing to the expense and greater mounting demands of a larger telescope.
Plus: Very sharp to the corners when imaging; diminutive size
Minus: Focuser adjustments can introduce camera tilt; short dovetail bar; no included case
Typical Retail: $500; $200 for optional 0.8x Reducer/Field Flattener
Website: www.sharpstar-optics.com/index.php?l=en
