The Stellarvue SV70ED Travelhawk is a doublet refractor this is available from Stellarvue for $399. It has an aperture of 70mm, a focal length of 420mm and a focal ratio of f/6. The doublet is hand figured, and made from extra-low dispersion (ED) glass. The SV70ED comes with a Vixen-style dovetail mounting foot that also is threaded for a 1/4 - 20 bolt for use on a standard tripod. A small metal foam-padded carrying case is included in the price. The scope weighs 4.5 pounds and is 12 inches long with the dewcap retracted. This evaluation was done with an SV70ED that was purchased directly from Stellarvue. Fit and Finish
The fit and finish of the SV70ED is excellent. It has a beautiful white powdercoat finish and screw-in lens cap with the SV logo on it. The dewshield is built in and slides back on the optical tube for storage. The rotating 2-inch Crayford focuser is excellent and one of the best that I have used on a small refractor. The dovetail mounting foot is made of relatively soft aluminum however, and indentations form when it is locked down into the dovetail by the two locking bolts in my Orion Sirius mount. These don't affect the use of the scope in any way, but they do mar the physical appearance of the foot. The scope doesn't quite balance perfectly in dec when a camera is attached, but it's not out of balance enough to be a problem with any kind of decent mount. The focuser rotates radially so that the focusing knobs cab be located at a more convenient position, or so that a camera's orientation can be changed once it has been focused and the focuser locked down. The focuser rotates by means of three nylon-tipped set screws that fit into a groove machined into the focuser. A thumbscrew locks down the focuser so it can't rotate, but this thumbscrew is not nylon tipped, and if tightened too much it will leave a gouge in the groove. I don't particularly like rotating focusers so I just tightened the three nylon-tipped screws down so the focuser won't rotate at all and removed the locking thumbscrew completely. If I need to re-orient the camera, I simply unlock the 2-inch adapter, rotate it, and lock it down again. When the scope was delivered, one of the nylon tips was broken off on one of the three screws, but this was quickly replaced by Stellarvue at no charge. Visual Performance I tested the SV70ED with a 32mm Plössl, 18mm Nagler Radian, 11mm Nagler and 7mm Nagler on several different nights. These eyepieces provide 13x, 23x, 38x, and 60x magnification. I also used a 1.4x Dakin Barlow with some of them. I observed one night from my driveway in suburban Philadelphia, with a naked eye limiting magnitude of about magnitude 4, on a night with a large bright gibbous moon. I was just able to detect M57, the Ring nebula at 13x magnification. At 60x, it was easily visible because of its high surface brightness. Epsilon Lyra, the Double Double, near Vega, was a nice tight little pair with each clearly elongated into a figure-eight shape at 60x. On an earlier night, the Trapezium in Orion was very easily separated with all four of the brightest stars cleanly visible, and hints of the E star visible sometimes. I could not detect the sixth star (F) that night, but this may have been more due to the seeing than the scope's performance. I also observed M13, the great globular cluster in Hercules, which was not quite resolved across the core at 60x, and the double star Albireo, which was a beautiful blue and gold. The Star Test The star test was performed using the scope without a diagonal to eliminate any possibility of the diagonal introducing any problems. Star testing the SV70ED on Arcturus and Vega, the scope produced an Airy disk with little color that was noticeable visually. Racking the focus at high power from outside of focus to inside of focus, the typical Fresnel ring pattern was visible outside of focus with the outer ring being brighter than the other rings. At focus, the Airy disk displayed a classic diffraction pattern with the first ring being a little large and a little on the bright side. Inside of focus, the Airy disc stayed a small spot as an out-of-focus hazy shell grew around it as focus was racked in. The Fresnel rings formed farther out, but the spot did not appear to be perfectly centered on the haze. Way inside and way outside of focus, the rings were not the same. Inside of focus, the outer ring was more fuzzy and bigger, and inner rings was larger also, compared to the rings outside of focus. In a perfect optical system, the Fresnel rings inside and outside of focus would be identical. However, identical patterns are rarely found even in excellent refractors, and I don't get them in my Astro-Physics 130EDT triplet apochromatic refractor. Inside of focus, as the focus was moved more inwards, the airy disk stayed the same size as the haze got bigger until the air disk finally blossomed out to the rings at about 1/4 turn of the fine focus knob. The inside-focus characteristics of a tight core surrounded by a hazy glow is indicative of spherical aberration. At low and medium powers, the SV70ED provided very nice views and tight stars. However, fine detail on Saturn's rings started to fall apart at high power due to the spherical aberration. Photographic Performance - Color Fringing Although the SV70ED has little color visually, it does have some blue/purple fringing around bright stars that is recorded photographically with a DSLR camera. This color fringing can be improved considerably by the use of a minus violet filter, but this will add $80 to $100 to the cost of the scope. If you regularly use something like Hutech's LPS filter, it will also remove this color fringing and you won't need to use a minus-violet filter. Below you can see the SV70ED's color performance on bright stars in M45, the Pleiades, one of the toughest objects for this kind of test. Hold your mouse cursor over the image below to see a comparison of the color fringing with and without a minus violet filter.
In Comparison 1, we can see that the use of the Lumicon minus violet filter greatly improves the blue/purple fringing around bright stars and reduces the size of the star diameters but causes a larger but fainter blue halo around the brightest stars. Photographic Performance - Field Flatness Like most telescopes, the SV70ED has a curved focal plane due to simple geometry. This is not noticeable when the scope is used visually with an eyepiece, but it is a problem for astrophotography of star fields. With this scope for astrophotography, stars become unacceptably out of focus about 1/2 of the way from the center of the frame to the corner of the frame of an APS-sized (22mm x 15mm) DSLR sensor. Stars become progressively more elongated and out of focus as you move to the corners of the field due to astigmatism and the curved focal plane. A solution that is sometimes suggested for astrophotography without a field flattener is to focus on a star that is 1/2 to 1/3 of the way from the center to the edge of the field. This is an awkward and less than satisfying solution however as stars in the center of the field will have less than perfect focus, while stars 1/2 to 1/3 of the way to the corner are in focus. Stars in the corner are improved, but still not in focus. The real solution is to use a field flattener. Stellarvue Field Flattener Stellarvue offers a zero power (0x) field flattener for $265, but this adds considerably to the price of the scope for astrophotography. Still, the combined price of $399 for the scope and $265 for the field flattener for a total of $664 is about 1/2 of what you would pay for a used Nikon 400mm f/5.6 ED IF manual focus lens which would provide a comparable field, aperture, focal ratio and focal length. And you can use the SV70ED for visual work as well as photography whereas the Nikon lens can only be used photographically because it does not have enough back focus for a diagonal. This evaluation was done with a Stellarvue field flattener on loan by Stellarvue. Hold your mouse cursor over the image below to see a comparison of the field with and without the Stellarvue zero-power field flattener.
In Comparison 2, we can see that stars in the corner of the field are greatly improved by the use of the Stellarvue field flattener, although they are still not perfectly in focus. Cheating the Focus Inward to Improve the Corners Because the field is still not completely flat with the field flattener, if we are willing to accept some degradation of the star images in the center of the field, we can improve the star images in the corner of the field by moving the focus of the camera inward. Below you can see images of stars in the center of the field and one of the corners enlarged 200 percent. This test was done with the SV70ED with the Stellarvue field flattener and no minus violet filter.
In Comparison 3, we can see that by cheating the focus inwards about 100 microns, or about 0.1mm, the spherical aberration becomes worse on the stars at the center of the field but the stars at the corner of the field do improve. Notice in particular the star in the lower right corner of the left hand frame in the mouse-over. As the focus moves inwards, we can see the core of the star stay about the same size, but the hazy spherical shell around it grows large. At the same time, the size of the stars in the corners get smaller, but a little bit of astigmatism begins to manifest itself. Astro-Tech Field Flattener Astro-Tech has just introduced a zero power field flattener for use with scopes in the f/6 to f/8 focal ratio range. I thought it would be interesting to compare the performance to the Stellarvue field flattener. The Astro-Tech field flattener costs $150. This evaluation was done with an Astro-Tech Field Flattener that was purchased directly from Astro-Tech. Hold your mouse cursor over the image below to see a comparison of the field with two different focus positions tested. The first shows star images with the camera focused in the center of the frame. The second is with the focus moved in about 0.2mm (200 microns).
By examining the stars in the corners of the frames in Comparison 3 to Comparison 4, we can see that performance is not nearly as good in the corners of the image taken with the Astro-Tech field flattener as those taken with the Stellarvue field flattener when the stars are focused in the center of the frame. In Comparison 4, we can see that moving the focus inward by 0.2mm produces similar performance as the SV field flattener in the center in terms of spherical aberration, and greatly improves the performance in the corners although some crosses are visible on the stars from astigmatism. Stars in the corner of the frame as still better with the Stellarvue field flattener compared to the Astro-Tech field flattener, even with the focus cheated inwards on both. This is not really a surprise since the Stellarvue field flattener was designed by Stellarvue to work with Stellarvue telescopes. William Optics 0.8x Field Flattener / Telecompressor William Optics offers a 0.8x combined field flattener / telecompressor. It reduces the focal length of the SV70ED from 420mm to 336mm and the focal ratio from f/6 to f/4.8. This gives a wider field of view and faster focal ratio. This evaluation was done with a William Optics 0.8x Field Flattener / Telecompressor that was purchased directly from William Optics. Hold your mouse cursor over the image below to see a comparison of the field with two different focus positions tested. The first shows star images with the camera focused in the center of the frame. The second is with the focus moved inward about 0.2mm (200 microns).
In Comparison 5, we can see that the William Optics field flattener / telecompressor does not flatten the field in the extreme corners of the frame very well when the image is focused on a star in the center of the frame. As with the Stellarvue and Astro-Tech zero-power field flatteners, performance in the corners is improved as the focus is moved inwards, at the cost of more spherical aberration in the center of the field. The performance of the William Optics 0.8x field flattener is not nearly as good in the corners of the field as either the Stellarvue or Astro-Tech field flatteners, but the William Optics unit is a combined reducer / field flattener. Conclusions
The Stellarvue 70ED Travelhawk is a beautifully made instrument that is a fine scope for visual use at low and medium magnifications. It is also a nice little instrument for astrophotography when combined with a field flattener or telecompressor. It is an excellent value for its price. Photographically, it has some blue/purple color fringing on bright stars that is recorded in images, but that is not that visible when the scope is used visually. A minus violet or an LPS filter with UV/IR cutoff greatly improves this color fringing for astrophotography.
As with many telescopes, the field of the SV70ED is not flat. This is not a problem visually. Photographically, stars in the corners are grossly out of focus unless a field flattener is used. In these photographic tests with an APS-sized DSLR, the Stellarvue field flattener worked the best and improved the size of the stars in the corners of the frame, although these stars still were not perfectly focused. In practical use in the real world, this will hardly be noticeable. The visual star test, and long-exposure astrophotography of star fields are two incredibly demanding tests of an optical system. They reveal every flaw, and very few telescopes are flawless. Camera lenses are even worse (see this test of Nikon's 400mm f/5.6 ED lens). Examining these tests at 200 percent magnification is also an extremely critical test. In the real world, you are hardly ever going to look at stars at even 100 percent magnification. The Bottom Line Considering the price of the scope, I would rate its photographic performance as a very good match for a beginner astrophotographer and I would recommend it as a great bargain. It must, however, be used with a field flattener for astrophotography. It is also a nice little carry-anywhere scope that can be used on a tripod for wide-field visual use. Ratings
Ratings are the highly subjective opinions of the reviewer. Notes All of the test images (except the North America Nebula) were taken directly from uncalibrated JPEG files shot in the camera. No post-processing was applied to them, only cropping and resizing for display here. The North America Nebula image is a stack of 42 two-minute exposures that were calibrated with dark and bias frames, and to which my normal image correction and enhancement techniques were applied. Images were focused on a bright star in the center of the field with the Live View focus feature on a Canon Digital Rebel XS (1000D). Focus was also tested inside and outside of the best visual focus on Live View and the best visual focus was, in fact, the best photographic focus for the star being focused on. |
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