A WORD OF CAUTION: I have noted some [other manufacturer's] optical manifolds on the market. One implies (by using deceptive wording) that you can achieve pin-point focus using an eyepiece. This is untrue! The human eye can't "see" critical focus for astroimaging (see ASTROIMAGING link for more details), especially with an astigmatism, which afflicts half the people in the world. This has been confirmed by conversations with Dr. Dale Anderson, MD, PC, who is an ophthalmologist and noted eye surgeon. Minimum pixel count, etc. is the easiest [and accepted] way to achieve accurate focus with a CCD camera, but there is no quick and easy [lazy] way to achieve critical focus when doing film [35mm] astrophotography, period! You must use a knife-edge or Ronchi grating type focusing device. Otherwise, all your film astroimaging efforts will be soft, fuzzy, or indistinct (unless you get lucky from time to time), instead of masterpieces worthy of publication. Of course, with computer controlled autofocus and modern CCD cameras, who uses this dying [or dead] film format anyway.
ANOTHER POINT OF CAUTION: VSI has always used first-surface mirrors in all their optical manifold devices - flip-mirrors, slide-mirrors and pick-off mirrors since day one. Why? Because they offer the most reflectivity, highest resolution, brightest and sharpest image possible, compared to prism-type diagonals. Before first-surface mirror flats were offered in ultra-high [1/10th wavelength or better] surface accuracies, prisms were extensively used in diagonals because they were easily mass-produced for binocular image-erecting applications and they were very cheap to simply "drop-in" a diagonal. Many decades ago, because of [hype] advertising touting prisms over mirrors, people thought prisms were better than mirrors, and many of them probably were back then, but not anymore! Old obsolete misinformation. Think about it. Prisms have three surfaces, instead of one like a first-surface mirror - 1) entering the glass prism, 2) reflecting off the hypotenuse of the prism (which should be silvered for maximum reflectivity, but usually isn't), and 3) exiting the prism. That's three optical surfaces your image has to traverse, compared to only one with a mirror. Image degradation is three times that of a mirror. During this intrinsic excursion, your image is traveling through pure glass, which reduced image brightness, simply because of the microscopic impurities in the glass itself, and the much higher density of the medium compared to "nothing at all." And, there are other disadvantages to a prism system. It's transparent so you can't distinctly see any incurred vignetting by the pick-off prism, just a fuzzy blop of blurred stars - Is that a galactic lens effect or........? With an opaque first-surface mirror, you [at least] know if your pick-off mirror is vignetting your imaging/film plane. It's obvious and, if need be, you can correct for the obstruction, instead of wondering if you need to call Brian Marsden.
AND YET, ANOTHER NOTE OF CAUTION: I have always been appalled at the industry standard manufacturing processes used by all the other companies that offer telescopes and especially imaging equipment. These industry standard techniques, that do apply to all the other terrestrial products in the world, can not possibly apply to any aspect of telescope or associated equipment manufacturing, but this earthly construction philosophy continues, and the astronomical community continues to purchase inadequate equipment that makes celestial observing and astroimaging a frustrating task, instead of a satisfying pleasure. And the quoted hype from so-called "people-in-the-know" (editors, well-known astroimagers, etc.) proclaiming these "wimp products" to be worthy of your hard-earned dollars, is ludicrous at best. I know of very few telescope products on the market today that construct their structurally supporting walls [boxes, tubes, etc.] from larger than 16 gage (or even 20 gage sheet aluminum) or the thinest of pot-metal castings - again, standard manufacturing techniques. With any extended, leveraged imaging train connected to the rear of one of these devices, you have to encounter a minimum of many arc minutes of metal flexure. This is not tolerable in astroimaging! From the beginning, all VSI flat-wall construction has been 1/4-inch plate aluminum or better (optical manifolds, etc.). Most VSI circular-wall construction is 3/8-inch wall thickness or better (focusers, turrets, rotators, etc.). VSI's massive construction policy is not overkill. On the contrary, it is only adequate for astronomical pursuits. It is an "astronomical standard" that needs to be adopted by all telescope and associated astronomy product manufacturers.
WARNING: Be aware that all [so-called] Crayford focusers, are not really Crayford focusers. Deceptive advertising by many dealers, trying to take advantage of the Crayford's reputation, offer Crayford-type, Crayford-like or Crayford-style focusers that have very little to do with the true Crayford design. NO radial ball bearings, NO zero-flexure, NO fluid motion, NO ultra-tactile sensitive control, NONE of the above functions that made the Crayford the finest precision focuser on Planet Earth. These non-Crayford focusers typically use plastic pads to simulate Crayford bearings and other inferior construction techniques that deminish the Crayford name. Only a focuser that uses four precision radial ball bearings can [rightfully] be called a Crayford focuser. - PBVS