by Jay Freeman
Astro-Physics shipped via United Parcel Service ground, and emailed me tracking numbers. The truck with the big parcel for the OTA showed up early enough on Monday, December 11, that I had not yet left for work. The UPS folks said they thought the other two packages were on the truck, but it was so full they couldn't find them. I asked to make the rest of the delivery "will-call", since the UPS depot is handy. The alternative was to deliver them later that day, but I was not going to be home.
The cardboard carton for the OTA was a bit larger than three feet long by a foot and a half square (90 by 30 by 30 cm). I could find no convenient location where a fork-lift truck tine had ripped into it, so I got a scissors and opened it myself. Closely fitted inside was a pale gray steamer-trunk style carrying case, with stout handles and four separate latches. The OTA lay within, wrapped in plastic film, protected by several inches of foam on each side.
It was gorgeous, with glistening white paint, close-fitting seams, and a style that finished edges and corners with nearly sharp edges (strictly, I mean small radii), not than rounded ones. There is always a problem getting something heavy out of a tight case, but Astro-Physics had left a couple of wide nylon straps, like the webbing on backpacks or handbags, wrapped around the tube with the ends loose on top, as an aid to pulling it free. I pulled, and was delighted how much lighter it was than my Celestron 14: I haven't weighed it yet, but I see no reason to doubt the specification of 33 pounds. I removed the plastic and slipped off the sturdy metal dust cap, to see whether shipping vibration had reduced the optics to powder. No, they were intact, though I did have to look hard to see that the corrector was installed, the low-reflection coatings were so good.
The bottom of the tube had the fittings I had ordered to use it with my Losmandy G-11. Astro-Physics had adapted a stock Losmandy dovetail plate to the OTA by specially machined adapter blocks, and all were in place. There were two brackets for detachable finders installed at roughly the 10:30 and 1:30 positions on the aft end of the tube -- one of the standard problems with Cassegrain configurations is that no matter where you put one finder, it is in the wrong place much of the time. Having two helps a lot.
I picked up the other two packages the next day. They included the rest of the OTA order and some separate items as well. There were only three more items in the main order; namely, the rather short dewcap, an interchangeable secondary obstruction which is larger than the aluminized secondary surface, to be used for correct baffling when viewing wide fields, and a few printed pages of hints how to use the instrument's features. The dewcap does not reverse for stowing on the main OTA, as with some other Astro-Physics products -- it would collide with the fittings that attach the stubby tube to the mounting. The extra items included a spare pair of machined adapter blocks, to fit on top of the tube, a MaxBright diagonal, and the "ring" portions of the two finder brackets I had ordered, which detach from the fittings on the tube when putting the instrument in its case.
There was also one item I had forgotten to order -- a power cord for the OTA's ventilation fans does not come with the OTA -- it turns out to be the same one as used with Astro-Physics mounts, and the company was giving people the opportunity not to pay for two of them, but I don't have an Astro-Physics mount, and I hadn't realized I needed to order a cord. The company courteously sent one with shipping upgraded to UPS air at no extra shipping charge, so I would get it in a hurry.
When I hefted an adapter block, I was pleased by how light it was. It was cut out and contoured to remove metal and save weight. Many such parts are made by die-casting or sand-casting, but the Astro-Physics units are milled.
I had plans for the extra adapter blocks. One worry with sleek, shiny OTAs is dropping them. The owner of a late-model Astro-Physics 155 mm EDFS told me he fears doing a watermelon-seed number with his OTA every time he lifts it, in which the tube squirts out of his frozen, trembling hands and crashes to the cold, hard ground, sending worthless shards and splinters of optical glass flying in all directions. (In this one regard, my 1987-model six-inch Astro-Physics refractor is superior to current models; it has a nice handle, mounted just below the focus knobs.) I had planned to use the upper adapter blocks as a place to install handles, but I didn't know exactly how they were going to work until I had the blocks in hand. It would be neat to have shiny brass handles, like door pulls, or perhaps folding or collapsing leather ones, like on fine luggage, but the blocks weren't quite big enough for handles long enough, and protrusions out from the box might damage the foam in the OTA case.
Therefore I ended up using simple rope handles, short loops of 1/4-inch polyester line tied under the adapter blocks, where there is about 3/8-inch clearance available. Each end of the telescope has two handles -- one each under the left and right side of the block -- paired like the handles on a shopping bag, so one hand at each block can grip both of its handles. I used the double handle arrangement for redundancy -- if one piece of line breaks, that end of the telescope will still be supported by the other. The loops are small -- when gripped, there is only an inch or two between my fingers and the top of the adapter block. The portion of each loop that goes between the block and the OTA passes through a length of 5/16-inch diameter clear vinyl tubing, to protect the line from chafing on the corners of the adapter blocks. With the handles installed, my level of nervousness when moving the slick, fragile, expensive OTA declined considerably.
I often fuss with commercial astronomical equipment, but there was little else to do to prepare the 10-inch Maksutov-Cassegrain for operation. I did paint the knobs of the 1/4-20 safety screws for the Losmandy dovetail plate day-glow orange. (These screws fit into the dovetail plate from the polar-axis side, as a last chance to block the telescope from falling off if the dovetail clamp gets loose.) You have to remove at least one to mount or dismount the telescope, and my rule is that small parts, that might get dropped at night, get painted a bright color.
Now I think it is time to describe some of the technology in the OTA. Much of this material is summarized from the Astro-Physics web site, or from various postings by Roland Christen here and there (and I am responsible for any errors), but some of it is based on my initial personal impressions of the equipment.
The optical configuration is a 10-inch Maksutov-Cassegrain, with the secondary mirror an aluminized spot on the convex inner surface of the corrector. The design is coma-free, with at least one aspheric surface, but I do not know which. This specific design is said to be insensitive to correction change with longitudinal displacement of the mirror along the optical axis, which is good, because focus is by moving the mirror. The focal ratio is 14.6 with the mirror at nominal position.
The primary mirror is quartz. The corrector is BK-7. Roland Christen has experimented with different means of reducing the time for a large telescope to come to thermal equilibrium, and considers a quartz primary to be an integral part of the solution Astro-Physics offers. He has stated that lack of equilibrium affects the telescope not only by distorting optical surfaces, but also by heating of air in the tube by the large thermal masses of the optical components. Thus choice of material requires a trade-off between coefficient of thermal expansion, heat capacity, and heat conductivity. The optical surface must not only deform minimally when not in thermal equilibrium, but also cool down quickly, so that tube currents and related effects are soon diminished.
The back of the primary mirror is shaped so the mirror is much thinner at the edge than at the center. The mirror is center-mounted, and slides on a central tube for focusing. The focusing mechanism is concentric with the mirror; the focus knob works it by a flexible belt. In contrast, in my Celestron 14, the focuser pushes and pulls on the mirror cell part way out to the edge. The belt design should be much less susceptible to mirror shift when focusing.
Mechanical means for quickly cooling the OTA are a mixture of plain and fancy. One simple trick, that will likely help enormously, is a removable dust cap at the *back* end of the tube. You loosen three knurled screws that stick out rearward from the OTA at the edges of the back, allowing the hat-box-lid shaped rear dust cap to slide aft a little way. After it has slid a few millimeters, its inner surface clears the thin collar that surrounds the focus knob, so the dust cap can start to rotate on its axis. The holes for the three screws are slotted, and one end is drilled out so the screw heads clear it. Thus when the outer dust cap has been rotated a few degrees, it slides aft and comes off. I first thought that procedure was too complex, but on second thought, you don't want the dust cap to come off unexpectedly.
With the dust cap off, there is still considerable structure protecting the mirror at its sides and back -- you don't need to worry much that something will whack the quartz -- but a great deal of the back of the mirror is exposed to circulation of air. Two muffin fans mount on the exposed structure, to force air across the back of the mirror. The fans, switch, and 12V power connector are not visible when the rear dust cap is in place. The gap between the outer periphery of the mirror and the inner surface of the tube is pretty well sealed -- it doesn't look as if there is any path for dust to get to the interior of the tube, other than by going up the baffle tube itself.
A massive threaded boss protrudes from the back of the OTA, through the rear dust cap. A reducing fitting provides a 2.00-inch focus tube, and a 2.00-inch to 1.25-inch bushing came with it. It looked as if the boss was planned to accept several other kinds of threaded fittings as well, but I don't know what they are.
The secondary spot diameter is 2.3 inches, proportionately much smaller than the obstructions in most commercial Schmidt-Cassegrains. Such a small spot should noticeably improve the telescope's performance when viewing low-contrast fine detail, compared to a larger obstruction.
A consequence of small obstruction is poor baffling: Sky light sneaks past the obstruction, down the baffle tube, to the field. The aluminized spot of this design is sufficient for a half-inch diameter field, and a removable, larger disc attaches to the center of the corrector to allow full baffling of a larger field. Wide fields are useful only at low magnifications, or for imaging, and in such circumstances the contrast improvement for fine detail is undetectable, so the larger baffle will not hurt when the telescope is so used. Several commercial Cassegrain-configuration telescopes have central obstructions much larger than the aluminized diameters of their secondaries, to provide more baffling. My Celestron 14 does, and so does the 3.5-inch Questar.
The rest of the baffling seems fairly conventional. The inside of the OTA seems not to have annular baffles or threading, rather, its surface is finished with some black matte finish that appears very dark, even at low angles of incidence and viewing. The inside of the dewcap is similarly finished. The sky end of the dewcap has a lip that rolls inward slightly -- the aperture at that end is narrower than the diameter of most of the length of the dewcap, which no doubt cuts down on grazing incidence reflections in the whole system. The OTA is two inches larger than the clear aperture, so that shielding of the tube walls by the periphery of the corrector cell likely has a similar effect. The central baffle tube, that protrudes up through the primary, seems to have the same kind of black matte finish inside and out, and has many fine slits in it. According to the telescope documentation, the slits aid in reducing the hot-air plume off the baffle tube.
December is an inauspicious time to receive a telescope in central California. I took delivery of my used 6-inch Astro-Physics refractor in December, 1998, and didn't get a chance to use it until the following April. So as waves of rain and cloud washed overhead, I settled down to wait and see when the sky would clear. I will pick things up there in the next part of this report.
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| Part I |
| Part II |
| Part III |
| Part IV |