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Astro-Physics 10-inch Maksutov-Cassegrain Report
Astro-Physics 10-inch Maksutov-Cassegrain Report
by Jay Reynolds Freeman ([email protected])
Photos Courtesy of Astro-Physics, Inc
Jay Freeman is an amateur astronomer of some forty years experience, who has done nearly all of his observing from sites in central California. He is well familiar with about thirty astronomical telescopes and binoculars.
On December 11, 2000, I exposed the central California coast to the prospect of rain, fire, deluge, conflagration, earthquake, plague, slug infestation, asteroid impact, invasion from zeta Reticuli, and premature entropy death of the universe, by taking delivery of an Astro-Physics 10-inch Maksutov-Cassegrain telescope. The approach of this instrument to official introduction has tantalized many amateur astronomers, and I hope my reports will be of interest. Yet high-end astronomy equipment is sometimes controversial, so I had better present such relevant humor as I have, before the shootin’ starts. Therefore I shall first offer...
Jay Freeman’s list of the ten best reasons to get a 10-inch Astro-Physics Maksutov, with footnotes. (Actually, the same reasons might suffice even if you got one that did not have footnotes...)
10) To settle a bet on whether the flagpole at Tranquillity Base is still standing.
9) White tubes require the least preparation before refinishing. (a)
8) It’s easier to spell “Maksutov” than “Apochromat”.
7) I needed a 10-gallon Wesson Oil container for leaky Christen triplets.
6) Who would have guessed the cure for refractorholism was worse than the disease?
5) Ordering provided a chance to ask Christine if she was the one Rich Neuschafer had named a telescope after.
4) The California water shortage requires heroic action.
3) At last, a telescope good enough to bear Refractor Red as finder. (
2) I really wanted a Stowaway and thought someone might trade.
1) Harvey needs a mate. ©
(a) E.g., my modified Brandon 94, “Juliette”, has been refinished pearlescent pink with gold trim. And has a
lacy garter to help keep the dust cap in place.
( “Refractor Red” is a 55 mm Vixen fluorite which has been refinished fluorescent red.
© “Harvey” is a white Celestron 14 who stands six feet, three and a half inches tall on his Losmandy G11, and who for a long time very few people had ever seen. Fans of Jimmy Stewart may recognize a certain similarity to a rabbit of the same name.
Seriously, you who know me might wonder whether I had any reason to possess such a telescope other than wanting to gloat -- not that wanting to gloat is necessarily a bad thing. After all, am I not the deep-sky weasel who keeps making comments about how “APERTURE WINS”? Yes I am, but there are a few other considerations.
First, our hobby is composed of many sub-hobbies, and most of us practice more than one. I certainly spend most of my amateur-astronomy time doing deep-sky work, yet I do occasionally look at solar-system objects -- that would be the Moon and the planets, for those of you who are also deep-sky observers. Sometimes I do so with pleasure, sometimes only to keep from going cold turkey on astro-photons during the weeks bracketing full Moon. If I am going to do that kind of work at all, I would like to equip myself to do it well, and though we seem to spend half eternity arguing about which telescopes are best, there is little doubt that Astro-Physics’s offerings are on most people’s short list of candidates for this kind of observing.
Second, there are non-solar-system objects for which high resolution is important. They include double stars and certain kinds of planetary nebulae.
Third -- and this point elaborates both of the first two -- where I observe, Harvey, my Celestron 14, is usually hit hard by atmospheric seeing. As I write these words, I have set the C-14 up nearly 100 times in the two and a half years since I recommissioned it, and only two of those nights have provided really first-rate seeing for a 14-inch aperture. Smaller telescopes at the same star parties have been hit less hard: High-end six- and seven-inch telescopes, such as the handful of large Astro-Physics refractors that are regulars, deliver most of their potential performance often. I was hoping that 10-inch aperture would offer a healthy resolution increase over these, which would be obtainable at the eyepiece much more often than is the performance that Harvey can deliver. (Harvey is a pretty good C-14, but not perfect. It wouldn’t surprise me if a well-figured 10-inch, with a smaller fraction of its aperture obscured by the secondary, offered higher absolute performance for seeing certain kinds of detail, yet this paragraph has more to do with sensitivity to seeing than with absolute performance.)
Fourth, Astro-Physics appeared to have been working quite hard to address some of the problems of cool-down time that often restrict the performance of high-end amateur telescopes; that also is a consideration in achieving high resolution, and one which has jaundiced my opinion of large Maks ever since a Questar 12 showed up at Riverside one year, and had almost cooled down enough to use by three in the morning, or so it was reported by those few die-hards who were still awake. I later encountered a 9- or 10-inch Mak from Intes or Intes Micro in the field, and it also exhibited severe sloth in cool-down.
Fifth, though the 10-inch Maksutov optical tube assembly weighs more than the optical tube assembly (OTA) of my 6-inch f/8 Astro-Physics refractor (not much more, considering I have equipped the 6-inch with a 15-pound counterweight near the eyepiece, to reduce the eyepiece height variation while observing), nevertheless, I expect that the smaller tube length of the 10-inch will make it quicker and easier to set up than the 6-inch. Thus it may well be not only a better telescope for the kinds of things I use the 6-inch for, but also an easier one to use. I may well end up substituting it for the 6-inch entirely, and find that I use it more often and with better results.
Sixth, the 10-inch Maksutov should work well with the rest of my observing paraphernalia. Its OTA is smaller and lighter than the OTA of my C-14, so I can certainly expect to use it on the same Losmandy G-11, and its eyepiece should be at least as convenient.
In a mid-2000 thread on sci.astro.amateur, some folks expressed concern about the processes whereby one gets onto an Astro-Physics notification list and then gets notified of an opportunity to buy. Although I own two other telescopes with Astro-Physics optics, I bought them used: The 10-inch Maksutov was my first purchase directly from the company, and I had no idea what to expect or what to do. Furthermore, I went through the “join list and get notified” process long before the sci.astro.amateur discussion. Thus it is probably worth reporting my experiences.
I first learned of the 10-inch Maksutov in mid 1998. I think I heard about it in postings by the proprietor of Astro-Physics, Roland Christen, on sci.astro.amateur, though perhaps my information came from some other part of the rumor mill. I wrote to Astro-Physics in November, 1998, and expressed interest in becoming a customer for one. Roland responded promptly, saying there was only a small, informal list of interested persons, and that he would put me on it. An opportunity to order might follow in six months. No money changed hands then, of course, for it was not by any means certain that Astro-Physics would produce the product, much less what its price was going to be.
It took lots longer than six months. Occasionally, during the next year or so, I sent mmail asking what was what. Someone always replied promptly. Twice I took extended vacations, and in each case I advised Astro-Physics, that in case an opportunity to buy came up while I was away, that yes, I still wanted one. I didn’t save the responses, but the general flavor was “thanks for telling us, not to worry.”
The opportunity to place an order came in January, 2000, a year and two months after I had put my name on the list. The Email arrived late on Friday, 21 January, after I had left for a weekend. I didn’t get it till Sunday, but the window to buy extended through Tuesday, so no problem. The Email advised that the first unit should be shipped in two or three months, and that others would shortly follow. The price was $9800 for a “bare” OTA, with a down payment of 50 percent requested then (in January). It was clear that bits and pieces, like tube rings and finder brackets, might have to be added to an order, later on.
I ordered without a second thought, even though $9800 is more than any three or four other OTAs that I have ever owned are worth, put together. To keep things in perspective, one could put an Astro-Physics 10-inch Maksutov and those three or four other OTAs in the Isuzu minivan that I bought to transport stuff for my hobby, and the van would still have cost more than all the telescopes inside it. Nobody who owns a late-model car should consider amateur astronomy an expensive hobby.
It was nearly a year till anything shipped. I sent occasional Email, mostly to make sure I had not missed something, and monitored sci.astro.amateur and the Astro-Physics users’ group for hints on what was happening. Roland was having diverse problems in getting things to come together -- subcontracted optics had to be re-worked, stuff came back from the coating company damaged or with inadequate coatings, and so on. I wasn’t particularly worried, even though I had no personal experience with Astro-Physics to go on; what I did have was a large number of friends and observing acquaintances who had dealt with Astro-Physics, who said that the company made fine products that were worth waiting for, and dealt with their customers with great fairness.It would take a lot of personal experience with any manufacturer before I could reach such a conclusion first-hand, but I had confidence in what my friends had to say.
I did not mind letting Astro-Physics have the use of $4900 of my money for the extra time. The problems Roland reported are the kind of thing that drives up price a lot; I suspect I would have paid a lot more, an increase of greater than a year’s interest on $4900, if the price had not been set until delivery time, or if I had waited for a later production run.
Anyway, I ended up getting a fine telescope, with many ingenious features and exquisite optical performance.
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.
Photos Courtesy of Astro-Physics, Inc
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 ¼-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 ¼-20 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.
Photos Courtesy of Astro-Physics, Inc
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.
My new Astro-Physics 10-inch Maksutov-Cassegrain had first light Saturday, 16 December, 2000, at Henry Coe State Park, not far southeast of Lick Observatory. It was a frustrating night -- seeing mostly so-so, transparency mostly poor, scattered to broken cloud intermittently crossing the sky. At no time that evening did I see anything resembling an Airy disc surrounded by diffraction rings, and I don’t believe anyone using any of the other telescopes present did, either. On the other hand, things could have been worse -- there were twenty thousand dollars worth of new stuff there, which should have invoked the curse of the new telescope in massive force. We were making jokes about using kayaks to paddle down the hill and get home.
I expect to use the AP-10 intensively during the next few weeks, weather permitting, and I will continue to report on its performance. Yet plenty of people are waiting to hear how it did, so I am not going to wait for razor-sharp seeing to post. In central California winter, that’s likely a long wait.
The OTA case fit sidewise in my van with a little wiggling, and made it all the way to the site without shucking around too much. I arrived while the Sun was still well up. That was part of the plan: I had foregone an opportunity to use the telescope the night before, because I wanted my first set-up to be when I could see what I was doing.
I configured my Losmandy G-11 with three 21-pound counterweights. I suspected I only needed two, but did not wish to shown wrong by having the OTA do a vertical reverse from being out of balance.
The key part of setup was dovetail alignment. Those of you who have read my postings about Harvey, my Celestron 14, may recall that I have spent moderate effort making special components, so I do not have to do a precision dovetail alignment of 52 pounds at shoulder height. The AP OTA was lighter and smaller, but was a dovetail alignment reasonable?
I set the latitude adjustment to 20 north, as low as I could get it without the counterweights hitting the tripod. With the dovetail clamp parallel to the polar axis, I would be sliding the dovetail plate up a 20 degree slope as I installed the OTA from the south. I could have gotten a shallower slope with the clamp pointed east and west, but at the expense of a rotating polar axis messing things up.
I cleared a spot on the floor of the van in case I needed to set the OTA down. I made sure the safety screw on the north end of the plate was out, then lifted the OTA by my new rope handles -- they worked great! -- and carried it around to the G-11. Installation was smooth and easy, and I was much relieved as I tightened the dovetail clamp and re-installed the safety screw. This telescope will not require any special hardware for me to set it up. (Your mileage may vary -- I do push-ups regularly.) Then I elevated the polar axis to the latitude.
I paused to remove the rear dust cover and let other people inspect the cell and focusing mechanism, then installed my new MaxBrite star diagonal, finder, and dew cap. Relative humidity was in the mid 40 percents, and temperature was 15 C, but I wrapped an anti-dew heater around the dew cap, just in case. Yet I did not need it that evening.
Both the front dust cap and the dewcap were tight fits, even with the latter’s setscrew backed way out -- I had to tug some to get them off. Too tight is better than too loose, but I have often wished there were a reasonable way to make the tightness of such items adjustable.
The telescope indeed required only two 21-pound counterweights, so the mounted weight was perhaps 35 pounds less than for my C-14. The longitudinal balance point of the tube was much further forward than for a Schmidt-Cassegrain, approximately half way from primary to corrector.
Thus variation in eyepiece height as the telescope looks at different parts of the sky is greater than for my C-14. With the telescoping legs on the Losmandy at full length, the eyepiece was convenient for most of the sky, but it required scooting way low in my observer’s chair to view the zenith, and using the finder then required much pretzeling of my spine. Perhaps I should put another finder on the front end of the tube, just for use in such circumstances.
The telescope had sat in the cool car most of the day, and I set up well before sunset, and it did not seem likely that the temperature was going to drop much, so I felt no need to use the muffin fans to cool the mirror, or even to leave the rear dust cap off for long. My guess about temperature was right -- it dipped only to 10 C during the evening.
I aligned my finder with a tree on a nearby hill. The Maksutov’s optics certainly worked, the belt-drive focuser felt smooth and solid, without a hint of backlash, and there was no image shift while focusing. Images appeared good all across the field, and indeed, I did not notice any off-axis aberrations of any kind the whole night long.
First light on a celestial object was a view of Jupiter at 309x (12 mm Brandon). I spotted the giant planet while the sky was still blue, and turned to it immediately, hoping that seeing would be very good at dusk. No luck, the image was turbulent. I could see several belts, and even some scalloping in them, but little detail. Other telescopes confirmed the problem was seeing, not turbulence in the tube. Saturn was similarly poor. I looked at Venus, whose half illuminated disc was spectacularly brilliant, utterly color free, and seemed to show a hint of shading variation across its surface, then decided to give up on planets for a while. But keep reading, I got back to them later.
I tried a few stars. There was no trace of an in-focus Airy disc, just swirling glare, but I could see enough out of focus to verify that collimation was okay, and what is more, what I could see looked the same inside and outside focus -- scarcely a critical measure of quality with so much turbulence, but encouraging nonetheless.
So I switched to a Vixen Lanthanum 8-24 mm zoom eyepiece to hunt bright deep-sky stuff. I logged over 70 objects during the evening, so I will only give highlights here. As I began, it still wasn’t quite dark, and I had the fun of looking at the ring nebula at 155x against a sky background that was distinctly pale blue. It was its usual oval shape, softer at the points of the oval than elsewhere. I could not detect the slightly ruddy periphery that I have occasionally seen with my C-14, even later, when the sky was dark. I ran magnification up with the zoom -- if the seeing had steadied I would have put in a better eyepiece and tried for the central star -- but had no luck on seeing.
For the next few hours, I chased sucker holes or peered through thin cloud at deep-sky objects, mostly using the zoom eyepiece at 155x (24 mm focal length), but occasionally with a higher magnification. M15 was notably higher up than the gas giants had been at sunset, and at 464x -- 8 mm on the zoom eyepiece -- it was resolved almost all the way across, with only a tight core that was merely granular. M2 was similar, though not as well resolved, perhaps because it was nearer the horizon.
H and chi Persei were pretty examples of open clusters, both blown more than wide open, showing numerous colored stars. The Auriga Messier open clusters were wide open and looked very different from one another.
M35 plus its smaller neighbors, NGC 2158 and IC 2157, provided an interesting contrast in size and compactness of open cluster.
Central Orion has plenty to test a telescope with. I looked at M42 and M43 several times, both with the zoom eyepiece and with my 40 mm Vernonscope Erfle, which gave 93x. M42 showed a pale green core around the trapezium, shading into dark reddish-purple tones in the “wings” of the nebula. There was not as much textured detail in the nebula as I have seen with better seeing, and a glance at the Trapezium showed why -- with Orion not far above the horizon, often, the edges of the images of its brightest four stars touched. The seeing was better at times, but even knowing where to look, I could not see stars E and F.
The late autumn sky also has some bright galaxies. I only had an unremarkable view of M74 and M77, through thin cloud, but M31 was pretty even through the same, showing dark lanes at 155x, and traceable out as far as star cloud NGC 206. M32 and M110 seemed to have different textures as well as different surface brightnesses. The view of M33 through cloud at 155x was not impressive -- all I could see was the central part of the galaxy, but a later view, at 93x through a break, showed the stretched-out S shape of its spiral arms unmistakably.
After I had looked at forty or fifty objects, I was beginning to be a little tired and thought it must be late. But no -- it was winter, and I had started observing at dusk. It was 8 PM. I took a coffee break and offered the telescope to some friends, who are experienced planetary observers, for a while. They promptly turned to Jupiter and Saturn and started playing with eyepieces. Seeing had improved a bit, and the spot formerly known as “Great Red” had come into view. They and a third observer, who had a late-model AP 155 EDFS set up ten meters away, took turns looking at the large planets. The consensus seemed to be that though the night was not first-rate, the AP-10 was showing more detail than both smaller telescopes and larger ones. (The larger ones were fast Dobson-mounted Newtonians, and I did not record whether the smaller one included the AP 155.) That is a very positive and strong statement. If true, it may indicate a combination of two things; first, that the telescope was of a useful size for the seeing that prevailed -- more aperture would have resulted in taking a much worse hit from poor seeing -- and second, that the telescope itself was not doing much on its own to ruin the image, that is, that its optics were very fine. All this is as I had hoped when I ordered it.
A wide sucker hole crossed Auriga and Taurus, so I tried a few objects there from my serious deep-sky list. I only looked at one faint galaxy, UGC 3374, but at 93x, I could see it. I also spotted the extremely faint planetary nebula, IC 2120 -- I could only suspect it at 93x, even with an Orion UltraBlock light-pollution filter, but at about 300x, with the zoom eyepiece, I could hold it without the filter. If the telescope can pull in these demanding objects, it can probably do most deep-sky work in my current C-14 program. I don’t think it is quite as good as the C-14 for deep-sky -- a factor of two in collecting area gives the larger instrument an enormous edge -- but Harvey’s coatings are twenty years old, and were never as high-tech as the AP-10’s, so the difference is closer than you might expect, and my current deep-sky program doesn’t usually push the C-14 to its limits.
I was still using the small central baffle, whose diameter matches the aluminized spot, but when I tried another deep-sky object in Auriga, diffuse nebula Sharpless 2-234, I found an annoying flare of light -- most likely from nearby Capella -- overlying part of the field, so I switched to the second, larger baffle, that came with the telescope. The baffles screw on to a threaded stud that protrudes skyward from the center of the corrector plate -- I did not look closely at how it attaches. Unfastening the small baffle was no problem, but installing the larger one was nerve-wracking. It was reluctant to thread on cleanly, and I certainly did not want to cross-thread it. I finally got it on, but it would have helped if there were some means to line it up squarely before engaging the threads. Perhaps the outer tip of the stud could be turned down so that it slipped inside the inner diameter of the female threads in the baffle, to guide the threaded portion into place? Anyhow, with the larger baffle in place, the flare went away, and I could see part of Sh 2-234 with no problem. The field of view was nice and dark -- I would say the baffling was excellent.
As the earth rotated, objects I had looked at before became better placed. I returned to Orion, and explored further. I had a nice view of NGC 2023 and 2024, and an unaesthetic but nonetheless convincing look at the Horsehead Nebula, all at 93x with no filter. Then I turned again to the Trapezium and put in my 8 mm Brandon. Seeing quality varied, and after some minutes waiting and tweaking the focus, I did have solid views of stars E and F. That is not much to cheer about -- I have seen six Trapezium stars with my 55 mm Vixen fluorite refractor -- but any bright star with a close, faint companion is a sensitive indicator of poor seeing or scattering in the optics, for the slightest smear of the bright star will wipe out the faint one.
Feeling optimistic, I tried Sirius, but no luck. I did see the Pup in 1999, with the C-14, and in seeing that was not perfect, so it is possible I could find it in the AP-10 on a good night.
I tried Jupiter and Saturn again, using the zoom eyepiece to pick the best magnification. They were just about transiting. I ended up with about 250x on Jupiter, but the Great Pink Spot was rotating out of view, and I am not a particularly experienced Jovian observer, so I am not sure what was special and what was not about what I saw. I did notice that the Galilean satellites were clearly small worlds of different angular sizes, though. And I switched to Saturn.
A little to my surprise, I found myself dialing the eyepiece to its shortest focal length, 8 mm, so I put in the 8 mm Brandon instead. At 464x, in moments of better seeing (but the seeing was far from perfect, even at best) the Cassini Division appeared crisply defined, changing abruptly from dark to bright at its boundaries, not the fuzzy-edged vague black smear of smaller apertures. The Crepe Ring was easy, like gauze. The B ring did not have constant brightness across its width. Within the A ring, the broad brightness minimum half way from its outer periphery to the outer edge of the Cassini Division, was easy, and I had occasional glimpses of a narrower dark feature between that minimum and the outer edge of the A ring, rather closer to the outer edge. Both these A-ring features have had the name “Encke” applied to them, but the nomenclature of the details of Saturn’s rings is so confusing I have given up using it. I looked for but did not see “spokes” in the rings.
On the disc, the dark band in the north temperate zone appeared narrower than usual, and even at 464x it showed a pronounced brown color. I also had a sense that the visible polar area of the disc was darker than most of the rest of it, though not as dark as the band.
The view of Saturn was startlingly good considering the so-so seeing, and also considering that I was still using the large, not-quite one-third-diameter, secondary baffle. I have had views approximately as good in my 1987 six-inch Astro-Physics f/8 triplet refractor, and in my C-14, but only on nights of vastly better seeing, for both telescopes.
I took the telescope down at Moon rise -- thickening cloud precluded doing anything else while waiting for Luna to get high enough to be a good target. Everything came apart as easily as it had gone together, and shortly the Mak-Cassegrain was stowed in my van for the ride home.
A fair summary of the night’s experience would be the following:
1) The telescope is well-designed, and easy to set up and use.
2) I saw no indication whatsoever of thermal problems, albeit on a night that was not very demanding in this regard. Nevertheless, the absence of such problems on a pretty good (thermally!) night, without using the muffin fans or leaving the back dewcap off, suggests that I will not have much to worry about from thermal difficulties in the future.
3) The optics are at least “very good”, and the night was not good enough to distinguish between “very good” and “excellent”.
4) The telescope was delivering a whole lot of resolution of low-contrast fine planetary detail on a night of ratty and intermittent seeing, more than many other telescopes of both greater and smaller sizes.
5) Between excellent baffling and high-technology coatings, the telescope offers better deep-sky performance than a typical 10-inch Newtonian or 10-inch Schmidt-Cassegrain. It is certainly capable of deep-sky work that will satisfy a certificated flaming whacko of a deep-sky weasel, such as I.
6) More detailed optical testing and performance evaluation will require a night of better seeing. I will report when that happens. Meanwhile, don’t hold your breath waiting.
All well and good, but what a lot of you probably want to know is, is it worth ten thousand dollars? There are two parts to the answer.
First, no telescope is worth ten thousand dollars if you don’t have ten thousand dollars to spend on a telescope, and many do not. Persons with a lower budget limit, or whose concern is performance per dollar, should not consider this telescope.
Second, for you who do have ten thousand dollars for a telescope, I can say with some confidence that the AP-10 is a viable choice. I did not order it on a whim, or out of pocket money. I was ready to spend a lot on new equipment, and I thought much about large Dobsons, possibly with tracking, as well as about collections of fancy accessories. Yet I ordered an AP-10. The one night I have had it out demonstrates that it almost certainly delivers more resolution on low-contrast detail than any other telescope I own, and that it provides enough capability for much of the deep-sky work I like do, all in a package that is easy to set up and convenient to use. That is a combination worth having. You should all hope that Astro-Physics produces some more of these, because I doubt I am going to sell you mine.
There is one more thing. I never have gone out of my way to name my telescopes, but many of them have nevertheless told me what their names are, or ought to be, and so it is with this one. I am not absolutely sure, but I think she is “Gillian”. Persons familiar with magical co-stars of Jimmy Stewart may be able to figure out why.
I will make this report part a running log of AP-10 experiences, with emphasis on things others might want to know.
December 20-21, 2000:
I observed at a close-in hilltop site above Palo Alto, all alone. Sound or motion in the brush became a sneaking mountain lion, and stillness meant it was poised to pounce. Yet the stars scarcely twinkled, so I hollared “Supper time!!” to any predators present, and set up. Seeing was better than during my first-light session.
Near the zenith, I could see Airy discs of stars often, though the rings were a blur, never even partially defined. It was easy to see six stars in the Trapezium, even at only 155x. The image of Sirius was better than on first-light evening, but I still couldn’t split it.
Better seeing made accurate focusing easier. When the seeing was momentarily at its best, there was a “snap” to focus; that is, there was a well-defined tiny region of focuser travel in which the image was sharpest. That is a very good sign about the optics’ quality.
At 309x (12 mm Brandon), Saturn showed disc detail new to me. The broad brownish belt in the south temperate zone was accompanied by a narrow equatorial one. The south half or third of the southern hemisphere was slightly darker than the rest, and more neutral in hue, not brown. The colors and subtlety of disc shading reminded me of a young Siamese cat, one whose “points” have not developed full intensity.
The Crepe Ring and Cassini Division were well defined, and the inner part of ring B had a slightly warmer color than the outer part. The broad minimum in the middle of the A ring was again present, but I did not see the narrower one further out, nor any spokes.
I looked at Jupiter but have no special report. I spent a while chasing faint galaxies, near the _Millennium_Star_Atlas_ chart limit, east in azimuth from Polaris, in Cepheus and Camelopardalis. The sky suffered from light pollution, yet galactic centers are generally relatively high in surface brightness, so I could add magnification to darken the background sky and increase perceived contrast. At 155x (1.64mm exit pupil), I found all the galaxies on my list, some twenty.
The AP-10 can indeed do deep-sky work that I call serious.
There was intermittent wind when I was looking at the planets, and the AP-10 was not in the lee of my car. The G11 got jiggly enough at 309x to interfere with seeing details. Yet the wind disturbed the seeing, so details went away, hence the mount was really not a problem.
December 22-23, 2000:
I set up at a parking area near a reservoir on the west side of California’s Great Valley. Intermittent high haze and cloud crossed the sky, but for part of the evening, seeing was good enough near the zenith that stellar images showed a solid Airy disc and a complete, but rippling, first diffraction ring, at all times. Those conditions permitted a convincing star test, made at 464x using a Vixen 8-24 mm zoom eyepiece, with the small central baffle in place, so the central obstruction diameter was only 2.3 inches. The test took place late, after the instrument had been set up several hours. I had not noticed thermal effects earlier, and the evening was not particularly challenging thermally -- the telescope had been stored at 15 C, and ambient temperatures were 9 or 10 C -- I just didn’t get around to a star test for a while.
First, the first diffraction ring was a great deal fainter, compared to the Airy disc, than with telescopes with one-third diameter central obstructions, like most f/10 or f/11 Schmidt-Cassegrains, and like my Intes 6-inch Maksutov-Cassegrain. Furthermore, I could not even see a second diffraction ring of this fifth or so magnitude star, though perhaps I might have if the image had been completely steady. The faintness of the rings not only reflects the AP-10’s smaller central obstruction, but also indicates that the optics are well-enough figured to exploit the advantage of the small obstruction.
Second, I compared out-of-focus diffraction patterns far enough out of focus that what was showing was a bright central spot surrounded by three bright rings. The rings were in motion, just as they had been in focus, but were complete, and as far as I could tell, considering the motion, the patterns were identical inside and outside of focus. I am sure I could do a better star test in more nearly perfect seeing, but what I saw during this one leaves little doubt that the optics of this particular AP-10 are excellent.
Jupiter and Saturn were well-placed, and I spent considerable time looking at them, and also showing them to the five or six other telescopists who were present. Jupiter showed rich belt detail, and provided a fascinating opportunity to watch an Io transit. The moon and its shadow crossed the face of the gas giant, giving views much like some of the Hubble shots that have recently been released. Someone remarked that he had been wondering if NASA had correctly balanced the colors in the released images, and we could verify that they had. The contrast in color between Io and Jupiter was striking; Io looked like a pale gold Christmas ball suspended in front of the ruddy and creamy colors of the gas giant. Even if I had not known about Io’s composition, I might have used the term “sulfur-yellow” to describe the hue. The color contrast was particularly striking just as the moon was about to begin its egress from the transit, when it was seen against the limb-darkened edge of the Jovian disc.
Saturn also showed pleasant colors, though subtler ones. The details were essentially as I saw them on December 20-21, though more steadily seen, as the seeing was much better. Several people present said that they had never had a better view of Saturn in any telescope.
The view certainly surpassed any that I myself had had, and its competitors include my own 1987 6-inch Astro-Physics triplet refractor in excellent seeing, and the 36-inch doublet at Lick Observatory in seeing that was notably less than perfect for it.
I looked at several double stars as well. Sirius was low enough that the seeing for it was considerably worse, though I did see the central Airy disc at times. I thought I caught a few glimpses of the Pup, as well, using an 8 mm Brandon for 464x, but the position angle did not check with an ephemeris for Sirius B, so I guess I did not.
I should have looked at gamma Andromedae earlier, but did not think of it till it was well down from the zenith and affected by declining seeing. The split between gamma-one and gamma-two was of course easy -- this bright yellow and blue pair is resolvable in many binoculars. I ran the magnification up to 742x (5 mm Pentax SMC-ED orthoscopic) to get a careful look at gamma-two, but although its Airy disc was clearly elongated, seeing did not let me check for any necking down or separation of this close, unequal double. Gamma-two Andromedae has been closing lately, it might not be within range of ten inches aperture now.
The good baffling and low scattering of the AP-10 appeared to make the colors of double stars particularly pleasant. Stars like gamma Andromedae, h3945, eta Cassiopeiae, and beta Cygni provided good examples of contrasting hues. I also used the instrument for numerous deep-sky targets, but have nothing to add to the initial impression I reported, that it is no slouch for this kind of work, though not quite the equal of my Celestron 14.
How The Grinch Went Cosmic:
I took the AP-10 out on Christmas eve, on the evening of Christmas day, and on the evening of the day after Christmas. Seeing was soft on all three occasions, so I did not get to try any more high-resolution tests. Rather, I used the telescope for deep-sky work of the kind that makes up most of my observing, and gathered more confirmation of my initial impression that it is not as good as my C-14 for such purposes, but closer than you would expect from mere consideration of clear aperture. The dark, well-baffled field, with crisp images from edge to edge, helped a lot with chasing and confirming faint fuzzies. There is no doubt that a deep-sky enthusiast could spend a long time with this instrument before running out of interesting things to look at.
I could see all five galaxies in Stephen’s Quintet at 155x, and could also see the four close companions of NGC 7331; namely, NGC 7335, 7336, 7337, and 7340, though the views were not quite as good as I remember from the C-14, even though the sky was dark and transparent when I turned the AP-10 on them. (I am not likely to be able to run a side-by-side comparison of the C-14 and the AP-10, since I only have one mount that can hold a telescope this large, so memory will have to do; however, the objects just mentioned are among my favorites, and I have looked at them many times with the C-14, usually at very nearly the same exit pupil.)
I also continue to be impressed with how much easier to use the AP-10 is than the C-14. The mounted weight of the former is only about 25 percent less than that of the latter, but the G-11 behaves much better with the smaller mass, it is lots easier to slew and set by hand.
Set up and take down are both also faster, and with the AP-10 in my van instead of the C-14, I have the front passenger seat available for a guest, if need be. (That’s where the C-14 OTA normally rides, but the AP-10 fits behind the front seats, crosswise, in its case.)
December 27-28, 2000: Slumming. I did not take the AP-10 out, but spent a while with my other 10-inch telescope, an f/5 Dobson with Nova optics that I built for airline transport to places like Hawaii. I tried a handful of deep-sky targets that I had recently looked at with the AP-10, at similar magnifications (106x in the Dobson, 93x in the AP-10), and insofar as memory permits, the images in the AP-10 were a hair brighter. That is to be expected considering the Mak-Cass’s high-tech coatings (the Dobson has plain aluminum on the primary but an enhanced coating on the secondary) and small central obstruction, and again confirms my impression that the AP-10 does somewhat better for its aperture than many 10-inch instruments that amateurs use.
This is probably the end of the initial series of my reports on the AP-10, though I will certainly write up any more spectacular sights that come with exceptional seeing. The conclusions I drew at the close of part III still stand, except with a little more confidence based on the additional experience. I shall restate them here:
1) The telescope is well-designed, and easy to set up and use.
2) I have seen no indication whatsoever of thermal problems, albeit none of the nights I have had the instrument out have been very challenging thermally. Nevertheless, the lack of such problems without using the muffin fans or leaving the back dewcap off, suggests that I will not have much to worry about from thermal difficulties in the future.
3) The optics are excellent, well baffled, and easy to focus.
4) The telescope delivers a whole lot of low-contrast fine planetary detail, even when seeing is so-so, and the more so when seeing is good. It regularly does better than many other telescopes of both greater and smaller sizes.
5) Between excellent baffling and high-technology coatings, the telescope offers better deep-sky performance than a typical 10-inch Newtonian or 10-inch Schmidt-Cassegrain. It is certainly capable of deep-sky work that will satisfy a beyond-the-fringe raving lunatic of a deep-sky weasel, such as I.
6) More detailed optical testing and performance evaluation will
require a night of near-perfect seeing. I will report when that happens, but don’t hold your breath waiting.
For persons with ten thousand dollars to spend on an OTA, an Astro-Physics 10-inch Maksutov-Cassegrain is certainly
one to conside