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Despite these difficulties of portability I would really like to try a larger
refractor on the MkIV mounting. Just to see how well it copes. The
MkIV's limitations are its 40mm/ 1.25" shafts, plain [bronze shell] bearings and the modest 6"
wormwheels. The pier, thanks to its massive construction, is certainly
adequate to the task. But, equally, is its own Achilles heel. The sheer weight
makes it anything but portable. On a more open site it would probably be fine. Where
high hedges and lots of trees obscure much of my sky it has been a disaster! Not helped by our house standing on the southern border of our property.The MkIV could be further improved with a stepped diameter, over-sized, polar axis shaft. Even have the shafts set in taper roller bearings. Sadly my lathe cannot cope with such demands in stainless steel. I became quickly bored with fighting the constant battle with rust on the original shafts. Just moving a counterweight became a lengthy farce even with generous greasing at each fitting. The 6" ring style wormwheels could be replaced by a much larger pair of Beacon Hill's offerings. This would change the nature of the MkIV and its shaft locking system. Which works by forcing a nylon plug against the inside of the wormwheels.
I could house my 5" f/15 lens in an over-sized OTA just to get a feel for a larger tube. Though it would not be the same as the highly desirable, increased aperture which went with the bigger plumbing. It might tell me something about the mechanics of large OTA support but provide none of the rewards for all the effort involved.
It should never be forgotten that any mounting is a combination of its own qualities and its means of support. OTA length, as well as its weight, is always a critical factor. I started a project on a 16" Dobsonian decades ago. I had a nice figure on the glass at f/5 but the PVC tube was so large it would not fit into my limited storage space. Regrinding the mirror blank to make it shorter focus proved that faster mirrors are far more difficult to figure. Particularly when using plate glass blanks and working in a shed with rapidly changing temperatures!
No doubt the amateur astronomical world has long been populated by such overambitious white elephants. With weight and moment arm rising rapidly it is no wonder that larger refractors have never become commonplace. The equally rapidly rising cost of larger apertures does not quite match that of Apochromats but has always been a major factor. The optical glass is increasingly costly the moment any attempt is made to use anything non-standard. The sheer size of the OTA makes storage, handling and mounting a nightmare. Even if the long tube were made of cobwebs the heavy lens and focuser bring their own weight toll with rising aperture. You can't just throw a tarpaulin over a 10' long OTA on a very tall pier and forget about until the next time you want to use the telescope. It is no wonder such instruments are few and far between and most are permanently housed.
As always, the relatively low demand for something different is reflected in the small number of commercial providers. The economies of scale in manufacturing never quite provide cheap enough products to remotely compete with the mass produced reflector. Buyers will pay a fortune for a tiny APO but will baulk at the cost of a larger, classical achromat.
Those with a long enough memory, or a telescope history book, will know that only 50 years ago the reflector suffered from exactly the same problems. It was a heavy beast of limited aperture and very expensive to buy from only a few producers. Chinese manufacturing and the Dobsonian design changed all that. Rapid progress towards lighter and more portable, ever larger apertures, still continues in reflectors. Sadly the same cannot be generally said for achromatic doublet refractors.
D&G continues to produce well respected optics with a rather long turnaround times from placing an order. In a world of instant gratification, waiting a year or more, to receive that coveted lens or OTA is a serious problem. Fortunately their reputation still provides a loyal customer base.
D & G Optical Home
iStar
tries to break the mould by providing a whole range of achromatic and APO optics and OTAs up to quite
remarkable apertures. They have probably made more lenses since start-up than D&G have made in nearly three decades. iStar have tried even more exotic glass choices to bring
shorter focal lengths at fairly modest price premiums. Their Rx lenses cost
about the same as the next lens size down in their range of standard achromats. The
advantages are obvious to those who recognise them. One can have an 8"
or even larger refractor without needing the "standard" 12' long tube on
an 8' tall pier. Or, one can replace a historical refractor achromat and have far better colour correction. This has occurred where iStar replaced large lenses in observatory class instruments. iStar Optical Home
iStar's optician is working on even better
colour correction using different glass combinations. The company is claiming
up to 50% reduction in CA compared with a similar, standard achromat.
The lens behaves as if it were 50% longer in focal length as far as
colour correction is concerned. Though the standard had been set at
about 35% on the majority of their offerings until now. Which equates to
owning an f/16 in [only] an f/12 tube. Given the complete lack of
affordable, but still serious commercial mountings, the f/15 R35 wolf in f/12 sheep's
clothing is potentially a game changer.http://www.istar-optical.com/astro-images.html
jp-brahic's gallery | AstroBin
Unfortunately the Chromacorr type correctors have not enjoyed wide enough acceptance to make these devices affordable. Nor even mass produced at present. Their exotic glass and difficulty of set-up has made their field of view limitations too costly for most amateurs to contemplate. Once the cost gets beyond a certain point one might as well "invest" in a "real" APO. The price of APOs continues to fall as Chinese optical production continues to improve. Mass production, advanced machinery and economies of scale, make the exotic glasses ever more affordable in finished form. Not to mention being demanded in sufficiently large quantities to actually warrant a melt. The special glasses can also be used for many other, more popular products. Things like binoculars, camera lenses and spotting telescopes sell in vastly greater numbers than the odd astronomical APO. Lunt has recently introduced a 6" APO with a price tag of just $3000.
Click on any image for an enlargement.
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