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Photo by John Hopper................. ...........................................................Photo courtesy of Sky & Telescope magazine................................................
The ARIES Chromacor is a revolutionary new product released in production quantities to the amateur astronomy community in 2001. It greatly improves the color correction of many achromatic refracting telescopes. It is specifically designed for 120mm f/8.3 and 150mm f/8 Chinese refractors sold under brand names such as Synta, Celestron, Skywatcher, Bresser, Hoon, and others. It also performs extremely well with the smaller 102mm f/9.8 achromats.
Customers have found success with it on a variety of other achromats, including 6" f/10 Jaegers, 215mm f/12 Chinese, 8" f/15 folded, and others. It's sure to be a killer setup on the new Meade 5" f/9.3 achromat as well.
Because the Chromacor costs more than the 150mm refractor optical tube assembly, its cost-effectiveness for the smaller, extremely inexpensive scopes is generally perceived to be lower than for the 150mm f/8 scopes.. However, the performance level of the 120mm scope with Chromacor is spectacular. I own both sizes myself, and some other 120mm scope owners feel no need to move up to the 150mm scope despite their investment in a Chromacor suitable for either scope. The 120mm scope is obviously much more manageable on light mounts, and an over-mounted 120mm has some advantages over an under-mounted 150mm. The Chinese optical tube assemblies are so inexpensive that owning both is well within the means of most Chromacor customers.
The ARIES Chromacor-II is a higher-end version of Chromacor, with even more expensively fine-tuned glass. For the additional 50% on the price tag, it removes approximately 2/3 of the remaining (slight) color left by Chromacor. The results must be seen to be believed. According to Valery, here is the performance:
1. 6" F/8 colors like in 102mm F/10 + Chromacor-I + 20mm extention tube. In focus not one object show any color be this Sirius or Vega or Venus.
2. 120mm F/8.3 + Chromacor-II. No colors on any objects. Very little out of focus colors.
3. (Note from JH: I'm 99% sure he's talking about the 102mm f/9.8 here) No colors on any object in or out of focus. The best possible color correction in the world today. Better than in Zeiss 100mm F/10 APQ! The ONLY APO which corresponds to Abbe's "definition of APOCHROMAT".
On the other hand, the original Chromacor gets chromatic aberration down to excellent levels itself, and individual customers must debate whether the quality of their individual achromatic refractor makes the extra investment of Chromacor-II worthwhile. On a 150mm scope with no significant problems, the answer is generally yes. On the smaller scopes, or 150mm scopes with zones, turned edges, serious astigmatism, mechanical problems, etc. the answer is often no. Getting rid of the color still works, but being left with a scope with poor or average performance after putting an extra $900 vs. $600 into it, helps but is of questionable dollar value. Buying a Chromacor-II for a bad OTA would make it that much more important to later find a better OTA with the same correction level to match your Chromacor-II. Looking for a Chinese OTA with a specific correction level can be frustrating...it might take 1 try or 5 tries, or more. Please find a good OTA first!
My advice is to find the best OTA you can, new or used. Try to get one that's trouble-free, excepting that some spherical aberration is OK. Then buy a Chromacor or Chromacor-II to match it. Not the reverse! Don't even think that buying a Chromacor-N followed by a "premium brand" achromat will guarantee a match. It won't. For example, the TMB achromats are said to be of better quality than Synta, and I expect this normally to be true. But wait for the individual scope's test results before assuming it's not slightly over or undercorrected. They're not all perfectly null-corrected, any more than the Syntas are, in my experience.
I've heard a number of sad stories from buyers of used Chromacors and new/used scopes (usually separately, and usually at least one on Astromart) who had a hard time after realizing one or more of the following: 1. The scope doesn't match the Chromacor. 2. The scope is bad. 3. When trying to replace a bad OTA, they have to lie about it at least as much as the person they bought it from, or take a loss. 4. It is hard to find a trouble-free OTA where the seller really knows the correction level. (If both of those things were true, they'd get a Chromacor and keep the whole setup.) 5. I don't routinely exchange Chromacors under any circumstances except for true/rare warranty issues, a mistake on my part, a rental unit, or by special prearrangement with a good customer or reviewer. And I certainly don't take trades or exchanges from anyone who didn't buy theirs from me originally/recently, or who switched scopes, or whose scope is having a seasonal variation from temperature, etc. 6. No, I don't want to buy any used Chromacors or used scopes, thank you.
For many years, optical designers have known that by plugging unrealistic glass properties of unavailable "phantom glasses" into their computer programs, causing near-unthinkable optical results, some extraordinary optical designs were possible.
Among these are designs which "magically" correct an achromatic telescope to apochromatic performance. Chromacor is the first example, to my knowledge, of a true "phantom glass" (not just an incremental improvement or a move to a previously known, higher-priced material) actually being produced in quantity for use in a consumer product. Surely there have been other examples of "phantom glass" with one strange property or another having been made in a laboratory and added to a catalogue of known materials. Some of those have probably found their way into spy satellites or other exotic applications, and others are perhaps just curiosities.
But Chromacor is now available for you to own, complete with "phantom glass" which caused the naysayers to confidently claim that this device would never work. Others agreed that it was physically possible, but knew it would be difficult and expensive to accomplish, and might not make economic sense unless inexpensive and optically excellent achromatic refractors were available on a continuing basis.
The Chromacor screws into your diagonal just like a standard 2" eyepiece filter.
What could be simpler?
Actually, it is recommended that you follow a fairly long but worthwhile collimation procedure when initially installing Chromacor. Part of the procedure should ideally be done before even ordering a Chromacor, to let you see if your scope is good, bad, or good but just miscollimated.
Link to Chromacor collimation procedure
You'll notice my own TV Everbrite diagonal with Vixen Lanthanum LV 6mm eyepiece above, with the eyepiece in the position I found gave the optimal color correction in a 150mm f/8 scope. The eyepiece is extended out of the 2-1.25" adapter slightly. If you're unlucky, you might even wind up with the eyepiece setscrew on its crazy recessed grooves. To avoid this, and allow the eyepiece be fully seated, add one or two 48mm spacer rings (from camera lens filters with the glass removed) between the Chromacor and diagonal. Astrobuffet provides 3 of these rings with the Chromacor, with additional ones available if your specific diagonal and eyepiece require different spacing. On my 120mm Skywatcher, I need either one ring or none, to fully seat most eyepieces. It all depends upon the eyepiece and diagonal, but I've not yet personally run across a situation where optimum correction required a special diagonal. Low-profile or recessed 2-1.25" adapters can be used if needed.
A word about eyepieces...
When used with Chromacor, the simpler the eyepiece, the better. Chromacor does not work as well with Barlows or self-Barlowing, complex eyepieces as it does with simple eyepieces which "look at" an unadulterated focal plane. The very best eyepiece I've found for use with Chromacor and Chromacor-II in fact, is a 10mm fully multicoated 51-degree Plossl which I import under the 1rpd brand name. Most people would claim to see more detail at above 120x in a 150mm scope, but if you do the math, it's already "enough power" to obtain ultimate resolution if you have 20/20 vision. That's 60 arcsec naked-eye / 120 = 0.5 arcsec potential eye resolution at 120x, and a 6" scope doesn't resolve 0.5 arcsec, so there you have it. But if you have worse than 20/25 vision, you can indeed see more detail at higher power, and even with 20/20 vision higher power makes it easier to see the same detail you'd be straining for at 120x.
I have these 1rpd P51 fully multicoated Plossls available in 4mm, 10mm, and 15mm focal lengths. They are $40 each, plus $8 per order (any number of Plossls) for shipping/insurance in 48 states. I can't wait until I get the 6.5mm length in FMC coatings also, but my first shipment of them was in fully coated rather than fully multicoated, so the 6.5mm is only $26, as are the 12.5mm, 20mm, and 25mm. The 30mm and 40mm (this one is under 51 degrees, but still called P51) are $32 each.
With Chromacor (or SAFIX) purchase, shipping is $1 additional per Plossl if by air, free if by ground.
Obviously a 4mm Plossl doesn't have much eye relief, and it has a tiny eye lens. Hence a good compromise is to use the Vixen LV series, which I don't sell, but they run a bit under $100 each and work well. When used with Chromacor, Ed Ting and I found that they outperform the more complex (not to mention very expensive) TV Radians, but fall short of Plossl performance. So there's a tradeoff between eye relief and absolute performance in the short focal length eyepieces combined with Chromacor. I keep forgetting to try my Brandons with Chromacor, but they should probably be pretty good too, I'd guess.
ARIES admittedly does the "heavy lifting" by producing this breakthrough product in the Ukraine, but Astrobuffet does add significant value to the transaction beyond simply drop-shipping or receiving and forwarding the products to our customers.
Here is what you get from Astrobuffet, with each Chromacor purchased:
You may buy any or all of the below accessories, or none if you prefer. I recommended the following $25 option package with each Chromacor purchased, in most cases:
Fixed-priced air shipping of $15 in the 50 states:
Options other than the normal package
International delivery available. Insured USPS Express Mail (EMS 2-3 day delivery) is typically $35 to the UK and most of Europe; $30 to Spain; $40 to Japan. USPS Global Express Guaranteed (which uses DHL) and UPS are also available at extra cost.
An APM Self-Centering Adapter can be added to the Astrobuffet package for an additional $70. This is a collet device which screws directly onto the Chinese focuser, replacing the setscrew diagonal-holding ring. Available separately for $80 plus shipping.
For the optional, recommended custom Ronchi grating mounted in 1.25" barrel, please add $75 with the purchase of a SAFIX or Chromacor, with no additional USA shipping; $1 additional international. The grating also available separately for $80 delivered in 50 states.
Additional (empty camera filter) spacer rings are available for purchase: $5 each, no additional shipping if bought with Chromacor. Separately, $5 each to prior Astrobuffet Chromacor customers, plus $2 shipping/handling for up to 6 in the 50 states via regular mail in a bubble or diskette envelope. In the future, I expect to also sell machined extension tubes of various lengths and materials.
The greatest weakness of achromatic refracting telescopes is "color" or more technically, secondary spectrum. This means that the focal length of the telescope objective varies for different wavelengths of light.
Apochromatic (APO) refractors eliminate this problem but cost a great deal more money than achromatic refractors.
The main design concept of the Chromacor was to reduce secondary spectrum throughout the visible wavelengths of light, to a level directly comparable to or better than many popular, true APO scopes...and far better than all ED semi-APO scopes claiming APO performance.
The results are in: Chromacor has proven the naysayers wrong, and exceeded the expectations of the amateur astronomy community in accomplishing this primary goal.
Below are technical diagrams and graphs showing Chromacor performance.
For a HIGHER-RESOLUTION view of a diagram, click on it; then return using your browser's "back" arrow key.
120mm f/8.3 achromat without Chromacor....120mm f/8.3 achromat with Chromacor
120mm f/8.3 achromat without Chromacor....120mm f/8.3 achromat with Chromacor
120mm f/8.3 achromat without Chromacor....120mm f/8.3 achromat with Chromacor
120mm f/8.3 achromat without Chromacor....120mm f/8.3 achromat with Chromacor
It should be noted that Chromacor is designed and recommended for visual use. For photographic and CCD work, true APOs still hold a number of significant advantages, including a flatter secondary spectrum into the ultraviolet, much wider usable field of view, compatibility with field flatteners, and so forth. Obviously there is nothing stopping a Chromacor owner from taking narrow-field photos which will be much better with a Chromacor than without, but this is an unsupported use of Chromacor, and should not be undertaken with expectations of being competitive with APOs optimized for photography.
Chromacor is available in 5 different levels of customized spherical aberration correction. Correctly testing and matching the Chromacor to an individual telescope's spherical aberration can thereby remove objectionable levels of undercorrection or overcorrection. The Chromacor can be ordered in two versions which add approximately 1/6-1/7 or 1/3-2/7 wave of undercorrection to an overcorrected scope, two versions which add that same amount of overcorrection to an undercorrected scope, or a neutral version which leaves spherical correction unaffected in an already-superb scope.
Hence, achromat owners can typically test their own scope and order a matched Chromacor to get spherical aberration down to the levels delivered with the finest APO scopes, or better....often, much better!
Because not all achromat owners want to become spherical-aberration-testing experts, Astrobuffet also offers pre-matched telescope/Chromacor sets, eliminating the need for customer testing. For anyone who doesn't already own a 150mm f/8 Chinese achromat, the matched set can be purchased, and many hours of testing and worrying can be saved, for a modest premium above the cost of buying a telescope and Chromacor separately.
For Chinese achromats of 100mm to 150mm aperture and 1000mm to 1200mm focal length, with focal ratios of f/7 or numerically larger, the reporting of star test and/or Ronchi test results will be sufficient to allow Astrobuffet to easily choose the best match of a Chromacor for your scope, and confirm in-stock status.
Upon request, ARIES can analyze other aperture and focal length scopes to help choose the most appropriate match. For instance, a 215mm f/12.5 achromat was theoretically found to need a correction level which is one level different from that of a 150mm f/8, for the same star test results. Those of you with a perfectly neutral (as both tested to date were!!) 215mm f/12.5 Chinese scope were thought to need a Chromacor-U1, not a Chromacor-N. Experience has shown that in fact the N is a much closer match when approximately 120mm of spacers are used, which also makes the color correction better than expected! Rather than use a stack of 24 Hoya filter-ring spacers, I recommend machining an extension tube with the 48mm x 0.75mm pitch threads. On English-system lathes, 34 tpi is a nearly exact match to this, certainly perfectly fine for the engagement length of thread used. One customer recommends using a 70mm threaded extension tube, plus a 50mm or 75mm long 2"-eyepiece extension tube such as found with the Astro-Physics Barcon convertible Barlow lens, or other 2" diameter extension. Performance with off-design-point scopes like this are not guaranteed, but we will work together with the customer to attempt to obtain a good result. My customers with the 215mm achromats are achieving great success with the scope..at least after putting the objective in right-side-up! For those of you with large-scope ambitions, the 215mm objective in cell, 2700mm focal length, is $1950 delivered in the 48 states.
In the end, what really made the commercial success of Chromacor possible, is this addition of spherical correction matching. Without this breakthrough, the slight variations in spherical correction of both the scopes and the Chromacors would have worked against each other, rather than working hand in hand. The need for perfectly neutral scopes 100% of the time is gone, and the need for perfectly neutral Chromacors 100% of the time is also gone, as the U1 and O1 Chromacors are in significant demand also.
Having said that, it must be noted that both the scopes and Chromacors fall within a very tight tolerance. Approximately 85% of the scopes to date have tested as correction level N or O1, only 1/7 wave apart. Thus the average Chinese scope is of the order 1/10 wave undercorrected, with a standard deviation of that same order. By necessity, Chromacors are made to match an equally tight range of values, rather than being randomly distributed among the 5 correction levels available. The U1 and U2 Chromacors were formerly in very short supply because of this, but they were made in small quantities to satisfy demand both by overcorrected scope owners and large-scope owners.
An APO refractor which has even zero spherical aberration in green light, will have measurable spherical aberration in blue and red light. This is called spherochromatism. Spherical aberration in matched Chinese-achromat/Chromacor systems are not only very close to zero, but are not subject to variation with wavelength. This is a true advantage over modern APO scopes.