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Chromacor?


Xiionn

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I have just bought a Skywatcher Evostar 150 off the bay for a pretty decent price. This is my first large frac and unlike my little 60mm Japanese frac (Which is over 40 years old) it does suffer from noticeable CA. Many years ago they used to do a lens called a Chromacor for large refractors which IIRC offered near ED APO performance. I cant find anything like this around now, only filters like the Baader Semi Apo filter (Which I am wary of because they simply block wavelengths of light rather than correct the optical path)

Anyone know of any Chromacor alternatives? Why on earth did they stop making them? I know they were expensive, but a 150 Apo ED is a LOT more expensive!

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I've owned 3 of those !

They are very rare now though and sell for £400+ apiece usually.

The advent of low cost ED doublet refractors from the far east killed hte market for them plus they were rather fussy to install and use.

They did work though and they corrected spherical abberation as well as CA if matched with the objective of the scope properly.

Istar were developing something similar called the Raycorr but I believe it's gone on a back burner for now, probably for the same reason that Chromacor's are no longer made.

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I have a EVO150 which does show CA around brighter objects as does any achromatic refractor but I found a semi-apo filter helps reduce the effects by about 50-65% while increasing the contrast on planets. Personally I can live with CA so whether a SA filter would work as well for someone who's eye is naturally drawn to purple fringing I can't say ?? Some people can't get on with newts because of coma and so buy coma correctors which satisfy their need and there are those who just can't seem to avoid seeing coma no matter what so get refractors or SCT's instead.

I think if CA is a problem your better off buying an ED or slow newt as there really is no cheap way of eliminating CA in an achromat. As John said Chromacor's are rare and because of that people can demand what ever price they want for them. Even so they are no silver bullet and require some faffing to achieve good results. £400 would be better put towards a 5" ED and have CA free views over the added resolution of the extra inch of aperture but that's just my opinion.

With regards to the EVO150 I think the biggest issue is mounting them as I can't seem to tame the shake when focusing. This is far more of a distraction that CA in my case.

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Does anyone/website have the design for a Chromacor? If an old one of these is going for £400+, then I wonder what the cost would be to make one of these - or a fleet of them...?

I think I know what I'll be researching today.

Clear & Dark Skies without CA,

Dave

After thought...

I wonder why I hear of some folk seeing CA in fast achro's, and others not seeing any to mention. In the same telescope? Just re-tooled my ST80 into a solar-scope, and the CA issue has never been a problem with it...

post-38438-0-31948600-1429186528.jpg

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CA is present in achromat refractors - it's the nature of the design and the physics of the glass used. Here is a table which illustrates how much you can expect with various specifications of crown flint achromat objectives:

post-118-0-94072100-1429194027.jpg

The point at which CA becomes objectionable is a personal one I reckon.

Here is a link to information on the Chromacor. It's out of date because it's been quite a while since they were produced. The design required very special glass types made to military specifications. I believe that the source of that has dried up as well:

ARIES Chromacor.doc

Here is my report on my experiences with a Chromacor, for what it's worth:

So what the heck is a Chromacor.doc

The improvement derived from the correction of the spherical abberation present in the objective is more significant than the CA reduction I reckon.

Filters remove something. The Chromacor actually corrected the CA and SA and got the focus point of the wavelengths of light much closer to the same point. It was not without it's issues though, eg: wide and ultra wide angle eyepieces tend to show CA that is still in the system away from the optical axis. There are some reports that the Chromacor could vignette / stop down the scope if positioned too far along the optical path.

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This is interesting. It cant be THAT hard to manufacture a corrective lens attachment like a Chromacor?!

Try it and see :wink:

If it was easy there would be lots of such devices on the market and folks would happily fork out £100 to convert their achromats into ED equvalents.

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:grin:

What i meant was, it cant be that hard for a company that makes lens equipment :smiley: . There are literally hundreds in China that make camera lens, binoculars etc. These are companies that normally excell in making little attachments and add ons for cameras. If you can buy a 70-200mm Tamron lens for a camera for £700, which is a 23 element / 17 group zoom lens with ED glass AND Ultrasonic drive, or a Nikon 50mm with 8 multicoated elements for £70....then it begs the question of how hard, and how much a fixed diameter short tube with a few elements and ED glass can possibly cost to make? Especially when you consider both of those have autofocus motors and aperture control built in on top, which a Chromacor would not. I know there is a mass production element in the price there, but still.

There is definaltely a market for these. Lets face it, If I can get near 150ED performance from my Evostar 150 for £350, rather than spending out ahem, £4000 on the Esprit 150, or £950 for a 120 Pro, and losing 50% light gathering, its a no brainer!

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:grin:

What i meant was, it cant be that hard for a company that makes lens equipment :smiley: .....

Who do you think Istar and Aries Optical are then ?. They are certainly top optical specialists

Here is the Istar website: http://www.istar-optical.com/

Aries Optical (who made the Chromacor) don't have a website but have a 10 year + history of making top line Fluorite and ED apochromatic objectives &

scopes up to 10" and, Maksutov-Cassigrain and Custom Mirror Optics.

In addition to the above, Roland Christen, the founder and owner of the renowned Astro Physics company has made 2 such correctors which I believe change hands for a lot more than the Chromacors. Here is the Astro Physics website:

http://www.astro-physics.com/

Remember these devices need to be individually matched to the objective lens that they will be used with because each objective has varying amounts of spherical abberration.

Anyway, by all means drop a line to some of the Chinese / Taiwanese manufacturers and see if they are interested :smiley:

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Top optical specialists certainly deliver serious kit, but normally at a premium cost. The Chinese companies today can often deliver 90% of the performance for 30% of the cost. Would I prefer to buy a tailor made Astro Physics optical assembly matched to my personal scope...yep! Can I afford it? Nope!

15 years ago, ED glass was expensive and pretty high end IIRC. Today, its far cheaper and easier to produce. Maybe it would be better to ping a request to Aries Optical :smiley: 

I would probably want to upgrade the focusing system on the 150 first. It's not horrible, but its not like my old Japanese smooth as silk one either. Focusers run expensive as well.

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There is a Chromacor II for sale on a Russian Website, for $1000... Too rich for my blood, at that price I probably would sell the scope and buy a 120 pro ED.

You can get a used ED120 for around £500 now (as I did). Thats what killed the market for the Chromacor's and their ilk.

ED glass types are still expensive. Ohara FPL-53 costs 17x as much as BK-7 (a crown glass) which an achromat would use. Flourite costs twice as much again.

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I only wish I could afford a Takahashi 150 Flourite Doublet. My 70's Japanese refractor is  small, but optically astonishing. I didn't buy it, I inherited it but i could not afford a new one! Trouble is, There just isn't enough light coming in and its now very old and worn. This 150 is very good, but I know a chromacor or similar would make it good as I will ever need!

I might go down the route of a Apo Filter, or fringe killer for now...see how that goes. Save up for an ED.

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A fringe killer will remove almost all traces of CA but the trade off is that the views become noticeably yellow. This appears to be the case for all the filters that claim to control CA with the exception of the SA filter which again only reduces CA not eliminates it.

Apparently a #8 light yellow filter stacked with an #80A blue filter does a similar job to a SA filter but at the fraction of the cost. I have no experience of this having not tried it myself but it may be worth giving it a go.

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A fringe killer will remove almost all traces of CA but the trade off is that the views become noticeably yellow. This appears to be the case for all the filters that claim to control CA with the exception of the SA filter which again only reduces CA not eliminates it.

Apparently a #8 light yellow filter stacked with an #80A blue filter does a similar job to a SA filter but at the fraction of the cost. I have no experience of this having not tried it myself but it may be worth giving it a go.

Ummm.... Yellow + Blue = Green the last time I looked.

Somehow having green objects without blue-purple fringes? I thing I'll take the fringes. But I'll grab my light yellow and 80A blue and take a look anywho.....

Clear & Trippy Skies?

Dave

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I'd be interested to hear the results Dave.

As for yellow and blue making green I'm not sure how it works? I don't think it's like mixing paint ?? The filters block certain light from getting through so in the case of the yellow it is stopping the blue / purple fringing and the blue stops the yellow / unsightly cast giving a more neutral colour star or so it has been said on CN. I agree in what your saying it should end up green but it's also about the light it is trying to correct / block if that makes any sense ??

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A fringe killer will remove almost all traces of CA but the trade off is that the views become noticeably yellow. This appears to be the case for all the filters that claim to control CA with the exception of the SA filter which again only reduces CA not eliminates it.

Apparently a #8 light yellow filter stacked with an #80A blue filter does a similar job to a SA filter but at the fraction of the cost. I have no experience of this having not tried it myself but it may be worth giving it a go.

I don't think there is a filter made that eliminates chromatic aberration without removing a lot of 'information' from the image.  Some filters 'mask' the presence of CA by blocking the visible evidence of the fact that the lenses in question are not capable of bring all wavelengths of light to a common focus. No matter what you filter out, the fact remains that all of the light in is not being focused and those wavelengths that are being blocked are essentially lost information thus leaving you an incomplete picture. 

That said, there are times when it is desirable to remove some information, such as with whitelight solar where the use of a Solar Continuum filter permits a very narrow band of the visible spectrum to be seen, that which allows fine detail to be resolved in sunspots and faculae.

As for the Raycorr and Chromacorr, I understand the required glass is just not available any more.  

Just thinking (typing ) out loud here ...............

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