Chromatic aberration in an apochromat.

[osbourne one-nil]

I've had a fantastic night tonight, firstly because it's crystal clear and I didn't expect that tonight, but also because I only got my new Tak FC-100 yesterday, so it's almost too good to be true. 

I've got it mounted on a Porta Mount but the tripod does let it down and is soon to be replaced. This, and a bit of a breeze, made focusing a bit tricky, particularly with my 9mm eyepiece. Is chromatic aberration more visible if the image is slightly out of focus? I ask because on Jupiter and Arcturus I could definitely detect it. It was very minor, but it was detectable. Perhaps I'm being hyper-critical and even apos show some false colour? It didn't stop me seeing Io's shadow cross Jupiter (doesn't it move fast!) or some very crisp detailed views, but what should I expect out of a Tak?

[lensman57]

I've had a fantastic night tonight, firstly because it's crystal clear and I didn't expect that tonight, but also because I only got my new Tak FC-100 yesterday, so it's almost too good to be true. 

I've got it mounted on a Porta Mount but the tripod does let it down and is soon to be replaced. This, and a bit of a breeze, made focusing a bit tricky, particularly with my 9mm eyepiece. Is chromatic aberration more visible if the image is slightly out of focus? I ask because on Jupiter and Arcturus I could definitely detect it. It was very minor, but it was detectable. Perhaps I'm being hyper-critical and even apos show some false colour? It didn't stop me seeing Io's shadow cross Jupiter (doesn't it move fast!) or some very crisp detailed views, but what should I expect out of a Tak?

The CA is probably more to do with the EP than the Tak, but the answer to your question is yes if the star is slightly out of focus you'd notice the CA more easily.

A.G

[osbourne one-nil]

Thanks - I did wonder about the eyepiece too, but they were a Panoptic 24mm and a Nagler 9mm. Still. as long as it could be something other than the scope, I'll stay calm and await my upgraded tripod. 

[Ags]

In an apo you can see color fringing when out of focus. The reason is that an apo brings light of different colors to focus at the same plane, but as you move out of focus the colors get less focussed at different rates - for example the blue light might defocus more rapidly, so you see blue fringing. You also see this effect in camera lenses where out of focus high contrast areas can have a green or purple tinge.

[John]

I think the scope is a doublet ?

I believe it's normal for ED / Flourite doublets to show a little CA either side of sharp focus around bright object. It should more or less dissapear at sharp focus although there may be just a trace left around the brightest stars.

Thats the way my Skywatcher ED120 and Vixen ED102SS behave anyway.

[osbourne one-nil]

Yeah - a flourite doublet and that pretty much sums up what I experienced tonight - thanks.

[Skyline]

There's a good article about this on Roland Christen's website, talking about colour correction on APO Refractors.

http://geogdata.csun.edu/~voltaire/roland/color.html

Al.

[Moonshane]

As far as I know, apochromatic means less CA, not none. My 120ed skywatcher shows very little when in focus (almost imperceptible even on Jupiter and the moon, a little more on only the brightest stars) and possibly more when out of focus but why would anyone worry about that?

The sky can create CA when there's a lot of moisture or the object is bright and low.

My newts are better in this respect than my Frac.

[osbourne one-nil]

Thanks - I think the problem last night stemmed purely from slightly inaccurate focusing as result of the mount being very wobbly; there were times when I couldn't see any at all. I certainly couldn't see any when I was enjoying the Leo Triplet, which looked stunning. I also have never seen a cloud transit as clearly as I did last night; I think I've seen them on many occasions, but last night, I definitely say it. 

I'll stop worrying!

[Moonshane]

Yeah just enjoy.

[mikeDnight]

I've had a Takahashi FS 128 and a FS 152. Both were amazing scopes by any standard, yet CA was visible in both, though it was so miniscule it wasn't worth bothering about and it really didn't bother me. I did notice however that dependant on which eyepiece design I used, the visible CA varied considerably. Naglers for example, which I love dearly, introduced colour to the image. Takahashi's own Hi LE eyepieces showed less CA. TMBs super monocentrics showed no noticeable CA at all.

As with the FS apo's, your FC is an doublet fluorite refractor, so it won't bring all colours to the same precise focus. Yet it will deliver better colour correction than an ED. Where the FS and FC apos really score is on contrast and definition. Even in reviews of top end triplets like Astro Physics and TMB vs Takahashi Fluorite doublets, the reviewers who love the triplets generally concede defeat when it comes to the contrast and definition shown in the doublet fluorites.

There is another thing to consider and that's the angle of the object you're looking at. The lower an object is in the sky the greater the effects of atmospheric refraction, which can introduce colour to the system. That and the choice of eyepiece coupled with variable seeing conditions can have a marked effect on the resulting image.

J.W. Sidgwick commented in his famous book, "Occulars are rather frequently overlooked as possible sources of inferior performance, which is then blamed on the objective".

That statement is as true today as it was when it was written all those years ago. I makes sense really as the eyepiece is half the telescope.

Anyhow! Give your beautiful world class refractor a couple of years of serious use and Im sure it will amaze you time and again, even when pitted against much larger scopes.

If you don't get along, I just started a Takahashi Rescue Society and would be happy to take it in and treat it kindly, taking it out for regular observing sessions to keep it happy and healthy.

Mike.:-)

PS. You might be wondering why someone who loves Tak scopes so much would sell them on. The answer is simple, I'm Stupid!

[osbourne one-nil]

That's how I felt when I sold mt TV85; regretted it almost instantly although for some reason, I'm not suddenly over it. ! Interesting what you say as I can't  remember noticing the CA through the 24mm or 15mm Panoptic, just the 9mm Nagler. I put it down to magnification making focusing harder. 

If it's clear tonight I'll get my Portaball out to and compare views of some fairly tenuous objects like the Leo Triplets and see what I think. I was surprised how easy they were to pick out. 

You sound kind a very kind and selfless man - I'll bear it in mind!!

[Joe G]

Hi

My Vixen FL102S, which is almost similar to your FC100 showed a little colour off centre on the moon & Saturn early this morning but none at the centre the my eyepieces, even when fully focused. It is probably because these targets are low in the sky as Jupiter showed no false colour. Viewing conditions in my area were pretty good at the time

Joe

[ronin]

OptCorp specify the FC 100 as "Refractor - Semi-APO".

The scope is a doublet so does not match the requirements of "apo", two lens cannot control 3 wavelengths.

I see that the Tak America site says:

Steinheil fluorite doublet Apochromat MCt

Which is incorrect for a doublet.

Unless someone alters the definition of Apochromatic.

[Mr Spock]

A doublet can't be an apochromat, but it can be a well corrected achromat

[Ags]

I always smile at the "two lens cannot control three wavelengths" argument. There are rather more than three wavelengths in play here :-)

Obviously three types of glass is an improvement over two, but it's still imperfect.

[osbourne one-nil]

It'll do for me - I'm almost relieved that it's not meant to be almost but not quite perfect because that is what my wife thinks of me....probably. 

[Skyline]

Swings and Roundabouts, enjoy whatever you have.

Thomas Beck's definition of Apochromatism.

http://geogdata.csun.edu/~voltaire/tmb/definition.html

[Piero]

Mine is also a doublet, but not a fluorite one. 

If I observe Jupiter or Sirius when they are very low, I can see some CA. This is just because I observe from a moderately light polluted place. 

As soon as they are a bit higher, CA disappears. 

If strong wind is there, I can have some difficulty on focusing properly and I can see some CA. Not much though. 

In those nights you cannot see the concentric airy rings in/out focus, just to give an idea. 

No instrument is perfect, you got a super one though! Just enjoy it! 

[Ags]

The CA you see when targets are low is due to atmospheric refraction, it is not related to light pollution. You would see the same from a dark site.

[mikeDnight]

Takahashi along with Canon developed the production of large aperture Fluorite lenses back in the 1970s. Many refractors existed at that time claiming to be triplet Apochromats, yet they were incapable of matching the colour correction of the Fluorite doublets produced by Takahashi and later Vixen.

Many of today's top Triplet Apo's use either Fluorite or a Fluorite composite such as FPL53 in their lens design. This was not possible even just a few years ago as such glass composites did not exist. Take a look through any early Astro Physics which people rave over and the CA will make you cringe when compared to a Takahashi doublet.

Even today, triplets and quadruplets claiming to be Apochromatic are far from perfect. A few years ago I purchased a TVNP101 from Venturescopes. It cost me £3,100.00 and it was not truly apochromatic. There was a noticeable colour fringing to bright stars and along the limb of the moon. It was barely visible but it was there. The TVNP101 also gave a decidedly warm view and gave the moon a somewhat nicotine yellow colour compared to the pure icey white of a Fluorite doublet.

Here's a direct quote from Takahashi:

Takahashi has dominated the worlds refractor market for decades, and continues as a formidable force in this field after the introduction of Fluorite models in 1977. Many other manufacturers pursued, often for reasons of cost, subjects of surfacing and other processes, following the questionable path of hazardous materials such as ED and Super ED. Over a wide frequency of spectrum Fluorite possesses the best characteristics of transmission, resolution and contrast, without equal. This reference is well appreciated by comparative observation tests, and it is probable it will never be surpassed in performance. With a very high transmission better than 99%, and perfect chromatic correction, the definition of the resultant image is an incredibly sharp image, difficult to obtain in large aperture reflectors which are often penalised through the effects of atmospheric seeing.

The superiority of this instrument becomes evident through comparison with other makes".

While it is true that Takahashi produce some of the finest triplet refractors the world has ever seen, they have to be thought of as a compromise as they were made primarily with the imager in mind. Even the great Takahashi cannot produce a triplet that delivers the same level of contrast and definition as the amazing Fluorite doublets of yesteryear, and that are desired by many a discerning visual observer. That after all is why they have returned to producing their FC series "Apochromatic" fluorite refractors that have such a cult following.

[Piero]

The CA you see when targets are low is due to atmospheric refraction, it is not related to light pollution. You would see the same from a dark site.

Thanks for the correction. I didn't know that. 

[mikeDnight]

You mentioned in your original post that you were having problems withe the ports mount being a bit unsteady. Well has about this as an upgrade?

Remember a mount can never be too solid! I love Takahashi, they never do anything by halves!

Mike

[Piero]

You mentioned in your original post that you were having problems withe the ports mount being a bit unsteady. Well has about this as an upgrade?

Remember a mount can never be too solid! I love Takahashi, they never do anything by halves!IMG_20150315_222323.jpg

Mike

I could see myself with that beast holding my TV-60 by bike and then on a field....!   

[nytecam]

Yep - all fracts show CA and why Issac Newton invented the Newt . OK Apos are better but still fracts and if you're into imaging with modern CCDs you need to kill cam IR sensitivity as IR won't be focused - all reflecting scopes bring all radiation to same ficus !