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Yet more questions about CA

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#1 oldtimer

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Posted 16 January 2013 - 01:03 PM

First what I DO understand abour achros. The closer an achro's focal ratio gets to the X3 ruke (ratio = 3 times the aperature in inches) the more controled CA becomes.

Now what I don't understand: I have a 4" F5 achro which achieves 42% (5 divided by 12) of X3 and a 5" F6.5 which achieves 43% of X3 (6.5 divided by 15). Since both are pratically the same and given that the optics are of similar quality, why is it that the CA seems much more controled in the 5" than the 4"?

#2 RogerRZ

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Posted 16 January 2013 - 02:02 PM

The lens was likely designed to give better correction on the blue end of the spectrum, at the expense of the red end, to which the eye is a bit less sensitive.

#3 oldtimer

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Posted 16 January 2013 - 02:38 PM

Does anybody know if this is true of the Explore scientific 127 F6.5?

#4 siriusandthepup

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Posted 16 January 2013 - 05:18 PM

It probably does not apply in this case, but I once had a 5" short focus achro (commercially made) that had amazing control on the CA.

I had some concern when looking in the focuser with no eyepiece about seeing the lens edges. After some analysis, it turned out that the drawtube was vignetting the light cone to the tune of stopping down the objective to 3.5" Well that certainly solved the mystery of amazing CA control. Sawing about 3 inches off the drawtube solved the issues with the light path and, of course, returned the CA to the expected levels.

Anyway, moral of the story: Always examine the light path closely on your short FL refractors for possible stopping down.

That said, I have noticed a big difference in CA on various achro designs which amaze me. Two examples of Achro's which have amazed me with their control of CA are the Omni 120mm f/8.3 and the Istar 8" f/8.8 . They both exhibit much less color than I expected. Lens design is a factor - but you aren't gonna design an achro into an apo without resorting to long, long focal length.

#5 jrbarnett

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Posted 16 January 2013 - 08:34 PM

It could be that the 4" does not have its lens elements spaced optimally. I'd take it apart and play with the spacing to see if I could improve color correction for the 4-incher.

Alternately, the 5" may not be full aperture. You can check by viewing the objective through the tube, looking in the focuser end with no diagonal or eyepiece in the optical train with the focuser set close to its actual position during in-focus use. If you can't see the lens edges (either due to the focuser draw tube cutting into the light cone or misplaced/overly aggressive baffles, then you'd need to estimate the *clear* aperture and redo you calculations based on that.

- Jim

#6 Jim Curry

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Posted 16 January 2013 - 08:35 PM

I had a FS-102 then built a 4" f/12 Istar lensed scope. Took them out, compared CA, sold the 102. It was a teeny, tiny, whinney bit less CA on Jupiter, certainly not enough to warrant keeping a somewhat expensive duplicate aperture scope in the stable. My 6" f/12 Istar lens is remarkably CA clean as well, not nothing but verrry little.

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#7 ValeryD

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Posted 17 January 2013 - 12:48 AM

Visible CA is not the only factor which must be counted when one choose the telescope for planetary works. In fact, the first parameter must be the CONTRAST.

Even in a very old instruction of how to choose a refractor telescope (only achros were available at that time) was written the following: "better buy a telescope in which you do see a light blue halo. A scope with trace of red or green or with no halo seen must be rejected".

In FACT I personally (with a group of observers) tested two chinese refractors D=127mm F/9.25 - both of the same manufacturer. First telescope was with close to the classical CeF correction and second one was corrected about as DeF (shifted to blue). First one showed classical purple halo around Jupiter as it must be in such an achromat. Second one showed very tiny of violet, almost color free! But, wait... with washed out of disk details! In the first telescope details in both equatorial bands were easily seen, while in the second one these details were barely detectable! Both scopes have very similar neutral correction for spherical aberration in a green filter.
Overall contrast in the second telescope was lower and this was immediately seen. Several other observers saw this also and we have a length discussion which scope is better and about CA in general. All observers easily agreed that the first scope with classical color correction was a clear winner contrast wise.
I can add that I immediately bought this second telescope (for about $300) as a sample for my further investigations what is what in this telescope. At that moment I didn't know it's exact color correction and measured it later in our laboratory with a kit of several narrow band filters from deep blue to deep red.
The results of my investigation were discussed on russian astro-forum with hot discussions. This test was also the trigger for my further comparision of two 5" APO telescopes, but both were apochromats. In this pair the scope with shifted to violet correction loose too, but no so much, because overall color correction was, of course much better than in achromats.

In the attachment one can see the copy from that old book with note which refractor to choose for planetary observing.
The text in russian and can't be directly copy - paste in the translator. It must be hand written with cyrillic board or whatever.

Attached Files



#8 ValeryD

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Posted 17 January 2013 - 01:29 AM

Here is a literal translation (by me) of the text of aforementioned instruction (in old russian, the book was early 19xx years).

"For Mars observing an EXCELLENT objective is required. We must especially recommend objectives made in Munchen, which usually made in the way that around bright object a LIGHT BLUE halo is seen. Mars delivers relatively little of blue light and their incomplete achromatisation is not a big fail. And Schiaparelly blocks them with very weak orange filter, which he threads in the eyepiece barrel. And wise-versa, the objectives wish shows no halo at all, or deliver reddish and greenish halo are not acceptable at all".

I can add, that Jupiter delivers more light blue rays and therefore a classical CeF correction will be better for it's observing than Cde (recommended in that book for Mars), but it will be still good for Mars too.
So, if to consider the best compromised achromat for planetary observing, this must be as longer as possible achromat with close to classical CeF correction. Any shift of color correction curve to blue part of spectrum will results planetary details contrast decreasing. At that ancient (already) times peoples have much less choice for image improvement in refractors and they tried all opportunities to do this, include different color correction curves. And finally all notable refractors manufacturers settled with CeF correction for achromats as the best possible compromise. I would not recommend to hesitate with their experience in achromats design and their applications to planetary observing.

#9 ValeryD

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Posted 17 January 2013 - 01:33 AM

It could be that the 4" does not have its lens elements spaced optimally. I'd take it apart and play with the spacing to see if I could improve color correction for the 4-incher.


Don't do this! It will not help with color correction and can only damage spherical aberration correction. And with huge probability one will not be able to adjust an objective again and a coma of decentering will be seen and damage the picture even worser!

#10 Rutilus

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Posted 17 January 2013 - 01:43 AM

My experience was very similar to Jim (Curry). After building my Carton 100mm f/13 and comparing it
side by side with a Tak TSA-102 and a friends FS-102, I sold my TSA.

The carton shows a tiny bit of C.A. on bright objects like Vega and Sirius, but it is the equal of the Tak scopes.
With the planets, I was seeing exactly the same detail in all 3 scopes, ditto lunar and double stars,
in fact the seeing of the F star in the Trap was easier with the Carton, and I have observed Sirius B
three times now since the start of the New Year.
I compared the Carton against several other ED scope and the Carton showed better contrast.
The sale of my TSA has now funded the building of my Istar 150mm f/15 Achromat.

#11 Jim Curry

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Posted 17 January 2013 - 06:41 AM

Hmmm, I wonder how many others have pushed the FS kool-aid away in favor of a quality achro?

Jim

#12 Mark Costello

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Posted 17 January 2013 - 09:12 AM

Does anybody know if this is true of the Explore scientific 127 F6.5?


The false color I see around Jupiter, Saturn, Mars, and the rim of the moon is a deep purple aura around the disks. Very bright stars show deep blue halos. It looks like a regular achromat with the aberration on the blue and violet end of the visible spectrum to me....

#13 ValeryD

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Posted 17 January 2013 - 11:00 AM

My experience was very similar to Jim (Curry). After building my Carton 100mm f/13 and comparing it
side by side with a Tak TSA-102 and a friends FS-102, I sold my TSA.

The carton shows a tiny bit of C.A. on bright objects like Vega and Sirius, but it is the equal of the Tak scopes.
With the planets, I was seeing exactly the same detail in all 3 scopes, ditto lunar and double stars,
in fact the seeing of the F star in the Trap was easier with the Carton, and I have observed Sirius B
three times now since the start of the New Year.
I compared the Carton against several other ED scope and the Carton showed better contrast.
The sale of my TSA has now funded the building of my Istar 150mm f/15 Achromat.


There are several variables in comparition between two scopes and if a color aberration is not huge, does not absolutely dominated, then other factors pay their roles.
For example - spherical aberration correction, wave front smoothness etc.
If, for any reason an APO has SA, say, 1/5 wave, then perfectly corrected for SA long focus achromat (especially such small one as your Catroon) will show more details on planets.

So, uncontrolled comparition has little real sense. We can compare CA influence if all other factors are nearly equal, better if they are equal.

#14 t.r.

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Posted 17 January 2013 - 11:52 AM

Those old references do add credence to the arguments today about the capabilities of the classic Fraunhofer achromats. I remember reading in a BAA book years ago that an achro's focal length can be pushed to 1.3 times the aperture in inches, up to 8inches(!!!) and still be usable for planetary observation. That makes most apo pundits croak! This means that an 8" F/12 is completely useful as a planetary instrument just as Roland Christen points out in his "Planetary Telescope" article here on CN. The common 6" F/8 fits this bill as well and is evidence for it great success. The new minus violet filters are another equation that wasn't available years ago in the classic period...wonder what they would have said about these? ;)

#15 chboss

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Posted 17 January 2013 - 11:05 PM

Hmmm, I wonder how many others have pushed the FS kool-aid away in favor of a quality achro?

Jim


Well you mileage may vary.... in my case owning a good Asahi Pentax f11 Achro and using a 110mm f15 Lichtenkneker Refractor I can safely say I would not bother changing. :)
My FS-102 beats them hands down optically as well as in practical use due to weight and length (mounting requirements).

best regards
Chris

#16 oldtimer

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Posted 18 January 2013 - 11:56 AM

Mark;

I agree. This brings me back to my original question concerning the 'better' color correction in the 5" F6.5 than the 4" F5. For those that have commented that my 5 inch may be 'stopped down' due to baffling or the intrusion of the focuser tube, I have already 'modified' the focuser tube that moved the baffling to allow a full inch of light cone to reach my wide field 2" eyepieces. So I'm still mystified!

#17 RogerRZ

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Posted 19 January 2013 - 06:09 AM

"Better" where? In the blue end? Red end? Middle? The lens was likely designed to give better correction on the blue end of the spectrum, at the expense of the red end, to which the eye is a bit less sensitive. I think ValeryD provided an answer where lack of CA on the blue end doesn't necessarily make a lens "better"...

#18 oldtimer

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Posted 19 January 2013 - 03:33 PM

This must be the answer. It shows much less blue and violet 'halos' around Jupiter and bright stars (than my 4" F-5). I have never seen any 'red'. I guess I'll just put this question to bed and be happy with my scope.

#19 RogerRZ

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Posted 19 January 2013 - 05:29 PM

"If you can't be with the one you love, love the one you're with..." ;)

#20 sg6

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Posted 19 January 2013 - 06:11 PM

There are lots of different glasses all of which would fall into the "standard" type for an achro, crown and flint and not ED.

The better scope could simply be designed and uses a glass that is a bit more towards ED then the not so good one. Still achro but a better glass and so less CA.

#21 Jon Isaacs

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Posted 19 January 2013 - 08:22 PM

There are lots of different glasses all of which would fall into the "standard" type for an achro, crown and flint and not ED.

The better scope could simply be designed and uses a glass that is a bit more towards ED then the not so good one. Still achro but a better glass and so less CA.


What glasses might those be? It's my understanding that with available glasses, you get the 1 part in 2000 achromatic correction. To improve upon that, you need to get substantially off the Abbe Normal line and those glasses just don't exist.

Roland Christen: Abbe Normal

Jon

#22 ValeryD

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Posted 19 January 2013 - 11:41 PM

There are lots of different glasses all of which would fall into the "standard" type for an achro, crown and flint and not ED.

The better scope could simply be designed and uses a glass that is a bit more towards ED then the not so good one. Still achro but a better glass and so less CA.


This is possible for a very few % of improvement of color correction and only for slow F/D objectives. This is absolutely useless for short F/D telescopes.

For shorter F/D, say, F/5-F/9 when we trying to use another glass combinations with lesser secondary color, then these glasses necessary have small difference in their Abber number - resulting lenses steep curvature and high order aberrations and sphero-chromatism starting to hurt the image - decreasing energy concentration in the Airy disk and increasing brightness of the surrounding diffraction rings = lesser contrast (the effect is the same as central obstruction increasing) - so, refractor loss it's greatest advantage over reflectors - the contrast due to lack of central obstruction.

There is no free lunch here! The only way to decrease secondary color in refractors is to use ED glasses in fast and slow F/D refractors and possibly short flint glasses in slow F/D refractors - F/15 and slower.

#23 aa6ww

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Posted 22 January 2013 - 04:10 PM

Hmmm, I wonder how many others have pushed the FS kool-aid away in favor of a quality achro?

Jim


I think what happens sometimes is that these long tube refractors are known for planetary and double star splitting, both of which does not required dark skies to observe them in, unlike deep space observing. Because of this, most of us can just set up a long tube refractor in our back yard and not worry about transporting them to dark skies to bring out their full potential.

Also, a long tube refractor is just fun to have, and look at, and look through. Its what most of us grew up wanting. I have a TSA-102 and a TOA-130 and I doubt Id ever sell either, but I would like to get my hands on a 8" F12 refractor, just for back yard planetary fun.
Like most of us who have different scopes for different reasons, I like the portability of my TSA. Last night for example, Jupiter was 1 degree from the moon. I got home from work at 6:15pm and had my TSA on my Vixen mount by 6:30pm, and spend a few hrs just having some fun with that fast set up. Sometimes thats all I need the scope for. A long tube refractor would have taken more time, and more set up, so the little shorty got the job done.
In the cold of this current winter we are having, I seldom venture away from home to observe, its just too cold and I cant get anyone to go out with me for an all nighter. Because of this, especially how Jupiter has been putting on an excellent show this winter, back yard planetary observing means we can just set up what we want and enjoy astronomy in our back yard where setting up a fun long tube refractor is a blast.
Finally, I think also, as more of us become more acquainted with these ultra expensive APO telescopes, and compare then to some of the very nice achromatic scopes that we are now seeing on the market, we realize that the APO isn't our only choice any more, if we just want a planetary scope, and don't want to pay for those ridiculously high prices people are asking for them.
Chromatic Aberration to me, is more of a forum topic, than a reality in observing, especially when we are looking through a long tube achromatic refractor. The tiny bit of color that does make it to the eyepiece is far less a factor any more, and the cost of these long tube refractors from D & G and now ISTAR, are making it fun to bring us back to the reality of how fun refractors can be, without the horrendous cost of an APO refractor.
I mean, seriously, over $11K for a TOA-150 or $10K for a TEC-160, when you can pick up a beautiful long tube ISTAR or D & G for one third to one forth that, and still have a serious beautiful planetary telescope that absolutely gets the Job done, not just on planetary observing, but for deep space and everything else you can point these beautiful scopes at.
Could we possibly be seeing the demise and lust for these ultra expensive APO's, because modern computer designed lens and excellent focusers and baffled tubes are now everywhere to be found at reasonable costs now, finally, with excellent optics?
I hope so, because Ive never wanted a long tube refractor more, and with ISTAR, D&G and even Skylight showing us that there are excellent alternatives. we now have more choices than ever.
Everyone is making a quality telescope now with quality optics, even at F/5 now!!

...Ralph






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