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Thread: Chromatic aberration on the Synta 6 F/5 achromatic refractor with eyepieces

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    Default Chromatic aberration on the Synta 6 F/5 achromatic refractor with eyepieces



    Hello all,

    In course of the consolidation of my EPs arsenal, I have frequently made tests of the chromatic aberration (CA),
    as observed when viewing through the eyepiece. Some preliminary results have been posted about one year ago,
    and now I have found time to repeat the tests with a higher precision, and to make a short documentation.

    The chromatic aberration problem

    The problem of the chromatic aberration (CA) of the single lenses, and of the achromatic doublets, has a good introduction:
    https://starizona.com/acb/basics/equ...efractors.aspx
    Chromatic aberrations
    The reference designer’s wavelength for an achromatic doublet is typically 546 nm (the green Fraunhofer’s spectral e-line), and the reference spectral lines for which the focus points should meet are the blue 486nm H-Beta, and the red 656nm H-Alpha (Franuhofer’s C and F spectral lines).
    The focus at 546nm is then the nominal focus length of the achromatic doublet according to the design. The light with other spectral wavelength is than focused along the optical axis in front of the nominal focus (longitudinal CA), resulting into the typical U-form deviations, as I have confirmed on the Synta achromatic doublet,

    CA on Synta 6inch 750mm achromatic refractor.jpg

    The graphic is to be understood as showing for example, that the red 656 nm light reaches the focus at the same distance of 0.4 mm from the ‘green’ nominal focus, as the blue 480nm light. So, the difference to the 486nm is not large, and the telescope optics can be considered as achromatic. The displayed U-form curve is the 3rd order polynomial best fit.

    An image taken with an achromatic doublet will therefore reveal the typical purple blur around the point-like light sources, and around the bright illuminated edges. This purple blur is due to the overlap of the red and blue wavelengths away from the nominal focus.

    Compensation of the CA

    Now, the question arises, which eyepiece designs are capable to deal with the CA, and to which extent.

    The first EP designs correcting to some extent the longitudinal CA have been the double achromatic Plssl,
    and the triplet loupe with an eye lens of Abbe – know as the Zeiss Abbe orthoscopic eyepiece. These both EPs have been originally designed to reduce the CA of the achromatic microscope objectives in order to achieve much higher useable magnifications, and eventually also to rectify the distortion (orthoscopic eyepiece).

    With increasing field of view, some of the later eyepieces designs have also tried to compensate not only the field aberrations, but also to compensate at least to some extent the chromatic aberrations. Therefore, the compensating eyepieces are of interest, especially when you have a fast achromatic refractor for the richiest field astronomy observations.

    Measurement results

    To illustrate the extent and the form of the CA compensation, I have chosen 2 groups of 3 eyepieces with different designs.

    1st group:

    Baader BCO ortho f=10mm (52 FOV), a budget but well performing orthoscopic eyepiece
    Pentax XW f=10mm (70 FOV), a popular workhorse among the wide field eyepieces for the fast telescopes at medium magnifications
    Zeiss D 30x/40x B WW f=12.8mm (68 FOV) eyepiece for the fast F/5.9 Zeiss Diascope Fl T* fluorite refractor

    CA on 10mm and 13mm EPs.jpg

    - All three eyepieces in the first group flatten the focus deviations in the spectral range of the dark adapted vision
    of 470 nm up to 546 nm, so they are capable of resolving details in the nebulae and in the galaxies, so far these details are within the reach of the 6” aperture and of the eyepiece magnification.
    - All three eyepieces fail to compensate the focus deviation in red, and therefore the bright red Miras and Carbon stars will require focus adjustment to get them into a pinpoint.
    - All three eyepieces tend to flip the focus at the wavelength shorter than 470nm, similarly to the eyepieces of other designs. The ‘blue flip’ is most pronounced on the Pentax XW 10mm.

    Among these three eyepieces as compared here, the Zeiss achieves a maximum on the possible amount of compensation, followed surprisingly by the Baader BCO. But even the Pentax XW is performing still well.

    2nd group:

    Baader BCO ortho f=6mm (52 FOV), a budget but well performing orthoscopic eyepiece, also optimized for the Zeiss Fl T* Diascope, see http://www.baader-planetarium.de/zei...op-Okulare.pdf
    Pentax XW f=7mm (70 FOV), a popular wide field eyepiece for the fast telescopes at medium to high magnifications
    Tele Vue Nagler Zoom 3mm – 6mm: setting at f=6mm

    CA on 6mm and 7mm EPs.jpg

    - The focus deviations on BCO f=6mm, and on the TV Nagler Zoom at f=6mm are very similar, and they are fine flattened in the spectral range of 470nm up to circa 550nm.

    - The focus deviations on the Pentax XW f=7mm are larger with a pronounced blue flip at 470nm. The polynomial fit fails to work, possibly due to the increased complexity of the EP design at f=7mm, and so the dip on the yellow curve at 580nm is an artefact of the polynomial. Nevertheless, The Pentax shows still a very good compensation of the CA in the spectral range of 486nm up to 546nm, where there are 4 wavelengths for the measurements.

    The measurement method used

    The measurements have been made on a thin lightening rod on the chimney of a nearby building, on the 7th and 8th July 2017.

    The following wavelengths of light have been chosen:
    656 nm: H-Alpha Astronomik filter
    540 nm: Baader Solar Continuum filter
    500 nm: OIII Baader Narrowband visual filter
    486 nm: H-Beta Astronomik filter
    470 nm: Baader bright blue bandpass filter

    Additional measurements to get more precise form of the CA on the achromatic doublet, and to check the applicability of the polynomial fits, have been obtained with the UV/blue B223 Zeiss bandpass filter, and with the Baader 685 nm long-pass deep red and NIR filter.

    The Synta achromatic refractor is documented here: http://www.astronomyforum.net/member...to-tripod.html
    The pictures of the eyepieces are here: http://www.astronomyforum.net/member...eyepieces.html

    Thanks for reading,

    JG
    Last edited by j.gardavsky; 07-23-2017 at 03:58 PM. Reason: added info on the BCO 6mm
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    Default Re: Chromatic aberration on the Synta 6 F/5 achromatic refractor with eyepieces

    Wow that's some in depth stuff! Nice job.
    j.gardavsky and Makuser like this.
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    Default Re: Chromatic aberration on the Synta 6 F/5 achromatic refractor with eyepieces

    Quote Originally Posted by j.gardavsky View Post
    Hello all,

    In course of the consolidation of my EPs arsenal, I have frequently made tests of the chromatic aberration (CA),
    as observed when viewing through the eyepiece. Some preliminary results have been posted about one year ago,
    and now I have found time to repeat the tests with a higher precision, and to make a short documentation.
    ......
    Great work and invaluable. It should dispel the idea that eyepieces can be bought without consideration of the scope. They need to be chosen together to make a good integrated system! I think your posts on these matters should be collected in one sticky thread BTW.

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    Default Re: Chromatic aberration on the Synta 6 F/5 achromatic refractor with eyepieces

    My 5" f/5.5 shows almost no CA under 100x. When you goto 150x that's about all you can use on the planets. Star clusters show a little purple haze and globulars can still be resolved. However 200x a lot of colors become apparent, double stars can still be split but colors are not what the descriptions say in the catalogs.

    Stay away from short focus achromats, if it's not f/8 or above don't buy it.
    Newtman but some refractors allowed.

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    Default Re: Chromatic aberration on the Synta 6 F/5 achromatic refractor with eyepieces

    Forgot to add that short focus achromats are really limited with what you can do with them. You are not going to change the performance of short achros into an f/8 or longer just with different eyepieces. The changes are very subtle.
    Newtman but some refractors allowed.

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    Default Re: Chromatic aberration on the Synta 6 F/5 achromatic refractor with eyepieces

    Quote Originally Posted by tomjones View Post
    Forgot to add that short focus achromats are really limited with what you can do with them. You are not going to change the performance of short achros into an f/8 or longer just with different eyepieces. The changes are very subtle.
    I hope nobody tells my 5" f/6.4 achro this, otherwise it may decide to stop observing galaxies, nebulae, globs, open clusters, planetary nebulae, double stars, carbon stars...

    Yeah, it throws a little CA on brighter planets, but no worries, it's 4" f/9.8 buddy on the same mount handles those duties quite well.

    I've got to go break the bad news to my ST80...no more rich field views and DSO's for you, young man!
    Bryan

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    Default Re: Chromatic aberration on the Synta 6 F/5 achromatic refractor with eyepieces

    Quote Originally Posted by tomjones View Post

    Stay away from short focus achromats, if it's not f/8 or above don't buy it.
    Unless you're not that objectionable to CA on the bright planets or you get some good filters to go along with some well corrected EPs or your main concern is DSOs. I absolutely love my short achros and the AR102 is my most used scope, great DSO performance. You can't beat a short achro for a good grab-n-go set up.

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    Default Re: Chromatic aberration on the Synta 6 F/5 achromatic refractor with eyepieces

    Quote Originally Posted by tomjones View Post
    Forgot to add that short focus achromats are really limited with what you can do with them. You are not going to change the performance of short achros into an f/8 or longer just with different eyepieces. The changes are very subtle.
    That seems just a bit harsh. Many folks find them quite useful and versatile. For instance DSOs.... there are a lot of them. Fewer planets. Also since 100mm binoculars are not everyone's cuppa... a short 6inch refractor might be just the ticket for those of us with binoviewers anyway.
    Last edited by not_Fritz_Argelander; 07-23-2017 at 12:45 AM.

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    Default Re: Chromatic aberration on the Synta 6 F/5 achromatic refractor with eyepieces

    Hi JG and all. I really enjoyed this in depth report, especially since I have the Celestron/Synta version of your 6" f/5 telescope. I also agree with nFA, and sent bladekeeper (Bryan) a PM two nights ago about collecting your great optical treatises for a sticky thread JG.
    As for me personally, I love my XLT150R refractor with 2" eyepieces. I enjoy the high contrast and wide angle views on the "faint fuzzies". And, it is the easiest for me to carry out and set up. Thanks JG for your excellent treatise, and to the rest of you for your fine inputs and responses as well.
    - Marshall

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    Default Re: Chromatic aberration on the Synta 6 F/5 achromatic refractor with eyepieces

    Hello all,

    and thank you for the nice lines.

    I have right now added a link to Baader into the thread:
    http://www.baader-planetarium.de/zei...op-Okulare.pdf
    This link just documents that the 6mm BCO has been matched to the Zeiss Fl T* fast Diascope refractor.
    So, here we are.

    Best,

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