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Astigmatism in a Triplet?

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#1 andythilo

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Posted 07 April 2021 - 12:21 PM

I’ve been offered a new 130mm triplet which is reduced as it has slight astigmatism. The image attached is from the scope. Anyone understand it, they say it’ll be good for photos but not so good for planetary or visual?

 

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#2 J A VOLK

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Posted 07 April 2021 - 12:47 PM

Imaging at prime focus or with a reducer is tolerant of small optical errors because of the relatively low magnification.  Optical errors manifest themselves increasingly as magnification rises, such as required for detailed planetary or double star observing.  Even a small amount of astigmatism is onerous at high magnification.


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#3 KBHornblower

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Posted 07 April 2021 - 12:57 PM

I would say that planets at the center of the field of view will be fine.  It is only toward the edge that astigmatism becomes apparent.


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#4 sg6

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Posted 07 April 2021 - 01:08 PM

Any indication on what the cause of the astigmatism cause is?

Thoughts are poor lens, lens at an angle, lens off center.

 

Latter 2 could be rectified, the first cannot.


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#5 Astrojensen

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Posted 07 April 2021 - 01:14 PM

 

The image attached is from the scope. Anyone understand it, they say it’ll be good for photos but not so good for planetary or visual?

 

The image doesn't show how the telescope performs in practice, it's just a computer-generated illustration to show how large the theoretical color spreads are, at different wavelengths (horizontal) and at different distances from the optical axis (vertical). The elongated images in the bottom row shows how the scope performs half a degree from the center of the field (edge of a one degree diameter field). 

 

On-axis astigmatism is a very harmful optical error for visual observing at high magnifications, but the scope might be acceptable for visual and photographic wide-field use, because the magnification used might be too low to show the astigmatism.

 

 

Clear skies!

Thomas, Denmark


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#6 andythilo

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Posted 07 April 2021 - 01:16 PM

This and that image is all the info I have unfortunately. It’s a brand new scope that I guess hasn’t quite made the grade.


Apochromatic refractor for astrophotography and observation without chromatic aberration due to the FPL-53 Triplet objective.

The Apo is new but the lens has a slight astigmatism. Photographically, the flaw is not visible and large field observation is also possible. Only a planetary telescope does not become of it.
♦ aperture 130mm - focal length 860mm - focal ratio F/6.6

♦ Adjustable triplet lens with air gap - FPL-53 (Ohara Japan) APO element for absolutely color pure imaging

#7 TOMDEY

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Posted 07 April 2021 - 01:30 PM

Any indication on what the cause of the astigmatism cause is?

Thoughts are poor lens, lens at an angle, lens off center.

 

Latter 2 could be rectified, the first cannot.

Those are computer-generated by ZEEMAX from the triplet formula, not measurements. Note that the astigmatism shown is the inherent  field aberration, not anything associated with the build or misalignments. It's actually a very good lens (design). If built to that, it would be a fine performer! Note that the polychromatic on-axis geometric spot cores are nice and tight, much smaller than the Rayleigh-Limit little circles surrounding each.

 

But if they are saying it manifests some as-built axial astigmatism --- yes, that would be a detractor, but consistent with the reduced price.   Tom


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#8 andythilo

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Posted 07 April 2021 - 02:33 PM

Those are computer-generated by ZEEMAX from the triplet formula, not measurements. Note that the astigmatism shown is the inherent field aberration, not anything associated with the build or misalignments. It's actually a very good lens (design). If built to that, it would be a fine performer! Note that the polychromatic on-axis geometric spot cores are nice and tight, much smaller than the Rayleigh-Limit little circles surrounding each.

But if they are saying it manifests some as-built axial astigmatism --- yes, that would be a detractor, but consistent with the reduced price. Tom


Hi

Would it be worth the risk? I suppose worse case I could return it.

#9 TOMDEY

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Posted 07 April 2021 - 03:59 PM

Well, a qualitative statement like "slight astigmatism" is in the eye of the beholder. All real telescopes and imaging lenses have some residual 1st order axial Zernike Primary Astigmatism. Some people would call a twentieth wave slight; some might call ten times that slight. If you want the option of returning it if dissatisfied, best announce that to the seller right up front, before asking them to ship it to you, though. There could be another buyer who is willing to just buy it outright as-is without added waffling conditionals. They are selling it at reduced cost because of the disclosed astigmatism. You really should either accept that accommodation or forget the whole thing.    Tom


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#10 Jeff B

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Posted 07 April 2021 - 04:57 PM

Off axis astigmatism is, with few exceptions, normal in refractors, especially fast refractors.  The design spot diagram looks great to me, even with a 1/2 degree field diameter, it's mostly diffraction limited.  Remember, reducer/correctors can typically be found to help correct for field curvature and, to an extent, astigmatism.

 

Does it display astigmatism on axis at high power?  If so, at what power does it become visible (oval, instead of round, out of focus diffraction rings close to focus)? 

 

If you have verified astigmatism at high power, the sample may be out of collimation and a simple tweaking of the three sets of push/pull collimation screws on the face flange of the lens cell (if it has them) and/or focuser alignment may dial it out.

 

Jeff 


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#11 Jared

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Posted 08 April 2021 - 12:48 AM

As others have mentioned, the scatter plot only shows the “as designed” performance of the scope. If this one is not up to their QC spec, I don’t think I would recommend you buy it. On axis astigmatism—enough that they feel they need to disclose it—means you wouldn’t want to use it visually for planets, double stars, or most galaxies. Photographically, it would be a ‘wide field only’ scope, so, again, not good for galaxies, planets, planetary nebulae, and the like. The whole reason people pay so much money for apochromatic refractors is to have trouble free, optically excellent, well corrected fields of view. This one won’t give you that.

 

I would like at it like a really good deal on the wrong size pair of shoes—a bad idea.


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#12 dan_h

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Posted 08 April 2021 - 10:07 AM

 It is only toward the edge that astigmatism becomes apparent.

If the astigmatism is due to the design, this will be true. If the astigmatism is due to an assembly error or mis-aligned element, is this still the case or can astigmatism affect the entire FOV?

 

Since the OP has been offered a discount on this scope, I would assume the lens is defective and we can't rely on what would be expected of a proper sample.

 

dan


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#13 andythilo

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Posted 08 April 2021 - 10:44 AM

I’ve decided not to go ahead with this. £1000 off a £2500 scope surely means there’s more than a slight issue with the image quality.

Thanks for all the posts and help 😊
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#14 Jeff B

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Posted 08 April 2021 - 11:03 AM

Good move. waytogo.gif


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