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Reducing Spherical Aberration In Our Scope Mods

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Reducing Spherical Aberration In Our Scope Mods

Post by marktownley » Tue Mar 18, 2014 6:59 am

Okey dokes, a bit of chatter has been generated about this over on the main page, so thought I would start a topic here about it. Not sure exactly where this will end up but let's see eh?

To me one of the easiest ways to see the results of spherical aberration is with our CaK images, quite simply the longer the native focal ratio the more contrast and sharper detail would seem to be visible. This is because scopes are often not designed to operate at 393nm, however f10 and upwards seems to give the best results here...

Question is though, how can we reduce spherical aberration at 656nm in our PST mods? Here we are often just throwing together components, and while they work within their own constraints, the softer images as a result of spherical aberration can be seen - question is what can we do about it?

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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Valery » Tue Mar 18, 2014 9:02 am

marktownley wrote:
Question is though, how can we reduce spherical aberration at 656nm in our PST mods? Here we are often just throwing together components, and while they work within their own constraints, the softer images as a result of spherical aberration can be seen - question is what can we do about it?

mark
From my personal point of view the only two ways to go:

1. Remain all as is with hope that a given objective in hands has such a SA at the Ha as an objective in a PST telescope or very close to this.
2. To use a specialized optics, specially designed for a specific compact and inexpensive refractor (120 - 150mm) and somewhat larger etalons
say, for SM 40 and LS50 and in some cases LS35.

On my own experience I know well the difference between optimized and occasionally available optics. The first one wins hands down over the latter.
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Peter Williams » Tue Mar 18, 2014 10:12 am

What a shame you can't apply the bandpass filter ahead of the objective. With only a single wavelength presented to the optical train you couldn't (shouldn't) get any chromatic abberation.
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Valery » Tue Mar 18, 2014 10:27 am

Peter Williams wrote:What a shame you can't apply the bandpass filter ahead of the objective. With only a single wavelength presented to the optical train you couldn't (shouldn't) get any chromatic abberation.
We now speaking about spherical aberration which has nothing to do with chromatic aberration.
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley » Tue Mar 18, 2014 10:39 am

Valery wrote:2. To use a specialized optics, specially designed for a specific compact and inexpensive refractor (120 - 150mm) and somewhat larger etalons
say, for SM 40 and LS50 and in some cases LS35.

On my own experience I know well the difference between optimized and occasionally available optics. The first one wins hands down over the latter.
Are some occasionally available optics better than others?

For instance, would there be any gain in say replacing the 200mm refocusing lens in a PST with one of a longer focal length eg 300mm?
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Peter Williams » Tue Mar 18, 2014 11:19 am

Valery wrote:
Peter Williams wrote:What a shame you can't apply the bandpass filter ahead of the objective. With only a single wavelength presented to the optical train you couldn't (shouldn't) get any chromatic abberation.
We now speaking about spherical aberration which has nothing to do with chromatic aberration.
Sorry my mistake. I understand now.
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Valery » Tue Mar 18, 2014 11:36 am

marktownley wrote:
Valery wrote:2. To use a specialized optics, specially designed for a specific compact and inexpensive refractor (120 - 150mm) and somewhat larger etalons
say, for SM 40 and LS50 and in some cases LS35.

On my own experience I know well the difference between optimized and occasionally available optics. The first one wins hands down over the latter.
Are some occasionally available optics better than others?

For instance, would there be any gain in say replacing the 200mm refocusing lens in a PST with one of a longer focal length eg 300mm?
No, I think it is better to stick with optimized pair of collimating - refocusing lenses. May be sometime some systems will work at it's best. But I am not sure.
However in many cases PST mod 1-2 are of acceptable quality with larger telescopes.

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Last edited by Valery on Wed Mar 19, 2014 5:02 am, edited 1 time in total.
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by peter drew » Tue Mar 18, 2014 6:00 pm

I have a 6" F10 Istar objective in one of my PST mods, the objective shows noticeable spherical aberration on a star test but gives excellent performance in Ha. Have I been lucky?.

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Re: Reducing Spherical Aberration In Our Scope Mods

Post by swisswalter » Tue Mar 18, 2014 7:44 pm

Hi Mark

a very much welcomed thread. I'll read it with care and great interest
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Merlin66 » Tue Mar 18, 2014 9:31 pm

My thoughts....
99% of all the astronomical lens data is multi-wavelength data (i.e. of importance to the "normal" visual/ A-P imager)
All the chromatic data normally shows that lenses are not fully corrected at ALL wavelengths. This then gives bloating and out of focus effects. However, when the data is reduced to a monochromatic wavelength (i.e. Ha/ Hb/ CaK etc) then refocusing to suit that wavelength it gives tight images. (I do this ALL the time with the spectroscope. There's a difference in focus between green and red/blue of about 1.2mm (!!))
Similarly, for spherical aberrations - this occurs when the central portion of the lens brings the rays to a different focus from the edge zone of the lens.
To compare various lenses for spherical aberration I think we need at least two things. The ray-fan plot (given in most of the data for commercial lenses (See Edmund Optics lens page for examples) and the target wavelength.
The analysis of the ray-fan plots is very well explained in "Telescopes Eyepieces Astrographs -Design, Analysis and Performance of Modern Astronomical Optics", by Hallock-Smith, Ceragioli & Berry, p 78.
By comparing the fan-ray plot for the different objectives we can quickly see if there are any significant differences.
According to "Fundamentals of Optics",3rd ed. Jenkins & White, p132-142 the spherical aberrations vary as the radius of the objective squared and the focal length cubed. Each lens combination is usually designed for minimal spherical aberration. They conclude "In lens design spherical aberration is always investigated by tracing a ray through the combination for the zone of radius 0.707 h" (where h is the radial height of the lens)
See also: http://www.telescope-optics.net/spheric ... #specified
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Valery » Wed Mar 19, 2014 5:41 am

Merlin66 wrote:My thoughts....
99% of all the astronomical lens data is multi-wavelength data (i.e. of importance to the "normal" visual/ A-P imager)
All the chromatic data normally shows that lenses are not fully corrected at ALL wavelengths. This then gives bloating and out of focus effects. However, when the data is reduced to a monochromatic wavelength (i.e. Ha/ Hb/ CaK etc) then refocusing to suit that wavelength it gives tight images. (I do this ALL the time with the spectroscope. There's a difference in focus between green and red/blue of about 1.2mm (!!))
Similarly, for spherical aberrations - this occurs when the central portion of the lens brings the rays to a different focus from the edge zone of the lens.
To compare various lenses for spherical aberration I think we need at least two things. The ray-fan plot (given in most of the data for commercial lenses (See Edmund Optics lens page for examples) and the target wavelength.
The analysis of the ray-fan plots is very well explained in "Telescopes Eyepieces Astrographs -Design, Analysis and Performance of Modern Astronomical Optics", by Hallock-Smith, Ceragioli & Berry, p 78.
By comparing the fan-ray plot for the different objectives we can quickly see if there are any significant differences.
According to "Fundamentals of Optics",3rd ed. Jenkins & White, p132-142 the spherical aberrations vary as the radius of the objective squared and the focal length cubed. Each lens combination is usually designed for minimal spherical aberration. They conclude "In lens design spherical aberration is always investigated by tracing a ray through the combination for the zone of radius 0.707 h" (where h is the radial height of the lens)
See also: http://www.telescope-optics.net/spheric ... #specified

I apologize, but let me say that this is not the way to go. Too many different telescopes use with PST mods. Too many variables. Lets take even one variable -
a SA in a given type and size telescope, say, Celestron 100mm F/10.
1. We don't know the amount of SA at 656nm because we don't know the objective design.
2. We don't know the error of the wave front of this objective SA wise - we don't know how it performs for SA even if we know the design. Objectives in such telescopes very rarely are nullified for SA. Mostly they are under corrected and this makes SA at 656nm even less certain.
3. We don't know the melt data for both lenses in an objective.
4. The real world is the combination of all three factors.

And this is the case of a given telescope! We can add a forth variable - different telescopes. What we will have? The mess! So, it works as good as you are lucky or not - adrift.

All telescopes with internal etalon were designed straight-through from the first element to the latter. They all are optimized for SA, astigmatism and absence of the field curvature. In other words they optimized as much as possible for a given design. Any arbitrary optical system design change will result in a deterioration of aberrations correction. The improvements will be in resolution and brightness increasing.
The softness of the image due to SA will vary - from barely detectable (if you are lucky) to significant. This image sharpness deterioration is not always quite noticeable because in almost all cases modified telescopes used to photograph followed by intensive image processing with sharpening. This makes all errors more forgiving and resolution + brightness increasing makes the mod absolutely worthwhile. The only question is how far we are from the best possible result.

BTW. Edmund lens stock is not for high quality optics and not for system design where aberrations must be nullified. First - manufacturing radii tolerances are very soft, lenses have no correction for melt data = SA will not be fully corrected even if data sheet of radii is OK for the design. Optics - an exact science, and for the construction of a precision instrument, we must have accurate data.


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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Merlin66 » Wed Mar 19, 2014 5:50 am

Valery,
OK if I accept your comments....where do we go next??
You're effectively saying we don't know what the PST etalon was designed for (in terms of SA of the PST objective) and we don't know what the SA of the possible donor will be - how then can we better match the optics???
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley » Wed Mar 19, 2014 6:50 am

How can we measure / find out / work out the SA @ 656nm for a given objective lens? Can we do this?
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Merlin66 » Wed Mar 19, 2014 7:01 am

Mark,
If the objective has a set of fan-ray graphs - it straight forward...it's on the graph.
If not, then I can only suggest using a resolution check - if the image is "diffraction limited" then the SA is effectively zero for our applications....
I think the issue is, we'd need to do the same test on the PST to find out if SA is a real or imaginary problem.
Based on almost 200 successful PST mods, I don't think it's the biggest issue we face....just my 2c
I'm more interested in Valery's comment - "rule of thumb" about the d/D stuff.....
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Valery » Wed Mar 19, 2014 2:35 pm

Merlin66 wrote:Mark,
If the objective has a set of fan-ray graphs - it straight forward...it's on the graph.
If not, then I can only suggest using a resolution check - if the image is "diffraction limited" then the SA is effectively zero for our applications....
No for both.

1. Fan-ray will not deliver the SA value.

2. You will be surprised - all systems from 1/2 wave SA to zero will show nice diffraction picture and the difference will be only in the amount of energy pulled out of central peak to the surround diffraction rings.


And we DO NOT NEED a zero SA in a donor objective! We need exactly (or very close) such amount of SA as in the 40mm F/10 objective taken as the native feed optics in the PST! If the donor objective will have zero SA at 656nm, then with PST internal optics we will have overcorrected SA at 656nm.

Again, there is no exact way how to make PST mod telescope mod with zero SA. Also ANY refractor telescope with focal length significantly longer than 400mm will have overcorrected field curvature and under corrected astigmatism - as farther from the central of the field of view, as more the sharpness will worsens.

All this is like a searching for the Holy Graal. For H-a it works in most case on the level we accept. In some cases very impressive (remember JP Brahic - if I spell his name correctly) and in some cases so so, but always significantly better than in a standard PST combo because of resolution and brightness increasing.
In a CaK line the things are much worser.

So, let just trying to catch the luck doing the mods.
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Merlin66 » Wed Mar 19, 2014 8:24 pm

Valery,
What about your d/D rule?
Any more info?
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Valery » Wed Mar 19, 2014 8:39 pm

Merlin66 wrote:Valery,
What about your d/D rule?
Any more info?
This rule does work.
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by swisswalter » Wed Mar 19, 2014 9:42 pm

Valery wrote:
Merlin66 wrote:Mark,

So, let just trying to catch the luck doing the mods.

Hi Valery

please I don't wont to die thumb. I'm most interested in getting good sharp, contrasty pics. So how can I achieve that in a straight way :oops:
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley » Thu Mar 20, 2014 6:50 am

Merlin66 wrote:Based on almost 200 successful PST mods, I don't think it's the biggest issue we face....just my 2c
I agree with you Ken, i'm just having one of my OCD moments to try and see if there are any ways the PST mod could be tweaked. Your average PST modder probably has no interest in this as the difference between the stock PST and say a 100mm mod is night and day, and people will always be happy with their upgrade, however, my curious mind is just seeing if there is any way to push the envelope ;)
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley » Thu Mar 20, 2014 7:04 am

Valery wrote:And we DO NOT NEED a zero SA in a donor objective! We need exactly (or very close) such amount of SA as in the 40mm F/10 objective taken as the native feed optics in the PST! If the donor objective will have zero SA at 656nm, then with PST internal optics we will have overcorrected SA at 656nm.

Again, there is no exact way how to make PST mod telescope mod with zero SA. Also ANY refractor telescope with focal length significantly longer than 400mm will have overcorrected field curvature and under corrected astigmatism - as farther from the central of the field of view, as more the sharpness will worsens.

So, let just trying to catch the luck doing the mods.
Ahhhh, now see Valery, you're giving some clues here I think we can work on: So, seeing as all PST mods are going to be longer than 400mm fl, like you say, they should have over corrected field curvature and under corrected astigmatism. If we know this fact is there an optical element we could introduce to bring the field curvature and astigmatism back to something like we would want it to be - not completely correct it or make it perfect, just make it better than it currently is?

I was wondering about the spherical aberration corrector plates that Edmunds Optics sell http://www.edmundoptics.co.uk/optics/wi ... ntryid=231

I know there is an element of luck with the mods Valery, but i'm wondering if we cannot make them perfect with off the shelf components, if there is a way of stacking the odds more in our favour to make them better?
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Merlin66 » Thu Mar 20, 2014 8:02 am

I still would like to know more about the d/D rule.
This has more potential to impact on the mod than other issues......
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley » Fri Mar 21, 2014 7:01 am

Merlin66 wrote:I still would like to know more about the d/D rule.
It's certainly something i've found from an imaging perspective in terms of what is a usable field before the bandpass at the field edges starts to get wider and blue shift...

Also, see the the PDF attached on the Edmunds spherical aberration correction plates, some interesting info.
photonik_intl_2011_032.pdf
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by Valery » Fri Mar 21, 2014 9:25 pm

marktownley wrote:
Merlin66 wrote:I still would like to know more about the d/D rule.
It's certainly something i've found from an imaging perspective in terms of what is a usable field before the bandpass at the field edges starts to get wider and blue shift...

Also, see the the PDF attached on the Edmunds spherical aberration correction plates, some interesting info.
photonik_intl_2011_032.pdf

Mark,

Read this from that PDF file:

[Spherical aberration plates work well in systems with little or no field of view. They are recommended for insertion in a pure collimated beam. If they are to be used in an imaging system with a field of view, it is recommended to put them near an aperture stop or a pupil. The stop is the one aperture in the optical system that limits the diameter of the beam. ]

Such an element must be of the same diameter as the refocusing objective and more important it must have the same SA as the whole system but opposite, so it will nullify the system SA. But how to measure the system SA?

The only real possibility to nullify system's SA is to use SAFIX as it is a variable SA corrector and no need to measure the SA of the system. Just change SAFIX SA as try and error process and see when the image becomes maximally sharp. However SAXIS is not cheap in the scale of money one typically spend for PST mod.

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Re: Reducing Spherical Aberration In Our Scope Mods

Post by marktownley » Fri Mar 21, 2014 9:44 pm

I thought about putting a EO spherical aberration plate after the PST etalon and before the refocusing lens, but, like you say how do you you measure the system SA, that is the problem.

SAFIX is not cheap compared to a PST mod, I agree, but sometimes it is not all about something being cheap.

I think you should offer the SAFIX for sale in the buy and sell here, i'm fairly certain there are a few people here who would be interested in buying one ;) :roll:
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Re: Reducing Spherical Aberration In Our Scope Mods

Post by swisswalter » Sun Mar 23, 2014 8:17 am

Hi Mark, Hi Valery

a very interesting thread. As I know a little bit about chemistry. but almost nothing about optics. I'm asking you wether the SA is a problem of the lenses only? Are mirrors free of SA ? :oops:


@Valery Yes some more Information about the SAFIX would be of great interest
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