Reducing Spherical Aberration In Our Scope Mods
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Reducing Spherical Aberration In Our Scope Mods
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?
mark
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?
mark
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Re: Reducing Spherical Aberration In Our Scope Mods
From my personal point of view the only two ways to go: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
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
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
We now speaking about spherical aberration which has nothing to do with chromatic aberration.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.
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Re: Reducing Spherical Aberration In Our Scope Mods
Are some occasionally available optics better than others?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.
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
Sorry my mistake. I understand now.Valery wrote:We now speaking about spherical aberration which has nothing to do with chromatic aberration.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.
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Re: Reducing Spherical Aberration In Our Scope Mods
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.marktownley wrote:Are some occasionally available optics better than others?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.
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?
However in many cases PST mod 1-2 are of acceptable quality with larger telescopes.
Valery.
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
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
Hi Mark
a very much welcomed thread. I'll read it with care and great interest
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
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
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
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.
Valery.
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Re: Reducing Spherical Aberration In Our Scope Mods
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???
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
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
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.....
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
No for both.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....
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
Valery,
What about your d/D rule?
Any more info?
What about your d/D rule?
Any more info?
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Re: Reducing Spherical Aberration In Our Scope Mods
This rule does work.Merlin66 wrote:Valery,
What about your d/D rule?
Any more info?
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Re: Reducing Spherical Aberration In Our Scope Mods
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
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Re: Reducing Spherical Aberration In Our Scope Mods
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 envelopeMerlin66 wrote:Based on almost 200 successful PST mods, I don't think it's the biggest issue we face....just my 2c
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Re: Reducing Spherical Aberration In Our Scope 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?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.
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
I still would like to know more about the d/D rule.
This has more potential to impact on the mod than other issues......
This has more potential to impact on the mod than other issues......
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Re: Reducing Spherical Aberration In Our Scope Mods
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...Merlin66 wrote:I still would like to know more about the d/D rule.
Also, see the the PDF attached on the Edmunds spherical aberration correction plates, some interesting info.
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Re: Reducing Spherical Aberration In Our Scope Mods
marktownley wrote: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...Merlin66 wrote:I still would like to know more about the d/D rule.
Also, see the the PDF attached on the Edmunds spherical aberration correction plates, some interesting info.
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.
Valery.
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Re: Reducing Spherical Aberration In Our Scope Mods
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
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
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Re: Reducing Spherical Aberration In Our Scope Mods
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 ?
@Valery Yes some more Information about the SAFIX would be of great interest
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 ?
@Valery Yes some more Information about the SAFIX would be of great interest
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Re: Reducing Spherical Aberration In Our Scope Mods
Walter,
Under corrected mirrors (think Hubble!) can cause SA in reflectors....
Under corrected mirrors (think Hubble!) can cause SA in reflectors....
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Re: Reducing Spherical Aberration In Our Scope Mods
Ah Ken
thank you very much , now I'm remembering
thank you very much , now I'm remembering
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Re: Reducing Spherical Aberration In Our Scope Mods
Here is a classic example in CaK: same filter throughout... (I was still dealing with reflections at this stage with the caK filter - ie the ghost overlain at the bottom of the image. First off 40mm f10Neo wrote:For Mark, do you have examples of images that suffer from this SA problem and how can you tell if it's not just seeing, tube currents or something else?
Makes me wonder if the proposed SA cure is outweighing the possible side effects that you may be introducing like internal reflections but that's another topic.
Interesting thread, like to see where it takes us.
Next, 50mm f8.4
Then finally 70mm f6
As the SA increases the contrast decreases and so detail goes.
With the Ha the raw image is quite soft (I think) with the histogram bunched up. Here us a raw unprocessed ha image from a couple of weeks ago...
Now I know I can sharpen and stretch this up in post processing, but where i'm going with this is would like to improve matters at source...
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Re: Reducing Spherical Aberration In Our Scope Mods
Mark,
You are assuming that the 50mm and 70mm objectives have more SA?
These could actually be better corrected than the one used in the PST?????
You are assuming that the 50mm and 70mm objectives have more SA?
These could actually be better corrected than the one used in the PST?????
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Re: Reducing Spherical Aberration In Our Scope Mods
The 40mm lens is one from a PST. The 50mm is a stopped down from the 70mm (same lens). While great at 540nm and 656nm, the 70mm scope is awful at 393nm.
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Re: Reducing Spherical Aberration In Our Scope Mods
Hi Neo, regards the increasing angle of a faster beam on the filter, there will be a shift associated with this, exactly how much I can't categorically say how much. We can get a little from this PDF from the Paris Meudon Observatory where it talks about their CaH filter: in a collimated beam the filter works at 396.85nm, at a 2.5 deg angle this is 396.80nm. There is of course greater thermal loading which also causes a centre wavelength shift - see their CaK PDF with the graph at 3c, 13c & 23c...Neo wrote:Thank you for the example images Mark. For the Ca images, to be honest I'm not convinced that the loss of contrast is completely because of SA. How do the Ca filters respond to ray patterns that enter at increasing angles? Are the 50 and 70 both the same objective, stopped down with front end diafragm?
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Re: Reducing Spherical Aberration In Our Scope Mods
Your CaK experiences you describe sounds like SA to me. The best scopes for CaK are long focus fraunhofer achromats, the worst triplets, and the faster the f-ratio (generally) the worse they are. To see if it was SA try stopping that meade down and see if matter improve as the effective objective diameter gets smaller.Neo wrote:The matter of SA is quite new to me but it reminds me of the days when I failed miserably at getting decent images from my new Lunt 1200 Calcium diagonal that I used on a Meade 127 f7,5 apo. From what I remember the Meade, although being a triplet, it suffered from soft images in the blue channel, being probably even worse in the UV regions. I never got to the bottom of it and have no idea if it could be a case of SA or just poor correction in a range that the budget APO was never designed for in the first place.
The Ha image I'm not sure what to make from that, for 100 mm it is lacking in detail sharpness. (even unprocessed as it is)
Yup, the Ha aint sharp is it. Seeing wasn't the best on that day and know I can get sharper shots than that, but I do think there is something going on with it, not sure what though...
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Re: Reducing Spherical Aberration In Our Scope Mods
It was taken using a Beloptik Tri Band ERF internally in the tube, i'm removing this because it was giving me reflections in CaK configuration, so, wouldn't surprise me if there wasn't something similar happening in Ha. The Baader D_ERF will be front mounted the next time it is used.Neo wrote:How is the 100 mm performing in visual spectrum, on it's own without the HA-gear, have you considered doing a startest in red channel?
No IR-leaking through your filter array?
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Re: Reducing Spherical Aberration In Our Scope Mods
With regards to minimising aberrations near 393 nm, I have heard that lenses from many photo enlargers are very good and also from old fax and photocopy machines (since near uv light was often used with them). However, I have often thought that the venerable RC design should work well here, particular if one also uses a Herschel wedge. There is no lens anywhere in the design and there is plenty of back focus.
I would imagine a properly moded Newtonian would make a good 393 nm scope. I recall seeing a design where the metallization is left off the main mirror so as to act as a built-in Herschel wedge. Some work might need to be done near the focuser to accommodate a stack of filters.
I would imagine a properly moded Newtonian would make a good 393 nm scope. I recall seeing a design where the metallization is left off the main mirror so as to act as a built-in Herschel wedge. Some work might need to be done near the focuser to accommodate a stack of filters.
Re: Reducing Spherical Aberration In Our Scope Mods
It is actually very simple to test a lens for spherical aberration at any wavelength you want ie check for spherochromatism. You setup the lens a double pass autocollimation mode us an optical flat. Since we are taking about small lenses, a good quality flat optics are available for not much money. One can also use a diagonal flat from a large Newtonian. The flat doesn't have to be very flat, just optical smooth so one with couple of waves of power will introduce less then a 1/20 of wave error in the test. It also doesn't need to be coated.
You can either use a white light source and look thru fliters of the wavelengths you wish to test at or use a LED's which produces the wavelengths of interest.
Here is a drawing of how you setup the test using a Ronchi Screen. If the lines are straight the lens has no spherical aberration at the wavelength your testing at. If they bow there are issues. Here is a picture of 4" f/15 lens that I'm testing in green light were it was designed to be perfectly corrected for spherical aberration. As you can see the lens has a problem.
- Dave
You can either use a white light source and look thru fliters of the wavelengths you wish to test at or use a LED's which produces the wavelengths of interest.
Here is a drawing of how you setup the test using a Ronchi Screen. If the lines are straight the lens has no spherical aberration at the wavelength your testing at. If they bow there are issues. Here is a picture of 4" f/15 lens that I'm testing in green light were it was designed to be perfectly corrected for spherical aberration. As you can see the lens has a problem.
- Dave
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Last edited by DavidG on Thu Apr 03, 2014 1:31 pm, edited 1 time in total.
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Re: Reducing Spherical Aberration In Our Scope Mods
Thanks for this information Dave. Yup, looks like there's a bit of trouble with that lens!
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Re: Reducing Spherical Aberration In Our Scope Mods
Dave, that reminder about the Ronchi was perfect!
You could use the same set-up to "match" different objectives i.e. similar Ronchi results = similar SA
You could use the same set-up to "match" different objectives i.e. similar Ronchi results = similar SA
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Re: Reducing Spherical Aberration In Our Scope Mods
Hi Dave
thank you very much for the hint- I'll give it a go ;-)
thank you very much for the hint- I'll give it a go ;-)
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Re: Reducing Spherical Aberration In Our Scope Mods
OK I asked the question on IIS and got the following reply re measurement of SA.
Record in and out of focus images of a star with a webcam/planetary camera and run the Roddier. If your objective is not an APO, or is a fast APO, use a green filter (I usually use an OIII). Spherical aberration is Zernike 11 (third order, which is the "main" spherical) and Z22 is higher order spherical (zones). It will also tell you everything else you want to know about your lens (astigmatism, coma, even colour correction if you use appropriate filters)
http://www.astrosurf.com/tests/roddier/roddier.htm
(use Google translate).
For example, this is the result of my 102/714 triplet, after collimation and centering.
Record in and out of focus images of a star with a webcam/planetary camera and run the Roddier. If your objective is not an APO, or is a fast APO, use a green filter (I usually use an OIII). Spherical aberration is Zernike 11 (third order, which is the "main" spherical) and Z22 is higher order spherical (zones). It will also tell you everything else you want to know about your lens (astigmatism, coma, even colour correction if you use appropriate filters)
http://www.astrosurf.com/tests/roddier/roddier.htm
(use Google translate).
For example, this is the result of my 102/714 triplet, after collimation and centering.
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Re: Reducing Spherical Aberration In Our Scope Mods
This tells nothing about performance at the CaK (393nm). SA at 393nm can easily be close to 1L.Merlin66 wrote:
For example, this is the result of my 102/714 triplet, after collimation and centering.
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Re: Reducing Spherical Aberration In Our Scope Mods
Valery,
yes, but...
If we apply a UV filter we should be able to do the same analysis around CaK region.
yes, but...
If we apply a UV filter we should be able to do the same analysis around CaK region.
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Re: Reducing Spherical Aberration In Our Scope Mods
Take a filter with about 50nm wide and centered at 415 - 425nm and see.Merlin66 wrote:Valery,
yes, but...
If we apply a UV filter we should be able to do the same analysis around CaK region.
Note, please, that this software is a placebo for telescopes owners and not an interferometer.
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Re: Reducing Spherical Aberration In Our Scope Mods
Thanks for the info Ken!
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Re: Reducing Spherical Aberration In Our Scope Mods
That is an interesting thread.
Some thoughts :
- refractors are to be optimized for 500-550nm. Actually there are a lot of dispersion in the real world. For example I measured an FSQ85 that was settled in the blue, and mine is near 700nm.
- Some other brand doesn't suffer from slight mechanical uncertainties. AP for example thanks to their oiled spaced design are right on spot, that is a yellow HENE laser line till mid 00', and then green HENE (543nm).
- The less spacing between the lens, and the less dispersion you will have between to samples of the same instrument for a serious manufacturer. Usually variation come from the mechanical variation, optics part are fairly accurate (for lenses, mirrors are a different story...).
- As I mentioned in another thread, SA follows a straight line vs wavelength. When I measure at 473, 543 and 635nm, I can provide the SA for any lambda down to 350 up to 800 nm. I do this for some labs who need to use several different lambda (tomography for example). If you look the report on Airylab website, you have in some of them graph of SA vs lambda. The slope of the line reflect the amount of spherochromatism. Do forget also that focus can partially compensate for SA (approx. one third).
- It is not possible to use Edmund SA plate unless being in a collimated beam. Do not try this on a converging beam : you would have a very strong coma off axis. Even in a collimated beam you must be just behind the collimating lens.
- Correcting SA requires a doublet. That is quite specific, I was thinking of doing that for large SCT in UV applications (Venus) but on SCTs the variation between different samples in quite great.
I'm not sure the PST lens has a lot of SA in the red. It's a 40mm F10 lens, it's spherochromatism would be very very small. I will measure mine some day out of curiosity.
Frederic.
Some thoughts :
- refractors are to be optimized for 500-550nm. Actually there are a lot of dispersion in the real world. For example I measured an FSQ85 that was settled in the blue, and mine is near 700nm.
- Some other brand doesn't suffer from slight mechanical uncertainties. AP for example thanks to their oiled spaced design are right on spot, that is a yellow HENE laser line till mid 00', and then green HENE (543nm).
- The less spacing between the lens, and the less dispersion you will have between to samples of the same instrument for a serious manufacturer. Usually variation come from the mechanical variation, optics part are fairly accurate (for lenses, mirrors are a different story...).
- As I mentioned in another thread, SA follows a straight line vs wavelength. When I measure at 473, 543 and 635nm, I can provide the SA for any lambda down to 350 up to 800 nm. I do this for some labs who need to use several different lambda (tomography for example). If you look the report on Airylab website, you have in some of them graph of SA vs lambda. The slope of the line reflect the amount of spherochromatism. Do forget also that focus can partially compensate for SA (approx. one third).
- It is not possible to use Edmund SA plate unless being in a collimated beam. Do not try this on a converging beam : you would have a very strong coma off axis. Even in a collimated beam you must be just behind the collimating lens.
- Correcting SA requires a doublet. That is quite specific, I was thinking of doing that for large SCT in UV applications (Venus) but on SCTs the variation between different samples in quite great.
I'm not sure the PST lens has a lot of SA in the red. It's a 40mm F10 lens, it's spherochromatism would be very very small. I will measure mine some day out of curiosity.
Frederic.
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Re: Reducing Spherical Aberration In Our Scope Mods
Hi Frédéric
interesting thoughts, thank you very much
interesting thoughts, thank you very much
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Re: Reducing Spherical Aberration In Our Scope Mods
Thanks for the information Frederic
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