Opinions - New Askar 103mm APO triplet

[Louis D]

9 hours ago, Elp said:

But if the colours dictated by mono LRGB filters are in focus they're in focus aren't they? If ones soft it'd be not in focus.

Not sure what your point is, but if it is that a particular channel needs a different focus to be sharp than the other channels, then so be it.  However, it could also be that due to design choices, the blue channel is less well corrected and doesn't ever reach a sharp focus relative to the other channels.

[Elp]

That's the way I understand it for OSC, not all colours would necessarily be in focus at the same point.

If you're using filters for mono imaging however you'd refocus for each filter, there should not be a colour channel which is "soft".

Edited March 9 by Elp

[Nik271]

No, there is also spherochromatism, that is spherical aberration which differs from colour to colour. So for example if green light is perfectly focused the blue  may not come to  a single focus point. The central part of the lens will have one focus point in blue and the outer part will have a slightly different focal point in blue. Minimising this aberration is the key distinction between a budget refractor and a premium one.

Edited March 9 by Nik271

[vlaiv]

13 minutes ago, Elp said:

That's the way I understand it for OSC, not all colours would necessarily be in focus at the same point.

If you're using filters for mono imaging however you'd refocus for each filter, there should not be a colour channel which is "soft".

If you shoot luminance you still use the whole spectrum, so it really needs to be RGB (rather than LRGB) and even then, there are reasons why blue might be softer than other colors.

One reason is spherochromatism, other is how the scope is optimized and third reason is that atmosphere impacts shorter wavelengths the most. In reality - I think that all of these three reasons combine with different contributions to make blue channel softer than the rest.

This is why I sometimes say that probably best way to do true color images is to do LRG imaging.

[Elp]

But the person mentioned they were using a mono camera...

[vlaiv]

9 minutes ago, Elp said:

But the person mentioned they were using a mono camera...

Yes, that is why all of the above is relevant.

- if using mono camera on less than well corrected scope - you will still shoot all the wavelengths at the same time - much like OSC is doing and some of those wavelengths will be out of focus - creating blur. This is why you are limited to LRGB type of imaging (LRGB filters with mono, but using only R, G and B filters - each with their focus position).

- even if you do that - there is a chance that one channel will be not be well corrected if scope is not well corrected. Usually that is blue channel as it contains shortest wavelengths - those are bent the most by refraction. Then there is spherochromatism - or fact that not all wavelengths of light have good spherical correction.

You can identify such situation from following graph:

This is ED scope 150 F/8 - but it shows what needs to be seen nicely.

On X axis - we plot defocus - or how much you have to move focal plane for particular point to be in focus. On Y axis - we have distance from optical center to edge of the lens. If you look above graph for say yellow line (620nm wavelength), at very bottom of the graph (center of lens) you will see that it focuses just a bit further away than what we have decided is optimal focus - but as it moves away from optical center - it defocuses more, but then at say 90% towards the edge of the lens, beams start to focus shorter than that and even focus shorter than our designated focus position.

When focal length of lens depends on distance from the center - that is what we call spherical aberration.  Here is graph from Wiki page on spherical:

In perfect lens - upper one, all rays intersect at the same point - have same focal length, but in bottom image - rays that are further away from center - have shorter focal length (intersect sooner).

Looking at top graph that shows ED lens performance - we can see that if line is straight - it has no spherical aberration. Further -if it sits on X=0 - there is no chromatism for that wavelength - it has exact focal length. Chromatism or secondary spectrum is when focal length depends on wavelength of light.

In any case - in above graph we can see that no wavelength is perfect - they are all bent and they are all some distance way from X=0 at some height (or even the whole time). But look at how bent the green line is and how bent the blue line is. Green is 500nm (green light) and blue is 436nm - or blue towards the violet. Blue line is much more bent than green - which means it has more spherical aberration.

In the end - here is another graph:

This graph shows difference between single lens, doublet lens, triplet lens and superachromat. Each of these lenses brings progressively more wavelengths into same focus. Simple lens / singlet will have any one wavelength at the focus at any time. Doublet will bring two wavelengths in focus at the same time, triplet will bring three and super achromat will bring four.

Each curve is progressively closer to true focus - which means less defocused wavelengths and less false color / secondary spectrum. However - even triplet (orange line in above graph) - won't bring all colors into focus and due to shape of the curve - some wavelengths will be more defocused than the others. Look at defocus at 400nm versus 700nm.  Curve shows much bigger distance from 0 on X axis at 400nm (this time Y axis shows wavelength rather than distance from center of the lens). But that is not important bit - what is important is range of defocus for each of R, G and B sections.

If we look at 400-500nm range we can see that defocus ranges from -4 to ~1.5. that is 5.5 arbitrary units of focus range. 500-600nm or green part will have from ~1.5 to ~0.5  and that is 1 arbitrary unit of range and red will have from ~0.5 to ~ -0.5 or again 1 arbitrary unit of range.

Depending on critical focus zone of the telescope - you might be able to adjust focus for green color 500-600nm range and red color 600-700nm range so that whole parts of spectrum seem like in focus (that one arbitrary unit), but if critical focus is say 3 arbitrary units wide - then you simply won't be able to have whole 400-500nm range in focus as it is 5.5 arbitrary units wide.

By the way - optical designer has freedom to tilt that S curve somewhat and this is what we call correction - wider focus range can be put in red part of spectrum - such scope is blue corrected or in blue part of spectrum - so we call it red corrected.

Most of the time optical designers opt for red correction - which leaves blue side somewhat softer.

 

 

[900SL]

10 hours ago, vlaiv said:

Yes, that is why all of the above is relevant.

- if using mono camera on less than well corrected scope - you will still shoot all the wavelengths at the same time - much like OSC is doing and some of those wavelengths will be out of focus - creating blur. This is why you are limited to LRGB type of imaging (LRGB filters with mono, but using only R, G and B filters - each with their focus position).

- even if you do that - there is a chance that one channel will be not be well corrected if scope is not well corrected. Usually that is blue channel as it contains shortest wavelengths - those are bent the most by refraction. Then there is spherochromatism - or fact that not all wavelengths of light have good spherical correction.

You can identify such situation from following graph:

This is ED scope 150 F/8 - but it shows what needs to be seen nicely.

On X axis - we plot defocus - or how much you have to move focal plane for particular point to be in focus. On Y axis - we have distance from optical center to edge of the lens. If you look above graph for say yellow line (620nm wavelength), at very bottom of the graph (center of lens) you will see that it focuses just a bit further away than what we have decided is optimal focus - but as it moves away from optical center - it defocuses more, but then at say 90% towards the edge of the lens, beams start to focus shorter than that and even focus shorter than our designated focus position.

When focal length of lens depends on distance from the center - that is what we call spherical aberration.  Here is graph from Wiki page on spherical:

In perfect lens - upper one, all rays intersect at the same point - have same focal length, but in bottom image - rays that are further away from center - have shorter focal length (intersect sooner).

Looking at top graph that shows ED lens performance - we can see that if line is straight - it has no spherical aberration. Further -if it sits on X=0 - there is no chromatism for that wavelength - it has exact focal length. Chromatism or secondary spectrum is when focal length depends on wavelength of light.

In any case - in above graph we can see that no wavelength is perfect - they are all bent and they are all some distance way from X=0 at some height (or even the whole time). But look at how bent the green line is and how bent the blue line is. Green is 500nm (green light) and blue is 436nm - or blue towards the violet. Blue line is much more bent than green - which means it has more spherical aberration.

In the end - here is another graph:

This graph shows difference between single lens, doublet lens, triplet lens and superachromat. Each of these lenses brings progressively more wavelengths into same focus. Simple lens / singlet will have any one wavelength at the focus at any time. Doublet will bring two wavelengths in focus at the same time, triplet will bring three and super achromat will bring four.

Each curve is progressively closer to true focus - which means less defocused wavelengths and less false color / secondary spectrum. However - even triplet (orange line in above graph) - won't bring all colors into focus and due to shape of the curve - some wavelengths will be more defocused than the others. Look at defocus at 400nm versus 700nm.  Curve shows much bigger distance from 0 on X axis at 400nm (this time Y axis shows wavelength rather than distance from center of the lens). But that is not important bit - what is important is range of defocus for each of R, G and B sections.

If we look at 400-500nm range we can see that defocus ranges from -4 to ~1.5. that is 5.5 arbitrary units of focus range. 500-600nm or green part will have from ~1.5 to ~0.5  and that is 1 arbitrary unit of range and red will have from ~0.5 to ~ -0.5 or again 1 arbitrary unit of range.

Depending on critical focus zone of the telescope - you might be able to adjust focus for green color 500-600nm range and red color 600-700nm range so that whole parts of spectrum seem like in focus (that one arbitrary unit), but if critical focus is say 3 arbitrary units wide - then you simply won't be able to have whole 400-500nm range in focus as it is 5.5 arbitrary units wide.

By the way - optical designer has freedom to tilt that S curve somewhat and this is what we call correction - wider focus range can be put in red part of spectrum - such scope is blue corrected or in blue part of spectrum - so we call it red corrected.

Most of the time optical designers opt for red correction - which leaves blue side somewhat softer.

 

 

Thanks Vlaiv, that was very helpful in understanding the causes.