70mm F6 doublet airy disk and rings question

[realflow100]

Whats a typical amount of rings and size of airy disk you would see?
Does star brightness matter for how many rings you see?
Or should it always be a max of 1-2 rings or something?

When I test my telescope dim stars have 1 diffraction ring.
moderately dim stars have 2 rings and a faint third ring.
and brighter and very bright stars have many rings. usually quite rainbowy rings too. looks colorful.
I dont see any evidence of coma or pinched optics.
Seeing isnt affecting my result too much
Using 140x magnification

if I go inside and out of focus just a tiny bit enough to notice a difference
one side looks almost the same as in-focus
and the other side the diffraction rings immediately disappear and a sharp bokeh ball/disk forms
if I defocus a bit further. the diffraction rings on one side of focus fade away and a faint fuzzy bokeh ball/disk begins to gradually appear
and on the other side of focus the sharp bokeh ball just gets larger
if I defocus quite a bit more (so the out of focus disk is at least 10 times the diameter of the in-focus star
in-focus shows a soft bokeh ball/disk with EXTREMELY faint rings inside it
and out-focus shows a sharp crisp bokeh ball/disk with moderately faint rings inside it

[notFritzArgelander]

Counting rings alone isn't much help. Any difference between inside and outside focus images indicates that there is some aberration like spherical aberration or astigmatism.

There is free software available for evaluating star test images. It can be found at http://aberrator.astronomy.net

There are also several dozen examples of infocus and outfocus images and some discussion of what they mean. Some examples:

http://aberrator.astronomy.net/scopetes ... _80_4.html
http://aberrator.astronomy.net/scopetes ... 100_2.html
http://aberrator.astronomy.net/scopetes ... r80_2.html

PS The Airy disk refers only to the image at the focus point.

[realflow100]

Cant download. Not found. error.

[realflow100]

Nevermind I got it.
Theres somewhere between 0.1 and 0.2 spherical aberration?
less than 0.3 maybe. just by guessing. and messing around with it for a while.

What would that mean?

[notFritzArgelander]

Nevermind I got it.
Theres somewhere between 0.1 and 0.2 spherical aberration?
less than 0.3 maybe. just by guessing. and messing around with it for a while.

What would that mean?

The fractions you are quoting are fractional wavelength errors at focus. The standard commercial grade optics are for one quarter or less wavelength error. So it means that your scope is "as expected".

[realflow100]

Oh.
So is that not particularly bad then? its pretty close to being perfect now? nothing too far out of tolerance?
I took an image at the absolute best focus I could possibly achieve (taking several minutes and many test photos to get it absolutely as perfect as possible)

and this is the result of what my stars look like (With very very slight trailing due to sky rotation and the alignment process of the 58 frames in DSS)
58 frames of 10 second exposures stacked and aligned. iso 1600
full image. and crop. of polaris.

[realflow100]

visually my telescope image seems almost perfect.
no signs of astigmatism. pinched optics. coma. or weirdness.
looks very sharp up to 70x magnification. and still decent at 140x
Star test is showing just slightly a tiny bit of under/over correction (objective lens element spacing) is a tiny bit off. but its not off too much. can see the bands on jupiter at least. and rings on saturn.

[notFritzArgelander]

Oh.
So is that not particularly bad then? its pretty close to being perfect now? nothing too far out of tolerance?

The word "perfect" has many meanings depending on context. It means different things depending on what criterion is used to specify the largest allowable error. Here's a list:

https://www.telescope-optics.net/effects1.htm

So your scope has no errors larger than permitted by the Rayleigh or Danjon-Couder criteria. My SV ED80A is more perfect than that by a factor of about 3-4 in terms of reduced wavefront error.

1/4 wave gives a Strehl (match of the diffraction pattern brightness to a truly perfect scope) of about 0.80. My SV ED80 A has a Strehl of 0.991.

How "perfect" an objective lens is going to have a big effect on price.

[notFritzArgelander]

visually my telescope image seems almost perfect.
no signs of astigmatism. pinched optics. coma. or weirdness.
looks very sharp up to 70x magnification. and still decent at 140x
Star test is showing just slightly a tiny bit of under/over correction (objective lens element spacing) is a tiny bit off. but its not off too much. can see the bands on jupiter at least. and rings on saturn.

So it pleases and you should consider not tinkering further and simply enjoy.

[realflow100]

so does that mean its close enough that the image is as sharp as it will realistically be for a 70mm F6 non-ED glass doublet up to 140x?
how close is it? is it within margin of error? or what exactly?
Does the out of focus pattern not matter when its below a certain level. with the in-focus image being as sharp as it will really get?

[notFritzArgelander]

so does that mean its close enough that the image is as sharp as it will realistically be for a 70mm F6 non-ED glass doublet up to 140x?
how close is it? is it within margin of error? or what exactly?
Does the out of focus pattern not matter when its below a certain level. with the in-focus image being as sharp as it will really get?

As long as the inside and outside focus patterns are consistent with less than 0.25 wave and 0.80 Strehl or more then you've achieved all that any mass produced commercial optics can achieve. To do better you'd likely have to repolish / refigure and recoat the lenses. You'd then get practical experience on why more perfect scopes cost more OR you might not have a telescope anymore. I'd advise against trying for more. Unless you want the learning experience more than the scope, in which case, go for it.

[realflow100]

I thought just changing the lens element spacing so that its properly corrected is the right way to get the smallest possible spherical aberration?
if its spaced wrong wouldn't that cause what im observing?
does it not matter if its 0.25 wave or less at that point?

[notFritzArgelander]

I thought just changing the lens element spacing so that its properly corrected is the right way to get the smallest possible spherical aberration?
if its spaced wrong wouldn't that cause what im observing?
does it not matter if its 0.25 wave or less at that point?

A lot goes into the 0.25 wave rms error at focus that you might see. It's not JUST about spacing. Spacing is the easiest fix to try that is all. It isn't any more than that.

The errors at focus are due to:

• suboptimal spacing

• suboptimal rotation of the lenses with respect to each other

• errors in the smooth curvature of the 4 curved surfaces in the doublet

• roughness in those four curves

• slight non uniformity in the coatings

• etc., etc. etc.

So spacing error is only one of many possible causes of what you are seeing. It is the ONLY one that can be addressed easily. Given that the lenses are not superfine polished it'd be astonishing if you got MUCH better than what you are seeing. For spacing to be the ONLY problem, the lenses would have to be of much better quality and too expensive to be put in this scope. Fine figuring and polishing of lenses takes time and money and perhaps specialized equipment.

I don't recommend trying for more. If you want to, hey, it's your scope.

[realflow100]

did a test with my telescope for rresolution/contrast detectability
I can very very VERY easily see contrast details much much smaller than the airy disk diameter in seeing
even smaller than the width of the diffraction rings around the airy disk
4 or 5 times smaller diameter contrast features detectable. like fine details in terrestrial or the moon. or maybe even smaller. almost the limit of my eyesight even at 105x magnification. even 140x still has gobs of fine contrast texture detail on things.
When the seeing is good enough that is. sometimes its so bad detail is blurred to the point even airy disk is not even detectable and its just a blurry heatwave over everything with a max useful magnification of like 40x
but in near perfect seeing I can see great contrast and sharpness much smaller than airy disk diameter or even the diffraction rings diameter up to 140x especially if i really nail the focus!

seeing/atmosphere turbulence is pretty much the only limiting factor at 105 to 140x magnification I think even higher than 140x could maybe work without getting blurry. at least on the moon when its more about half-phase

[notFritzArgelander]

did a test with my telescope for rresolution/contrast detectability
I can very very VERY easily see contrast details much much smaller than the airy disk diameter in seeing
even smaller than the width of the diffraction rings around the airy disk
4 or 5 times smaller diameter contrast features detectable. like fine details in terrestrial or the moon. or maybe even smaller. almost the limit of my eyesight even at 105x magnification. even 140x still has gobs of fine contrast texture detail on things.
When the seeing is good enough that is. sometimes its so bad detail is blurred to the point even airy disk is not even detectable and its just a blurry heatwave over everything with a max useful magnification of like 40x
but in near perfect seeing I can see great contrast and sharpness much smaller than airy disk diameter or even the diffraction rings diameter up to 140x especially if i really nail the focus!

seeing/atmosphere turbulence is pretty much the only limiting factor at 105 to 140x magnification I think even higher than 140x could maybe work without getting blurry. at least on the moon when its more about half-phase

The Airy disk resolution only applies to point sources, stars. Finer contrast details on extended objects, terrestrial, lunar, planetary is "as expected". It's easier for the eye to detect a very thin line than the separation between two points.

[John Baars]

Whats a typical amount of rings and size of airy disk you would see?
Does star brightness matter for how many rings you see?
Or should it always be a max of 1-2 rings or something?

When I test my telescope dim stars have 1 diffraction ring.
moderately dim stars have 2 rings and a faint third ring.
and brighter and very bright stars have many rings. usually quite rainbowy rings too. looks colorful.
I dont see any evidence of coma or pinched optics.
Seeing isnt affecting my result too much
Using 140x magnification

if I go inside and out of focus just a tiny bit enough to notice a difference
one side looks almost the same as in-focus
and the other side the diffraction rings immediately disappear and a sharp bokeh ball/disk forms
if I defocus a bit further. the diffraction rings on one side of focus fade away and a faint fuzzy bokeh ball/disk begins to gradually appear
and on the other side of focus the sharp bokeh ball just gets larger
if I defocus quite a bit more (so the out of focus disk is at least 10 times the diameter of the in-focus star
in-focus shows a soft bokeh ball/disk with EXTREMELY faint rings inside it
and out-focus shows a sharp crisp bokeh ball/disk with moderately faint rings inside it

Good optics show:
-no rings on dim stars.
-no rings on moderately dim stars, maybe one very faint and thin one depending on the brightness.
-one faint and thin ring on bright stars and the faintest of all as a second ring if seeing permits.

More rings mean spherical aberration. There is no way of encountering top-optics in a departement-store lens. Individual testing, correcting, retesting and recorrecting takes a lot of time of the optician working on it. His wages count. In a factory which produces the cheapest telescopes, usually only one of lenses from an entire batch is tested. If correction is needed, it is applied to the whole batch at once.

The startest you describe sounds like spherical aberration 0.5 wave or bigger. There is no way of correcting this by distance between lenses, only refiguring.

Resolving point sources is different from linear resolvement on objects with a certain surface area; this is expressed in lines/mm, and always outruns resolving pointsources in the way nFA already said. ( this is why we see the Cassini division and cloud belts on Jupiter) If you don't have comparison material in the form of top optics, the daytime image in a telescope will quickly appear sharp. An observation from which you would quickly reconsider if given the same conditions you were looking through top optics at high magnification.

What I know from previous threads of yours about this telescope, you've already taken it to the limit. It couldn't be better, given the circumstances. Time to just start enjoying it?

[realflow100]

I was testing it at night though
and running through aberrator the wave error doesnt resemble 0.5 at all. way too different looking
it resembles closer to 0.1 to 0.2 waves of error fairly closely
I havent been able to see cassini division yet. but maybe that could just be seeing/atmosphere needs to be extremely good?
on jupiter i see very rare over the course of a few minutes periods of fleeting sharpness where I see slightl detail in the cloud belts. but I havent located the red spot yet. though it should be visible just need to look harder and longer for it

[John Baars]

moderately dim stars have 2 rings and a faint third ring.
and brighter and very bright stars have many rings.

There is no way a 0.1 to 0.2 wave system produces this.
Intra-and extra focal images in Aberrator start at 2 waves defocusing; try more wavelengths defocussing (as in real life) and you'll be surprised at the multitude of differences. For a better overall view which fits in the images you can even change the aperture to 200mm.

My 0.25 wave undercorrected 70mm Televue Pronto (not the least among small refractors) could only just distinguish the Cassini Division. A 0.1 wave corrected instrument (which is very good) can.
The fact that this has not yet succeeded in your instrument, I attribute, partly in view of your remarks about the rings, not to seeing or other circumstances but to the instrument itself.

Believe me, in a batch of a hundred factory ground and polished lenses there is not one that suddenly stands out head and shoulders above the rest, as if it had been hand-corrected for hours.
As a matter of fact is is even not of the greatest importance, as long as you enjoy the views your own telescope gives you. Much more important.

[realflow100]

I finally took my telescope outside just after dark. full moon and all. and took some good 20 minute looks at both saturn and jupiter at 140x magnification
the seeing/atmosphere seemed to be very decently stable jupiter was a clear disk with 2 easily identifiable bands. and looked very nice. fleeting moments I saw maybe 1 or 2 additional bands.
Saturn had very clear ring separation from the planet. and maybe a faint band just above the rings of the planet too.
and I'm almost certain I saw cassini division for a few brief split-second moments in moments of exceptional atmosphere seeing. more than 50% certain
Also double double was confirmed 100% identifiable as two pairs of two stars very close together (a lot closer than I expected. but I was able to detect the orientation of the stars at least.)
The image is flipped left-right since im using a refractor. so they are the other way around compared to stellarium.
One pair was more vertical and the other pair was more horizontal.
and I'm pretty sure one pair was very slightly closer together
The left pair was roughly horizontal orientation. and the right pair was roughly vertical. though both are slightly tilted. and it took a minute or so to really see them. as they are very very dim and fairly faint due to the full moon and lots of light pollution.
Does this sound like typical results for a 70mm achromat doublet telescope? (non-ED)

[John Baars]

(...)
Does this sound like typical results for a 70mm achromat doublet telescope? (non-ED)

Sorry to dissapoint you. To me It doesn't sound fully like a 70mm achromat.
I know that in many reviews the double double is put forward as a kind of accuracy test. However, they are not, especially for larger instruments. With 2.3 and 2.4 arcsec separation, they are at most a good test for a 50mm instrument. With a good 70mm instrument they are already separable well below 100X. At 70X or 80X you already get the impressions as you describe above.

Nice that you managed to separate the A and B ring of Saturn, you can expect that from a 70mm achromatic. A good achievement. The thin line of the Cassini separation itself is more difficult of course. Remember Cassini must have seen it worse than you and I saw it through our 70mm telescopes. Cassini would have squinted with envy.

All in all, you should expect the performance on Saturn and Jupiter as you described from a machine-made lens. On double stars, however, it lags behind. I expected the latter when you described the diffraction rings. A well made 60mm f/15 lens at 120X does a better job.
But please don't let my remarks restrain you from a pleasant view of the sky.