171 replies to this topic

[stevew]

Would a 6 inch refractor with a .90 strehl ratio out perform a 5 inch refractor with a .98 strehl ratio on the planets ?

I understand that the extra light gathering would be noticeable on deep sky objects, and I'm assuming that the higher resolution of the 6 inch should show more details.

But when does quality out perform quantity?

[TxStars]

When looking at fine detail a higher quality scope can provide more detail than a larger scope that is not as good.

It also depends a lot on local seeing conditions supporting a larger scope.

[RichA]

Would a 6 inch refractor with a .90 strehl ratio out perform a 5 inch refractor with a .98 strehl ratio on the planets ?

I understand that the extra light gathering would be noticeable on deep sky objects, and I'm assuming that the higher resolution of the 6 inch should show more details.

But when does quality out perform quantity?

Good question.  You could stick them in a test facility measure them, do resolution and contrast tests, but that would be eliminating the variables seen in real observing.  Also, the kinds of aberrations leading to the lower strehl ratio have different impacts on resolution and contrast. 

[stevew]

Good question.  You could stick them in a test facility measure them, do resolution and contrast tests, but that would be eliminating the variables seen in real observing.  Also, the kinds of aberrations leading to the lower strehl ratio have different impacts on resolution and contrast. 

This is true, however the .90 strehl scope is still more than diffraction limited, and being an inch larger should have a higher resolution. Not to mention the extra light gathering that would tickle the color receptors in your eye, to show more detail on Jupiter's belts..

[PowerM3]

From personal experience I would say that its almost certain the higher quality scope will show more detail if we are only talking about 1". It its a larger difference(say like 3+") and the seeing if excellent then it becomes harder to tell. 

[Codbear]

For your specific example, I would find it hard to believe that the .08 difference in Strehl rating could offset a 44% increase in light-gathering power in either brightness or detail. 

 

It's an interesting and a good question. At some difference in Strehl ratings the answer would of course favor the smaller aperture but, like you, I don't know where that is.

 

Perhaps an optical heavyweight might weigh in with a more informed analysis.

[bobhen]

From the designer and telescope maker R.F. Royce…

 

“From this optician's viewpoint a reasonable standard for telescopic objectives of high-quality would be those producing a Strehl ratio from preferably .96 (1/9 wave, P-V) with .97 (1/10 wave, P-V) as representing the best that is reasonably attainable.

 

.90 Strehl is around 1/5 wave or a touch better and according to a Royce chart is “below” what is considered “even good” for a high-quality optic.

 

Consider that it has been well documented that high quality Tak and AP 6 and 7-inch refractors have delivered better planetary performance than much larger 9.25, 11 and 14” SCTs, which of course have much larger apertures but are also less well figured.

 

Consider also this statement (HERE is a link) from Carl Zambuto about optical quality and contrast. Contrast is important for planetary observing.

 

From Roland Christen…

“Compare the roughness of the wavefront of the different types of scopes. The effect of roughness is to scatter light from the Airy disc into the diffraction rings. In the case of planetary viewing, it creates a thin veil of light that lowers contrast.”

 

Consider that Roland Christen, in his article on telescopes for planetary observing, when talking about refractors for planetary performance compares smaller 7 to 9” apo refractors to larger 8 to 12” achromatic refractors.

 

Considering the above, I think a reasonable conclusion “in your example” is that for planetary performance and “observing the finest details and subtle tonality differences ” the higher optical quality telescope will have better contrast and would deliver more.

 

Bob

Edited by bobhen, 25 January 2021 - 07:59 AM.

[tgreene]

I think we can supply an answer to the OP's question that is a bit more quantitative.

Let's consider 3 areas: brightness (or signal-to-noise), spatial resolution, and contrast.

We have already covered brightness; the 6" collects 44% more light.

Both telescopes are diffraction limited, so the 6" has ~20% better spatial resolution.

Contrast is the interesting one. If the wavefront errors are randomly distributed (not into 1 particular
aberration), then the 5" would have contrast ~(0.98/0.9)^2 = 20% better than the 6". Would this be obvious
on the planets (OP's question)? I'm not sure given that the 6" has 20% higher resolution. I think it would
be close but the devil is in the details of how the WFE is distributed.

Edited by tgreene, 25 January 2021 - 04:17 PM.

[gwlee]

In using a variety of small scopes (60-210mm) that varied from good quality scopes to sensibly perfect scopes, my larger scopes have always performed better. 

[bobhen]

I think we can supply an answer to the OP's question that is a bit more quantitative.

Let's consider 3 areas: brightness (or signal-to-noise), spatial resolution, and contrast.

We have already covered brightness; the 6" collects 44% more light.

Both telescopes are diffraction limited, so the 6" has ~20% better spatial resolution.

Contrast is the interesting one. If the wavefront errors are randomly distributed (not into 1 particular
aberration), then the 5" would have contrast ~(0.98/0.9)^2 = 20% better than the 6". Would this be obvious
on the planets (OP's question)? I'm not sure given that the 6" has 20% higher resolution. I think it would
be close but the devil is in the details of how the WFE is distributed.

The "20% resolution" advantage of the 6" in your example will not be realized because it has lower optical quality. If the 6" had "the same" optical quality as the 5, then the 6" would have a full 20% resolution advantage.

 

Bob

Edited by bobhen, 26 January 2021 - 02:02 PM.

[starcanoe]

One thing to consider regarding resolution. A 6 inch resolves 20 percent better..for double stars and looking at stuff like lines for whatever reasons you have. But a 20 percent resolution is 44 percent MORE information on the same TWO dimensional images. Or think of it this way. If you have a lower resolution monitor with an image of Jupiter where you can see the pixels...if you increase the number of pixels by 44 percent you are going to notice a lot more detail (think of airy disks as pixels).

[RichA]

This is true, however the .90 strehl scope is still more than diffraction limited, and being an inch larger should have a higher resolution. Not to mention the extra light gathering that would tickle the color receptors in your eye, to show more detail on Jupiter's belts..

Resolution is dictated by aperture, all things being equal.  But a 1/4 wave scope, (diffraction-limited)  would reduce contrast and thus resolution for areas of similar intensity would be reduced.  In other words, equal double star resolution wouldn't be impacted but planetary features likely would.  You need considerably better quality than diffraction-limited to maximize contrast and therefore resolution in most cases.  The colour point is true;  better-defined colours can lead to increased visibility of detail, no question so aperture is again important.  There have to be other things at work (seeing, for example) to make a six-inch scope perform at the level (or below) that of a 4 inch scope.  A good way to see that is to observe the effect magnification has on images in a smaller and larger scope.  As magnification climbs, delineation of colours drops, for all scopes.  But beyond a certain point, the small scope visibly loses colour  tonality owing to dimming of the image.  A 4 inch refractor at 300x produces somewhat drab-looking planetary images compared to a 6 or 8 inch at the same power.

[bobhen]

One thing to consider regarding resolution. A 6 inch resolves 20 percent better..for double stars and looking at stuff like lines for whatever reasons you have. But a 20 percent resolution is 44 percent MORE information on the same TWO dimensional images. Or think of it this way. If you have a lower resolution monitor with an image of Jupiter where you can see the pixels...if you increase the number of pixels by 44 percent you are going to notice a lot more detail (think of airy disks as pixels).

But in your example the "pixel quality" is the same so more is better.  But that is not the case with the example of the 2 refractors. The 5" will have "better pixel quality".

 

One has to account for the lower optical quality of the 6” optic. 

 

For an extreme example: A 10” telescope “on paper” has a resolving POTENTIAL of around .46 arcseconds. However, if the optic is figured to only one wave, that .46 arcsecond resolving potential will never be realized. A 6” telescope with optics that are 1/12 wave will show far more planetary details where the larger 10” with the very poor optics might show just a plain white disk.

 

Bob

[tgreene]

The "20% resolution" advantage of the 6" in your example will not be realized because it has lower optical quality. If the 6" had "the same" optical quality as the 5, then the 6" would have a full 20% resolution advantage.

I believe that is incorrect if you look at the numbers given in the OP's question and in my response. The question was whether a 6" with 0.90 Strehl would outperform a 5" with 0.98 Strehl. 0.90 Strehl is still diffraction limited, and the Airy PSF would still have almost exactly the same FWHM as a telescope with perfect Strehl (assuming randomly distributed WFE). Therefore a 6" telescope with 0.9 Strehl will have superior resolution (~20% better) than a 5" one with 0.98 Strehl because Airy disk size determines resolution. The difference is that the lower Strehl telescope will put more light into the diffraction wings outside of the central disk, lowering contrast. This is nicely illustrated here: https://www.telescop....net/Strehl.htm (see Strehl and encircled/ensquared energy section).

Edited by tgreene, 26 January 2021 - 04:31 PM.

[Stopforths]

Good question.  You could stick them in a test facility measure them, do resolution and contrast tests, but that would be eliminating the variables seen in real observing.  Also, the kinds of aberrations leading to the lower strehl ratio have different impacts on resolution and contrast. 

There will be finer detail say on jupiter more easily seen in the .98 of lessor aperture.  Seen this with brother kevs zeiss APQ 100/1000 versus my 5 inch f-15 D &G.  I ended up selling the D &G which was a fine scope because an intes micro 715 Delux noticeably outperformed it interestingly.

 

Never forgotten this the image in perfect seeing was like an etching only 100 mm of perfection

 

Now lusting after a higher end apo its a terrible disease....................

[junomike]

I believe that is incorrect if you look at the numbers given in the OP's question and in my response. The question was whether a 6" with 0.90 Strehl would outperform a 5" with 0.98 Strehl. 0.90 Strehl is still diffraction limited, and the Airy PSF would still have almost exactly the same FWHM as a telescope with perfect Strehl (assuming randomly distributed WFE). Therefore a 6" telescope with 0.9 Strehl will have superior resolution (~20% better) than a 5" one with 0.98 Strehl because Airy disk size determines resolution. The difference is that the lower Strehl telescope will put more light into the diffraction wings outside of the central disk, lowering contrast. This is nicely illustrated here: https://www.telescop....net/Strehl.htm (see Strehl and encircled/ensquared energy section).

All theory aside I'd take the 6" apo as IME along with potentially resolving more, the added brightness (44%) is constant provided it's diffraction limited (or better).

 

We need Eddgie with one of his MTF graphs.....

[mikeDnight]

I read some time ago that Takahashi's FC100DZ has a 0.99 Strehl, which when looking at the graph below it seems to be in the centre. Then today I read that the DZ has a 0.97 across the visible spectrum. I've no real idea how this would impact the performance of a well figured refractor like the Tak, but could anyone actually see the difference between a 0.97, 0.98, 0.99 Srehl?

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[Jeff B]

The answer is it depends on what's degrading the larger aperture.

 

Case in point recently for me was an APM 152ED I recently lived with for a while, which has a bum edge.  Images of the moon, Jupiter and Saturn were soft, lacking low level detail and contrast at high powers.   Stopping it down to 5" increased image quality of the objective.   The larger aperture had an obviously lower strehl than that of the smaller aperture, so much so that the smaller aperture with its higher Strehl easily out performed the larger aperture with the lower strehl in every way except brightness at equal magnifications.  The paperwork that came with the scope showed a strehl of .943, which I do not believe as the surface contour plot shows no hint of the turned edge and looked rather like what I would have expected if the objective was stopped to 5".

 

Jeff

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[stevew]

The answer is it depends on what's degrading the larger aperture.

 

Case in point recently for me was an APM 152ED I recently lived with for a while, which has a bum edge.  Images of the moon, Jupiter and Saturn were soft, lacking low level detail and contrast at high powers.   Stopping it down to 5" increased image quality of the objective.   The larger aperture had an obviously lower strehl than that of the smaller aperture, so much so that the smaller aperture with its higher Strehl easily out performed the larger aperture with the lower strehl in every way except brightness at equal magnifications.  The paperwork that came with the scope showed a strehl of .943, which I do not believe as the surface contour plot shows no hint of the turned edge and looked rather like what I would have expected if the objective was stopped to 5".

 

Jeff

Jeff, Great example. It reminded me that I once had a Meade 127ED that had enough spherical aberration that it barely showed any more planetary details than a 4 inch Televue I had. 

As was mentioned by someone earlier in this thread, It depends on what type of aberrations the larger scope has, and the severity of them.

 

How is it possible that the APM 152 came with a strehl rating of .943 yet your test show obvious edge problems.

What do you think the actual strehl ratio is of the full six inch aperture of the APM? 

[Stopforths]

below .8 I reakon well below it.

[TxStars]

The Add:   Our 8" Seemore scope has  0.99 strehl 

Reality :  "The center 1" was 0.99 strehl"  but the image is bright..  LoL

[mikeDnight]

The answer is it depends on what's degrading the larger aperture.

 

Case in point recently for me was an APM 152ED I recently lived with for a while, which has a bum edge.  Images of the moon, Jupiter and Saturn were soft, lacking low level detail and contrast at high powers.   Stopping it down to 5" increased image quality of the objective.   The larger aperture had an obviously lower strehl than that of the smaller aperture, so much so that the smaller aperture with its higher Strehl easily out performed the larger aperture with the lower strehl in every way except brightness at equal magnifications.  The paperwork that came with the scope showed a strehl of .943, which I do not believe as the surface contour plot shows no hint of the turned edge and looked rather like what I would have expected if the objective was stopped to 5".

 

Jeff

Thats a great example Jeff. Is the APM a rebadged Skywatcher?  I'm only asking as I'd imagined APM to be better spec. 

[Jon Isaacs]

The "20% resolution" advantage of the 6" in your example will not be realized because it has lower optical quality. If the 6" had "the same" optical quality as the 5, then the 6" would have a full 20% resolution advantage.

 

Bob

 

Not true. The advantage may not be fully realized but splitting doubles, the 6 inch will split closer doubles.

 

But in your example the "pixel quality" is the same so more is better.  But that is not the case with the example of the 2 refractors. The 5" will have "better pixel quality".

 

One has to account for the lower optical quality of the 6” optic. 

 

For an extreme example: A 10” telescope “on paper” has a resolving POTENTIAL of around .46 arcseconds. However, if the optic is figured to only one wave, that .46 arcsecond resolving potential will never be realized. A 6” telescope with optics that are 1/12 wave will show far more planetary details where the larger 10” with the very poor optics might show just a plain white disk.

 

Bob

 

Both optical quality and aperture are important. 

 

I think we are talking about realistic telescopes. A 10 inch with a 1 wave mirror will "blow" away a perfect 10 mm optic. 1 wave mirrors and 10 mm apertures should not be part of this discussion.

 

In terms of resolving power, a "decent" 10 inch will out perform a 5 inch on the planets, on trouble stars..

 

I've split doubles under 0.5" with my 10 inch.. Of course it's not just an average, run of the mill 10 inch Dob.  It cost me $240 on Astromart 18 years ago and I had to find the pinched secondary.. after that, it was just another made in Taiwan 10 inch F/5 that lives where 0.5" seeing does happen. 

 

Last night I was getting clean wide splits on doubles around 1.0"  in a 5 inch, those are nearly Dawes limit splits and involve overlapping Airy disks.. not clean and wide.

 

When one looks at the relative sizes of the Airy disks and considers the actual meaning of Strehl, concentrated energy, judged as a 10 inch, a 5 inch puts a small fraction of the energy into the region the size of the 10 inch's Airy disk.

 

Jon

[CHASLX200]

I would rather have a near perfect 3" scope vs a not so good 18" scope.

[Deadlake]

Thats a great example Jeff. Is the APM a rebadged Skywatcher? I'm only asking as I'd imagined APM to be better spec.

If this is the 152ED I believe they are made by Sharpstar. APM from what I can tell has moved its non LZOS production to Long Perng, I leave you to speculate why. However once a scope has left the Sharpstar factory they will not take it back if any issues so some dealers will not touch them.