Aperture steps and their real world impact on visual performance

[John]

I was going to post this under an existing thread on whether a poster needed a 5 inch refractor but I thought it might benefit from a thread of it's own, so here it is.

As a rule of thumb, it is said that, with newtonian telescopes, to get a really consistent and noticeable increase in visual optical performance, assuming the optical quality is comparable, you need a 4 inch step up in aperture eg: 6 inches to 10 inches, 8 inches to 12 inches etc, etc.

I wonder what the equivalent step for a refractor is ? 

Are schmidt-cassegrain and maksutov-cassegrain step changes closer to refractors or newtonians ?

To clarify, this is for a performance gain that is noticeable each time you use the scope, rather than something that has to be teased out or only shows on certain targets or under the best conditions. 

Personally I get this when I compare the views with my 100mm refractor with my 130mm but it's somewhat less marked between the 100mm and the 120mm. 70mm to 100mm is a significant performance jump as well. With newtonians my experience seems to match the rule of thumb above - the differences between an 8 inch and a 12 inch were consistently clear but somewhat less so between a 10 inch and a 12 inch.

I'd be interested to hear others experiences on this, from a visual observing perspective 🙂

On a practical front, the physical size, mounting requirements, and general manageability of telescopes do also seem to take a significant step upwards with the related aperture increase. This needs to be factored into a final decision of course. And then there is the little matter of paying for the thing ...... 🤔

 

Edited December 10, 2023 by John

[GrumpiusMaximus]

As a relative beginner, I'd be interested in this too.

My thoughts (such as they are) would suggest that you get the most benefit from an increase in aperture from a refractor due to the lack of obstruction and the percentage increase in light transmission inherent in a non-obstructed system.  I have very limited experience with refractors in general but may get to play around with an 80ED next week and do a side-by-side with my smaller scope.

For SCTs, a Celestron C5 has around a 38% obstruction and similarly the C6 has a similar 37% obstruction, so I'd expect the difference in observability to be a direct function of the increase in aperture.  A C8 with 35% might get slightly better viewing than a 2-inch increase in aperture over a C6 would suggest.

However, a number of years ago I did a Messier marathon and the two scopes that we were primarily using were a C5 and a C6.  We had a very successful night (over 90 objects) and the C6 did have a significant advantage over the C5, even though there was only 1 inch in it.  Perhaps at the smaller end of the aperture range you get a disproportionate benefit with an SCT and from 8 inches up, it's a matter of diminishing returns?

That also doesn't necessarily take into account the quality of the view in terms of contrast.  My experience is limited but I currently own a 5" SCT, a 4" (parabolic) reflector and a 70mm ED refractor.  In terms of the quality of the view, the refractor is the winner, hands-down.  To the point where I have been using it in lieu of the C5 on Jupiter, despite the native focal length being less than half that of the C5.  I can make out the same if not superior levels of detail with the 70ED, even though the object is much smaller.  A 20mm WO Swan eyepiece with a 2x barlow gives very nice views indeed.  The C5 also delivers excellent views but with a slightly lower contrast, although I really do need to do a proper side-by-side with both mounted next to each other on the same night.  That could just be my eyes and my limited abilities as a beginner though.

Tl;dr, in my view refractors may theoretically offer the greatest benefit from an increase in aperture (although this isn't something I have empirically observed due to my lack of experience).  SCTs do offer quite a difference by small increases in aperture but I suspect have diminishing returns and between a good small refractor and a good small SCT, the refractor gives a better 'quality' of view in terms of contrast, even if the aperture and focal length are much smaller to the point where it becomes a personal preference.

Edited December 10, 2023 by GrumpiusMaximus

[Nik271]

I have noticed a significant increase in resolution between my 102mm refractor and 120mm Mak when planetary observing, but no so much for DSO observing.

My take on this is that even small 20% increase in aperture it noticeable when trying to tease out fine details on planets.

For DSO observing I need about 50% extra aperture to see one magnitude deeper.  Since large reflectors are mostly used for DSO observing this may explain why the aperture jumps for Newtonians are at 50% increments, whereas for refractors (where fine resolution is the key quality needed) smaller jumps in aperture are required. 

[Elp]

Being bortle 7 I have always wondered whether aperture will make much difference to my deep sky searching, I've kind of resigned to the fact it likely won't make much difference at all, any aperture increase will only magnify surrounding light pollution. A nice test for this is looking at m31 or m13, not more than a dot in my 60mm refractor to the point it looks like you're just looking at a star field (the short FL also contributes to this). Jumped up to a 130pds, can see them more faintly as you'd expect but still extremely faint smudges, and need averted vision to make out any discernable shape. C6 not much difference, but the central obstruction and contrast reduction it introduces plays a part. The best visual experience I've had was with my 102mm refractor, so I suspect if I went up to 120 or so, I'd see a resolution difference for sure, but only marginal in magnitude due to light pollution which is the driving factor of what anyone is able to see. I dont use filters so maybe they'd help a little, but they won't be anything like what you can see with a 10s camera image, 30s more so, 2 minutes and more reveals the good stuff.

[Zermelo]

I think you need to break the improvement down across different dimensions, as other commenters are already doing.

Light grasp (hence image brightness)
Limiting magnitude
Resolution
Contrast
Colour rendition
Image stability? (that question about whether an increase in aperture can actually degrade an image, in indifferent seeing)

And the type of target may have a bearing on this too, for example larger aperture resulting in smaller Airy discs, which some observers prefer and some do not.

[Stu]

Interesting post John.

As I’m sure you know, I’ve had a ludicrous number of small refractorsranging from 60mm up to 150mm

60mm FS-60C

66mm WO ZS 66SD

72mm TS 72

76mm FC-76DC and TV76

80mm Vixen 80M and Stellarvue 80ED

85mm TV85

90mm Sky 90

100mm FC100DC

120mm 120ED

130mm LZOS 130mm

150mm Startravel 150 and Vixen Atlux

Have probably missed some too 🤪🤪

I found that even the small steps up between say 66mm and 76mm creates a meaningful improvement whether it is in splitting doubles or planetary detail. I guess in percentage terms a 10mm increase is quite significant on a 66mm scope (15%) for example. Likewise the 100mm is definitely a big step up from 72 or 76mm.

Strangely I’ve probably found less from 100mm to 120, with 130mm providing more meaningful improvements. I found that the 120ED was a lot bigger than the 100DC whilst not providing me with enough extra for things like Solar white light or lunar observing so got used less.

The 150mm did also give a benefit on globs and planets in terms of resolution but was a beast at 20kg and f9 so again didn’t see the use it might have got.

The FS-128 still hasn’t seen that much use, but is very manageable and shows significantly more than the FC100DC so when the weather improves it will be ideal. I do think 100mm and 130mm are kind of sweet spots, and also somewhere around 72mm also in terms of compactness whilst still showing something worthwhile. It’s not often I can fit a 60mm in but not be able to take a 76mm, so I find that more useful for spotting and very quick sessions.

 

[Stu]

25 minutes ago, Nik271 said:

I have noticed a significant increase in resolution between my 102mm refractor and 120mm Mak when planetary observing, but no so much for DSO observing.

My take on this is that even small 20% increase in aperture it noticeable when trying to tease out fine details on planets.

For DSO observing I need about 50% extra aperture to see one magnitude deeper.  Since large reflectors are mostly used for DSO observing this may explain why the aperture jumps for Newtonians are at 50% increments, whereas for refractors (where fine resolution is the key quality needed) smaller jumps in aperture are required. 

I think that makes sense Nik, in that the obstruction won’t affect resolution benefits for planetary but will impact light gathering area/brightness for DSO?

[vlaiv]

I think it really depends on type of object in question.

For stars - it is almost straight forward - faintest magnitude has direct relationship to aperture size (if conditions are fair and optics is good).

Doubles and planetary - again, it is related to resolving power of telescope - and here we have direct relationship between resolving power and size of aperture.

For DSOs - well this is very controversial topic, or rather topic where we don't have full understanding of what is going on.

Simplified model says that there should be no difference in surface brightness or contrast between two different apertures as long as exit pupil is kept the same. However, there are other factors that contribute that have nothing to do with aperture of telescope directly (but more with other properties) - like size of object in question, and minimum photon count per resolving area element to trigger psycho physical response of the brain.

In another words - for same exit pupil, sometimes more and sometimes less magnification can give better view (which involves focal length of scope or F/ratio rather than anything else).

Sometimes small telescope, although having the same exit pupil won't show object because there is not enough total light for brain to "allow" seeing the object (again, this is somewhat controversial as it would mean that putting object half way out the field stop should make it disappear - but that never happens. Maybe because once brain "sees" the object - it keeps it "switched on" because brain has tendency to keeps reality consistent).

Anyway, that is difficult and probably not fully understood topic, so not sure if we can determine rule of thumb here.

[vlaiv]

2 minutes ago, Stu said:

I think that makes sense Nik, in that the obstruction won’t affect resolution benefits for planetary but will impact light gathering area/brightness for DSO?

I think that comparing Mak and refractor, F/ratio and thus often used exit pupil have more contribution than central obstruction. Central obstruction is small enough (even large one) to fall below just noticeable difference (which is around 7-10%). Take for example 30% CO - it makes only 9% by surface or light gathering area.

I think that having two mirrors and corrector plate - removes more light than that central obstruction (or about the same if coatings are enhanced at 96% reflectivity).

 

[Mr Spock]

If we are saying a noticeable improvement for refractors it a step from 100mm to 130mm, would then the next step be from 130mm to 170mm? Is there an improvement from 130mm to 150mm? Also given that a quality 150mm is three times the price of a 130mm.

[Stu]

34 minutes ago, Mr Spock said:

If we are saying a noticeable improvement for refractors it a step from 100mm to 130mm, would then the next step be from 130mm to 170mm? Is there an improvement from 130mm to 150mm? Also given that a quality 150mm is three times the price of a 130mm.

There is an improvement, but the size, weight and cost go up steeply so unless permanently mounted they become quite a pain to setup. I tend to think 150mm and above as newt or SCT/Mak territory.

[Louis D]

I generally think in terms of percentage jump.  Going from a 60mm to a 72mm is a 20% increase in aperture.  Going from a 72mm to a 100mm is a whopping 39% increase in aperture.  However, going from 100mm to 120mm is only a 20% increase once again.  Going from 120mm to 150mm is 25% increase, so slightly larger, but not massively so.  Certainly not enough in my mind to justify the cost, weight, and difficulty of mounting a 6" refractor (APO or not) over a 6" fast-ish Newtonian.  A 150mm APO refractor is going to run you about 20x or more the price of an f/5 Newtonian which is APO by definition.

By contrast, going from a 200mm reflector to a 240mm reflector would be a 20% increase.  However, the increase in number of observable objects is not nearly as much as the jump from 60mm to 72mm.

Increases in aperture are thus much more noticeable at the lower end in my experience than at the higher end.  For instance, going up in 25% increments starting at 50mm rapidly opens up the number of objects the human eye can detect or resolve.  However, once you get up around 10 to 12 inches, the jumps need to be larger in my experience to open up a significant number of new objects.  Thus, I see mostly 16 inch and under Dobs at star parties because of this.  You also need really dark skies to justify using 18 inch to 36 inch Dobs.

Edited December 10, 2023 by Louis D

[Stu]

1 hour ago, Zermelo said:

And the type of target may have a bearing on this too, for example larger aperture resulting in smaller Airy discs, which some observers prefer and some do not.

I think this is where it becomes more about the aesthetics of the view rather than outright performance. A small refractor shows very beautiful, and large airy disks which are easily visible in average seeing so they give the appearance of beautiful bullseyes.

To see the same in a big dob, you need higher power, very good seeing and everything such as cooling and collimation to be right, whereas more often than not whilst you may split tighter doubles, the stars are still less than perfect as you can’t quite see the airy disk clearly.

[Mr Spock]

Having used a C9.25 for many years I wouldn't choose that over, say, a 130mm apo. Going big for me means Newt. I'm very happy with the views I get with the 12". SCTs have their uses - planetary imaging is one are where they are a good choice.

I think a good quality 130-140 apo is the right size and a good step up from 100mm.

[Elp]

The issue with all this is subjective personal opinion and with all environmental factors being equal, everyone's personal view will differ not only due to ones eyesight.

However small they are, I still like the sharpness I get on planets with my Z61, can still see some banding on Jupiter. Some may look through it and say "is that it". Someone said that when looking at Jupiter through my C6 even though it was at nearly four times the focal length and three times the aperture. So number don't really translate to real world experiences, the bias of the viewer also plays a part.

[Ricochet]

1 hour ago, John said:

As a rule of thumb, it is said that, with newtonian telescopes, to get a really consistent and noticeable increase in visual optical performance, assuming the optical quality is comparable, you need a 4 inch step up in aperture eg: 6 inches to 10 inches, 8 inches to 12 inches etc, etc.

I wonder what the equivalent step for a refractor is ? 

I suspect this 4" rule comes from the same basis as the idea that for DSO observing you only need to make eyepiece steps that at least double or halve the brightness of the image, which is done by choosing root 2 (1.41) or larger jumps in eyepiece focal length. To double the brightness of an image with mirror size changes we need to double the area of the mirror, which again means we should be considering at least a root 2 change in aperture. For a 6" scope this would require at least an 8.5" scope, so we have to skip the 8" and choose the 10", for an 8" scope we need 11.3" so we have to choose the 12", and for a 10" we have a borderline situation where the maths says we need 14.1" but it is probably close enough that it still works in the real world. If we go smaller than this then it suggests there is a suitably large gap between a 5" and an 8", which is smaller than our 4" rule, and I think we see enough people with both an 8" dob and Hertitage 130p to say that this probably still holds. What I need the dob mob to weigh in on is how this holds up for the really large apertures. If you've got a 14" is a 20" a suitably large step up or is 18" large enough? Is the next worthwhile step from a 20" an 28"?

When it comes to refractors then I suspect that if you are using it for DSOs then the same rule will apply, so it probably isn't worth buying a 120ST if you already own a 102ST, but a 150ST would give a different enough view to be worthwhile. Of course the 120ST is still an improvement that you could buy it and not keep the 102ST (and the same applies to the Newt/Dob examples above). However, frac usage isn't so heavily biased towards DSO observing and we see a lot of people using them primarily for double star and planetary observations. In this case it isn't the light gathering capability that matters but the resolution of the telescope, which is proportional to the aperture. The ideal increment is probably not the doubling/halving that we assume with DSOs else the next step up for someone who owns a 4" frac would be an 8" frac. I think @Nik271's suggestion of a 20% increase in aperture sounds reasonable, but that is bad news for the financial controllers of everyone who owns a 4" frac as that would mean that having a 5" frac in addition to the 4" is a worthwhile propsect.

 

1 hour ago, GrumpiusMaximus said:

In terms of the quality of the view, the refractor is the winner, hands-down.  To the point where I have been using it in lieu of the C5 on Jupiter, despite the native focal length being less than half that of the C5.  I can make out the same if not superior levels of detail with the 70ED, even though the object is much smaller.  A 20mm WO Swan eyepiece with a 2x barlow gives very nice views indeed.  The C5 also delivers excellent views but with a slightly lower contrast, although I really do need to do a proper side-by-side with both mounted next to each other on the same night.  That could just be my eyes and my limited abilities as a beginner though.

Focal length is not an important characteristic for visual planetary, unless in terms of focal ratio of telescopes of the same type. I would suggest that you should really be considering aperture (good) and central obstruction (bad), and that your experience with the 70ED vs C5 is largely a result of the large central obstruction of the C5.

 

1 hour ago, Elp said:

Being bortle 7 I have always wondered whether aperture will make much difference to my deep sky searching, I've kind of resigned to the fact it likely won't make much difference at all, any aperture increase will only magnify surrounding light pollution. A nice test for this is looking at m31 or m13, not more than a dot in my 60mm refractor to the point it looks like you're just looking at a star field (the short FL also contributes to this). Jumped up to a 130pds, can see them more faintly as you'd expect but still extremely faint smudges, and need averted vision to make out any discernable shape. C6 not much difference, but the central obstruction and contrast reduction it introduces plays a part. The best visual experience I've had was with my 102mm refractor, so I suspect if I went up to 120 or so, I'd see a resolution difference for sure, but only marginal in magnitude due to light pollution which is the driving factor of what anyone is able to see. I dont use filters so maybe they'd help a little, but they won't be anything like what you can see with a 10s camera image, 30s more so, 2 minutes and more reveals the good stuff.

Actually, I think it will make a big difference for some types of objects, but you need to consider which objects you are observing. When observing DSOs we have two sorts of objects: extended objects and point sources. For extended objects the brightness of an object is a function of exit pupil whereas with point sources the brightness is more a function of aperture. The background sky is a type of extended object so if you try to observe an extended object like M31 the ratio of object and sky brightness will remain the same so there is only a marginal improvement from in increase in magnification (at the same exit pupil) when moving to a larger scope. However, if you wish to observe open star clusters then you are observing a group of point sources. In this case a larger scope will make them brighter and by decreasing the exit pupil you can make the background sky darken more quickly than the stars and so they become more visible. In the case of globular clusters like M13 then you have an interesting case that at small apertures they appear as extended objects but once you step up to a medium and large scopes they start to be resolved as point sources and then you can see more with larger scopes (minimum of 8", 10" or 12" will open up more "smaller" globulars).

[bosun21]

Having recently gotten involved in planetary imaging it's all about the resolution for me. I used to own a 8" dobsonian then made the jump to the 12" for visual. I immediately noticed the difference both in brightness as well as resolution. The faint DSO's that required averted vision with the 8" were visible by direct vision with the 12". I started planetary imaging with a 6" Maksutov and have now moved to a 10" go to dobsonian. I consciously kept the step up ratio to the 4" which is recommended.

[John]

Some very interesting and thoughtful responses - many thanks folks 🙂

Taking the refractor design, I find it interesting that the benefits that additional light gasp brings, which is 44% in the case of the step from 100mm to 120mm as an example, seems harder to detect (for me) in terms of the appearance of a deep sky object than the more modest increase in resolution that the larger aperture delivers - around 17% better. I can usually clearly see the larger scope resolving tighter double stars which simply don't split in the smaller aperture scope, for example.

I guess this is impacted by the nature of the challenge that these different target types present. In the main, a double star being split or not split is fairly clinical to detect but improved contrast or extension in a target that is already nebulous by it's nature is somewhat harder 🤔

 

[Mr Spock]

3 minutes ago, John said:

in the case of the step from 100mm to 120mm as an example, seems harder to detect (for me) in terms of the appearance of a deep sky object

The magnitude difference is only 0.4, so faint extended objects are still going to be faint. I find it makes a difference in brighter objects though. My 120mm shows the Orion nebula much better than the 100mm. Though with these sorts of objects the limiting factor for me is always the heavy light pollution I have here.

[bosun21]

My best views of M42 was with the 12" and an Astronomik UHC filter. So much detail on display.

[Mr Spock]

It's not come round into my 12"'s arc yet. Soon... soon 

[Paz]

From the scopes I've used myself from 60mm to 350mm I find that doubling the aperture gives a striking improvement, a 50% increase provides a strong improvement , 25% is clear but not strong, and once down to 10% or so I would have to go back and forth comparing to really notice. I find the impact on observing to be stronger on dsos than on planets/doubles.

[globular]

I think this question is similar to eyepiece jumps. For eyepieces I, broadly speaking, went for gaps of 1.4x, i.e. the next EP 40% longer focal length than the one below.  This 1.4x is actually √2 and means that you double the area in each step.  This seems to give a meaningful jump in FOV and exit pupil at the lower power end, and sensible magnification jumps at the high power end.

Applying to telescopes would suggest jumping from a 6" to 8.5",  an 8" to 11.3", a 10" to a 14.1", 12" to 16" and so on.
And in the very low aperture refractor world,  60mm to 85mm,  80mm to 113,  100mm to 140, 120mm to 160mm and so on. 

In practice I ended up with slightly more EPs (so slightly less than √2 gaps) and I think I'd go for slightly larger jumps when it comes to telescopes.  I think @Paz's "strong improvement" of 1.5x is what I'd aim for.

All that said, in reality other considerations like weight, length, mountability, aesthetics, whether you a 'collector', observing preferences, sky conditions, finances, etc, etc all come into play - often to greater extent than any "meaningful noticeable difference" argument.

Edited December 10, 2023 by globular
typo

[Louis D]

1 hour ago, Paz said:

From the scopes I've used myself from 60mm to 350mm I find that doubling the aperture gives a striking improvement, a 50% increase provides a strong improvement , 25% is clear but not strong, and once down to 10% or so I would have to go back and forth comparing to really notice. I find the impact on observing to be stronger on dsos than on planets/doubles.

My thoughts exactly.  I went from an 8" tube Dob to a 15" truss Dob and found the improvement massive on DSOs and planetary detail (remember, steady Texas skies here).  Of course, after a massive auto accident 23 years ago that ripped up my back, I have barely been able to lift the 15" out of the back of the hall closet because the mirror box weighs 60+ pounds.  I figure I'll move it to a dark sky vacation/retirement home at some point and place it on a roll-out platform with jacks to keep it steady.

[Voxish]

I certainly noticed a difference between a couple of old SW ED scopes. But when I foolishly sold my ED120 for a OO dob and didn’t get on with it I bought another 4 inch refractor (a Vixen) since then I have owned a number of other refractor’s, all of them 4 inch and for whatever reason never bought another 5 inch. I do think 4 inches is something of a sweet spot. Perhaps if I had an observatory I would buy something big, a 6 inch apo would be nice wouldn’t it!