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Can a 4” APO ‘beat’ an 8” SCT? (yes and no)
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Can
a 4” APO ‘beat’ an 8” SCT? (yes and no)
by Gary S. Strumolo
This article is derived from a talk I gave my local astronomy club many years ago. This competition, and variations of it using different types and sizes of telescopes, seems to be a perennial question. Our goal in writing it was not to disparage any type of scope; all have their use and, like beauty, their value to an individual is in the eye of the beholder. We’ll not bring up issues like portability, weight, cool-down time, cost, or any of the many other factors that go into a purchase decision.
Rather, we’ll focus on the optics and their ability to render visually appealing images of both low and high contrast objects. We are not going to address what could be generated after recording, stacking, and processing thousands of video images. This analysis is not for astrophotographers; it’s for visual astronomers!
The tool we’ll use to conduct this analysis is called the modulation transfer function, or MTF for short. The MTF curve for an optical system, like a telescope, determines how much contrast of the object being observed is maintained after passing through the optical system. It can be illustrated here:
The MTF curve reflects the ratio of image contrast to object contrast.(This and all subsequent MTF curves were generated by the Windows program Aberrator).The type of curve above is typical for an unobstructed scope, like a refractor. If you compare two refractors of different apertures you get:
basically showing that the larger scope can reveal finer details on the object than the smaller one can. Aberrator can analyze more than an unobstructed lens, however:
In this example, we consider the case of an 8” aperture with a 33% central obstruction, as found in your typical 8” SCT. The downward bend in the curve reflects the fact that low contrast resolution is degraded because of the obstruction. To determine how much we consider the previous graph and ask how much smaller in diameter must the aperture be so that the new (unobstructed) curve will line up with this dropped down portion. That will give us the ‘effective’ size of an unobstructed scope that will match this one for low contrast features.
For example, consider this for an obstructed scope:
The black ideal curve is for an unobstructed aperture (refractor). The red actual behavior curve is for an obstructed aperture of the same size. The low contrast blue line, which matches the red curve drop, is for a smaller unobstructed scope. To determine its size we look to see where the blue curve hits the X-axis. Here it is around 0.7, which means the original scope performs like an unobstructed one 70% of it’s diameter.
The effects of a central obstruction on the ability to resolve low contrast features can be illustrated in the following diagram (20% for a typical Newtonian and 33% for a typical SCT):
So, e.g., the 20% obstructed scope (blue line) behaves like an unobstructed one 85% of its diameter (blue dashed line). Now obstruction isn’t the only factor in making the final decision about which is better. There are two others: collimation and seeing (turbulence). Let’s consider each, and how they are linked to the obstruction issue.
Collimation (for more, see T. Legault astrophoto.fr/collim)
Most people are familiar with the first phase of collimation, where you pick a star and deliberately rack it out of focus. You will see a set of rings (if you have an obstructed scope like a Newtonian, SCT or MCT there will be a dark central circle in the middle). The goal of ‘rough’ collimation is to make everything perfectly concentric. But that isn’t the end!
(and the bane of MCTs and Newtonians as well). The second stage is to use high powers on a focused star to see the Airy disks. These must also be concentric. As the figure above indicates, fig A is ideal while Fig D is still off. Now, how bad is it? Well, consider the pattern in Fig C above. It doesn’t look that bad, right? Well …
The figure above is an example for a Newtonian. We can see that that a C level of miscollimation produces an MTF curve that is similar to one from a spherical aberration of 1/3.5 wave, and one with a 43% obstruction. These are ‘equivalent’ to a 20% obstructed scope at 63% diameter (again, for resolving low contrast features).
So returning to our SCT:
we can see that while a perfectly collimated 8” SCT under excellent seeing conditions might behave like a 5.6” APO (left), poor collimation can reduce it to the level of of a 4” APO or worse! (hence the ‘yes and no’ in the title)
Seeing conditions (turbulence)
We will equate seeing conditions to turbulence in the atmosphere, ignoring things like poor transparency due to clouds, etc.. Now different aperture scopes are affected differently by turbulence:
(source:
https://skyandtelescope.org/astronomy-equipment/beating-the-seeing/)
This behavior might actually help the smaller scope over the larger one during visual astronomy because the eye can better follow detail as it moves a bit vs being steady but blurry. And unless you live in areas blessed with frequent steady skies …
But given the observation above, the effective turbulence for a larger scope is greater than that for a smaller scope. So we could be in the following situation:
So under fair-below average seeing conditions (which many people deal with most of the time) it’s possible for a reasonably well collimated 8” SCT to simply match a 4” APO.
So how do these analysis results compare to observations?
l An 8” beats a 4” on high-contrast objects (moon craters, Cassini division, shadow transits, edge of planet)
l A 4” can match (or possibly beat) an 8” on low-contrast objects (surface of Jupiter and Saturn, possibly Mars) except under very good-excellent seeing conditions.
To test this, we can use the Aberrator program to simulate the effects of turbulence on image quality:
We can see that under no turbulence the SCT beats the APO even for the low-contrast surface features of Jupiter, but when there is turbulence the image quality is equal.
I hope this helps explain the factors behind the quality of what you see in the EP. Of course, size matters, and if your goal is DSOs then the larger scope will always win out (after all, they are called ‘faint fuzzies’ for a reason). But, as we all know, the ‘best’ telescope is the one we use the most!
Thanks for reading and clear skies!
Gary S. Strumolo
- B McCandless, ed100, Max T and 18 others like this
71 Comments
I can tell you that my 70 mm F/6 Stellarvue refractor beats my former Skywatcher 150P 6" F/8 Dobson on anything related to Solar System and doubles.
I see more detail on the Moon, Sun and planets with my refractor than with the old reflector.
On the other side, the Dobson beats the refractor on planetary nebulae (I could see a hint of colour and a more defined shape on NGC 7009 and NGC 3242 with the 6", on the 70 mm they're fuzzy and grey) and galaxies (I could -barely- see half a dozen galaxies from my backyard on the 6", but they're completely invisible on the 70 mm)
None of my telescopes beat the other ones. They're all kind and non-violent.
I've actually compared a 120mm and 140mm refractor to a good 8" SCT. On a few things the 120 bested the SCT. But the better comparison was with the 140 refractor. It was close on the moon with the SCT just edging the refractor out, but on low contrast planets like Jupiter regardless of conditions the refractor won by a big margin. Some galaxies tend to do better in the refractor like M31. And many nebulae did much better in the refractor. I didn't need a filter on M27 as it showed plenty in the refractor, but one was needed with the SCT. Splitting stars was better with either refractor but one area where the SCT was king and really pulled ahead of both refractors was with globular clusters. My conclusion is one that is often stated here and that is you need and require different tools for different jobs. Although I no longer have my SCT I do miss the gorgeous views it gave me of some of the globs that my refractors just can't touch.
Sorry, but I don't believe you on this. The difference in aperture is far too vast to be made up with "superior-refractor-contrast". The difference between a 70 mm refractor and a 6 inch Newt is the difference between a 6 inch APO, and a 12 inch Dob, and that is an immense difference in good seeing.
Read Joe Bergeron's review of the Omni XLT, a 6 inch F/5 Newt, in which he compares it with an Astrophysics 155EDT, and an AP 92 mm Stowaway. He clearly states that the small Newt was superior to the 92 mm refractor, and pretty close to the larger 155 mm AP triplet. I highly doubt that your 70 mm ED scope is superior to the 92 mm Stowaway, let alone a 6 inch Newt. And this was a 6 inch F/5 Newt, with a 2 inch secondary. A 6 inch F/8 Dob has a smaller obstruction, and thus, even better contrast.
Link: http://www.joebergeron.com/omni150.htm
Either your Dob was a really bad sample, or it wasn't collimated, or you have really, really bad seeing. But it would take truly abysmal collimation or seeing before a 70 mm scope is seen as better than a 6 inch F/8 Dob.
Believe me, difference is that brutal.
By the way, I'm in Montreal, right below the jet-stream.
Seeing is always poor, mediocre during the best nights.
I try to avoid jumping in like this, but I'd have to agree with you that the Dob must be way out of collimation to lose out to a 70mm refractor by anyone. Sorry.
I would have to say that a 100mm apo generally can’t beat a well collimated 8 inch sct. I have been able to look through both types of scopes on the same night quite a few times and it it very rare that the 100mm refractor was better. My 140mm apo comes a lot closer to being as good as my C8, but it depends on the target and seeing conditions. Sometimes good refractors can make star fields look really amazing, which is due to the fact that they have a wider field of view and have a bit more contrast. I really like my refractors. They are awesome for imaging, but to me bigger is just better for visual, even if bigger is an sct with the dreaded central obstruction.
The above is basically what anyone comparing smaller, high quality refractors to larger SCTs over the course of months or years comes away with.
What it shows is that SCTs do not have high quality optics. SCTs do well on objects that are already high in contrast: globulars, the moon, etc. but fall down and can be bested, even by smaller and much smaller high quality refractor optics, on objects that are low in contrast like planets.
High quality, high contrast, smooth, unobstructed refractor optics can deliver on low contrast planetary details where SCT optics with their obstruction, light scattering correctors and less well figured optics will all conspire to cause those details to wash out.
It takes the larger aperture of the SCT to make up any difference. Compare a same size 8" apo to an 8" SCT and you will easily see why an SCT must be larger than a scope with high quality optics to deliver more, and even then, not in all cases.
Bob
Obviously! Which is why the article isn't about "Can an 8" APO 'beat' an 8" SCT?"
bobhen, on 12 May 2023 - 06:09 AM, said:
It takes the larger (but lesser optical quality) aperture of the SCT to make up any difference. Compare a same size 8" apo to an 8" SCT and you will easily see why an SCT (with its lesser optical quality) must be larger than a scope with high quality optics to deliver more, and even then, not in all cases.
Bob
If my statement above is so obvious, why write an article? And if obvious, why do these 4" apo versus 8" SCT and 7" Mak threads keep popping up? There are 2 running right now.
You might think that statement is obvious but post that statement in the Cats and Casses forum and you'll get a dozen pages of responses on why it "isn't obvious at all", and plenty of responses praising SCT optical quality.
Bob
There are 5 reasons why it is not intuitive- collimation (has to be perfect), seeing (varies not only from night to night but sometimes minute to minute), tube currents, not comparing scopes at the same time, and not matching eyepieces and magnification. For instance, refractors generally need minimum time for equilibration compared to a sealed SC (vs my ventilated ones...why don't they manufacture them with vent holes just behind the corrector????). Imagine an unequilibrated C8 tilted 30 degrees. There is warmer air floating in the highest part of the scope with a horizontal boundary layer so you are looking through a WEDGE of warm air (most people imagine a vertical scope with a symmetrical warm boundary layer just behind the corrector) plus a plume of warm air rising vertically from the front of the baffle tube, plus cool air falling from the tube walls facing the sky. No wonder people describe blurry stars.
Because the article wasn't about comparing two scopes of the same size where one is unobstructed and the other is. It was comparing a smaller unobstructed scope (the APO) to a larger obstructed one (the SCT). Was that not clear? I have no doubt that an 8" APO would best an 8" SCT for the obstruction reason alone. But since an 8" APO is in the unobtainium category, and there are plenty of 4" APOs and 8" SCTs out there ...
I can't explain what may go on in the different forums, and there are enthusiastic supporters in both camps. I try to avoid those religious wars as much as possible.
Years ago I was the observatory director at a college campus. In the dome we had a 20" Ritchey Cretien Cassegrain telescope. One night I set up a 4-inch apo outside near the dome. We could split more double stars with the 4-inch apo than we could with the 20-inch. The reasons were the 20-inch took too long to reach ambient temperature and the large secondary and secondary spider created too much diffraction.
Sure, it’s perfectly possible
So, if the scope was thermally equilibrated and the atmosphere was still enough that you could use a significant fraction of the possible native magnification, there is NO way a 4 inch scope should out perform a 20 inch scope, of any design. The fact that you state the large RC never reached thermal equilibrium invalidates any comparison of the optics.
Now if the point being made is that the small scope is ready to go quickly and the large one never got out of the starting blocks, then that IS a reasonable comparison to be made. The advantage of small scopes is that the are easily portable and DO reach thermal equilibrium more quickly. They are nice and convenient and provide much more detail than a casual observer often appreciates. That last part is also true for the larger scopes too:-). There are many nights when I just leave the SCT's inside, since some seasons I can see 10 degree temperature drops in 3 hours. Wrong design for those conditions, unless you do some things to control temperature loss like insulation, which does work.
Cheers!
JMD
when I had my 2 story 16 foot dome observatory (divorces LOL) I precooled it to the expected temperature an hour after dark. Why would a college observatory not do this is beyond me
Should the title read:
"Can a PREMIUM 4" APO beat an 8" SCT?"
I'm thinking that an AT102ED will have a harder time competing than a Tak.
I think excellent seeing is rare for me - i would go with a AT102.
On the other hand - I suppose im already there with a 5 inch Mak.
No, it shouldn't
The conditions set up in Aberrator (see above) are already perfect. So it's a super-duper, premium, first-class APO refractor. Doesn't matter. It's seeing conditions and collimation (for the SCT) that does. But the AT102EDT isn't quite an APO, so yes, it would perform a bit worse than a Tak I suppose.
BTW, in the article I included a 5" MCT as well.
Observatory did have air conditioning. But in the winter when it's 20 degrees at sunset and the sun heating up the inside to 40+ degree °F, the air conditioners don't work. Major observatories, like on Mauna Kea refrigerate the inside of the observatories 24/7 to keep the optics at the expected night time temperature. Electric bills at these observatories cost $10Ks/month. A small college cannot afford to refrigerate the inside of an observatory. Summer were not a problem with the window A/C. Winters were!
Correct that if the 20-inch was at ambient temperature it would outperform a 4-inch, and it did. But if a binary companion happen to be under a diffraction spike from the secondary spider, you wouldn't see it when you could in the refractor!
When I had my APO of 6.9" it compared very closely to my SCT of 11", just as predicted in your article.
Indeed you did mention a 5" MCT - I missed that. Thanks for pointing it out!
Best
Diffraction spikes can be an issue for newts of any aperture. All scopes have design compromises. Diffraction spikes are one of the reasons I do not have a lot Newts. Curved spider vanes are a potential solution to that issue. In the end, you need to pick the right tool for the right job. As Jon Issacs is found of pointing out, small refractors do somethings well and large Newts do other things well. the right tool for the right job. If I want to look at something really dim, or at high magnification, I would favor a well acclimated 20 inch scope of any design over a 4 inch scope. My 10 inch dob will out perform my 5 and 6 inch AP scopes on most nights on most targets.
JMD
Really? Have you compared a “ mid priced apo” with your tak? Most comparisons that I have read have stated that mid priced apos are embarrassingly close to the higher priced offerings. I’m not talking astrophotography here just visual. The capabilities of my Stellarvue access don’t have me thinking that I want a tak, rather I’m thrilled to get this level of performance for. The price I paid. And my experience with multiple outings that on planetary my Stellarvue performs about 75 percent of the c8. Not bad for a much lighter scope. On the moon where there is no shortage of light I feel there is no advantage to the c8 and because the Stellarvue is so much lighter and easier to use than my fork mounted c8 it’s a no brained to use it. I wouldn’t turn down a tak for the right price but I don’t believe most observers will be giving anything up by going with the astrotech.
I haven't compared - and i don't own a Tak or plan to. I would favor a mid-priced as well.
I was just musing based on what I've read about magnification / sharpness etc of higher end refractors. I would think the higher end MUST perform better - at least by some margin... or people wouldn't buy them. Whatever margin that is - it would add more leverage against a C8