Can of worms - Fracs and Dobs

[niallk]

15 minutes ago, estwing said:

now,now mate lets not get into a Frac bashing thread.....

Absolutely! Only kidding - just joking in response to those cartoons

"Will throw in a good used car" ha ha. Actually sounds like a great deal...

As said, I've been pondering a frac to complement the dob adding more possibilities.  When funds permit I could see myself going for it.

[Stu]

9 hours ago, JOC said:

Hi Stu (& all), but def. Stu - wow!  I've read it three times and it seems to make sense although I feel as though I might turn all that into a diagram tomorrow.  So its almost counter intuitive: The F6 frac in your example with the 10mm EP shows more of the sky because the magnification is less - thats OK,  I've got  my head round the concept that magnification is really the amount of sky you see tgrough the EP.  So a frac is OK if you want to see lots of sky, but not for seeing really small or distant objects?  Yes/no?

Don't worry, this stuff is complicated and it takes time to fully get your head round it. I am learning still all the time, including from this thread

Magnification affects the amount of sky you see, but is not the only factor at play. The apparent field of view of your eyepiece is also key.

Let't take a few examples based on the previous scope types. This time we will use the same 10mm eyepiece example, but in two types. One is a 10mm Baader Classic Ortho, with an apparent field of view of 50 degrees. The other is a 10mm Ethos, with an apparent field of view of 100 degrees.

These will give the same exit pupil in each scope because they are both f6. In the 70mm, both eye pieces will give x42, but the true field of view will be 1.19 degrees in the Ortho and 2.38 degrees in the Ethos. In the 300mm the two eyepieces will give x180 with the Orthos giving 0.28 degrees and the Ethos 0.56 degrees.

You can calculate True field of view by dividing the Apparent field of view by the magnification, so for instance 50/42=1.19. There is another, more accurate calculation using the field stop but let's keep that for another day.

Snapshots from SkySafari attached to illustrate the above. First one shows all the figures. Then we have the 70mm with the two eyepieces, then the 300mm at the same scale, then all together so you see the differences, with M42 as the target.

[Ricochet]

39 minutes ago, Stu said:

@Ricochet I agree with this, but if you have 1 unit area of brightness (surface brightness) in the small scope, and five units of area at the same surface brightness in the large scope, what is the measure of total 'output' if you like? I think that is what Doug is referring to.

There is presumably a difference if the object either fills or does not fill the field of view? If just fills the field in the smaller scope, but overfills it in the larger scope then I would expect the views to look identical (excluding any detail differences due to the mag)

Think of it that you have brightness and you also have area. And then you have brightness x area =  "output". Increasing magnification increases area and output but your eye perceives brightness and not output (I think). 

11 minutes ago, cloudsweeper said:

Yes Stu, this is exactly what I have been referring to.  The total/overall/integrated brightness might be expected to be greater, while the brightness per unit area is constant.  As I said before, the analogy is the LED torches - 1 LED is the same brightness/area as 5 LEDs, but the 5 LED torch has more output, and would lighten your shed up more in the dark.

Apart from this consideration, it is undoubtedly true that the larger image at the same SB would appear  brighter to the brain.

I find all this interesting and challenging, and I still have nagging doubts, but the friendly exchange of ideas and knowledge can help to clear the mists!

Doug.

 

 

The 5 LED torch is brighter because all 5 LEDs are pointing at the same spot. A better analogy would be to take five 1-LED torches into the shed and then aim them at the wall so that none of the beams are overlapping. In this case you have five areas of the wall all individually at the same brightness as a single torch but covering five times as much of it. 

[cloudsweeper]

9 minutes ago, Ricochet said:

 

The 5 LED torch is brighter because all 5 LEDs are pointing at the same spot. A better analogy would be to take five 1-LED torches into the shed and then aim them at the wall so that none of the beams are overlapping. In this case you have five areas of the wall all individually at the same brightness as a single torch but covering five times as much of it. 

Good consideration!

[Stu]

9 hours ago, John said:

I don't think that is the case or that Stu was saying that. He gave some examples of scopes with certain specifications and then explained the impact that those specs would have on image scale, field of view size and brightness. I think those examples would apply regardless of the design of scopes, ie: with a refractor, newtonian or catadioptric etc. I may be incorrect but I don't think Stu was implying anything specific about the refractor design as a whole ?

Back to my Maltesers I think - I'm probably confusing things rather than making them clearer !

 

 

 

Totally right John, I wasn't implying anything regarding to scope type and focal length in my post.

To explain my thinking for the OP a little...

Different scope designs have an impact on the focal ratio, and so the focal length for each scope type. An SCT by definition seems to have a focal ratio of f10, Maks have focal ratios of f14 and above generally, Mark Newts seem to be around f5 and a bit, large dobs are often f4.5 and below, getting faster as they get bigger, smaller newts can be anything from f4 ish including longer focal ratios up to about f11. So a 10" SCT will have a focal length of 2540mm and a 10" dob will normally be f4.8 ish and have a focal ratio of 1200mm.

Fracs seem to be the one type which can be in a wide range of focal ratios, anything from very fast imaging fracs at f3.5 ish, up to long focal length 'achros' at f15, but more normally in the f5 to f10 range.

So I think scope type does have a role to play, but there are contrary examples all over the place.

My two examples represented fracs in the widefield corner, and newts in the high mag corner, but that's not necessarily the case.

Want a widefield newt? I have a very nice TAL Alkor 65mm Newt, one of the smallest available. At f7.7 it is not fast, but because the aperture is so small, the focal length is only 500mm, so it gives relatively widefield views despite only using small format eyepieces 0.9..", can't remember the exact number.

Want a high power frac? Well there are plenty of old (and new) long focal ratio fracs out there. Take the Istar Perseus 150 f12. Want a widefield frac? Don't look here, it has the same 1800mm focal length as the 300mm newt in our example, so the fields of view will be the same.

[Stu]

1 hour ago, Ricochet said:

Think of it that you have brightness and you also have area. And then you have brightness x area =  "output". Increasing magnification increases area and output but your eye perceives brightness and not output (I think). 

That's an interesting concept, and I guess we are now getting into very complex stuff about what your eye responds too. I'm about to enter the Malteser zone too, but seeing as I have given up chocolate for Lent, I'll carry on

I think this is where contrast comes into the frame too. For instance if you look at the North America Nebula in a large dob, there is a chance that the whole fov will be filled with the same surface brightness so you will see nothing.

View that object in a widefield frac and you will see the contrast between the object and the darker background sky at the edges so will be able to detect it.

Take this to the other end of the scale. View a faint galaxy in a small frac, and it will likely be too small for your eye to detect it at all. View it in a large dob (lets assume same exit pupil rules again for simplicity) and whilst the surface brightness will be the same, the image scale will be much larger so again, your eye will be able to perceive the contrast at the edges of the object and so you will see it.

At the end of the day, both do a different job very well!

I do wish the clouds would go away so we could try some of this stuff out!

[Ricochet]

22 minutes ago, Stu said:

I think this is where contrast comes into the frame too. For instance if you look at the North America Nebula in a large dob, there is a chance that the whole fov will be filled with the same surface brightness so you will see nothing.

View that object in a widefield frac and you will see the contrast between the object and the darker background sky at the edges so will be able to detect it.

This is in fact a very good point. What your brain perceives is not brightness but the contrast between areas of different brightness. 

This also probably leads back into the OP's question of why use a frac with a smaller aperture. The aperture/tube edge of all telescopes diffracts star light from the Airy disk into the surrounding rings. In designs with a secondary mirror the obstruction diffracts even more light into the rings. In a Newtonian the spider also diffracts some light into spikes. All this diffraction must cause a drop in contrast between the star and background sky and so the design with the least diffraction (refractor) has the most contrast. Well in an apo anyway, chroma in a short achro probably undoes any diffraction advantage. 

[mikeDnight]

18 hours ago, JOC said:

 

Question is now launched:

Why do frac users deploy those long thin telescopes when they could be using a Dob with a huge mirror and lots of possibility to resolve detail.  Where does the Frac design score over the Dob. design?

  Need an emoticon with a tin hat!

It's a sensible and interesting question, as on the face of it aperture is King and spending several thousand pounds on a small aperture frac seems counter intuitive. Yet countless experienced observers and imagers do so!

From an imagers point of view (I'm not an imager!) the  beauty and power of a good refractor is obvious and undeniable!

Visually though, why choose a refractor at all? In a nut shell, they are better optically than anything else! Inch per inch they have greater light throughput and deliver sharper visual images than any reflector or catadioptric. It's true that they lack the light gathering ability of the large Dobs  but they deliver laser etched views of the Moon and planets. Because of having no central obstruction and virtually zero light scatter from a reflecting primary they produce an as near to perfect image as is possible, producing the tightest Airy disc. As an extended image of say a lunar or planetary surface is essentially made up of multiple Airy discs, the instrument that has least interference due to scatter, central obstruction, spider diffraction and coma, will produce the more aesthetically pleasing and sharper view. Thats where refractors score over other designs!

Personally I'd choose a 4" high end frac over a 6", 8" and nine times out of ten a 10" reflector for visual unar and planetary study. However there is a major variable to seriously consider, which is that we dont all see the same. W. F. Denning noted in his wonderful book Telescopic work for Starlight Evenings, " What one man sees in a 5 inch glass, another man needs a 10 inch."  I have been blessed with good visual acuity, and so am more than happy with a small refractor. Others may need larger apertures to get the same level of visual performance.  So, the observer becomes the greatest variable in the telescopic equation!

Also, if my interests lay in observing distant galaxies I'd opt for the large aperture reflector without question. My own interests lie in visual lunar and planetary, and so I've opted for the sharpest, least hassle scope scope in my price bracket. Horses for courses!

Mike

 

 

[faulksy]

7 hours ago, jetstream said:

I just had some excellent lunar views @ 350x in my 15" dob and binoviewers I would have got my frac out but I didn't want to waste the good seeing

brilliant gerry lmao

[SilverAstro]

4 hours ago, mikeDnight said:

will produce the     more     *  aesthetically pleasing

 So, the observer becomes the greatest variable in the telescopic equation!

pretty much nail-on-head all the way !

My own eyesight used to be top notch, but even then I cherished a faint seyfert over a crater and one of these days I may get a gravitational lens ! Seeing some of the things being talked about in the science of it all is 'more'  for me ,but now I need considerable assistance to distinguish close doubles

PS. I thought it was all a nightmare   dream from last night, till I found that the universe had expanded out of the AstroLounge !!

[mapstar]

22 hours ago, JOC said:

Question is now launched:

Why do frac users deploy those long thin telescopes when they could be using a Dob with a huge mirror and lots of possibility to resolve detail.  Where does the Frac design score over the dob design?

I've been reading with interest and the op asked the question above.

Each scope design has its own merits as detailed by many above. 

MikeDnight actually puts across what would've been my take on the refractor. It has no central obstruction so you will get a purer sharper image on point sources of light. They are less weildy, require less maintenance and cool down. On planets, stars, and Lunar they give good contrast. Splitting doubles they have no diffraction spikes so easier to split until resolution is taken into consideration. Once the aperture size gets above 4" though the cost goes up exponentially as the different coatings required to keep the chromatic aberration in check are expensive to apply, test and refine.

The newtonian can be used many different ways but unlike its refractor relative its a harder beast to live with. It needs tweaking occasionally and as they become larger are harder to transport. On DSO's the aperture pound for pound will wipe the floor with a refractor. Newtonians are much cheaper to make and don't suffer any of the chromatic abberation of the cheap/larger refractors. More aperture means much more resolution and based on cost reflectors are king.  

Imaging is a different thing and not in my realm of experience. I do know most imagers use refractors. Mainly as it is one less thing to fiddle with I guess (they do enough of that with the dodgy electronics). One thing that does baffle me is why they add diffraction spikes when processing??? Probably another thread on "why I'm ashamed to post non newtonian images?" 

It all boils down to the old 'there isn't a one for all scope'. Pick what you want to concentrate on and buy the scope that best fits the bill.

I have to admit I use a refractor as much as I do a reflector. Just happens to be 2" aperture and gives lovely star images but I have regularly seen M81 and M82 in it. Only problem is its missing another 20" to see them properly 

Which ever scope is decided on the atmospheric conditions on a clear night have the biggest impact on any scope design. 

[Stu]

2 minutes ago, mapstar said:

I've been reading with interest and the op asked the question above.

Each scope design has its own merits as detailed by many above. 

MikeDnight actually puts across what would've been my take on the refractor. It has no central obstruction so you will get a purer sharper image on point sources of light. They are less weildy, require less maintenance and cool down. On planets, stars, and Lunar they give good contrast. Splitting doubles they have no diffraction spikes so easier to split until resolution is taken into consideration. Once the aperture size gets above 4" though the cost goes up exponentially as the different coatings required to keep the chromatic aberration in check are expensive to apply, test and refine.

The newtonian can be used many different ways but unlike its refractor relative its a harder beast to live with. It needs tweaking occasionally and as they become larger are harder to transport. On DSO's the aperture pound for pound will wipe the floor with a refractor. Newtonians are much cheaper to make and don't suffer any of the chromatic abberation of the cheap/larger refractors. More aperture means much more resolution and based on cost reflectors are king.  

Imaging is a different thing and not in my realm of experience. I do know most imagers use refractors. Mainly as it is one less thing to fiddle with I guess (they do enough of that with the dodgy electronics). One thing that does baffle me is why they add diffraction spikes when processing??? Probably another thread on "why I'm ashamed to post non newtonian images?" 

It all boils down to the old 'there isn't a one for all scope'. Pick what you want to concentrate on and buy the scope that best fits the bill.

I have to admit I use a refractor as much as I do a reflector. Just happens to be 2" aperture and gives lovely star images but I have regularly seen M81 and M82 in it. Only problem is its missing another 20" to see them properly 

Which ever scope is decided on the atmospheric conditions on a clear night have the biggest impact on any scope design. 

Well said Damian  

[SilverAstro]

The diffraction pattern of a Newt can assist the splitting of very close doubles that have a large difference in magnitude.

[JOC]

5 hours ago, mikeDnight said:

It's a sensible and interesting question, as on the face of it aperture is King and spending several thousand pounds on a small aperture frac seems counter intuitive. Yet countless experienced observers and imagers do so!

From an imagers point of view (I'm not an imager!) the  beauty and power of a good refractor is obvious and undeniable!

Visually though, why choose a refractor at all? In a nut shell, they are better optically than anything else! Inch per inch they have greater light throughput and deliver sharper visual images than any reflector or catadioptric. It's true that they lack the light gathering ability of the large Dobs  but they deliver laser etched views of the Moon and planets. Because of having no central obstruction and virtually zero light scatter from a reflecting primary they produce an as near to perfect image as is possible, producing the tightest Airy disc. As an extended image of say a lunar or planetary surface is essentially made up of multiple Airy discs, the instrument that has least interference due to scatter, central obstruction, spider diffraction and coma, will produce the more aesthetically pleasing and sharper view. Thats where refractors score over other designs!

Personally I'd choose a 4" high end frac over a 6", 8" and nine times out of ten a 10" reflector for visual unar and planetary study. However there is a major variable to seriously consider, which is that we dont all see the same. W. F. Denning noted in his wonderful book Telescopic work for Starlight Evenings, " What one man sees in a 5 inch glass, another man needs a 10 inch."  I have been blessed with good visual acuity, and so am more than happy with a small refractor. Others may need larger apertures to get the same level of visual performance.  So, the observer becomes the greatest variable in the telescopic equation!

Also, if my interests lay in observing distant galaxies I'd opt for the large aperture reflector without question. My own interests lie in visual lunar and planetary, and so I've opted for the sharpest, least hassle scope scope in my price bracket. Horses for courses!

Hi Mike, this^^^ is probably the easiest to understand explanation for me so far.  I didn't realise that the vanes in the ends of the reflectors had so much effect, but even I can see that it clearly is an issue that the fracs don't have.  The end tube 'obstruction' must be an issue for these Cadioptric devices that I've seen pictures of too.   I must admit I'm still pondering the 1 and 5 LED lights issue.  True object magnification vs. object size in the field of view is making more sense though.

As for........"What one man sees using 5 inches .........., another man needs a 10 inches"    Well its a good job we are talking telescopes!

[jetstream]

igure 2: Six different views of the airy disk produced by six different apertures sizes of identical quality with no obstruction or atmospheric turbulence. Its very apparent here why larger telescopes show much finer detail than smaller ones. A: 10cm. B: 15cm. C: 20cm. D: 25cm E: 30cm F: 40cm.

Figure 3: Six different views of Saturn. 

A: 10cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

B: 15cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

C: 20cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

D: 25cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

E: 30cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

F: 40cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

This info is borrowed from Peach illustrating MTF resolution- given equal optics and the same conditions the larger aperture will show more detail than a smaller one. It is impossible under these conditions for a smaller scope (frac )  to show the same or more than a larger one.

http://www.damianpeach.com/simulation.htm

[JOC]

Airy disk?  not heard of this before this thread.  I read here https://en.wikipedia.org/wiki/Airy_disk still none the wiser as to what its going on about.  In the images about F is clearer and more defined than A the others are A moving towards the quality of F.  What is 1/8th wave spherical aberration please?  In simple sentences and words no longer than two syllables please.

[jetstream]

What all this says is that more aperture gives more detail on lunar/planets and also provides more resolution on these objects than the Dawes limit etc suggest, which describes resolution for point sources (stars).

There is a very important statement at the end of Peachs article and explains why 4" class refractors work so well in the UK. From Peach...

"While smaller high quality apertures, a good amount of the time can give a performance that is very close to the instrument theoretical limit, larger apertures can not due to the limitations imparted by astronomical seeing."

[mikeDnight]

13 minutes ago, jetstream said:

igure 2: Six different views of the airy disk produced by six different apertures sizes of identical quality with no obstruction or atmospheric turbulence. Its very apparent here why larger telescopes show much finer detail than smaller ones. A: 10cm. B: 15cm. C: 20cm. D: 25cm E: 30cm F: 40cm.

Figure 3: Six different views of Saturn. 

A: 10cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

B: 15cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

C: 20cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

D: 25cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

E: 30cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

F: 40cm aperture. 1/8th wave Spherical Aberration. Unobstructed. No atmospheric turbulence.

This info is borrowed from Peach illustrating MTF resolution- given equal optics and the same conditions the larger aperture will show more detail than a smaller one. It is impossible under these conditions for a smaller scope (frac )  to show the same or more than a larger one.

http://www.damianpeach.com/simulation.htm

Damian's illustration is simply incorrect and utterly misleading! This is definitely a case of people speaking with great authority about something they know little about. Books, journals, papers and the internet are full of such junk.

First of all, the eyepiece view is different in different types of scope. The image in A is more akin to the view through an 8" Schmidt Cassegrain on a good night. If a refractor gave a view like that, except on a very poor night, it would be bin fodder! Image A is simply a grossly out of focus view.

 Second, the height of Saturn above the horizon, in part determines how clean the view will be. On a night of excellent seeing and with Saturn riding high, a 102mm fluorite apo will, with good quality eyepieces, show a visual view of Saturn as in F. Those who argue otherwise have never truly observed with such an instrument. I have seen the Encke gap many times using a Vixen FL102 and also with a SW120ED. In fact such refractors reveal so much subtle detail in the Saturnian ring system that its impossible to sketch it! 

A similar misleading statement appeared in a recent modern day book on the subject of visual observing I had chance to glimpse at, only two days ago. In it the author stated that a 6" telescope is the smallest useable aperture for someone who wishes to seriously observe Jupiter. "What a load of utter junk!!!" 

Years ago George Alcock was ridiculed for his Jupiter observations using a 4" refractor, by those who should have known better. Things apparently havent changed! The trouble was that those critics, who likely used large reflectors, simply couldn't believe that a small telescope could reveal such detail, because they lacked the experience. The same mindless opinions still permeate modern astro literature etc. What is sad is that people believe these claims, to their detrement. 

Mike.

[jetstream]

9 minutes ago, mikeDnight said:

Damian's illustration is simply incorrect and utterly misleading!

The key word is illustration Mike, and it does not change the fact that more aperture gives more planetary/lunar detail assuming equal optics etc and seeing that supports the aperture.

 

[mikeDnight]

11 minutes ago, jetstream said:

The key word is illustration Mike, and it does not change the fact that more aperture gives more planetary/lunar detail assuming equal optics etc and seeing that supports the aperture.

 

That is true Gerry, but the images are still misleading! They dont take into account scope design, or location on earth for example. A SCT will never produce an image as sharp as a high end reflector. It simply can't do it due to the nature of its design. Similarly, a Newtonian can never perform like a refractor, though I'd much prefere a Newtonian over a SCT any day. 

My real gripe with Damians image descriptions is that some may be mislead into believing that all 4" telescopes will deliver images visually as in A, which is simply untrue. The biggest problem as i see it, is that most of those who publish things like this, lack the hands on experience at the eyepiece with the small apertures they so vehemently resent. Also, what an observer experiences in Arizona USA will be completely different from what he would experience in Lancashire UK. That is why many visual planetary observers here in the UK go for the small to medium aperture refractor as their instrument of choice.

Anyway Gerry, wasn't it only a few weeks ago that you were considering a 100mm Takahashi for yourself, or did I imagine it?  

Mike

[John]

Another issue that I find quite interesting (QI ) is the way the light path of different designs of scope work. With the newtonian the light comes down the tube, hits the primary and then goes back up the tube in a sort of tapering cone until it hits the secondary and is diverted sideways out of the tube. So it passes through the tube, and any warm currents of air, thermals etc twice plus a bit more before it reaches the eyepiece. Similarly with an SCT, the light traverses the tube 3 times before it exits through the rear port. With a refractor the light passes down the tube once narrowing as it goes until it hits the diagonal mirror and is deflected into the eyepiece.

It would seem to my mind that the single pass of the refracting design, narrowing and moving away from the tube walls as it goes, leaves less chance for the light cone to be disturbed by warm air / thermals within the tube ?.

Probably not a big issue but it might be part of the whole picture.

[mikeDnight]

My last sketch of Mars was made on the 1st March 2017 while using a 100mm refractor and a magnification of X370. Considering the very tiny apparent diameter at present, and the rediculous high power I needed to obtain a respectable image scale, the view was quite nice. (Prism diagonal used!)

Not bad for 3.9 inches! 

Mike

 

[Peter Drew]

43 minutes ago, John said:

Another issue that I find quite interesting (QI ) is the way the light path of different designs of scope work. With the newtonian the light comes down the tube, hits the primary and then goes back up the tube in a sort of tapering cone until it hits the secondary and is diverted sideways out of the tube. So it passes through the tube, and any warm currents of air, thermals etc twice plus a bit more before it reaches the eyepiece. Similarly with an SCT, the light traverses the tube 3 times before it exits through the rear port. With a refractor the light passes down the tube once narrowing as it goes until it hits the diagonal mirror and is deflected into the eyepiece.

It would seem to my mind that the single pass of the refracting design, narrowing and moving away from the tube walls as it goes, leaves less chance for the light cone to be disturbed by warm air / thermals within the tube ?.

Probably not a big issue but it might be part of the whole picture.

I think it's a very big issue. 

[Peter Drew]

I'm not a fraction of the observer that MikeDnight is and my eyesight is not what it used to be but I have access to and have used many apertures and types of telescope.

Currently my C8 se gives as good lunar and planetary images as any of these. The one difference is this, the C8 is in Tenerife. The mostly clearer and steadier atmosphere as well as the 28 degrees latitude seem to make this possible so location is a large part of the equation. I'm due to be out there from Monday and am looking forward to making the comparison again. 

 

[mikeDnight]

59 minutes ago, John said:

Another issue that I find quite interesting (QI ) is the way the light path of different designs of scope work. With the newtonian the light comes down the tube, hits the primary and then goes back up the tube in a sort of tapering cone until it hits the secondary and is diverted sideways out of the tube. So it passes through the tube, and any warm currents of air, thermals etc twice plus a bit more before it reaches the eyepiece. Similarly with an SCT, the light traverses the tube 3 times before it exits through the rear port. With a refractor the light passes down the tube once narrowing as it goes until it hits the diagonal mirror and is deflected into the eyepiece.

It would seem to my mind that the single pass of the refracting design, narrowing and moving away from the tube walls as it goes, leaves less chance for the light cone to be disturbed by warm air / thermals within the tube ?.

Probably not a big issue but it might be part of the whole picture.

From what I remember, and Peter will hopefully correct me if I'm incorrect, the light reflected from the F2 primary is then again reflected back down the tube from a X5 amplifying secondary. But its the square of the amplification factor that makes things more complicated, as it means an SCT is 25 times more sensitive to internal tube currents than a Newtonian or refractor.

Mike