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Yuri at TEC recent comment regarding APO vs. Mak

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#26 Eddgie

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Posted 19 November 2013 - 05:24 PM

Here is a followp picture that shows diffraction at work.

The two star pairs are from a program called "Aberrator" and represent how a pair of double stars seperated by two arc seconds would look in a perfect 155mm aperture (top) and and a 30% obstructed 200m aperture.

Now from center to center, the distance is two arc seconds.

Now, imagine that this pattern was formed by two points, exactly oppsite one another at the edge of a 2 arc second wide black line running from top to bottom.

Notice that the Airy Disk in the 155mm apeture is larger, but less energy is going into the ring.

The middle is the 200/30 apeture. You can see that the Airy Disk themselves are slighly smaller, but more energy is going into the rings. On your line between them, the line would appear "darker in the top telescope because there is less energy overlapping it than in the bottom.

But this only holds up so far.

When the larger telescope produces a diffaction disk and first ring that fits within the diameter of the smaller scope, the bigger scope will have better contrast tranfer (assuming no other optical issues).

So, if you put a 310mm aperture in the place of the 200mm, even wiht the central obstruction, the biggest apeture will transfer the most contrast.

Bottom pair is a 310mm apeture with 30% obstruction.

Again, this is only diffration, not optical quality.

But it is clear to see that the 310mm apeture will easily have better contrast transfer. ALmost no light is falling on your line...

You can very easiy see that your black line is no longer "Black" in the middle picture. It has lost a lot of contrast. Think back to the MTF chart and look at the deepest drop or "Elbow" and you are looking at the size detail on the MTF plot that is represented in this picture!

The top image though shows that the line is no longer "Black" either. If you look closely, you can see that the contrast is not perfect. The space on the line is now very dark gray, but not black.

In the 310mm apeture though, the line appears black, but I assure you that it is not. Remeber, there is some light out there in the other airy disk rings, and if this were really the edge of a white line, it would have an infinite number of overlapping airy disks.

You would see the line as gray in all three apetures, but it would be darkest gray in the bottom apeture, medium in the top apeture, and grayest in the center apture.

The bottom apeture will have done the best job of transferring contrast, the top will do the second best, and the center will come in last for detail this size. For different size details the MFT chart shows that much smaller than this and the effects of the obstruction become meaningless (past the elbow, or for detail that is perhaps less than .5 arc seconds wide in this case).

And this.. Here, I am assuming a black line on a white background. If the line starts with 50% contrast and you loose 50% contrast, it will only show up as having 25% contrast at the focal plane. And it gets more compliacated if you use irregular shape and various colors.

it is just the physics of diffraction at work. Diffraction of the apeture is absoute and 100% concrete scientific fact, and every time you look at the Airy Disk in a telescope, you are witnessing it first hand. The smaller the apeture, the bigger the disk. It is an absolute consequence of diffraction of the aperture itself.

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#27 Dwight56

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Posted 19 November 2013 - 05:29 PM







Quote:
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I did the same thing with My own Apo AP 155 F7 and a Ultima 9.25 SCT around 12 years ago. The SCT was rated better than 1/4 wave. The night or early evening observing Jupiter with both scopes at high power in very excellent seeing. The AP pulled ahead of the larger Celestron in Contrast. I could descern more detail on the disk of Jupiter. Now for deep sky of course I was getting brighter images with the 9.25 SCT but when it came to resolution and contrast the AP 155 won out.

Dwight


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Back in 2001 several of us set up in Blair Valley in eastern San Diego Co., CA, to view the Leonid Meteors. I brought my then-new Meade 10-inch SCT OTA (on a G11 mount), and another guy brought a scope with a 6-inch Astro-Physics APO objective. Be it conceded, the refractor was not an A-P scope, but a scope built around the A-P objective. My 10-inch SCT blew the APO out of the water when viewing Jupiter. I currently own a TV NP101is refractor. Although it provides excellent contrast, and members of the public often remark about its crisp image when I use it at school star parties, it doesn't perform above its station on planets or DSOs. Saturn's satellites, Rhea, Tethys and Dione are at the very limit of visibility, whereas they are easily seen in my Nexstar 8se. Some of the comments in this thread strike me as hyperbole.

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Yes back in the 1980's before the EDT and EDF's you could purchase the lens from Astro-Physics and roll your own tube. After some really poor examples surfacing Roland stopped selling lens sets by themselves and did only complete tube assy's since the do it yourselfers really did not seem to know what they were doing.

#28 Eddgie

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Posted 19 November 2013 - 05:33 PM

And finally this..

Jupiter is maybe 40 arc seconds in diameter right now.

a 2 arc second wide detail will be 1/20th the width of Jupiter!!!.

So, as you can see, the limit for a 155mm scope would be a detail that is maybe 1 arc seconds across if it is very dark.

If it is very light, it will have to be larger to easily see it

The 310mm aperture though as the potentil to show a much smaller detail with much more contrast.

This should be intuidive just by looking at the diffraction patterns in the picture.

And Jupiter is pretty big as planets go. Mars often fails to get to be much bigger than 20 arc seconds, so this line would reprsent a line that is 1/10th of the angular size of Mars.

This essay does not include optical qualiity. This is best case for all apettures.

It also does not include polychromatic strehl in the refractor, but no refractors are perfect because of their inability to bring all wavelenghs of light to the same focus.

Refractor forums never talk about it, but if you really want to be fair to reflectors, you really shoud include it.

A 6" f/8 achromat has worse contrast tranfer than a 6" MCT by a long shot. But polysthrel is never really mentioned during Refractor/reflector comprissons, even though the effect is the same (removes energy from the centarl disk of an in focus star and broadcasts it into an area around the star).

#29 ATM57

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Posted 19 November 2013 - 05:53 PM

**When we talk about apo-eaters, most people argue that it either has to be a mak or a newtonian, but what about a Schiefspiegler? No central obstruction and 100% perfect color correction by default. A bit long, but the eyepiece is at the comfortable end. Price is very reasonable.**


I built a 6" 2 mirror Schiefspiegler many years ago (Lorraine Precision Optics). The performance of that scope was nothing short of stunning. At F/29 it was limited to mostly high power targets but I can state that an unobstructed reflector with excellent optics will give views of high power targets that get burned permanently into your memory.

Scopejunkie

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#30 t.r.

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Posted 19 November 2013 - 06:55 PM

You are so easy to spin up Ed. :poke: :lol:

I remember when you thought binoviewing was a senseless pursuit due to the light loss the books proved existed. Now you claim it is the only way you care to observe! In time, you may be convinced of a smaller apo beating a larger sct as well. I guarantee if you brought your sct to the Northeast, your opinion would change. Never say never Ed. I've concluded that there are NO absolutes in this hobby because there are simply too many variables for the book answer to account for. As a matter of fact, I prefer to have both a large sct and a small apo as they compliment each other perfectly and perform when the other cannot. YMMV. ;)

#31 Jon Isaacs

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Posted 19 November 2013 - 06:56 PM

A few thoughts/experiences:

For deep sky objects like the Veil, contrast differences which results from central obstructions and differences in apertures simply cannot be resolved by the eye, at those low light levels, the rods are working together and the eye's resolution is no longer measured in arc-minutes but degrees..

For those who disbelieve: Try this simple demonstration. Look at the moon through a solar filter. The moon is no longer bright and easily resolved. Rather, it is now of a brightness that qualifies as a brighter DSO, the surface brightness of the full moon is a couple of magnitudes brighter than the Ring Nebula. It's relatively easily seen but the details are a different story..

- I have to think that a properly done comparison, either equal exit pupil or equal magnification would show the Veil to be better in the larger scope. I also have to think that a comparison between the refractor and a decent Newtonian of significantly larger aperture would show the Newtonian to provide the superior views of the Veil. My 13.1 inch F/5.5 Starsplitter has provided me with surprisingly good views of the Veil from my red zone backyard.

- When one is talking planetary, comparing a 6 inch unobstructed with a 8 inch SCT, there's room for the CO to cause problems. Comparing a 6 inch with a scope twice it's size means the Airy disk and first diffraction ring of the 12 inch is the same size as the Airy disk of the 6 inch.. that's a lot of concentrated energy for the CO to spread out.

Jon

#32 azure1961p

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Posted 19 November 2013 - 07:55 PM

Here is a followp picture that shows diffraction at work.

The two star pairs are from a program called "Aberrator" and represent how a pair of double stars seperated by two arc seconds would look in a perfect 155mm aperture (top) and and a 30% obstructed 200m aperture.

Now from center to center, the distance is two arc seconds.

Now, imagine that this pattern was formed by two points, exactly oppsite one another at the edge of a 2 arc second wide black line running from top to bottom.

Notice that the Airy Disk in the 155mm apeture is larger, but less energy is going into the ring.

The middle is the 200/30 apeture. You can see that the Airy Disk themselves are slighly smaller, but more energy is going into the rings. On your line between them, the line would appear "darker in the top telescope because there is less energy overlapping it than in the bottom.

But this only holds up so far.

When the larger telescope produces a diffaction disk and first ring that fits within the diameter of the smaller scope, the bigger scope will have better contrast tranfer (assuming no other optical issues).

So, if you put a 310mm aperture in the place of the 200mm, even wiht the central obstruction, the biggest apeture will transfer the most contrast.

Bottom pair is a 310mm apeture with 30% obstruction.

Again, this is only diffration, not optical quality.

But it is clear to see that the 310mm apeture will easily have better contrast transfer. ALmost no light is falling on your line...

You can very easiy see that your black line is no longer "Black" in the middle picture. It has lost a lot of contrast. Think back to the MTF chart and look at the deepest drop or "Elbow" and you are looking at the size detail on the MTF plot that is represented in this picture!

The top image though shows that the line is no longer "Black" either. If you look closely, you can see that the contrast is not perfect. The space on the line is now very dark gray, but not black.

In the 310mm apeture though, the line appears black, but I assure you that it is not. Remeber, there is some light out there in the other airy disk rings, and if this were really the edge of a white line, it would have an infinite number of overlapping airy disks.

You would see the line as gray in all three apetures, but it would be darkest gray in the bottom apeture, medium in the top apeture, and grayest in the center apture.

The bottom apeture will have done the best job of transferring contrast, the top will do the second best, and the center will come in last for detail this size. For different size details the MFT chart shows that much smaller than this and the effects of the obstruction become meaningless (past the elbow, or for detail that is perhaps less than .5 arc seconds wide in this case).

And this.. Here, I am assuming a black line on a white background. If the line starts with 50% contrast and you loose 50% contrast, it will only show up as having 25% contrast at the focal plane. And it gets more compliacated if you use irregular shape and various colors.

it is just the physics of diffraction at work. Diffraction of the apeture is absoute and 100% concrete scientific fact, and every time you look at the Airy Disk in a telescope, you are witnessing it first hand. The smaller the apeture, the bigger the disk. It is an absolute consequence of diffraction of the aperture itself.


Of all the graphs and such explaining contrast, those three illustrations say it far and away the best. That line can represent any number of contrast features . Very well done Ed.

Jon, I agree deepsky contrast res is a different animal - same rules but different playing field all together.

Pete

#33 saemark30

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Posted 19 November 2013 - 08:14 PM

I have only done a comparison between the two in my backyard which suffers from light pollution. What has really surprised me is that the APO out performs my SCT on DSOs in my sky conditions. Although the SCT is a C11 with 4 more inches of aperture, the APO will consistently show me DSOs that are invisible in the C11 and will show those that are visible in the C11 with more detail in the APO. The only explanation

Can you provide a few examples of DSOs invisible in the C11 but visible in your APO?
While M42 looks real good in my 6" refractor, galaxies are brighter in a C11 or 10" Newtonian.
In fact a 10" optimized Newtonian will be superior on all objects including planets when seeing permits. Otherwise it can also be a fuzzball because of thermal effects inside the tube.

#34 BillP

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Posted 19 November 2013 - 08:42 PM

I have only done a comparison between the two [7" AOP & C11] in my backyard which suffers from light pollution. What has really surprised me is that the APO out performs my SCT on DSOs in my sky conditions. Although the SCT is a C11 with 4 more inches of aperture, the APO will consistently show me DSOs that are invisible in the C11 and will show those that are visible in the C11 with more detail in the APO.


I am glad you are believing your eyes and what you see. Yuri, evidently likewise. Theory and modeling is of course very important as it gets us closer to understanding. However, most theory and models I often read here neglect a great many factors. As a result, they do not model the issues completely, and therefore as accurately as needed. But there is no fault here in all of this, as the model is well more complex than can really be addressed here. IMO there is much anecdotal accounts to confirm what you state and see, and the best tact is to let the theory and models be a guide, but in the end confirm or adjust based on your own direct observations, as you have done. On DSO in particular, only with a 10" Newt vs a 6" Refractor, I like you see "more" in the DSO from the refractor vs the Newt. The perceived contrast in the image was and is probably why (from my observations and deductions). Contrast of course is impacted by a number of things, all of which you will find no complete model for here, at least not in any post I have read. There are inherent differences between refractive glass and the coating technologies for it, vs mirroring glass, and the coating processes used for it. So one really needs to model this, as well as the various impacts from diffraction from spiders, and COs and the such to really get to the accurate analysis. Anything less IMO leaves out too much to be fully predictive, so why I will always defer to what I see, over what I read. All that is going on when one observes with their eye through any given instrument is highly complex, much much more than simply a matter of apertures and COs. So given this, indivudal observed results mean more. So can a 200mm Mak get beaten by a 155mm APO? I'm sure it can, just as I'm sure a 155mm APO can be beaten by a 200mm Mak! So both can be true...and the answer as to why lies in much more than simply an analysis and modeling based on apertures and obstructions.

Btw, on the topic of scatter from coatings and substrates and coatings and mirroring, here is an interesting paper that begins to discuss the issues, which a nicely concise and macro level conclusion, It all just points out that proper modeling of all of this, is much more complex than the basics of the scope and its aperture and obstructions, one really neads to go into the materials level as well to get the proper picture -- An Analysis of Scattered Light in Reflecting and Refracting ... Also read this. Important from the perspective that there are not theories yest established to adequately explain some things related to scatter.

In the end always believe your eyes, because if you don't then really what is the use! :lol:

#35 Muffin Research

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Posted 19 November 2013 - 09:03 PM

In any case.. The ease with which contrast is achieved when viewing with a an apo refractor sure comes to play?
I mean you take it out and it's there.
The times that viewing with cats and newts really all come together and deliver etching views are well on those rare nights when you are viewing with a cat or newt and go wow! this is really an excellent night seeing and scope both cooperate and the views are stunning.

With the refractor you just go blammo! pin point stars and objects, and you start of slow and wide and by the time you are at high mag the lens is there as well delivering pin point stars all across the range.

Even if theoretically and practically more detailed is observed with the larger mirrored instrument it's always possible for the smaller refractor to give the more aesthetically pleasing look. the ones that give the wow.

I've had memorable photographic views of Saturn and Jupiter and the moon with my C8, very very fine detail still the view in my 3" refractor devoid from the finer planetary detail like bands, swirls, bit of color, shades and rings always gives a gratifying look.
you just point it and focus and go hey it's there, that's Saturn allright, still doing fine.. just look at it hanging there, amazing isn't it?.. whilst with the C8 you are always chasing perfect vision, focus and detail and seeing, changing eyepieces and waiting for that moment that everything snaps.. with the refractor it's point focus observe.

#36 mgwhittle

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Posted 19 November 2013 - 09:11 PM


I have only done a comparison between the two in my backyard which suffers from light pollution. What has really surprised me is that the APO out performs my SCT on DSOs in my sky conditions. Although the SCT is a C11 with 4 more inches of aperture, the APO will consistently show me DSOs that are invisible in the C11 and will show those that are visible in the C11 with more detail in the APO. The only explanation

Can you provide a few examples of DSOs invisible in the C11 but visible in your APO?
While M42 looks real good in my 6" refractor, galaxies are brighter in a C11 or 10" Newtonian.
In fact a 10" optimized Newtonian will be superior on all objects including planets when seeing permits. Otherwise it can also be a fuzzball because of thermal effects inside the tube.


Yes, I would be glad to. Since the Veil was mentioned by someone else I will use that as an example. In my C11 only the eastern section is visible without using a filter at my moderately light polluted backyard site. In the 175EDF both sections are visible without any filtration. In addition to being more visible, I am able to see structure in the Veil with the 175. With the C11, the Veil is bright but not as tenuous as in the 175.

I can name other examples, this spring I was able to detect fainter galaxies in the 175 that could not be coaxed out of the C11. And galaxies that were visible in both had more detail in the 175. One that comes to mind is NGC 4565. In the C11 it is a silver streak against the sky. In the 175 it was the same silver streak but I could detect a "fattening" in the middle.

I understand that this is fantasy according to another poster but it's a fantasy that occurs in my backyard on a regular basis. Maybe my backyard is the Nexus of the Universe and the laws of physics don't apply here.

The C11 is f/10 and 2800mm, the 175 is f/8 and 1400mm. I have a nice selection of eyepieces and can match exit pupils pretty close and magnifications even closer.

Couple of points here. The C11 can out resolve the 175 on doubles by a substantial margin. Fainter stars are visible in the C11. For point sources, the C11 beats the 175. It (the C11) also shows a tremendous amount of detail on Jupiter, however I have stayed away from making any comments regarding planetary performance between the two because I haven't done that kind of comparison yet. Lunar performance is close enough between the two that I would almost call it a draw.

My experience is that on DSOs from a moderately light polluted area, my C11 cannot compete with a 7 inch APO. I attribute that difference to the APO's contrast that Yuri mentioned in his comments.

#37 mgwhittle

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Posted 19 November 2013 - 09:17 PM

I have only done a comparison between the two [7" AOP & C11] in my backyard which suffers from light pollution. What has really surprised me is that the APO out performs my SCT on DSOs in my sky conditions. Although the SCT is a C11 with 4 more inches of aperture, the APO will consistently show me DSOs that are invisible in the C11 and will show those that are visible in the C11 with more detail in the APO.


I am glad you are believing your eyes and what you see. Yuri, evidently likewise. Theory and modeling is of course very important as it gets us closer to understanding. However, most theory and models I often read here neglect a great many factors. As a result, they do not model the issues completely, and therefore as accurately as needed. But there is no fault here in all of this, as the model is well more complex than can really be addressed here. IMO there is much anecdotal accounts to confirm what you state and see, and the best tact is to let the theory and models be a guide, but in the end confirm or adjust based on your own direct observations, as you have done. On DSO in particular, only with a 10" Newt vs a 6" Refractor, I like you see "more" in the DSO from the refractor vs the Newt. The perceived contrast in the image was and is probably why (from my observations and deductions). Contrast of course is impacted by a number of things, all of which you will find no complete model for here, at least not in any post I have read. There are inherent differences between refractive glass and the coating technologies for it, vs mirroring glass, and the coating processes used for it. So one really needs to model this, as well as the various impacts from diffraction from spiders, and COs and the such to really get to the accurate analysis. Anything less IMO leaves out too much to be fully predictive, so why I will always defer to what I see, over what I read. All that is going on when one observes with their eye through any given instrument is highly complex, much much more than simply a matter of apertures and COs. So given this, indivudal observed results mean more. So can a 200mm Mak get beaten by a 155mm APO? I'm sure it can, just as I'm sure a 155mm APO can be beaten by a 200mm Mak! So both can be true...and the answer as to why lies in much more than simply an analysis and modeling based on apertures and obstructions.

Btw, on the topic of scatter from coatings and substrates and coatings and mirroring, here is an interesting paper that begins to discuss the issues, which a nicely concise and macro level conclusion, It all just points out that proper modeling of all of this, is much more complex than the basics of the scope and its aperture and obstructions, one really neads to go into the materials level as well to get the proper picture -- An Analysis of Scattered Light in Reflecting and Refracting ... Also read this. Important from the perspective that there are not theories yest established to adequately explain some things related to scatter.

In the end always believe your eyes, because if you don't then really what is the use! :lol:


Thanks Bill, you are bringing up some very interesting points that help me in my struggle to understand exactly what is going on here when it doesn't agree with the graphs.

#38 PowellAstro

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Posted 19 November 2013 - 09:29 PM

@ Bill

That was a great read and very good info, thanks for posting the links. I had stated this in a previous post just based on some testing I had done on a unit I made. This really explains it in great detail. So many blame what they see in the SCT on the CO when it's more the coatings, if the scope has a very good figure, that is. The difference I have seen on mine by just getting the best coatings is night and day where contrast is concerned. I always had QSP redo my mirrors but they are no longer in business. The new coatings always seem to almost double the contrast.

#39 Muffin Research

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Posted 19 November 2013 - 09:37 PM

I'll have to say when it comes to Galaxy detection the SCT really is a poor performer.
They are indeed much more easily hinted in a refractor.

With the C8 except for the bleeding obvious ones it's always straining to detect a smudge with even a smaller refractor it's more obvious to detect there is something 'there' and I guess contrast and sharpness is what it boils down to. The large instrument CAN provide more resolution and finer detail given the circumstances but the refractor gives more ease in making the faint fuzzies stand out, a good contrast that makes it easy to discern them in the star field you are looking at.

#40 PowellAstro

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Posted 19 November 2013 - 09:52 PM

Even Roland Christian said in one of his papers that he had a SCT the inside and out star test was perfect but the in focus image was bad and a lot of scattered light out side the airy disk. My 8" SCT displayed this same character until the mirrors had new high quality coatings. The airy disk it shows now is bright and sharp and only the slightest first diffraction ring can been seen. The only change were the coatings. The old coating made the glass look like it had 1/2 wave of SA error.

#41 mark8888

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Posted 19 November 2013 - 10:08 PM

I understand that this is fantasy according to another poster but it's a fantasy that occurs in my backyard on a regular basis. Maybe my backyard is the Nexus of the Universe and the laws of physics don't apply here.


Ya... in these recurring threads, I think people should consider adding "at my location", "in my experience" a little more often, and considering that we have different eyes and different perspectives, rather than telling people that what they're seeing is inaccurate. Here's my experience:

In reflectors, including presumably well-collimated ones, in my experience, stars most often look like lifeless dots. To me, stars look much better in refractors. The colors are much deeper and more obvious and shinier and more pleasing to look at in refractors. And thus probably also appear much more like they really are. The double cluster, to me, in a refractor, is pulsing with energy. It's vibrant. In a reflector, even one with much more aperture and with more stars viewable, it's like looking at dots or salt on a piece of black paper... a clinical view as opposed to a realistic one. Subtle variations I can see amongst them in a refractor are gone. I totally accept that others might have a different experience, but this has been my experience. In these threads, when people post that they see more contrast/a better view in refractors, others post that what they're seeing isn't real (a myth!), because they themselves don't see it. Here's another possibility: perhaps some people's eyes can more easily pick up the presence of the central obstruction in reflectors, regardless of aperture?

#42 maknewtnut

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Posted 19 November 2013 - 10:34 PM

Mark makes a great point in the first sentence of the preceding post. It should ALWAYS be remembered, regardless of the subject.

To move on from that, I know the thread is titled apo v Mak, and early on folks have added SCT's to the mix. I will add this. An extremely well executed Newtonian will give any apo a run for it's money.

If we're considering one design type compared to another, it's certainly worthwhile to consider the Newtonian. Why? First off, once you start increasing aperture, the strong points of the apochromatic refractor fade very quickly. One of those is affordability. One can stick strictly to image quality, yet to rule out the simple fact that a vast majority cannot or will not fork out the long dollar for even a 6", much less a 7" or larger, is unavoidable. Mount considerations are also quite expensive. I could go on to mention cooldown issue, which by virute of few owners, few know of firsthand.

To avoid harried debate, let me get to the point. Build a mid-aperture Newt around a relatively flawless mirror, and compare it head to head to a top notch apo of the same or up to one inch less aperture. There shouldn't be that much difference for the visual observer.

#43 jrbarnett

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Posted 19 November 2013 - 10:35 PM

Simple question: have you seen Sirius B in an SCT? Any SCT? It's easy pickings in a 3" apochromat at under 100x. Nuf said. At least SCTs are compact and mountable per inch of aperture.

For planets, SCTs are among the worst of choices. The large CO, no matter how good the optics are, always makes the scope far more sensitive to poor seeing than scopes with smaller or, better yet, no central obstruction. Seeing is king when it comes to planets and a 6" APO will perform well on planets delivering most of its aperture potential most of the time irrespective of seeing. The better figured it is, the better it will cope with poor seeing. An SCT, on the other hand, will deliver its aperture potential (resolution) almost never in practice. Part due to the physics of a big CO under seeing shifts and part due to, on average, worse quality optics than premium APOs.

My best planetary views were with a 12" Newtonian. Second best a 5.5" APO. I've yet to see any SCT of any size that came close to either. And I understand why that is so.

- Jim

#44 PowellAstro

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Posted 19 November 2013 - 10:40 PM

Contrast and stars do look better in the refractors but for many reasons other than just no CO.

Lenses have a more stable image because:

1..The mirror surface of a reflector must be fashioned to 4x the surface quality of a refracting lens to produce the same wavefront error. This means it is more likely on average, that the refractor lens will be of better quality. Not just for this reason alone but also because most times lenses are also smaller, which is easier to produce to a high level of accuracy. If the Reflector and a Refractor lens(of ordinary crown glass) both have the same defect on their surface, the lens would produce a wavefront 4x smoother. This is confirmed by Jean Texereau's Book (How to Make a Telescope).

2..For reason 1, the wavefront disturbance from seeing will be more profound on the reflector as the error is doubled due to reflection compared to the refracting lens. This includes turbulence.

3..Reflectors suffer from tube currents much more than a refracting type because the light passes close to the walls of the OTA to get to the mirror. This is where the most severe tube currents reside. In the refracting lens, as soon as the light passes through the lens, the light starts to move away from the OTA walls as it travels to the focal plane and therefore is much less effected by the most severe tube currents at the OTA walls. Not only this but the light in the reflector must pass through this twice.

4..These same reasons contribute to the Refractor having a more stable image and more concentrated. With a refracting lens, the image, due to turbulence, is only effected to 1/2 the level it would be in the reflecting scope as the mirror will double the error and the tube current will increase it yet again.

5..Contrast is also lost in the coatings used for reflector surfaces as well. many studies have shown much scatter in these coatings that damage the MTF of the systems.

These are things that you must deal with using a Reflector and take into account. On average, as I have seen for years, the refractor outperforms the reflector. In all but the best of nights, my 6" refractor shows a much better image than my 14" SCT. On those very rare nights, a handful a year, the 14" shows it true colors and outperforms all my other scopes by a large margin. However, I would like many more good nights, so I use the refractor!

#45 core

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Posted 19 November 2013 - 10:53 PM

And thus probably also appear much more like they really are. The double cluster, to me, in a refractor, is pulsing with energy. It's vibrant. In a reflector, even one with much more aperture and with more stars viewable, it's like looking at dots or salt on a piece of black paper... a clinical view as opposed to a realistic one.


I would say then, that salt-on-black-paper stars in a reflector is reality, and stars getting all 'vibrant' and 'pulsing with energy', you have bigger issues than optical quality ;)

Honestly, how do you qualify 'pulsing with energy' and 'vibrant'?

#46 Bill Barlow

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Posted 19 November 2013 - 10:59 PM

What coatings did your 8" SCT have before it got re-coated? Was it the XLT from Celestron or Starbright or UHTC/UHC from Meade? I had read that the XLT and UHTC has a very high amount of light transfer and reflection..something on the order of 95-98%. How much did the re0-coating process cost? Did you re-coat all the optical elements?

Bill

#47 mark8888

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Posted 19 November 2013 - 11:06 PM

I would say then, that salt-on-black-paper stars in a reflector is reality, and stars getting all 'vibrant' and 'pulsing with energy', you have bigger issues than optical quality ;)

Honestly, how do you qualify 'pulsing with energy' and 'vibrant'?


Stars are sources of energy. They are vibrant. They shine, they are not dots. That's reality. I see this more clearly in refractors. They look brighter and colors of stars look deeper. If you think they are dots, you have your own issues you need to deal with ;) .

#48 PowellAstro

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Posted 19 November 2013 - 11:12 PM

The main and secondary mirrors were re-coated. The corrector was not re-coated as this would require it to be refigured. They had the UHTC and they are no where near 95%. In fact they only would get to the upper 80s as light through the system. This is however much better than around 73% for the whole system with the standard coatings. The new coating was not dielectric but QSP's next coating down. The dielectric can damage the optic due to the heat used. The new coatings are rated at > 98% and there is a big difference from the stock. The cost was around 350.00 with the shipping for both ways. You would never believe it's the same scope.

#49 The Ardent

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Posted 19 November 2013 - 11:39 PM

If you have this experience when observing, you are doing something right. Far too many "serious" observers are so preoccupied with looking for, or at "objects" that they don't see anything else.

Stars are sources of energy. They are vibrant. They shine, they are not dots. That's reality. I see this more clearly in refractors. They look brighter and colors of stars look deeper.

:like:

As far as the Mak / refractor debate, I've had both. Due to my circumstances and preferences, I still have the refractors. As time passes, I find myself doing more with less aperture (excellent optics)

#50 Asbytec

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Posted 20 November 2013 - 12:26 AM

Maybe this isn't the forum, especially discussing TEC quality, to post a view through a cheaper MCT (Strehl ~0.94.) But, if the proof is in the seeing, here's a sketch of Jupiter through an Orion 6" MCT. It's not my best, but it's rather typical. It was done in Ant II to III seeing with a waning gibbous moon "at my location" and, of course, cooled and collimated. (10mm UO HD Ortho, Celestron Barlow) How does detail and contrast compare in terms of APO aperture? I've seen 4" to 4.5" images that are comparable (and have observed 5 white ovals which were absent this night.)

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