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5" APO verses BEST 7" MAK CASS

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#176 Erik Bakker

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Posted 12 May 2019 - 03:55 PM

Here is a front view of the corrector and CO of my Q7, which measured to around 33% CO in real life. The deeply curved meniscus lens makes it appear smaller though.

 

IMG_1574.jpg


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#177 Arminovski

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Posted 13 May 2019 - 06:13 AM

I compared my Skymax 180 and TS 152/900 achromat on orion nebula at similar but low magnifications, up to about 100x. I thought that mak could bring out slightly more details in nebula clouds. Not by much but just a hint. I didn't compare them extensively but I did it under same night and under city pollution. That's my initial verdict non-apo vs mak. At those mags an apo should not be much different but somebody else should be the judge there as I don't own one.

#178 Kevin Barker

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Posted 14 May 2019 - 03:18 AM

An Interesting thread. I own a 7 " f-10 IM Mak and a 5.1 inch triplet fluorite apo.

 

The Mak is definitely better on deep sky but I have yet to see better planetary low contrast details on Jupiter in the Mak. The Mak will also out resolve the apo and split tighter doubles but only in the best of seeing.

 

The smallest scope to outdo my 5.1 inch apo is my 8 inch f-15 Mak. And this was only on two nights of testing.

 

This is not to say that in perfect conditions the 7 " Mak might do better on planetary. I just have'nt seen it yet.

 

I struggle with the concept of using excessive magnifications. It really does come down to optimal exit pupil. To see really low contrast detail one does not use 0.33 or 0.25 mm exit pupils. From my experience an exit pupil in the  0.6 mm-0.8 mm range will show everything that is to be seen.

 

Central obstruction effects contrast, 33% will reduce the low contrast transferred by the aperture minus the central obstruction. a Q7 would be like a 120 mm apo if this holds true. If the obstruction gets below about 20% then obstruction will have  minimal effect. So it would not surprise me if a well made Mak Newt or long focus Mak Cass would match an apo of the same aperture.

 

The quality of the optics also comes to bear. From my experience scopes with Strehl ratios in the low to mid 90% 's or better will outperform barely diffraction limited scopes with Strehl ratios in the low 80%'s if they are cooled, collimated and have the same aperture and obstruction. 1/4 wave of SA has a similar effect on contrast as a 33% obstruction when comparing to a non obstructed scope of similar quality.

 

I wonder sometimes with refractor comparisions to reflectors if the refractors are really apochromatic or just good achromatics or semi apochromats. A lot of so called apos have SA that changes with wavelength.

 

I have not often had perfect seeing conditions in NZ, maybe only 12-15 nights in 30 years observing. Some observers are lucky. They might be wiser due to the old adage "seeing is believing"


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#179 Asbytec

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Posted 14 May 2019 - 06:57 PM

Folks say quality allows higher magnification before the image breaks down, and that seems true. But, I am not sure quality is really the reason because, as Kevin states above, it really is about the exit pupil. Images might be cleaner, about as clean as they can possibly be especially without obstruction effects. However, images still dim with smaller exit pupils. I am not sure quality can do much about that. 



#180 fred1871

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Posted 14 May 2019 - 07:41 PM

The exit pupil in effect sets a limit to what the eye can see (yes, it can be spelled out in detail). What better optics do is show you more, and are less sensitive to seeing, before you hit the exit pupil limits. What those limits are for particular objects (Jupiter or Mars or a double star) will vary with the contrast sensitivity of the eye relative to the exit pupil for that type of image.

 

Hmmm... I'm sure somebody can find better words for what I'm pointing to though not fully describing.


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#181 CHASLX200

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Posted 15 May 2019 - 06:09 AM

Folks say quality allows higher magnification before the image breaks down, and that seems true. But, I am not sure quality is really the reason because, as Kevin states above, it really is about the exit pupil. Images might be cleaner, about as clean as they can possibly be especially without obstruction effects. However, images still dim with smaller exit pupils. I am not sure quality can do much about that. 

Better optics can handle high power much better than so so optics.



#182 Asbytec

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Posted 15 May 2019 - 09:23 AM

Better optics can handle high power much better than so so optics.

I tend to agree and say as much above. We can all see star diffraction at ludicrous magnification be it nearly perfect or aberrant and obstructed. That becomes apparent, in planetary images too, at much less magnification. At some point it's less about the optics and more about the very small exit pupil and physiology.

At some point we hit a wall depending on the image and object itself. It becomes less about quality and more about the inverse square law. As Frank said above, the quality image shows more (better contrast) before we hit the wall. The wall is not the scope's fault, it's our ability to observe it's large dim image. It's our fault.

If quality can surpass the wall by a bit, I'd think it has to do with some improved throughput allowing a somewhat brighter image so we can more easily see it. But, I don't believe anyone observes Jupiter at 0.3mm exit pupil. Doubles and other bright high contrast, absolutely. But, again, most any scope can go that high, although the image is not as sharp.

At some point and with some variability, the exit pupil governs what we see. Not so much the quality or lack of. So, saying quality scopes can go much higher producing a sharp image is certainly true, but the small exit pupil exacts a toll on all scopes and our eyes (including floaters). When we say we go to ludicrous magnification, surely any of us have done so, we should specify on which objects.

Edited by Asbytec, 15 May 2019 - 09:29 AM.


#183 Richard Whalen

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Posted 15 May 2019 - 05:51 PM

Image brightness between a 5" and 7" scopes is largely immaterial on objects like Jupiter. Contrast and resolution is what matters. I use 450x to 525x to purposely dim it down some to see small low contrast details in the polar regions in my 8", that at 262x to 350x are really hard to make out  as the planets brightness can be overwhelming on the eye.

 

This also applies to the moon and to a lesser degree Saturn and Mars. I find on Jupiter a .38 to .45 exit pupil works best for my eyes and seeing conditions. Larger exit pupils might be better for bringing out colors and large scale details, but you will miss the small low contrast details. On a really poor night I might be at .7 to .8 exit pupil and only see medium to large scale detail. I dont care how big your scope is, if it does not have excellent contrast you will be missing out on a lot of detail.


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#184 Kevin Barker

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Posted 15 May 2019 - 07:03 PM

Image brightness between a 5" and 7" scopes is largely immaterial on objects like Jupiter. Contrast and resolution is what matters. I use 450x to 525x to purposely dim it down some to see small low contrast details in the polar regions in my 8", that at 262x to 350x are really hard to make out  as the planets brightness can be overwhelming on the eye.

 

This also applies to the moon and to a lesser degree Saturn and Mars. I find on Jupiter a .38 to .45 exit pupil works best for my eyes and seeing conditions. Larger exit pupils might be better for bringing out colors and large scale details, but you will miss the small low contrast details. On a really poor night I might be at .7 to .8 exit pupil and only see medium to large scale detail. I dont care how big your scope is, if it does not have excellent contrast you will be missing out on a lot of detail.

Richard your powers used and exit pupil size are a lot higher and smaller than what I would commonly use. Even in as close to perfect conditions I would never go more than 200-250 X on Jupiter with my 5.1 inch apo, I have tried to, it just does not work for my eyes.

 

For example what I describe as low contrast features like the hard to see equatorial banding on Jupiter I find easier to confirm with exit pupils from 0.6-0.8 mm. In fact I cannot see the features when the magnification gets too high and the exit pupil too small, they tend to be not visible. 

 

Perhaps you have seeing that is a step above what I usually experience. I also tend to observe planets for reasonable periods of time, trying to discern subtle low contrast features as they spin into view and as the seeing fluctuates.

 

Perhaps we have different eye physiology. I'd like to think I have pretty good vision 20/20 with only a wee bit of astigmatism. Astigmatism of course is not such an issue with small exit pupils.


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#185 Kevin Barker

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Posted 15 May 2019 - 07:17 PM

Jason

Your Q7 looks lovely. Are you sure the secondary obstruction is 26%. I would have thought the Q7 Astro would have a 33% spot baffle on the outside of the corrector ? The secondary mirror might be 26 % but the baffle would increase the effective obstruction??

 

Kevin

I have owned many telescopes, to include refractors (e.g., 130mm and 178mm TMB scopes, really beauties) - but my Questar 7" Astro provides the best views of the planets, moon, and sun - given my skies.  The Q7 Astro has a 190mm primary, with a 178mm corrector.  The secondary obstruction is about 26% - with a focal length of about f/14.  Its compact size makes it a great addition to my large AG Optical 17" iDK - both mounted together on a Paramount ME 2.  It has been my dream setup for sometime.

 

attachicon.gif JPEG image.jpg

 

attachicon.gif IMG_0030.jpg



#186 Asbytec

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Posted 15 May 2019 - 07:20 PM

"Contrast and resolution is what matters." 

 

Totally agree, Richard. Great seeing contributes to both. 

 

"Larger exit pupils might be better for bringing out colors and large scale details, but you will miss the small low contrast details."

 

waytogo.gif

 

I am not a believer in the 25 to 30x per inch rule of thumb - presumably for Jupiter or empty magnification more generally. Some folks prefer it due to the *impression* of better contrast, better color saturation, and maybe limited by seeing to some extent. I prefer 40x per inch or more, if I can get it, for the reasons you mention. Small, bright low contrast features show better - purposefully dimmed and a larger image. That puts me at 244x or 0.6mm exit pupil presuming my scope is really working at f/13 (and 280x or 0.5mm is working at f/15 - long story as to which it really is). Much above that Jupiter is too dim to see anything other than it's main features.

 

Up that high the exit pupil is too small for my eye. The problem is the small relative aperture at 0.3mm and associated slow focal ratio at about f/70 on the eye as we view the magnified afocal image. The point being, the small relative aperture on the eye applies to all scopes at some point. Especially on Jove's bright low contrast detail, so at some point magnification is limited and usually well below that for other bright high contrast objects. As you say, the moon, Mars, Saturn, and even Ganymede are different. As are double stars. Each can take more magnification, and all scopes can do 100x per inch or more. It's only a matter of what we see that high. Usually, only star images can survive it. 



#187 Asbytec

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Posted 15 May 2019 - 07:42 PM

Richard your powers used and exit pupil size are a lot higher and smaller than what I would commonly use. Even in as close to perfect conditions I would never go more than 200-250 X on Jupiter with my 5.1 inch apo, I have tried to, it just does not work for my eyes.

 

For example what I describe as low contrast features like the hard to see equatorial banding on Jupiter I find easier to confirm with exit pupils from 0.6-0.8 mm. In fact I cannot see the features when the magnification gets too high and the exit pupil too small, they tend to be not visible. 

 

Perhaps you have seeing that is a step above what I usually experience. I also tend to observe planets for reasonable periods of time, trying to discern subtle low contrast features as they spin into view and as the seeing fluctuates.

 

Perhaps we have different eye physiology. I'd like to think I have pretty good vision 20/20 with only a wee bit of astigmatism. Astigmatism of course is not such an issue with small exit pupils.

You and I are in the same optimal magnification range. I have excellent seeing and modest ambient temperatures like Chas and Jupiter crossed our tropical zenith in recent years. I also find observing Jove over a reasonable period of time (about 45 minutes to produce a sketch) allows us to catch small soft detail all over the planet as even the best seeing fluctuates a little and Jove "etches" from time to time. I also have some astigmatism, but also lots of eyepiece time studying Jupiter. 

 

I do believe a quality refractor or any scope with higher throughput can produce a brighter image affording a bit higher magnification. Sometimes I ponder whether a brighter Airy disc (at near perfect Strehl) acts like a bright pixel adding to image brightness, but the total light seems pretty much the same even with aberration and obstruction effects. Only contrast is affected. And there is some variability in our personal acuity. So, there are no hard and fast rules, only ball park performance at optimal magnification ranges. But, not at ludicrous magnifications on Jove. In this case, it's not the image that breaks down, it's our acuity of that image. I think that is important to understand when we talk about image breakdown at very high magnification. Sometimes "image breakdown" is not the scope's fault no matter how good it is. 

 

Aberrant and obstructed scopes tend to "breakdown" due to lower contrast effects, and that can normally be seen at more modest magnification and still magnify aberrant and diffracted star images along with the best scopes. Better scopes do not appear to breakdown at that same modest magnification or higher because they hold their better contrast. Importantly, however, all objects dim due to the inverse square law and smaller relative apertures no matter the scope. To repeat, that's not the scope, that's our (range of) personal acuity. In this case, even Jove is affected as our planetary benchmark because of it's wealth of detail and color. So, talking about image "breakdown" does require some qualification. Sometimes it seems as if perfect scopes can do no wrong, but they can and do because all observers using those scopes hit an acuity wall at some point. Not all image breakdown is the scope's fault, especially at high magnifications. 


Edited by Asbytec, 15 May 2019 - 07:46 PM.


#188 Richard Whalen

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Posted 16 May 2019 - 08:41 AM

Richard your powers used and exit pupil size are a lot higher and smaller than what I would commonly use. Even in as close to perfect conditions I would never go more than 200-250 X on Jupiter with my 5.1 inch apo, I have tried to, it just does not work for my eyes.

 

For example what I describe as low contrast features like the hard to see equatorial banding on Jupiter I find easier to confirm with exit pupils from 0.6-0.8 mm. In fact I cannot see the features when the magnification gets too high and the exit pupil too small, they tend to be not visible. 

 

Perhaps you have seeing that is a step above what I usually experience. I also tend to observe planets for reasonable periods of time, trying to discern subtle low contrast features as they spin into view and as the seeing fluctuates.

 

Perhaps we have different eye physiology. I'd like to think I have pretty good vision 20/20 with only a wee bit of astigmatism. Astigmatism of course is not such an issue with small exit pupils.

Kevin, I consider equatorial banding to be medium contrast, where the differences in color make it pop. I like you find a .6 exit pupil works well for these details. Low contrast features I like to observe are ovals and other items in the polar regions. I tend to observe Jupiter for hours at a time, always sitting if conditions are very good (less than .6 arc seconds).

 

When seeing is just "good" in the .7 to .9 range I am in the .6 to .8 range also as these details usually are not visible anyways. I think the difference is the seeing conditions, when it is excellent here there is no fluctuation on Jupiter, everything is dead still. This can last a few minutes to several hours at my location. When this happens Jupiter really pops and looks like a small Hubble image with fine details from pole to pole. I never see this much detail in any of my other scopes, even my 18" which is why I sold it though colors and large scale details were much more vivid in it. Seems my 8" hits a sweet spot for my viewing site/conditions. My 5" shows a nice Jupiter, but does not have the resolution to see these details even when  conditions are perfect. I think 8" to 10" high quality aperture is required and less than .5 arc second seeing from my experience. 

 

We all have to make do with our observing site conditions, and that really can limit us to what works best. Other choice is to travel to a better location to get our fix, which for me is the Florida keys, Cedar key or St Georges Island which all get excellent laminar flow more often than my home site depending on prevailing winds. 


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#189 Asbytec

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Posted 16 May 2019 - 09:10 AM

Kevin, I consider equatorial banding to be medium contrast, where the differences in color make it pop. I like you find a .6 exit pupil works well for these details. Low contrast features I like to observe are ovals and other items in the polar regions. I tend to observe Jupiter for hours at a time, always sitting if conditions are very good (less than .6 arc seconds).

Yea, I think the white ovals on Jove are a great example of small, low contrast detail. They are basically white on different shades of grey and they are small near the resolution limit of a modest aperture. I remember very well the first time I caught sight of one, "Oh! That's what they look like! Man, they are small." I think around 1" arc give or take, if memory serves. Normally, when discussing Jove observations, these seem to be a good test of our scopes, seeing, and everything else. So, sometimes I'll ask how many of them have been seen. To date, I managed 5 of the 9 or so more prominent ones in a 6" aperture in pretty good seeing. Not all at once, but over time...one here, two or there, and another one later on. Once I got three of them near the GRS. That was based on maps of Jupiter on ALPO form a few years ago. I find the north polar region to have a bit more interesting detail to see depending on what's going on. The south Pole has a few low contrast white bands across it, sometimes easy sometimes not. 



#190 vahe

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Posted 16 May 2019 - 09:50 AM

 I use 450x to 525x to purposely dim it down some to see small low contrast details in the polar regions in my 8", that at 262x to 350x are really hard to make out  as the planets brightness can be overwhelming on the eye.

 

 

On Jupiter I usually use powers in the range of 200x to 300x with 155 APO or 8"Mak, the 250x is my personal sweet spot with either of these two instruments, as for "overwhelming brightness" I do not experience excessive brightness mainly because I use binoviewer all the time, also the use of binoviewer helps detect difficult details at lower powers compared to mono view.

.

Vahe



#191 Richard Whalen

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Posted 16 May 2019 - 04:16 PM

On Jupiter I usually use powers in the range of 200x to 300x with 155 APO or 8"Mak, the 250x is my personal sweet spot with either of these two instruments, as for "overwhelming brightness" I do not experience excessive brightness mainly because I use binoviewer all the time, also the use of binoviewer helps detect difficult details at lower powers compared to mono view.

.

Vahe

Vahe,

 

I have tried bino viewers and your right, they dim enough to use lower powers. My problem is my eyes since childhood dont play together well, more so when tired. Had an operation for wall eyes back in the mid 1960s, which helped but did not totally align eyes 100%. So most of the time I have a hard time merging images, when I did it was amazing, but caused a lot of eye strain so I stick to mono viewing. My right eye is heavily dominate, and am lucking it still has no floaters or astigmatism. My left eye has gone to crap in comparison as I have aged with significant astigmatism and a few floaters.




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