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Equipment Discussions >> Cats & Casses

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orion61

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I cant believe the size of the LX600........
      #5600071 - 01/01/13 06:13 PM

Secondary obstruction! HOLY COW!! How can that thing have enough clear aperture to have any contrast at all?
IS THIS SCOPE mostly aimed at the AP crowd?
Does anybody have one of them yet for a review?


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Cepheus Elf
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Re: I cant believe the size of the LX600........ new [Re: orion61]
      #5600128 - 01/01/13 06:45 PM

What percentage area obstruction is it? I couldn't see anything in the specs. on the Meade website. I guess the f8 focal ratio makes for a bigger CO maybe??

Mick

Edited by Cepheus Elf (01/01/13 06:48 PM)


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Darren Drake
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Re: I cant believe the size of the LX600........ new [Re: Cepheus Elf]
      #5600150 - 01/01/13 07:06 PM

Looks like it's about a 42% or so obstruction.

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orion61

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Re: I cant believe the size of the LX600........ new [Re: Darren Drake]
      #5600235 - 01/01/13 07:57 PM

I'd say at least!
Why isn't it possible to change the Primary F ratio?
Then reduce the Aspheric diopter correction and lengthen the tube and defrease the secondarys size?
You would think F 4 Primary and a 25% longer tube
which would allow a 40% secondary size reduction should be doable? Or are the pairs "locked" into the ratios due to the restrictions of the SCT format?


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Darren Drake
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Re: I cant believe the size of the LX600........ new [Re: orion61]
      #5600260 - 01/01/13 08:15 PM

I think it would increase the fl and redude fov...

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Jared
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Re: I cant believe the size of the LX600........ new [Re: orion61]
      #5600289 - 01/01/13 08:34 PM

As you surmised, it is aimed primarily at the astrophotography crowd. For deep sky imaging, a large central obstruction is not very important. It reduces throughput somewhat, but the loss of contrast can be easily addressed in post processing. Most photographers are happy to give up some contrast to gain some speed. For visual users, the f/10 LX200 is likely to be the more popular choice.

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GlennLeDrew
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Re: I cant believe the size of the LX600........ new [Re: Jared]
      #5600413 - 01/01/13 10:02 PM

The contrast loss due to the larger obstruction is only at the arcsecond level, not arcminute/degree/radian. It will be evidenced more at the small scale of planetary detail. There is no loss at large scale, such as a brighter sky 'background'. For prime focus DSO work, no difference can possibly be seen, for the scale at which the damage is done is lost in the undersampling, not to mention atmospheric seeing.

I get the impression that some folks mistakenly believe secondary obstruction-induced contrast diminution applies at all scales.


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David PavlichAdministrator
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Re: I cant believe the size of the LX600........ new [Re: GlennLeDrew]
      #5600434 - 01/01/13 10:11 PM

What Glenn said! Some of the high end RCs are close to a 50% obstruction. The larger secondary in these astrographs is to allow the scope to illuminate the big chips. A smaller secondary would cause pretty hefty vignetting on chips like the 11000 and 16803 class. I'm on the waiting list for a 12" version of the f8.

David

Edited by David Pavlich (01/01/13 10:12 PM)


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GlennLeDrew
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Re: I cant believe the size of the LX600........ new [Re: David Pavlich]
      #5600512 - 01/01/13 11:08 PM

Indeed, the Ceravolo f/4.9 field corrected Cassegrain (a CDK) has a 55% secondary obstruction. But examine the *unprocessed* images produced by it, available on the web site.

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rmollise
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Re: I cant believe the size of the LX600........ new [Re: orion61]
      #5601035 - 01/02/13 11:01 AM

Quote:

Secondary obstruction! HOLY COW!! How can that thing have enough clear aperture to have any contrast at all?
IS THIS SCOPE mostly aimed at the AP crowd?
Does anybody have one of them yet for a review?




This is, I believe, unchanged from the RCX400 version of the f/8 optical tube. Despite the obstruction, the 10-inch I had the pleasure to use worked well and produced outstanding images visually.


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JoeR
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Re: I cant believe the size of the LX600........ new [Re: rmollise]
      #5601225 - 01/02/13 01:02 PM

I got nice visuals with a 6" Ritchey–Chrétien that had a 50% CO. The DSOs I viewed looked good and the planetary views had a contrast similar to an 80mm apo refractor. When low power was used that's when the CO was apparent the sky was not as black as it could have been.

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GlennLeDrew
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Re: I cant believe the size of the LX600........ new [Re: JoeR]
      #5601633 - 01/02/13 05:29 PM

Quote:

When low power was used that's when the CO was apparent the sky was not as black as it could have been.




This was due to the larger exit pupil, which delivers an image of higher surface brightness.

As I pointed out earlier, a large obstruction affects contrast at very small scale, of order an arcsecond. It *does not* cause a brighter sky, in the way widely scattered light can, such effect being called veiling glare.

Again, I'm astonished at the vast ocean of confusion out there in amateur land, as regards even the fundamentals of contrast transfer in optical instruments. I really wish the community as a whole would finally get these matters straight, and not propagate the same old misinformation generation after generation.


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orion61

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Re: I cant believe the size of the LX600........ new [Re: GlennLeDrew]
      #5601711 - 01/02/13 06:28 PM

Quote:

Quote:

When low power was used that's when the CO was apparent the sky was not as black as it could have been.




This was due to the larger exit pupil, which delivers an image of higher surface brightness.

As I pointed out earlier, a large obstruction affects contrast at very small scale, of order an arcsecond. It *does not* cause a brighter sky, in the way widely scattered light can, such effect being called veiling glare.

Again, I'm astonished at the vast ocean of confusion out there in amateur land, as regards even the fundamentals of contrast transfer in optical instruments. I really wish the community as a whole would finally get these matters straight, and not propagate the same old misinformation generation after generation.



So if the CO makes no difference, why do Compound Camera lenses never have the contrast levels of a similar sized non obstructed one? I have never seen one or heard of one mentioned in the Photo mags.
Also my 102mm ED refractor has much better Planetary images than my 6" SCT, which shows very good defraction patterns,
but the background sky isn't as inky black as the non obstructed one even working at the same or faster F ratio?
Can you show me lab data that will help me understand this?harts or graph form, or even a controlled head to head test?
Don't get me wrong I am a Schmidt and Mak lover, I was just blown away by the refractor I literally stumbled onto as a throw in on a trade?
I'm just having difficulty grasping the concept, I had read in S&T that the larger secondary has a thicker defraction ring, that gets spread across the entire image that decreases contrast on a scale determined by the % of the obstruction?
Also why is it always such a big topic over in the Refractor Threads?
Please remember the vast majority of us are Blue collar workers and not Theoretical Physicsts by trade. Were on the viewing end not the developmental one.


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DesertRat
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Re: I cant believe the size of the LX600........ new [Re: orion61]
      #5601732 - 01/02/13 06:45 PM Attachment (33 downloads)

With a 42% obstruction the visual use for planetary would be impacted a fair amount. The biggest impact occurs at about 0.5 of the normalized MTF as seen below. For the finest details it would be ok for imaging however, as seen from the MTF on the right side. I've seen some great images of Jupiter from a fellow in Germany using a big RC with a significant obstruction.

No doubt a good 4" refractor displays a more contrasty image visually than a 6" with largish obstruction. But the 6" will compete imaging wise.

For details on how to read an MTF consult Suiter's book.

Glenn


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gillmj24
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Re: I cant believe the size of the LX600........ new [Re: DesertRat]
      #5601744 - 01/02/13 06:54 PM

I'll have you know the 10" and 12" rcx400 were some of the best cassegrain optics I have looked through. At the top if you're only considering "mass produced" ones. Visually they were a treat. Large obstruction and all.

Edited by gillmj24 (01/02/13 07:17 PM)


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orion61

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Re: I cant believe the size of the LX600........ new [Re: gillmj24]
      #5601828 - 01/02/13 07:47 PM

I can believe they have good optics I had a regular 10" Meade 200 with optics that were fantastic! I have only heard of one type of cpmplaint with them and it wasn't anything to do with optical quality. I'd love to have one some day, If it didn't have a focus/colimation issue.
That chart made perfect sence to me, I was just getting conflicting storys


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GlennLeDrew
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Re: I cant believe the size of the LX600........ new [Re: orion61]
      #5602301 - 01/03/13 02:18 AM

Orion61,
The extra energy deposited into (primarily) the first diffraction ring affects contrast, by definition, over the scale of said ring's radius. For most amateur scopes this is something in the region of one arcsecond. At image scaleslarger than this, the effect becomes negligible.

What do you mean when you say "spread over the full image"? The full field of view? If diffraction were of such magnitude as to actually brighten the sky, this would imply that all stars, and indeed all brighter-than-sky extended objects in the field would have to be diffracted into invisibility.

There is no such thing as a 'pitch black' sky, except when the exit pupil is less than 1mm (even for a truly pristine, zenithal sky), or a narrow-band filter is employed at moderate to small exit pupils.

If you select at random any patch of sky of arbitrary size (arcminutes to tens of degrees across), statistically the sky glow will be at least 3 magnitudes, or 16 times brighter than the light from the resolved stars. And that's for a pristine sky of 22 MPSAS! A suburban sky of 21 MPSAS will therefore be fully 4 magnitudes, or 40 times brighter.

What this means is this. Even if you were to completely, utterly and evenly spread out the light of the resolved stars, the light therefrom could at best only brighten the sky by a few or several per cent. This bears pausing and cogitating upon.

Therefore, if the *tiny* bit of energy robbed from the stars' Airy disks is not the source of the brightening you observe, what is? Could it be the sky glow itself? No. A uniform glow cannot augment upon itself when the light is further diffused, be it by diffraction or other means. If you focus upon and then defocus an evenly illuminated wall, for example, your camera will deliver the identical surface brightness for given aperture and exposure time.

If anything, the larger central obstruction should further *darken* the sky--at given exit pupil diameter-- due to light subtracted thereby. And indeed, this is just what occurs. In order to actually brighten the sky to a visibly sensible degree, light must come from *outside* the FOV, or from a very bright source within the FOV, and be scattered throughout the field as veiling glare.

So the question I put to you; *exactly* what is the source of light which brightens the sky further due *only* to an increase in central obstruction?

I invite any and all to address this, if an opinion is held.


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GlennLeDrew
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Re: I cant believe the size of the LX600........ new [Re: GlennLeDrew]
      #5602382 - 01/03/13 05:26 AM

It should have occurred to me to phrase the matter thusly.

If a central obstruction causes a brighter sky 'background' (actually, foreground), then the following must be in accord.

A refractor and some particular eyepiece produce an exit pupil of some particular diameter and hence an image of some particular surface brightness.

One installs a circular obstruction centrally on the objective whose relative diameter is 0.5. The light subtracted thereby is thus 25%, leaving 75% to contribute to image formation. Yet according to the thesis espoused, the sky is not to become dimmer, but instead actually to get brighter!

By what mechanism of arcane physics is this to be achieved? Whence comes the light?


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Joe Cepleur
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Re: I cant believe the size of the LX600........ new [Re: GlennLeDrew]
      #5602721 - 01/03/13 10:50 AM

Please could someone lable the axis on that graph? Something about apparent brightness versus percent central obstruction?

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GlennLeDrew
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Re: I cant believe the size of the LX600........ new [Re: Joe Cepleur]
      #5602840 - 01/03/13 11:56 AM

Joe,
Brightness has has absolutely nothing to do with it. That graph is concerned only with resolving power. This is an affirming example of the kind of ignorance against which I am continually fighting. Just to be able to distinguish between the basic parameters of optical performance can prove to be a chore for the majority of backyard observers.

I fully know that I can be something of a 'pill' for most to swallow, but if it takes a a figurative 'pounding' on the ol' bean to hammer the point home, so be it.

Is it not possible to disabuse the community of at least the more egregious falsehoods, thereby easing the lot of the next generation? The continued promulgation, decade after decade, of the same ol' ludicrous 'facts' is a source of unending wonder to me.

The first edition of the Backyard Astronomer's Guide, from some twenty years ago, lists the top ten telescope myths. Well, they're still haunting the amateur community. Why?


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Alph
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Re: I cant believe the size of the LX600........ new [Re: GlennLeDrew]
      #5603023 - 01/03/13 01:36 PM

Quote:

What do you mean when you say "spread over the full image"?



It means that the diffraction effects spread over the full image. In principle, this is a correct statement. You just can't take it literately without qualifiers. In practice you will not see it, although the MTF graph captures that effect quite well.


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DesertRat
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Re: I cant believe the size of the LX600........ new [Re: Alph]
      #5603282 - 01/03/13 03:59 PM Attachment (23 downloads)

There is no way an obstruction will brighten the sky in practical terms. If that was seen something else was going on.

Below is an annotated MTF labeled with Suiter's terminlogy. Basically what is shows is that a perfect telescope with increasing obstruction continues to have a high resolving power. In fact resolution is slightly enhanced as can be seen in the graph. However contrast suffers for larger structures or lower spatial frequencies. This graph has an annotation for the Dawes criteria which is at approximately 0.98. That is a normalized figure. To convert to arcseconds recall the Dawes criteria being Theta = 4.56/D with D in inches and Theta in arcseconds. You can convert from there. Rayleigh is at 5.45/D. Pardon the imperial units.

A fraction or normalized X-axis is used so the graph applies to any aperture size within reason.

Glenn


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orion61

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Re: I cant believe the size of the LX600........ *DELETED* new [Re: GlennLeDrew]
      #5603493 - 01/03/13 06:34 PM

Post deleted by orion61

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cn register 5
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Re: I cant believe the size of the LX600........ new [Re: orion61]
      #5603522 - 01/03/13 06:57 PM

If you ask a question that needs a technical answer then you can't complain when you get technical answers.

Chris


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orion61

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Re: I cant believe the size of the LX600........ new [Re: cn register 5]
      #5603609 - 01/03/13 07:51 PM

I dont mind a technical answer,
I just dont like being refernced as stupid for asking it.
or seeing it for the first time.
I merely referenced an article that had other information.


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budman1961
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Re: I cant believe the size of the LX600........ new [Re: orion61]
      #5603873 - 01/03/13 11:17 PM

Based on the temperature of all the responses to any new Meade thread, why did you ask the question? Not to call you anything, or infer anything, but it seems to me these Meade threads all end up the same way......

Sorry.....


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GlennLeDrew
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Re: I cant believe the size of the LX600........ new [Re: budman1961]
      #5604023 - 01/04/13 02:46 AM

Oops! I had mistakenly posted here a lengthy test result on *spider* diffraction, meant for a thread in the ATM Forum. Silly me...

Edited by GlennLeDrew (01/04/13 06:21 PM)


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Asbytec
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Re: I cant believe the size of the LX600........ new [Re: orion61]
      #5604158 - 01/04/13 07:05 AM

Quote:

So if the CO makes no difference, why do Compound Camera lenses never have the contrast levels of a similar sized non obstructed one?

I'm just having difficulty grasping the concept, I had read in S&T that the larger secondary has a thicker diffraction ring, that gets spread across the entire image that decreases contrast on a scale determined by the % of the obstruction?

Also why is it always such a big topic over in the Refractor Threads?




Well, Glenn is right. Both answers are a result of scale. The CO affects least images on the scale of the first ring, then creates some loss out to about 3 times the Raleigh limit. This equates to features smaller than Jupiter's belt thickness. So, for finer planetary detail, the contrast response can be less that full aperture.

On larger scales, where cameras presumably operate, diffraction on a fence post, for example, is practically none existent. Once you get beyond 10x the Raleigh criteria, all scopes perform about the same.

The trick to reading the MTF is to realize all of the right half are very small features at or near the Airy disc. Much of the left side are also very small features. If you think about it, diffraction effects of a 1" arc pattern have little effect beyond 3 or 4" arc. Contrast improves beyond this range - on the very far left of the MTF graph (spacial frequencies around 0.3 and less.)

The larger COs in SCTs are really designed for general purpose use and do give very good views. No doubt. And they are optimized, really, for resolution across all frequencies, not just a specific region of the MTF.

The problem they present is on peak intensity of the diffraction image. To maintain 80% (diffraction limited equivalent) peak intensity, the CO must remain pretty small and not divert too much light into the rings (along with any aberrations present.) This also requires a good optical figure. Actually, a perfect scope can have a CO approaching 45% and still maintain peak intensity of about 80% or better. Lessor correction requires a smaller CO to maintain that level of intensity (peak vs rings.) This is where refractors have an advantage, they have no CO and their peak intensity (pretty much) equals their (normally very good) Strehl.

But, again, most of that is occurring on scales where diffraction matters, maybe out to the angular dimension of third ring or so. Beyond that and contrast improves dramatically and and obstructed scopes begin to catch up to unobstructed apertures...and camera lenses.

http://www.telescope-optics.net/telescope_central_obstruction.htm


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mmalik
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Re: I cant believe the size of the LX600........ new [Re: Darren Drake]
      #5604227 - 01/04/13 08:27 AM

Quote:

Looks like it's about a 42% or so obstruction.




Obstruction is calculated by two ways, by area and by diameter. Meade uses 'by area' method. A while back I had come up with following figures for LX800 which should be the same for LX600 given the same OTAs:

10" f/8 ACF:
Obstruction size=4.58" [Derived]
Obstruction by area=20.95%
Obstruction by diameter=45.80%

12" f/8 ACF:
Obstruction size=4.93" [Derived]
Obstruction by area=16.86%
Obstruction by diameter=41.08%

14" f/8 ACF:
Obstruction size=5.1" [Derived]
Obstruction by area=13.28%
Obstruction by diameter=36.43%


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mmalik
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Re: I cant believe the size of the LX600........ new [Re: Darren Drake]
      #5604238 - 01/04/13 08:34 AM

Following is the math behind central obstruction calculations; following example applies to 10" f/8 ACF:

Aperture=10"
Secondary Mirror Obstruction=4.58"
Pi=3.14
Area of entire aperture=3.14*(10/2)^2=78.50
Area of secondary mirror obstruction=3.14*(4.58/2)^2=16.47

Obstruction (by area method)=16.47/78.50=0.21=20.95% (Meade's way of calculating)
[Easy calc: 0.458*0.458=0.21=21%]

Obstruction (by diameter method)=4.58/10.00=0.46=45.80% (Others vendors' way of calculating)
Obstruction (by diameter, alternate method)=SQRT(16.47/78.50)=0.46=45.80% (Others vendors' way of calculating)
[Easy calc: 4.58/10=0.458=45.8%]


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Cotts
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Re: I cant believe the size of the LX600........ new [Re: budman1961]
      #5604550 - 01/04/13 11:04 AM

Good morning, everyone. Let's make an effort to stick to talking about the science of optics in this thread without certain kinds of editorializing that makes us all uncomfortable.

Dave


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orion61

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Re: I cant believe the size of the LX600........ new [Re: budman1961]
      #5604590 - 01/04/13 11:23 AM

Quote:

Based on the temperature of all the responses to any new Meade thread, why did you ask the question? Not to call you anything, or infer anything, but it seems to me these Meade threads all end up the same way......




Yeah i see your point, it was just a knee jerk reaction while reading my new S&T and saw the front of one for the first time under the new products out! I was actually looking for a note by someone that had one, to give a "mini review"
In the long run I still think the SCT's and related systems are still the best over all system


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Asbytec
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Re: I cant believe the size of the LX600........ new [Re: Joe Cepleur]
      #5604603 - 01/04/13 11:32 AM

Quote:

Something about apparent brightness versus percent central obstruction?




The MTF does not plot brightness, but there is some peak intensity loss spreading the rest of the light into the rings. If you crunch the numbers using some approximations (assuming the same aperture and aberrant Strehl of .95) you get...

http://www.telescope-optics.net/obstruction.htm

CO 46%
EE 42% (Airy disc, max = 84%)
Intensity 62% (Normalized to 1)
Nominal 59% (Working Strehl assuming aberrant Strehl 0.95.)

CO 41%
EE 50%
Intensity 69%
Nominal 66%

CO 36%
EE 57%
Intensity 75%
Nominal 71%

CO 29%
EE 67%
Intensity 84%
Nominal 80%(Essentially the diffraction limit peak intensity.)

CO 0% (Refractor)
EE 84%
Intensity 100%
Nominal 95%

You can see how a smaller CO reduces light loss due to it's diffraction effect and obscuration of the light path. An unobstructed aperture does not have this "problem." However, a little contrast loss is nothing a little more aperture cannot cure.


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Joe Cepleur
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Re: I cant believe the size of the LX600........ new [Re: Asbytec]
      #5604701 - 01/04/13 12:37 PM

Quote:

a little contrast loss is nothing a little more aperture cannot cure




That I understand, despite that I'm still learning to understand all the numbers. This is the quirk that makes SCTs great. Sure, there are trade-offs, but look how big the gains are. A relatively inexpensive, portable telescope that performs pretty much like a giant, costly, heavy refractor! Not exactly, but dollar-for-dollar and pound-for-pound plenty good enough. Gotta love 'em!

I read recently that, compared with an unobstructed scope, an 8" C8 has the resolution of an 8-inch, the brightness of a 6.25-inch, and the contrast of (I forget exactly...) a 5.5-inch. That's a fabulous deal for the size, weight, and price!


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GlennLeDrew
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Re: I cant believe the size of the LX600........ new [Re: Joe Cepleur]
      #5605912 - 01/05/13 01:40 AM

Earlier in this thread I had given vent to my general frustration regarding the ongoing propagation of myths in the amateur community. My comments were directed at no one in particular, but rather the nebulous, amorphous masses comprised of us all. I have to remind myself than in a community where the participants cannot see nor hear one another, adherence to diplomacy is the watchword. My apologies to any and all who I've offended.

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Mike Harvey
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Re: I cant believe the size of the LX600........ new [Re: GlennLeDrew]
      #5607338 - 01/05/13 09:33 PM

Quote:


Again, I'm astonished at the vast ocean of confusion out there in amateur land, as regards even the fundamentals of contrast transfer in optical instruments. I really wish the community as a whole would finally get these matters straight, and not propagate the same old misinformation generation after generation.




I'm SO GLAD you posted that! There are, indeed, way too many iron-clad "truths" that are no more than uninformed "beliefs"!

Unfortunately, trying to 'set the record straight' on these forums only leads to flame-wars launched by the Luddites who just will not listen to anything that disturbs their firmly-entrenched worldview.

Wouldn't it be great if someone with widespread credibility wrote a book correcting these misconceptions?

Mike


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Joe Cepleur
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Re: I cant believe the size of the LX600........ new [Re: Mike Harvey]
      #5607492 - 01/05/13 11:10 PM

Quote:

Quote:


Again, I'm astonished at the vast ocean of confusion out there in amateur land, as regards even the fundamentals of contrast transfer in optical instruments. I really wish the community as a whole would finally get these matters straight, and not propagate the same old misinformation generation after generation.




I'm SO GLAD you posted that! There are, indeed, way too many iron-clad "truths" that are no more than uninformed "beliefs"!

Unfortunately, trying to 'set the record straight' on these forums only leads to flame-wars launched by the Luddites who just will not listen to anything that disturbs their firmly-entrenched worldview.

Wouldn't it be great if someone with widespread credibility wrote a book correcting these misconceptions?

Mike




Does this constitute "highjacking the thread," or may we accept that, in order to discuss the size of the LX600's obstruction, one must be willing to discuss whatever is known about the size of obstructions?! Personally, I consider the philosophy behind a topic to be part of the topic, because one must understand how one understands.

I, too, was shocked upon first seeing the LX600's obstruction, and asked myself the same question, so I was glad to see the question appear in a thread. I see the problem as essentially cultural. Highly technical people with scientific training ask each other questions that include all relevant details, including those likely to be known. They also ask, "How do you know?" questions. People outside of this cultural circle sometimes misconstrue this as arrogance. It would help if people knew each other's styles, and so discussed topics fluidly, independently of personal style. Trouble is, it's not clear how they might come to know what they do not know, so the flame wars will likely continue.

The gap is well illustrated by the comedian's gag (wish I could credit this; anyone recognize it? maybe Eddie Murphy?). It's unprintable in this forum in its original form, so let's hope this variant will suffice: "The tech folk be always measuring their biceps. The Romantics just look at their arms and say, 'Wow! They're big!'"

Personally, I'd love to learn the standard misconceptions and the physics behind them. It would be part and parcel of learning the actual truths, but a more thorough way to learn.

It's late. I'm tired. Sorry if this important point is tending toward a rant.

Maybe there should be a new forum dedicated to myths about telescopes!

Anyway, the thread has helped me. As I see it, contrast is the range between the brightest and dimmest regions of an image. Sure, a scope of the LX600's aperture could be built for higher contrast, but with such large aperture and the efficiency of modern coatings, apparently the LX600 works well enough as is, with wide fields of view and fast exposure times as part of the package.


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GlennLeDrew
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Re: I cant believe the size of the LX600........ new [Re: Joe Cepleur]
      #5607666 - 01/06/13 01:17 AM

Joe,
The topic is the large central obstruction and its impact on contrast transfer. Bringing to light the confusion which leads to the very kinds of misinformation brought up during the discussion is most definitely relevant, and not 'hijacking' the thread.

One thing I've long advocated is the treating of contrast at two scales.

Small, where diffraction effects impact the image within the limit of the inner 2-3 rings surrounding the Airy disk for any image point.

Large, where scattering floods all or some portion of the field with veiling glare.

The former is relevant to the observation of fine detail at reasonably high levels of surface brightness, where the eye's resolving power is maximal, or not much reduced. Such targets as the Moon, planets, double stars and compact clusters benefit from minimal degradation introduced by additional diffractive edges within the aperture.

Objects having low to moderate surface brightness are not visibly impacted by diffraction, simply because the eye's resolving power is too low. Galaxies and virtually all nebulae fall into this category; they're called faint fuzzies for a reason.

Another aspect to bear in mind is that the effects of diffraction become visible only when the exit pupil has become sufficiently small. Even for systems having a whopping central obstruction, it requires something like a 1.5mm (perhaps a bit larger) exit pupil to see the Fresnel pattern. At larger exit pupils--certainly by 2mm or so--the image is about as good as unobstructed as regards perceivable detail.

At this and larger exit pupils, veiling glare is the only contrast robber of concern (assuming optical quality is otherwise good.)


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Asbytec
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Re: I cant believe the size of the LX600........ new [Re: GlennLeDrew]
      #5607696 - 01/06/13 02:07 AM

"Simple minds discuss people. Good minds discuss events. Great minds discuss ideas."

Indeed.


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Bill Barlow
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Reged: 12/03/07

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Re: I cant believe the size of the LX600........ new [Re: mmalik]
      #5608054 - 01/06/13 10:50 AM

I find it interesting from your calculations of the CO by area/diameter in the new Meade f/8 OTA's. It seems that as the aperture gets larger, the CO gets smaller. The M14 f/8 has about the same CO in area and diameter as the C9.25.. The C9.25 is an excellent scope visually, so would the M14 f/8 also be good visually? But maybe the different optical design of the C9.25 primary and secondary mirror f ratios contributes to this?

Bill


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freestar8n
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Re: I cant believe the size of the LX600........ new [Re: Joe Cepleur]
      #5608072 - 01/06/13 10:59 AM

Quote:

Personally, I'd love to learn the standard misconceptions and the physics behind them. It would be part and parcel of learning the actual truths, but a more thorough way to learn.




I recommend references to primary sources. Astronomical imaging is complex enough - and when you combine it with human vision in a changing atmosphere - it becomes a even more complex and touches on many disciplines. This makes it unrealistic that you can draw a single graph and make broad claims to compare the expected performance of two very different systems.

Like most threads on this topic, this one refers to Suiter and to various web pages, but it does not refer to textbooks on Fourier optics or journal articles. Suiter is a good intro to aberrations and helps interpret telescope behavior in the context of things like MTF, but it does not go in depth on ways in which MTF is very limited. MTF would be great if the object being viewed were a single, perfect sine wave pattern - but instead even something like the bands of jupiter involve many spatial frequencies that overlap and interfere, and imperfections in both phase and amplitude can generate artifacts that would spoil the view even though the mtf looks "good" in the "frequencies of interest."

Two references on Fourier optics are Gaskill and Goodman, where MTF is treated in more detail and caveats are provided on its usage. I have cited them previously in other threads.

On the topic of large CO in SCT's, I think it's useful to refer to a key early paper on design options for SCT's where some field curvature is allowed. Sigler's 1975 paper in Appl. Optics fits in well here, although it pre-dates more high res. imaging work with ccd's as opposed to film. I can't quote the whole thing, but will provide a brief excerpt:

"It is ... unfortunately true that the flat field designs, due to the Petzval constraint, have large secondary obstruction ratios, small secondary magnfication... These constraints are not too serious for strictly photographic instruments where the secondary obstruction is frequently as large as T=0.5 without unduly affecting performance. However, for photo/visual instruments, the large field of view and low effective focal ratio are sacrificed for higher secondary magnficiation ratios... The attainment of high resolution, especially in the intermediate spatial frequencies of the MTF curve, is facilitated by having the smallest possible secondary obscuration ratio."

So here is a journal article that points out that you can flatten the field by reducing the power of the secondary, but it ends up being bigger - and that will impact high resolution performance in visual work. In summary, a big motivation for the larger secondary in imaging Cassegrains is to reduce Petzval curvature and flatten the field - but doing so is undesirable due to the loss of resolution. Anyway, you can flatten the field and reduce the f/ratio with lenses near the focal plane, while keeping the secondary small.

With regard to comparisons of contrast and sky blackness with a refractor vs. an sct - key differences are the much better baffling that you can have in a refractor, and the potential for less scatter by using only lenses rather than mirror surfaces. If some people feel the overall contrast is better in refractors vs. sct's, it may not be easily explained in terms of mtf - but it may be nonetheless true due to other factors.

People have different feelings for how important the CO is to the view, and my attitude is simply that smaller is probably better. But how much difference it makes depends on many factors that are not captured in an MTF plot or simulations - and also depend on each user's visual system and preferences - and how the optical characteristics combine with the object and visual system to cause the thumb on the human to point up or down in response.

Frank


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Cotts
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Re: I cant believe the size of the LX600........ new [Re: freestar8n]
      #5608155 - 01/06/13 11:40 AM

My take-away from all of the above posts is this:

If you have a 45% (by diameter) obstruction in your scope and you take a photo of a 1.5 x 1.0 degree field surrounding the Rosette nebula there will be zero contrast loss - as any number of astrophotographers will tell you - the contrast loss is entirely at arc second scales, a level of detail that the pixels on the chip cannot resolve. The telescope is designed with this in mind. Ceravolo and Officina Stallare and others make some very expensive, state-of-the-art astrographs with central obstructions approaching 50%. Visit the websites of these companies to see the astonishing, contrasty images they produce. I bet the farm that if you buy a scope of this configuration you won't even get a diagonal with it - it is not meant for visual use.

If you take those same telescopes and throw on a diagonal and a 6mm eyepiece for a 250x view of the detail in Jupiter's cloud bands, you're going to run into some serious degradation of the contrast.

This seems straightforward to me. To discuss contrast loss in greatly obstructed telescopes without referring to the angular size of the target leads directly to 'myths'.

Dave


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