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Larry Carlino
member


Reged: 06/05/06

Re: EIGHT-INCH SHOOTOUT new [Re: Mauro Da Lio]
      #5320032 - 07/16/12 10:28 AM

My sincere thanks to everyone who made comments on my review. A few points do need clarification, especially regarding the use of "too low" magnification and the primary mirror boundary layer effect. To put things in perspective, I have been an active, serious visual observer for over 45 years. I have owned over 150 telescopes in that time and have looked through hundreds more. I have built a dozen or so scopes, including 16, 17.5, 22.5, and 28-inch Dobs. From the late 1960's to the early 2000's, I contributed regularly to the various ALPO planetary sections with detailed visual observations and sketches. The generally poor seeing conditions in Western New York where I reside are somewhat compensated by the blessing of 20/15 vision in my observing eye. I am very familiar with optical physics and can spot an optical system not in thermal equilibrium with ease. My larger scopes are generally equipped with fans and other devices to ensure that a boundary layer on the primary evacuates expediently.
As to magnifications used in the review, my fortunate visual acuity allows for lower powers to be adequate and conformal to less-than-excellent seeing conditions. Please note, that all four of the telescopes in the article have been tested on the night sky and with an artificial star at magnifications in the 400x to 500x range, though this was not mentioned in the review. The result was the same. Nowhere in the review did I say anything about intrinsically poor performance of reflective telescopes; on the contrary, the inexpensive Newtonian put on a fine performance - but it is absolutely NOT the equivalent of an 8-inch refractor. The secondary obstruction, spider vanes, and nature of light itself make this impossible given equal optical quality. Many of my most detailed planetary sketches in the past few decades have been made with 10" and 12.5" Cave Newtonians, so I DO recognize the virtues of this simple, effective optical system. The whole point of this "shoot-out" review was to illustrate that differences in optical design and arcane technical details are secondary to experience, some basic knowledge, and enjoyment of our magnificent universe with the equipment one has. I have no other "agenda," nor can I be relegated to the "uninformed" or "inexperienced" category.
Again, my thanks for all of the kind comments.
For Mauro: Because you have mildly offended me, I'm not going to share my Grand Unification Theory equations with you. (hint: the 6 quark varieties "lock up" the 6 "missing" dimensions from the Big Bang)


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KeithC
super member


Reged: 08/23/06

Loc: Atlanta GA
Re: EIGHT-INCH SHOOTOUT new [Re: Larry Carlino]
      #5320120 - 07/16/12 11:09 AM

Larry,

One of the things that I liked most about your review was that it was so gentle. I picked up a keen sense of awe, wonder, fun and discovery....the very things that drew me to this hobby as a child. I applaud your work and would love to see more of it!

KeithC


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Mauro Da Lio
professor emeritus


Reged: 09/12/04

Re: EIGHT-INCH SHOOTOUT new [Re: KeithC]
      #5322231 - 07/17/12 04:15 PM

Sorry Larry. It was not my intention to offend you. Criticism is not offense.

I disagree with the conclusions and the way they are drawn.

I pointed out that magnification was too low to be meaningful, and I still believe so (despite higher acuity 140x is far lower than the roll-off magnification of 20 cm apertures). By the way I also have high visual acuity but still find advantageous going in the 30-50x per inch whenever possible. If that magnification is not achieved the bottleneck is not the scope (or it is a bad scope).

As for compensating bad seeing with visual acuity, sorry, but that is another incorrect statement in my opinion: seeing degrades the MTF of the scope and you cannot recover lost contrast with visual acuity (you can do somewhat on photography by running deconvolution filters). With an example: take a photo on your desktop, apply a gaussian blur and look it with a magnifying glass: it does not help.

Edited by Mauro Da Lio (07/17/12 04:29 PM)


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Larry Carlino
member


Reged: 06/05/06

Re: EIGHT-INCH SHOOTOUT new [Re: Mauro Da Lio]
      #5322435 - 07/17/12 06:30 PM

Mauro,
If you would please re-read my last response, you will see that I DID use magnifications of 400-500x and found the results to be the same as for those obtained at 240x(30x per inch of aperture). This was not mentioned in the original review as I wanted to give an impression of the telescopes' performance at powers normally used under mediocre seeing conditions, something, unfortunately, many of us in the NE U.S. are plagued with. I stand by my original conclusions.
I absolutely did not say or imply that visual acuity compensates for poor seeing conditions. Excellent visual acuity, however, DOES allow the use of somewhat lower magnification to obtain the desired angular resolution. That is a scientific fact.


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grom
member


Reged: 02/23/09

Loc: Spain
Re: EIGHT-INCH SHOOTOUT new [Re: Daniel Mounsey]
      #5323260 - 07/18/12 09:09 AM

Quote:

This is a great comparison because it teaches beginners what really happens in real world situations. Members should take note of this and start making comparisons of their own. There's WAY too much optical theory going on in the forums and way less hands on experience in real world comparisons and it shows. There's not enough reviews like this anymore.



I couldnt agree more. Great article.

Quote:

I disagree with the conclusions and the way they are drawn.

I pointed out that magnification was too low to be meaningful, and I still believe so (despite higher acuity 140x is far lower than the roll-off magnification of 20 cm apertures). By the way I also have high visual acuity but still find advantageous going in the 30-50x per inch whenever possible. If that magnification is not achieved the bottleneck is not the scope (or it is a bad scope).




And this is an example of the "way too much optical theory". Mauro, you can throw all the theory you want, and you probably will be right theoretically. However, in my experience of years using scopes between 3" and 8", the vast majority of Jupiter observations I settle for less than 200x, in order to get the most pleasing image. So, I am not surprised at all the author used 140x.

In this other 5" apo shootout (link below), Jay Reynolds Freeman states that, on Jupiter, "We all independently settled on magnifications between about 125x and about 175x as providing the best views".
http://www.cloudynights.com/item.php?item_id=506

So, again, it is practical experiences at the eyepiece what I sometimes miss in the forums, not theory.


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Daniel Mounsey
Vendor (Woodland Hills)
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Reged: 06/12/02

Re: EIGHT-INCH SHOOTOUT new [Re: Mauro Da Lio]
      #5323415 - 07/18/12 10:57 AM

Quote:

If that magnification is not achieved the bottleneck is not the scope (or it is a bad scope).




Really? I've seen plenty of situations where great scopes can't be magnified due to several factors we could discuss all day.

Mauro, Larry's review was a sincere effort and in my opinion, you're not giving him enough credit. You found an area where less magnification was used in the review and you're discrediting him for it, yet Larry brought up several interesting areas that experienced observers would pay attention to. It's easy to sit back in the simulator and go over your notes to prepare for the expectations, and it's different situation when you're actually flying the plane. I can usually tell just by the way people write, who has the experience making comparisons and who's sitting at the computers looking at MTF's.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: EIGHT-INCH SHOOTOUT new [Re: Mauro Da Lio]
      #5323607 - 07/18/12 01:20 PM

Quote:

I think that comparison has been carried out at too low magnification to be meaningful. I read of 140x on Jupiter and 240x on Mars.

A well cooled scope, decent figured, on fair seeing should be able to get crisp images at higher magnifications.






I did notice that the fans were only used intermittently, full time is what works for me even here in San Diego where the temperatures are mild. "Well cooled" is critical, as we all know.

Jon

Edited by Jon Isaacs (07/18/12 01:23 PM)


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planet earth
Carpal Tunnel
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Reged: 09/07/10

Loc: Ontario Canada
Re: EIGHT-INCH SHOOTOUT new [Re: Larry Carlino]
      #5323843 - 07/18/12 04:34 PM

Quote:

mediocre seeing conditions, something, unfortunately, many of us in the NE U.S. are plagued with. I stand by my original conclusions.



I'm situated not too from you across the border, and your quite right about the seeing conditions.
With my 8 f7.6 1530mm fl. Newtonian of good quality I have 5 eyepieces for planets, giving magnifications of X145,170X,190X,219X and 255X.
I judge the seeing conditions by the highest magnifications I can use on a given night on the various planets and the moon. I found that even with a increase of 20X magnification on a given night, throughout the year, can make a huge differance as far as seeing finer detail and not seeing it.
IMHO with 145X on Jupiter you should be able to see more detail then you could possibly draw with good optics and less then perfect seeing.(maybe even less then 145X)
In Book One or was it Book Two ATM, it mentions a observers favorite magnification as X145 using a 18 inch Newtonian.
(possibly less then excellent seeing, or very good eyes? I don't know)
Anyway's Good review!, but one should read it more then once!
Clear Skies.
Sam


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Mauro Da Lio
professor emeritus


Reged: 09/12/04

Re: EIGHT-INCH SHOOTOUT new [Re: grom]
      #5327293 - 07/20/12 06:11 PM

Quote:


And this is an example of the "way too much optical theory". Mauro, you can throw all the theory you want, and you probably will be right theoretically. However, in my experience of years using scopes between 3" and 8", the vast majority of Jupiter observations I settle for less than 200x, in order to get the most pleasing image. So, I am not surprised at all the author used 140x.

In this other 5" apo shootout (link below), Jay Reynolds Freeman states that, on Jupiter, "We all independently settled on magnifications between about 125x and about 175x as providing the best views".
http://www.cloudynights.com/item.php?item_id=506

So, again, it is practical experiences at the eyepiece what I sometimes miss in the forums, not theory.




Believe it or not, on Jupiter I use about 400x (380x) regularly with a 24" reflector at home. In my preferred dark site, which is also on a elevated place above most of the planet boundary layer it is not rare to use 500x with images still crisp. With a previous 10" newtonian I used way more than 200x on Jupiter (~300x). I am not the only one. I found that there are many factors that are similar to seeing but that can be controlled ( http://www.skyandtelescope.com/howto/diy/3304176.html ). Since then I used maniacal control of thermal effects within the tube, ground, surroundings.

As for 200x being sufficient here is another anecdote: on Mars last year, below 200x the image produced by a 8" GSO and the 24" were virtually non distinguishable (except for brightness). However the 8" image breaks at about 400x whereas the 24" was able to go quite regularly at 800x (on Mars). The magnification that a scope can hold before image breakdown IS the quality of the image at the focal plane.

Quote:


Really? I've seen plenty of situations where great scopes can't be magnified due to several factors we could discuss all day.





That is the point. Many of these "factors" are not in the scope and not the seeing (seeing is the only factor that limits magnification, which cannot be worked around).

I do not say that Larry's review was biased. Nonetheless, there is no indication of why one scope was inferior and why it was not possible to use higher magnification (on extended low contrast sky objects, not the artificial star). Was it poor optical figure? (It has to be really poor to be detectable at 140x). Was it seeing? Was it local thermal currents (could be considered that the best performer had closed tubes)?
We only know that three scopes compared that way, but without telling why no conclusion can be really drawn.

Edited by Mauro Da Lio (07/20/12 08:11 PM)


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Mauro Da Lio
professor emeritus


Reged: 09/12/04

Re: EIGHT-INCH SHOOTOUT new [Re: Mauro Da Lio]
      #5327359 - 07/20/12 07:11 PM

Just to clarify. Here is Jupiter at different scales. The top line is blurred. On my PC the right images correspond to approximately 300x when viewed at 60 cm of distance. The leftmost images are 1/3 the size (ie 100x). Set distance as to obtain the same magnifications.



Comparing the pictures on the left one can hardly tell the difference, which instead becomes obvious at the the right. Differences are hidden at low magnification.
BTW The second column from the right is about 150x. Looking with one single eye reveals what can be captured (the limit being the low magnification, since details are there).

Edited by Mauro Da Lio (07/20/12 07:23 PM)


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David Knisely
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: EIGHT-INCH SHOOTOUT new [Re: mwedel]
      #5328589 - 07/21/12 05:26 PM

mwedel posted:

Quote:

Or are you saying that it's not Larry's shootout in particular that is unfair, but that any comparison of three scopes at such wide price points would be unfair?




Yes, the latter to be sure (no knock at all against what Larry did). I often hesitate to compare three different designs as they are each good and bad in their own ways. I know many SCT's can be bested by a very good Newtonian mostly due to the larger secondary obstruction and compromise design of the SCT, and I also know that a well-made Mak Newtonian has its share of advantages and problems as well. The f/ratios of the three instruments were a bit different (f/10 for the SCT which is more forgiving of eyepiece problems), although the f/5.9 and f/6 of the Mak-Newt and classical Newtonian were close enough for a decent comparison provided conditions were the same. Also, when discussion the Intes Mak-Newt, stating, "Is it the equal of an apochromatic refractor of equal aperture? No. " may have been a little unfair, as I don't know of any commercial 8 inch f/5.9 Apochromatic refractor on the market today that could be even remotely affordable to the amateur. Stating that the six in APO "beat it" is also doing something I don't exactly like: comparing scopes of differing apertures. The six inch refractor won't show seeing disturbances quite as much as the larger aperture Mak-Newt, and the refractor does not have a central obstruction. These are two obvious differences which may make a statement about a comparison with a refractor slightly less than completely fair. The Intes instruments I have seen generally have been pretty good (mostly in the "premium" class), but put a good coma corrector in the Newtonian and I think the two probably should have been a lot closer to being equal.

The most valid thing Larry said was, "After this test, Im convinced that the real key to enjoyable observing is making the most of what one has, sprinkled with a healthy dose of patience and practice.

That, at least to me, means a lot more than the telescope itself.
"

On this, we are in full agreement. Clear skies to you.


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Daniel Mounsey
Vendor (Woodland Hills)
*****

Reged: 06/12/02

Re: EIGHT-INCH SHOOTOUT new [Re: Mauro Da Lio]
      #5329210 - 07/22/12 02:17 AM

Mauro,

I appreciate your efforts, however, I think you should do some research on the observers you're questioning. The reason these kind of reviews can be helpful is because it helps others see that everything isn't what they might expect for whatever the reason and that's fine, because that's what happens in the real world. But, to discredit Larry's efforts because a couple numbers don't agree with your statistics is a low blow in my opinion. You should at least have had the courtesy to allow him and opportunity to elaborate a bit more on the points in question and instead you just threw a blanket statement to discredit his efforts. You weren't there during the observations were you?

It really amuses me when statisticians try to follow up on real world observations they question in fact most of them don't even have the experience to back up their own statements. I'd like to see others make some comparisons of their own and see if it all comes out as their theories dictate they should. It's easy to sit back and nit pick about something, but that doesn't discredit Larry's review, not by a long shot. His general consensus of what telescope performed best over all, was credible and pretty consistent throughout the review. A lot of these number crunchers are clueless on real world situations and its the same old broken record I keep hearing again an again in some of these forums. I can provide several red flags in your own comments that make less sense than the ones you're complaining about.

You stated "In my preferred dark site" How does that make any sense when you're trying to observe planets with a 24"? That's a recipe for planetary disaster. How can you speak so highly on this topic and not even know that? Steve Kennedy is a master optician who brings world class 28" and 32" telescopes to our star parties and the best views of Jupiter consistently occur right after sunset when very few stars are even visible. As soon as the skies darken and the eyes dark adapt, its pretty much over and done and an ND filters can't compete with the twilight factor experience, not even close if you want to talk about world class images. There is such a thing as too much contrast. Should we tune the contrast on our television sets to maximum?

Then you go on to state that it "isn't rare" to use 500X with images still crisp for Jupiter in your 24"? Getting a stable view of Jupiter at any magnification in a 24" is challenging, let's get real here. Your idea of tack sharp, high definition could be totally different than another observers. I've tested some of the most incredible telescopes in the world under world class seeing here on the west coast with small temperature variances and those numbers are absurd for Jupiter. Anyone can put an eyepiece in a telescope and say they're doing 300x, 400x 500x. That's not telling the whole story. I don't care if it's 20" or 25" telescope doing 800x on Saturn. I've seen detail on Jupiter's surface at a measly 135x in a 4" telescope with 13 color variations, top to bottom. Staggering detail. 135x is a little number but the detail was amazing! Now we get into exit pupils, aperture, central obstructions, MTF's, etc etc etc trying to make sense of it all. Let's just get out and observe and see what happens. 99% of the time I observe planets, I don't even care what magnification I'm using. I just take it as high as I can until I can't take it any further. Whatever that number happens to be is what it happens to be. The numbers I use are mostly put in place for reviews. Let's give this forensic analysis approach a rest and get on with some real world shootouts for a change, can we.


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Mauro Da Lio
professor emeritus


Reged: 09/12/04

Re: EIGHT-INCH SHOOTOUT new [Re: Daniel Mounsey]
      #5329520 - 07/22/12 09:53 AM

Daniel. You are using with me what you complain I supposedly did with Larry. I did not like Larry's review because it does not explain anything about the cause of the observed ranking.
In the other link http://www.cloudynights.com/item.php?item_id=506 you will see a completely different approach to explain why not being able to use more magnification (and you will see the Lick's scope was able to use much more). I did not want to throw any low bow. The same question (as you read the latter link) was asked them and they answered. Larry can elaborate a bit more on the point of magnification as well as on which were the causes (eg connecting star test to observed image quality).

Low magnification compresses image difference below the detection threshold of the eye (take the two Jupiters on the last column http://i45.tinypic.com/35le043.jpg and move back from the screen until you re no longer able to see the difference. Then compute the equivalent magnification: it is about 150x). At low magnification the bottleneck is the Contrast Sensitivity Function of the eye, which incidentally you may measure. http://neurovision.berkeley.edu/Demonstrations/VSOC/izumi/CSF/A_JG_RobsonCSFchart.html

As for the"dark site" issue, I mentioned it because it is also a site with good seeing. It is a parking lot just at the bottom of these famous peaks in the North Eastern Alps http://www.lemiemontagne.it/sitopubblico/images/Imm_Desktop/TreCime_Lavaredo.jpg
The site is 2320 m elevation, high enough to be above most of the planetary boundary layer (except if wind comes exactly form the peaks, which is rare) I did never measure seeing objectively, but my estimate is it to be sub arc second regularly.
Once I compared a TV 101 NP in parallel with a 16" dobsonian there. The 101 was able to achieve 250x of "roll-off" magnification. I stress term "roll off" the actual meaning of which is defined by Mel Bartels (scroll down) http://www.bbastrodesigns.com/ratemirrors.html . Incidentally, in this other link http://www.cloudynights.com/item.php?item_id=506 what people do is just finding the roll-off magnification. I do no use useless magnification. I graually increse magnification until no new details are visible. So believe it or not roll-off is between 400 and 500x on that place (which means r0 approximately 20-25 cm, typical of good sites - where excellent sites have r0 = 30-40 cm.
Conversely at home, which is open countryside on a large flat land, roll-off is less but still often in the 300-400x. I am not the only one using such magnifications. A friend of mine, not far and in similar conditions makes pictures like these http://www.marcoguidihires.com/aggiornamenti/ingrandimenti/Giove/20100922.jpg .

There is however a trick in being able to use such high magnifications. It is controlling the boundary layer. My method is this http://autocostruttori.blogspot.it/2009/08/yet-another-method-for-cooling-mirrors.html and Alexis made a nice comparisons of different methods here: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4122861/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1

I can telly you that it makes the difference. When I star observing images are often blurred at 160-200x, but then they gradually become crisper and crisper and the limit becomes the REAL seeing, which often means 400-500x on Jupiter. Of course a smaller and thinner mirror such as 10" 1" thick will be faster to equalize with the ambient temperature. Not so a 24" 2.25" thick unless some effective boundary layer control is adopted. But if that is used you get such high magnifications.

Anyway the discussion is not about how much magnification can be achieved by 24" but if 140x was enough. Larry does not explain why it was not possible to use higher magnifications. Here they explain http://www.cloudynights.com/item.php?item_id=506

Last comment is about seeing better in twilight. That is a problem related with the apparent size of the object. at 100-200x Jupiter looks 1.5-2.5 apparent degrees wide. If t stands onto a dark background the eye does not adapt to the brightness of the object. The object is like "over exposed" because the area considered bye the eye for "exposure adaptation" is wider. However things change when the object covers an apparent size of 5-8 (ie at 400x or more). In this case the planet is wide enough that the eye adapts to the brightness of ts surface without considering the foreground.

Finally, as for how many details you can see at low magnification you can never see more that your CSF permits. Take one picture of a planet (eg http://www.marcoguidihires.com/aggiornamenti/ingrandimenti/Giove/20100922.jpg ) full of details and move at distance such as the apparent size correspond to the magnification you have in mind. Look with one eye. That is what you may at most see at that magnification. The real picture may have a lot more details, but the bottleneck is your CSF. If the picture remains crisp (ie we are below the roll-off magnification) at 250x, then there is NO WAY to see the same details at 135x.
Moreover: the roll-off magnification IS the quality of the image.

Quote:

I just take it as high as I can until I can't take it any further. Whatever that number happens to be is what it happens to be.



You find the "roll-off" magnification. However ending up with 400x is a totally different story than stopping at 150x. By definition if you stop at 400x that means that with less magnification less was visible. Believe it or not, with exactly your same method, I got 250x with the TV 101 (on perfect seeing) and usually get much more with greater apertures.

Edited by Mauro Da Lio (07/22/12 10:25 AM)


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cheapersleeper
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Re: EIGHT-INCH SHOOTOUT new [Re: Mauro Da Lio]
      #5329579 - 07/22/12 10:42 AM

240x was used on Mars and the resulting images were discussed.

Some of you seem to confuse a simple Review on a busy public forum with a paper written for a scientific journal. This venue is not a scientific journal and you are not "peer reviewing" this piece. In a place like this, it is not unreasonable to just regard the scopes as a black box and report what is seen. It is only if an author is trying to make definitive claims from their observations that they may need to be questioned.

Larry provided a reasonable review appropriate to giving non-scientists some information regarding how these scopes perform under conditions that are similar to what an average amateur might encounter.


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Daniel Mounsey
Vendor (Woodland Hills)
*****

Reged: 06/12/02

Re: EIGHT-INCH SHOOTOUT new [Re: Mauro Da Lio]
      #5329654 - 07/22/12 11:49 AM

Mauro,

Probably one of the greatest views of Jupiter I ever saw was while using a 20" at just 218x. That's just 218x! or about 11x per inch. We were using a stereo viewer and the image looked like something from a Voyager craft. Detail and colors beyond description in fact we could clearly see streams of violet ammonia ice crystals beneath Jupiter's troposphere. It was truly amazing and the amount of angular resolution we had on tap delivered the goods. According to you, Larry's magnifications of about 140x or about 18x per inch were invalid. I'd like to hear more about "your" real life experiences making some of your own comparisons without any premeditated bias as to what the calculations should dictate. I'm an observer and most of the things I cover come from real life comparisons I've made and too often, people are forming opinions they have little experience with in reality and there are often conflicts they don't realize themselves. That's pretty much my assessment of all this. I respect and understand if you prefer all the science. That's what most people appear to be into and part of the reason I spend less time in the forums debating these days.

Here's an example. All I have to do to determine of a particular food product is healthy or not is to just turn the package over and read the ingredients. As soon as I start seeing terminology like glycerin, riboflavin, filic acid, niacine, sodium bicarbonate or things my spell check doesn't even recognize, I already know it's unhealthy. On that same note, whenever I continually hear people talking about optics that sound as ridiculous as the ingredients in these food products, I immediately raises red flags with me and I begin to question their amount of actual hands on experiences. It's very simple. Sure, that may not always be the case, but more often than not, it is.

In my opinion, it would have been wiser to say your "preferred seeing site" rather than your preferred dark sky site. There's a huge difference between the two when you're viewing planets. I realize Larry's review is not your style and that's okay. I like Larry's review because he said it like he saw it and made some efforts to throw in some interpretation. After the reviews are done, it can be interesting to discuss the possibilities constructively, but to say his report was invalid was pretty insulting in my opinion, especially since Larry has actually made a number of comparisons himself.

Why are we not discussing focuser placements on dobs? You are probably asking yourself what focuser placement has to do with any of this yes? Exactly my point. I've tried to explain countless times how the focuser placements on dobs can degrade image quality because of heat plumes pouring off the observer. Even William Herschel spoke of this problem, yet it's continually ignored. It can make a telescope with world class optics perform like something with 1/2 wave optics, yet nearly every ATM puts focusers on the same side as usual. I know for a fact, based on true hands on experiences that my argument is valid. The visual comparisons I've made, verify this fact, yet no one hardly speaks about the issue. In the meantime, they're bickering over central obstructions and a few decimal points on some strehl ratios MTF's and it's simply ridiculous. It just gets old listening to it all sometimes and in the meantime, more meaningful issues are being ignored. Regardless, I think we both made our points.


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David Knisely
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: EIGHT-INCH SHOOTOUT new [Re: Jon Isaacs]
      #5329742 - 07/22/12 12:48 PM

Quote:

Quote:

I think that comparison has been carried out at too low magnification to be meaningful. I read of 140x on Jupiter and 240x on Mars.

A well cooled scope, decent figured, on fair seeing should be able to get crisp images at higher magnifications.






I did notice that the fans were only used intermittently, full time is what works for me even here in San Diego where the temperatures are mild. "Well cooled" is critical, as we all know.

Jon




Yup, I too keep my fan on my 14 inch Newtonian running even at high power (one of the things Orion did right on the scope was mounting the fan so it would not induce any vibration even at high power). Even small tube currents can also make comparisons between instruments more difficult, as one telescope may have minor ones and another may not. I have even reviewed a special Newtonian design that had an optical window get a thermal plume that wrapped around the secondary mirror causing problems. Fans on Newtonians can indeed be very helpful. Clear skies to you.


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Mauro Da Lio
professor emeritus


Reged: 09/12/04

Re: EIGHT-INCH SHOOTOUT new [Re: David Knisely]
      #5330109 - 07/22/12 04:59 PM

Minor thermal effects may pass unnoticed, yet their effect may affect the image when it comes to pushing the limits. I often find myself looking ad intrafocal images to check the status of the boundary layer. There are nice videos published by Bryan Greer.
The fact is that even 1C temperature difference on the front surface of the mirror can cause appreciable degradation. One of the reasons why I ultimately preferred having the fans extracting air instead of blowing is that in this way I get a uniform flow of air entering on the upper opening of the tube and slowly descending cleaning all the internal of the tube (and ultimately swiping the mirror face in a laminar flow). That make the air inside the tube as close as possible to perfectly equalized closed tubes. In addition air enters from the top (which is 3 meters above the ground in my case) where some of the ground effects are already vanished.
I use faster fan speeds in the first hour or two and then switch to lower speeds. That is the time when I can often see hints of the airy disk at >600x magnification, or at leas I see very few speckles slowly dancing.

Daniel. The CSF of two eyes is higher. That makes bino-views grossly equivalent to 1.5 times the magnification. You ca check the gain yourself by repeating the Gabori test with one and two eyes.

Las year I visited an observatory naer my home http://dobsoniani.forumfree.it/?t=54392804 (you can decipher something with the help of online translation). It host a 80 cm Keller telescope (1M+1M the observatory). The mirror is Lomo and has a (realistic) Zygo interferometer stating lambda/33 rms. Sky is not really dark (~20.6). Seeing has been measured several times and found to be often 0.8 arc second with 0.6 not rare. That night however it was wors than average according to the director of the observatory. The temperature outside was -9C. Inside the dome it was -7C. However the mirro was perfectly equalized with the inside. 2C temperature gap may be huge if they happne to be on the front of the mirror (in few mm) but they are not so critical on a larger scale. That night the useful magnification on Jupiter was "only" 320x.

Zambuto advertises mirror as being able to achieve 50x per inch. This "magic" number is the magnification that corresponds to the best tuning of the eye CSF and the telescope MTF. You may find an interesting explanation here: http://www.cityastronomy.com/rez-mag-contrast.htm There is no reason for being unable to get them if the mirror is fine and if the seeing permits. If you do not get them then there is some problem (either optics, atmosferic seeing,mlocal thermal effects, your eyes, etc.)
So, when I observe from 0.5" site with sufficient aperture and thermal effects controlled there is no reason for not being able to use >400x.

I had a 8" SC too. I remember being able to use nearly 200x on regular basis (with 140x showing definitely less) and sometimes 280x on Jupiter.

As for the "dark site" it happens it also is the best site as for seeing. However I do not move there for high resolution. The site is Bortle 2 to 3 depending on the situation (SQM > 21.5 on the Cygnus MW). So it is BOTH Bortle AND 0.5". When I move there I make deep sky observing for most of the night (hence the name dark site). If there are planets we look at them at the end of th night (not to compromise dark adaptation) and we close our observing session with one hour of high resolution (that is also when the scope have reached maximum performance).

As for focuser placement, and body heat entering the light path, light shroud and extracting the air from the bottom is countermeasure. In addition it may be a problem for a 1.5 meter long scope. For a 3 meters tall scope the air entering the light path is less likely. Nevertheless I happened to notice body heat of the line of people waiting to look into the scope (and stand just below it open mouth) during one recent event. So I asked people to form the line elsewhere.

What I do not like in Larry's review is that one has to accept it as is. These scopes behave that way. Besides magnification, there is, for example, poor explanation of what was causing the differences. Could it be that the Newtonian had a barely acceptable star test and the others were better figured? In this case one could understand that a Zambuto in the same place could have obtained a completely different outcome. Or was it that there still were some residual heat problems (the fact that closed tubes behave better could hint this)? In this case a Zambuto would not be the answer and fans could. None of these considerations are however possible.

Do I believe Larry did a bad/biased/unfair review? No. However the review is lacking elements that can help understanding what was going on.

Edited by Mauro Da Lio (07/22/12 05:20 PM)


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Javier1978
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Re: EIGHT-INCH SHOOTOUT new [Re: Mauro Da Lio]
      #5344938 - 07/31/12 09:45 AM

Larry,

Thank you for posting this review, I really enjoyed it!

Clear skies,

Javier.


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Mark Harry
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Re: EIGHT-INCH SHOOTOUT new [Re: Javier1978]
      #5377098 - 08/20/12 07:51 AM

In my opinion and personal experience, Larry in using the scopes 'primarily' in the 120-150x area of magnification, likely revealed everything he should have with the 3 scopes. I think of it in this way:
Thru experimentation, I found that generally the most detail, though small (airy disc and up) are revealed at 15x/inch. Yes, you're going to have to work a bit to glean this info.
But on a target like Jupe, there is quite a bit of bright, but LOW CONTRAST detail. The slight difference in delicate detail can be most easily seen at low powers wihout exception.
Think of it this way-
In raising the powers to a 2-3 fold increase, will spread the light across more area. The size will increase 2-3 fold, but the contrast difference will be 4-9 times LESS. How many times has everyone tried to slip a slightly higher power eyepiece in the focuser, only to see a delicate, faintly visible feature evaporate?
***********
I liked the report Larry wrote, and think it's well written for anyone with multiple levels of experience. Thank you.

Regards,
M.


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Re: EIGHT-INCH SHOOTOUT new [Re: Mark Harry]
      #5511300 - 11/09/12 09:40 AM

Excellent article, Larry.

Now for those questioning the use of low magnifications. I have done a lot to reduce the thermal problems around my telescope which is kept in a well ventilated barn. The truss structure has spirally wrapped layers of ripstop nylon with air spaces in between and topped with four mil plastic. The entire telescope is mounted on a cart with a plywood floor to get away from ground heat and the observing area is on the north side of a barn where the sun does not get to heat the ground. The ground is actually part of our driveway and is composed of crushed stone. My wife hates it because it looks like an extension of our lawn with all of the grass and weeds growing on it but it remains cool during the day. This observing area has to be mowed in the summer. Much of my planetary observing is done towards the east up to the zenith since this gets my focuser on the downwind side and results in no heat plumes from my presence.
Around eighty percent of the time I use a magnification of 121x with a 25" telescope and sometimes I remove a Paracorr to get 105x. Why? Because it is the only way to obtain nearly pinpoint stars and see the spider diffraction pattern to insure that optimum focus has been achieved. Rare are nights when I can observe at 181x without feeling like my eyes have just opened for the first time in the morning. When I lived in Buffalo, New York I had but three nights in twenty years where I viewed at 656x and 755x. My views of Saturn and other objects were exquisite on those nights when the seeing was steadied by the heat island effect of a sizable city. Now I have moved to hillier terrain where local seeing conditions are worse than Larry's although the sky is much darker.
My eyesight is still good (it used to be 20/10) and I have trained myself to eke out detail that some observers may miss at low magnifications. One has to make do with the observing conditions that one has.


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