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Refractor vs. Reflector ?

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#26 Tommy5

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Posted 02 March 2005 - 08:44 PM

Great report,I think that as aperture size increases any advantage that refractors may have in scopes less then 5 or 6" rapidly evaporates,too many things working against a large 7" or more aperture refractor,the secret to reflector performance is getting collimation and thermal issues worked out and then getting a premium ultra smooth mirror,you can make a mirror that is a perfect parabola with a machine, but even though the mirror is corrected perfectly it will be rough,making a smooth mirror is an art form and takes a skilled craftsmen many hours to get right, hence its expense and difficulty in finding one,a longer focal ratio also helps to get everything "tuned",also it is hard [impossible] to find steady enough seeing conditions to allow a 24"mirror to fully use its resolving power,so a smaller mirror may display a more appealling image, but probably won't display more detail, thanks again for sharing your wonderous observing experiences with us.

#27 Daniel Mounsey

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Posted 02 March 2005 - 09:03 PM

Well said Tommy. I agree. I'll get back to this forum real soon with some close up pics of Grissoms Newt and an explination on how they improved these scopes. By the way, the 13.5" combined all the aesthetics of the apo with all the detail and color seen in the 24" into one solid package. It was just too unbeatable. I love when people walk over to Grissoms scopes, not exspecting much and then they drop their jaw in disbelief at how beautiful and detailed the images are.

#28 Ron B[ee]

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Posted 03 March 2005 - 12:24 PM

Once again, these Newts just floor me! Refractors will always be beautiful, but it's hard to justify a $250,000 refractor to an $8,000 Newtonian that's beating it. The fact is, is that if you dial the Newt right, your gonna win. Pons has been observing planets for 50 years. He's earned the right to decide what he thinks is best and he's got the best of both worlds to prove it.

When I asked him which scope he liked better on planets, he said the Newt was king, hands down and it's as simple as that. He has no shame in saying so, dispite the fact that he's spent a good part of his life and a lot of money building the refractor.


Well, if I may be permitted to inject some thoughts along this line, which I read as "aperture is king" ;). We who are economically deprived do not need to have access to a $250K APO vs $8K Newt to see for ourselves. If you have access to a $2.5K 4-inch APO vs a $0.8K 8-inch Newt (with good mirror of course such as Discovery or better yet Zambuto, Royce, etc.) which are more readily available, you'll be able to see the planetary difference yourselves (providing you get the seeing blessing ;)). Judging from Daniel's informative report, the extrapolation seems to work backward fairly well.

Ron B[ee]

#29 Silvio

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Posted 03 March 2005 - 01:24 PM

I'm an absolute novice compared to the folks mentioned in this thread so I hope that I am not putting my foot in my mouth.

I have always thought that a perfect Newtonian was hard to beat. What I have personally experienced is that I just do not have the skills to perfect a Newtonian - centering the optical path right in the middle of the tube, squaring the focuser, squaring the diagonal, baffling the OTA, baffling the focuser, installing the proper thermal equalibrium equipment, collimation, etc etc.

I find that refractors are much more simple and therefoe easier to use at their full potential.

So in theory, Newtonians are the best. But for me, a good APO will most of the time deliver a better image than I am capable of coaxing out of a Newtonian.

My respectful opinions only....

Dark skies to all,
Silvio.

#30 Daniel Mounsey

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Posted 03 March 2005 - 03:20 PM

I'm an absolute novice compared to the folks mentioned in this thread so I hope that I am not putting my foot in my mouth.

I have always thought that a perfect Newtonian was hard to beat. What I have personally experienced is that I just do not have the skills to perfect a Newtonian - centering the optical path right in the middle of the tube, squaring the focuser, squaring the diagonal, baffling the OTA, baffling the focuser, installing the proper thermal equalibrium equipment, collimation, etc etc.

I find that refractors are much more simple and therefoe easier to use at their full potential.

So in theory, Newtonians are the best. But for me, a good APO will most of the time deliver a better image than I am capable of coaxing out of a Newtonian.

My respectful opinions only....

Dark skies to all,
Silvio.


Silvio,
You did not put your foot in your mouth, you are absolutely correct!
Best Regards,
Daniel Mounsey

#31 Daniel Mounsey

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Posted 03 March 2005 - 03:22 PM

Once again, these Newts just floor me! Refractors will always be beautiful, but it's hard to justify a $250,000 refractor to an $8,000 Newtonian that's beating it. The fact is, is that if you dial the Newt right, your gonna win. Pons has been observing planets for 50 years. He's earned the right to decide what he thinks is best and he's got the best of both worlds to prove it.

When I asked him which scope he liked better on planets, he said the Newt was king, hands down and it's as simple as that. He has no shame in saying so, dispite the fact that he's spent a good part of his life and a lot of money building the refractor.


Well, if I may be permitted to inject some thoughts along this line, which I read as "aperture is king" ;). We who are economically deprived do not need to have access to a $250K APO vs $8K Newt to see for ourselves. If you have access to a $2.5K 4-inch APO vs a $0.8K 8-inch Newt (with good mirror of course such as Discovery or better yet Zambuto, Royce, etc.) which are more readily available, you'll be able to see the planetary difference yourselves (providing you get the seeing blessing ;)). Judging from Daniel's informative report, the extrapolation seems to work backward fairly well.

Ron B[ee]


Hey Ron,
I'll have to eat my words. You are also absolutely correct as well in my opinion. Thank you for the wisdom.
Best Regards,
Daniel Mounsey :)

#32 Greg Morrison

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Posted 03 March 2005 - 05:01 PM

Hi Ron:

Speaking of refractor vs reflector how does your TMB 130 stack up against your newt on the planets? How does the 4" doublet compare to the 5" triplet while we're at it as well?

Thanks, Greg

#33 Ron B[ee]

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Posted 03 March 2005 - 05:21 PM

Hi Ron:

Speaking of refractor vs reflector how does your TMB 130 stack up against your newt on the planets? How does the 4" doublet compare to the 5" triplet while we're at it as well?

Thanks, Greg


Good questions, Greg. I did setup my 8-inch Dob next to my TMB130LW Super Light Cup one night but the seeing was so poor that I could only make out that Saturn was brighter through the Dob ;). This year's weather has been aweful but I'm sure and hoping I'll have good quality time with both later this year. So when good seeing hit, I've been expending it with my Super Light Cup for good reasons ;).

Regarding the Super Light Cup vs Light Cup (i.e. my 4" TV-102), I'll borrow a CG4 and am waiting on seeing Thomas Back's new "invisible" ;) TMB/WO quartz diagonal for a side-by-side shootout. But one night I did a comparison with mount/unmount/mount (sweating :bawling: ...) and you can find the result here.

Ron B[ee]

#34 mirage

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Posted 03 March 2005 - 09:37 PM

So I'm speculating that it's all about a well-ventilated finely figured mirror coupled with an exceptionally stable three-vane sixty-degree spider...

#35 half meter

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Posted 03 March 2005 - 09:45 PM

...and stand downwind of the focuser :lol:

#36 Ron B[ee]

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Posted 03 March 2005 - 09:47 PM

So I'm speculating that it's all about a well-ventilated finely figured mirror coupled with an exceptionally stable three-vane sixty-degree spider...


Nope, it was my lowly Discovery (premium ;)) PDHQ f/6 Dob with 4 vane spider. It does have a 19% secondary size though. I use the table fan to cool the mirror really fast. You can get some idea of what the $0.8K Dob can do on the planets here, here and here.

Ron wanna B[ee] 8-inch Tall Evangelist

#37 mirage

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Posted 03 March 2005 - 10:16 PM

Nope, it was my lowly Discovery (premium ) PDHQ f/6 Dob with 4 vane spider.


Whoops! I quick-replied to the wrong post. I was referring to the tuning tricks Daniel plans to share.

I've heard nothing but great things about those PDHQs, in whatever case. Enough so that I've more than once considered going that route in lieu of my planned SS10 Truss-Tube. Well, with my recent layoff it might take a bit longer to work that out...

I'd bet that there's not much required to 'Grissom Supertune' a PDHQ accordingly beyond replacing the spider and running a nice fan. Maybe add couple of ventilation holes in the the OTA near the primary?

#38 Greg Morrison

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Posted 03 March 2005 - 11:50 PM

Thanks for the comparison Ron. I look forward to hearing how the newt and TMB stack up against one another.

Greg

#39 sixela

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Posted 08 March 2005 - 12:36 PM

centering the optical path right in the middle of the tube, squaring the focuser, squaring the diagonal,

All unnecessary, as long as your axial collimation is good: all these things only affect illumination, and for planets you're not using that edge of the field at newtonian focus anyway.

That doesn't mean that an engineer with a newt won't go for all that diagonal offset tuning (believe me, I should know), but that's because he has a warped mind, not because it gives better on-axis images.

baffling the focuser,

I assume you mean flocking. Well, I guess that's not different from refractors, with the one difference that refractors are well (or badly) baffled in the factory ;).

installing the proper thermal equalibrium equipment,

There are cool down issues on large refractors as well. Unfortunately, large apertures are what you need for resolution.

collimation, etc etc.

Got me there. But practice makes perfect - if you have only refractors, you'll never learn the art of wielding the cheshire or - lo and behold - the mighty autocollimator.

#40 bierbelly

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Posted 08 March 2005 - 01:57 PM

Daniel was making all that stuff up. It's easy to have perfect seeing and incredible sharpness and clarity...when you're writing about it. :grin:

#41 Daniel Mounsey

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Posted 08 March 2005 - 08:01 PM

As promised, I'm going to share the way Pons and Grissom set up these Newtonians. Many observers believe that refractors are sharper and yield higher contrast than reflectors. This is true only up to a certain degree at which point, the limited aperture of the refractor is going to prevent it from revealing the contrast of subtle details, to a slightly larger Newtonian that isn't working as hard per inch to produce an image. What good is contrast if you're lacking angular resolution. Another thing is experience. I always get these mad scientists trying to debate me on the issues which will fallow, yet only a handful of observers have actually witnissed up close and personal what these Newtonians are actually like compared to very large apochromats. I've seen this with my own eyes on numerous occasions. The reflectors will win out in every catogory.

You can make this comparison at any aperture, small or large, as long as the execution of the Newtonain is properly dealt with. The first problem most often seen in Newtonian design is the use of metal tubes, which have become popular these days. The other major problem is the clearance. For example, about 1/2" to only 3/4" around the primary. This is not enough clearance. The other issue is that if you add a cork inlay, it will close the clearance even further. I'll explain the tube issue and get to the next part after.

Pretend we have two cups, one is made of metal and one made of styrofoam. If we pour ice cold water into each cup, you'll notice that the metal one will start to reveal major condensation on the outside, while the styrofoam is completely uneffected because it is completely dormant. Many observers are under the impression that because metal cools more quickly, that's better, and that it will transfer cooler air into the tube, right? WRONG. This is what's actually causing one of the problems which hinder the Newtonain design.

Since the temperature inside the tube will usually be warmer as the ambient temperature drops, the primary and the inside of the tube will be behind schedule as far as temperature. You've got a boundary layer pouring off the primary for quite some time. Once this warm air starts crawling along the upper, inside of the tube, it starts to mingle with the optical path.

When this cold metal interacts with this warmer air inside the tube, it causes unwanted eddies. It's like trying to mix water with paint thinner and the two just don't mix. Pons and Grissom prefer to use carboard sono tube because it behaves a lot like styrofoam and it's dormant. You want a material that does not react to temperature variances. I strongly suggest you have a minimal of 1.5" of clearance around the primary for 10" on up. The disadvantage of sono tube is that it can warp over time, so metal end rings could be considered. Bryan Greer at protostar claims metal is fine as long as you use a cork inlay. If you decide to do this, then make sure you have clearance around the primary.

One thing I don't like about metal is that it tends to resonate when used with fans. This is another advantage to sonotubes. Sonotube doesn't look as pretty, but it performs the best in our opinion and the images have proven this. Notice in this picture looking down the tube, how much clearance is around the mirror. I'm not trying to argue with anyone, but I'm telling you what works in the field. So the rule of thumb here is a dormant material and a minimal of 1.5" of clearance.

Try and ignore the curved spider at the moment as I will explain it in better detail later. Look down the tube and see the clearance around the primary. This is very important if you plan to work at 10" to 14.5". Tomorrow I will explain fan placements and I'll tell you what not to do. :cool:

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#42 bierbelly

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Posted 08 March 2005 - 08:08 PM

Finally! I had to shame him into it. One question Daniel. How did they get that camera down at the other end of the scope? :grin:

#43 Ron B[ee]

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Posted 08 March 2005 - 08:25 PM

One thing I don't like about metal is that it tends to resonate when used with fans. This is another advantage to sonotubes. Sonotube doesn't look as pretty, but it performs the best in our opinion and the images have proven this. Notice in this picture looking down the tube, how much clearance is around the mirror. I'm not trying to argue with anyone, but I'm telling you what works in the field. So the rule of thumb here is a dormant material and a minimal of 1.5" of clearance.


I'm glad you're continuing this thread, Daniel. My Discovery Dob is a sonotube and I really like it. One small thing I would add: the sonotube is gentler on the hand while nudging than the cold numbing metal tube ;) (not to mention the heat transfer you already talked about).

Ron B[ee]

#44 Guest_**DONOTDELETE**_*

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Posted 08 March 2005 - 09:04 PM

Dan-Awsome stuff! Thanks very much for sharing.

#45 NHRob

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Posted 08 March 2005 - 09:25 PM

What material are the spider vanes made from?
I once made circular vane spider from steel stove pipe.
I had to cut it up with a hacksaw. Nasty job!
It worked though.
Rob

#46 Patrick

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Posted 09 March 2005 - 11:18 AM

Daniel,

Given the concern for thermally induced tube currents, wouldn't it be better to go with a truss tube style Newtonian and avoid the problem altogether?

Patrick

#47 Chriske

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Posted 09 March 2005 - 12:17 PM

Silvio,

"...centering the optical path right in the middle of the tube, squaring the focuser, squaring the diagonal" , maybe not necessary, but it makes collimation very simple, just do it..!

#48 NHRob

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Posted 09 March 2005 - 12:32 PM

I had a 10" truss dob and I found that, while it avoided tube currents, I had problems of body heat causing thermal currents drigting across the optical path. This was in cold weather, of course ... which is a good percentage up here in NH.
Solution? Add a shroud ... which is nothing more than a cloth tube anyway.

Rob

#49 Daniel Mounsey

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Posted 09 March 2005 - 12:46 PM

Now the fan placement. This is the most controversial issue regarding planetary Newtonians. The way I determine which method works best is based on the views I see while comparing them to other methods. We've tried just about all of them and I will tell you how Pons and Grissom do theirs.

It has been suggested that elastic rubber bands will help filter out vibrations, however in my opinion, this also has much to do with the tube material and the speed of the fans themselves. During our tests, you can go without the elastic bands just as Pons and Grissom have done as illistrated in these pictures but there's a crucial element that makes a HUGE difference and I believe that this is where the main problems stem from. You should have a thermostat of some sort to regulate the speed. In some cases you can be pushing too much air or not enough.

You can also place a separate 8" fan behind a primary of 12.5" which is not attatched to the scope at all, however you will need a thermostat in either situaton. Remember, I'm talking about the tube first. There are differences in fan placement for truss's and tubes.

Many people have attempted different positions of fan placement either sucking down, forward or sideways but I will tell you what has proven to win in the field beyond the shred of a doubt. I've seen magnifications up to 600x using 12.5" scopes with Pons and Grissoms fan placements. You are better off getting Boxer fans which are brushless to minimize vibrations. In these designs there are three 4" fans placed directly behind the primary and attatched to the tube with a solid wood board. The side fan is another option but I have some issues and I'll give numerous reasons why depending on the situation, but first I'll explain what's going on with the rear fans pushing forward, which the best way to go in our opinion.

I remember waking up to test Grissom's fans and the differences were so amazing, I could see it right before my eyes. I pointed the 13.5" at Saturn without the rear fans on. I had asked one of my friends to turn the fans on. Within about five seconds, the image literally froze rock solid. I then told him to turn the fans off. Within seconds, the image started to soften.

We kept doing this over and over under the sub arc second skies of Mt. Wilson. At one point I stood in front of the tube while the fans were on and could actually feel the subtle breeze flowing against my face. These fans remain on at all times because these large scopes have nearly endless thermal drawbacks when compared to refractors.

I was so impressed with fans I became obsessed with them and tried different methods. Bryan Greer at Protostar has conducted numerous tests and is very well informed. I have had numerous conversations with him and I believe he's an amazing source of excellent information on these issues if you have questions.

Think about the tube for a moment and what's going on inside of it. Depending on how long it takes things to settle, some of these issues may play an issue, while some may not. But, a proper fan set up eliminates practically all the thermal gremlins no matter what time you observe. You can role these Newtonians out and get startling images right off the batt. The system as a whole does not have to depend so much on settling down. These fans work so efficiently, that there isn't any time for thermal gremlins to cause these problems.

There's heat exchange coming off the secondary, heat exchange at the top of the tube, there's a boundary layer in front and back of the primary, tube currents, etc. Think about light having to contend with all these issues before it even enters the eyepiece, it's no wonder people like their refractors better and I don't blame them. These combination of fans along with Grissoms curved spiders are unsurpassed in visual planetary performance. The boundary layer gets sucked off the face of the mirror and the diffraction spikes, the scintillation and thermal gremlins are completely flushed out of the picture like a vacuum. The remaining image is a steel ball that looks like a Voyager picture. You wouldn't even want to look through refractor in a side by side after you've seen through on of these scopes, it's truly amazing.

So what's up with the side fans? Side fans basically help to speed up the cooling process and break down the boundary layer to produce a more laminar effect. In Alan Adler's Sky&Tel review, he had a push/pull mirror system. This prevented him from being able to use a rear fan. The concept of the side fan was to help break the boundary layer and have it flow out the holes residing at the opposite side of the tube. This sounds cool, but it's not that simple. Both Pons and I tested this method in a tube and on a truss during observations. Even though some observers claim it helps during visual observations, our tests proved otherwise.

During a star test, you can rack the image out and see the boundary layer and watch how it behaves. But more importantly you can watch a planet with them on and off. Personally, I thought the images were not as impressive with them on. The air isn't just flowing out the other side. Those gremlins ricochet iside the tube. They don't just majically flow out the other side and the star test proves this. Nothing beats having no boundary layer at all. If you have a full thickness mirror, side fan can help speed up the process of cooling.

I know some observers will object to my opinion on this matter and that's OK. I'm not searching for the perfect image. I've already found it, so it's up to you. Thes pictures show Ed Grissom and Pons fan set up.

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#50 Daniel Mounsey

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Posted 09 March 2005 - 12:49 PM

another pic

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