184 replies to this topic

[Mike Lockwood]

A high F ratio is inherently better for high power views. A slow scope with excellent optics will give better views of planets than a fast scope with equivalent optics. This is a fact that owners of fast scopes need to accept.

No, it is not a fact, and it is quite wrong, so we will not accept it.  What Jon said above is correct.

 

The laws of optics and physics don't lie - they will give the same view, assuming similar quality eyepieces and equilibration and the the eyepiece is designed for the faster cone, not including some miniscule effects (which are almost always overblown) from a larger secondary in the faster scope.  This is easily overcome by adding a small amount of aperture if one wishes.  Then the slightly larger fast scope wins.

 

Let's not ban certain terms, (i.e. planet killer) let's educate people about where they came from and why they are misleading or wrong.

 

My planetary scope?  My 20" f/3.0.

[Galicapernistein]

No, it is not a fact, and it is quite wrong, so we will not accept it.  What Jon said above is correct.

 

The laws of optics and physics don't lie - they will give the same view, assuming similar quality eyepieces and equilibration and the the eyepiece is designed for the faster cone, not including some miniscule effects (which are almost always overblown) from a larger secondary in the faster scope.  This is easily overcome by adding a small amount of aperture if one wishes.  Then the slightly larger fast scope wins.

 

Let's not ban certain terms, (i.e. planet killer) let's educate people about where they came from and why they are misleading or wrong.

 

My planetary scope?  My 20" f/3.0.

 

So someone starting out in astronomy who wants to see Saturn’s rings should buy a 6 inch F5 because they’re so much more convenient than an F8? I don’t think so.

Edited by Galicapernistein, 21 January 2020 - 11:53 AM.

[Jon Isaacs]

The point of calling slow newts planet killers is that they give killer views of planets. A high F ratio is inherently better for high power views. A slow scope with excellent optics will give better views of planets than a fast scope with equivalent optics. This is a fact that owners of fast scopes need to accept.

Well...

 

The resolution and contrast are strong functions of aperture, weak functions of focal ratio.  What this means is for a given focal length (and OTA length) a well fast scope will be a better planetary scope than a well made slow scope, it has the aperture advantage.

 

In the old days, when scopes were relatively small, my 12.5 inch F/6 Meade RG was a monster scope when it was manufacturers, and the skills to make fast mirrors were lacking, slow scopes were considered planet killers.

 

That was 40 years ago.  

 

Today, a 12 inch is a medium size scope and people are using much larger scopes for viewing the planets.  I have to think that Jeff Morgan's 16 inch F/7 was a slightly better planetary scope than my 16 inch F/4.4, smaller secondary, that sort of thing.

 

The problem is that 112 inch focal length.  That takes a serious ladder.  Jeff wrestled with that for a number of years. He worked on the low rider concept and a periscope but I believe I recently saw the mirror for sale on Astromart.  my 16 inch is still alive and going strong.. No ladder needed.  

 

So in terms of a planet killer, the practical aspects have to be considered. It is about the view and the viewing experience.  A 12 inch F/6 has a focal length of 72 inches, a 16 inch F/4.5 has a focal length of 72 inches. Both will be flat foot scopes for a 6 footer.  Certainly the 16 inch F/4.5 has more potential and if properly made can realize that potential.  

 

So what fans of slower scopes have to realize that it's different world today that it was 40 years ago.  Mirrors are thinner,  better materials, quartz instead of pyrex, properly supported and thermally equilibrated.  They cool quickly, they use interferometers to test and figure the mirrors. 

 

This is a comment made by Roland Christen:

 

"...How about a custom 17" Cassegrain with precision quartz optics, small secondary, 100% US made? Optics by Lockwood and AP.  We made a large Cassegrain with Lockwood mirror, which I showed at NEAF this spring.  The mirror has the smoothest, most accurate curve on it that I have ever seen.  The scope is light enough to be placed on a mount by one person."

 

-Roland Christen, Astrophysics, 16" f/3.5 lightweighted quartz primary mirror by LCO, other optics and telescope by Astrophysics"

 

When Roland says the mirror has the smoothest, most accurate curve he has ever seen, not much can top that. 

 

Planetary is about aperture, seeing, optical quality and care.  It's not about focal ratio.

 

Jon Isaacs

[Jon Isaacs]

So someone starting out in astronomy who wants to see Saturn’s rings should buy a 6 inch F5 because they’re so much more convenient than an F8? I don’t think so.

Think outside the box.. 

 

A 6 inch F/8 is 48 inches long, an 8 inch F/6 is 48 inches long, a 10 inch F/5 is 50 inches long.  They're the standard dob configurations.. 

 

I can tell you which one provides the better planetary views... 

 

Jon

 

P.S.:  A 6 inch Newtonian is not what I consider a planet killer.  A good scope but not enough aperture.

[Galicapernistein]

Think outside the box.. 

 

A 6 inch F/8 is 48 inches long, an 8 inch F/6 is 48 inches long, a 10 inch F/5 is 50 inches long.  They're the standard dob configurations.. 

 

I can tell you which one provides the better planetary views... 

 

Jon

 

P.S.:  A 6 inch Newtonian is not what I consider a planet killer.  A good scope but not enough aperture.

A slow newt will give better high power views than a fast newt with equivalent optics. It would be nice if we could accept that fact without bringing in these other factors.

Edited by Galicapernistein, 21 January 2020 - 12:31 PM.

[Jon Isaacs]

A slow scope will give better high power views than a fast scope with equivalent optics. It would be nice if we could accept that fact without bringing in these other factors.

 

I accept that a slow scope with equivalent optics will provide slightly better views than a fast scope.  

 

But it would be good if you would accept that those other factors are far more important in providing killer planetary views than the focal ratio.  

 

Today, a slow scope that provides the planetary views possible with a large aperture, fast scope is impractical.  

 

It is about the views.. 

 

Jon

[spereira]

OK, folks, this yes-it-is, no-it-isn't disagreement is becoming tedious. 

I suggest you agree to disagree on this subject before this topic goes off track.

 

smp

[Galicapernistein]

OK, folks, this yes-it-is, no-it-isn't disagreement is becoming tedious. 

I suggest you agree to disagree on this subject before this topic goes off track.

 

smp

Agreed.

[ltha]

I agree with John that the term “Planet killer” is anything but meaningless, but if you prefer: optimized for planetary viewing. I have read many glowing accounts about fast Newtonians providing stunning planetary views. Over the years I have owned many “slow” Newtonians that in side-by-side comps ran right with top APOs (though not aperture to aperture). But my experience with fast Newts has been more mixed. I have a beautiful 18” Starmaster f4.3 that provides wonderful planetary images, but from my observing notes:

 

“Side-by-side I found the Takahashi FC-125 every bit as sharp as the larger FS-152 on Saturn and the moon, and the contrast far better than the large Newt (strehl 96.4). I was looking at the lunar terminator through the Starmaster at a small crescent section of mountains extending just beyond the terminator, it appeared to be floating in space. When I looked through the FC-125 I could see the base of the mountains extending onto the lunar surface, so I went back to the Starmaster but simply could not pull out that detail. Chocked it up to superior contrast in the FC. Between the FC and FS it was a push in every way. I was very impressed!”

 

Unfortunately, the night I had the scopes out I had not yet assembled the Cave 12.75” f/5.8. Since then it has beaten everything in planetary viewing, including my TEC200ED. 
 

We literally just put the TEC on the pier Joe Nastasi built for me. My friend Larry Myers tightening things up:

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[Ihtegla Sar]

It seems like the term "planet killer" may not mean the same thing today as it did 40 years ago, which is leading to a lot of confusion and arguments about what it means.  

 

40 years ago a "planet killer" by default would have been a slow telescope that was properly configured and collimated (easier to do with a slow telescope) and properly cooled (easier to do with a small telescope given the mirror thicknesses that predominated 40 years ago).  So your typical "planet killer" 40 years ago would have been a slower and smaller (by today's standards) telescope, since the large, fast, thin, well configured mirrors and the mirror cells needed to support them didn't exist in the amateur market 40 years ago. 

 

Today the term "planet killer" also includes large, fast reflectors (and expensive) telescopes that are well figure, polished, collimated and cooled.  And this new breed of "planet killers" might be even better "planet killers" than the "planet killers" of 40 years ago due to increased aperture.

 

But 40 years ago, a "planet killer" might not have been the best at galaxies, nebula, globular clusters, etc. because of narrow field and/or small aperture.  In contrast, today's 20 inch f/3 "planet killer," would be a "killer" of everything.  Not just planets and tight double stars, but nebula, galaxies, globular clusters . . . basically anything and everything in the night sky that you point it at, limited only by the field of view, which is fairly ample given the fast focal ratio.

 

So, while the term "planet killer" is not "meaningless" it does seem overly broad and confusing, given the historical context as modified by the modern technological advances.  As a result, the term "planet killer" seems more like an outdated colloquialism than a term of art.

 

Also, applying the term "killer" to any telescope seems somewhat silly to me.  Telescopes don't "kill" anything (except time and money). So the term "planet killer" seems like incongruent chest beating in a hobby of people who stare passively at the night sky.  But each to their own.  

 

 . . .

 

Let's not ban certain terms, (i.e. planet killer) let's educate people about where they came from and why they are misleading or wrong.

 

 . . .

Well put.  Even if the term "planet killer" is confusing and somewhat silly (my own opinion) there is no reason to start language policing everyone and banning words.  Education is the way to go!

Edited by Ihtegla Sar, 21 January 2020 - 03:58 PM.

[Galicapernistein]

“Slow Newtonian” is probably a more accurate and less threatening name than Planet Killer.

[nirvanix]

I've gotten many extraordinary views of the moon/planets with an optically excellent 10" f/5 dob. The whole idea of some longish newt or hyper-expensive APO as the only planets killers has one foot in mythology and the other in the grave.

 

Largish aperture of high quality is your best bet.

[Asbytec]

 

A planet killed can be a small scope that generally makes the best of a bad situation or it can be a larger scope that can take advantage of a very good situation.. 

I think this is an interesting point. Some of the best views of planets I have seen were through a modest 6" slow aperture performing to it's potential under very good seeing (which is aperture dependent favoring smaller apertures). In other words, the modest aperture was perfect for the conditions, so planetary images were as sharp as they could be and a wealth of detail available was seen. To see the planet etched in the sky should mean the scope was killing it. However, it was still a modest aperture and lacking some resolution, contrast, and color of a larger one.

 

My slightly larger 8" faster aperture, operating under slightly less favorable seeing but still good enough most of the time, is still pushing some boundaries taking planet killing a step above the smaller 6" aperture. I can push higher magnification in the larger aperture at the same productive 0.6mm exit pupil and even squeeze a little more out of it at a slightly smaller 0.5mm exit pupil because seeing permits it. Even if seeing does not permit, I'll wait for those moments it does. Granted, I am probably beyond the magnification required to see all the resolution provided by the aperture, but I assert an ideally large and bright enough image can show smaller bright low contrast extended features to the point the image becomes too dim to see well. 

Edited by Asbytec, 21 January 2020 - 07:11 PM.

[CHASLX200]

So someone starting out in astronomy who wants to see Saturn’s rings should buy a 6 inch F5 because they’re so much more convenient than an F8? I don’t think so.

If anyone knows it is Mike my man.  I can also tell ya i had my best views of the planets with mirrors F/5.6 to F/4.3. This was with Starmaster 11 to 18" with Zambuto optics and two OMI mirrors of 12.5 and 15". I have owned around 260 scopes and have had many slow Newts as well as faster Newts.  Colliamtion is what gets people with fast scopes and not using a Paracorr. 

 

 

[CHASLX200]

I’m talking about the name “planet killer.” Hasn’t that always meant a slow Newtonian? Just like a rich field scope is a fast Newtonian, even though other scopes can obviously provide rich field views. A scope can give great planetary views and still not be called a planet killer. Maybe the term itself should be killed, and we should specify if we’re talking about slow Newtonians, or just any scope with good optics.

Planet killer can be any scope from a Mak, APO, Achro or Newt.  I have owned all and some of each gave some crazy views of the planets. But still my faster Newts F/5.6 and faster gave my best all time views. Once you get into the 14.5" size and have dead still seeing and a top notch mirror the view can be a heat stopper.

 

 

Most people think the old school long and slow Newts are planet killers and they can be.  Before i owned my first Starmaster in 2000 i also did not think fast Newts were worth a hoot for planets as every fast Newt i have owned in the 70's and 1980's gave some sorry, mushy and coma filled views. This was before the Paracorr and good collimation tools. I am sure the mirrors were also not that great.

 

It took some arm twisting but i did order a Starmaster 12.5" F/5 Dob with a Paracorr in 2000.  Boy how wrong i was.  First look at Jupiter at 500x was crazy. 

Edited by CHASLX200, 21 January 2020 - 07:25 PM.

[CHASLX200]

Slightly off topic, but is there a certain time of year when the atmosphere is the most stable in Florida?

My best all time view was a very warm Feb nite in 2001 when what i call the seeing went away.  It was dead still and i used 1150x with Zambuto 14.5" Starmaster.  I maxed out the power with the barlow and eyepiece combo and could have gone higher if i had more power to give it.

 

Feb seems to be the best for seeing when it is a very warm nite and sea fog is about to move onshore.  On cold nites with big temp drops just after a front you just as well stick to sweeping at low power. Summer also seems to have many nites with not so great seeing as planets look they are in fast moving water.  I am right on the gulf. So warm winter nites are best for me.

[MitchAlsup]

Mike L felt very strongly when he sold me my 14.7" thin quartz (20mm) mirror that it would benefit from/require the whiffletree support. I had to twist Nate at Aurora's arm a bit but he built me a nice custom cell for it.

Mike L also convinced me to use a whiffletree edge support for my 20" F/3 1.3" thick.

The Cruxis edge support calculator indicated 2 points at 45 degrees was losing just a smidgeon of strehl.

[a__l]

Not going to happen. I do this hobby for fun, and that sounds like work.

I do not think this is work. This can be done in 10 minutes with a smartphone and Internet access. Mathematical video processing can be done by anyone.
This would be an objective the confirmation to words about the quality of the "destroyers" telescopes (optics, design, collimation, cooling).
So far, it looks like a fear of investment protection. Suddenly something goes wrong. Perhaps other reasons.

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Edited by a__l, 21 January 2020 - 09:57 PM.

[turtle86]

I do not think this is work. This can be done in 10 minutes with a smartphone and Internet access. Mathematical video processing can be done by anyone.
This would be an objective the confirmation to words about the quality of the "destroyers" telescopes (optics, design, collimation, cooling).
So far, it looks like a fear of investment protection. Suddenly something goes wrong. Perhaps other reasons.

Well, I think it's work, so we'll have to agree to disagree on this.

[Mike Lockwood]

Mike L also convinced me to use a whiffletree edge support for my 20" F/3 1.3" thick.

The Cruxis edge support calculator indicated 2 points at 45 degrees was losing just a smidgeon of strehl.

I really liked how well the retrofit whiffletrees worked on my own 20" f/3.0.  It's described in an "In the Shop" installment.

 

If you're making a cell with 2-point edge support, it's not that much more difficult to just add whiffletrees and reduce the pressure at each support point.  This is why I like using them on thin mirrors - though there is less weight, they are easier to distort with point loads.

[Magnetic Field]

I accept that a slow scope with equivalent optics will provide slightly better views than a fast scope.  

 

Why?

 

It cannot be the reason that a faster scope is more prone to collimation issues.

 

I fail to understand why people think a slow scope is better for planets.

 

Edit:

 

A 20" f/5 vs 20" f/10 with the same optical error and flaws interferometrically measured at focal point. So, why should I use the 20" f/10 for observing the planets? For the same magnification is it the choice of eyepiece which would favour the f/10?

Edited by Magnetic Field, 22 January 2020 - 02:33 PM.

[ltha]

I think it depends on what you mean by “better". My favorite planetary scopes are refractors and Newtonians.  During the Mars close approach in 2003 I was out regularly. The absolute best view I had of Mars was through my 10” f/5 Portaball. Detail was absolutely etched. A neighbor happened to be out and took a look after which she said, “Wow! That looks like the cover of Time magazine!” The scope was well collimated, cooled and out on one of the best nights of seeing we had. Simply nailed it!

 

A few nights later I set up my D&G 8" F/12 achromat and the same 10" PortaBall for a casual evening showing Mars to friends.  I let both scopes cool down for nearly an hour and a half before doing any observing. Checked collimation on the 10” PB and it was great. Right from the gate I preferred the image in the big refractor. There was some false color, in an 8” at F/12 it is something to contend with, but the image detail was a notch above the Portaball. The image was more “solid" in the D&G, probably due to tube currents in the PB. The image of Mars when unfiltered demonstrated enough more readily observable planetary detail to give the D&G the edge. And everyone there saw and commented on the difference. Once I added the #21 orange filter, the advantage was even more pronounced. Part of the edge is no doubt due to the greater depth of focus enjoyed by the long focus refractor. Focus is a far more critical issue in the F/5 reflector and even though Pete's focuser is a great unit, it was harder to hit sharp focus and stay there. Much more image focus wander in the PB where the D&G held firm images throughout. And what images! Syrtis major and minor were stunning and the nearly dot-like polar cap sharply presented. Plus the longer focal length allows the use of more "relaxed" eyepieces: I was running a 14mm Pentax XL in the D&G and a 8mm Clave in the PB. The Moon was also present and gave the opportunity to hit powers that were silly under the conditions: 5mm TMB mono in the D&G for roughly 480x. Surprisingly nice image! The PB did not show any more detail but the lack of color gave it the more aesthetically satisfying image. Power was limited in the PB to 315x with the 4mm TMB. Another great image.

 

Around 4:00 AM I had a chance to view Saturn through both scopes. When I was a boy, probably 10 or so, I attended a series of lectures on astronomy at Griffith Park Observatory. After one of the lectures I had a chance to view Saturn through the Zeiss 12" refractor and it was the most impressive view of the planet I have ever had. The memory of that view has stayed with me and after owning many excellent scopes it still remained unmatched. Until early this morning that is: I had the 8" set up with the new TMB Super Monocentrics in 10mm and 8mm focal lengths and that long cherished memory lived again. The detail was stunning even under less than perfect conditions. The TMBs and a 12mm Pentax SMC Ortho really let the lens loose and it just nailed Saturn. The PB was turning in its usual excellent image of Saturn as well but I kept going back to the D&G. The lack of diffraction spikes was a decided plus and Saturn is not a problem as far as spurious color. Plus, Saturn was pretty low and the PB, due to its short length, was quite close to the ground which made for less comfortable viewing.

 

My preferences come from side-by-side comparisons under identical conditions. In my experience, longer focal length gives you better "depth of focus”; it is easier to reach high power with more relaxed eyepieces; minor collimation errors are more easily tolerated (not a reason to skip obtaining the best collimation possible), and there is a smaller secondary obstruction. I have never used a Newtonian shorter than F/4 but even at f/4 the focus becomes more critical and to get high powers you need short focal length eyepieces and/or barlows, or the newer eyepieces that incorporate both. When I lived at the beach I had pretty good seeing most of the time, and killer seeing reasonably often. I would love to have tried a 20” f/3 on one of these nights. I totally agree that aperture is queen, but seeing is king.. Most of us live in areas where average seeing is the rule and no amount of “optical paper perfection” can change that. I own a 12.75” F/6 Newtonian with quartz optics that has killer specs and was figured by one of the best mirror makers in the US. On nights of best seeing it kills my TEC200ED, even at f/5.8. But on average nights it is another story entirely.

Edited by ltha, 23 January 2020 - 09:49 PM.

[25585]

http://apm-telescope...m/item/33353539

 

http://scopeviews.co...ptics200Dob.htm

[Jon Isaacs]

http://apm-telescope...m/item/33353539

 

http://scopeviews.co...ptics200Dob.htm

 

I would not buy a scope from Orion Optics UK.  A friend bought a 6 inch F/5 with a "1/10" wave mirror.  The spider was mounted in such a way that the secondary could not be properly positioned.  They did not respond to his emails.  It was sad.

 

Jon

[Magnetic Field]

I would not buy a scope from Orion Optics UK.  A friend bought a 6 inch F/5 with a "1/10" wave mirror.  The spider was mounted in such a way that the secondary could not be properly positioned.  They did not respond to his emails.  It was sad.

 

Jon

They have a very bad reputation.