407 replies to this topic

[Jon Isaacs]

 

 

Apos?  Why bother?  

 

Show up at a star party with an AP or Takahashi, especially of 5 inches or more, and people will line up to see M13 looking badly because the telescope is not big enough to show the stars in globulars well.

 

+1. Even inexperienced observers can tell the difference. When someone brings a frac to a local outreach event, as the evening progresses, the lines form at the newts, dobs, and sct's...

 

 

Star parties and outreach are an anomalies..  I do my "serious observing" alone.   Still the same principles apply, the right instrument for the job at hand.  A 4 inch refractor pointed at the Pleiades can get a long line, a 16 inch Dob pointed at the Pleiades, not so much.. A 16 inch Dob pointed at M13 can get a long line, a 4 inch refractor, not so much.

 

Discussions like this, whether one scope is better than another, it's like arguing whether a jewelers hammer is better than a sledge hammer, it's senseless.  Working on a watch, a sledge hammer is useless, breaking up a concrete slab, a jewelers hammer is worthless.  In my tools, I have both small hammers for the tiny jobs, big, heavy hammers for big jobs.. the right tool for the job..  

 

The same is true with when it comes to my astronomical tool set, expect that the roles are reversed, I have big telescopes for the tiny objects, small telescopes for large objects.. 

 

Jon Isaacs

[precaud]

 

+1. Even inexperienced observers can tell the difference. When someone brings a frac to a local outreach event, as the evening progresses, the lines form at the newts, dobs, and sct's...

 

Star parties and outreach are an anomalies..  I do my "serious observing" alone.

 

Anomalies? Maybe your "serious observing" is an anomaly. Who knows.

 

Besides, so what? That comparison doesn't negate the legitimate data point I brought to the conversation. Compared to reflectors, fracs are special-purpose instruments that have, within the whole scheme of observing, a more limited subset of usefulness than reflectors. There is hardly a time where I take out a frac and don't wish that I had more aperture at hand. (Last night, for example). But there is rarely a time when I set up a reflector and wish it were a frac.

 

For someone like the OP who is evaluating the worth of a decision between the two, these are user data points worth considering, IMO. You may feel otherwise.

[Jeff B]

Actually, IMO, for amateur owned stuff,  the practical upper limit in size for a triplet APO is ~10" and 14" for a fluorite doublet, regardless of cost.   10" triplet APOs are BIG chunks of glass and can have huge thermal issues due to airgaps and element flexure issues for the oiled ones.  APM has made much bigger airspaced ones but I'd like to see how well they work in real life.   

 

So to me, there's no question that beyond 10-12", big Newts can rule the day.  

 

But, there are those pesky off-axis reflector designs that have never really gained commercial traction but seem to keep hanging around in the amateur fringes.

[Eddgie]

A number of years back, I went into the Refractor Forum and got into some polite discussions and some real brawls (but nothing really nasty) about the pros and cons of APOs and Newtonian Reflectors.  I listened very carefully and discussed these and really got beat up when I said that a Newtonian could be built that would equal the performance of an Apochromatic telescope of equal aperture with regard to that fundemental building block of any image, a star's diffraction pattern..

 

Oh, how the fur flew on that one!    After recording many statements from that forum's membership, I quietly stepped aside and started a project to put my money where my fat mouth was.  It took a long time as I became increasingly ill with RA, but I finished that Newt and wrote a scientific article outlining my reasoning.

 

The article was accepted and is now in print in Astronomy Technology Today, (www.astronomytechnologytoday.com) , and I am sorry to report that at this point, you've got to subscribe in order to get access to my work, but that's the deal these days.

 

In any case, please note that I have no intention to going to the Refractor Forum to listen to science fact, science falsehoods and other remarks about what I did.  I'm simply too crippled in my hands to answer.

 

My best advice for the APO owners was this.  If they disagreed with my findings, they should consider writing their own scientific paper to refute my claims, with footnotes and all (!), get it accepted and printed for all to see.

 

That is how science is done.  My article, while provocative, should be a prod for people to think about what they are saying, and what the manufacturers are claiming.  I've added this topic here for the poor benighted Newtonian owner who has to listen to the superiority of refractors in general.  All the time.

 

And with a little bit of luck, better instruments will follow.  Even by ATMs.  Even with Newtonians.

 

ed_turco

 

At small apertures (8" and below) it is difficult to argue with the benefit of refractors unless the system needs to be very fast, where it becomes hard to argue the fact that it is difficult to make refractors at speeds much faster than f/5 that can match a similar sized mirror in contrast performance.

 

Once the aperture becomes much larger than 6" though, there are a great many practical factors that outweigh any slight advantage an APO might have over a similar focal length reflector.

 

It simply does not matter if a 12" f/10 Apo would be slightly superior to a 12" f/6 reflector (which is going to take a very unique refractor by the way because of the aspherizing required to control the spherochromatism, and doubt that it could be done easily even at f/10) if no one can afford the telescope, the mount, and the observatory required to hold it.

 

It is a practical thing.  Professional astronomers figured that out 100 years ago.  Once you get over about 7", the gap narrows, and the practical aspects over-ride the tiny difference in performance that he refrator would enjoy.

At smaller apertures though, I give the nod to the Apo.  If you can handle a 6" Apo and a mount for it, it is a more capable instrument than a similar sized Newtonian can be.

[george golitzin]

 

Apos?  Why bother?

 

Imaging

 

 

Yes, for that application they make sense.  But I'm just a visual observer.  While I enjoy the work of astro-photographers and have great respect for what they do, I can't imagine putting in that kind of work on a hobby.

[Eddgie]

Just about all of the scientific images taken by professional astronomers within the last 100 years were done with reflectors.

 

It simply becomes to technically difficult and expensive to get a big refractor to achieve equal performance.

 

Once the aperture gets above 8", it becomes almost impossible to make a refractor than can achieve the same performance of a similar sized reflector when imaging without the system being required to work at maybe double the focal length of the reflector.

 

Again, on small instruments, the Apo is simply the best possible scope design because it is (relatively) cheap to make, and it is easy to control the amount  of CA, and at these sizes, spherochromatism is not an issue.

 

Above 8" though, and it gets very difficult.

 

Ask Roland Christen, who uses a 10" MCT for planets.   

 

And once again, this completely omits the practical aspects of the physcial implementation of a very large imaging refractor.   A suitable mount for a 10" Apo would cost more than the most customized and highly figured 12" mirror ever made.

Edited by Eddgie, 29 April 2015 - 12:17 PM.

[Eddgie]

And these arguments completely ignore the fact that people don't want to spend a lot of money on reflectors.  

 

If you were dedicated to the cause and hired someone to make a perfect optical figure on a 12" reflector, and put the most perfect coatings possible on it, you could match the performance of a 12" Apo.

 

But who wants to spend$10,000 on a 12" reflector?

 

And that is where these arguments always break down.   Once the system gets large and fast, the reflector reigns supreme, and if someone wanted to match the performance of the similar sized Apo working at an optimal focal ratio (maybe f/6 for the reflector vs the f/12 that would be required for a 12" Apo to match the contrast, and it becomes clear that a big Apo simply is not worth doing.    Again, professional observatories learned that long ago.

[Eddgie]

And with the advent of the ED glass triplet, I have not seen any professional observatories commission the building of any big Apos.

 

At small apertures, the ED Apo triplet is the unquestionable best performer for most people for most applications.

At  7" and larger, it really doesn't matter because most of us would not want to spend the money. Some do, and Astro-Physics meets their needs in about 10 years...

 

But most builders won't match Astro-Physics by aspherizing the lenses of their scopes, and even at 10", the spherochormatism will limit the performance that would not really be any better than a similarly sized reflector that had been built at the same investment level and focal ratio.

 

Who would want to spend $5000 on a 10" reflector though, to eek out that last bit of performance?

 

And make no mistake.  A 10" refractor requires a level of investment that simply makes the entire argument kind of silly.  It requires an observatory...    I can keep a 12" Newt on my back porch.

 

 

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[nirvanix]

If you want to stay at 7" or less, why not opt for one of those newts that have the secondary mounted to the side so there's no obstruction. That would theoretically give you better than apo quality. Orion sold one a few years back that was 3.6", and there was another maker that made them up to 7". They're more expensive, but <<< $$$ than APOs. I wonder if anyone still produces them?

[Jon Isaacs]

If you want to stay at 7" or less, why not opt for one of those newts that have the secondary mounted to the side so there's no obstruction. That would theoretically give you better than apo quality. Orion sold one a few years back that was 3.6", and there was another maker that made them up to 7". They're more expensive, but <<< $$$ than APOs. I wonder if anyone still produces them?

 

 

I am not sure why you would say "better than APO" quality, Off-Axis Newtonians have issues of their own. They are reflectors so they have reflective losses and thermal issues.  They are also limited to rather slow focal ratios since the primary is a section.  An F/11 OA Newtonian does not have the large diffraction free region of a standard Newtonian because it is cut from an mirror about F/4.  It's better than F/4, Dan McShane of DGM who made them (and whom Orion ripped off but that's another story) said the coma was about like that of an F/6 Newtonian.  It always seemed to me if you were going to build and F/11 Newtonian, just build it, don't bother with the OA part.. 

 

Jon

[Jon Isaacs]

 

 

+1. Even inexperienced observers can tell the difference. When someone brings a frac to a local outreach event, as the evening progresses, the lines form at the newts, dobs, and sct's...

 

Star parties and outreach are an anomalies..  I do my "serious observing" alone.

 

Anomalies? Maybe your "serious observing" is an anomaly. Who knows.

 

Besides, so what? That comparison doesn't negate the legitimate data point I brought to the conversation. Compared to reflectors, fracs are special-purpose instruments that have, within the whole scheme of observing, a more limited subset of usefulness than reflectors. There is hardly a time where I take out a frac and don't wish that I had more aperture at hand. (Last night, for example). But there is rarely a time when I set up a reflector and wish it were a frac.

 

For someone like the OP who is evaluating the worth of a decision between the two, these are user data points worth considering, IMO. You may feel otherwise.

 

 

My point was that all telescopes, refractors, reflectors, big and small are special purpose instruments.. Big hammers and small hammers. Apparently you believe that big hammers are more useful than small hammers. I like having both.

 

When I out where the skies are dark and clear and I am viewing through a refractor, I am not wishing I had setup a larger aperture instrument.  When I am out where the skies are dark and clear and I am viewing though a large reflector, I am not wishing I had setup a small refractor...

 

The reason is simple, I have setup a small refractor and a large reflector, telescope choice is not THIS or THAT dilemma, it's a THIS and THAT win-win solution...  

 

Jon

[Talsian]

If you want to stay at 7" or less, why not opt for one of those newts that have the secondary mounted to the side so there's no obstruction. That would theoretically give you better than apo quality. Orion sold one a few years back that was 3.6", and there was another maker that made them up to 7". They're more expensive, but <<< $$$ than APOs. I wonder if anyone still produces them?

 

That would no longer be a newtonian telescope.

[Jon Isaacs]

 

If you want to stay at 7" or less, why not opt for one of those newts that have the secondary mounted to the side so there's no obstruction. That would theoretically give you better than apo quality. Orion sold one a few years back that was 3.6", and there was another maker that made them up to 7". They're more expensive, but <<< $$$ than APOs. I wonder if anyone still produces them?

 

That would no longer be a newtonian telescope.

 

 

It's an off-axis Newtonian.. Essentially what happens when you use an aperture mask with a standard Newtonian to eliminate the central obstruction and the spider vanes.  The difference is that the mirror is cored and so one large mirror provides 4 smaller mirrors and that is put in a tube with the secondary that is no longer blocking the light path of the primary.

 

DGM Optics: What is an Off-Axis Newtonian

 

Jon

[macdonjh]

 

 

Star parties and outreach are an anomalies..  I do my "serious observing" alone.   Still the same principles apply, the right instrument for the job at hand.  A 4 inch refractor pointed at the Pleiades can get a long line, a 16 inch Dob pointed at the Pleiades, not so much.. A 16 inch Dob pointed at M13 can get a long line, a 4 inch refractor, not so much.

 

Discussions like this, whether one scope is better than another, it's like arguing whether a jewelers hammer is better than a sledge hammer, it's senseless.  Working on a watch, a sledge hammer is useless, breaking up a concrete slab, a jewelers hammer is worthless.  In my tools, I have both small hammers for the tiny jobs, big, heavy hammers for big jobs.. the right tool for the job..  

 

The same is true with when it comes to my astronomical tool set, expect that the roles are reversed, I have big telescopes for the tiny objects, small telescopes for large objects.. 

 

Jon Isaacs

 

 

Boy, Jon, it's sensible talk like this that has ruined many an irrational argument on many a forum.  I think questions like this are rooted in the idea that it's possible to be satisfied with one scope, if you pick the right scope.  I also think for many amateurs it is possible to be satisfied with one scope.  The popularity of SCTs is a bit of evidence for that.  I don't have one scope and I know you don't, either.  But then I consider myself to be more of an enthusiast than a hobbyist: I spend as many clear nights outside as my family obligations will allow rather than remembering to use the scope I have in the closet a couple of times a year.  From the posts of yours I've read, you're even more dedicated than I am.

 

Another aspect of this that I've noticed is even for those that are satisfied with one scope, taste may change over time.  A long time SCT user may try a fast Newtonian and become a convert for a while, or forever.  That hypothetical observer may now be satisfied with his new Newtonian and sell his SCT.  Or he may turn into a collector and keep both.  My first scope was a Mak and I thought I'd be a Mak observer forever.  Then I added a 6" refractor and I thought I'd be a refractor guy forever.  Then I found out what is involved in the care and feeding of an 11" refractor, so I became an SCT guy.  Then I found an f/4 Newtonian.  Then...  Then...

 

Oh, I almost forgot, given the timescales astronomers deal with, a jeweler's hammer is a perfectly acceptable tool for breaking up a concrete slab...

[dag55]

Oh, I almost forgot, given the timescales astronomers deal with, a jeweler's hammer is a perfectly acceptable tool for breaking up a concrete slab...

[Thomas Karpf]

 

Apos?  Why bother?

 

Imaging

 

Specifically, low power imaging.

[Niklo]

Hi,

in my experience it hard to beat an APO up to 5" with a newton that size.

Comparing my 113/900 (Vixen GP-R114E) with 0,885 Strehl has much less contrast on Jupiter than a Vixen FL102S APO. So visually I see more details and better colours. If the seeing is not good I see gray bands on Jupiter in the 113/900 newton and in the APO I already see brown bands at the same time.

Taking videos and making photos by stacking, sharpening ... Playing with the camera and with the software I can let the contrast rise and show things that were visually below the contrast border for my eyes so I can compensate the lack of contrast.

 

I have a 6" f/8 newton with 31 mm secondary. The mirror has 0,95 Strehl which is pretty good in my eyes. I bought it to avoid buying a 5" APO. I think from the contrast it's in the range of a 120 mm or 5" APO. What I somehow dislike is that it takes long time to be on surrounding temperature. During that time my Vixen 80L shows much better pictures. OK to avoid that I bringt the Newton 1,5 or 2 hours out before I start watching. Every time I had my 102/920 Vixen APO and the 6" f/8 Newon out I saw the same details in the APO and in the newton. The festoons and the Jupiter moons looked better in the APO and one small band showed a little bit more contrast in the newton. The viewing esthetics was always better in the APO.

For the Jupiter moons I often saw a ring like a diffraction ring around them in the newton and in the APO I don't. Jupiter show four beams coming from the spider that holds the secondary in the newton. OK I could use a curved spider but then the spider arms need to be thicker with more obstruction and some told me that the near area around bright objects get's brighter too. Up to now the larger 6" haven't visually showed me more than my 4" APO so for visually watching Jupiter I prefer the smaller APO as the esthetics of viewing is nicer. For taking planet videos the 6" newton shows it's real power. The beams, spikes are not visible but you can see the larger resolution there. Probably with better seeing conditions the 6" would show more than the smaller APO but to bring it up to a good 6" APO view I cannot believe.

For me that isn't important. I like watching with both my refractors and my newtons

If you want to have an APO > 5" or 6" you pay too much in my eyes. In this case I would prefer a reflecting system with maybe 1 " or 2" larger. For home use a 8" APO would be too heavy and much too expensive but a 10" newton is normally no problem.

So I needn't have an 6" newton that shows 6" APO views but of course it's interesting what you did to tune your newtons to get the most performance out of your newton to watch planets?

Clear sky,

  Roland

Edited by Niklo, 30 April 2015 - 04:17 AM.

[Astrojensen]

 

I think questions like this are rooted in the idea that it's possible to be satisfied with one scope, if you pick the right scope.

It is eminently possible to be happy with just one scope (the alternative, to have NO scope, is certainly far worse!), as long as you accept that it can't do everything well and concentrate on what it does well, take the things it can barely do, as a challenge, and accept that there might be things it can't do at all.

 

It's when people can't accept that there's no scope that can do it all, that their problems really begin.   

 

 

Clear skies!

Thomas, Denmark

[Pinbout]

 

It is eminently possible to be happy with just one scope

 

a 6inf5 beats all for just one scope.

 

this is one I won't get rid of.

 

[Daniel Mounsey]

Are apos better than Newts?

 

No.

 

If I want a killer view of a planet, it's going to be in a newt of 12.5 inches or larger.  Pretty hard to pony up the cash for that size apo.

 

If I want detail on DSOs, it's going to be in a newt of 16 inches or larger.  The larger the better.  They don't make apos that big.

 

What's left?  Pretty wide-field views of deep sky?  A 6-inch f/5 newt with coma corrector does the trick.

 

So why would I care to compare apos and newts of equal aperture?  An academic interest?  The only pertinent question is, what instrument are you going to use for this or that application?

 

Apos?  Why bother?  

 

I love large reflectors for particular targets. I'm also heavily into doubles and multiples, so how would your post apply to that category? I could use an aperture mask, but that still does not negate the procedures neccesary to produce exsquisite airy disks from a thermal aspect when one comes home from work and these timely procedures still have to be followed. What about portability? What about not having to collimate? What about imaging (not that I care about it). There are certain aspects of telescope procedures, relative to particular targets not addressed. 

[aatt]

All I know is that on a side to side contest a month ago, my 15" reflector smoked a very expensive Tak 5" on Jupiter.No one should, perhaps, be surprised by that, but never-the-less there was no contest between the superior detail via increased resolution of a large reflector with a good mirror. For visual astronomy newts trounce  refractors, financially and weight-wise, with the lighter and cheaper extra aperture/resolution.

[precaud]

OK, just to play "devil's advocate for a moment:

 

 

Are apos better than Newts?

 

No.

 

If I want a killer view of a planet, it's going to be in a newt of 12.5 inches or larger.  Pretty hard to pony up the cash for that size apo.

 

If I want detail on DSOs, it's going to be in a newt of 16 inches or larger.  The larger the better.  They don't make apos that big.

 

What's left?  Pretty wide-field views of deep sky?  A 6-inch f/5 newt with coma corrector does the trick.

 

So why would I care to compare apos and newts of equal aperture?  An academic interest?  The only pertinent question is, what instrument are you going to use for this or that application?

 

Apos?  Why bother?  

 

I love large reflectors for particular targets. I'm also heavily into doubles and multiples, so how would your post apply to that category?

 

I have a much greater selection of observable doubles/multiples with my dobs than my fracs.

 

I could use an aperture mask, but that still does not negate the procedures neccesary to produce exsquisite airy disks from a thermal aspect when one comes home from work and these timely procedures still have to be followed.

 

Is this an exaggeration? I thought SoCal has a mild temp gradient from sunset to darkness, certainly more gentle than ours in the high SW desert.

 

How does observing "exsquisite airy disks" fit in to the normal regimen of visual astronomy? Is it a normal activity, or a troubleshooting / QA procedure?

 

What about portability?

 

I'll take my 8" dob any day.

 

What about not having to collimate?

 

A minute or less? No big deal. Compared to a GEM setup, its a snap.

 

What about imaging (not that I care about it).

 

That is probably the exception.

 

Not mentioned is ergonomics, which is heavily in favor of the dob-mounted newt.

[Jon Isaacs]

 I have a much greater selection of observable doubles/multiples with my dobs than my fracs.

 

 

Only if your scope is properly cooled and the seeing permits. What is the closest double you have split with your 16 inch? How often is the seeing sufficient to allow a larger scope to out perform a 6 inch on doubles?

 

Daniel was pointing to the hassle required to get a larger scope ready for observation.. That's certainly one reason I enjoy refractors. Splitting 0.5 arcsecond doubles is one reason why I like larger Newtonian.

 

Jon

[Thomas Karpf]

 

What about not having to collimate?

 

 

What about it? 

 

Collimating my 12.5" Meade Starfinder takes me perhaps a minute with a laser.  I typically set up during dusk so I have at least tens of minutes to spare.  As my scope is going to be sitting a while waiting for the mirror to acclimate, who cares about the time? 

Total cost: $75 for the laser.

 

How much time do you spend adjusting the altitude setting of your equatorial mount if your drive to observe takes you north or south any distance?

 

How much time do you spend polar aligning?

 

Is my dob perfect?  No, of course not; no scope is.

Does it have better resolution than the best 4-10" APO on the planet?  Absolutely.

[GShaffer]

Hi,
in my experience it hard to beat an APO up to 5" with a newton that size.
Comparing my 113/900 (Vixen GP-R114E) with 0,885 Strehl has much less contrast on Jupiter than a Vixen FL102S APO. So visually I see more details and better colours. If the seeing is not good I see gray bands on Jupiter in the 113/900 newton and in the APO I already see brown bands at the same time.

(clipped)

Clear sky,
  Roland

I want to see the 6" newt that will best my FL102S on Jupiter......