117 replies to this topic

[claytonjandl11]

A while back I had talked to Al Nagler and asked a question concerning Refrators VS Reflectors. I asked him if one had an 11" premium Reflector with a 20% obstruction would it equal a 9" premium Refractor in visual observing. His answer and I quote:" the 9" refractor would blow away the 11" Reflector" My question then is at what point then does a Relector with a 20% obstruction or less would it equal a Refractor, in other words what size Reflector equals a Refractor? Assuming both are premium scopes.

Thanks for reading,
Nick T.

[snorkler]

Nick,

I saw the same post in the refractor forum, and it's apples v. oranges to me, too. I've never had the chance to view through a 9" refractor, but suspect an 11" reflector would give it a run for its money, and a 12" would probably beat it (on bright objects). On dim objects, I'd bet on the reflectors right off. My closest comparable comparison was at Deep Impact on Comet Tempel 1 last year. My buddy and I were using his 12.5" Portaball, set up between a guy with a 10" Dob, and another guy with a 6" Astro Physics refractor. Both reflectors easily beat the refractor on seeing the Deep Impact result.

[121601]

A while back I had talked to Al Nagler and asked a question concerning Refrators VS Reflectors. I asked him if one had an 11" premium Reflector with a 20% obstruction would it equal a 9" premium Refractor in visual observing. His answer and I quote:" the 9" refractor would blow away the 11" Reflector" My question then is at what point then does a Relector with a 20% obstruction or less would it equal a Refractor, in other words what size Reflector equals a Refractor? Assuming both are premium scopes.

Thanks for reading,
Nick T.

Nick,

I think this is alluding to the old question of which is better in terms of refractors vs. reflectors.

Aside from central obstruction issues, I think that what really makes APOs appealing is the degree of accuracy that it can be collimated at the factory relative to a Newtonian. A very recent article (D. Rickley, Sky and Telescope, 111, #6, pp. 92-95(2006)) clearly illuminates this topic regarding collimation. Although this was a essentially a mathematical modeling, I thought the paper was one of the few readable articles which unambigously highlight the role of precision focusers and perfect collimation. The data presented are not only semi-quantitative but also graphical. The stringent requirements for attaining planetary detail and resolution were graphically illustrated on modeling experiments using Saturn as a model.

As such, the article also reveal that even the use of "precision" laser collimators for fast (f/5, etc.) Newtonians is inadequate for achieving the level of precision required to reach optimal optical resolution. The author describes that the margin of allowable error for the return beam is smaller (!) than the actual diameter of the point sources found in typical laser collimators. If I read it correctly, the author also concludes that collimation and focuser alignment outweigh the foctors of aperture and
obstruction for achieving highest possible degree of image integrity. Vic Maris of Stellarvue has often emphasized this point very well.

Renato

PS There is the first time I've seen 3 posts from a small area in the East Bay Area. I hope this is not the reason you're selling your Starmaster 20". I posted this note on the Stellarvue refractors forum and even there the general consensus was that aperture rules.

[Starkler]

I look at it this way....

Take the money for the cost of any given refractor and spend it on a newtonian using premium components, then collimate it well, and there is no comparison !

P.S My ed80 just got replaced by a 5" newtonian as my grab and go scope.

[Bob W6PU]

Go to the top of this forum"The Best Of Reflectors, and scroll seven articles down on "Reflectors versus Refractors" Must Read...it says it all!

I found it to be absolutely fascinating!

Cheers!
Bob

[claytonjandl11]

PS There is the first time I've seen 3 posts from a small area in the East Bay Area. I hope this is not the reason you're selling your Starmaster 20". I posted this note on the Stellarvue refractors forum and even there the general consensus was that aperture rules.

Hi Renato,
No that is not the reason I'm looking to sell my 20", I've had it for 2 years now, and it's given me great views. But alas I've been using my smaller starmaster, and my NP-127 for my observing they're much more easier to set up, especially when the seeing hasn't been that good, I've been able to get more use out of them.

Nick T.

[Cosmosphil]

Nick,
What would expect from a visionary who has staked his whole business on reaching for perfection with refractors to say?
Aperture rules?!!

[Mike Harvey]

I think I can speak with at least a little credibility on this subject, having owned numerous large refractors:
TeleVue 140
NP-127
One of only two Christen 6" f/15 folded Triplets
Takahashi FC-125
Takahashi FCT-150
8" Alvan Clark
and having used several other premium 8", 10" and 12" refractors.
As a recovering "refractor-holic" I still crave their look, fit and finish. And inch-for-inch they can't be beat.
But an 8" or larger refractor w/mount is a true BEAST to own unless you can leave it permanently mounted. Topping it off there's that little thing called "COST"!
Once we started having master mirror makers like Carl Zambuto et.al. turning out mirrors that were simply without compromise, optically - the paradigm shifted!
A premium 12.5" reflector today will simply blow away that beautiful old 8" Clark. My present 14.5" Ed Stevens mirror produces planetary images that are superior to anything I ever saw in a 10" Zeiss triplet and pretty darn close to the 12"! And don't even get me started on comparisons with my 28" Starstructure w/Steve Kennedy mirror! This scope is fully driven and features GOTO, yet I can set it up, take it down and transport it (easily) all by myself. Try to picture what a 20" refractor would look like and cost...that's about what it would take to begin to match the performance!
Would I love to have a giant TMB set up in a dome behind my house? Sure! But not for 'performance' reasons.

[Darren Drake]

One reason that refractors are percieved as being so much better than reflectors of equal aperture has little to do with the actual capabilities of each scope. Instead it has in large part to do with something rarely discused; thermal currents. Refractors light path's are a ONE WAY trip down a protected tube which isn't exposed to outside air currents. Newt's light paths have to go through an unprotected path TWICE which doubles the problems and also often mixes with the observers body heat.
I've done many comparisons with equal aperture refractors vs reflectors and the result always shows the newt to have much more turbulence.

[dave b]

one guy in our club just got a 6" APO, massive mount, palm computer, ect and with shipping it came to $18k. we were drooling all over it (he thought it was dew). for the same money he could have a GOTO 30" scope with premium optics.

best bang for the buck? nope, but it sure is sweet to look at!

[Mark Harry]

:" the 9" refractor would blow away the 11" Reflector"

I made a customer a 6" F/10 a couple of years ago, and he compared it to a 5" TV refractor. He thought the Newtonian edged out the TV on most counts. I believe he used a 3/4" secondary with it. The mirror's test indicated exceptional stats, and I believe if the above 9 to 11" comparison was similar, the above quote wouldn't be true. Mark

[Bill McHale]

Hmm, all do respect to Uncle Al, but he might be a bit biased in this regard; after all refractors are a big part of his market. I believe it is absolutely true that in small sizes (below 6") that a refractor has a huge advantage over a reflector, even if the reflector is significantly larger (I believe a TV-85 should easily beat a 4 1/4" Newtonian). In larger sizes the advantages are still there, but greatly lessened. My personal belief is that the biggest thing holding the performance of Newtonians back is the willingness of consumers to spend money on them. If we were willing to spend $3000 for a 6" Newtonian, most of the common Newtonian problems could be mitigated or eliminated and Newtonians would probably also be viewed as premium instruments. Alas though, most look at Newtonians as a source of relatively cheap aperature.

--
Bill

[Tom1175]

$3000 to build a highend 6" Reflector? Thats a ton of cash. Im willing to be that for $1500, you could build a 6" reflector that would blow away a 4" Refractor.

heck, I built my 8" for about $900, only has an old Meade F6 mirror in it, and I would put it up against a 6" Refractor in a heartbeat. With one of those top end mirrors from Royce or somewhere else, it really would put a hurting on the refractor.

[NHRob]

I agree. A good 4" apo goes for $2000-3000.
I could easily build an 8" newt, with primo optics, that would blow the apo away. Also, invest in the best focuser, secondary, nice tube,powder-coating, finder, rings, and fan-cooling system. I think this could be done for < $2000easily.

Rob

[Bill McHale]

Thats ultimately my point, pretty much all the Newtonian's shortcomings can be addressed if people were willing to pay anywhere close to as much for a Newtonian as they were for an Apo. Look at how well the MNTs perform. I am sure it has less to do with any inherent advantage in the design (because for on axis viewing they have none) than it does with the fact that certain limitations of the Newtonian are addressed by the existence of the Corrector. The Corrector seals the tube which limits tube currents. The scopes have small secondaries, and the better ones are even baffeled. If someone started selling Newtonians with optical windows, baffeled them well, designed a better cell for the mirror, etc the 6-8" Newtonian could easily compete with other "more serious" instruments in that size range.

[Alan French]

If you seal up a Newt, how do you get rid of all the heat stored in the mirror? My 8" SCT simply never cooled down enough to perform well because there was not good way for the primary to shed its heat.

Clear skies, Alan

[mistyridge]

You get rid if the heat the same way a similar size Mak Newt would. Or an electronic heat sink system can be designed similar to those used on CCD cameras. This could be added to the sealed tube. I suspect you could build a 10" f/6 that would perform as well as 10" APO. It would make a great planet killer. I don't know how much 10" window would cost.

[snorkler]

I think everyone agrees that at some point, a bigger Newt's views will always beat a smaller refractor's views. We just quibble about where the break point is. I don't care if it takes an 11", 12", or 14" Newt to beat a 9" refractor, since they'll all come in costing far less than the 9" refractor. I can just buy an 18" truss Dob (with Zambuto, Hall, Stevens, OMI/Torus, Kennedy, Swayze, Royce mirror), a van to haul it around in, get better views, and still have $50,000 in the bank compared to buying the 9" refractor - let alone the massive mount required to support the refractor.

[Bill McHale]

If you seal up a Newt, how do you get rid of all the heat stored in the mirror? My 8" SCT simply never cooled down enough to perform well because there was not good way for the primary to shed its heat.

Clear skies, Alan

I will agree that it is a problem, but not, I think, an unsolvable one. Certainly a easy way would be to have a removable back and perhaps vents at the front that would allow good air circulation early in the evening to get the mirror down to ambiant. Fans and other active cooling could help the process along as well.

Ultimately, I think the problem can be solved, probably very effectively. I am kind of spinning a fantasy here, but just imagine what could be done with the Newtonian if we were willing to spend as much on a 6" Newtonian as we would on a 4" high end apo. Active Cooling, baffeled tube, optical window, very high quality mirrors with very good coatings, excellent cells that facilitate cooling, but also help hold collimation better than the current average. Heck even with out the optical window, I think there is alot that can be done to improve the general reputation of the Newtonian if we were willing to spend more on them.

[mistyridge]

A newt with high quality mirror, quartz diagonal, optical window, active cooling, super cell that requires no more collimation than a SCT, and a tube that has the superior characteistics of the cardboard tube but none of the drawbacks would still cost less than a similar sized APO I think.

[Cosmosphil]

I would put up my 10" f/6 against any 9" refractor. I think it either would come very close or match it for tens of thousands of dollars less. At one point, you have to ask how much the secondary really intrudes into the actual visual image in the end. I have gotten stunning planetary images from a C14. Certainly, no planet killer equipment with its almost 40% obstruction! And, Mak-Cass' have the same obstruction as well.
I can only refer this back to the excellent posts from Daniel M's ideas on how to maximize newts that blow away large refractors consistantly!!!

[galaxyman]

First I want to add also that the refractor will have a cleaner or crystal clear image compared to the reflector. No matter how small the secondary obstruction there is, it still will not be a tight or clear as a refractor. So I think the clearer and greater contrast image does give the refractor a different perspective in deep sky viewing.

Fortunately I do have both a large reflector and now a large refractor. As soon as the next dark window with hopefully clear skies comes around, I will put the BIG refractor through a thorough test on many deep sky objects, and will report it to Cloudy Nights.

Below is my response from the same question posted in the refractor forum:

Actually a refractor that is close to the size of a reflector will outperform the larger scope on almost all deep sky objects.

When seriously viewing DSO's there are many variables to consider. First as everyone knows dark skies makes a world of difference. As for the telescope three things must be taken into consideration. First, very good optics, second, is good eyepieces, and third is the key ingredient on why refractors perform so well. It's called the human eye and how does the first two parts of this equation help's the eye in observing DSO's.

Now the good eyepieces are used in both types of scope, but the refractor does give sharper and more contrast images then the reflector. This allows the eye of an experienced observer to pick out detail more easily. Basically, the refractor helps our eyes more then the reflector at the eyepiece. Also most refractors can be used at higher magnification, which also is essential with serious deep sky observing.

I'm not saying that my new 8" f/9 refractor can outperform my 22" dob, The BIG dobs just overwhelms it on sheer size, but reflectors in the 10" to 12" can much indeed be challenged by a large refractor.

Karl

22" f/4.5 Dob
12.5" F/4.8 Dob
8" f/9 Refractor
4.7" F/5 Refractor

[Daniel Mounsey]

One reason that refractors are percieved as being so much better than reflectors of equal aperture has little to do with the actual capabilities of each scope. Instead it has in large part to do with something rarely discused; thermal currents. Refractors light path's are a ONE WAY trip down a protected tube which isn't exposed to outside air currents. Newt's light paths have to go through an unprotected path TWICE which doubles the problems and also often mixes with the observers body heat.
I've done many comparisons with equal aperture refractors vs reflectors and the result always shows the newt to have much more turbulence.

[Jarad]

Karl -

I think everyone here agrees with you. You said an 8" refractor can compete with a 10-11" reflector, and most of the replies here say the same thing (and that also matches the D-d rule, since a 10" will have around a 2" secondary, 10-2=8).

The point is for the same cost as the 8" refractor, you can get a MUCH larger reflector, and the aperture will overwhelm the no CO advantage. I would say an 8" refractor will probably match a good 10" newt on planets (and be slightly behind on DSO's). It will lose to a good 12.5" reflector or larger on everything. By good, I mean quality optics, collimated, and thermally equilibrated. You yourself said that you 22" will blow away the 8" (as it should), but which costs more?

That said, I do have a nice 4" refractor, and I love it's images. They are very nice, but they don't compete with either my 10" or 14.5" reflectors except on wide field objects.

Jarad

[Bill McHale]

[quote name="galaxyman"]First I want to add also that the refractor will have a cleaner or crystal clear image compared to the reflector. No matter how small the secondary obstruction there is, it still will not be a tight or clear as a refractor. So I think the clearer and greater contrast image does give the refractor a different perspective in deep sky viewing.
[/quote]

I think one needs to be careful about overstating the difference made by the central obstruction, particularly when the c.o. is small. Generally when its below 20% and especially when it is <= 15% it has virtually no visible impact on the image.

The areas where the refractor has the advantage tends to come from the issues that are often left unaddressed in a Newtonian. Thermal currents, baffeling, the surface smoothness of the mirror, etc. Even when some of these issues remain unaddressed, I have seen 8" Newtonians perform as well as or a 6" Astro-Physics refractor (the Newtonian had an excellent mirror that was very smooth). I looked through another Newtonian that was made by an ATMer... he addressed many of the common shortcomings and it was a real stunner; no Astro-Physics or other large refractor was on that field that night to compare against, but its performance certainly would have given them a good run for the money had they been there.

[quote]
Fortunately I do have both a large reflector and now a large refractor. As soon as the next dark window with hopefully clear skies comes around, I will put the BIG refractor through a thorough test on many deep sky objects, and will report it to Cloudy Nights.
[/quote]

The only problem here is that, I would guess that your Newtonian is not baffeled nearly as well as the refractor, and probably hasn't had the Newtonians additional thermal issues addressed either. It makes it hard to do a very good apples to apples comparison. Even really good Newtonanians like Starmaster and Portaball often make compromises to make the scopes easier to transport.

[quote]
Below is my response from the same question posted in the refractor forum:

Actually a refractor that is close to the size of a reflector will outperform the larger scope on almost all deep sky objects.
[/quote]

I think I would qualify this statement carefully. A good reflector will trounce a poorly made refractor. Further, I think I would limit it to scopes of equal size. An 8" refactor can outperform an 8" Newtonian, but I am not sure that given the right scope that an 8" refractor would beat a 10" Newtonian (Indeed reports on the Better Mak-Newtonians seem to show that a really good 6" MNT is the essentially the equal of a 5" apo).

[quote]
When seriously viewing DSO's there are many variables to consider. First as everyone knows dark skies makes a world of difference. As for the telescope three things must be taken into consideration. First, very good optics, second, is good eyepieces, and third is the key ingredient on why refractors perform so well. It's called the human eye and how does the first two parts of this equation help's the eye in observing DSO's.

Now the good eyepieces are used in both types of scope, but the refractor does give sharper and more contrast images then the reflector. This allows the eye of an experienced observer to pick out detail more easily. Basically, the refractor helps our eyes more then the reflector at the eyepiece. Also most refractors can be used at higher magnification, which also is essential with serious deep sky observing.
[/quote]

Ok, I agree that the refractor tends to have a slight edge in sharpness, but unless the reflector is poorly made, it will only be a slight edge... and that is only if we are comparing the Optical Tubes alone. Reflectors are often easier to mount since they can be mounted much closer to the ground. As a result it is usually substantially easier to get the Newtonian to be steady enough to take advantage of its capabilities. Regarding the magnification advantage, the Newtonian, again if well made, should support magnifications of up to 50X per inch, where the refractor might be able to go to 60-70X per inch. For Deep Sky work, I really don't see that extra boost to be that crucial.

[quote]
I'm not saying that my new 8" f/9 refractor can outperform my 22" dob, The BIG dobs just overwhelms it on sheer size, but reflectors in the 10" to 12" can much indeed be challenged by a large refractor.

[/quote]

Karl, no offense, but if your 12" reflector is challeneged by an 8" refractor then there is something seriously wrong with the reflector. Even assuming the reflector uses standard aluminum coatings and the refractor the latest coatings, the 12" scope will be pulling in twice as many photons as the 8" scope.