Is bigger better?

"When you say "better", better at what? "

Better contrast, better resolution and more surface details were visible.

We were also able to see 2 more of Saturns moons with the 12.5 Reflector.

The observation was made around 2am and these scopes had been setup from 8pm so they had all reached thermal equilibrium.

DaveG -- How would I begin to learn to test optics? I'd love to know how good or bad my scopes are! Maybe I should spare you the effort of answering, because I'm unlikely to take apart a scope to bench test it, although I am learning how to star test

Sasa -- Why do you star test with a green filter?

"Image Scale" -- Please could someone explain that? I think it means that, for the same aperture, the smaller field of view of a longer focal length means any given object fills more of the field of view, so more of the scope's resolution is naturally devoted to the object at the image plane. It's already bigger, so it looks bigger with less magnification from an eyepiece. This makes it resistant to becoming over-magnified. It could be (could it be?) that, for the same magnification, smaller aperture with longer focal length could thus get better resolution than bigger aperture with shorter focal length. If true, there should be equations for this.

As to what I buy: Aperture, schmaperture. If I can't lift it, I won't buy it!

http://starizona.com...asicsscale.aspx

here is a good explination

Post deleted by RLTYS

It also depends on what you are observing. Planets or wide field deep space objects, double stars or small, faint fuzzies. All telescope designs have certain compromises and design limitations such as long or short focal length or chromatic aberration. I don't think there is such a thing as the perfect-all purpose scope.

Yes.. for what objects? Planets? M33 is a large LSB object. Sky transparency and large exit pupil are the key. To me, it looks best in a 8-10 inch reflector at 5mm exit pupil. I was observing M1 a good bit lately. It is better a higher magnification from my light polluted back yard. It's barely visible in any thing under 4" and I wasn't able to see it in a good 60mm.

Sometimes the telescope choice is out of our control like right now I switched to a dew resistant newt because of dew.
All forms of finders have been useless lately and my 6" MCT was shut down and then my Edmund 4"f15.
I brought out my RV-6 and I'm OK now except no finder.
I've had some very strange weather.
The RV-6 is stunningly good, funny how we forget. Actualy I knew the RV-6 is very good but I was using a $1000 MCT why bother with the old RV-6? Now I'm scared to bring the Mak back out I think the RV-6 will beat the expensive exotic.

Robert

Nothing wrong with a RV6. Really good all around scopes.
Depending on what you are viewing determines the scope selection. However, throw in a high end 6" APO with a focal length of about F7 you have a really good all around scope that will provide detail that will require a lot more aperture in a reflector before you will be able to see the same detail. That is what I mean is bigger better. Also, that black velvet background that many of us love is so hard to obtain with a reflector. The Tak CN212 as a cassegrain provides that black velvet background. That is one fine reflector. That is the only reflector I have seen that in.

However, throw in a high end 6" APO with a focal length of about F7 you have a really good all around scope that will provide detail that will require a lot more aperture in a reflector before you will be able to see the same detail.

Errm. If the CO is less than 20%, no difference on detail same size.
Scattering from the mirror coatings will limit the black background.

Best coatings on both, best baffling ... no diff.

a 6" F8 or F10 Newt with a really good mirror and small secondary will surprise you. Really, I looked through a 6" F9 newt with a 1.25" Quartz diagonal an 1/12 wavefront mirror. It was a dead heat with a AP 6" F12. The AP was a tiny bit better. TINY. My 10" F6 kicked the 6" AP. You need to use a well collimated newt with a GOOD mirror. That AP was a fantastic scope, but I have to agree with WFJ. 6" APO will not best a very good quality 8"+ reflector. Don't compare an AP 6" to a Orion Dob. Compare it to a long focus Zambuto mirror scope.

I will say, a 6" F7 AP would be a killer all around scope. It will outperform an average 8" or maybe a not so good 10" dob.
Nice wide fields.. No too hard to mount. Sure would trade my 6" F10 Jaegers for one.

Well, I had a 10" Royce dob with a protostar secondary and tuned to perfection and it could not outdo my 6" AP. The dob was good but not that good. Maybe if you have an exceptional mirror it would be good contest. Even the Intes MN76 I had would not outdo the AP. I have not had any of the other scopes out long enough to do a fair comparison. The Quantum 8 and the Tak CN212 should be a good comparison to the AP. The portaball so far has given me the best view of Jupiter ever. Must have been a rare night of good seeing. I would love to see Jupiter in a 25" dob on a night of good seeing with a great mirror. Yes, bigger is better on nights of good seeing as long as the optics are also excellent to allow seeing that fine detail that only good optics will provide.

Sasa -- Why do you star test with a green filter?

I'm not sure if one needs green filter for long achromats, probably not, for example star test of C63/840 lens looks good also in white light. But some of my former telescopes were pretty fast, like Stellarvue 80/480mm triplet or Vixen 130 ED SS. There is usually fair amount of spherochromatism in such fast refractors (dependence of spherical aberration on wave length). As a result, the defocused star pattern does not look like in textbooks and it is hard (at least for me) to judge the quality of the optics. With green filter you are checking the optics at the wave length at which the optics was optimized.

Hello Jon, yes, I meant NGC5053, a globular close to M53. Sorry for the typo. I was chasing it for several nights with 250mm Newton but with no luck. To my surprise, I glimpsed it through ED100 just on the first try from the same location.

It is not important that it is globular. I meant it as a typical member of class of objects with low surface brightness. It was just a first one that came to my mind.

It's been a while since I looked at NGC-5053 but it's pretty easy to find since it's about 1 degree from M53. I have to think it was an issue with light from a street light because it should be easy pickings in the larger scope..

Jon

Personally I don't think there is an answer to you original question. There are too many variables and qualifications. I remember a night where I had one of my best views of Jupiter thru a C14. Next in fairly dark skies, M33. Very dim view the galaxy which filled the eyepiece field even tho it was long focus. The contrast was not impressive. So I move to my 4 inch f/5 refractor and look at M33. Perfectly framed, spiral structure much more evident and contrast superb. Your question is like asking the better of apples and oranges. But I enjoy the thread.
Bill

That is basically an issue with a large aperture, slow scope. Celestron specs the C-14 at F/11 so the wides possible field of view is about 0.66 degrees. M-33 is about 0.7 degrees x 1.1 degrees, it over flows the eyepiece. But in a similar sized Newtonian working at F/5, it can be properly framed against the dark sky and it can be quite impressive. I like to wander around M33 with my 16 inch F/4.42 (F/5.07 with the Paracorr), there are some interesting details to be seen.

Jon

It really is not impossible to make a Newtonian perform as well as an equivalent aperture APO Refractor. If both have equivalently well figured optics and the design for the Newt was on the stingier side in terms of diagonal size (i.e. below 20%), one would be hard pressed to tell a difference if the target was Jupiter or Mars. One would need to carefully collimate the Newtonian, flock / baffle the tube interior, have quality focusing components, anything is possible. A central obstruction of 30% is noticeable on the image (but not as awful as it has been made out to be), 25% is tolerable for all but the most fussy fusspots, 20% is barely perceivable by most people, 15% may only be perceived by the slenderest percentage of expert observers, 10% is absolutely impossible - I mean absolutely and totally and utterly impossible for the eye to detect a loss of contrast.

I have looked through a few Newtonians with obstructions of 12 - 15% - they may as well be considered the equivalent in image sharpness / contrast to a high end APO of equal aperture. It is a myth to talk of the APO >> Newtonian - they can in fact be very nearly equal. There is no question though that a Newtonian would give an equivalent sized Achromat a severe hurting (provided apertures >= 6" otherwise the Achromat is generally superior). A 6" achromat just cannot cope with good CA correction unless the tube was absurdly long. A 6" f/10 Newtonian can have a diagonal of 1" (even .75" if one used the scope exclusively for planetary / double stars) and thus give a very good contrasty image, a 6" achromat would have to be at least f/15 to give similar contrast if one is prepared to ignore the indigo/violet halo that surrounds bright objects. 6" f/10 is troublesome in size but manageable, 6" f/15 is a nightmare! 6" and above are magic numbers for the Newtonian and many 6" f/8 instruments are very fine, as is the 8" f/6.

However one thing a Newtonian cannot do: It will never match up, aperture for aperture with an APO when one factors in the focal ratio. A 6" APO could be very well color corrected down to about f/5. A Newtonian on that same focal ratio would get absolutely hammered in terms of contrast, fine resolution, details etc... To get a Newtonian to perform as well as the APO, pound for pound, aperture for aperture, would be if the design was for larger mirrors / objectives. At the small end of the scale, a refractor absolutely smokes a Newtonian out the water (particularly if one wishes to keep the focal ratio reasonable, i.e. f/10 or below). Once one hits around 6", particularly 8", the differences become academic rather than practical. The difference between a TEC 8" f/8 APO and an 8" f/8 Newtonian could be made negligible. The comparison between a 6" f/8 APO and a 6" f/8 Newtonian with a premium mirror errs in favor of the APO - the APO edging the Newtonian out slightly on contrast and fine color. At f/10 though, with one making a sacrifice on 100% illuminated diameter (perhaps down to a quarter inch or even an eighth of an inch), the 6" Newtonian would hypothetically rival the APO on planets. However, the APO would comfortably beat the Newtonian on illuminated area / field of view.

However, whatever the case one can make for good quality Newts, Maks, SCT's, Achromats and so on, there is something so utterly appealing about the APO. It is its mystique, its prestige, its poetry that makes all other telescope designs seem like something ugly to own and use. APO performance advantage over other telescope designs has been grossly overestimated by the majority of people, but lets face it, it's appeal extends far beyond the notion that it performs far better than anything else. We all know it does not, we all know it is a placebo type effect (it costs more, everyone says it is better, therefore my brain will say its better), we all know that even if it does perform ever so slightly better, it certainly is not worth $20K more.

Whatever the case, the APO versus everything else debate will go on endlessly. If low dispersion glass became dirt cheap and APO's end up costing as much as Achromats, most people would not get so hung up on them. The fact they are expensive means those that really spend a fortune will justify it's cost by whatever means possible - even ignoring basic optical theory / physics when a 4" APO is faced up against a Cassegrain or Newt of double the aperture. There are a plethora of threads along these lines "4inch APO better on planets than 8 or 10inch scope" etc... We all know basic optical theory should trump bias, emotion, or ignorance.

Peace.

"It really is not impossible to make a Newtonian perform as well as an equivalent aperture APO Refractor."

I haven't used APO's enough to know for sure, BUT Pre-APO / ED, amateurs on a budget built or bought Newtonian "Planet Killers" that I have looked through, and these were excellent performers. Even the "general purpose" Newts, like Jim Vice's 8" Cave, produced bright planetary images with truer color than an equivalent aperture ACHRO.

I remember an article years ago in Reflector about taking a run of the mill 6" f/8, masking off the outer 1/2" of the mirror (where most of the error occurs) and installing a smaller secondary and low profile focuser. I believe the obstruction was below or around 20% and compared favorably to a 5" APO.

Cheers,
Bill

Sorry to break the news
"All Things Being Equal" In other words any really well built telescope with optimizes optics, good seeing and optimal ground thermal conditions / control.
This not being a question of Newt Vers. Refractor, Acro / Apo or Cass.

"Always" is the answer to this "Resolution" question.


Aperture will always wins the race. No doubt about it. Why else would some of us drag around a 20" DOB!!
(aveman

PS:
Of course this is not taking into account my bias passions and the million reasons a well built large or small APO refractor will alway be my favorite to actually use!!

So there we have it. The myth that bigger is better has been busted. We all agree the scope with the better optics always wins no matter what the size of the scope.

So you are saying that a 50mm Ashai Pentax will best a run of the mill C8? I don't know that I would agree with that, though I do know that I would use my Zeiss Telementor or even my Russian Lomo Mak more than either and also enjoy the experience more.

As I have said time and again, my thresholds of aperture at both ends of the scale are determined by aesthetics and practicality.

OK let me reword that a bit
The scope with the better optics will alway better the scope of similar size or the unobstructed scope will most of the time beat a scope of similar or larger size. so the question of is bigger better has been busted. Bigger is not really better, take a coulter 17" for instance. We all know that they were not better. a lot of big meade newtonians were not better, well a lot of meade gear was not better I know that I kept talking about high end scopes and now I realize that I started this thread because of all the really bad experiences I have had with large cheap reflectors and refractors that I kept hoping would give me that thrill of large aperture cheaply. It only thought me a lesson that aperture comes at a cost and there is no substitute for quality.

And your last sentence I totally agree with. That being said, be careful not to re-kindle last week's firestorm. I am sure there are still glowing embers lying about.

Terra :flower: