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4" refractors vs 6" reflector on planets


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this is just out of interest because i have no idea what is better because a 4" retractor has an unobstructed view where a 6" newt has a central obstruction of about 1.5-2 inches.

so what is better for planets?

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Id say A good refractor will be the better choice.

I used to read that a 3 inch refractor vs a 6 inch reflector was the usual, but i dont see that at all now.

Edited by Earl
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I'd say the frac as well - my TAL 100RS can blow my 8" Newt out of the water on planets and I have seen it give a 6" SCT a good run for its money as well.

I love my TAL but keep a sharp eye out and you may see it up for sale soon cos I am seriously (very very very seriously - in a serious way - so its serious) thinking of selling it to get a TAL 125R.

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I am interested in this as I am considering a 6" f8 or even f11 (or even an 8" f8 dobsonian) as my next scope to effectively replace my 120mm refractor. I'll be hoping to make a direct comparison with someone else's first though!

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Aperture, aperture, aperture. The smaller 'frac may give a tidier image especially when the seeing is poor but there will be more detail in the view given by the larger reflector (assuming they're of similar optical quality, properly collimated and allowed to cool to ambient temperature).

A typical 6" reflector tube is also lighter than a typical 4" refractor, giving the mount an easier job.

A 4" short focus ED / APO frac is a superb instrument for low power wide field deep sky work (on a portable altaz mount) or for small scale deep sky imaging (on a good equatorial) - but the 6" Newtonian is much better as a first/only instrument for visual use, and will turn in a good performance on the moon & planets with a webcam.

If you're serious about solar work, you NEED a 4" frac. Otherwise the Newt makes much more sense. (Cheaper too!)

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Aperture wins on planets, in my opinion. Provided, of course, that the optics are of a similar quality and the scope is well maintained. More aperture means more resolution, and a higher practical magnification.

The smaller refractor may give a slightly crisper image, and you needn't worry about collimation and diffraction etc, and its performance will be more consistent through bad conditions, but on a night of good seeing, the extra aperture will make the difference.

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I am solidly in the aperture camp! Many, if not all of the best images in the planetary imaging section are made with large reflectors.

Refractors, either very slow achromats or (very expensive) apochromats can give a slightly more contrasty image, but in terms of pure resolution aperture is everything. The 6" F/8 Newtons from some manufacturers are often referred to as apo-killers, because due to their small central obstruction they suffer much less from spurious diffraction.

Some purists went with Kutter or Schiefspiegler telescopes, but they have problems of bulk, slow speed, etc.

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I agree with everything posted by Brian on this topic. You can't alter physics, all other things being equal, a 6" will have twice the light grasp and 50% more resolution than a 4" and a Newtonian will be an APO. The 3" refractor verus the 6" reflector myth originated from the early days when good refractors were available but small reflectors were of doubtfull commercial quality or homemade.

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The 3" refractor verus the 6" reflector myth originated from the early days when good refractors were available but small reflectors were of doubtfull commercial quality or homemade.

Even then the 3" frac was twice the price .... and you needed to be really serious to afford a 4" f/15 achromat .... the going price for these in 1970 was around £500 which is at least £10K in today's money, and you'd only get manual slow motions for that (if you were lucky). Reflectors have improved more than refractors in the meantime, modern dielectric coatings (99%) mean that reflectors are now MORE efficient in light gathering than an equal size refractor (90%) despite the central obstruction, and they last ~20 years rather than 2 or 3 as with plain aluminimum coating (80%) or ~ 1 month with silver (90%).

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this is just out of interest because i have no idea what is better because a 4" retractor has an unobstructed view where a 6" newt has a central obstruction of about 1.5-2 inches.

so what is better for planets?

Hi jordan20,

not every reflector has a central obstruction. TCT do not suffer from this.

When it comes to refractor/reflector comparisons make sure that you

do not compare a lemon to an apple. Bad collimation brings down

even the best scope.

Cool down is another issue that has to be solved. If all these things

are good a good 6" Newt beats the best 4" apochrmat on planets.

To equal a good 8" newt you will need a good 7" apochromat.

A 6" apochromat is not sufficient.

An achromat suffers from longitudinal colour aberration, that lowers

the contrat transfer.

Many fast so-called apochromats suffer from spherochromatism,

sperical aberration that varies with the wavelength, this too

lowers contrast transfer.

You better look for very long focal length achromats.

Or for good triplet apochromats, or reltively long focal length doublet apochromats.

Or a slightly bigger apertured well-made newt. Or an equally apertured

well-made TCT.

Cheers, Karsten

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An interesting thread this and one which I answered for myself a few years ago with a 4" F15 refractor and a 6" F7 newtonian side by side

I found the image quality from the refractor far superior with better contrast and finer detail, the newt image was a little brighter therefore more able to take filtered observing on fainter planets.

I agree that theory dictates the newtonian should be better but IMO the practicalities take over.

Another example, I had a C11 which I used for planets regularly, sold it the other year when I saw the views through my new Meade 127 ED triplet were that bit better, dimmer but better, (and that was with proper collimation on the C11, clean mirror and correctors same eyepieces and diagonals)

I would recommend to go along to a star party or club where you can try this out for yourself and form your own opinion.

Philj

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I would recommend to go along to a star party or club where you can try this out for yourself and form your own opinion.

Actually, whilst going to a star party is a good idea, forming an opinion on the basis of what you see is a bad one. Probably 90% of the SCTs and 70% of the Newtonians I've seen at star parties have been seriously miscollimated, and star parties rarely allow time for proper cooling of larger instruments.

The prejudice in favour of refractors goes back to Victorian times when speculum metal mirrors which tarnished rapidly and were at best 60% reflective were common. Even so, Webb ("Celestial Objects for Common Telescopes", 1859) states quite categorically that the resolving power of a reflector is not inferior to that of a refractor of the same aperture, and later editions published after silver-on-glass films were developed state that the light gathering power is not much less either.

The 6th edition (1917,footnote on page 2) contains "Buffham assigns equal light to silvered Newtonians of 9, 6.5 and 4.5, and achromatics of 8, 5.75 and 4 inches respectively. Sa. finds 6.5 in silvered reflector equal to 6 in achromatic."

Page 4: "Reflectors are delightfully exempt from this defect (chromatic aberration); and as now made with specula of silvered glass, well deserve, from their comparitive cheapness, combined with admirable defining power, to regain much of the preference which has of late years been accorded to achromatics."

Of course it is ridiculous to compare a budget 4" Newtonian with a premium 4" APO refractor. But then it's just as ridiculous to compare a budget 6" achromatic refractor with a premium 6" Newtonian reflector. Sorry, but you get what you pay for; and, whilst serviceable quality optics have dropped in price recently, top quality optics have not. Optical quality matters far more than design!

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The Thing is right, even though there are only two options under discussion!!!!

This is so often discussed that I am beginning to wonder if different eyes (and brains) respond differently to different scopes. You can't alter physics, as has been said, but there is engineering as well as physics in play here and you can alter that.

And then there is the way the observer sees.

As a rule I like refractors and I simply cannot for the life of me see how anyone could prefer the planetary view in my 10 inch Meade SCT to that in my 5.5 inch TEC apo. But maybe some would. Brian made an interesting point on this a while back, saying the SCT image was 'scruffy' but held more information. This may be true. But give me the neat refractor any day because I feel I can see more and it makes me happier, all that crispness! But that's me.

Olly

Edited by ollypenrice
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I simply cannot for the life of me see how anyone could prefer the planetary view in my 10 inch Meade SCT to that in my 5.5 inch TEC apo. But maybe some would. Brian made an interesting point on this a while back, saying the SCT image was 'scruffy' but held more information.

How scruffy it is depends on the figure of the mirrors & correcting plate, plus collimation & cooling.

I have little doubt that your TEC is near perfect optically. I very much doubt that your Meade 10" SCT is any better than 1/4 wave RMS, if it is it's a lucky accident.

Larger scopes also have longer cooling times (commercial Meade & Celestron SCTs with unventilated tubes are particularly slow) and the bigger volume of air being looked through exacerbates the effects of bad seeing - at some point the aperture of the scope exceeds the size of the turbulent air cells, then you get smearing rather than movement of a sharp image. In my experience - with normal northern UK conditions (i.e. not good) - boiling predominates up to 6", smearing being rarely noticed, whilst smearing is normal with 10" or more, any residual boiling being local heating effects and/or tube currents caused by incomplete temperature stabilization.

It's not design, it's construction quality plus the unavoidable effects of the jet stream.

give me the neat refractor any day because I feel I can see more and it makes me happier

OK, fine, after all it's your money!

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Actually, whilst going to a star party is a good idea, forming an opinion on the basis of what you see is a bad one. Probably 90% of the SCTs and 70% of the Newtonians I've seen at star parties have been seriously miscollimated, and star parties rarely allow time for proper cooling of larger instruments.

Hello Brian,

you have listed some essential points why many reflectors give bad views.

Add bad mirror surface quality to it. A german amateur mirror maker

recently tested a 130mm f/5 chinese mirror with the foucault test:

Astronomie Forum - ASTROTREFF - Der Treffpunkt mit Foren und Chat für die Amateurastronomie - Astrotreff - Astronomie Treffpunkt - Skywatcher Heritage 130P Flextube Testbericht

That 0.38 strehl ratio was really bad.

But as soon as you compare a perefectly collimated, well cooled down

quality mirror newt or reflector to an apochromat or a long focal length achromat

the newt begins to show his capabilities.

I compared an 8" f/6 ATM Newt to a 7" f/8 TMB apochromat.

We compared Jupiter and M13 and the views of both were equally good in both scopes. Seeing was pickering 8/10 at best.

On an other night a friend of mine and me compared his 140mm f/7 TEC Apochromat

to his 180mm Takahashi Mewlon Dall -Kirkham-Cassegrain, We observed Saturn.

he planet´s banding and ring and rings division was equally good in both.

The mewlon showed 1 additional moon right at the limb of the ring

that the TEC failed to show.

One night another friend and me compared his 115mm f/7 TMB to my 8"f/6 Newt.

We observed Saturn. Allthough seeing was 7/10 prckering at best

it was no test at all, the 8" ATM Newt was clearly superior.

One night with superb seeing two other stargazers and me compared

a 6"f/9 Meade ED to the 8"f/6 ATM newt. The newt showed more detail

on Jupiter and had far better colours.

So if you know what to do you can achieve views with a Newt

as good as with a slightly smaller apertured apochromat.

It takes mor effort to solve cool down issues and local seeing issues,

and collimation needs to be perfect, but if all is well the Newt is very

hard to beat on-axis.

Cheers, Karsten

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I'd suggest the one place a newt would be cooled sufficiently is a star party. Lets face it if the scopes been left out all night its not going to get any colder.

Every star party I have been to the newt is left covered unti early evening when its stripped of its covers and uncapped to allow cool air in (this year it will be fan assisted). If it not cool by 4am then frankly its never going to be cool and ergo no one whould ever own a newt cos if it cant cool with all night to cool down then whats the point.

Miscollimated ? Me ? Nope - I always collimate in daylight on the firts day and tweak the mirror as the first stars show to make the collimation as accurate as possible.

Fact is on both recent outings with the newt and the frac the frac has consistently shown planets with more contrast and finer detail than the newt can manage with double the aperture. For deep sky the results are exactly what you'd expect and the reverse - the newt can make a decent view of faint stuff while the frac cant get the quality.

On bright objects yes the newt will accept filters better than the frac BUT with the frac I scarcely need them - maybe a light yellow or a light blue at most.

I can only go by what my eyeballs are telling me which is on bright objects the frac shows them better - more detail, more contrast. Saturn a few months ago with the frac was easily the best view i have ever had of it and that inlcudes a few long tube newts.

That means either the Skywatcher 200 mirror is duff (not true because other observers have found its views to be good).

Or its miscollimated (deffo not because its always perfection)

Or it cant hack the sky conditions - while that may be true that would suggest that sky conditions are rarely ever suitable for the newt and therefore its redundant.

I kind of believe the latter which is why I am giving all the scopes one last bash at SSP this year and then - some of them will go to fund a TAL 125R.

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I had a Tal 125R and I still have a Skywatcher 8" Newtonian.

Without any doubt what so ever the 8" Newtonian is the better planetary scope.

The 8" has better resolution, no chromatic aberration and excellent active cooling. During the last winter I regularly used x300 on Mars.

On the 21st April I observed Mars at x420 with the 8" under good seeing.

The 8" showed more planetary detail than the Tal 125R which is why I sold it.

I believe a good 8" Newt is the most cost effective planetary scope going for our UK sky conditions.

Edited by dweller25
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Heres one to through into the mix then!!..

I have a 200P DS & 150P. Both f5's the planet image was better in the 150P, hence why I swapped the dual speed crayford for the Mars & Saturn session. The 200P DS is disappointing to say the least on planets, no complaints on DSO's at all. In fact a while ago I had a SPX200 f6, now that was a different story on the planets.. the bet ever views of Jupiter.

Rob

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Hi folks,

Miscollimated ? Me ?

a friend of mine, the one with the 4"f/8 Lomo apochromat, 140mm TEC apochromat,

the 180mm Mewlon, the 180mm TEC apochromat, the 12" Newt, told me that he

allways collimates his newt. He used his collimating laser for that. But he was allways

disappointed by the Newt`s performance.

One day I was there too. He showed me how he collimated his newt. Seemed to be o.k.

I looked at a star with high agnification and the star showed coma. So his newt was miscollimated.

He inserted the collimating-laser again and the newt was out of collimation,

He collimated and again the newt showed coma.

So i inserted the laser and turned the laser around. The red dot ran round in a nice big cirle.

So the laser was decollimated and with a tool like that one cannot acheive god collimation.

On a star party I go around and look through other newts too.

I do not think that there are more than 10% perfect collimated.

Maxbe aditional 20% are quite o.k.

The others are so much decollimated that they will not give acceptable views.

I asked a young lady if I could have a lok through he new 10" Meade truss tube newt.

She agreed and even in the 30mm exepiece coma showed up in the center(!)

I asked who collimated the scope and she told me a fellow stargazer did it for her.

At 125x only one pair of the double double could be split:

The one where the comatic fans were side-by-side, the other one could not be split

because the coma fan reached the other star.

After collimation in the newxt night it was not problem at all to split the double double

at about 80x. But the star test showed that this scope suffered from astigmatism:

About 1/3 wavelength. I recommended to ask to exchange the secondary miror.

I'd suggest the one place a newt would be cooled sufficiently is a star party.

I think at a star party cool down is better than at home. But:

The metal tubes of the low-cost newts are too narrow.

Every uprising warm air from the mirror will lead to tube seeing.

In addition if placed under the night sky the upper side of the tube will get colder

than the side of the tube facing the ground, due to radiation.

A friend of mine mearured up to 4°C temperature difference(!)

This will cause a different type of tube seeing.

All in all most modern Newts are not constructed inthe right way.

I have an ATM Newt with a tube made of 0mm thick hard foam, lined by thin aluminum layers on both sides, interally flocked, wit active cooling, precision mirror made of

glas-ceramic material, with 0.985 Strehl. I needs only collimation after long periods,

or if I banged it against something. It is a ugly looking but nicely performing scope.

My experience is:

If well collimated and if cooled down a quality newt performs as well as a slightly smaller apochromat.

I have several fellow stargazers with quality apochromats to compare with.

But atmospherical seeing needs to be good when you want to compare scopes with

6" and more aperture!

Cheers, Karsten

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Well said Karsten ... I agree with almost everything you say. There's no better method of guaranteeing that a scope is miscollimated than using one of those laser collimators without collimating it first!

The difference between Newt owners and SCT owners is that most Newt owners at least realise that their scope is supposed to be collimated by the user. Few SCT users bother to even try.

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Yeah - aperture wins...every time.

Several times, an AP180EDT was pointed at Saturn or Jupiter...a nearby 11" Starmaster reflector blows the 7.1" astrophysics out of the water...every time. The AP owner agrees. When the seeing is good, my 22" is amazing.

Perhaps the "poor" performing Newts have bad mirrors? That could be the case. The 11" has a Zambuto, my 22" is a Pegasus and my 30" Starmaster has a Swayze mirror. All good glass.

Another case...we worked with a friend's 12" Newt and collimated that thing dead on and tried to figure out what is going on with his scope...then we tried to put another secondary mirror and the poor performance disappeared. Turned out that the secondary was bad and his primary was killer. Spent $150, problem fixed.

CS,

Alvin

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