4" refractors vs 6" reflector on planets

[russ]

Ask 14 different people (and counting) and you'll get 14 different answers!

Absolutely spot on! Which is why i say you can't beat a good Tasco 50mm achro for planets.

[DeejayP999]

So if the Newt has a really good mirror, is really well collimated and really has cooled down, then it might give a better view than a refractor.

But since those things rarely coincide in the real world, most of the time it probably won't!

[michael.h.f.wilkinson]

Absolutely spot on! Which is why i say you can't beat a good Tasco 50mm achro for planets.

50mm? achro? Luxury!! When I was little, all we could afford was a tiny 30x40 toy telescope with single lens objective and EP. And we enjoyed it too!

[russ]

50mm? achro? Luxury!! When I was little, all we could afford was a tiny 30x40 toy telescope with single lens objective and EP. And we enjoyed it too!

Sounds a nice scope Michael, nice specs I what i would give for a single lens objective!

[michael.h.f.wilkinson]

On a different note, I had a peek at Saturn a while back with the 16" RC on top of our building at the uni. Nothing messy, murky or scruffy about the image. Jupiter should be nice too. A well-collimated, high quality piece of kit, producing perfect planetary image once the dome seeing has cleared up (usually after 2 a.m.).

I do feel the discussion is being clouded by comparisons between say a TEC or similar high quality apochromats with cheep 300-500 GBP Newtons. This is unfair. Try and compare a 300-500 GBP fast achro with a similarly priced, larger aperture Newtonian (especially say the 6"F/8 which is more tolerant to slightly off collimation), and you have a fair comparison. If you put GBP 5000+ apos into the mix, then compare to similar, high quality SCTs or Newtonians.

[TheThing]

...I have an ATM Newt with a tube made of 0mm thick hard foam,...

How's that then? Must be non-existant (or more likely a typo!)

How thick is the foam really please, out of interest?

[michael.h.f.wilkinson]

How's that then? Must be non-existant (or more likely a typo!)

How thick is the foam really please, out of interest?

at least the thickness is not negative

[Astro_Baby]

I'm surpised you say that about the 125 dweller - truly. And a bit alermed because my intention is to sell one or two scopes to fund the 125 and naturally the 200 newt is the one most likely to go.

As for my collimation its always perfect and I know that because one of the first things I do when I am out is take a star test with it as its portable and potentially thrown out of kilter by transportation.

We could all I am sure coax fantastic performance out of a custom built newt with perfect mirrors, under brilliant skies etc and compare it to an average frac being used next to the M25 motorway but I thought we were discussing like with like - ie £300 newt versus £300 frac.

Both the 4" TAL and my 8" SW newt will be at SSP and I am perfectly happy for anyone to come take the taste test on them and see what you think. My money will be on the frac for sure. Now that has to be a fair test - both are roughly the same cash and the newt is x2 the aperture of the frac and half its F ratio.

It cant be just me cos I was talking to someone today who also has an 8" newt and was also expressing disappointment with it for planetary work hence them coughing up for a small WO frac.

[KaStern_Former_Member]

Hello Stargazers,

sorry for my typing errors.

The 8-cornered tube is made of 10mm thick aluminum lined hardfoam.

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.

Alvin, thank you very much for your important point! I can say that the secondary

of a standart newt I bought had only 0.66 strehl when tested with an interferometer.

I practice it is not that bad as it sound, because only a part of the secondarxy mirror

surface is used for planetary viewing. That avoids the worst outer parts of the secondary.

But the view improoved markedly by adding a quality secondary, That one was tested too

and strehl was 0.96 over the whole surface area, the inner 3/4 have about 0.99 strehl.

I thought we were discussing like with like - ie £300 newt versus £300 frac.

Oh sorry, I did not know this. So a 300 british pound newt vs a 300 british pound refractor?

Maybe a 200mm f/5 or f/6 Newt vs a 120mm f/8 achromat?

Or a 150mm f/8 Newt vs a 100mm f/10 achromat?

Cheers, Karsten

[dweller25]

Mel,

I am comparing like for like in terms of cost.....

My Skywatcher 8" F/6 Dobsonian (£269) outperformed my Tal 125R (£275) on the planets and the Tal 125R I had was VERY good.

I live just North of Manchester so I'm sure you can imagine the quality of my skies - although this is not usually an issue with the planets.

Sorry to be blunt but If your 4" refractor can outperform your 8" Newt then something is wrong with the Newt.

I suspect that what we are talking about here is either optical variability in Skywatcher 8" mirrors or inadequate removal of the boundary layer on the primary mirror.

HTH

I'm surpised you say that about the 125 dweller - truly. And a bit alermed because my intention is to sell one or two scopes to fund the 125 and naturally the 200 newt is the one most likely to go.

As for my collimation its always perfect and I know that because one of the first things I do when I am out is take a star test with it as its portable and potentially thrown out of kilter by transportation.

We could all I am sure coax fantastic performance out of a custom built newt with perfect mirrors, under brilliant skies etc and compare it to an average frac being used next to the M25 motorway but I thought we were discussing like with like - ie £300 newt versus £300 frac.

Both the 4" TAL and my 8" SW newt will be at SSP and I am perfectly happy for anyone to come take the taste test on them and see what you think. My money will be on the frac for sure. Now that has to be a fair test - both are roughly the same cash and the newt is x2 the aperture of the frac and half its F ratio.

It cant be just me cos I was talking to someone today who also has an 8" newt and was also expressing disappointment with it for planetary work hence them coughing up for a small WO frac.

[GazOC]

It might be worth pointing out that, unless you are looking for small details in the ring system, Saturn isnt the toughest test on the lack of resolving power of the smaller telescope relative to a larger one.

My ED120 shows the colour in the bands of Saturn very well but thats not the same as being able to show more detail than a larger scope on (say) Jupiter.

[Astro_Baby]

Well you may be right dweller and the 8" newt is duff - I do have a fan for breaking down the boundary layer but I cant honestly say it ever seems to make much diference. Perhaps seeing conditions have never been favourable to the newt - in which case as I said in an earlier post - if seeing conditions in two years have never been favourable then I guess the newt is a waste of time at least for me and maybe it should be sold off. Perhaps sky conditions have never been kind enough for the newt - after all DSOs tend to be less at the mercy of seeing conditions than planets.

I dont mind you being blunt

I'm curious on this thread because I have been so smitten with the 100RS that my natural inclination is to get a bigger version precisely because the views are so much better - at least for planets. On deep sky the newt will win out because of its bigger aperture.

Perhaps the difference between F5 and G6 makes the difference - I honestly dont know. All I can say is I was happy with the views it gave until I tried the 4" frac out. So far the only target the 4" frac wont resolve well is Venus but the seeing has been terrible so far.

Gaz - fair point on Saturn.

[Capricorn]

Seem to be comparing 4" refractors to 8" newtonians now.

Started out as 4" refractors compared to 6" newtonians.

Probably a few SCT's thrown in somewhere if I read back.

For a 4R:6N comparison I guess little in it, probably sharper in the 4 and a bit brighter in the 6.

From experience I prefer the sharper image even if smaller.

Would say in a 4R:8N comparison then the 8 would win out.

[LukeSkywatcher]

I have a 3.5" refractor (f/l 1000mm,f11) and a 5" reflector (f/l 650mm,f5). Hand on heart i have to say that the 3.5" refractor gives me better views of planets

[KaStern_Former_Member]

Hi Paul,

I have a 3.5" refractor (f/l 1000mm,f11) and a 5" reflector (f/l 650mm,f5). Hand on heart i have to say that the 3.5" refractor gives me better views of planets

there are some really bad chinese made newts out there.

I see you list the skywatcher heritage in your sihnature.

The well-known german amateur mirror maker Stathis Kafalis

tested the mirrorof a 130mm f/5 heritage with a really bad result:

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

http://www.astrotreff.de/upload/Stathis/20100525/heritage130_fxp.gif

He calculated the strehl ratio as 0.38 wich would be awfully bad.

Cheers, Karsten

[great_bear]

That's interesting data, since that mirror is the same one as used in the Explorer 130P as well. I can't say I'm surprised since even at only 130x I've always felt that it's right at the limit of its capabilities.

Great for wide views though.

[great_bear]

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.

People blame the obstruction because it seems like the obvious culprit for poor performance. The science however, doesn't back this up at all - not even as a cause of reduced contrast.

This article <click> by respected mirror-maker RF Royce may be of interest.

[GazOC]

GB, unless I'm missing something (very probably!!) that article says that the obstruction DOES degrade performance.

[great_bear]

50mm? achro? Luxury!! When I was little, all we could afford was a tiny 30x40 toy telescope with single lens objective and EP. And we enjoyed it too!

Objective? Luxury!! When I were young all we 'ad were toilet roll tubes blacked with charcoal - and we were grateful !

[great_bear]

GB, unless I'm missing something (very probably!!) that article says that the obstruction DOES degrade performance.

Oh, sorry - I think I confused things by providing a link of general interest, next to my comment about the science of things - apologies!

What I meant was this: People usually compare good-quality refractors with budget, off-the-shelf, Newtonians. When the refractor wins, some "knowledgable", over-respected amateur will say "Ah yes... That will be the effects of central obstruction" without any scientific back-up whatsoever. What's worse is when people unquestioningly believe such statements.

As brianb has pointed out in the past, even top-quality glass doesn't pass through all light unobstructed - on the contrary, the absorption of light by a refracting objective's glass thickness, typically exceeds the amount of light lost by an equivalent aperture reflector's secondary obstruction - so to say that refractors offer "unobstructed aperture" is very misleading.

Similarly, with regards to "loss of contrast" by central obstruction, within reasonable limits, the figures in this article <here> don't back that up either.

I think all that RF Royce is really saying on this subject in his article is that there is (of course, since it's obvious) a certain size of central obstruction beyond which its effects become an issue. My point is - with the quality of off-the-shelf telescopes - there are other problems that need addressing before you start questioning the basic design of reflecting telescopes.

I have no problem when people say: "I can only go by what my eyes tell me!", that's fine, but when - with no expert knowledge or scientific training - they start attributing what they see to specific telescope design attributes, someone has to say "prove it!" sooner or later. In most cases it's poor baffling or badly-figured optics that's at fault, not the basic choice of telescope design.

It seems clear to me, that aperture-for-aperture, a quality, hand-made telescope will run rings around any low-cost scope that's rolled off a production line - irrespective of the basic design chosen.

[GazOC]

No worries GB.

Another great article on the subject.

Thierry Legault - What are the effects of obstruction ?

[brianb]

It seems clear to me, that aperture-for-aperture, a quality, hand-made telescope will run rings around any low-cost scope that's rolled off a production line - irrespective of the basic design chosen.

Ain't that the truth

[bombassa]

Hi Paul,

there are some really bad chinese made newts out there.

I see you list the skywatcher heritage in your sihnature.

The well-known german amateur mirror maker Stathis Kafalis

tested the mirrorof a 130mm f/5 heritage with a really bad result:

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

http://www.astrotreff.de/upload/Stathis/20100525/heritage130_fxp.gif

He calculated the strehl ratio as 0.38 wich would be awfully bad.

Cheers, Karsten

Whats this? Bad mirrors?

How common is this?

I was looking for a larger 'production line' newton for next year but now you scare me if I pay a lot of (for me nevertheless) money and risk getting a bad mirror?

[GazOC]

0.38 is shaving mirror standard!

[michael.h.f.wilkinson]

Objective? Luxury!! When I were young all we 'ad were toilet roll tubes blacked with charcoal - and we were grateful !

Blacked with charcoal!! We dreamt about charcoal. Our dad closed the end of our tube with duct-tape and told us to imagine seeing the stars!! And we had better be good at it!!

The problem with the kids today is that when you tell them they don't believe a word you're saying