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planet15

150mm Refractor vs. 200mm Reflector?

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planet15    13

Hello All

I am a newbie who started astronmy again earlier this year, and thinking of upgrading from a 80mm Refractor to either one of these. Which one would give better views in general? I used to hear "Apeture is king", then surely the 200mm reflector should give better views? However, I am more fond of the refractors as they do not need collimating and just, I don't know, they seem easier to operate.

But more importantly, 150mm refractors might give superior views in general, I feel. But I have never had one of these in my life, so I am not sure. Certainly the price seem a lot affordable for the 200mm reflectors unless I could find a used 150mm refractor, which might takes some time. Thanks for your advice.

Jay

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Visually, I would go for the 200mm reflector any day. A 150mm refractor that can compete with a 8" Newtonian needs to be an apochromatic one (ED or triplet), and that is going to cost a great deal of money. If you have that kind of money, a 16" newt could probably be bought as well (and be better visually on most counts). I had a 15cm F/8 Newtonian with small central obstruction. These are very close to the performance of a high quality refractor (they are sometimes referred to as APO-killers). Nevertheless, the 8" SCT I have now bests it visually (even on planets: it has a tad less contrast perhaps, but finer detail can be seen), despite its larger central obstruction. Central obstruction is not the bugbear people often believe it is. Chromatic aberration in fast achromatic refractors is worse for fine detail, I find.

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Schorhr    316

I second that, and especially for deep-sky objects aperture is the only thing showing more... No matter how great a smaller refractor might be.

150/200/320mm comparison http://www.clarkvision.com/visastro/m51-apert/index.html (under good condition and long and careful observation)

About collimation: Don't worry, 8", especially the f/6 (1200mm focal length) newton reflectors are not that critical, and even a f/4 telescope can be collimated within a minute or two (if the mirror is aligned already).

Depending on the storage/transport and construction you don't even have to collimate it every time, and at least in a dobsonian rocker-box mount, a reflector is a inexpensive way to get a great telescope.

Either way, both a 150mm and a 200mm telescope will show a lot of things, it's amazing to have such a device for hobby use.

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umadog    358

The reflector/refractor thing seems to be one of those areas of visual astronomy where people have very firm preferences. Sometimes the preferences expressed by people on one side of the fence aren't understood at all by those on the other side. Personally, I'm in the larger Newtonian camp. A good 150 mm refractor is expensive so if you're only considering it, why not buy a second hand 8" Dob instead and see how you like it. If you don't like it, you can always sell it for what you bought it for and down-grade ;) to the smaller refractor.

Edited by umadog
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Note that I am not anti-refractor. I have a few nice ones. For wide field it is hard to beat a small, short refractor, like my 80mm F/6. If your current refractor is a fast one, keep it for wide field. For the vast majority of DSOs, a wide field of view is not needed. You just need aperture. Bang-for-buck wise, Newtonians are very hard to beat. I used Olly Penrice's TEC 140 (140mm apo frac) and his 20" dob on the same night. These two scopes cost roughly the same (the TEC is more expensive if you include the EQ mount). The TEC delivers some stunning views, and is an awesome imaging scope. The 20" dob blows it out of the water for visual observation of faint fuzzies.

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Naemeth    1,039

I second Michael's and Umadog's advice. It is because 6" refractors are expensive (especially if ED or even APO) that people often go for larger dobs. With a 16" dob, you would see a great deal more, and for more planetary detail you can always mask some of the aperture, which increases the focal ratio, but more importantly, allows the aperture to be controlled in a way so that seeing is less of an issue, and therefore you can see more detail. You also get better contrast as the aperture mask is placed off-axis so the secondary mirror supports are no longer in the way.

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DirkSteele    1,279

You will be able to see from my signature I am a refractor man, and despite having recently dropped £20,000 on a 7" triplet Apo, unless you have that kind of budget, and are as easily separated from your money as I am (in other words, a bit silly), I would suggest you would be better served by going for a larger aperture Newtonian. You mention lack of collimation as part of the appeal, but it really is nothing to fear, and once you are well practiced, can be done in just a few minutes.

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John    18,851

Having owned a number of 6" refractors and dobsonain mounted newtonians from 8 to 12 inches I have to agree with the above advice. I'm a refractor enthusiast as well but for the sort of budget that I have available a newtonian delivers the best £-performance ratio.

I've compared my ED120 refractor (4.7") with a number of newtonians and have concluded that it will match a good 6" newtonian in planetary and lunar detail but the additional aperture will show deep sky objects just a little better.

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NGC 1502    855

You will be able to see from my signature I am a refractor man, and despite having recently dropped £20,000 on a 7" triplet Apo, unless you have that kind of budget, and are as easily separated from your money as I am (in other words, a bit silly), I would suggest you would be better served by going for a larger aperture Newtonian. You mention lack of collimation as part of the appeal, but it really is nothing to fear, and once you are well practiced, can be done in just a few minutes.

What a seriously honest post from a guy who's dropped £20k on a frac :smiley:

To the OP, your current 80mm refractor would make a nice pair with a 200mm reflector.

Regards, Ed.

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umadog    358

It's worth pointing out that if the central obstruction is below 20% by diameter then the telescope will perform indistinguishably from an unobstructed instrument of the same aperture (barring diffraction spikes from the vanes, of course). In other words, if a Newtonian is well designed for visual use you won't experience a contrast loss by having the secondary there. The secondary is a bit of a red herring, therefore. As I see them, the downsides of a Newtonian are:

1. The design is more prone to thermal issues than refractors. These can be tamed by allowing the instrument to cool and being mindful of the problems. Nonetheless, refractors don't have the boundary layer issues that Newtonians have.

2. Newtonains are harder to baffle against stray light.

3. The coatings are more delicate in a Newt.

4. The need for collimation puts off some (although with experience it's quick and easy to execute well)

There is a final issue. It's often claimed that refractors produce more contrasty views. Personally, I'm a little doubtful of this. I'm doubtful because the statement is often expressed in rather general terms. I think what people are actually responding to is the typically smaller exit pupil found in refractors since they are generally smaller aperture than reflectors. Smaller exit pupil means darker sky background. I suspect that a refractor and reflector of similar specifications will produce views with very similar contrast if the target is something like an open cluster or a DSO. On a planet or bright star, the diffraction spikes of the Newt will give the impression of a brighter sky background so the refractor may have the more pleasing view in this case. It probably won't show more detail on the object, though. I don't have a pair of such similar instruments to test out these ideas, though.

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66mikeg    28

I chose a 200mm reflector and I'm not disappointed. How heavy is a 150mm reflector anyway? We have a 19th century 6.5 inch reflector at the observatory and just the tube isn't exactly portable ( weighs about 50 kilos)

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Jay , great thread start up .

UMMM ?

I have a Takahashi Mewlom 210 ,

and ,

A 150mm 'Chromocorred ' f/8 Saxon refractor .

I aint got 2 set's of eyes .

For maximum power the Takahashi M210 . 800x easy .On a good night .

150mm f/8 Chromocored Saxon , 200x on the Moon , large field of view , same as the Tak , but bigger field , nice .

Welcome home .

If I hade to take one , it would be the Takahashi M210 with the 150mm f/8 Chromocorr'd , as a guide scope .. on a ...

I have both, mate .

Brian.

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planet15    13

Wow thank you for all your great replies. Very helpful, useful and at the same time interesting. As my budget is about £300 right now, it wouldn't be bad move to go for the 200mm reflector then, rather than waiting to save up for the refractor. Many thanks. I will be reading these posts over and over again as they all teach me tremendously. :) cheers.

Jay

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umadog    358

Or one of these.

One of what? It's some sort of cat, but beyond that I can't tell.... What is it?

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My Nephew in Hamilton New Zealand . is looking after this scope . Here is my sisters lounge .post-18525-0-78856300-1368011860_thumb.jpost-18525-0-78856300-1368011860_thumb.j

Brian.

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John    18,851

A chromacorred 6" F/8 is a very potent instrument. I've owned 3 of them !.

The chromacor's are now very, very difficult to get hold of though and have got very expensive too. The result is still a 6" scope.

On the question of weight (asked by 66mikeg above) my Synta 150mm F/8's weighed around 11kg and the Meade AR6 around 13kg. Thats as tube assemblies only.

For £300 the 8" F/6 dobsonian is as good "bang for your buck" as you can get though :smiley:

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Moonshane    10,782

I am a confirmed dobaholic so the only choice is an 8" dob between these two. the issue really is that unless spending 10x your budget or more, a 150mm refractor will provide less impressive views of low power faint objects and more faults with high power bright objects than an 8" newtonian. I'd also consider it more unweildy and heavy/difficult to use than an 8" dob.

the one exception to the above is for wide open clusters where the frac will provide slightly more attractive views I think.

for me at that sort of budget an 8" newt/dob is about as good as it gets.

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It's worth pointing out that if the central obstruction is below 20% by diameter then the telescope will perform indistinguishably from an unobstructed instrument of the same aperture (barring diffraction spikes from the vanes, of course). In other words, if a Newtonian is well designed for visual use you won't experience a contrast loss by having the secondary there. The secondary is a bit of a red herring, therefore. As I see them, the downsides of a Newtonian are:

1. The design is more prone to thermal issues than refractors. These can be tamed by allowing the instrument to cool and being mindful of the problems. Nonetheless, refractors don't have the boundary layer issues that Newtonians have.

2. Newtonains are harder to baffle against stray light.

3. The coatings are more delicate in a Newt.

4. The need for collimation puts off some (although with experience it's quick and easy to execute well)

There is a final issue. It's often claimed that refractors produce more contrasty views. Personally, I'm a little doubtful of this. I'm doubtful because the statement is often expressed in rather general terms. I think what people are actually responding to is the typically smaller exit pupil found in refractors since they are generally smaller aperture than reflectors. Smaller exit pupil means darker sky background. I suspect that a refractor and reflector of similar specifications will produce views with very similar contrast if the target is something like an open cluster or a DSO. On a planet or bright star, the diffraction spikes of the Newt will give the impression of a brighter sky background so the refractor may have the more pleasing view in this case. It probably won't show more detail on the object, though. I don't have a pair of such similar instruments to test out these ideas, though.

I tend to compare views with the same exit pupil, as the size of the central peak of the diffraction pattern is essentially the same. An SCT like mine does have stronger "wings" lowering contrast a bit. Having said that, the amount of detail visible is generally higher.

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umadog    358

I tend to compare views with the same exit pupil, as the size of the central peak of the diffraction pattern is essentially the same. An SCT like mine does have stronger "wings" lowering contrast a bit. Having said that, the amount of detail visible is generally higher.

That's to compare different apertures or different scopes with similar aperture? What, qualitatively, is the effect of lowered contrast? I hadn't thought about it until now, but you say that contrast goes down but detail goes up. I find that counter-intuitive. What is it with respect to? A smaller aperture refractor?

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That's to compare different apertures or different scopes with similar aperture? What, qualitatively, is the effect of lowered contrast? I hadn't thought about it until now, but you say that contrast goes down but detail goes up. I find that counter-intuitive. What is it with respect to? A smaller aperture refractor?

Ideally you would want to compare scopes of about the same aperture, but failing that, you can use the same exit pupil for the reasons I stated. The reason my big C8 gives more detail at an exit pupil of e.g. 1mm than the 80mm APM is simply that the magnification is much higher. Suppose I take two craters on the moon, both on the terminator, but differing in sizes by a factor of 2.5 (hypothetical case, I know). Will show detail in the smaller crater that the APM cannot resolve, but the larger crater will show better contrast than the smaller in the SCT, because the SCT will diffract more of the light of bright structures into the areas of darkness. This reduces the brightness of the bright structures and increases the brightness in the dark areas.

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ollypenrice    18,798

Michael mentioned our TEC140 and 20 inch Newt. In fact they didn't cost the same, it is madder than that! The 20 inch was £1400 second hand and the TEC was £3800, also second hand. The big one does blow the smaller one out of the water on faint stuff but, equally, the big refractor blows the Newt out of the water in terms of finesse. If you are hooked on refractor finesse then you know you are. If you are not, stay where you are and don't develop an apo habit!!! It is an expensive kind of madness.

On your budget you could very easily afford a second hand six inch Synta (Bresser/Skywatcher/Helios etc etc) F8 achromat. They go for a song, quite honestly. They are really not bad at all. I still have one but they fetch so little that I have never bothered to sell it, yet the views are good. A well collimated 8 inch Newt will beat it though, I suspect. If you just fancy a big cheap refractor the second hand 6 inch F8s are lots of bang for your buck. I'd avoid the faster ones because of serious CA.

Olly

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ollypenrice    18,798

I am a confirmed dobaholic so the only choice is an 8" dob between these two. the issue really is that unless spending 10x your budget or more, a 150mm refractor will provide less impressive views of low power faint objects and more faults with high power bright objects than an 8" newtonian. I'd also consider it more unweildy and heavy/difficult to use than an 8" dob.

the one exception to the above is for wide open clusters where the frac will provide slightly more attractive views I think.

for me at that sort of budget an 8" newt/dob is about as good as it gets.

I'm a refractoholic but I have to admit that Shane is perfectly right on all counts.

Olly

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