Reflectors vs Smaller Refractors

[gareththegeek]

I noticed whilst reading Sky at Night magazine's star guide thingy that it considers a 'Small' telescope to be less than 150mm reflector or less than 100mm refractor.

For the most part from what I have read aperture is considered king so is it the case that in general a 100mm refractor can be considered to be equal to a 150mm reflector?

Thanks,

G

[Wurzil]

hmmmm youve opened a can of worms here. expect interesting and contradicting replies to this question.

ps -my own hunch is that even if refractors are better mm for mm than relectors of equal quality; i dont think the difference would be that great.

warren

[pvaz]

Refractors don't have a central obstruction and have a higher light transmission then reflectors.

But you also have to consider that cheap refractors (non ED/APO) cause different colors to focus at different points causing color aberration on bright objects (planets/moon) while reflectors don't.

So for the price of a decent 80mm refractor (over 400£) you can buy a 250mm reflector and it will be far superior, except on bright objects and splitting double stars where the refrator may have a slight advantage. Even a 8" reflector for less then 300£ will out perform the refractor on DSOs and be almost as good on planets, when properly collimated. So, unless your interest is really focused on planets or imaging, in general a reflector will give you more value for your money. The larger aperture on the reflector will however allow higher mags even if the contrast is slightly worsth.

PS-> As Wurzil said this is a very touchy subject... Better buy one of each!

PS2-> In sum, if you compare a 80mm refractor to a 80mm reflector, the refractor will win. But if instead you fix a budget and ask what's the better scope I can buy to see a bit of everything then a reflector will always be the answer as they cost much less to produce, and you get much more scope for the same money.

[GazOC]

IIRC Sky at Night used to call "small" scopes either 4" refractors or 8" reflectors which was way off.

I think 4" vs. 6" is still pushing it on most objects but at least its arguable.

[Wurzil]

I was out with a 120mm frac and a 250mm reflector last night and the frac won out on Mars (more colour and detail) and on stars (more contrast and sharpness on open clusters etc).

The reflector won out on everything else (saturn, golbular clusters, galaxies and nebulas).

APETURE IS KING (errr...sort of....)

warren

[brianb]

Refractors don't have a central obstruction and have a higher light transmission then reflectors.

The central obstruction has very little effect at all on light grasp. The fact that, with cheap achromatics, much of the light goes "the wrong way" & has to be filtered out, and that refractors have four, six or eight air/glass surfaces instead of two in a Newtonian, and that the light has to go through two, three or four thick lumps of glass (or fluorite), makes at least as much difference to light grasp as a 30% CO.

When mirror surfaces were 80% reflective, they weren't as efficient as refractors. With modern 99%+ mirror coatings, and despite the ultra high transmission coatings available for lenses, I suspect that reflectors are now actually more efficient in terms of light grasp than refractors. And that's before you add on the fact that you'll probably be using a diagonal with your refractor; no need for one with a Newtonian.

The central obstruction does reduce contrast - slightly - which means that, for visual resolution of low contrast detail on planetary surfaces, a refractor works slightly better, but even so I'd rather have half an inch of extra aperture than no CO.

Refractors are kings for solar work for reasons which have lots to do with the dissipation of the heat arriving from the sun. But otherwise the difference between designs is the least important of the three major factors (the most important being, in order, aperture & optical quality).

You do see references to reflectors being equivalent in light grasp to a refractor half the size, and the people who originally wrote this (in the early 19th century) were not being ridiculuous - they were comparing long focus cemented doublet achromatic refractors to reflectors made with speculum metal mirrors (around 60% reflective, per surface, when fresh) and in those days no-one dreamed of using a diagonal with a refractor because of the heavy light loss.

[gareththegeek]

Interesting, that's pretty much what I figured. As I probably should have said in my OP, I was not considering cost here, but assuming a good quality scope in each case.

I was surprised that the magazine said a refractor which has only 66% the aperture diameter (and 44% the surface area) of the reflector could see roughly the same objects.

I guess its complicated since it depends what object you want to see!

G

[F15Rules]

The other important factor is the seeing on a given night...often a larger reflector won't give of its' best if the seeing is not great, wheras a refractor will more often perform near its maximum potential.

Although I am a refractor fan myself, don't own a reflector, I don't believe any one scope can be a perfect all rounder, and to get the best possible tools for all applications (visual), you would probably need or want to get a good 4-5" frac and a good 7-10" reflector for all the reasons outlined in the thread above..

Dave

[russ]

I have a 90mm f11 refractor and a 200mm f6 Newtonian. Love both scopes, they both have their pros and cons. There's something very right about using a refractor even though in practice the Newt outperforms on almost everything.

One area where the refractor is streets ahead is star images. Stars are true pin points of light and this really comes into play when observing double stars. Looking at Albireo really allows the refractor to strut its stuff and show the newt who is boss. Stars are tighter and the colour contrast more pronounced. The newt is no where near as good.

But other than doubles, the newt wins out on almost everything. Things are close with saturn and the moon.

[albedo0.39]

Interesting comment about albireo. When at school (a lifetime ago) I recall one observing session when we had the "big" school scope out - an 80mm Apo. We saw Albireo and the sight has never left my memory. Sure, my current scope is better in every way except portability, but there is something so right about a nice refractor and a decent double star.

[GuillermoBarrancos]

Well I am still dubbing between a 80ED / 100ED and a 200mm F/5 Newt.

I want to get into Astro imaging and know both these scopes will do well when it comes to that.

It's tough, as on the web you see people making stunning (DSO) images on both type scopes.

Making the choice even harder. /sigh

Altho when it comes to portability, then the choice for the 80ED / 100ED is quickly made I guess hehe.

But then you also got the 150 Mak Pro .....grrrrrrr

[gareththegeek]

Looking around ED refractors seem to list the maximum potential magnification as 3x aperture size in mm, reflectors list maximum potential magnification as 2x aperture size in mm. Is this what the Sky at Night magazine is basing things on perhaps?

[brianb]

often a larger reflector won't give of its' best if the seeing is not great, wheras a refractor will more often perform near its maximum potential.

Perfectly true - but we have to differentiate here between tube currents resulting from incomplete and/or uneven temperature stabiliztion (systems with a closed tube have a small advantage here, compounded in refractors by light only passing through the tube once) and atmospheric seeing (where the smaller scope will usually be less affected by unstable seeing, irrespective of the design & construction).

Medium sized Newtonians - and other systems with open tubes - can suffer quite badly from tube currents; insulating the tube and/or adding a fan to draw air through the tube can help a lot. Large scopes (over about 12") tend to need some sort of forced ventilation to perform well irrespective of design & construction.

The effect of a large scope's extra light grasp can be reduced by poor seeing - when the seeing is bad enough to smear stars out into blobs, you can't see faint ones as easily as if they were sharp, and you need magnification to dilute the sky background. Smaller scopes are less affected because the magnification you need to darken the sky is lower in proportion to the aperture.

The actual limiting magnitudes for my 11 cm triplet APO and my 11 inch SCT should differ by almost exactly 2. This is what I find, in practice, when the seeing is reasonably steady. When the seeing is smeared (those of you with observing experience will know exactly what I mean) the difference is reduced, but still well over one magnitude. Again, on a night with awful seeing, the big SCT might not show any more detail than the small APO, but it's no worse (next to nothing in either!) and, if the seeing does settle down momentarily, detail beyond the capability of the smaller scope "pops out" with the bigger aperture.

The real downside of the larger scope - apart from its physical bulk & weight - is the fact that it will usually have a much smaller field of view. A scope can be too big for viewing of large, relatively bright DSOs; this, together with portability considerations, is why you will ideally have more than one instrument. A "small" refractor of good optical quality makes an ideal companion to a "medium" reflector or SCT.

[brianb]

Looking around ED refractors seem to list the maximum potential magnification as 3x aperture size in mm, reflectors list maximum potential magnification as 2x aperture size in mm

Both hopelessly optimistic. 1.2x per mm (30x per inch) will show everything the scope is capable of ... irrespective of everything else.

[gareththegeek]

Both hopelessly optimistic. 1.2x per mm (30x per inch) will show everything the scope is capable of ... irrespective of everything else.

I know that in practice these magnifications won't be useful but my point is that it seems to imply that a higher useful magnification can be achieved using a refractor than a reflector of the same aperture size, by a factor of 3/2. I am trying to account for the claim made in the magazine that small is 100mm- for refractor and 150mm- for reflector.

[russ]

I think the magazine is referring to DSO objects visible in those size scopes. In which case aperture will be the deciding factor rather than max mag.

I think you'll find a lot of people will say a good refractor will punch above its weight class, more so than a reflector. But then the reflector is really up against it when the aperture is 200mm plus because the UK skies won't support 2x aperture regardless of the scope.

[crazyjedi]

I was about to offer my two pence worth but the above answers seem to cover nearly all angles.

If anyone reading all this gets too confused, they can always get an SCT or a MAK instead of either eh?

[brianb]

they can always get an SCT or a MAK

Yes, the advantages of neither with most of the disadvantages of both .... the great thing about an SCT is how compact it is, I have one because it's the only way I can get a 10"+ aperture instrument that's reasonably portable.

And the fact is that many of the super high resolution lunar & planetary images posted in this & other forums are made with SCTs. Planetary purists used to scoff at the "huge" central obstruction but the proof of the pudding is in the eating, so to speak, and the design works when it is properly constructed, allowed to cool to ambient, accurately collimated and being used by someone who knows about planetary imaging.

[Astro_Baby]

From a practical standpoint I was out earlier this week with the TAL (4" Achromatic Refractor) and its view of Saturn easily blew away the SW 200 (8" Reflector) - how much tube currents were the cause of this I cant say as the refelector wasnt in use on the same night. What I did notice is that with loose hazy cloud floating about the refractor seemed better able to pull a decent view out of the murk than the reflector ever can.

[russ]

Interesting comment about albireo. When at school (a lifetime ago) I recall one observing session when we had the "big" school scope out - an 80mm Apo. We saw Albireo and the sight has never left my memory. Sure, my current scope is better in every way except portability, but there is something so right about a nice refractor and a decent double star.

Funny you should say that because the most lasting image of Albireo i have was after seeing it with an ED80. The background sky was black, the stars were true pin points and the colour contrast will stay with me forever. I think it must have been an exceptional night as i've never been able to replicate that night, not even with the same scope. Got close but never matched it.

[crazyjedi]

I wasn't advocating SCT's or MAK's i'm a reflector man (although i wouldn't turn my nose up at a couple of grands worth of APO) My own opinion is - it's horses for courses depending on your viewing pleasure. My choice of reflectors is more budget driven. I have to agree tho, most of the best amateur pics I've seen have been thru SCT's. they seem to be the way the wind is blowing for imagery, despite the fact that i thought they were something of a half way house between the two traditional scopes. Beggars can't be choosers tho eh? any one giving away a couple of grand worth of APO let me know tho