reflector vs refractor?

[lostinspace]

can a reflector scope show as much detail as a refractor? if not, at what point should i be thinking of purchasing a refractor?

[kniclander]

depends what you want to look at but generally a reflector will show you more per £££ you spend (especially if you buy one on a dobsonian mount).

[michael.h.f.wilkinson]

Agreed. All refractors suffer from chromatic errors, which need careful correction (achromatic refractors are good, are apochromatic better at this). This requires careful grinding and polishing of 4 to 6 optical surfaces. A reflector has no chromatic errors, but has a central obstruction, which means the aperture is anular rather than a solid disk. This degrades the image slightly. On the upside, only 2 optical surfaces need to be shaped properly, for a Newtonian, which is why they are cheaper for a give aperture.

If you have an obsession with planets, and a huge budget, an apochromatic refractor is best.

[matt c]

Would I be right in thinking a 12" dobsonian is more practical than 6" refractor. The dob breaks down into two managable pieces and can be carried out I assume very easily, It's then just a case of putting the tube on the base.....easy. I'd imagine a 6" refractor on an eq mount would take maybe half an hour to setup. I think the refractor is a spiecalist scope as said above for planets and double stars while the reflector performs well on these aswell it's more a general scope that can handle most things (if not all). In terms of price you're talking twice the aperture for about half as much.

[haitch]

If you want to gather lots of light for seeing faint things then you will probably want a reflector.

If you want to look at brighter object (planets & possibly double stars) or are imaging and can gather light for longer from a smaller aperture to compensate then you may benefit from the extra contrast you get by avoiding the central obstruction and going for a refractor.

Of course once you get into large diameter reflectors you have to consider the ratio of aperture to focal length which, as the ratio gets lower and the cone of light you are focusing gets fatter, causes problems such as coma and brings about the need for more expensive eyepieces or just comproming on the views. Personally I'm a reflector guy and wouldn't pass up my 14" dob for anything but a bigger reflector.

[brianb]

can a reflector scope show as much detail as a refractor?

For the same aperture? IMHO, yes. But everything depends on the optical quality. You tend to get what you pay for.

[michael.h.f.wilkinson]

For the same aperture? IMHO, yes. But everything depends on the optical quality. You tend to get what you pay for.

I would qualify that. If you have a very fast primary mirror (like my C8's F/2 primary), you need a large central obstruction and this does not just lose light, it lowers contrast and resolution. However, a slower 6" F/8 Newton has a small central obstruction and is sometimes refered to as an apo-killer, i.e. it has a performance almost as good as a apochromatic refractor for a fraction of the cost. I believe my old 6"F/8 was almost as sharp as my new 8" SCT, but the latter gathers far more light.

However, for a given amount of money you can indeed buy a reflector of twice the aperture of a refractor (certainly in th ecase of an apo), and the reflector will beat the refractor in resolution. However, the contrast will tend to be lower.

[brianb]

a large central obstruction ... does not just lose light, it lowers contrast and resolution.

The effects are very marginal. The old adage about refractors giving better contrast than reflectors was explained by the central obstruction but was in fact more to do with the lower amount of light scatter in refractors in the days before broadband multicoating became the norm.

A central obstruction does indeed divert some of the light from the outer part of the central Airy disc into the diffraction rings, thereby causing a small contrast loss, but the obstruction has to be huge (over 50% by diameter) before the effect is obvious to the eye. The central disc is actually reduced slightly in size, increasing rather than decreasing the resolution, though the effect is so small that you're unlikely to be able to measure it in the real world.

I would expect a SCT or Mak to have marginally lower contrast & light grasp than either a refractor or a Newtonian of the same aperture because of the extra optical element - scatter & absorbtion in the corrector plate dwarfing the effect of the central obstruction.

It's perfectly possible to make off-axis compound reflectors (with a clear, unobstructed aperture) and, if there was a significant performance advantage in doing so, such instruments would be on the market. Fact of the matter is, you see the very occasional ATM project, but no commercial offerings.

Meanwhile, just look at the many, very fine, high resolution images of low contrast targets (planetary surfaces) made with SCTs with central obstructions of the order of 30% by diameter....

I say again, these days the design hardly matters in comparison with the optical accuracy of the construction (and the quality of the coatings used).

[michael.h.f.wilkinson]

True, the build quality, and optical perfection are paramount. The "Schiefspiegler" or Kutter telescope I assume you refer to had problems of its own, in particular coma and astigmatism, due to its off-axis design. I studied the design a while (which was quite popular for a while her in the Netherlands, but it was impractical. It could generally only be made with slow F-ratio (F/22 to F32 was common) which did not help. The other off-axis designs have similar problems.

I always favour good Newtonians for their aperture/price ratio, and SCT, Maks or RC designs for their compact build.

[Leegsi]

Ideally you want one of both.

One for planets and one for deeper/fainter stuff

[haitch]

It's perfectly possible to make off-axis compound reflectors (with a clear, unobstructed aperture) and, if there was a significant performance advantage in doing so, such instruments would be on the market. Fact of the matter is, you see the very occasional ATM project, but no commercial offerings.

Why go compound? - That's CLANTS to you!

Orion USA Reflecting Telescopes

Someone thought avoiding obstruction in a reflector was commercially viable.

[brianb]

Someone thought avoiding obstruction in a reflector was commercially viable.

Thanks for pointing this out. I wasn't aware of it.

However I'm not surprised it hasn't taken off. The long focal length (f/12) suggests that the primary mirror is spherical rather than an off-axis paraboloid, and the 91mm aperture rules it out as a serious planetary instrument (too small). I like the oversize tube with excellent ventilation though.

[haitch]

Yeah, I knew about them but would have never considered them - not enough aperture and imagine trying to collimate that thing!!!!

"Now let me see if I did have a centre spot it would be ummm somewhere there."

If you are only going for something that size I would have thought you'd get a reasonably colour corrected refractor or a nice little mak for somewhere in the ball park price.

I first spotted that over a year ago and have barely seen a mention of them since so obviously they're not well thought of.

Just thought I'd bring it to the discussion table

[brianb]

imagine trying to collimate that thing

If it is a spherical mirror, no issue as there is no "centre", just line the thing up well enough that the secondary diverts all the light from the primary to the centre of the eyepiece tube and the primary is seeing reasonably coaxial with the tube. You can hardly go wrong at f/12 anyway!

In any case, final collimation is done by examining the diffraction pattern. People who depend on laser collimators copy the collimation error of the tool into their "accurately collimated" scopes then wonder why they only get soft images. The best that can be done with a tool - be it a simple one like the cap, which works well enough at f/8, a Cheshire eyepiece or a laser collimator - is to get somewhere into the ballpark.

[haitch]

Oh yes. Of course - spherical surfaces by their nature have no centre.

[Macavity]

From posting experience, I sense it easier to look at practical examples of 'scopes, and compare their individual (de)merits, rather than (albeit interesting!) "asymptotia" - Moreover reflect on your own real [iMO, dominant] PERSONAL (physical, financial etc.) limitations?

I suspect, give a modicum of experience, a significant amount of reading (between the lines?) and a knowledge of my practical (local site etc.) limitations, perhaps a 6", F5 triplet APO, weighing around 15lb (with optional GoTo) would suite me just fine, but...

There seem to be two (legitimate) approaches: Buy SOMETHING, experience it, then either sell it on (If you don't like it, or can't fund it!) OR try to "improve" a particular scope to it's maximum potential. I'm not sure either is the route to lasting finacial prosperity though...

[Macavity]

I think ultimately one has to look at practical examples of 'scopes, and their individual (de)merits - And moreover include [iMO, dominant] PERSONAL (physical, financial etc.) limitations?

I suspect, give some experience, a significant amount of reading (between the lines?) and a knowledge of my practical (local site etc.) limitations, perhaps a 6", F5 triplet APO, weighing around 15lb (with optional GoTo) would suite me just fine, but...

There seems to be two (legitimate) approaches: Buy SOMETHING, then either sell it on (If you don't like it, or can't fund it!) OR try to "improve" a scope to it's maximum potential. I'm not sure either is the route to lasting finacial prosperity though...

[pvaz]

My personal take on this is the following:

If you have a general interest to observe any astronomic object and you don't want to spend a fortune, then a reflector will suit you better since they are cheaper and give a good performance on any target.

If you have no funds limitation or you specifically want to observe solar system objects then another scope might suit it better.

As said before, the best of both worlds is to have one of each.

My personal choice was to get a decent, but still portable, reflector (8" dob). Now I'm focused on getting the best of it so I'm upgrading my EPs to televue and also got a hotech collimator. I want to squeeze every last photon I can get from my current scope. In the future moving on to astrophotography or getting more light power is something to consider. I think in the light gathering department a 16" dob is the cap for me. Anything above is just too much hassle.

[brianb]

There seem to be two (legitimate) approaches: Buy SOMETHING, experience it, then either sell it on (If you don't like it, or can't fund it!) OR try to "improve" a particular scope to it's maximum potential. I'm not sure either is the route to lasting finacial prosperity though...

If you can't fund it, then buying new is not the way to go as there is such a drop in value when you sell it on.

Buying second hand is a great way for people who have some experience to get their second scope at a great price. But you do need the experience to make sure you're not being handed down a lemon.

High end accessories are a great way of splurging money that might be better saved & used on a OTA upgrade. Or a better mount. The difference between a mid price EP and the TV Nagler / Pentax XW is not that huge in performance terms, but you can easily blow a thousand or two extra in a set of high end eyepieces. OTOH the difference in performance between an 8" scope and a 12" scope with similar quality optics is huge, and the difference in observing convenience between a wobbly mount and a nice solid one is immense.

Of course practical considerations come into the issue - what's the biggest scope you can cope with physically - that's why my main scope is a CPC1100. And will remain so until I can afford a 20" or 24" Dob on an equatorial platform with accurate tracking.

[ombos]

This a very simple view: refractor = around F7 nice and crisp views for close objects and best of all stars, good all so for some nebs. No or little maintenance, weakness DSO. You will find that most people have one of each type. Reflector lots of scope for your cash, pain to lug about and set up from time to time, fast around F5 and sometime faster, can be demanding on eyepieces. IMHO buy a nice second hand refractor you just want go wrong, dont get a cheap one or they will give you false colour and other undesirable effects. Now if you only want one scope (depending again on want you want it to do) consider a SCT, I dont know your budget but once again hold out for a min 8" (theres some cracking deals on the 9.25") which again IMHO is a scope for life. Then do you want to take pics?? it just goes on and on my friend, but a nice 4" refractor is a scope for life. At star parties everyone wants to look through my tak and I go for the sct 11 or 9.25?????? I dont even set up the 250mm its just too much trouble to take about, so really think about it or you'll be swopping and loosing money.

[Eyeball]

This a very simple view: refractor = around F7 nice and crisp views for close objects and best of all stars, good all so for some nebs. No or little maintenance, weakness DSO. You will find that most people have one of each type. Reflector lots of scope for your cash, pain to lug about and set up from time to time, fast around F5 and sometime faster, can be demanding on eyepieces. IMHO buy a nice second hand refractor you just want go wrong, dont get a cheap one or they will give you false colour and other undesirable effects. Now if you only want one scope (depending again on want you want it to do) consider a SCT, I dont know your budget but once again hold out for a min 8" (theres some cracking deals on the 9.25") which again IMHO is a scope for life. Then do you want to take pics?? it just goes on and on my friend, but a nice 4" refractor is a scope for life. At star parties everyone wants to look through my tak and I go for the sct 11 or 9.25?????? I dont even set up the 250mm its just too much trouble to take about, so really think about it or you'll be swopping and loosing money.

Will someone tell me what IMHO means before I go nuts!!!!

[michael.h.f.wilkinson]

In my honest opinion

(afaik )

[Capricorn]

Alternative: In My Humble Opinion.

Take your choice.:D

[Eyeball]

In my honest opinion

(afak )

Thanks for that Michael, it was driving me to distraction.

[Eyeball]

Alternative: In My Humble Opinion.

Take your choice.:D

For novices like myself it is not always clear what an abbreviation means. Where the first instance of an abbreviation is used, its full meaning should be stated: In My Humble Opinion (IMHO) etc...

A little thought would make it so much easier for newcomers.