Refractors or Reflectors?

[gurney]

I have to be honest - I'm confused!

I thought refractors were best for planetary work, and reflectors were best for DSO's!

If so - what are reflectors best at then? And why do more people have reflectors if refractors are so good, easier to set up and easier to maintain?

I must be missing something! (no surprise there then!)

[Kaptain Klevtsov]

Reflectors are, size for size, much cheaper than refractors. Even SCTs are cheaper, though not by as much.

The down side is that you lose contrast to a degree because of the central obstruction which is part of the reflector design, so its not win-win with a reflector.

Better to be able to image something at low contrast (that's what Photoshop is for) than not at all though.

Kaptain klevtsov

[supernova]

If your a looker, like me, then Dob or reflector

[gurney]

My primary aim is definitely for astrophotography and CCD imaging for spectroscopy and so on.

As far as resolution goes - aren't reflectors better due to a larger aperture - or does the law work differently for refractors? (Can't imagine why it would but since I'm rather rusty on all this I thought I should ask!).

Basically I'm looking to get back into the sciencey side of things, and hooking up to a computer for imaging and spectroscopy. That's my main aim. I'll be wanting to study DSO's AND I'll be wanting to do some planetary observations. In fact I'll be wanting to do a bit of everything to be honest - but ultimately I'll be wanting to get very geeky, and not just simply observing stuff for the sake of it (that isn't meant to sound derogatory by the way - there is no greater pleasure than gazing at the heavens!)

So are you saying that with enough money - refractors are better than reflectors?

In terms of optics - what is the equivalent sized refractor in terms of resolution and limiting magnitude of your average12" reflector? (...knowing that reflectors are not all equal, having different focal ratios etc etc - but just trying to keep it simple for now)

Sorry for the dumb questions!

[Astro_Baby]

I just like the 'essence' of a reflector myself. Its probably cultural in that when I started in astro years ago ( and then packed it in ) everyone of consequence had a reflector

I can never shake the image of refractors being a bit 'boring'. I know, I know everyone is going to shout that modern refractors are the business etc etc ( and the Williams fans are probably going to lynch me ) but being a woman I am entitled to buy on emotion.

[Kaptain Klevtsov]

In terms of optics - what is the equivalent sized refractor in terms of resolution and limiting magnitude of your average12" reflector? (...knowing that reflectors are not all equal, having different focal ratios etc etc - but just trying to keep it simple for now)

Sorry for the dumb questions!

There are no dumb questions Gurney, dumb answers yes, but not dumb questions.

A nice 8" apochromatic refractor is about equivalent to a 12" reflector, so you'd better start saving your pennies.

WRT spectroscopy, I really don't have a clue. (for discussion) For example, is chromatic abberation no longer a bad thing when you want to spread the colours out? Do you just need to collect loads and loads of light from a single source?

Be interesting to find out such things, so no, its not a dumb question at all. In fact its a rather heavy duty kind of question.

Kaptain klevtsov

[gurney]

Nice one!

Right then - off to check out 8" refractors! *takes deep breath! - and pleased the missus has nipped out for a bit!*

[gurney]

GULP!

I'm thinking I'm not going to get one of those past management somehow! Ah well!

[Kaptain Klevtsov]

Thought not.

Kaptain Klevtsov

[gurney]

Several grand is one thing - but over £15k!!! Woof!

[lunator]

Hi Gurney

To do spectroscopy will probably require a pretty big aperture so the only game in town is the SCT/Newt.

Have a look at the secondhand market as at this time of year itis quite slow and you may pick up a bargain.

Cheers

Ian

[Jeff RV]

You could always try one of these > http://galaxyphoto.com/max.htm

Then if its not up to scratch JUMP ON IT evidently!

Jeff.

[Astroman]

To do deep sky spectroscopy, you need as much aperture as you can get, and a refractor is simply not capable of gathering enough photons in a reasonable amount of time to do the job. The world's largest refractor is what, the Yerkes 40" Clark? That's ~1 meter. You won't find many pros scrambling for spectra from that one, I'm afraid.

Of course, it depends on what kind of spectroscopy you want. Local stars in our galaxy would be fine with a more modest scope, even a refractor of 5"-6". If you just want to class stars or make a catalog of some sort. (It's what I did.) But if you want to measure redshifts of distant galaxies, you'll need at least a meter class reflector.

And here's another thing-I don't know why this myth of reduced contrast from central obstructions keeps being perpetuated. A slow Newt can easily keep up with a slow refractor. Some of the best planetary views I've had have come through a 10" F/6 or F/8, (like my F/7). It's aperture that enables resolution, not the type of scope or central obstruction.

[gurney]

Thanks! That's what I always thought was the case - which is why I was doing some head scratching over the refractors thing. I wondered if I was just missing some important piece of info!

Now then - a meter class reflector. That sounds nice!

I just showed my wife a picture of the 20" Meades and said "can I have one of those please?", to which she replied "Do you think he'd just making up for what he hasn't got in his pants?"

[Astroman]

Concerning KK's question about chromatic aberration in refractors and spectroscopy; it shouldn't make any difference, as you are doing that on purpose anyway. You'll have a greater concern with not enough light, once it's spread.

[gurney]

And on that point I'd have thought a large aperture reflector would be better simply because you've got more photons coming through the aperture due to larger surface area, unless refractors are heaps better at transmitting a much larger percentage of incoming photons all the way through to the focal point than reflectors.

[Astroman]

And on that point I'd have thought a large aperture reflector would be better simply because you've got more photons coming through the aperture due to larger surface area, unless refractors are heaps better at transmitting a much larger percentage of incoming photons all the way through to the focal point than reflectors.

And you'd be right. I was saying you'd have trouble with a refractor as they can't be made as large. A lot of pro research will use images from a smaller, (1-4 meter), scope to find an object of study, then switch to a larger scope, (6-10 meter) for spectra. I get a lot of info from Keck and Gemini for GRB research, (not that I do anything with it, I just see it), while location and discovery images are mostly from smaller stuff. It's all good!

[Kaptain Klevtsov]

there was a link to some guy's website here some time ago, but I'm struggling to find it. Was very interesting as I remember it though. Will post a link if I turn anything up.

Kaptain Klevtsov (putting the wrong words in the search box again)

[Merlin]

The relative differences in performance between refractors and reflectors has been blown-up out of all proportion.

There's very little to choose between them and, as far as I know, when it comes to reflector designs, a well-made Newtonian is superior to any folded- beam 'scope.

SCTs are popular because they can take a lot of attachments.

[gurney]

.... and because they're much more compact, no doubt.

[Sam]

Gurney, For most people the main thing is the price. You can buy a big reflector for not much money (relatively), this means you can spend a bit more on a good mount - which is key for imaging - and the camera of course.

Sam