Why would I want a refractor?

drmorbius (Randall) · #1 06-08-2006, 07:30 PM

This is a rhetorical question and not meant to offend anyone or ignite a flame war, but this has perplexed me for a while now.

While researching my first scope (an 8" dob), the major rule seemed to be "aperature rules"... the more light you get in the better off you'll be when peering into the cosmos.

So if I can buy a newtonian reflector with 200mm of light gathering power (albeit a reversed image) for $400, why would I want to buy a 90mm refractor at over twice the price? Overall, refractors seem to be much more expensive and to my limited thinking, you get less scope???

Obviously there's more to this equation than simply light-gathering power? Thanks in advance for your enlightenment...

mick pinner · #2 06-08-2006, 08:00 PM

to me it's the quality of the view, large aperture will help you see deeper into the sky but not necessarily give you a better image.
you have to deciede how deep you want to see and what you want the quality of the image to be, a good quality refractor will give you much better views of the nearer objects than a cheap reflector but will lack deep space power.

yagon · #3 06-08-2006, 08:28 PM

An issue for some people (myself included) is cooldown time: a refractor has a short cooldown time compared to CATs and dobs.

My observing tends to be for half an hour after work during the week: very casual. If I had a CAT I'd observe half as often.

R

mickoking · #4 06-08-2006, 09:16 PM

A refractor also has the best contrast of any other telescope design due to the unobstructed optic's (ie no mirrors in the way). My Dob and refractor are a good double act

bojan · #5 06-08-2006, 09:25 PM

Also, refractors are mechanically stronger and more stable in general.... Easier to collimate and they keep the collimation much longer...
With really good lenses, contrast is better (no central obstruction, wich hampers the image slighty).
IMO, the refractor vs reflector is non-issue.. some people simply like refractors, and if they can afford them, it is OK. It is like the difference between the ride in Ferrari and Toyota: both have 4 wheels and will take you from A to B, but it is cool to ride in Ferrari....
But the difference between 200mm reflector and 90mm refractor is huge.... refractor of this size is not worth even considering if you can get 200mm for the same money.
Go for 200mm reflector (or bigger).

Starkler (Geoff) · #6 06-08-2006, 11:25 PM

Quote:

Originally Posted by mick pinner

a good quality refractor will give you much better views of the nearer objects than a cheap reflector but will lack deep space power.

People tend to spend a lot of money on refractors but prefer their newtonians to be cheap

If one was prepared to spend half the asking price of a tak fs102 and build an 8 inch newtonian with premium optic components and then compare, I wonder how much the quality of view would differ?

Gargoyle_Steve (Steve) · #7 07-08-2006, 12:45 AM

Randall as you know it's all a matter of what suits each individual and meets their requirements. Refractors with higher f ratios typically than our dobs will tend to give higher magnification easier ... but again the light gathering capability is much lower so the image itself is fainter.

If I can borrow bojan's example of above of a Ferrari and a Toyota - yes the Ferrari is much cooler, but if the Toyota is a Landcruiser for instance it can go places, do things, carry loads (inc multiple large bore telescopes) that the Ferrari never will. I'll take "useful and functional" every day over "cool".

Thats why I have a dob .... and a 4wd!

bojan · #8 07-08-2006, 08:25 AM

Quote:

Originally Posted by yagon

An issue for some people (myself included) is cooldown time: a refractor has a short cooldown time compared to CATs and dobs.

My observing tends to be for half an hour after work during the week: very casual. If I had a CAT I'd observe half as often.

R

I am not sure about shorter cooling time of the refractor.... The tube of the refractor is closed on both ends, so there is no air circulation to help cooling down... Of course, if the tube is made of metal (usually the case for older instruments, I am not sure about these days), that may be advantage over cardboard or fiberglass tubes usually used for reflectors.
On the other hand, the reflector, having a tube open at both ends, should reach thermal equilibrium faster, but because the mirror is usually much thicker than equivalent lens, it will take more time.
So, all this depends on many details of the design of particular instrument.

iceman (Mike) · #9 07-08-2006, 08:31 AM

Quote:

Originally Posted by Starkler

People tend to spend a lot of money on refractors but prefer their newtonians to be cheap

If one was prepared to spend half the asking price of a tak fs102 and build an 8 inch newtonian with premium optic components and then compare, I wonder how much the quality of view would differ?

Very good points from wise ol' Geoff there.

If you're on a budget, the best value in terms of aperture and quality optics is still an 8" dob.

You can spend a lot more, if you get servocat, argonavis, premium optics, premium focuser, etc. For the same money as a quality refractor you'll get a lot more in a newt.

acropolite (Phil) · #10 07-08-2006, 08:36 AM

One point that hasn't been mentioned is that refractors are ideal for imaging. Their light weight (for piggybacking) and good optical properties make them ideal for imaging.

OneOfOne (Trevor) · #11 07-08-2006, 01:20 PM

I guess another point is portability. If you buy a 100mm refractor, it is a "nice" scope, and pretty portable, while an 8 inch dob is less so. But then again, for the cost of a 100mm refractor, you could get a 250mm dob and have money to spare. Meanwhile, a 100mm reflector (would have to be an EQ1 for that sort of size) would be not much more than a toy! Once a refractor gets much above 100mm, it starts to get pretty big....an 8 inch refractor would need a truck...but the truck would cost a lot less!

I guess this reinforces Starkler's view, you may be able to justify $500 on a dob and get a pretty big bucket. If you were interested in a refractor, you may be able to stretch the budget to a couple of grand and get a bucket that will hold a quarter of the photons! Four times the money to get a quarter of the light....factor of 16x. Mind you, I wouldn't say no to a nice "little" 120-150mm apo...would you?

Satchmo · #12 07-08-2006, 01:29 PM

Apart from the portability issue, I don't understand the refractor /reflector debate.

If you want to see images as fine as the finest medium aperture apo -refractor, for a similar price, just put a cardboard mask with an 80mm off axis aperture over your 250mm dob, aligned in between the spider vanes. When you are bored with seeing seeing galaxies and nebulae as fairly featureless blobs, just take off the mask and enjoy the aperture .

Yep I'm a bit of a stirrer, but there is a lot of truth in this.

Mark

bojan · #13 07-08-2006, 02:41 PM

Spot on, Mark.
The bigger, the better

ving (David) · #14 07-08-2006, 02:44 PM

not a truer word was said mark

drmorbius (Randall) · #15 07-08-2006, 02:53 PM

Whew... that's a diverse set of answers... let's see if I can distill this down a bit...

If I like to view DSO's with my very own set of eyeballs, then I need as much light as possible and I should get the biggest reflector my money can buy. If I were to pay the same amount of money for a refractor, it would probably be significantly smaller, so there'd be less light, so the DSO would appear much fainter.

But if I was interested mainly in planetary and moonetary(?) viewing, then a refractor would provide me with a better image because of the better contrast.

And if I were into imaging, the lower light level of the refractor doesn't really matter as I would rarley be looking through the eyepiece.

Now assuming what I've said is roughly true... one last question... If I were imaging and therefore the photon collection capability of the scope was not a major factor... would the image typically be better from a refractor? Is it a case of optics quality? In other words, if I wanted the same image quality from a reflector I would end up spending the same amount of money to get similar quality optics (mirrors)?

I know there's no such thing as a dumb question, but I must really be pushing the envelope here...

thanks everyone -- randall

bojan · #16 07-08-2006, 03:02 PM

Quote:

Originally Posted by drmorbius

If I were imaging and therefore the photon collection capability of the scope was not a major factor... would the image typically be better from a refractor? Is it a case of optics quality? In other words, if I wanted the same image quality from a reflector I would end up spending the same amount of money to get similar quality optics (mirrors)?

I know there's no such thing as a dumb question, but I must really be pushing the envelope here...

thanks everyone -- randall

Lets get it straight: Mirrors are NOT inferior.
They are cheaper because there is only one surface to be figured, polished and aluminized.
For lens there are at least 4, or 6, depends on the design.

For imaging, if you are into wide field, the design to go for is Catadioptric, the combination of mirrors and lenses, to correct for various problems simple optical systems normally have.
And the reduction in contrast is very small, really.

But, the size does matter.. because with bigger aperture your exposure time is shorter.
And. the size is important for resolution as well. Bigger aperture, better resolution (of course, there is a limitation here, atmospheric turbulence will limit the resolution, but the stacking of number of frames will sort this problem to the certain degree).

So, there is no issue here: go for the biggest mirror you can afford, money wise and transportability wise

ving (David) · #17 07-08-2006, 03:14 PM

hi randall,
dont go giving kyourself a headache there

basically the bigger the aperture the more detail you will see... planets or DSOs, astropics or visual.... thats about it.
more aputure = more resolution.

janoskiss (Steve H) · #18 07-08-2006, 03:19 PM

1. DSO fainter in smaller aperture: YES!
2. Refractor better on planets because of contrast: in general NO, but for same aperture (and a lot more $) YES. Also see Mark's suggestion re use of off-axis aperture mask: same as unobstructed smaller aperture and with bit of care will give you the performance incl. high contrast of a small-to-medium apo.
3. Aperture is mainly for visual not for imaging? In general, NO, but it deepends what you want to image. you still need aperture if you are going to image small things, e.g. detail on the surface of planets. Resolution is always limited by aperture due to diffraction. Features smaller than the resolving limit of the scope will be completely blurred out no matter how long the exposure. (Resolving limit in arc seconds ~= 116/aperture in mm.)

As I see it, imaging with a small refractor is a matter of convenience. As so many of our members have shown, they are great for wide field astrophotos where the resolving limit is not an issue. And you won't need to spend a fortune on the mount for good results as you would with a large Newt or Cassegrain.

btw. Hubble is a reflector with as much aperture as NASA could afford at the time.

Dennis · #19 07-08-2006, 03:21 PM

Hi Randall

Just thought I’d jot down some of my experiences.

Imaging with my 4" f9 (918mm fl) Vixen ED refractor on my Vixen GPDX mount is very pleasurable, forgiving and feasible. Wide field, DSO and hi-res can be done with this set up without too much angst.

For long exposure DSO imaging with a C9.25 at f10 (2350mm fl) on my Takahashi EM200 mount, it is more of a black art. A permanent set up would help reduce the long list of niggling, tuning, variables I seem to struggle with every time I set up in the back garden or go to an astrocamp.

So, if you want to enter into the realms of high resolution, long focal length or long exposure (=auto guided) imaging, the mount requires as much serious consideration as the 'scope.

Cheers

Dennis

rmcpb (Rob) · #20 07-08-2006, 03:22 PM

Quote:

Originally Posted by drmorbius

Now assuming what I've said is roughly true... one last question... If I were imaging and therefore the photon collection capability of the scope was not a major factor... would the image typically be better from a refractor? Is it a case of optics quality? In other words, if I wanted the same image quality from a reflector I would end up spending the same amount of money to get similar quality optics (mirrors)?

A smaller refractor will work BUT there is a formula for the resolution of a scope and its based on the aperture, the larger the aperture the finer resolution possible. This extends to better resolution in your final image from a larger aperture, whether its from a refractor or reflector, and this means a better image. Size matters!!

Another important factor is that a larger aperture will be gathering more light in a given time so your exposures will be shorter for a larger aperture. This will give you a better chance of grabbing more images in times of good seeing in a larger scope than a smaller scope allowing for a better image and to actually get to use the better resolution of the scope. Again, size matters

After saying that whatever scope you use most is the best.

Cheers