Why would I want a refractor? [Archive]

drmorbius

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. :astron:

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??? :shrug:

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

mick pinner

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

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

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

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

06-08-2006, 11:25 PM

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

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

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

07-08-2006, 08:25 AM

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

07-08-2006, 08:31 AM

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

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

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

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

07-08-2006, 02:41 PM

Spot on, Mark.
The bigger, the better :D

ving

07-08-2006, 02:44 PM

not a truer word was said mark :)

drmorbius

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... :P

thanks everyone -- randall

bojan

07-08-2006, 03:02 PM

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

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

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

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

07-08-2006, 03:22 PM

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

xstream

07-08-2006, 03:26 PM

Randall, The points Mark and bojan have stated in their previous posts are part the reason why Anna and Petra have ordered a 20" Suchting mirror.

You just can't beat aperture for dollar value.

Dennis

07-08-2006, 03:27 PM

Hi Randall

Here are a couple of example images:

M16 through Vixen 4" f9 ED refractor at prime focus.
M16 through Celestron C9.25 with F6.3 Reducer/Corrector.

Both with the same ST7E ccd camera.

Cheers

Dennis

mickoking

07-08-2006, 03:59 PM

I reccomend owning both (if the budget extends that far) As Refractors and reflectors have their own strengths and weaknesses.

As a young kid I always wanted to own a large sleek refractor but in the late 70's I these types of scopes were prohibatively expensive. Now days we live in a golden age of amateur astronomy we can own all types of goodies ;)

It is also my experience that a refractor will reach thermal equalibrium a lot quicker than a large Dob and is certainly more forgiving of bad seeing.

ballaratdragons

07-08-2006, 04:14 PM

And don't forget that to the Layman a Refractor is a 'real' Telescope.

A Dob is a water heater.

Perfect example: I had my 12" dob set up in the front yard one night to check out the low Eastern Sky. A neighbour walking past asked what I was doing. I said "just using my telescope to see down low over that way, would you like to have a look?" After umming and ahhhing he finally relented and looked into the finderscope!!!

When I said that's the finder and pointed out the Dobs Focuser he said: "Oh, I wondered why you had the Telescope stuck on a Water heater tank" !!!!

Nuff said.

IanW

07-08-2006, 04:17 PM

Been there done it a few times over. The key to building an APO killing Newtonian is very simple, don't build a jack of all trades short focal ratio Newtonian. Building a planetary Newtonian with an 8" f.8 to f.10 is easy and you can obtain a superb figure to the primary mirror without much effort. Throw in a slightly undersized diagonal and you'll end up with a 15% or so central obstruction and that is for all intents and purposes invisible and lowers the contrast by so little that from a good dark site it's hard to notice it.

The only hard bit is finding a reasonably priced mount that can swing a long tube without effort.

mickoking

07-08-2006, 04:25 PM

300mm, f15 newtonian. Now that would be an awsome bit of kit :thumbsup:

Striker

07-08-2006, 06:12 PM

heheh...300mm F15...that would be about 5m long...lol...I think you mean F5.

[1ponders]

07-08-2006, 07:10 PM

I don't know Tony, an f15 300 newt would be an absolute planet killer. Man the image scale you could achieve with that would be awesome :eyepop:

drmorbius

07-08-2006, 07:29 PM

Thanks everyone... I think I understand. I'm content with my 8" dob and won't rush out and spend lots of money... for now. Building my own scope with a 20" mirror sounds like awesome fun!

[1ponders]

07-08-2006, 07:45 PM

There is another reason Randall that no-one has mentioned, and seeing as I am working towards it I will bring it up :P

It's called filling the quiver. You know sort of like making sure you have the right fishing rod for every situation. I have 4 refractors in different sizes and f ratio, plus an SCT, now I'm working towards a couple of newts, so I never find myself in that situation of never having the right scope for any given situation :lol:

Paul
(AKA Striker Junior)

mickoking

07-08-2006, 09:13 PM

Nope, I did mean f15 ;) If I was rich, a large Newt f10-15, in a purpose built observatory would be nuts :thumbsup:

AstroJunk

07-08-2006, 10:07 PM

In answer to the original question, You want a refractor because it's a long way to go up an 8' ladder only to find the dob is pointing in the wrong direction!

(photo borrowed from another post)

janoskiss

07-08-2006, 10:08 PM

Here are some of my observations relating back to earlier comments re big fast dob vs smaller slower dob and poor man's apo ie off-axis masked newt:

I just came in from observing Jupiter and the Moon and a few doubles with my f/5 12" and f/6 8". Seeing was lousy to average. To the best of my ability to judge both scopes were sufficiently cooled.

Now, the 12" showed no advantage and as Ken noted earlier the view was qualitatively better, i.e. more pleasing to look at but showing no more discernable detail, in the 8" scope. This I figure to be due to the atmosphering blurring of the image being more noticeable with the increased brightness of the larger scope. This makes the limb of the planet appear less sharply focussed in the larger scope at the same power. Chucked a variable polariser on the EP to reduce the brightness and tweaked it till the image looked about as bright as the small scope (both at about 120x) and voila, the image in the bigger scope looked as good as in the smaller.

Also tried the off-axis circular aperture mask (60-70mm) that Mark mentioned on both scopes at powers in the range of 50-120x and indeed it is very much like viewing through a small apochromat. (I don't own an apo any more but I used to have an ED80 so I have some idea.) The brightness is drastically reduced of course, but the spider diffraction spikes are also gone and contrast appears significantly higher with a much darker sky background also. The effects of bad seeing are virtually unnoticeable (because the small aperture cannot resolve the motion of the atmosphere). You quickly run out of light and resolution though as you up the power.

asimov

07-08-2006, 10:19 PM

Hah!

Give me a refractor anyday...A real one I mean!

http://www.iceinspace.com.au/forum/showthread.php?t=2920

avandonk

08-08-2006, 12:29 AM

While all you people are arguing, the rest of us will be looking and exposing.

Bert

shahgazer

08-08-2006, 12:58 AM

With the recent trend of video astronomy, and the use of integrated videos such as Stellacam & Mallincam, we can achive quiet a remarkable depth even when we use refractors or any other smaller aperture telescopes. Of course, you have to get use with the additional power cables, video cables and other tangling stuffs in the dark! ;)