Reflector vs. Refractor - Differences?

prova · #1 24-04-2008, 08:27 AM

like to know for my own information, what the fundamental differences are between the two in regards to:

Visual observation of the planets and DSO's

and

Astrophotography

good and bad's of both types of scopes basically
thanks

Craig.a.c (Craig) · #2 24-04-2008, 10:22 AM

Being a relative newby to astronomy, I would think it comes down to personal preferance really.

programmer · #3 24-04-2008, 10:29 AM

Refractors give more contrast and I think sharpness, but can give false colouring (chromatic abberation). There are limits with reflectors (obstructed by secondary mirror) with low power widefield views. Coma more prevalent in reflectors I believe (affects edges of view, mainly in 'fast' scopes). Also depends a lot on your focal length, f/ratio and eyepieces, not just basic reflector vs refractor design. Can't speak for astrophotography.

ving (David) · #4 24-04-2008, 10:48 AM

you can easily get a reflector of 16" in most astro stores. try and get a 16" refractor in the sasme place....

cost. inch for inch the reflector is much cheaper.

Kokatha man · #5 24-04-2008, 11:36 AM

Quote:

Originally Posted by ving

you can easily get a reflector of 16" in most astro stores. try and get a 16" refractor in the sasme place....

cost. inch for inch the reflector is much cheaper.

No problemo! Just wave your 16 figure bank balance in front of their faces.....watch 'em move....!

I have this thing about refractors; but I've got my reflector for real aperture.....

dannat (Daniel) · #6 24-04-2008, 01:03 PM

Prova,
I think reflectors being bigger capture fainter objects - like dso's- though one thing not in their favour is colimation. Most refractors are way to expensive cf. similar aperture, though for the same magnification they give tack sharp views provided they are made with good quality glass.

Most i think find refractor's easier for astrophotography of dso's, as they usually have shorter focal lengths - less imaging time required, & the reflectors with short f/l tend to have coma problems, which need to be adjusted for.

For photography of s/system stuff most use reflectors - as they can give the magnification of the planets etc they are after.

prova · #7 24-04-2008, 01:13 PM

Quote:

Originally Posted by Kokatha man

No problemo! Just wave your 16 figure bank balance in front of their faces.....watch 'em move....!

I have this thing about refractors; but I've got my reflector for real aperture.....

so what would looking through a 4" refractor be the equivelent of a 4" newtonian, is that how it works?
to be precise, im looking at the skywatcher 100ED pro opposed to an 8" reflector ..

if it's a case of seeing a little less with the 100ED but getting more clarity on the image .. fine

Kokatha man · #8 24-04-2008, 01:55 PM

Quote:

Originally Posted by prova

so what would looking through a 4" refractor be the equivelent of a 4" newtonian, is that how it works?
to be precise, im looking at the skywatcher 100ED pro opposed to an 8" reflector ..

if it's a case of seeing a little less with the 100ED but getting more clarity on the image .. fine

Hi Provo - I think my thing about refractors is an "oldies" phenemenon more than anything else: the 8" newt would definitely outperform the 4" refractor of whatever lens system (achro/apo.)

However, the 4" refractor would definitely outperform the 4" newt: forgetting about other aspects that could start a raging "war" if I go too far here, there is the simple issue of "clear aperture." Simply, the refractor doesn't have to contend with a secondary mirror in its' light path.

Refractor afficianados will speak of clarity of definition/contrast, especially re planets: I have a 6" f8 refractor, only an achromatic (2 doublet objective lens cells, adjustable) but even without the Baader "semi apo" filter, CA (chromatic aberration) was almost non-existent - with this filter in, I can confidently say it is virtually non-existant. And I've seen my fair share of achro refractors with their share (and more) of CA!

The Synta job I have gives me absolutely great views (I call him "Long Tom") and I am almost about to christen a special pier I have just about finished for him: there is a review of one of these particular refractors (not mine!) in the "Articles, Reviews etc" section of IIS where the frequently used description of these refractors as "planet-killers" is used to describe its capabilities!

Long Tom really works a treat on Jupiter and Saturn (and no, with the Baader in I get no CA with Venus) and I darn well reckon it'd give most 8" newts more than a run for their money imho!

ED's and "true apochromatics" are wondrous sharp and clear in their definition: just ask some of the lucky owners of these units what they think!

The longer focal length refractors (and if they're only achromats the general consensus is that longer f/l reduces CA) are a bit of a pain when obbing the zenith (you're on your hands and knees almost) but my special ergonomic designed pier (Pierre!) will sort that all out.

As I said, I think it's because when I was young the only refractors were cheap and really nasty "department store" types or hideously expensive astro units: I could (and did) grind my own mirrors and make a newt of good aperture (6" & 8") but there was allways that allure of owning a quality decent sized achromatic refractor. (ED's and true apochromatic weren't invented then!)

Notwithstanding any of the above, price and light-gathering/aperture competitions with reflectors leaves the refractors for dead, particularly if you want state-of-art apochromatics: however, this old fella is still going to purchase another "Long Tom" in the near future (along with an upgrade from HEQ5 to EQ6) for his already designed 6" achromatic binoculars....!

Cheers, Darryl.

prova · #9 24-04-2008, 02:58 PM

Thanks heaps Darryl ..

dannat (Daniel) · #10 24-04-2008, 03:10 PM

You should find someone in ACT who has both or either and check for yourself - there are a few ACT IIS'ers who you could advertise for a free look

prova · #11 24-04-2008, 03:13 PM

Quote:

Originally Posted by dannat

You should find someone in ACT who has both or either and check for yourself - there are a few ACT IIS'ers who you could advertise for a free look

funny you say that because I was just thinking the same thing ..
*goes off and creates another post*

lknowlen · #12 25-04-2008, 06:58 AM

The most popular telescope system that is used by most amature astronomers is an 8" F/10 Schmidt Cassegrain telescope which incorporates the best qualities and features of both the refractor and the reflector. The 8"F/10 is best also for astro-photography. If you want to become a serious lunar and planetary observer a nice 4 inch refractor may be a good starting system. For deep sky either a 6" or 8" reflecting telescope is a good choice because you get a lot of aperture for low cost. In my opinion the best way to make your selection or gain information is to view through a variety of telescopes at a star party. A good book to help explain telescope differences is "Night Watch" by Terence Dickinson. Hope the information is helpful to you.

AstralTraveller (David) · #13 25-04-2008, 04:37 PM

In terms of aperture for dollar you won't beat a reflector, but they take more maintenance than the other designs. Commercial jobs often require a bit of tweaking to get the best out of them. Thing you may want to do include: a tube extension to deal with stray light and dew; flocking or otherwise blackening the inside of the tube; light baffles in the tube (there are pros and cons with these two ideas - see other threads); cooling fan; and light baffles behind the primary. None of this is difficult.

Do that and a 20cm newt will beat a 10cm refractor comfortably for DSO and it should also be better on planets. Having said that my 15cm refractor (achromat) was slightly better than a GSO 25cm newt on Saturn the other night but it was close and I think the newt was over-magnified.

Having used reflectors all my life I found the chromatic aberration on my 15cm f/8 Synta to be unacceptable (unlike Kokatha man) but a Baader fringe killer fixed that.

There is a view that newts must have poorer contrast than refractors. Modeling I saw years ago in S&T indicated that that is only true when the central obstruction in larger than about 20%. Short ratio newts typically have about 25% obstruction. If you fit a small secondary to one of these you will have a narrower usable field of view, probably too narrow. The alternative is to have a longer ratio scope. Then you can have a smaller secondary without sacrificing field of view. For instance my 25cm f/8 newt optics with a 5cm secondary still have a usable fov of at least 1.2 degrees. The problem is the 2m tube length!

It's all swings and roundabouts I'm afraid.

mark3d · #14 25-04-2008, 07:52 PM

up until the eyepiece all a reflector or refractor does is gather the light and focus it into a small area. so objectively there should be no difference. but like guitars made of different wood that apparently sound different, the light might appear to be different

Kokatha man · #15 25-04-2008, 08:13 PM

Quote:

Originally Posted by mark3d

up until the eyepiece all a reflector or refractor does is gather the light and focus it into a small area. so objectively there should be no difference. but like guitars made of different wood that apparently sound different, the light might appear to be different

(my embolding in your quote) - I take it that's an intended pun Mark....!?! And don't those different guitars actually sound different....!?!

Cheers, Darryl.

mark3d · #16 25-04-2008, 08:26 PM

lol i didnt notice the pun!

and actually the guitars are probably a bad anaology.. they probably do sound different .. different wood would vibrate differently and produce different sound.

focused light should be just light.. but im prepared to accept there might be a difference for some reason

fraunhofer · #17 26-04-2008, 12:55 AM

Refractors get pretty big when you get to large aperture but oh the views they can give. Please enjoy the enclosed photos of several telescope systems

Kokatha man · #18 26-04-2008, 08:16 AM

Great images fraunhofer: I particularly liked the very portable one by the lakeside - I could see myself slinging this one over my shoulder to take up onto my observing deck!

Thanks for those, any more details on specifications and vintage etc?

Regards, Darryl.

fraunhofer · #19 26-04-2008, 01:46 PM

Yes if you would like to know more abut the refractor pics, please visit the website Clintstars.com You can also GOOGLE: "Zeiss refractor"

csb (Craig) · #20 26-04-2008, 04:34 PM

Quote:

Originally Posted by mark3d

up until the eyepiece all a reflector or refractor does is gather the light and focus it into a small area. so objectively there should be no difference. but like guitars made of different wood that apparently sound different, the light might appear to be different

That doesn't seem quite correct, because newtonians, SCT's, etc have an obstruction directly in front of the light path - secondary mirror.

The secondary mirror has slight detrimental effect on the contrast of the image. This seems to be more noticable in the smaller telescopes - perhaps upto 6".

So if compare a 4" refractor side by side with a 4" SCT, the refractor will have higher contrast and some sky objects will show better.