Refractors vs Reflectors, focal ratio and magnification

[Pat (Patrick)]

Hello IIS members,

My first search was for a forum, and it took me here earlier this week. I have already learnt quite a bit thanks to this site. My first post and I'd like to say a great forum with a great community. Well done!!!
I have never looked through a telescope before. I have my 15 x 70 binoculars on their way from Andrew's. All thanks to the research done here.

My questions-

To what degree are reflectors more suited to DSO's and refractors suited to planet/moon observation?

How does the focal ratio and magnification affect what is viewed when comparing the two scopes in respect to DSO's and the planets?

Is a high focal ratio simply inferior to a low focal ratio? Or does a high focal ratio allow for sharper images at higher magnification compared to a low focal ratio?

I know I should look through a scope to help my understanding and I will, perhaps visit the pony club?... when I understand the sky a little more so I'm not too overwhelmed.

Last questions- how heavy is an 8" and 10" dob and does the OTA detach from the mount? I'm wondering how a dob is transported, back seat/boot/trailer?

So, until I have a chance to see for myself, I await your answers.

Pat.

[RobF (Rob)]

Welcome Patrick. I'll have a go at helping - I expect many will suggest you find a local meeting to join in so you can have a look and learn for youself as well as chat with people who've already made some choices buying gear.

1. You can't really say reflectors/refractors are better at DSO/planetary. Aperture rules for collecting light to see faint fuzzies (DSOs), but also gives you better resolving power. For a same size refractor you generally get much sharper images than the reflector, but then you can buy a much bigger reflector for the same $. Most of the planetary gurus seem to use either 12" or larger reflectors, or 11" or larger SCTs.

2. Focal ratio - for the same size (aperture), the exposure time varies roughly as the square of the FR. So an F5 8" (FL 1000mm) reflector will gather light 4 times faster than an F10 8" (FL 2000mm) SCT for example. Incidentally this is why ED80 refractors at F7 are so highly prized - fast, tight imaging with a great field of view and decent price.

3. Have already started to answer - longer FR not inferior - gives a tighter FOV and longer imaging times, but yes - can be very handy when trying to see more detail on bright objects (planetary).

4. Haven't owned a dob, but remember most of the weight is in the glass and will depend on area/volume of mirror - so 10" has at least 50% more weight than 8". Most people say 10" is largest size feasible for fitting in the back seat of a sedan.

Have fun!

R

[Pat (Patrick)]

Hello Rob, thanks for the reply.

Is exposure time and imaging time the same thing?

What is the difference between long and short? Clarity?

Still very green with all this.

[RobF (Rob)]

The comments on time were really just to compare the speed at which light is gathered. Most people take lots and lots of images to stack and combine for DSOs, so you potentially end up having to take a lot more for a longer total imaging time with a longer FR.

There's no right or wrong about long and short FL's. Like a lot of things in astronomy, its just horses for courses - what things you discover you're interested in (or if you've got the money, just buy one of everything). Short FRs are more of a modern trend, as they're usually more technically difficult to make well. Even so, shorter scopes will usually show up shortcomings in cheaper eyepieces much more than longer FL scopes.

Hope that helps and doesn't make things more confusing...

[Jen]

Hi Pat welcome to IIS
you've come to the right place your gonna love it here
By the look of most of the guys around here they just have both reflector and a refractor

[Terry B]

Welcome Pat
The replies above talk about exposure time etc. This is only relevant for taking photos through the scope. Visual is completely different.
For visual then the light gathering ability of the scope is all important. ie aperture. A 300mm reflector will collect the same amount of light as a 300mm refractor (negating the loss from the secondary mirror in the reflector)
A 300mm reflector will cost you under A$1000. A 300mm refractor would be somewhere near A$20000.
The refractor will have better contrast and slightly brighter image because of no secondary mirror but will cost 20x more and weight 3 times as much.
Essentially to see deep sky objects(with the exception of the few bright widefield objects) you need a decent light collecting ability ~200mm or better. Refractors of this size are prohibitavely expensive so most people opt for reflectors.

The focal length is not terribly important visually because you vary the magnification of the image by changing the eyepiece. However short focal length eyepieces are not as easy to use as the longer ones.
So to see deep sky objects (or planets) that are small and need a bit of magnification, using a longer focal length scope (~1800mm or longer) is easier with a 10mm eyepiece than using a 900mm scope with a 5mm eyepiece.
Bright widefield objects can be seen with a short focal length scope of any design but there are not that many of these to see.

Hopefully this helps.

[RobF (Rob)]

Hmmm - Terry is right - I really did fly off on an imaging tangent there.....

[Shano592 (Shane)]

Pat, welsome to IIS.

If you have a couple of spare hours on Saturday evening, why not come up to the Pony Club?

If the weather holds, everyone up there will be happy to give you advice, opinions, and share the views with you.

[ausastronomer (John Bambury)]

Originally Posted by Jen

[COLOR=darkorchid][B]By the look of most of the guys around here they just have both reflector and a refractor

NNNNNNNNNNNNNNNNOOOOOOOOOOOOOOOOOOO OOOOOOOOOOO !!!!!!!

Jen you have it all wrong

I have two high quality scopes and they are both reflectors. I have another two premium scopes being built as we speak; and they are both reflectors. Refractors are useful. Every one of my four reflectors will have a refractor on it as

"A FINDERSCOPE"

Then again I don't take pretty pictures

Cheers,
John B

[Jen]

well i did say MOST of the guys

[marki]

Thats right Jen, If you are into visual observing, refractors make great finderscopes, simularly for imaging purposes they are best used as guide scopes .

Mark

[Enchilada]

Originally Posted by Jen

well i did say MOST of the guys

I have both. Does that make me astronomically ambidextrous or an unusual metro-sexual male who cannot make up his mind??

[Jen]

Originally Posted by Enchilada

I have both. Does that make me astronomically ambidextrous or an unusual metro-sexual male who cannot make up his mind??

ummmmm do you really want me to answer that
(ok Jen runs and hides now)

[Wavytone]

Some like them short & fat, others like long and slim. And many guys need one of each, when the mood arises !

[Marclau (Marcel)]

Originally Posted by Terry B

Welcome Pat
The replies above talk about exposure time etc. This is only relevant for taking photos through the scope. Visual is completely different.
For visual then the light gathering ability of the scope is all important. ie aperture. A 300mm reflector will collect the same amount of light as a 300mm refractor (negating the loss from the secondary mirror in the reflector)
A 300mm reflector will cost you under A$1000. A 300mm refractor would be somewhere near A$20000.
The refractor will have better contrast and slightly brighter image because of no secondary mirror but will cost 20x more and weight 3 times as much.
Essentially to see deep sky objects(with the exception of the few bright widefield objects) you need a decent light collecting ability ~200mm or better. Refractors of this size are prohibitavely expensive so most people opt for reflectors.

The focal length is not terribly important visually because you vary the magnification of the image by changing the eyepiece. However short focal length eyepieces are not as easy to use as the longer ones.
So to see deep sky objects (or planets) that are small and need a bit of magnification, using a longer focal length scope (~1800mm or longer) is easier with a 10mm eyepiece than using a 900mm scope with a 5mm eyepiece.
Bright widefield objects can be seen with a short focal length scope of any design but there are not that many of these to see.

Hopefully this helps.

Awesome Terry............a few things I had totally forgotten over the years and your advice will help me when I upgrade towards end of the year.