How do I know what magnification I'm using

[narky (Sam)]

Hi everyone,

I'm getting myself a barlow for planet watching.

I have a Skywatcher SW102 f/5
Here's the scope in question : http://www.ozscopes.com.au/skywatcher-102mm-510mm-az3-refractor-telescope.html

The lenses that came with the scope are Plossl 10mm and 25mm.

I figured I'd get the 3x magnification barlow available from Andrews.

How do I work out what magnification this will give me?

I assume it'll be well within the limits of the scope?
Sorry, i'm feeling extra lazy today so I can't be bothered looking for the answer myself. Can someone please just throw the answer my way.

[mental4astro (Alexander)]

Simple maths gives you the magnification,

Mag = focal length of scope / local length of eyepiece

So, your scope has a focal length of 500mm, with a 25mm eyepiece (EP) this gives:

500 / 25 = 20X

With the 10mm EP:

500 / 10 = 50X

Using the 3X Barlow you triple the magnification of a given EP, so using the Barlow with the 25mm will give 60X, and the 10mm EP it is 150X.

[narky (Sam)]

Originally Posted by mental4astro

Using the 3X Barlow you triple the magnification of a given EP, so using the Barlow with the 25mm will give 60X, and the 10mm EP it is 150X.

Thank you so much !!

Yay, I was pretty confident I would be within the ballpark of my scopes specs. Thanks for confirming it.

Appreciate the quick response.

[Wavytone]

With all lengths in mm here are the relations:

Focal length of the scope = F
Eyepiece focal length = f
Aperture of the scope = A
Maximum pupil of your eye when dark adapted = a, which we will assume is 6mm (in average conditions).

Focal ratio = R = F/A or alternatively F= RA
Magnification = m = F/f = RA/f
Exit pupil of the beam from the eyepiece = e = A/m = f/R

Lowest useful magnification occurs when the exit pupil = the eye pupil, i.e. e=a. Or, when choosing your lowest power eyepiece f = Ra, which for your scope is 30mm. As you have a 25mm Plossl, that's close enough.

For a person with normal vision, the maximum useful magnification occurs between 1.1X to 2X per millimetre of aperture. For refractors and f/15 Maksutovs it's 1.5X to 2X. For SCT's and Newtonians, 1.1X to 1.5X. This is the highest magnification at which you should just be able to discern the diffraction pattern around bright stars. At higher magnifications, objects get dimmer and fuzzier and you won't see any more detail.

So for your refractor m = 1.5A and the eyepiece focal length required is f = R/1.5 or perhaps even R/2, which suggests an eyepiece in the range 3-4mm. The Barlow works as described by Alex. A 3X Barlow and those eyepieces will give effective focal lengths of 8 or 3.3mm. 8 will be fine but 3.3 is very short and probably awkward to use.

You may find a 2X Barlow more useful which, with the 10mm eyepiece is equivalent to 5mm. If you really want the maximum magnification, acquire a 7mm eyepiece which will also give 3.5mm with the 2X Barlow, giving a nice selection of magnifications with just 3 eyepieces.

[Draco (Draco)]

Mental4Astro is right on the money

[louie_the_fly (Stew)]

Sam, I downloaded a simple excel sheet. Can't remember where from, but it's on my other computer. I'll have it tomorrow. PM me your email & I can send it to you. It's simple to use.

[narky (Sam)]

Appreciate the answers.
I got home and decided I'd do a little research of my own. Figured I'd put this here in case someone else comes along in the near future.

http://www.rocketmime.com/astronomy/...ification.html
Says:

The Dawes Limit determines the smallest separation between two stars that the telescope can resolve. For a person to see that separation, the telescope needs to magnify the separation to one the eye can resolve. The maximum magnification of the telescope can be found by just looking at the diameter of the scope in mm.


It goes on to say :

The eyepiece focal length to get the maximum magnification can be found by just looking at the f-ratio for the scope! So if I look at the front of an 8-inch scope where it says "f/5" and I know the smallest eyepiece focal length to use with it is 5mm.

It also says you can go to higher magnifications which will not provide any addional details, but can make it easier to see because it is larger despite being fainter.


I found this really nice calculator .. http://www.stargazing.net/naa/scopemath.htm

Damn. I guess I should have looked at this thread again before I ordered the 3x. Oh well, if it is blurry I'll just grab a 2x.

I'm just hoping to see Saturn/Jupiter with the barlow, so hopefully I'm good. The calculator above says the only issue will be loss of light due to small exit pupil. The reassuring news for me is the first link I posted the guy says he finds 140% above maximum magnification is best for Saturn. I'm pretty close, so fingers crossed.

[narky (Sam)]

Originally Posted by louie_the_fly

Sam, I downloaded a simple excel sheet. Can't remember where from, but it's on my other computer. I'll have it tomorrow. PM me your email & I can send it to you. It's simple to use.

Wow, that's neat.
Thanks for that.

Handy little spreadsheet indeed.