Eyepiece "size"

[ThunderStorm]

I saw some eyepiece stating 5mm 1" and 2.5", what are the differences? Do they affect magnifications?
5mm is the focal length, right?

Sorry if this is a trivial question.

[Kunama]

5mm is indeed the focal length.
1.25" diameter or 2" diameter nosepiece

Diameter does not affect magnification only the focal length.
Telescope focal length divided by eyepiece focal length = magnification factor

Depending on the scope, 5mm eyepieces usually give quite high magnifications which then require decent seeing and possibly tracking capability.
For example my 128mm F8.125 scope has a focal length of 128 x 8.125 = 1040 and a 5mm ep gives 1040/5= 208X

There are no trivial questions ........... :question::question:

and Welcome to IIS

[ThunderStorm]

Sorry my bad.
Yes, it should be eyepiece adapter. How do they used for (different size)?

[Allan_L]

This is the other end of what Matt said.
The 2" adapter is for bigger 2" eyepieces,
this adapter is then removed from the focuser and the 1.25" adapter put in for use with regular 1.25" eyepieces.
Edit: the above relates specifically to Skywatcher DOBs.
Not so silly a question as you may think...
you will see many posts of people who could not focus their new SW DOB because they put BOTH the 2" and the 1.25" adapter in together.

As you imagine, 2" eyepieces give a slightly bigger image or true field of view.

This is discussed here, with some useful link.
http://www.iceinspace.com.au/forum/showthread.php?t=77748

[ThunderStorm]

So 1.25" adapter can be used on 1.25" eyepiece.
So as 2" adapter can also be used on 1.25" eyepiece?

[astro744]

A picture is worth a thousand words. See http://www.bintel.com.au/Accessories/Adapters/Bintel-2----1-25--Adapter/409/productview.aspx

When using 1.25" barrel eyepieces, first insert the 2"-1.25" adapter into the focuser. Then insert the eyepiece into the adaptor.

When using 2" eyepieces just insert the 2" barrel of the eyepiece into the focuser. You do not use the adaptor for 2" eyepieces.

Interesting. Iceinspace dictionary says adaptor is incorrect and should be adapter. Australian Macquarie Dictionary 5th edition says adaptor. Shorter Oxford English Dictionary 6th edition has both but says adaptor is more for a device and adapter for a person. (You learn something every day!)

[ThunderStorm]

So normally what is the size of the focuser that come with a telescope?
Then are we putting the adaptor onto the focuser if our current eyepiece does not fit the focuser?

Any article describe the structure of a telescope?
Thanks.

[astro744]

Normal depends on the telescope and I perhaps incorrectly assumed you had a 2" focuser hence the need for an adaptor. Some telescopes will have a 2" focuser and some will have a 1.25" focuser. A long time ago there was also a 0.965" focuser and matching 0.965" eyepieces.

Clave also made 27mm and 50mm barrel eyepieces to suit their local market I would think but also made 1.25" and 2" for the rest of the world. (Metric in France!)

Use your favourite search engine to find out all you want about telescopes. There's a lot of info available.

[AstralTraveller]

Some scopes will only accept 1.25" eps (eg small maksutovs) but the majority accept either size. Typically the top of the draw-tube (the bit that moves in and out) has a 2" sleeve. The tube will be slightly larger and the sleeve, which is never removed, steps it down to 2". The 1.25" adaptor is placed into the 2" sleeve as necessary to accommodate the smaller eps. There are also some 25mm (0.995") eps around but they are typically quite old and now uncommon. The only example I have has a 25mm to 1.25" adaptor permanently attached to it.

[julianh72]

You still see the 0.965" / 24.5 mm eyepieces in some of the smaller "department store" telescopes (e.g. 50 mm refractors), but most of the brand-name "beginner" telescopes seem to ship with 1 1/2" (31.8 mm) eyepieces these days, and a lot of those have 2" focusers with 1 1/2" adapters.

[astro744]

That would be 1 1/4" not 1 1/2" and just to be really picky, 31.75mm often rounded to 31.7mm since 31.8mm definitely will not fit.

Being in Australia we have had the luxury of both the imperial system and the metric system and in amateur astronomy both seem to mix seamlessly. e.g A 12" telescope with 1500mm focal length.