Eye piece

[CedrikG]

Hey folks !

I just received my Orion XT10 Classic Dobsonian Telescope but need to upgrade the eye piece, as mine is just a 2x. I want to be able to see planet in very great detail, as well as nebulae, double stars and galaxy. With my scope I got a 2x barlow.

I got 2 question :

- I see there are different kind of eye piece, some are much cheaper than other for the same number of MM. I was wondering what was the difference between a 40$ 25MM and a 90$ 25 MM.

- With my 2x barlow, will I be happy with a 25 MM ? That way, I would use the 25 MM for the planets, then add the barlow for deep space, giving me a 50 MM.

Thanks !

[jabeoo1]

This thread is a very useful guide.  

http://stargazerslounge.com/topic/43171-eyepieces-the-very-least-you-need/

[CedrikG]

Amazing post, thanks for sharing! That will answer all my question.
I just got something I dont understand, english is not my primary language so I just need a clarification.

What mean ... exit pupil ?
 

[jabeoo1]

This is from the Televue website:

Exit Pupil

The exit pupil is the image of the objective that is formed by the eyepiece. It's where you place your eye to see the full field of view. You can calculate the diameter of the exit pupil by dividing the focal length of the eyepiece by your scope's focal ratio:

Exit pupil = eyepiece focal length ÷ telescope f/#

For reflector telescopes, it's best to avoid exit pupils larger than 7mm or smaller than 0.5mm. Refracting telescopes have no upper limits on exit pupil sizes.

[RikM]

Amazing post, thanks for sharing! That will answer all my question.

I just got something I dont understand, english is not my primary language so I just need a clarification.

What mean ... exit pupil ?

The exit pupil is the diameter of the beam of light coming out of the eyepiece. If this is too small, it is difficult to place your eye in the correct position and the view will quite dim/dark. If the exit pupil is large the view will be brighter but so will the background sky so less contrast. If the exit pupil is too big you can see the shadow of the secondary mirror as a dark blob in the centre of the view. Not very nice!

For 10" f/5 Newtonian eyepieces between 30mm and 5mm should work well. The focal length works the opposite way to how you are thinking. A long FL eyepiece (30mm) shows a large area of sky, low magnification, good for nebulae etc. A short eyepiece (5mm) shows a small area of sky, high magnification, good for planets.

The Barlow works on the telescope, not the eyepiece. For the telescope it is x2 for the eyepiece it is /2. E.g. 25mm becomes 12.5mm with the Barlow. (Actually the 25mm is always 25mm but the telescope changes from 1200mm to 2400mm. It is very confusing at first but will become clear over time).

The XT10 is a nice telescope. Your 25mm with the Barlow should be a good combination to start with

[jabeoo1]

If you wear glasses CedrikG you should consider the distance your eye needs to be away from the lens to be able to view through it properly.  If the exit pupil is too big you may see the secondary mirror obstructing the view in the eyepiece.  If too small you may pick out 'floaters' in the view as dead cells float across the internal fluids of your eye ball. 

[CedrikG]

ahhhh now it become so clear.

Yeah I was completely wrong about the MM and magnification! Thanks for making it clear.
Do you think a 3.5 MM 1-1/4" eye piece would work great on my telescope ? Theres one for sell, and I am very interested in deep field so maybe this could be a good one for me ?

[ronin]

Using sort of minimum magnifications for Jupiter 80x will make a clear image and for Saturn you will need to push up to 120x or more.

If the image is clear and sharp at 120x then use that on Jupiter also.

Now 120x may not display as big a Saturn as you would like so consider 150x, after that I suspect you will be trading off size against image quality a bit. This very inconvenient thing called an atmosphere causes a problem.

120x-150x should be farily easy on the scope.

If you want to upgrade then going to say throw in a budget it is relevant as eyepieces go from $30 to $500.

And the f number of the scope.

For deep field you will want something like 25mm or 18mm, not 3.5mm.

Think 3.5mm will be too much, or useable once or twice a year.

[jabeoo1]

ahhhh now it become so clear.

Do you think a 3.5 MM 1-1/4" eye piece would work great on my telescope ? 

1200mm / 3.5 = 342

My advice would be, but its only my advice ! 

x342 magnification will be too high to be useful in just about every situation !  Aim for around x150 or so.  Barlow a decent 15mm plossl to start with.  That will give x80 & x160 & a very useful starting point.  As you see what results this gives you will want to follow your desire for more magnification (I only really go to x200 most the time), and lower wider field maybe.  Follow your instinct but start at the x150.  

[CedrikG]

I do have a 25mm, which I can transform in a 12.5 with the barlow and I cannot see any detail of jupiter, I barely see the stripe as it is so small, I was quite disappointed. I would like to see it in great detail, I want to clearly see the eye, and would like to see the different layers of saturns ring.

I will follow the advice and go with a 5 or 6 MM, see what happen from there.

Thanks so much for all the help everyone

[CedrikG]

Is it possible to use a different manufacturer eye piece than the Scope I have ?
In other word, can I buy a Celestron Eye piece for my Orion ?

[bingevader]

Most definitely yep!

Take a look at the mongrel collection in my sig., they all work perfectly well with both my 'scopes.