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Lens question


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I recently purchased an Orion 8" Dob which I love.  It came with a 25mm Plossi eyepiece and 2x shorty barlow.  I don't want to go crazy, but would be interested in any recommendations for an additional lens (lenses).  I'm specifically thinking about viewing Jupiter with this lens.

cheers!  :smiley:

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Hi Dan,

With the 25mm and 2x barlow together, this will give you 12.5mm EP. Thats enough to start getting good views of Jupiter. If you want to add to your collection, i always think (as do others) that an 8mm eyepiece is a good one to use for observing planets. Which one you buy depends on budget and other things such as eye relief of the EP and FOV (field of view) offered by the EP.

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How about the Seben 8-24mm zoom EP?. A couple of people here use it and it seems good. Usually when the name Seben comes up we all say avoid like the black death, but the zoom for about 50 quid seems to be one thing Seben do right.

I'm just chucking out suggestions. I really am not technically minded when it comes to scope speeds and eyepieces that work well with certain scopes. I'm purely visual and if i like the views i get with any gear, i'm happy.

Edited by LukeSkywatcher
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So if the scope is 1200 mm FL, and my current  eyepiece is 25mm, that would give me 48X ,( 96X with a 2X barlow).

If I went 8mm, that would give me 300X with the barlow ( 1200/8  x  2).

Is 300X a reasonable amount of magnification for this scope?  I understand that 400x is the uppermost (unrealisitc) limit.

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You have 8 inch dob. That is where 2000mm comes from

so 2000 ÷ 25 will be magnification

and 2000 ÷ 12.5 (with barlow)

I have been told 7mm is the lowest to go with 8inch. I havent as yet got 7mm

i have used 8mm and 6.7mm and have been pleased with them.

al

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You have 8 inch dob. That is where 2000mm comes from

so 2000 ÷ 25 will be magnification

and 2000 ÷ 12.5 (with barlow)

I have been told 7mm is the lowest to go with 8inch. I havent as yet got 7mm

i have used 8mm and 6.7mm and have been pleased with them.

al

I think 8" is 200mm aperture :smiley:

The focal length is 1200mm so it has a focal ratio of F/6.

400x is the theoritical maximum. Practical reality means that 150x / 200x / 250x are much, much more useful high powers. 300x might be OK under very good conditions on the Moon, Saturn, Mars and double stars.

On deep sky objects low to medium magnifications are the way to go much of the time.

When I had a 200mm F/6 dobsonians I've found eyepiece focal lengths between 32mm and 5mm a useful range :smiley:

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I am a bst starguider fan from stl.

how about bst 15mm, with your barlow giving 7.5.

Al

The BST's are not available in Northern California Al. They are marketed as Astro Tech Paradigms or Orion Epic II ED's in the USA.

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I think 8" is 200mm aperture :smiley:

The focal length is 1200mm so it has a focal ratio of F/6.

400x is the theoritical maximum. Practical reality means that 150x / 200x / 250x are much, much more useful high powers. 300x might be OK under very good conditions on the Moon, Saturn, Mars and double stars.

On deep sky objects low to medium magnifications are the way to go much of the time.

When I had a 200mm F/6 dobsonians I've found eyepiece focal lengths between 32mm and 5mm a useful range :smiley:

thanks john, i was just about to correct my error, but you have explained a lot better.

al

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The add to what others have said, go get the Astro-Tech 8mm Paradigm, $60 usually, also sold by Agena Astro as the Agena Astro ED and sometimes a whole $1 less. :grin: :grin: :grin:

If you have the normal 25mm and 10mm then you have the normal 2x barlow, as that is not generally good there is little point pairing a poor barlow with a 15mm Paradigm.

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The add to what others have said, go get the Astro-Tech 8mm Paradigm, $60 usually, also sold by Agena Astro as the Agena Astro ED and sometimes a whole $1 less. :grin: :grin: :grin:

If you have the normal 25mm and 10mm then you have the normal 2x barlow, as that is not generally good there is little point pairing a poor barlow with a 15mm Paradigm.

Are you saying not to use the Barlow?

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Apologies, just read you have the Orion and a single 25mm plossl. My presumption being it was the "standard" 25mm+10mm+barlow that are usually supplied here. Over here the 25mm is often the only partially reasonable item. The barlow is often, and better, used as an extension tube by removing the lens. :grin: :grin:

Think Orion are better with their items then Skywatcher here, and the 2 items are usually better quality, so yes I think the barlow will work with the 25mm.

Going to say try them out:

So next clear night scope+25mm+(Jupiter or M42), then Scope+barlow+25mm+(Jupiter or M42).

And decide how good/bad/acceptable the barlow is.

After the post I realised that in the US the Celestron X-Cel eyepiece are very close in price to the Astro-Tech Paradigms.

The Paradigms and the X-Cels perform almost identically but for planetary work the X-Cels have a better selection of shorter focal lengths.

The Paradigms are 5mm, 8mm 12mm.

The X-Cels are 5mm, 7mm, 9mm, 12mm.

I would say that you will eventually need 4 eyepieces to cover all aspects and in the X-Cels I would go for 7mm, 9mm, 18mm, 25mm. The 7mm and 9mm being for higher magnifications and planets. The 5mm in either will be for when conditions allow.

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Fairy simple - some people like a barlow some do not. I prefer just an eyepiece and no barlow. So I tend to answer assuming just an eyepie will be used.

The decision is yours but saying get an 8mm is easier then saying a 16mm and a barlow and swap around. One aspect is if you take out a barlow and eyepiece to put in another you have 2 bits to juggle with and store. One bit is easier.

Also a barlow and a 5mm will not work so not all eyepieces can be mated and a 25mm and 2x barlow match a 12mm eyepiece. To me it seems too much bother to works out which combination so I use individual eyepieces. If I had 3 eyepieces and a barlow it is the same cost as 4 eyepieces and I guess I would not really have 6 different focal lengths a 15mm X-Cel barlowed is the 7mm so do you not buy the 7mm when the 7mm alone night reveal a bit more of planetary detail.

As said 2 sides to the thought and I simply find individual ones suit me. There are in effect 6 Paradigms and 6 X-Cels either set of 6 covers probably everything so I have the set of Paradigms (BST's here).

One advantage of a barlow was with plossl's, a plosssl has eye relief of about 2/3 (70%) so at 8mm you get close to the eyepiece, a 5mm plossl was/is almost unusable. A barlow and plossl maintains the eye relief of the longer focal length plossl. So a 2x barlow and 10mm plossl gives the magnification of a 5mm and the eye relief of a 10mm.

The Paradigms and X-Cels have eye relief of 15-18mm so that is not a problem with these.

So if you bought the TV plossls - very good eyepieces - to get below 8mm you would have to include a barlow. If you wear glasses then an 8mm is likely unusable so a barlow and 15mm would be a possible combination.

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