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Make a Seben worthwhile?


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Hey all..

Been reading, reading, reading here. A wealth of knowledge, one of the best I can find. Hoping I can pick some brains, here.

Just got my first scope - What looks to be a Seben brand 6" Newt, Short tube (150-1400). I've read the scary reviews here and elsewhere and was wondering about a few things.

I received the scope second-hand, and for free. It was supplied with "PL6.5" and "PL25" eyepieces, eyepiece moon filter (green tint), eyepiece sun filter (scary), plastic cased erecting eyepiece, and plastic cased barlow(2x).

Things were *quite* dirty, so I had some restoration to do. I started with cleaning up the mount, which was quite dusty. Everything moves very smoothly, although tightening the RA axis fully still allows motion without applying much pressure to the tube. Also, the declination setting circle indicator ring spins very freely on the mount - this isn't normal, is it?

Luckily the mirror looked pretty much pristene, so I attacked the eyepieces. I was under the impression that a Plossl EP contained 4 lens elements... So I was surprised to find only one element (plastic seeming) in each EP. I found the barlow to be total junk (I can see abberrations clearly), and the erector's element isn't seated properly in its plastic housing. Garbage.

Comments on any of the above would be GREATLY appreciated; but here are my real Q's:

What can I do to make this thing at least somewhat usable? It is a first scope, and was free, so I don't expect the world.

-EP's I would assume.. But what brand/type/power(s)?

-A real barlow?

-Is it possible to remove the corrector lens (and assumably some of the poor image quality), and operate at the lower focal length? Pros/Cons of this? If I can/do, how would that change the EP lineup I'd want?

-Any other hacks/modifications to increase quality?

I can't spend a fortune... And I don't know if I could in good conciense sell this thing for $ towards a better scope.. But there's gotta be somethin to help - right?

Thanks so very much!!

-Matic

Cleveland, OH USA

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Hi Matic, as you've discovered Seben isn't held in high regard in astronomy circles, however as you received your equipment for free then I wouldn't feel too bad!

The 6" aperture newt you have is a good size and much better than the standard beginners fair 114 scopes. I'm a little concerned that your telescope is rated at 1400mm focal length though which makes me think that it's a 700mm OTA with a poor barlow element fixed into the focuser tube (a big no no in the quality department) However again, as it's free i wouldn't be too stressed about it.

The eyepieces you already have sound like Plossls which actually makes them half decent eyepieces, however if the eyepieces have plastic lenses as you say, they may not be plossls at all. The 6.5mm plossl will give you a 215x magnification and the 25mm wil give you a more useful 56x magnification. My advice would be to throw away the solar filter rapidly. I'd also throw away the erecting eyepiece and the barlow but they might be of some use to you.

The mount will benefit from your cleaning and if you are mechanically minded you can strip the whole thing down and re-grease the moving parts. i would ignore the setting circles on the mount as they dont have enough resolution to be of any use.

I would look on ebay for some branded GSO plossls. The rule of thumb is to use a 1mm exit pupil as your "workhorse" eyepiece and a 5mm exit pupil as your wide angle/finder eyepiece. The exit pupil can be found by dividing the eyepiece focal length by the focal ratio of the telescope. In your case your telescope has a f/9 focal length (maybe!) which means to give a 1mm exit pupil you need a 9mm eyepeice. To give a 5mm exit pupil you need a 47mm eyepiece but a more practical eyepiece would be a 32mm plossl. You can find both a 32mm and a 9mm eyepiece readily from suppliers or from ebay.

A barlow is a great investment and effectivly doubles your eyepiece collection, I would recommend the TAL 2x barlow as being the best combination of quality and price.

As for removing the barlow in the focuser draw tube, it IS possible and I've read of many people who have done it, however i'm unsure of the quality of the image after. It might be better to save your pennies and purchase an Orion 6" newt in replacement. That being said, you can't make things worse so have a go!

As an alternative, take a look at Orions offerings:

http://tinyurl.com/4ml4o

dont forget that looking on astromart can make things a lot cheaper than buying new!

I'm sure that others here can give you more advice

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I wouldn't buy anything for it yet. Borrow a real Plossl and try that first. What you see will tell you whether it's worth keeping the scope.

Yep. Also, are the eyepieces 1.25" in diameter, or .965" (just a little larger than the width of your thumb, and about the width of your thumb, respectively?) If they are .965", see if there is an insert in the focuser tube which can be removed to allow the insertion of 1.25" eyepieces. Then get one or two decent plossls, say, Celestron or Antares, or borrow one, and see if things improve. If you are interested in continuing in this mad hobby, you can take any parts you buy for this scope to your next one.

Columbus is a big enough city that you probably have a store in town that sells telescopes and stuff, and can also give you some advice. I would suggest a 32mm and a 10mm eyepiece as a start. Next thing you would need is a decent Barlow, but hold off until you decide what you are going to do with the scope. Try it with and without the corrector lens that we are assuming is in the focuser tube.

When you get outside, choose a medium bright star, and focus it to a point. If the point has some rays coming off it, that's OK, but if it isn't a point, then you either have to adjust your mirror (collimate) or you have a poorly figured mirror.

Let us know how you get on, and we'll take it from there.

BTW, I'm only a half-day's drive from you, in Niagara, Canada. Welcome to the forum!

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  • 3 years later...

The mirrors in Seben scopes are spherical and if you remove the internal barlow lens it will probably be awful at that focal length.

My father-in-law has the same scope and I've used it when visiting him in Spain and had some reasonable views of the Moon, Saturn and the Andromeda Galaxy through my own eyepieces that I took with me (his is 1.25"). Get a half decent eyepiece or 2, avoid short focal lengths and you just might enjoy it as a stop gap 'til you can get something better.

Almost everyone wants a better scope. At least you are safe in the knowledge that your current one has cost you nothing!

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