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Aggers

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Aggers

Aggers

Posted
Posted

Hi all,

          I have recently taken charge of a short tube newtonian,which I have discovered has no eye pieces.This I thought would be easily resolved until I scratched the surface.

Can anyone help in plain english the best place to start as I am eager to get started.Im not sure which model it is either,as the booklet covers 3 models I believe.Any advice will be useful as i am starting from the ground.

Thanks all

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Herbig

Herbig

Posted
Posted

Hi and welcome. See the 'sticky' on eye pieces at the top of this topic area.

As you have a Newtonian you will also need some sort of collimation device even if you just use it to check the instrument.

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Schorhr

Schorhr

Posted
Posted

Welcome!

I too would be curious to see a image of your telescope :-)

Especially before you spend money on a telescope.

Simple cheap Plössl eyepieces can be bought for 7-15gbp, or a small zoom eyepiece for 15-20gbp that is far from perfect but a low budget start.

If you have one of those laying around, or any other magnifying glass with short focal length and multiple lens set-up: They can be used as (simple) eyepiece.

http://www.ebay.co.uk/itm/10-x-Pocket-Magnifier-Magnifying-Glass-Loupe-/261302742007?pt=LH_DefaultDomain_15&hash=item3cd6db6bf7

http://www.ebay.co.uk/itm/New-Hot-10x-21mm-Jewelers-Eye-Loupe-Magnifier-New-10-Times-Magnifying-Glass-DZ88-/370921263457?pt=LH_DefaultDomain_0&hash=item565ca15161

But only low magnification and with reflections and chromatic abberations. But you could start to view moon, orion nebula, Andromeda galaxy and open star clusters right away if you have one already.

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Schorhr

Schorhr

Posted
Posted

Oh no, the Seben catadioptre :-)

I always have issues with judging sizes on pictures. There are several Seben telescopes like this.

The pictures are to small to read any imprints / stickers / markings.

Infront of the WWW imprint seems to be a sticker. Is this just the sun-warning? Is there another sticker near the focuser?

Seben has a 114/1000 (Star Sheriff), 150/1400 (Seben big boss) and a 203-800 (Orbinar 203).

http://www.astroshop.de/seben-teleskop-n-150-1400-big-boss-eq-3/p,3997

I *think* it could be the 150/1400. If you can, could you measure the tube diameter and length to better judge the size?

They have in common:

-weak mount

-catadioptre design with poor, short focal length mirror (with a lens in the focuser to increase the focal length, trying to decrease the problems with the spherical mirror and coma, but making it harder to collimate and introducing chromatic aberration)

Don't worry, despite all these problems you will still be able to see moon and planets and brighter deep sky objects, though the telescope is far from beeing ideal optically.

If it is the 150/1400 that's quite a focal length... A 1.25" eyepiece would only show a little over 1 degree of field (two times the apparent diameter of the moon, that's not much).

This makes searching objects other then planets and moon a bit difficult.

While the Seben telescopes are usually to be avoided, some of their eyepieces are okey for the price.

30mm Plössl + 10mm Plössl + 2x achromatic Barlow would give you the largest variety for the lowest price (around 35gbp total) though 10mm + X2 = 280x magnification is already a bit over the edge regarding optics quality and the usual atmosphere seeing condition.

Other combinations would either be redundant (30+15+barlow) or too close together (20+30+barlow).

30+15+6.5 without barlow could work even though the gaps are a bit large and the 6.5mm has a short eye relief.

The combination of the cheapo-Zoom and a 30/32mm Plössl could work but the cheaper of their zooms is really not good, especially as the apparent field of view at 22.5mm is so narrow.

If you just want to look at moon and planets, just the zoom is a valid way to start, though later (with a new telescope or decent eyepieces) the zoom would probably get little use. It has reflexes, a narrow field of view on one side, and is nothing perfect... So it basically matches the telescope :D

You can save a bit by ordering multiple seben/orbinar eyepieces directly at seben.com , as on amazon.co.uk and at ebay they offer "free shipping" and the combined price is higher.

cheapo-zoom http://www.amazon.co.uk/Seben-Telescope-Eyepiece-7-5-22-5mm-Astronomy/dp/B00EL5XU9S/ref=sr_1_13?ie=UTF8&qid=1387147393&sr=8-13&keywords=orbinar+mm

Don't get too excited about it's reviews here on the forum, ( A ) some people confuse it with their other zoom and ( B ) I have it here, it really isn't great if you have viewed throurgh decent eyepieces.

The problem with their 30mm eyepiece (http://www.amazon.co.uk/Orbinar-Plossl-30mm-eyepiece-31-7mm/dp/B00858IF1A/ref=sr_1_6?ie=UTF8&qid=1387147393&sr=8-6&keywords=orbinar+mm ) is that it lacks of an eyecup, so either get the 25mm or make one out of a pipe insulation (hardware store) or foam rubber sheet.

If you hav a local star gazers club or observatory, you may find someone that can borrow you a few cheap old eyepieces.

There are better wide angle eyepieces from 27 gbp (66 degree wide angle) to 50gbp (HR Planetary/BST explorer), but I think you should not invest so much money in the seben telescope.

IMHO the best option would be to sell it at ebay with some nice pictures (and buy the zoom as accessory, people will buy anything with good pictures and zoom ! ;-) ).

If you are lucky you'll get enough from the auction to either buy a Heritage 130p (stablie rockerbox mount and less questionable optics) or save for a 8" dobsonian that will show much more.

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Schorhr

Schorhr

Posted
Posted

Oh, so I guessed it right :-)

Unfortunately it's not. Mount, optics and collimation are problematic.

But still you can see a lot with it, if you buy one, two, three eyepieces...

To sum up my big answer from before: I would not invest a lot in eyepieces/upgrades, it's not worth it. But the best telescope is the one you have! ;-)

Either use it or sell it. Don't let the mount and optics frustrate you, it can be an exciting hobby.

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Schorhr

Schorhr

Posted
Posted

For buying eyepieces:

Maximum magnificaton

The rule of thumb for maximum magnification is aperture/optic diameter in milimeter times 2.

So for a 150mm mirror that would be 300x. BUT the catadiopter will probably limit you to 200x?

As seeing conditions usually won't allow more anyway, 200x should be plenty.

1400mm focal length devided by 200 magnification equals 7 ... a 7mm eyepiece.

A 6.5mm Plössl has a short eye relief, but it could be one option. The zoom eyepiece another.

A 9-10mm eyepiece would be usable more often (with bad seeing conditions and also if the optics are not good... but offer less magnification).

~

Minimum magnification (for finding objects)

The iris can't open further then 6-7mm depending on age, eye and light pollution.

A eyepiece with a larger exit pupil "wastes light".

Your telescope has an aperure ratio of f/9.3.

The exit pupil is => eyepiece [devided by] focal length.

A 6mm exit pupil => 9.3 x 6 = 55.8mm eyepiece.

Sadly the telescope only has a 1.25mm focuser and the usable limit is around a 32mm Plössl with 52 degree apparent field of view.

There are 40mm Plössl for 1.25" focusers but the tube limits the apparent field of view to a narrow 40 degree.

The 32mm Plössl (or 30mm Plössl) has a exit pupil around 3.2mm, that's good for many nebulae and galaxies.

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cantab

cantab

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Posted

A decent 32 mm Plossl as a first buy I reckon. I've heard good things about GSO Plossls, their 32 mm one goes for about £35. From what I've heard of these scopes they don't perform well at high magnification, so I wouldn't buy a high-power EP right away, check things work OK at low and medium magnification first.

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