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martinj
Joined: 11 Aug 2006
Posts: 3
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 Posted: Fri Aug 11, 2006 11:07 am Post subject: Buying my first eyepiece for an 8" dob |
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I have a Meade Lightbridge 8" and I have been happy with it so far but I would like to increase the magnifiaction. As I haven't brought an eyepiece before I don't have a clue on what I should be looking for.
Any Ideas? |
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Joined: 09 Aug 2006
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bauzl
Joined: 22 Aug 2006
Posts: 6
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| Posted: Tue Aug 22, 2006 6:27 pm Post subject: Re: Buying my first eyepiece for an 8" dob |
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Hi,
I bought a Lightbridge 8" last month as well.
I allready had some "high power" eyepiece before, so I didn't immediately need to buy an additional one.
My tip:
The Lightbridge 8" is an f/6 scope. This is not extremely fast but lets say reasonably fast focal ratio. So there is a good chance that only the center of your field of view is sharp, as the image plane is not realy a plane but curved. I have the de luxe model which came with an QX 26mm wide angle eyepice which suffers from this effect.
So there are three options:
- avoid wide angle eyepieces (O.K., this is not really an option)
- buy eyepieces which are specialised on short focal ratio Newtonians (e.g. Nagler, which might turn out to be more expensive than the scope). I have good experiance with a 12,5 mm (and borrowed 9 mm) ED eyepieces.
- use a Barlow Lense to increase the power of the existing eye piece. This effectively leads to a larger focal ratio and therefore flattens the image plane. Be sure to use an achromatic Barlow unless you like to add the achromatic shortcomings from refractors to your Dobsonian  CHECK before you buy a Barlow, whether you can still reach focus! In fact with my Lightbridge I cannot reach the focus if I fully insert some of my eyepieces.
I read some recommentations for having a good mix of eyepieces and a good Barlow Lense. If you have a 26 mm eyepiece and a 2 x Barlow, you will probably not need a 12 mm eyepiece.
best regards
Bauzl
There are some dark sites in Austria.
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There are some dark sites in Austria! |
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martinj
Joined: 11 Aug 2006
Posts: 3
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| Posted: Wed Aug 23, 2006 11:50 am Post subject: |
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Thanks Bauzl
| Quote: |
| CHECK before you buy a Barlow, whether you can still reach focus! In fact with my Lightbridge I cannot reach the focus if I fully insert some of my eyepieces. |
I am very tempted by the barlow but this has made me a little concerned. I would have thought Meade would test that sort of thing?
Whats achromatic? Sorry to sound dumb but I am very new to this. 
Thanks again |
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bauzl
Joined: 22 Aug 2006
Posts: 6
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| Posted: Wed Aug 23, 2006 7:56 pm Post subject: |
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Hi,
I hope and also expect that Meade is doing these tests as well, but you do not necessarily buy everything from one vendor. That's why there are standards. In fact I have to admit, I do not know whether Meade offers a Barlow suitable for the lightbridge.
Achromatic means for any kind of lense that the chromatic aberration is limited to an hopefully acceptable level. Every lense system - including Barlow Lenses and eyepieces have the problem that the focal length is not the same for all wavelength of light. Different wavelength means different colors. So if you have light of multiple colors coming in to the lense - and white starlight consists of more or less all possible colors - you cannot set the focus precisely for all colors. Therefore you do not get a sharp spot of the star you are looking at, but something nebulous and colorfull. The more power you use the worse this effect will be.
This is something a mirror does NOT produce. There is no chromatic aberration in mirrors. Thus it would be a shame to introduce this shortcoming to a Dobsonian.
So be sure not to buy a departement store Barlow Lense but a good achromatic one, which consists of more than one glass lense but a lense system which limits this color error 'chromatic aberration' - na achromatic Barlow Lense.
Please do not rely on my sole opinion before you buy. Consult the web. Wikipedia is your friend: http://en.wikipedia.org/wiki/Eyepiece
clear skies
bauzl
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henry
Joined: 08 Mar 2007
Posts: 3
Location: Cromer Norfolk
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| Posted: Thu Mar 08, 2007 1:43 pm Post subject: |
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I have just bought a Celestron Starhopper 8" (OK, it's for my daughter, but I had to check it out, didn't I?).
I used it for the first time last night, aiming at Saturn. I found the accessory spotter scope almost completely useless so had to seek the target using the eyepiece alone. It's amazingly hard to find even a bright object you can see quite plainly with the naked eye, when you're using the 25 mm eyepiece (x48) supplied. And when I found Saturn, it was very nice, but in underwhelming detail.
I suppose the 25mm is a kind of compromise eyepiece to be sold with the scope. So yesterday I ordered two more eyepieces -- a 40mm (the biggest I could find, which will give 30x), for wide-field and spotting, and a 9mm (x133) for more detailed viewing, once the target is finally in my sights.
So I think you need a range of eyepieces -- one as wide-field as you can, just to help you hit the target; one fairly high power (but not too high) so you can enjoy the target; and something in the middle, for extended objects. |
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henry
Joined: 08 Mar 2007
Posts: 3
Location: Cromer Norfolk
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| Posted: Wed Mar 28, 2007 8:41 am Post subject: |
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I see what you're saying, Ambermile, but I am using the low-mag piece simply to spot things, which I'll then look at in more detail at higher mags.
Since I last posted, I got two Plossl eyepieces, a 40mm and a 9mm, and both are fine. I do not notice any darkening in the centre of the 40mm (though now you have mentioned it, I shall look out for it). However, the 40mm makes it much easier to scan for objects before homing in on them, which compensates somewhat for the somewhat seat-of-pants operation of the Celestron dob mount. |
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