About Eyepieces

[vindictive666]

hi all


mike and or moderators i came across this site today i found it full of usefull info and i thought i may off use for newbies like myself and others perhaps ?

if it is can it be made into a sticky or added to the links ?


http://www.belmontnc.4dw.net/optics.htm#Barlows


the link below is actual the home page


http://www.belmontnc.4dw.net/index.htm

[chunkylad]

Thanks for the link John

What a great resource for a newbie like me!!!!!

[ballaratdragons]

That's a fantastic find John!!! Thanks. I just put it in my favourites. (I 'bookmarked it' I think is the correct term?). It will compliment the 'Filters' site nicely.

[ving]

cool site! :)

[ausastronomer]

Just be aware that what you’re reading is someone's opinion which may or may not be 100% correct. For instance:-

"A 2x Barlow functions by dividing the focal length of any eyepiece by two"

That is not correct, the functioning of the barlow has nothing to do with the eyepiece its used with. A barlow is a concave tele-negative lens and works by acting on the light cone prior to it reaching the eyepiece, that’s why its inserted before the eyepiece in the light path. A standard design 2X barlow works by halving the angle of incidence of the light cone thereby effectively doubling the focal length of the telescope objective. You could look at the effect of the barlow as halving the focal length of the eyepiece in terms of its effect on magnification, but that’s not how it works. The article correctly states that inserting the barlow earlier in the light path will increase its magnification effect. ie. In a SCT or refractor a 2x barlow inserted before the diagonal effectively becomes a barlow of "about 3X" magnification.

Another example of a clear error:-

Erfles
Invented in the .1940's, the 5 or 6-element Erfle delivers a wide 60 to 70 degree apparent field of view."

Heinrich Erfle designed the type I Erfle whilst working for Zeiss in 1917, for military use. The types II and III Erfles, were designed sometime between 1917 and Erfles death in 1923. The 3 designs were all patented by Zeiss immediately after Erfles death in 1923 so its anybody's guess how this bloke thinks they were invented in the 1940's. Wrong war maybe ? :)

The article does however contain some helpfull tidbits for beginners, but to be honest I would recommend a good book like Starware by Phil Harrington where you know the content has been properly researched and checked.

Clear Skies
John B

[elusiver]

top link.. alot of info on that site...

el :)

[elusiver]

also.. as a quikc side note.. is there a better type of eyepiece for the dob. I think it was BD who said the kellners or huygens(can't remember which one) were better suited to a reflector as it matched the curvature of the mirror or something along those lines. Is there a better type of eyepiece(don't say nagler either :P) for a dob.. or what i mean is is there a type of eyepiece that is optically better matched to our dobs? Or should the dob owners just stick with plossls until we can afford naglers?

el :)

[ausastronomer]

Elusiver,

Plossls or orthoscopics of good quality (not cheap crap) will work well in your dob and can be purchased 2nd hand for less than $100 or new for less than $150. A good plossl or ortho will be infinitely better in your dob than the eyepieces it was supplied with. A good plossl or ortho will work well with scopes as fast as F4.

CS-John B

[rmcpb]

Now that is a great site :)

[ausastronomer]

I have now come to the conclusion that this article has not been properly researched and checked before posting to the internet, it contains a lot of errors.

"Orthoscopics
Four-element "orthos" were once prized as the best all-around eyepiece, but they've lost ground among the amateur ranks because of a narrower field compared to other modern designs like Plossls. Orthos yield excellent sharpness, color fidelity, and contrast. They have longer eye relief than Kellners, though not as long as most Plossls. They are especially suited to planetary, lunar, and double star observing."

An orthoscopic in fact has slightly longer eye-relief than a plossl, not the other way around as stated in the article. The eye-relief of an orthoscopic is generally 75% to 80% of its focal length and a plossl is 70% to 75% of its focal length.

I am not even going to bother reading any further, clealry the person who wrote the article is no expert and is regurgitating tid bits he has picked up. Go buy a copy of Starware, the current edition is #3

CS-John B