newtonians.....

Kamikazer · Post by **Kamikazer** » Sat Nov 01, 2003 8:16 pm

got a little qns:

fast newts such as an f/5 tend to produce noticeable comas...
a 2x barlow would effective slow it down to f/10...
would this essentially eliminate comas?

gwenyi · Post by **gwenyi** » Sat Nov 01, 2003 9:11 pm

focal ratio would be calculated by dividing the focal length by it's aperture

for example , the aperture of ur scope is 127mm and it's focal length is 1500mm. therefore, the focal ratio would be 11.8.

by adding a barlow, it would double the magnification of ur eyepiece.... i don't think it will affect the focal ratio ...

Post by **weixing** » Sun Nov 02, 2003 11:29 am

Hi,
A 2x barlow will double the magnification of the eyepiece by double the scope focal length, but I wonder will it improve the image quality.

Anyway, may be those got a short focal length and a long focal length scope can perform a test and post the result here??

Post by **Airconvent** » Sun Nov 02, 2003 11:49 am

weixing wrote:Hi,
A 2x barlow will double the magnification of the eyepiece by double the scope focal length, but I wonder will it improve the image quality.

Anyway, may be those got a short focal length and a long focal length scope can perform a test and post the result here??

A Barlow is good for achieving higher mag without the shorter eye relief. Good for those who wear specs. In addition, it is also a good way to "double" your eyepiece set as you can have "2" FL for each eyepiece now!

However, in reality, you are adding another piece of glass, (typically 2), hence
image will be slightly dimmer, i.e. a 16mm eyepiece would be brighter than one with 32mm + x2 Barlow (for same quality eyepieces) but the latter combination offers better eye relief. This is important as you go for shorter FL like 6mm. However, that being said, good barlows like the TV or Celestron Ultima do their job with minimum loss...

rich

harlequin2902 · Post by **harlequin2902** » Sun Nov 02, 2003 12:08 pm

got a little qns:

fast newts such as an f/5 tend to produce noticeable comas...
a 2x barlow would effective slow it down to f/10...
would this essentially eliminate comas?

_________________
Kun Loong

Hm, maybe. Never really tried though. But Televue does have a 'Paracorr' designed specially to correct that. But despite coming up with such an interesting device, strangely Televue itself does not sell Newtonians...hm....

MooEy · Post by **MooEy** » Sun Nov 02, 2003 2:36 pm

not likely, let's look at the different types of mirror design, spherical, parabolic, hyberbolic.

for a average f/8 mirror, the shape is usually parabolic, and this is usually known as the perfect mirror. should not show over/under correction.

for faster mirrors, like in the case of the epsilon, it uses a hyperbolic mirror. without the corrector in the focuser, the scope will show serious overcorrection and coma at the edges.

in the case of spherical mirrors, usually it shows undercorrected images.

televue's parracor is a coma correct just like those used in sct and mcts, just that it's small and placed after the focuser.

"In addition, it is also a good way to "double" your eyepiece set as you can have "2" FL for each eyepiece now! " hehe with a barlow and a diagonal, u simply get 3 fl per eyepiece, not just 2 =D

~MooEy~

Kamikazer · Post by **Kamikazer** » Sun Nov 02, 2003 8:27 pm

how does a coma corrector actually work?
how much does a good one usually cost?

comas appears when viewing stars...
do they appear in views of dso or planets?

elton · Post by **elton** » Sun Nov 02, 2003 8:59 pm

Coma increases from the middle of the field of view. With a barlow and using the same eyepiece, you will be viewing at a higher magnification a smaller area of the sky around the middle of the field so coma will appear to be less. Coma correctors are expensive (a few hundred US$) and cumbersome to use (paracorrs), the aries screw-on ones are an alternative. Sam, got any lobang from APM to get any of these? In newtonians, coma is present on all objects and appears as a elongated distortion, not really noticeable in DSOs and planets unless it is very bad (near the edge, but edge distortion of eyepieces also add to this). On bright stars, they are obvious as sea-gulls, but not very obvious on dimmer ones. If you wanna see coma just check out any newtonian at a ob session. It really isn't that bad cos mostly you view stuff at the center of the field and not the edge.

kayheem · Post by **kayheem** » Sun Nov 02, 2003 10:11 pm

Kamikazer wrote:how does a coma corrector actually work?
how much does a good one usually cost?

comas appears when viewing stars...
do they appear in views of dso or planets?

Take a look at this:

http://www.licha.de/AstroWeb/reviews_fu ... ?iRevId=27

Kay Heem

Kamikazer · Post by **Kamikazer** » Mon Nov 03, 2003 8:18 pm

any idea where i can find a 6" newt f/6 (preferably f/7) or slower, made by a reputable manufacturer? (like celestron or meade)(just looking around...no money yet)