purpose of spider vane ?

carlogambino · Post by **carlogambino** » Sat Sep 11, 2004 4:23 pm

hihi, a few questions here cos i need to widen my knowledge about optics from you experts:):

a parabolic shape doesn't introduce coma, making the mirror too fast introduce the coma. if one were to make a 8" f/10 parabolic mirror, no coma would be visible.

hmm... not sure why this is so. Can anybody explain?

But i thought that's only in the centre of the FOV. There would still be coma at the edges. As quoted from http://www.opticalmechanics.com/about_coma.htm,

The surface of a Newtonian telescope’s primary mirror is parabolic. The parabola is the only geometric Surface that will focus a bundle of parallel rays to a point. Except for the limitations due to diffraction and atmospheric turbulence (seeing), a perfect parabola would focus a bundle of parallel rays to an infinitely small point. However, this is only true if that bundle of rays is parallel to the axis of the parabola or parallel to the optical axis of the telescope. When you center a star in the fov of a telescope the above condition is met and there is no coma in the image.

When a bundle of parallel rays from a star strikes the parabolic mirror at an angle, the star image is formed off the optical axis, or decentered in the fov. The parabola does not reflect this bundle of angled rays to a perfect point. Instead, the star appears as a bright spot with a V-shaped comatic flare.

Also, can anybody explain how a coma corrector works?

My third question is this: Since good parabolic mirrors produce excellent images, why do SCTs and MCTs have undercorrected mirrors with correcting plates instead of parabolic mirrors with no correcting plates? Has it got to do with the f-ratio of the SCTs and MCTs?

4:The f-ratios of SCTs and MCTs confuse me. It's stated that the Fastar Lens assembly for prime focus CCD imaging allow the telescopes to be optimised for a ratio of f/2. Also, a focal reducer lens reduces the f/ratio to f/6.3. But for visual observing, f/10 configuration is used instead. How come the f-ratio can change so dramatically?

5:still not sure what mirror shift in SCTs and MCTs is caused by.(moving of the primary mirror?)There's this crayford focuser which solves the problem of mirror shift, how does it work?

Only a sec 4 student, so dont know much:( Thanks for any answers.

gwenyi · Post by **gwenyi** » Sat Sep 11, 2004 6:28 pm

lol.... deviating from the topic liao...

for qns 4:

from: http://perso.club-internet.fr/legault/focal.html

'A focal reducer is a group of lenses with a positive power, which decreases the primary focal length of the telescope. As in the case of a Barlow lens, its reduction factor depends on its focal length and its distance from the focal plane. When this distance increases, the reduction is more pronounced. Because of aberration correction problems, it is advised to use a focal reducer at a reduction factor very close to its nominal factor.

The relation between the reduction factor R, the focal length FR of the reducer and the distance D between the lens and the focal plane is:

R = 1 - D/FR

Determining the focal length of a focal reducer is easy: point the Sun (or the Moon) with the reducer alone, and measure the distance between the lens and the solar (or lunar) image.

The Meade/Celestron F/6.3 reducer has a focal length of about 230 mm. Therefore, the nominal distance between the lens and the focal plane is about D = FR*(1 - R) = 230*(1 - 0.63) = 85 mm. It is only at this distance that this reducer gives its nominal reduction factor of 0.63x.

The drawing above shows that a focal reducer moves the focal plane closer to the telescope. On some instruments, the focusing range may not be sufficient to reach the point with a focal reducer. Furthermore, specially on SCTs, vignetting problems are usually more critical with a reducer than without.'

for qns 5:

'http://easyweb.easynet.co.uk/~chrish/lx_ms.htm'

2 cents,
wenyi

Kamikazer · Post by **Kamikazer** » Sat Sep 11, 2004 6:28 pm

i have a qns too:
how expensive is a coma corrector?

gwenyi · Post by **gwenyi** » Sat Sep 11, 2004 6:36 pm

well.. some of them can realli empty ur pocket ... but then again, it will have to depend on the scope u are using.

televue coma corrector http://www.buytelescopes.com/product.asp?t=&pid=2776&m=

vixen r200ss coma corrector http://www.buytelescopes.com/product.asp?t=&pid=4706&m=

MooEy · Post by **MooEy** » Sat Sep 11, 2004 7:17 pm

"The surface of a Newtonian telescope’s primary mirror is parabolic. The parabola is the only geometric Surface that will focus a bundle of parallel rays to a point. Except for the limitations due to diffraction and atmospheric turbulence (seeing), a perfect parabola would focus a bundle of parallel rays to an infinitely small point. "

like i said earlier, a spherical mirror is undercorrected, a parabolic mirror is just nice as it focus all the light to a single point(aka perfect sperical aberation) and a hyperbolic is over corrected.

coma is only seen off axis, when the star is not in the middle of the fov. normally in smaller newts, less than 10" coma is not too bad. one usually see the coma at lower powers, where more of the "image circle" is being used.

a coma corrector is usually not needed till ard the 20" ranges. to give u a rough idea, the 8" f/4 r200ss doesn't show any coma with a 20mm eyepiece. might show a little with maybe a 30mm. i don't see any noticable coma in weixing's 6" f/5 with a 28mm eyepiece.

as for ur 3rd qns, i have stated in my first post here, a spherical can be made easily with machines, thus prefered in mass produced scopes. a parabolic mirror would need too much input from the optician thus avoided. but recently it seems that those machines are getting better, esp those china scope makers. but consistency is still not really there, can vary a little.

~MooEy~

kayheem · Post by **kayheem** » Sat Sep 11, 2004 7:24 pm

MooEy wrote:"

a coma corrector is usually not needed till ard the 20" ranges. to give u a rough idea, the 8" f/4 r200ss doesn't show any coma with a 20mm eyepiece. might show a little with maybe a 30mm. i don't see any noticable coma in weixing's 6" f/5 with a 28mm eyepiece.

~MooEy~

I think coma correctors are needed in faster scopes e.g. F/5 or faster, not so much the size of the mirror. To see coma, I think you should do it by taking a picture at prime focus, rather than using an eyepiece as cheaper eyepieces can cause edge distortions on its own.

I used to own a Taiwanese 8" F/4 and it did show coma with an eyepiece that was improved when the Vixen coma corrector was used.

Kay Heem

MooEy · Post by **MooEy** » Sun Sep 12, 2004 7:40 am

true la, but it's the bigger scopes that have really fast f/ratio due to size problems. really hard to justify spending $600+ on a paracorr on a $1k+ scope.

oh ya, how gd is the vixen coma corrector? and how much u paid for it?

~MooEy~

kayheem · Post by **kayheem** » Sun Sep 12, 2004 9:37 am

MooEy wrote:true la, but it's the bigger scopes that have really fast f/ratio due to size problems. really hard to justify spending $600+ on a paracorr on a $1k+ scope.

oh ya, how gd is the vixen coma corrector? and how much u paid for it?

~MooEy~

Hi,

If I remember correctly, there are currently 4 brands of coma correctors on the market, viz Televue (Paracorr), Lumicon, Vixen and Baader. The Vixen one is attached directly to the Vixen focuser. Hard to describe it, until you see it. It does not come in the standard 2" or 1.25" flavour.

THe Baader is the most inexpensive of the lot. I think it costs about Euro 120

http://www.cloudynights.com/reviews3/GSO.htm
http://www.baader-planetarium.de/zubeho ... ferenz.htm

Hope that helps.

Kay Heem

MCYM · Post by **MCYM** » Sun Sep 12, 2004 6:16 pm

IMHO the 2" MPCC is something i could vouch for. Practically i use it on a F3.5 fast refractor for imaging and it reduces the coma and astimism to acceptable level. To top it off it comes with a number of attachment accessories. It is no wonder it's call Multi-Purpose CC. For the price, i regard it as a steal.

Picture here:
http://pachome1.pacific.net.sg/~audioworks/CCD/MPCC.jpg

The TV Paracorr have a 1.2 magnification factor with in reality defeats the purpose of coma correction on a fast mirror.

Good Luck in finding one.

Matthew

Kamikazer · Post by **Kamikazer** » Mon Sep 13, 2004 9:42 am

MooEy wrote:"

a coma corrector is usually not needed till ard the 20" ranges. to give u a rough idea, the 8" f/4 r200ss doesn't show any coma with a 20mm eyepiece. might show a little with maybe a 30mm. i don't see any noticable coma in weixing's 6" f/5 with a 28mm eyepiece.

~MooEy~

i thought coma should be magnified in higher power?
and there is less apparent coma in low power?
how come its the other way round?