purpose of spider vane ?

[gwenyi]

hello all. i'm not too sure what's the purpose of a spider vane(is it to decrease the time taken for thermal equilibrium ?). at the same time, why is it that some maksutovs have spider vanes, while some don't ?

please enlighten me on the above qns. thanks !



cheers,
wenyi

[kayheem]

hello all. i'm not too sure what's the purpose of a spider vane(is it to decrease the time taken for thermal equilibrium ?). at the same time, why is it that some maksutovs have spider vanes, while some don't ?

please enlighten me on the above qns. thanks !

cheers,
wenyi

Hi,

The spider vanes keep the secondary mirror in the centre of the OTA in the regular newtonian. Some catadioptric scopes e.g. Schmidt Cass (e.g. Celestron C5, C8 etc) and Maksutov Cass (e.g. Meade ETX-90, Skywatcher 127 MCT) have a corrector plate to hold the secondary in position, so they do not need a spider.

Other Cassegrains e.g. Vixen VMC200L have a spider as the corrector lens is built-in elsewhere:

http://www.vixenamerica.com/Products/De ... Cat=VC200L
http://www.vixenamerica.com/Products/De ... at=VMC200L

There was a guy who tried to build a regular newt using a glass optical window to support the secondary for commercial use. Unfortunately, the design did not take off.

I think there was a thread on straight spider vanes creating spikes on bright stars. Some like it, others don't.

Each design have their pros and cons.

Kay Heem

[carlogambino]

hmm... so the straight spider vanes actually cause the spikes due to diffraction? So MCTs and SCTs will never have diffraction spikes?

There was a guy who tried to build a regular newt using a glass optical window to support the secondary for commercial use. Unfortunately, the design did not take off.

not too sure what's the difference between this and schmidt/maksutov newtonians. pls enlighten me. thanks!

[kayheem]

hmm... so the straight spider vanes actually cause the spikes due to diffraction? So MCTs and SCTs will never have diffraction spikes?

not too sure what's the difference between this and schmidt/maksutov newtonians. pls enlighten me. thanks!

That is correct. MCTs and SCTs do not have diffraction spikes..The difference between Schmidt and Maksutov Cass are the design of the primary and corrector plates. The mak corrector plates are a curved mensicus type while the schmidt ones are a very complex curve but they look like thin plain optical windows.

Curved spiders have diffraction spikes but they are not obvious as they are all spread out.

Kay Heem

[MooEy]

hmm..normal newts try to correct for spherical abberation by using a parabolic mirror. but it's not easy to make a parabolic.

mass producing using machines usually make a big mess out of the mirror. normal polishing machines are only gd at making spherical mirrors, not parabolic. the optician himself often have to do most of the grinding to get a precise parabolic shape, which is often avoided in mass produced telescopes.

the idea behind the sct and mct is that the mirror can actually be a spherical shape, which is much easier to produce. but the problem with sct is that the corrector is actually an apsherical(complex curve as mention by kayheem), which make it no easier to make a perfect sct corrector compared to a parabolic mirror.

commercial sct maker often compensate for this problem by making the central obstruction bigger to hide most of the airy disc during a star test. not really a real fix, but it makes the star test much nicer.

the maksutov is a much more elagant design. by using a menicus corrector, which is actually in the shape of a sphere. this make the corrector extremely easy to produce. combine with a slighter longer focal length, it allows gd control of spherical abberation.

in the case of a mak newt, the menicus is still there, the primary mirror is still a spherical, but the secondary mirror is a flat diagonal instead.

i have no idea how schmidt newt works, in fact i do not know if they really work. seems like meade is the only one making them, and i do not hear much raved reviews or anything close. seen thru one which wasn't really collimated, can't really comment, neither do i wish to comment.

~MooEy~

[carlogambino]

Thanks for the answers!

in the case of a mak newt, the menicus is still there, the primary mirror is still a spherical, but the secondary mirror is a flat diagonal instead.

hmm... then in a regular newt, what is the secondary mirror shaped like?

hmm..normal newts try to correct for spherical abberation by using a parabolic mirror. but it's not easy to make a parabolic.

I heard that parabolic mirrors introduce coma. To solve the problems of this abberation(coma) and and the same time prevent spherical abberation of the spherical mirrors in a cassegrain, corrector plates are used instead. I wonder why the newtonian configuration risks the abberation of coma and uses parabolic mirrors instead of spherical mirros with correcting plates. Anybody can answer this?

[gwenyi]

coma can be easily solved with a coma corrector and parabolic mirrors produce excellent images. that's why takahashi epsilon astrograph series uses hyperbolic mirrors to reflect the light to exactly a single point, so that all the images will appear pinpoint. super crisp images when coupled with cam


cheers,
wenyi

[MooEy]

in a regular newt, the secondary mirror is flat, tilted at 45 degree.view from top, it looks like a eliptical(oval shape)

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.

in a newt, there is only 2 mirror, which one of them is completely flat and doesn't correct for anything. the primary itself does a gd job of delivery on axis images, but off axis images suffers. by adding a coma corrector removes most of the coma.

the tak epsilon uses a hyperbolic mirror, which is suppose to be overcorrected. the corrector in the focuser fix the coma and also the overcorrection.

~MooEy~

[MooEy]

"I heard that parabolic mirrors introduce coma. To solve the problems of this abberation(coma) and and the same time prevent spherical abberation of the spherical mirrors in a cassegrain, corrector plates are used instead. I wonder why the newtonian configuration risks the abberation of coma and uses parabolic mirrors instead of spherical mirros with correcting plates. Anybody can answer this?"

simple, a spherical mirror is undercorrected. a parabolic mirror is just nice and a hyperbolic is overcorrected. newts are simple, so it makes sense to work with a well corrected mirror rather than one that is not and try to correct for it.

the schmidt newt is suppose to use a spherical mirror and a corrector as u describe. dun ask me abt it's performance.

at any rate i rather have a 1/8 wave parabolic mirror and correct for the coma with a coma corrector than having a 1/4 wave undercorrected sperical mirror and have a schmidt corrector.

~MooEy~

[weixing]

Hi,

I wonder why the newtonian configuration risks the abberation of coma and uses parabolic mirrors instead of spherical mirros with correcting plates. Anybody can answer this?

There is not much problem with using spherical mirror if your f-ratio is f/9 or higher. But can you imagine a 10" newtonian with a f-ratio of f/9... it will be at least 2m in length, so in-order to make the newtonian shorter, the f-ratio have to be reduce. But Spherical mirror will have spherical aberration with low f-ratio, so parabolic mirror is use. A good parabolic mirror will only have coma at the very edge, so basically it is negligible... anyway, you can aways use a coma corrector to correct most of the coma. You can use spherical mirrors with correcting plates to correct the spherical aberration, but it will be impractical if you are building a 20" or larger telescope.

Have a nice day.