Parabolic mirrors

[alowen]

Hi, is the sw130p really that much better than the sw130?

Does the 'parabolic' mirror actually make the images clearer? I also noticed the 130p has a shorter tube than the 130, why is this?

Thanks for all your help

Al

[acey]

Can't comment on those specific scopes, but a newtonian reflector needs to have a parabolic primary mirror for proper performance, because this is the shape that brings reflections from all parts of the mirror to focus at a single point. Under some circumstances you can get away with a spherical mirror because the errors will be small enough, but all other things being equal, parabolic will always outperform spherical.

[Cornelius Varley]

The 130P has a focal length of 650mm and the 130 has a focal lenght of 900mm hence the difference in tube length. Spherical mirrors do not work so well at fast f numbers because the reflected light doesn't focus at the same point on the focal plane, you get an area in the middle of the field of view which is in focus and then towards the edge it becomes less focused. With longer focal length telescopes with a small aperture this is less pronounced.

Peter

[brianb]

OTOH you don't have coma with a spherical mirror (or as much with a longer wavelength) ... and with a mirror made of anything other than Zerodur, the curve deepens whilst it is cooling because the centre is thinner than the edges - it used to be said that it was best to leave a glass mirror undercorrected by about 20%.

With a 5" mirror at f/7 the difference is very small - within the manufacturing tolerance. At f/5 spherical is not accurate enough & optical performance definitely will suffer. But do bear in mind that a spherical mirror is a great deal easier to collimate & with "average" collimation the theoretical performace advantage of a parabolic mirror will be lost.

[ED209]

That's very interesting. I was wondering how on Earth people ever ground mirrors by hand to be perfectly parabolic, I guess the mirror has only to be somewhere between parabolic and spherical to work to some extent.

[barkis]

An 8" f5 mirror left spherical, will not be able to bring parallel light to focus. It will suffer from that well known description, Spherical Aberration. Such as the Hubble mirror did when it saw first light. That's the reason the remedial missions to the telescope happened, to correct the mirror with intervening lenses.

The 8" f5 mirror with a paraboloidal shape, assuming the corrections are as they theoretically should be, will focus parallel rays, and no smearing will result..

The difference between the two curves, is in the order of 22 millionths of an inch. Not much, but so important.

Ron.

[alowen]

Thanks guys, so of the 2 scopes (130 and 130p) theres not a great deal of difference because of the difference in f/l then????

Al

[barkis]

The longer Focal length scope will perform well enough, just don't do magnification overkill, it won't respond.

Ron.

[ollypenrice]

OTOH you don't have coma with a spherical mirror (or as much with a longer wavelength) ... and with a mirror made of anything other than Zerodur, the curve deepens whilst it is cooling because the centre is thinner than the edges - it used to be said that it was best to leave a glass mirror undercorrected by about 20%.

With a 5" mirror at f/7 the difference is very small - within the manufacturing tolerance. At f/5 spherical is not accurate enough & optical performance definitely will suffer. But do bear in mind that a spherical mirror is a great deal easier to collimate & with "average" collimation the theoretical performace advantage of a parabolic mirror will be lost.

Funny how these things seem to come at once. A guest who made his large primary watercooled said that he did not undercorrect his mirror, knowing that the cooling would prevent the distortion Brian was referring to. This was all new to me three days ago.

Olly

[Jonmc]

I recently got a 130p as my first scope and what I have looked at so far has been in focus and crisp from edge to edge of the field of view

[Astro_Baby]

Theres also the advantage that a long focal length speherical mirror will have a big sweeet spot which makes collimation a doddle. I am restoring a very badly abused TAL 1 at the moment and even though when I got it the collimation was out by a mile it still showed reasonable views of the moon and a few other things. Not perfect but pretty good considering the state of its collimation.