focal length

[steelfixer]

Hi all.

Can anyone tell me the mathmatical equation for working out the focal langth of a primary mirror.

I like so many beginners purchased a 150mm Seban telescope which I am quickly learning is a pile of ****.

It has a focal length of 1400mm using a focus tube.

What I am thnking of doing is to ditch the focus tube but I need to know how to work out where to put the secondary mirror in relation to the primary.

The first thing I intend to do is buy a decent telescope but rather than put this one in a corner to gather dust I am thinking of using the parts to build one myself.

Any thoughts ideas ect would be greatly appreciated.

Thanks

Graham

[Demonperformer]

A mirror will have a fixed focal length, depending on the curve on its surface, so if it has a focal length of 1400mm, it will always have a focal length of 1400mm. That length sounds a bit unusual for a 150mm mirror. If you are saying that 1400mm is the distance from the mirror to the focussing tube, the focal length is actually longer (roughly by the length of the focussing tube), and I would guess that it is a 150mm f/10 (i.e. actually 1500mm focal length).

HTH (Sorry if I am completely misinterpreting what you are saying).

[Naz]

If you mean the part that holds the eye piece, then you only need to replace focuser in the same postion. If you mean the body of the scope, you need to keep the same measurments as the original tube. You can not alter the optical properties by changing the physical size of the metal work.

You will find that the objective mirror is sphereical, cheap to produce, and not a parabolic mirror which is required to bring all the light into focus at the same point.

You would be throwing good money after bad, I am sorry to say.

Save up, read the comments and reviews on SGL, ask questions, and if possible go to a local astro club / star party / or find some one who can show you their scopes. Then you can make an informed choice.

[steelfixer]

Hi there

The 1400mm focal length is including the tube which has a correction lens in its base.

The actual distance between the two mirrors is about 700mm.

I have tried just removing this correction lens but to get an image the eye piece has to be inside the main body of the telescope.

I have deduced wether correctly or not that the secondary mirror will have to be moved closer to the primary mirror to bring the focal point outside the main body.

As there is no information on the primary mirror as to its focal length I was hoping that there must be a mathmatical formular to work this out.

I.e diameter of mirror + - x / curve of mirror ect.

thanks

graham

[Demonperformer]

focal length = (radius squared)/(4xsagitta)

the sagitta is the maximum depth of curve

[steelfixer]

your are a dimond thanks for that.

[swamp thing]

Hi Graham

Correct me if I'm wrong but if your scope is as you say "A pile of ****"

Then using the parts to make another one would result in another "pile of ****", as a scope is only as good as the sum of it's parts.

If you want to build one start with some quality parts. That way all your hard work will be rewarded.

Regards Steve

[Moonshane]

I wonder if you could buy a used 6" f5 or f6 mirror and replace the current mirror and corrector with that, using the same secondary and adapting the cell slightly?

This might be a cost effective way of making a big improvement to the scope.

[steelfixer]

In answer to your points.

Steve you are probebly right but i think the main fault with this scope is the poor quality of this correction lens if that is what it is called.

After reading several threads on here about focusing issues I am amazed to hear from people that adding a barlow lens lengthens the focal length thus if I am correct means moving the focuser out to obtain the new focus. With mine if I use a barlow lens I have to move the focuser in to get the correct focus.

My train of thought is to get rid of this correction lens and shorten the body of the scope thus moving the secondary closer to the primary.

I asume it will reduce the scopes magnifacation but I am hopeful it will clear up the faults with poor focus.

I wish I had done a little more research before rushing headlong into buying this scope but then we live and learn.

I am looking into buying a 200mm newt/dob, So I do not rearly care about chopping this scope about. If nothing else I should learn a bit about telescopes, mirror focal lengths and the like so it will not all be in vain.

thanks

graham

[Naz]

How about removing the "correction lens" altogether, and replacing it with a better quality Barlow, after all, that is all it is. Just slide it into the eyepiece holder. You should get an improvement, but will still suffer from the problems of the main mirror.

The tube should be the correct length for the basic focal length of the mirror, the corrector lens acts like a barlow to give you the 1400mm focal length. So no need to do anything drastic to the scope. Hope this helps.

[steelfixer]

that is not a bad idea.

my only concern is the corrective lens is at the base of the tube some 5 to 6 inches away from the base of the eyepiece.

will that make a difference to the barlow lens.

i guess there is only one way to find out.

i will let you know soon.

thanks for the idea.

[steelfixer]

i have tried the barlow idea to no avail.

what I have found out is this correction lens for want of a better name is a double concave lens about 15mm thick. by that i mean both sides of it are concaved.

it does not magnify or diminish so i presume it is there to straighten out the light rays as they come off the seconday mirror up the focus tube.

Some one on here must know what it does.

help !!!!!

[Naz]

Barlow lenses are concave on both sides, the one fitted in the factory maybe of a different focal length. Sorry I have run of suggestions other than taking the main mirror out of the tube and mounting it on a bench and playing with different combinations.

Kind of back to the drawing board.

[Bazzaar]

Yes, these types of scopes are Jones-Bird designs(or Bird-Jones)

They use a simple spherical primary giving them a cost advantage over other scope designs. This causes spherical aberration so a corrector lens is placed a little way inside of prime focus, this in turn extends the apparent focal length of the system. The true focal length of the primary is approximately the physical distance between the primary and eyepiece holder.

I dont think it is correct to consider the corrector lens as just a Barlow.

The quoted focal length for these scopes is the effective focal length including the effect of the corrector.

The performance of this design is now dependant on the quality of the corrector, as I think you are now finding!

If it is made achromatic or better, this design can work well.

Regards

Barry