Increasing focal length

[dawson]

In theory, could one just have an empty tube coming off the visual back of the scope, the same length as the scopes focal length (say 2700mm) and double the focal length that way? Physically would it work?

James

[Guest CodnorPaul]

You need someone who knows what there talking about here James but my initial impression is the optical systems are designed to focus to a point of light at a specific point so the image you got would be tiny

[dawson]

This is where i need to read an optics book.

But isn't the light passing that focal point, and on its way spreading out again, enlarging the image once more?

Where are all these clever boffins at half seven in the morning

James

[BAZ]

The thing is the corrector plate does not act as an objective lens, in the same way a refractor objective does. It is there to correct the light path for the mirrors, which do the focusing of the light. If however you had a refractor lens of that focal length, then yes, but it would have to be a substantial tube to stop any flex, or the collimation would be shot.

[philjay]

Simply extending the tube is not increasing the fl James. To increase the fl you must regring the mirror or lens. However one can cheat by sticking a barlow, telextender, powermate or whatever the makers want to call them. These amplify the image and give the effect of increasing the fl but with consequeences, extra glass and surfaces, amplifying ca in achro or poorly corrected apo refractors, etc Thats why its nice to start with a long fl primary to begin with

[dawson]

That's useful, thanks.

James

[T A WOW]

I think that phil has said it all. Simply sticking a tube on the back would do nothing other than look daft.

You could probably stick a long f length refractor to the back of it, but I wouldn't of thought it'd do much for your viewing pleasure. You may have some balancing issues with your mount to

[Brantuk]

The focal length for any scope is fixed unless you refigure it. The tube simply contains it and supports the optics - just get a longer fl scope like that 10" refractor at the show lol

[dawson]

I see. Now i feel really thick.

So adding that extra inch or two of empty tube onto my tatty old refractor (which is no longer tatty), didn't do anything with the focal length of the optics, it simply allowed the eye piece to be held in a position where it coincided with the focal point of the optics. I see.

Can we delete this thread?

James

[BAZ]

I think it's a useful thread James, it's a question which may be asked in the future, and here it is for others to see.

 

If you don't ask, you don't learn, so keep on asking James. You may be asking the question someone else may not want to, so in effect you are doing a public service.

[Brantuk]

Lol - afraid we can't delete it mate - it'll be a useful point of learning for other newbies - and it'll encourage them to ask questions without fearing a non friendly answer hehehe.

[dawson]

So when i'm an international expert on the Encke Dision of Saturn you lot can send this thread to The Sun or The Mail and out me as a fraud who knows nothing about optics haha

On a semi serious note, when i turn the focus knob of my mak-cassegrain, and the primary mirror goes towards the corrector plate, that presumabely shortens the focal length slightly? But what happens to prime focus then, it must also move, but does that negate the change in focal length the mirror has introduced?

James

[xanthic]

Does this help? - http://en.m.wikipedia.org/wiki/Optical_focus

[dawson]

Erm...

[T A WOW]

I believe that adjusting focus on a Mak cass or Schmitt (spelling) cass does alter the effective f length of the scope. Not sure by how much though as I don t know how far the mirror actually moves. As for prime focus. I don't know what that is. I assume it's something to do with taking photos?????

[+Graham]

James

you are missing the point.

The Focal length of a scope is a fixed length set by the curve ground into the mirrors / lenses.

It makes no difference by how much you change the distance between the mirrors the focal length will never change.

Think of it as a piece of string.

No matter what you do to it be it fold it or roll it up into a ball the length of the string  will still be the same length.

[dawson]

Ah. So the distance between the corrector plate and prime focus IS fixed; moving the primary mirror in on a Mak-Cassegrain must just then increases the distance from the 'secondary' to prime focus.

 

I thought the focal length of a Mak-Cassegrain did vary slightly, because the mirror moved, but what you are saying makes sense.

 

Thanks.

 

James

[T A WOW]

Interesting metaphor graham. Well explained though.

[Brantuk]

I think I see what James is asking......

 

For some eyepieces or attachments we do put spacers in to hit the focal point (which is always in the focal plane somewhere). Whilst the curve on the mirror of a mak/sct is a constant determinant of the focal length - moving the primary back and forth can move the focal point to a different position within the focal plane.

 

We sometimes also do this with Newtonians eg for focusing binoviewers or using barlows - we may need to move the primary up the tube slightly to get the focal point in the right place. But the focal length remains the same in both types of scope.

[Tweedledee]

I'm not sure exactly what James is trying to work out, and I also think that other interested posters on this thread may be looking at this from slightly different angles.

 

My post may help some or confuse others, but here goes.

 

This is how I see it...

 

In a newtonian or a refractor there is only one curved optical element to focus light, the primary mirror or the objective lens, assuming no barlow or reducer is in use. Any flat diagonal mirror only has the effect of changing the direction of the light. Therefore in newts and fracs, the focal length is always fixed and remains the same. Also the focal plane will always be in the same position give or take a few microns due to temperature changes.

 

In a cassegrain telescope, whether Classical, Schmidt or Maksutov there are two curved elements that both focus the light, the concave primary and convex secondary mirrors, (we can ignore the corrector plate since this does not affect focal length). If the cassegrain telescope is focused by moving the primary mirror, then the distance separating the primary and secondary mirrors will change. This will slightly change the effective focal length of the telescope and also change the position of the focal plane at the back of the telescope in strict accordance with some mathematical formulae.

 

If you don't like the maths please don't read further.

 

Distance from secondary to focal plane = ((f1-d)*f2)/(f1+f2-d)

Effective focal length = (the above formula)+M*d

Where;

f1= primary focal length

f2=secondary focal length (secondary is convex so negative in value)

d=separation of primary and secondary

M=magnifying power of secondary

 

The following numbers are calculated using reasonably good approximations of focal lengths and separations of primary and secondary mirrors, since I do not have access to the exact figures, so the following serves as an approximation only, although the formulae used are set down in the physics of optics.

 

I did the following spreadsheet to illustrate this with both the mak and sct. The formulae are the same for both. Please don't take the numbers as exact, they just show the trend, but probably aren't too far out.

 

 

A typical 180mm Mak has a focal length of 2700mm and therefore a focal ratio of F15.

 

The above spreadsheet shows that if the primary mirror is moved just 5mm closer to the secondary using the focusing knob, the effective focal length will be increased by about 155mm and the focal plane will pushed approximately 150mm further back and the focal ratio will become about F15.9. Conversely a similar move of the primary in the opposite direction will result in a focal ratio of about F14.4.

 

The focal length (and therefore F ratio) can be varied by some 10% plus or minus using mirror focusing and can also allow a large amount of extra back focus if required which can be useful for imaging.

 

Since most eyepieces need the focal plane in very roughly the same position (some are even parfocal with each other), this change of focal length or focal plane position will be far less apparent and barely noticeable since refocusing between eyepieces will change the primary to secondary distance only slightly. So with eyepieces the differences will be minimal and you will always be working pretty close to F15 on the mak.

 

If changing from say a 1.25" diagonal to a 2" diagonal, then the light path would need to be extended substantially to allow for the extra light path through the larger diagonal. Such a change would make a more significant difference to the focal length and ratio, which would be worth taking into account.

 

Exact focal lengths can be worked out at the scope using other methods if required, but thats a whole other set of calculations.

[dawson]

Peter, you have answered my second question perfectly; the focal length of my Mak is not fixed and will vary (albeit slightly) when moving the primary mirror.

Thanks for taking the time to do the math to show this; i've not looked at the soread sheet as i'm on my phone, but the text makes sense.

Thank you.

James

[philjay]

Nice work Peter, yep Im guilty of thinking purely on the refractor side of things and forgot about the convex secondary effect when shifting the primary closer to it. Came across this awhile back when I had my C11 and forgot about it. 

 

I was musing this awhile back and was thinking of the parallels. 

I was wondering about the closer you move the 2 mirrors together the longer the focal length but if they were to be wound in very close then you would lose aperture due to the secondary being too far into the primary mirror light cone, thus missing some of the aperture of the primary. Similar effect to stopping down a refractor in a way I s'pose. In a mass produced scopes this shouldn't really happen as the focus mechanism will no doubt be designed to have the correct travel for the optics.  

 

Probably total twaddle but that's the way my brain cell goes occasionally

[Tweedledee]

You're welcome James.

I enjoy crunching the numbers .

 

Cheers Phil.

The way the numbers seem to work, indicates that only a small movement of the primary is needed to cause a large movement in the the focal plane. Therefore, unless you are pushing things right to the limit, there should be little if any vignetting or detrimental effects caused by the relatively small mirror movements.

 

My brain cell enjoyed the workout, and although I was already aware of roughly how mirror focusing affects the focal length and focal plane position in a cassegrain, it was enlightening and quite satisfying to see the way the formulae actually describe this.

 

I got the formulae from a book called Telescope Optics : A Comprehensive Manual For Amateur Astronomers, by Rutten and van Venrooij, Willman Bell Inc. When I first opened it, even I found it a little daunting, there is barely a page not at least half filled with long winded formulae, tables, graphs and optical diagrams. After a good flick through the book, I could see that it was definitely right up my street, especially once I'd established there was no calculus in it  .

[dawson]

I was reading a book (The Modern Amateur Astronomer (1995, so not that 'Modern' but still an interesting read) http://www.amazon.com/Amateur-Astronomer-Patrick-Practical-Astronomy/dp/3540199004) and there was a section talking about the changing focal lengths of Cassegrain variant telescopes. Here are the three pages which say pretty much our own Peter Sull had said. It starts half way down the first page:

Edited May 27, 2013 by dawson