2" diagonal and focal length

[andrew63]

I guess a diagonal will increase the focal length of a telescope. Just realised when looking at magnifications field of view etc. - with say a 2" diagonal will in effect add some 30 to 50mm to your telescopes focal length?

I have not really considered this before, but would increase the mag a bit with short focal length lens in particular.

andrew

[John]

I don't think a diagonal acts as a focal extender. A 2" diagonal will swallow up around 50mm of inwards focus travel though as the light path through it is longer than a 1.25" diagonal. The effective focal length of the scope does not change though, as far as I'm aware.

[DirkSteele]

I don't think a diagonal acts as a focal extender. A 2" diagonal will swallow up around 50mm of inwards focus travel though as the light path through it is longer than a 1.25" diagonal. The effective focal length of the scope does not change though, as far as I'm aware.

I agree, it does not alter the incoming light cone as a barlow or powermate would, but instead eats into the back focus of the scope.

[andrew63]

Thank you both - so the mirror bringing the focus to a different point of focus will not increase the length like say a 2" extender will. And conversely not using a diagonal will only effect how much focus you have to use.

andrew

[faulksy]

i have often wondered this my self

[JamesF]

I was investigating this when I built my binocular objective based finders recently. I discovered that 1.25" and 2" diagonals have different effective optical lengths, and mirror diagonals and prism diagonals also have different effective optical lengths. Prisms have shorter effective optical lengths, which is logical when you think about how they work. That is, assuming all other things are equal, such as the length of the eyepiece holder on the top of the diagonal and the fit of the diagonal to the back of the focuser, and that we're talking about 90 degree diagonals.

I can't recall the figures for anything other than a 1.25" mirror diagonal unfortunately. That works out at about 75mm. I measured the diagonals I used and to within the couple of millimetres error in my measurements that was consistent.

I'll see if I can find the other figures again.

James

[JamesF]

Aha! That was easier than I imagined.

The other figures I have are around 110mm for a 2" mirror diagonal, 55mm for a 1.25" prism diagonal and 80mm for a 2" prism diagonal. Again, these are approximate figures as they'll depend on the exact fit of the diagonal and the length of the tube that receives the eyepiece barrel.

It's correct that a diagonal doesn't change the focal length. What's interesting though is that if you have a mirror diagonal and you're a bit short of in-focus then swapping to a prism may sort you out, and if you have a prism diagonal and are short of back-focus then the extra optical length of a mirror diagonal may give you enough to avoid needing extensions.

James

[John]

Thank you both - so the mirror bringing the focus to a different point of focus will not increase the length like say a 2" extender will. And conversely not using a diagonal will only effect how much focus you have to use.

andrew

Yep, thats it.

With some scopes you have to have a diagonal in place to bring them to focus. Either that or add an extension tube. The Japanese apparently prefer to use refractors "straight through" though - my Japanese made Vixen has an extension tube supplied with it to enable straight through use.

When I tested the TAL Apolar 5" refractor, despite accepting 2" diagonals, the scope did not have enough inward focuser movement to bring eyepieces to focus when using it so it was effectively a 1.25" only scope despite having the larger diameter drawtube. Somewhat frustrating !.

[Adrian]

If you are using an SCT, Mak or similar scope with a compound system then by moving the primary mirror to focus then you ARE changing the focal length of the system to match the fixed eyepiece focal point.

I had a 10" SCT (nominal focal length 2500mm) and did some testing:

with no diagonal the focal length was ~ 2460mm;

with 1.25" diagonal the focal length was ~ 2725mm;

with 2" diagonal the focal length was ~ 2840mm

so quite a significant change.

For reflectors & refractors the focal length is fixed and you move the eyepiece to focus.

[JamesF]

The point there though is that changing the relative positions of the mirrors and corrector is changing the focal length, isn't it? If you don't move the primary mirror then the focal length with a diagonal installed is exactly the same as without one.

James

[Adrian]

Absolutely.

To labour the point, the focal length of any individual element (mirror/lens) cannot change without refiguring the element. For compound scopes it is how the different elements with differing focal ratios interact that changes the focal length, but the primary remains (for the 10" SCT) a 250mm f2 and the secondary a -f5, however they are positioned in the system.

For single element scopes there is no such interaction to change the FL.

[Carbon]

As mentioned, it will alter the focal length in a SCT. I see a noticeable difference in image scale of a full moon, for instance, switching from one diagonal to the other, or using no diagonal.

[Dunkster]

Sorry for the stupid question but could anyone explain infocus and back focus in a practical sense

[JamesF]

As mentioned, it will alter the focal length in a SCT. I see a noticeable difference in image scale of a full moon, for instance, switching from one diagonal to the other, or using no diagonal.

As I pointed out before though, it is not the diagonal that changes the focal length however. The focal length changes because you move the mirror. A diagonal is (should be) optically flat. It has no properties that change the focal length.

James

[JamesF]

Sorry for the stupid question but could anyone explain infocus and back focus in a practical sense

It's easier to explain with reference to a newt or refractor than SCT or Mak because their focusing mechanisms are different, so let's ignore those for the sake of this explanation They can suffer from the same problems, but they're perhaps not as easy to visualise.

The primary mirror/lens produces a focused image at a point called the image plane. Typically this will work out to be somewhere within the length of the focus tube and diagonal assuming the latter is fitted. To get an eyepiece in focus you need to get the focal point of the eyepiece to coincide with the image plane. To get a camera in focus you need to get the camera sensor to coincide with the image plane.

It can happen that the image plane is not far enough out of the focus tube to allow you to move the focuser in far enough to get your eyepiece or camera to reach the image plane. This is a lack of in-focus. It's a common issue with some newts when used with DSLRs. It can sometimes be fixed by using a low-profile focuser, or using a barlow which has the effect of moving the focus point further out of the focus tube. Sometimes the only option is to move the primary mirror further up the OTA.

If you're not using a diagonal on a refractor because you have a camera on it for imaging then it may be that you can't move the focuser out far enough for the camera sensor and image plane to coincide. This is a lack of back-focus. The usual fix is to add an extension to the front of the camera, effectively moving it further from the objective lens and within reach of the image plane.

Does that make sense? Anyone think of anything I've forgotten?

James

[AndyH]

I made some measurements a while back..........

Andy.

[Dunkster]

So it's an actual physical movement in or out of the tube - that makes sense - thanks

[JamesF]

I wish I'd found that when I was looking for information, Andy

I don't think it's too far out of kilter with the figures I have though. It does amply illustrate that the optical length of a diagonal may make a fair bit of difference at the extremities of focuser travel. It's a shame that sort of information rarely seems to get listed as part of the spec when you're looking to purchase one.

James

[username]

I've been wondering about the effect of a diagonal in my maksutov too. When they say the focal length is 1300mm where do they actually mean. The back of the telescope or with the digonal included somewhere.

I'm starting to think they mean the back of the telescope as I was using a measuring tape to look at the exit pupil of a 15mm vixen npl plossl the other day (was cloudy ) and it looked pretty dead on 1mm which assuming the plossl size is correct would mean it's operating nearer f15 rather than f13 with the 1.25" diagonal in

I need to find some calipers for a more accurate check

[JamesF]

Forget the diagonal. It just doesn't matter. The focal length is the same whether it's present or not.

Focal lengths of compound lens/mirror systems are not as straightforward as those for a newt or simple refractor. Let's say, for the sake of illustration, that you have a Mak with a 1.25" diagonal connected. We'll say that the diagonal has an optical path length of 75mm and that the image plane is flush with the top of the diagonal which should be about the right place for a standard 25mm plossl eyepiece. If you then swap the 1,25" diagonal for a 2" model with a 110mm optical path length, the focal plane is somewhere down inside the diagonal, 35mm below the top (110mm - 75mm). To return the image plane to the top of the diagonal you need to move the focal plane 35mm further "out". With a Mak (or SCT) you can do that by moving the primary mirror closer to the secondary. It's a bit like moving the mirror up the OTA of a newt instead of moving the focuser outwards.

But here's the thing... Unlike a newt, the secondary in a Mak or SCT is not flat. It forms part of a compound optical system together with the primary and the corrector. And the focal length of a system of optical components is dependent on the distance between them. So, by moving the primary you're partly causing the image plane to move because the focal length of the system is changing.

If you have a Mak or SCT with a diagonal and eyepiece where the image is in focus then you could remove the diagonal and replace it with an extension tube of exactly the same optical length, refit the eyepiece and the image would still be in focus. The diagonal does nothing do the focal length, but, if you remove or replace the diagonal with something of a different optical length then by moving the primary to get the image back in focus you are also unavoidably changing the effective focal length of the scope.

At least, that's the way I understand it

James

[JamesF]

To further answer your question about the how the quoted focal length of the scope is measured, Jim, it's probably not done with a tape measure

It's likely to be calculated from the optical characteristics of the primary and secondary mirrors. For example, if the primary was an f/3 mirror and the secondary gave 4x magnification then you'd have an f/12 scope, so if your aperture were 100mm then the focal length would be 1200mm.

James

[alan potts]

I don't think a diagonal acts as a focal extender. A 2" diagonal will swallow up around 50mm of inwards focus travel though as the light path through it is longer than a 1.25" diagonal. The effective focal length of the scope does not change though, as far as I'm aware.

I was always of the belief that in a SC scope it did change the focal length and therefore the magnification, I remember reading this, I thing it was in a review on Cloud Nights site. I was of the opinion of 'how can it' myself. This is one for some extra homework.

Alan.

[John]

I was always of the belief that in a SC scope it did change the focal length and therefore the magnification, I remember reading this, I thing it was in a review on Cloud Nights site. I was of the opinion of 'how can it' myself. This is one for some extra homework.

Alan.

I think this may have been addressed earlier in this thread. An SCT's focal length does change when the focus is adjusted as this action moves the primary mirror, unless a replacement focuser has been fitted (see below). As a 2" diagonal needs more inwards focuser movement, the SCT focus needs to be adjusted when the 2" diagonal is inserted, with the consequential alteration in focal length. This is a factor relating to the way that most SCT's focus rather than the diagonal itself. If you lock the standard SCT focus and attach a 3rd party focuser, as some folks like to do, I reckon this would remove this issue. It's also a way to stop the "Mirror flop" that some older SCT's suffer from.

Hope that makes some sense !

[JamesF]

It's also made me wonder about imaging with Maks and SCTs. I tend to use a diagonal when imaging with my Mak because it makes swapping between eyepiece and webcam easier. If I didn't, and instead put the camera directly into the end of the OTA, I wonder if the required mirror movement would increase the focal length and give a larger image and if so by how much?

James

[JamesF]

And another thing

Celestron (for example) say that if you're imaging with one of the Edge scopes then the image plane should be a specific distance from the rear of the OTA. I wonder if that's because it's the point at which they consider the spacing of the mirrors gives the best-corrected image?

James