2" diagonal and focal length

[Adrian]

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?

The opposite, in fact. No diagonal creates a shorter focal length. The further you extend the focal plane behind the scope, the longer the effective focal length (and if the secondary is not oversized, the lower the effective aperture of the scope). As you move it forwards, the fast primary light cone goes around the secondary, effectively 'cropping' the fov & aperture.

Also, as you state above, the correction is best at a specific point; outside this point you will get increasing spherical abberation. So putting binoviewers in a diagonal is probably a bad idea

But you could investigate, using various spacers, the best corrected point to put the camera on your scope.

[alan potts]

John,

Sorry, didn't read the full thread, it's so hot I'm half asleep, need some of your rain. Yes of course the main mirror move and the secondry is static, so it must change. I have a 3rd party focus but still use both, just for variety.

Alan.

[Astro_Baby]

Nebula, its a good question as to where the focal length is measured from. There is a hellishly long thread in another place about this with regard to the Skymax 127.

This all comes into play when you add an external visual back focuser and or 2" diagonal. Conventional wisdom would say that as you add extra length on the back, and this is only for a Mak or SCT then you run the risk of clipping the light cone resulting in an effective smaller aperture which esults in less light grasp and consequently a dimming of any object. My own observations we that it made no difference to the image whether or not I used the standard 1.25 adapter and diagonal OR an external focuser and 2" diagonal. This was disputed.

The recent long thread showed that on the 127 and 180 maks from skywatcher in fact the effective aperture is mostl likley 10mm smaller than stated. The math and polemics here are somewhat long, complicated and boring but the upshot is because the scopes operate at about 10mm aperture less than their theoretical max there is in fact no dimming of images caused by the light cone being truncated. Thats because the image is in effect already truncated by the scopes own design.

In a Mak or SCT as Adrian has explained the focal length is variable based on where the main mirror is focused AND any external length to the light path caused by an external focuser or 2" diagonal. So I can hear you ask why dont you just add a 6' length of pipe and have massive focal length. The answer is because if you add much length you WILL cause the light path to be compromised and you will see dimming of the object.

If you want to see the other thread and cant find it on the other place hit me up by PM and I will send you a link.

[username]

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.

Thanks for your explanation and sorry about the late reply. What you've said above is sort of what I was wondering, exactly what is the focal length with the diagonal in? So when they quoted 1300mm on the side of the scope do they mean with eyepiece in the back where the diagonal goes or 1300mm with a 1.25" diagonal in. There must have been a standard extension or diagonal plus standard eyepiece for them to come up with that figure as it is dependent on the optical train out the back.

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

Hehe The other halves measuring tape was the closest thing to hand I was actually using it to see what the exit pupil size was (by pointing it at lit wall) with different eyepieces and was suprised to see that with a 15mm plossl that (within the accuracy of the measuring tape!) it was more like 1mm raher than 0.85 meaning that it was giving me 1x the aperture magnification and the focal length was nearer 1500mm. Calipers would have been more effective though.

Nebula, its a good question as to where the focal length is measured from. There is a hellishly long thread in another place about this with regard to the Skymax 127.

This all comes into play when you add an external visual back focuser and or 2" diagonal. Conventional wisdom would say that as you add extra length on the back, and this is only for a Mak or SCT then you run the risk of clipping the light cone resulting in an effective smaller aperture which esults in less light grasp and consequently a dimming of any object. My own observations we that it made no difference to the image whether or not I used the standard 1.25 adapter and diagonal OR an external focuser and 2" diagonal. This was disputed.

The recent long thread showed that on the 127 and 180 maks from skywatcher in fact the effective aperture is mostl likley 10mm smaller than stated. The math and polemics here are somewhat long, complicated and boring but the upshot is because the scopes operate at about 10mm aperture less than their theoretical max there is in fact no dimming of images caused by the light cone being truncated. Thats because the image is in effect already truncated by the scopes own design.

In a Mak or SCT as Adrian has explained the focal length is variable based on where the main mirror is focused AND any external length to the light path caused by an external focuser or 2" diagonal. So I can hear you ask why dont you just add a 6' length of pipe and have massive focal length. The answer is because if you add much length you WILL cause the light path to be compromised and you will see dimming of the object.

If you want to see the other thread and cant find it on the other place hit me up by PM and I will send you a link.

Hi, thanks for the heads up. I found it in the other place and it's a long read and absorb! Very good thread and it helped me.

So looking at other people data I can assume (I won't say safely..) that my Skymax 102 is really a Skymax ~96 (by doing a dodgy interpolation with quoted skymax 90mm and 127 numbers) due to the diverging meniscus lens being same size as the mirror

Then looking at the OP's star drift timings for their 127 effective focal length with different amounts of back focus and scaling it down to the 102 with a 1.25" diagonal I'm probably operating at an effective focal length of 1400mm giving me a focal ratio of f~14.4.

Interesting their 127 has a focal length of 1640mm if you interpolate for a 1.25" diagonal back focus

Which puts my mind at rest and will help me in my eyepiece decisions and doing my own accurate measurements. Thanks again (and sorry to sort of hijack this thread)