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(Another!) C11HD Back Focus Question

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#1 DwainM


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Posted 12 February 2013 - 11:04 PM

It’s ANOTHER back focus question…
This one goes the other way though. Instead of concentrating on the effects of going too far back I’d like to know what happens when you go the other way. I have searched the forums for a while now and just can’t seem to find the answers I’m looking for. :help:

The recommended back focus for my C11HD is 146mm but the factory supplied diagonal and 23mm eyepiece only measure ~100mm back focus from the rear cell. :confused:
Does this difference mean any thing to the visual user? I’m guessing it’s the difference between what our eyes can perceive versus the camera; but what exactly is the difference?
If the field stop of the eyepiece was @ 146mm as recommended for AP could you visually notice a difference between this and 100mm? I guess this will be related to the posts I see about locking the primary and using an external focuser.
And related to all that, I’m wondering which way does the primary mirror have to move when moving the focus point from 146mm to 100mm.
How does that change the illumination of the secondary and the visual image?

I hope this isn’t too many questions for one post.

Thanks folks! :bow:

#2 Eddgie



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Posted 13 February 2013 - 12:00 PM

I originally answered based on the C11, and that info would be incorrect for an EdgeHD 11 which is a different design.

If you chage the focal plane from 150mm to 100mm, you move the primary mirror to the rear (turn the focuser knob clockwise). This decreases the focal lenght of the scope at the same time.

The changes visually will be to small to see.

For imaging in standard SCTs, the baffles set the back focus. Less back focus is better because the size of the blur at the focal plane is a function of focal ratio, and the faster the focal ratio, the smaller the angular size of the blur will be.

But if you move the primary mirror back to far (in standard SCTs, this means less than about 100mm of back focus), the light coming from the inner part of the mirror close to the baffle will start to hit the baffle on the way to the primary mirror. In essence, you get vignetting at the center of the field.

The EdgeHD (and again, I re-wrote this after posting because I realized that I had made a mistake) is a much different design, and my guess is that because of the lenses in the system, the blur diameter is smallest across the feild at a specific mirror spacing and this may be for something like chromatic abberation which was not an issue in the standard SCT design.

Imagers don't really see an Airy Disk in their images. Instead, they see a blur circle. If the blur circle is kept small enough, stars look like points.

For visual use we see an Airy Disk, and the size depends not only on the focal ratio of the scope, but on the magnification of the eyepeice being used.

It is unlikley that changing the back focus more than 50mm (difference between a 1.25" or 2" diagonl) will make enough difference in the visual performance that anyone could be able to see it. Even a 100mm back focus change is likely impossible to see visually other than the very slight magnification change in the image.

For imageing though, we want the smallest possible blur diameter, and Celestron has designed this to occur at 146mm.

There are other issues though. The older SCTs would start to vignette the center of the field if you used mirror spacing less than 100mm. This is why Celestron always specified about 100mm of back focus for imageing.

As you move the primary further and furter to the rear (decreasing back focus), the light coming from the center of the mirror will start to hit the outside of the primary baffle on its way to the center of the seconary mirror.

The SCT design is really optimized to work with a rather small range of mirror spacings. To much mirror spacing and you vignette the center of the field, and once the mirrors get to close together, you start to vignette the outside (and even closer even the inside) of the field.

The standard SCTs were optimized for mirror spacing between 100mm and 200mm or so (The C11 could accomdate more than 200mm before the baffles started to reduce the aperture).

I don't have any analysis for the EdgHDs though.. I know that Celestron has moved the optimal back fucus further out and I was told by someone at Celestron that this was done to allow imagers to put in filter wheels.

But clearly this means that the baffle geometry would be affected in some way.

But since they have traditionally provided between 100mm and 200mm of vignetting-free back focus, my guess is that this is still the case.

You can measure the aperture yourself to be sure using a flashlight test, but the change in visual performance but my bet is that visually, the difference in perfmance otherwise is so amall as to be meaningulss for visual.

For imaging though, once again, the system usually will give the smallest blur diemeter at the manufacturer's recommended back spacing.

#3 DwainM


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Posted 13 February 2013 - 11:42 PM

Thank you.
You always take time to answer my questions in detail and that means a lot to me.
Whenever I post to CN, it's only after I've searched for the info I need in this and the other forums. I figure that most of the questions I come up with have already been asked a bunch of times already plus I enjoy the hunt. Digging around for the info is something I enjoy because I come across so much other info in the process. CN is really a special place because of folks like you.
I'm glad to be here.

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