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Metal back (back focus) tolerance

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#1 Anding

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Posted 17 October 2020 - 07:29 PM

Cassegrain telescopes have their focal plan at a fixed distance from the rear of the mirror cell / rear of the instrument.  From a CCD/CMOS sensor point of view, what is an appropriate tolerance (+/- mm) in the metal back distance for getting images with no loss of definition?  Can I work it out from the focal ratio and pixel size for example?

 

The reason I ask: the Tak reflector I am looking at requires a metal back distance of 56.2mm while ZWO state that their ASI imaging train has a length of 55.0mm.  I can replace one of the Tak couplings with a custom-made alternative made 1.2mm longer to resolve this.  This is not expensive and will be rigid, but of course is not adjustable.  However I'm wondering whether I can really expect a machine-shop to deliver such an adapter within a tolerance in length better than +/- 0.1mm.

 

Many thanks for your thoughts.

 

 

 

 

 



#2 michael8554

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Posted 18 October 2020 - 04:14 PM

Some Cassegrain designs - Celestron Edge for example - have a back-focus "sweet spot".

 

Yours is 56.2mm.

 

I would imagine that a 1.2mm error wouldn't be a problem, but you haven't said what Tak you are referring to.

 

Again, you haven't said what thread size you are joining your "ASI Imaging Train" to on your Tak,  but 1mm M-threaded spacers are readily available, which would get you really close.



#3 TxStars

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Posted 18 October 2020 - 09:06 PM

I would just use one of the tilt adjusters as you will likely need to make some tilt adjustments any way..



#4 Michael Covington

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Posted 18 October 2020 - 09:49 PM

Are we talking about a Schmidt-Cassegrain or a classical Cassegrain?

With classical Cassegrain, the question does not come up.  Just like a refractor, the focal plane *is* in one *particular* position and you focus the telescope by moving your camera or eyepiece in or out to reach that position.

Schmidt-Cassegrains are a different beast.  Their focal plane can be moved quite a lot by changing the separation of the primary and secondary mirrors.  So you can choose where you want the focal plane to be, fix the eyepiece or camera in position, and focus the telescope to put the focal plane there. There is a sweet spot that produces the best images, and as I recall, a reasonable tolerance for it is plus or minus 5 mm.


Edited by Michael Covington, 18 October 2020 - 09:50 PM.


#5 Anding

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Posted 18 October 2020 - 10:24 PM

Thank you all for the suggestions. This would be a modified classical Cassegrain with no front lens, but with a three element corrector behind the secondary and finally a reducer at the rear port. Unfortunately I am quite ignorant of how such focusing arrangements actually work and the technical details pertinent to the metal-back distance. I would like to find a resource to explain that!

Simple trigonometry suggests, treating light as rays, that light rays coming to a single point on the focal pane will spread to a disk on a parallel plane in front or behind. Let's say the sensor is at a distance x from the true focal plane and the focal ratio is f then the diameter of the disk will be y=x/f. Putting some numbers in, say x = 0.1mm or 100um and say f = 4 then y = 25um. That's equivalent to 5 pixels of 5um. To me this seems impossibly demanding, so there must be more to it. I guess the image sensor has some depth, which relaxes the constraint very slightly, and stellar images have a spread due to seeing anyway.

Tilt adjusters seems like the best option, provided of course that the path length is slightly "under" rather than "over" with the tilt adjuster fully tight ended.

Edited by Anding, 19 October 2020 - 05:22 AM.


#6 Michael Covington

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Posted 19 October 2020 - 08:18 AM

Basic question:  How do you focus the telescope?

If the focal plane is in a fixed position, you *must* be able to move the eyepiece or sensor forward or backward.  There is no other way to focus the telescope.

 

You cannot simply build the telescope to be correctly focused, because thermal expansion and contraction will cause it to vary.



#7 Anding

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Posted 19 October 2020 - 04:37 PM

I guess that’s true Michael, so perhaps the optical performance of the scope is better at the specifically quoted metal back distance? I’m still searching to find out how that degrades if the sensor is +/- mm away from that


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