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Orion 180 Mak with 1" back opening - considering focuser

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#1 MichiganSV102ED

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Posted 10 July 2019 - 08:31 PM

Orion 180 Maksutov Cassegrain with the smaller 1" rear opening.  (Scope is a few years old and not the new design with the 2" rear opening.)   QUESTION:  If I purchase the SCT thread adapter (Scope Stuff) and a GSO focuser will there be any issues with the performance of the telescope?  Does the focal length change (get longer) because of the added length of the focuser?  Please comment on the effect of the added focuser.  Thank you. 



#2 Garyth64

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Posted 10 July 2019 - 08:40 PM

The focal length is doesn't really change.  If you add a longer focuser, that does not change the focal length of the scope.  With a longer focuser you may never be  able to bring anything to focus.



#3 Bean614

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Posted 10 July 2019 - 08:43 PM

Actually, the opening is 1.25", not 1".



#4 Eddgie

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Posted 10 July 2019 - 09:56 PM

The focal length is doesn't really change.  If you add a longer focuser, that does not change the focal length of the scope.  With a longer focuser you may never be  able to bring anything to focus.

As I recall, the scope in question uses a moving primary, and it that is the case, then the focal length does change, so your statement would perhaps be more correct if you were to say that the focal length really does not change that much.

 

Any scope with a moving primary and negative secondary will have an increase in focal length if the primary is moved closer to the secondary.

 

Now, this does not mean that the increase in magnification will be enough to matter, but it will change.

 

Typically, the change is a couple of millimeters for every millimeter that the focal plane is moved away from the scope. There is a formula for it somewhere on the forum, but I will leave that for someone else to find.

 

Anyway, as I recall, it is a couple of millimeters for every millimeter of back focus, so adding a focuser with 50mm of light path could raise the focal length by 100mm or more. 

 

Also (and I don't know if this would be a problem with this particular scope) some MCTs will start to loose aperture if the focal plane is pushed too far to the rear. 

 

Now, the difference may not be enough to bother the OP (which is what you may have been implying), but technically, the focal length does change, and depending on the back focus that is added, there could be aperture loss. 

 

(I know that for f/10 SCT the focal lenght change is 3.1mm per every millimeter the focal plane is moved to the rear, but I don't remember where to find the formula.)



#5 luxo II

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Posted 11 July 2019 - 05:17 AM

The backfocus moves by approximately da = dA * m^2, where

 

da = the change in backfocus,

dA = the change in separation of the primary mirror and secondary, 

m = the magnification of the secondary mirror which equals the final focal ratio (f/15) divided by the primary mirror focal ratio, or about 4.

 

So the backfocus moves by about 16X the movement of the primary, which is why moving-mirror focussing poses a non-trivial mechanical challenge to implement in a way that works nicely with minimal axial or lateral sloppiness.

 

In an f/10 SCT m is about 3 so the movement is around 10X.

 

The exact formulae are at http://bobmay.astron...assformulas.htm (see da/dA)


Edited by luxo II, 11 July 2019 - 05:30 AM.

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#6 Garyth64

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Posted 11 July 2019 - 07:17 AM

"As I recall, the scope in question uses a moving primary, and it that is the case, then the focal length does change, so your statement would perhaps be more correct if you were to say that the focal length really does not change that much."

 

Yes, I know that.  I thought by saying "it really doesn't change", was more correct than me saying, "it doesn't change".  I thought that would be enough doubt, without going into a long explanation.  I guess not.

 

Still the question was if he added in a longer focuser would that change the f.l., and the answer is no.  Just adding a longer focuser will not change the f.l.

 

And many things you said, weren't exactly correct.  But, I'm not going to return the favor.



#7 Garyth64

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Posted 11 July 2019 - 07:33 AM

I'm not going to go into any detailed explanation, but In my Gregorian, I wanted more BFL.  I changed the separation of the mirrors.  The original parameters had p = 6.74" and p' = 39.19".  I changed p to 6.66" which changed p' to 42.13".

So, in my system, moving the secondary .08" moved the focus 2.94".  (36x) 

 

I use a different formula than on the website link.   And there is a lot of good information there, that is worth reading.  Especially, the part where the overall correction of the system could suffer if the changes are too great.


Edited by Garyth64, 11 July 2019 - 07:39 AM.


#8 luxo II

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Posted 11 July 2019 - 07:50 AM

Still the question was if he added in a longer focuser would that change the f.l., and the answer is no.  Just adding a longer focuser will not change the f.l.

Disagree. 

 

That would be true if the mirrors were fixed, but they're not - as one who has owned the SW version of this scope.

 

The Orion MCT has moving-mirror focussing which the OP will have to use to get the focal plane within the range of the GSO focusser.

 

A longer focuser pushes the focal plane back and to achieve focus at that position the mirror spacing must change - and in doing that the focal ratio - and the effective focal length - changes.

 

The change is enough to be noticed if you test it (for example) by measuring the time for a star to drift across the field of view with the GSO focusser vs without. It's not huge, but it is measurable.


Edited by luxo II, 11 July 2019 - 07:56 AM.


#9 Garyth64

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Posted 11 July 2019 - 08:06 AM

Yes, yes, yes, I know about the mechanics.

 

Geez, you guys.  It is simple. 

 

Just adding a longer focuser does not change the focal length!

 

Yes, of course the separation of the mirrors would have to change to get anything to focus. But that is not was asked. 

 

The original question was, " Does the focal length change (get longer) because of the added length of the focuser?"

 

But just the act of adding a different focuser does not change the focal length.  That is all I was saying no to.

 

If I went to a longer focuser of my newt, I would not be able to focus anything.  The f.l. didn't change, to get it to focus, I would have to move the primary.

 

This is akin to another guy here on CN who had a short refractor,  he wanted a longer refractor and thought he would just buy a longer tube and remount the parts.  It had to be explained to the novice that it just didn't work that way.  changing the tube will not change the f.l.


Edited by Garyth64, 11 July 2019 - 08:12 AM.


#10 junomike

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Posted 13 July 2019 - 10:24 AM

Yes, yes, yes, I know about the mechanics.

 

Geez, you guys.  It is simple. 

 

Just adding a longer focuser does not change the focal length!

 

Yes, of course the separation of the mirrors would have to change to get anything to focus. But that is not was asked. 

 

The original question was, " Does the focal length change (get longer) because of the added length of the focuser?"

 

But just the act of adding a different focuser does not change the focal length.  That is all I was saying no to.

 

If I went to a longer focuser of my newt, I would not be able to focus anything.  The f.l. didn't change, to get it to focus, I would have to move the primary.

 

This is akin to another guy here on CN who had a short refractor,  he wanted a longer refractor and thought he would just buy a longer tube and remount the parts.  It had to be explained to the novice that it just didn't work that way.  changing the tube will not change the f.l.

It did/does for me in my SCT's.

Adding anything to the back of mY C11/C14 increases the magnification using the same eyepiece.  If the eyepiece didn't' change and the magnification increased, then the F/L had to have increased.

 

Note in a Newt and Refractor the Focal Point can't change (as it's fixed), therefore the Focal Length has to remain the same.




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