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focal length too short or too long

ATM dob equipment
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#1 G00se

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Posted 20 July 2019 - 11:17 PM

Built a 12" F5 dobsonian.  Measured the focal length to 60inches.  Set it up.  Aimed it at Saturn.  Saturn is in the eyepiece, except it is massively out of focus.  I am guessing that the focal length is not actually 60 inches.  Is there a way to tell if I need to increase or decrease the focal length?

 

Any help appreciated.  Thanks

 

Matt



#2 oldstargazer

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Posted 20 July 2019 - 11:28 PM

Is the focuser all the way in or all the way to get to what would be considered the best you can get right now? If you have already tried max in both directions you can either try some object in the neighborhood or put the focuser at max out and then slowly pull the EP out of the holder to see if that makes it better. If it gets better then you need to get some more length in the light path however you think is best for your project.


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#3 GoFish

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Posted 20 July 2019 - 11:29 PM

If the focal length is longer than you expected:

  • point scope at moon
  • no eyepiece in focuser
  • use a piece of white tissue paper to project the moon image through the focuser
  • measure the location of the tissue when the moon image is focused

Not so straightforward to measure the other possibility, i.e., if the focal length is shorter than expected. 


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#4 sg6

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Posted 20 July 2019 - 11:38 PM

How did you measure the focal length.

Oddly it tends to be a not so easy measurement to determine. Just a 50mm error means it is way out, and as you say should be 60" that is 1524mm so 50mm is just 3.2%. That is a fairly tight tolerance.

 

I suppose the other question is how sure are you that the optical path is 60 inches, and does the 60 inch focal plane position make sense. Where does, or should, it fall in respect to the focuser. Basically where did you design it to be.



#5 G00se

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Posted 20 July 2019 - 11:50 PM

Thank you for the reply and the suggestion. 

 

The image looks best when the focuser is all the way in.  Pulling the eyepiece out makes it worse.  I will try shortening the focal length.

 

Is the focuser all the way in or all the way to get to what would be considered the best you can get right now? If you have already tried max in both directions you can either try some object in the neighborhood or put the focuser at max out and then slowly pull the EP out of the holder to see if that makes it better. If it gets better then you need to get some more length in the light path however you think is best for your project.


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#6 G00se

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Posted 20 July 2019 - 11:51 PM

I will try this when the moon comes up.  Thank you for the suggestion!

If the focal length is longer than you expected:

  • point scope at moon
  • no eyepiece in focuser
  • use a piece of white tissue paper to project the moon image through the focuser
  • measure the location of the tissue when the moon image is focused

Not so straightforward to measure the other possibility, i.e., if the focal length is shorter than expected. 



#7 G00se

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Posted 20 July 2019 - 11:53 PM

My measuring is fraught with problems.  Basically a tape measure plus the thickness of the mirror cover.

 

My rig has room for up and down play so I can increase and decrease an inch or two each way.

 

I bought the mirror used.  The seller was confident is is F5.  I am 99% sure it came from a 12inch Zhumell.  I found the specs on 12 inch Zhumells and they have a 1500mm focal length.  I am going to try that.

 

Thank you for the reply.

How did you measure the focal length.

Oddly it tends to be a not so easy measurement to determine. Just a 50mm error means it is way out, and as you say should be 60" that is 1524mm so 50mm is just 3.2%. That is a fairly tight tolerance.

 

I suppose the other question is how sure are you that the optical path is 60 inches, and does the 60 inch focal plane position make sense. Where does, or should, it fall in respect to the focuser. Basically where did you design it to be.


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#8 kathyastro

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Posted 21 July 2019 - 05:04 AM

Thank you for the reply and the suggestion. 

 

The image looks best when the focuser is all the way in.  Pulling the eyepiece out makes it worse.  I will try shortening the focal length.

Your focal length is too short.  It is the tube length - the light path - that you want to shorten.  Changing the focal length would require re-grinding the mirror.


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#9 watchplanets

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Posted 21 July 2019 - 12:22 PM

Norm the focus is 3 inches past the focuser where your ep would be. Did u measure from the primary to 2nd then to ep location or just the tube height is 60 inches? Cause I cant really tell since u didn't really say


Edited by watchplanets, 21 July 2019 - 12:23 PM.

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#10 Eddgie

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Posted 21 July 2019 - 12:56 PM

The focal length does not matter at all.

 

What matters is how far away the primary mirror is from the secondary mirror.

 

If the focuser does not have enough inward travel to reach focus, the primary mirror has to be raised so that it is closer to the secondary.   This can sometimes be done with the existing collimation screws, simply by running them just short of fully clockwise, but if there is not enough travel, longer screws could be a solution.

 

If the primary mirror cannot move closer to the secondary, then the next possibility is moving the secondary to the primary.. If the scope has truss polls, they can be shortened as required.

The last option is to use a lower profile focuser like the Kineoptics helical focusers.

 

http://www.kineoptics.com/HC-2.html

 

Now, here is my fear for you....  I would guess that you did not use a Newtonian design program because had you done this, you likely would not be having this problem, because the program would have asked the focuser height. The fear is that if you have to move the primary mirror too far far up,  depending on the size of the secondary, you could cause aperture loss.   For example, if you selected a very small secondary mirror with a very small fully illuminated circle, even a small decrease in the primary/secondary spacing could cause aperture loss. 

 

From your description though, your problem is not a "focal length" problem, it is a primary to secondary spacing problem.  
 

If your secondary is particularly large (25% or 30%) this fear is probably not in play, but if you started with a very small secondary, my advice would be to find a Newtonian program and make sure that when you move the primary up, you are still going to have full aperture. 


Edited by Eddgie, 21 July 2019 - 12:58 PM.

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