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Need some help regarding focal length

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Posted 19 January 2013 - 11:49 AM

Hello everyone, i am new here :)

Am making 6" F/8 and My neighbour is also making 6" but F/6 mirror not F/8, i am sharing grits with him, but question is that my mirror when seen in sun its point is small sharp dot like playing marbel, but my neighbour mirror is giving sharp bigger dot. Bigger than my dot.
Can you tell me why so difference? his dot is bigger than me.

Who is wrong me or my neighbour?


#2 gpelf


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Posted 19 January 2013 - 12:23 PM

Hi Ksinghboy,

Even though I am a newbie here as well, Welcome to Cloudy Nights (You will love the people and the help you get here).

Are you at rough grinding or fine grinding stage? If you are at the courser grits it may be nothing more than one mirror is closer to a sphere than the other, At this stage of grinding you cant be to critical about what you see optically. Have you tried directly reading the Sagitta (straight edge and feeler gauge method)? The focal length for a 6" F8 is 48" for a 6" F6 it is 36", Is this close to what you are seeing?

Hope this helps somewhat, even if it does not, keep checking, I am sure some of the pros here will respond.

Clear Skies,

#3 Mike I. Jones

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Posted 19 January 2013 - 03:38 PM

A couple of old ATM tricks for getting your focal length during roughing:
(1) Spray WD-40 on the mirror, then use the sun to measure your focal length. Make sure the shadow of the tape measure is centered on the mirror.
(2) You can also use shoe polish. Dab a little glob in the middle, then spread it evenly over the surface. Wipe off most, then buff polish to a nice sheen, and you'll get a good sharp sun image.

#4 Pinbout



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Posted 19 January 2013 - 11:36 PM

Neither of you are wrong. it will all work out in the final figuring, that's where it counts.

My 8in f4.5 isn't marble like and its fine.



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Posted 19 January 2013 - 11:44 PM

Thank you my friend. Can you all tell me that on long FL does point of sun image is small or bigger?

My direct question is what is the diameter of short FL and long FL when seen in sun?


#6 dan_h



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Posted 20 January 2013 - 10:17 AM

The size of the image at the focal plane is dependant on the focal length of the objective whether it be a lens or a mirror. For both the moon and the sun, which are nearly 1/2 degree,(30 arcminutes) in size, you can calculate the image size as 0.009 X focal lenght.

For example, a 1000mm focal length scope creates an image of the sun that is 1000mm X 0.009 = 9mm in diameter.

For a real quick estimate I use 1/100 the focal length so I can do the arithmentic in my head without thinking. For a 6" f8, focal length is 48", image of the sun is about 0.48". For a 6" f6, focal length is 36", image of the sun is about 0.36".

Hope this helps,




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Posted 20 January 2013 - 10:51 AM

Thank you Dan very useful informations you have provided.

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