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Old 19-06-2020, 01:48 PM
bluesilver (Peter)
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Primary focal length measurements

Hi, I am sure this is a very basic simple question, but i would like to ask just to be sure.
I am trying to find out what the true f number of my scope is, ( sorry, not quiet sure what the correct term is, like f5, f.4.5 )
It is a Skywatcher 16 inch Dobsonian.
I know it says diameter 400 mm ( 16 inches ) Length 1800 mm ( 70 Inches )
But i would like to find out what it truly is.
I have read that you simply divide the Primary focal length by the primary mirror diameter.

My question is, how do you actually correctly measure the Primary focal length?
1. Is it just simply the full length of the outside tube from top to bottom?
Or 2. is it from the reflective surface of the primary mirror to the centre of the eyepiece?
Primary mirror is simple enough, just run a tape across it to get the true diameter.

If someone could please just correct me on which one is the correct method for finding the true primary focal length it would be greatly appreciated.
Thanks.
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Old 19-06-2020, 02:59 PM
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Merlin66 (Ken)
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Peter,
It's your #2 the distance from the mirror surface to the focus at the eyepiece.
If it says it's 1800mm, this is probably close enough.
The f ratio is FL/ mirror diam = 1800/ 400 = 4.5


(you can double check the FL - at the next bright moon remove eyepieces and focus the moon's image on a card. measure the distance to the focuser, then the focuser to the middle of the secondary and finally secondary to mirror ; add these numbers and you have the FL)
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Old 19-06-2020, 08:18 PM
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Another way to use Moon is to measure the diameter of the Moon's image when focused properly on piece of paper.
From this measurement and angular size of the moon at this time you can easily calculate the FL of your mirror.
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Old 20-06-2020, 09:15 AM
bluesilver (Peter)
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Thanks for the replies, appreciated.
So have gone and measured everything up and came up with something interesting.
So to start with, the scope is the Skywatcher 16 inch Dobsonian, the goto collapsible version.
https://www.ozscopes.com.au/skywatch...telescope.html
As you mentioned, it works out to be f 4.5, it is listed as a f 4.4
So when measured it is actually a f 3.5 scope.
The mirror is 400mm diameter, ( 16 inches )
From the mirror surface to the centre of the eye piece it is 1400mm ( 55 inches ) This is with the scope fully extended.

So it is a very interesting outcome, unless i have done the maths wrong, I have double and triple checked the two measurements, but fairly sure it all works out to be f 3.5
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Old 20-06-2020, 09:25 AM
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Merlin66 (Ken)
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Peter,
Hmmmm. To have a f3.5 Dobbie from SkyWatcher would be very, very, unusual.


Can you point the just the main mirror (no trusses, secondary etc) towards a wall while also being in the Sun? You must obviously take care!!!
Measuring the distance to a focused image on the wall from the mirror would give a very close measure of the mirror focal length.
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Old 20-06-2020, 09:35 AM
Rainmaker (Matt)
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When measuring the focal length you need to measure from the primary mirror to the centre of the secondary mirror and then add half the diameter of the upper assembly and the height of the focuser when the focuser is wound out about 3cm.....

I’m sure that you will find it close to 1800mm
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Old 20-06-2020, 10:24 AM
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Merlin66 (Ken)
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Peter,
As Matt says and I mentioned previously:
""measure the distance to the focuser, then the focuser to the middle of the secondary and finally secondary to mirror ; add these numbers and you have the FL.""
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Old 20-06-2020, 10:55 AM
bluesilver (Peter)
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Thanks again for the replies, Yes looks like i missed quiet a few steps overall.
Been very cloudy and overcast here for the past few days, but interested to give the image version a try, never heard of those ones before.
All very new to me in working this out, simple as it seams.
I am sure the spec are true and correct from Skywatcher, just thought out of interested i would measure them up.
Going from the reply from Matt :
When measuring the focal length you need to measure from the primary mirror to the centre of the secondary mirror and then add half the diameter of the upper assembly and the height of the focuser when the focuser is wound out about 3cm.....
Primary mirror to centre of secondary mirror = 1400mm
Half the diameter of the upper assembly 450/2=225mm
Height of the focuser when the focuser is wound out 5mm
Total comes to 1630mm
Getting there, but obviously missing something.
Can't seam to pin point where i am going wrong.
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Old 20-06-2020, 10:58 AM
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Peter,
Ok
I think the focuser will be much more than 5mm.
Measure the total height of the focuser mechanism from the tube to the end.
This is probably about 100 to 110mm
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Old 20-06-2020, 11:30 AM
Rainmaker (Matt)
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Peter, the focal length of your mirror is the distance from the centre of the mirror to the point where light bouncing off the mirror comes to focus.
If the secondary wasn't in the way, that cone of light would focus 1800mm above the primary.
The secondary's job is only to bend that cone of light to the side so that we can view the image without getting in the way of the incoming light path.

The 'intercept' is the distance from the centre of the secondary to where the focal point is. The exact distance of the focal point above the racked in focuser
varies depending on scope designs. On my Dob it is only 11mm above but most of the SW/Orion/Bintel/GSO scopes have it about 20-30mm .

Adding the intercept distance to the distance between the primary and secondary will give you the focal length.
If you want to see where the focal point is on your scope you can simply place some translucent tape across the focuser tube (NO EYEPIECE) then point the scope at the moon and bring the image of the moon as sharp as possible on the tape by adjusting the focuser outwards. That will be where the focal point is.
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Old 20-06-2020, 02:54 PM
bluesilver (Peter)
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Thanks again, yes that is the part i was over looking, the focuser tube.
Appreciate the patience.
Yes works out without doing the sun or moon light method comes to f4.4
Appreciate the time and patience by all.
Thanks.
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Old 20-06-2020, 03:23 PM
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All's well that ends well.
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