How to Measure telescope Focal Length?

[pgc hunter]

While cleaning the mirror on my 12" dob, it's got me thinking about its actual focal length. The previous owner said it was 1415mm (F4.65) but he also mentioned that he wasn't 100% sure, only that someone gave him that "ballpark" figure.

When I removed the mirror, someone wrote "F/5" on the back, presumably the person who made it. If that's true, then the actual F/L would be 1500mm. However, I tried measuring the approximate distance from the primary mirror to the focuser via the secondary mirror and got around 1390mm but I assume measuring it in that manner is innaccurate. So you can see the conundrum here.

Does anyone know how to measure the focal length of a telescope if it's unknown?

[mswhin63]

The full length is within the focuser, possibly the at the point of the lens. I am sure there is a diagram around that shows the point within the focuser. I varies slightly with the type of lense thus the need for focusing.

[Satchmo]

If you've been told its 1415 and done a rough measure of 1390 , then why not call it 1400mm ?

if you want to know more accurately , set up a torch with cover over it and say a 6mm hole punched in the cover. Set the scope up at roughly 2800mm from the source to the mirror and find the reflection of the hole, which may be coming to focus in front of or behind the torch.

Move the tube until the image of the hole is focussed sharply onto the torch cover , just next to the hole. Measure the distance from the hole to the primary and your FL is half this.

Whatever is written on the back of the mirror is meaningless as the glass may have been reground at sometime or it was a commercial mirror that was supposed to be `nominally ' F5 as marked.