Checking primary mirror focal length (PADAS Newtonian)

[moriniboy]

Hi

Before we decide on a re-coat (see photo) we need to confirm the focal length if this 8.5" Newtonian primary mirror as there is nothing written on the rear

What's the best way to do this, I have thought about using the sun and projecting an image onto a wall, any safer suggestions?

[Peter Drew]

The Sun projection method you mention is the easiest and safe as long as the focal plane surface is non combustible!  

[Uplooker]

54 minutes ago, Peter Drew said:

The Sun projection method you mention is the easiest and safe as long as the focal plane surface is non combustible!  

Sorry, that made me laugh out loud - the combustible bit

[moriniboy]

Ok, seeing as its cloudy again, just used a single LED torch in the garage so I can see when its focused well and its working out at  215/2300mm or f10.7, bit of a planet killer!

[Peter Drew]

A light source that close will produce a longer focal length reading than the true one, I'd recheck that against the Sun if the dimension is important. 

[moriniboy]

Ok will do, thought it was i bit long

[Mr Spock]

Try the moon - much safer

[moriniboy]

Had a go on the moon but the coatings (or lack of them) did not help, came in at approx 48" so the revised figure is a more manageable f5.64.

Will check on the sun tomorrow and confirm.