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F12, 4" newt Build.

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Ok so, an up[date.

I have tried all the suggestions above.

loosening the mirror clamps - this didn't seem to change anything. (they actually weren't that tight anyway when I looked)

shortening the focal length - this also didn't work, the image produced was similar to before.

then Agnes, I tried your suggestion - originally to determine if the mirror was a dud I'm guessing, but.....and this is were I feel more of a nooob than ever, for not checking......

the focal length was 2.5m on the nose.   meaning (I think) that I have a scope tube that is too short by around 5cm.  Incidentally, the mirror came to a lovely single spot from a torch light in a dark room.

So, this leaves me a little disillusioned, as I had tested the focal point (but in a different way).   but like Dyson I am willing to get this right so I put it out there....what would you guys/girls do?

I have already suggested a Serrurier truss before and was warned against this.  Perhaps a small purposeful addition to the back end of the tube in black?  essentially just a new cell. 

if at all possible I don't want to go Dob with this as I want it to attach to my Eq6.

thanks for any suggestions.


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I think Agnes was asking you to test it as a sphere which means that the led and paper will be twice the focal length of the mirror away from it. Rather than just using a led it would be better to put say a 1mm dia hole in something opaque and place the led behind that. Smaller the hole the better really. Your F12 mirror should be so close to a sphere that the difference will be negligible and it will focus pretty distinctly when it's at the right distance - more usual F ratio paraboloids wont. The Faucault test makes use of that fact. They might with a suitable aperture mask.

I'm going to ask something really silly so please don't take offence. I assume both mirrors are front surfaced coated? If not multiple images are more or less to be expected. When the coating is behind glass the glass reflects surprisingly well so usually 2 images are formed from one surface.

It's possible to make a decent pin hole with a LED. Cover the end with aluminium foil and the rest with say blue tack. Make a hole in the foil with a pin, rotating it rather carefully keeping it vertical usually makes it round. The problem with testing a mirror as Agnes outlined is that the size of the image of a planet on the focal plane of your mirror is really tiny. A led is way way bigger and size could very easily hide any problems. People have been known to strop good quality needles on news paper to sharpen them further and get holes down to 0.002in by making it over lightly ground glass. If it's a led just press very lightly. If the return image doesn't look round rotate the led to see if that is causing odd shapes. Or rotate the mirror to see if it moves with that.



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