Moving primary miror for DSLR Focus

[hazegood]

I cannot get focus on my 1100d using my Astromaster 130, I see a few options discussed,

1, change the focuser,  probably cost more than the original cope, so that's a no go.

or move he primary up the tube, using either cut the end off the tube and remount the mirror cell,  r move the mirror using new bolts springs and washers (I like this idea as it can be reversed easily)

my questions are how far would I need to move the primary up the tube, to get focus,  (that's probably the easy answer)

The other question is, why does this work, after all the image the focuser is looking at is reflected off the secondary mirror so that's where the image you are looking at comes from,  so surely you are focusing on that image or am I missing something

 

[steppenwolf]

The

The other question is, why does this work, after all the image the focuser is looking at is reflected off the secondary mirror so that's where the image you are looking at comes from,  so surely you are focusing on that image or am I missing something

The secondary mirror simply intercepts the light cone reflected and focused by the primary and turns it through 90 degrees to divert it out through the side of the optical tube. The focal plane is the position of focus at the focal length of the primary so moving the primary up the tube pushes the point of focus outwards.

[D4N]

When the collected light hits the secondary it is still converging to the focal point, it isn't yet in focus. The focal point is somewhere around the focuser tube.

In fact the focal point of the scope will be its focal length away from the primary mirror.

The focal plane on a DSLR is often marked on with an arrow and flat line, it's quite near the back of the camera. You can measure from there to the primary mirror and work out how far away from the focal point you are.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

[laser_jock99]

Start with a small distance - say 10mm. Sawing the tube is a bit drastic unless you need to move the mirror up the tube a lot try longer springs first.

http://stargazerslounge.com/topic/188976-8-f4-newtonian-mirror-cell-mods/?hl=%2Bsix+%2Bspring+%2Bmod

[hazegood]

Thanks for the replies, I understand why it woks now,

Yes my camera does have the focal plane marked on the body,  how do ui measure the distance from this to the primary, I assume I measure from camera focal plane to secondary, then from secondary to primary and add the two measurements?

 

[D4N]

Yeh that's it, the amount you are over the focal length of the scope is the amount you need to move the mirror.

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