Should I take a hacksaw to my focuser housing?

[themos]

Hello, I have had enough of not reaching infinity focus with my Newtonian+DSLR. Either I hack away a bit of the tube and bring the mirror up or I hack away a bit of the focuser housing (the top 10mm) thus allowing the focuser tube to get closer to the secondary. The latter seems easier but I am a bit concerned about making the focuser too wobbly. This is a starterscope, a Celestron 114EQ "PowerSeeker" so it's no great loss if things go badly, I guess. What do you think? Has anybody taken off a bit of the focuser housing ?

[Merlin66]

Probably better ( an more elegant) a solution to move the main mirror up the tube.

[themos]

I've been thinking that the focuser tube will come so far towards the secondary that it will start obstructing a fair bit of the aperture, so the negatives are piling up...

[Merlin66]

By moving the main mirror the position of the final focus could be moved sufficiently out from the focuser that the draw tube shouldn't encrouch into the tube that much??

[themos]

Yes, that is one advantage of moving the mirror up. Well, here we go....

[themos]

Bother! I didn't hack away enough, still can't get to focus. will try again tomorrow.

[The Warthog]

I am sure that Merlin`s solution is the better one.

[themos]

Can you believe it, the second attempt left me THIS close to focus... might be able to do the rest with the collimation screws...

[themos]

By the way, here is a quick note to help you calculate how far away from infinity focus your sensor is if the most distant object it can focus is a known distance away. You need to know your focal length, f. If the most distant object you can focus is L (say 30 meters), you take the ratio L/f (in my case, f is 900mm so L/f = 30/0.9 in meters = 33.3). Then you divide f by this ratio (you should add 1 to the ratio, strictly speaking, but in most cases the ratio will be large enough not to need that correction) and that gives you the distance you need to move the sensor to take it to the focal plane. So when I could focus something 30 meters away, I was 900/34 = 26 mm away. I think I can focus clouds now, they are about 1 km away, so the ratio is 1000/.9 = 1111. That means I'm only 900/1111 = 0.8 mm away. Which means I SHOULD be able to play with the collimation screws to give me the extra distance.

[Merlin66]

Themos,

Did you actually hacksaw your focusser

or did you move the main mirror cell up the tube???

[themos]

No, the focuser is intact :-) I've shortened the tube by about an inch.

[Merlin66]

Well done!!!!

You had me worried there for a while!!!!

[themos]

Well, I claim success! A few quarter turns of the collimation knobs and the Bahtinov confirms that I can just about get the Canon through focus. Problem is, the eyepieces now need an extender tube...OK, a slow 4.5" inch Newtonian is not the kind of rig made for imaging but I wanted to have a little play at that focal length.