Is my tube uneccesarily large?

[hes not well again]

Apologies for the somewhat ambiguous nature of the title but I couldn't help myself. I have a newtonian that was home built about 40 years ago to a design published in a book from the fifties [i think]. It has always seemed to me that the tube is way too long. The secondary mirror is at least 18" down from the mouth of the tube which makes it a nightmare to adjust as you can't see through the focuser when you are adjusting it. From what I have read the recomended distance is about the width of the tube after the secondary. This summer i am going to recoat the mirror and make a new pier for it as well as a respray for the tube so if the extra tube is not needed I will cut it down. What think you?

[Merlin66]

The tube seems to be designed for a longer focal length mirror than the one you appear to currently have....The black section at the top seems to show the holes to support a spider....is there an "extra" focuser hole in the tube??

[NGC 1502]

The extra tube length will have two benefits - the secondary will be much less prone to dew, and ambient light will be shielded from reaching the inside of the focuser and reducing contrast.

But the tube length does seem excessive, and as it's making collimating the secondary harder, then if it were me, I'd be reducing the tube length when the other work is in progress. You can always make a foam dewshield to extend the tube when it's in use. The shorter tube will be more portable should you wish to transport the scope in a car.

All the best with the renovation, Ed.

[Chris H]

So that's how the car jack works, when you mentioned it the other day I could not envisage how it worked.

[Moonshane]

The extra tube length will have two benefits - the secondary will be much less prone to dew, and ambient light will be shielded from reaching the inside of the focuser and reducing contrast.

But the tube length does seem excessive, and as it's making collimating the secondary harder, then if it were me, I'd be reducing the tube length when the other work is in progress. You can always make a foam dewshield to extend the tube when it's in use. The shorter tube will be more portable should you wish to transport the scope in a car.

All the best with the renovation, Ed.

just what I was going to say Ed.

[hes not well again]

The 'holes' at both ends are actually bolts. There is no extra hole for a focuser. The tube seems to be heavy duty ducting and the black rings are just to trim the ends and avoid sharp edges. Having got the tape out it measures up as about f5.5. I had to remake the mirror cell as the screws only screw in against the mirror clips and ultimately start to bend them straight. I bought a teleuve plossl 8mm and had to remove the cell to move it up the tube because it wouldn't focus. Thats when I found out two of the clips had nearly been bent straight and were only just holding the mirror. One quick redesign later and a quick collimation by eye [cheshire on order] and its amazing how well it works.

[hes not well again]

The extra tube length will have two benefits - the secondary will be much less prone to dew, and ambient light will be shielded from reaching the inside of the focuser and reducing contrast.

But the tube length does seem excessive, and as it's making collimating the secondary harder, then if it were me, I'd be reducing the tube length when the other work is in progress. You can always make a foam dewshield to extend the tube when it's in use. The shorter tube will be more portable should you wish to transport the scope in a car.

All the best with the renovation, Ed.

Thanks for the post. I spent an hour or two this evening setting up the secondary and actually it wasn't that bad. As you say the extra length will keep out stray light so I may keep it. As I have a hatchback it fits in the car no problem. The pier is the most awkward item but as I am a woodturner will turn a nice elm one. Will post some pics in the diy section when I do it in the spring.