Calling for Experienced Refractor Builders

Well Guys and Gals, I could use some help.

You might remember a recent thread in which I sought some opinions on a 4" f15 project I was thinking about undertaking. Well I dived in and now find myself at a point that I could use some help.

I was lucky enough to be offered an Edmund 4"f15 tube that had a broken objective. After considerable research I ordered and received an Antares 105mm f14.2 objective with an adjustable cell.

After some lathe work on the cell, it now fit the tube, but despite trying to match the objective to tube distance on the new cell, their must be enough difference in the cell and objectives to give me a focus problem.

I cannot reach focus with a diagonal in place with my Televue plossls, not enough in travel If I remove the diagonal, I can reach focus with the 1.25" focuser racked out 2" of it's 6.25" travel. That seems to measure very close 60 inches focal length, as expected.

I obviously need to shorten the tube, so being that the thought of a mistake in cutting the tube is making me a little nervous, I am seeking counsel here. These are my questions.

1. The tube is nicely baffled in three places. If there is an error in the placement of the objective introduced by the new cell, I believe it to be that it is too far forward by less than an inch. It therefore seems to me that the best course is to shorten the tube at the back so as not to change the light cone and cause a problem with the baffles. Do you agree?

2. By shortening at the back, is there a chance that the long tube of the 1.25" focuser may vignette?

3. I was able to achieve focus with no diagonal using my TV 32mm Plossl and my 13mm TV plossl. Those were the only two that I tried because the focal point was unchanged between them, so I expect that the others will focus nearby. Would it be prudent to plan for the shortening to place the focus point at mid point of the focuser travel?

4. I use plossls almost exclusively right now, but if that changes in the future how does the focal point of TV Plossls compare to others like orthos or wide field types? Does using these plossls as the model make sense?

5. Does adding Barlow affect this calculation?

6. So assuming my thinking is correct the amount to cut from the back would be:

Current Tube Length - (Length of the Light Path of the Diagonal - 2 inches) - Half of the Focuser Travel)

OK I thank you for coming this far, Now Please Help me not screw this up!

Thanks

1. It therefore seems to me that the best course is to shorten the tube at the back so as not to change the light cone and cause a problem with the baffles. Do you agree?

Agreed.

2. By shortening at the back, is there a chance that the long tube of the 1.25" focuser may vignette?

Unless there is a baffle in the focusser tube, you are
probably OK. The focusser tube must be about 9"
long. So the beam at the front is about
9/14.2=0.63" in diameter. A 1.25" opening at the
front should leave lots of room.

If there is a baffle in the focus tube and it causes
trouble, you could remove it and put flock paper
inside the front of the focus tube.

I have often seen focusser tube baffles that were
designed assuming no diagonal, and then when
you add a diagonal the focusser moves in acouple inches,
and then there is lots of vignetting.

3. I was able to achieve focus with no diagonal using my TV 32mm Plossl and my 13mm TV plossl. Those were the only two that I tried because the focal point was unchanged between them, so I expect that the others will focus nearby. Would it be prudent to plan for the shortening to place the focus point at mid point of the focuser travel?

If you think you might do binoviewing or
photography later, I would leave lots of
available in-travel. You might set it so with
no diagonal + 13 Plossl you are about 1/2" from
the outer end of travel.
This should cover most eyepieces. The with a
1.25" diagaonal you will then be near the middle
of the travel. With a camera or bino, you'll
be near "in" end of travel. Binoviewers
typically need about 5.5 inches of in-travel.

4. I use plossls almost exclusively right now, but if that changes in the future how does the focal point of TV Plossls compare to others like orthos or wide field types? Does using these plossls as the model make sense?

Yes, the TeleVue Plossls are fairly typical.
Some other brands will need about 1/2" in-travel
from there.

5. Does adding Barlow affect this calculation?

Some Barlows need in-travel, some need out-travel.
Needing more out-travel is less of a problem,
since you can slide it out of the focusser
part way. Needing more in-travel is a real
problem.

Good luck!