Where does infinity start?

Okay, that is not really the question--just an interesting title.

The real question is how far away does a terrestrial object have to be before it has substantially the same focuser position as something at infinity?

Specifically, we have rebuilt a 22 inch F4.2 or so Dob, and will be setting it up for the day for the first time this weekend. I know with the work we did on the mirror cell that we will probably have to shorten the truss tubes (since the mirror cell and collimation system is now an inch and a half "shorter" and the mirror is that much further back).

There is a cross on the hill a mile away that we can focus on. How much difference will there be between the focuser position for that cross and a star at infinity?

If it is essentially the same, then I can focus as well as possible to determine where the focuser would need to be, shorten the poles during the day, and observe that night. Other wise, I will have to check things out at night, and not really be able to observe until then next night after modifications.

By the way--anybody have any helpful hints about how to do this operation?

Alex

Alex,

Simple formula, for an object at least 10 focal-lengths away:

Focuser position (relative to infinity) = FL * FL / d,

where FL is the focal length and d is the distance to the object.

For FL = 22 x 4.2 = 92.4" and d = 1 mile = 63360", the position is 92.4 * 92.4 /63360 = 0.135".

In general, the formula for a simple lens (or mirror) is:

1/FL = 1/d + 1/e,

where FL is the focal length, and d and e are the distance to the object and image, respectively. Solve for e:

e = 1/(1/FL - 1/d) = FL/(1 - FL/d)

We want e - FL = FL^2/d/(1-FL/d)

If FL/d is small compared with 1, then we can ingore the last term and say:

e - FL ~= FL^2/d.

I hope this helps.

Allen

I just tested the backfocus with my 10" F6. I focused on a tree not quite half a mile away. When I pointed the scope at a star that night I found I needed to rack it in about 1/8" further to get the star to focus. Eventhough its a pain, I would test it on a star just to be safe.

Alex, if you use a 31mm Nagler and paracorr that would be the most intravel you would need unless of course you use something like the Denk II binoviewer with the "low power" OCS. For that you would need about another .25 to .50" of in travel. That's the way it is in my 20" F4.5 anyway. But if you dont use a binoviewer that wont matter.

Thanks, all.

Turns out, we set that big old telescope out, and the focuser was completly discombobulated. Apparently the two bearings that held the knobs and move the tube had fallen out. That took a while to fix, and when we finally got it together, the scope worked very well. There was no need to adjust tube length, even with the paracorr and whatever eyepeice we used. I assume that before we shortened the mirror cell, the eyepieces had come to focus further out.

Anyway, thanks for the help.

Alex