Daft question about optical finders

[JOC]

So as I understand things an optical finder is very much like a refracting telescope.  In all but one respect, focussing.  So here is the daft question, why don't optical finders need to be focussed to get an acceptable view, yet refracters with an eye piece do? 

[Scooot]

You usually can focus finders, but I suppose as the eyepiece is built into them they’re already very close to infinity so only need tweaking.

[Ricochet]

(Usually) finderscopes are refractors. They require focusing just as a normal refractor does, except there is one difference. On the cheaper finders you only have one eyepiece. Because you only have one eyepiece, and everything you are viewing is effectively at infinity, once you set the focus it will always be in focus (for you). This means that a regular focuser can be excluded (for cheapness) and you can focus simply by screwing the objective or eyepiece in and out. The caveat is that if you need glasses, but don't use them to observe, where you set your finder focus will be different to where someone else will set the focus. As a result, you may not be able to use each other's finders.

[Gfamily]

They can be focused, in my experience by loosening a locking ring just behind the objective, so you can adjust the position of the objective lens holder, but as the magnification is generally low, the focus isn't particularly critical. 

[Louis D]

7 minutes ago, Gfamily said:

They can be focused, in my experience by loosening a locking ring just behind the objective, so you can adjust the position of the objective lens holder, but as the magnification is generally low, the focus isn't particularly critical. 

Exactly.  Loosen the locking ring, rotate the objective cell until it's focused at infinity, and then tighten the locking ring.  If you need to wear eyeglasses to see the sky, I'd recommend wearing them while setting focus so the image in the finder and the sky will both be in focus as you home in on your target.

[DS24]

On my finder scope there's a focusing ring just behind the lens at first I didn't know how to do it but then I saw this ring that turned

[JOC]

Many thanks for the replies, well I read above and thought "I must be missing something", so I've just taken a look.  Well no I wasn't going crazy, I've got a Skywatcher RACI finder and there is no adjustment whatsoever anywhere on the finder to change the focus.  I reckon it must just be fitted with an EP pre-set to 'infinity'.  Yet somehow it is always in focus for me and I wear (fairly strong) glasses when I use it, I am not aware that the views are particularly out of focus either even though you might imagine it was pre-set for someone with 20/20 vision.

[Gfamily]

Unless the design has changed you'll see that the cell that holds the objective has a front part and a rear part; the rear bit is the locking ring that fixes the position of the objective.

Loosen that off and your can move the front part on its thread to adjust the focus. When set, move the locking ring forward again to fix it in position.

I very much doubt they've changed the design in the last couple of years, so it's worth double checking.

[Scooot]

Thats strange, there’s a thread about a Skywatcher raci finder here. Maybe yours isn’t the same, or they’ve changed them?

 

[JOC]

No, mine is not like that there is a picture I've already loaded which shows the objective end

Whilst I took the picture to show the dual mount you can clearly see there is no adjustment at the EP end

 

 

[Cosmic Geoff]

I have a Skywatcher 9x50 RACI finder as described above, bought a few months ago. I managed to adjust the focus but it was a bit of a faff. It is set so that it is in focus when I wear my glasses.

[JOC]

There is a big milled ring on the large end, but tbh if I have to twist it any harder than I am currently trying I think I'll cause damage!

[Ricochet]

The milled ring is the lock ring. Twist it so that it turns "away" from the objective and then the objective cell can twist forward and back to focus. If your eyesight is really such that it is set so that you have run out of travel on the thread then you will have to twist the objective so that it moves away from the lock ring.

Having said that, if it is currently in focus, just leave it and take away the knowledge of how you would focus it should you need to. 

 

 

[Gfamily]

6 hours ago, JOC said:

... you can clearly see there is no adjustment at the EP end

Correct, the adjustment is made at the other (objective) end.

[barkis]

As previously stated in the thread,  the finder's eyepiece is fixed and can't be moved.

focusing is tuned to an object in the sky, by unlocking the ring on the front lens cell, to allow

the the cell which holds the objective lens to be screwed in, or out in order

to  achieve a sharp focus of the star you are lining your main scope to.

you can use a target in daylight, such as a TV aerial or distant chimney  to line up your finder.

However, make sure your main telescope is located on the same target.

when you return to a sky target, you will need to screw the finder cell in towards it's eyepiece to sharpen the image,

as you are now focusing at a target at infinity, which can differ a bit from a terrestrial target.

Ron.

[JOC]

1 hour ago, Gfamily said:

Correct, the adjustment is made at the other (objective) end.

So how difficult should it be to turn?  Should it require grasping in vice and the application of a set of pipe pliers which I what it feels like it needs?

[barkis]

47 minutes ago, JOC said:

So how difficult should it be to turn?  Should it require grasping in vice and the application of a set of pipe pliers which I what it feels like it needs?

Concentrate your effort the narrow locking ring.   Your right forefinger and thumb wrapped around tightly should suffice. Turn it anticlockwise  using as much  pressure as it takes to shift it. Once it loosens off,unscrew it a number of turns, then adust the the main lens in or out to achieve focus. Once you're happy with it, screw the lock ring back down and tighten it only slightly, It can easily be undone whenever any More  adjustments need to be made.

The video above explains exactly how to do this.

Ron.

 

[Gfamily]

54 minutes ago, JOC said:

So how difficult should it be to turn?  Should it require grasping in vice and the application of a set of pipe pliers which I what it feels like it needs?

If it's in focus already, it doesn't need to be moved at all.

If you do need to move it, are you turning it backwards? The locking ring needs to go towards the rear of the finder to release the ring that holds the objective.

[JOC]

19 minutes ago, Gfamily said:

If it's in focus already, it doesn't need to be moved at all.

No it doesn't I guess, I just found it rather remarkable that it was already spot on for me and that it looked like they didn't need focusing as I couldn't shift anything on it.  I still can't, but at least the thread answers my question that they do need focussing and there is nothing unique about them - they are as I though tiny refracting telescopes