Refractor Drawtube Vignetting ?

[John]

When I'm using a 2inch diagonal with my 102mm F6.5 refractor, the focus point for my eyepieces requires the drawtube to be racked almost fully in. I'm slightly concerned that the drawtube and the 4 baffles in it are cutting into the light cone, in effect stopping down the scope slightly.

I guess one way to see if this is happening would be to make a scale diagram of the light cone and see how wide the cone is at the cirtical positions and then measure the diameter of the baffles and see what the match is. Alternatively, does anyone know of a visual way of telling if this is happening ?.

Thanks,

John

[Kaptain Klevtsov]

A rough way might be to leave the focuser where it reaches focus, then put your eye behind the lens at the focal length of the 'scope and see if you can see the edges of the front lens. It should be obvious if the vignetting is severe, though I don't know where you would measure from as the optical centre (if that's the right term) of the objective.

Kaptain Klevtsov

[John]

Thanks for the suggestion KK.

As the clouds have rolled in this evening I have been doing some measuring and it looks like my fears are unfounded. The narrowest baffle in the drawtube has an aperture of 45mm and even when the focusser is racked fully in there are a few mm to spare before the baffle would even begin to cut into the light cone.

I should have known that Vixen would have got that right

John

[barkis]

I would have thought the light cone would clear the focuser when fully racked in. It would be a poorly designed scope if that was not the case.

Unless of course you have carried out a modification of some sort John.

Ron.

[John]

I would have thought the light cone would clear the focuser when fully racked in. It would be a poorly designed scope if that was not the case.

Unless of course you have carried out a modification of some sort John.

Ron.

I'm sure you are right Ron, but I got this idea stuck in my mind that I might not be seeing the full benefit of the 102mm aperture.

The scope is 100% unmodified as far as I know.

John

[Jamie]

The scope is 100% unmodified as far as I know.

John

The only mod done to that scope to my knowledge is the focus knobs have been replaced (by Tom Yates).

Other than that it is in its original condition...

[barkis]

John,

Your scope f/length at f6.5 = 262mm Approx.

Having drawn out the light cone from those figures, I measured the width in two places.

1 At 152mm from the focal point the width of the beam is 22.5mm

2 At 203mm from the focal point the width of the beam is 30.5mm

If you also mark these points on the tube, it should be farly easy to establish if the baffles in the drawtube coincide with at least the 152mm point. Depending on the diameter of the baffle window, you can establish vignetting or not, as the case may be.

Hope you can get some result from this.

Ron.

[John]

Thanks Jamie - thats what I thought, bog standard apart from the focus knobs.

Thanks Ron - I'll give that a go. I'm pretty sure I just being paranoid about this !.

John

[GazOC]

Ensuring the drawtube doesn't cut into the light cone is basic telescope design, I'm sure Vixen have this covered.

[John]

Ensuring the drawtube doesn't cut into the light cone is basic telescope design, I'm sure Vixen have this covered.

Well I would've thought that as well Gaz but I've just read a review of the WO Zenithstar 80 Short Tube where the reviewer found that poorly placed / sized baffles (in the main tube) reduced the effective aperture to 65mm when the focusser was racked in.

I'll do the checks that Ron suggests but I'm now fairly confident that my Vixen does not suffer from this.

John

[GazOC]

To be fair though WO have fixed the problem in the second run and replaced scopes of people who bought the affected scopes. Still very amateurish to allow the problem in the first place though.

It was pretty big news at the time and I'm sure that if Vixen had 'dropped the ball' in similar circumstances then we would have heard about it.