Telrad question

[a10ken]

Hi all, i have just bought a telrad for my newt and wondering what the optimum distance from eye to glass would be to maintain the correct size at infinity of the degre circuls

[spaceboy]

As far as I am aware it doesn't matter how close or far your eye is to the circles as they remain in the same perspective. IE: the closer you get to the circles the bigger they get but the more stars and the wider the FOV you see. The further away the smaller the circles but the smaller and tighter the FOV. So basically what is comfortable for you.

[a10ken]

Thats my point. if your eye is close up then the 4deg circle will be covering a larger area of sky than if it were farther away. so the dia of the viewed area will be greater than 4 deg i think...

[John]

Thats my point. if your eye is close up then the 4deg circle will be covering a larger area of sky than if it were farther away. so the dia of the viewed area will be greater than 4 deg i think...

You tend to view a Telrad from around 6-12 inches back. I believe the circles maintain a constant size when viewed against the sky whatever distance you view from - come to think of it, it would be a pretty useless finder if that was not the case and Telrad's are just about the most popular there is worldwide so I can't see such a simple issue would have been overlooked by all the 1000's of owners.

[cgarry]

The FOV is the same whatever the distance you are from the Telrad - that is the point of it!

Cheers,

Chris

[a10ken]

Thank you:icon_salut:. i just needed that explaining out in simples terms .

[tom714uk]

I intend to get one of these but it won't be for a couple of months and in the meantime I'm very curious - how do they obtain that effect?

[John]

I intend to get one of these but it won't be for a couple of months and in the meantime I'm very curious - how do they obtain that effect?

The red circles are projected onto a flat glass panel. When you look at the sky through the glass the circles appear to be projected against the sky. There is a slight knack to using them, I find both eyes open works best for me, but once you get the hang of it they work really well

[JoLo]

The correct distance from eyeball to Telrad is that space in which your neck does not pain you from unnatural positions of your spine....this point in space is further back at zenith, closer when the tube is nearer to the horizontal. FOV doesn't change, it's all about comfort, IMO.

[tom714uk]

The red circles are projected onto a flat glass panel. When you look at the sky through the glass the circles appear to be projected against the sky.

I thought that was the case, but how do they get the circles to subtend the same angle no matter how far away your eye is?

Cheers,

Tom

[cgarry]

I thought that was the case, but how do they get the circles to subtend the same angle no matter how far away your eye is?

Cheers,

Tom

I have absolutely no idea! In fact I just had a quick play with my Telrad to convince myself that it does actually work and it does.

Chris

[spaceboy]

Thank you:icon_salut:. i just needed that explaining out in simples terms .

I thought that's what I did

[tom714uk]

I guess if the glass were somehow manufactured to scatter light from every point on its surface down the sides of several imaginary cones, of appropriate angles (one for each desired circle) and a common axis, the effect would be as observed. I don't know any materials science though so this may be impossible! A very clever device though by the sounds of it.

[spaceboy]

Thats my point. if your eye is close up then the 4deg circle will be covering a larger area of sky than if it were farther away. so the dia of the viewed area will be greater than 4 deg i think...

The circles would have to physically get bigger to cover a larger part of the sky. They only look bigger because you are closer to them. The sky and the 4' Telrad circles are proportionate to each other and so remain equal in perspective to your viewing distance no matter what.

[great_bear]

Originally Posted by tom714uk

I thought that was the case, but how do they get the circles to subtend the same angle no matter how far away your eye is?

I have absolutely no idea! In fact I just had a quick play with my Telrad to convince myself that it does actually work and it does.

There's a lens immediately between the reticle and the glass. This makes the reticle appear to be at infinity.

[cgarry]

The circles do physically get bigger on the plastic they are projected onto as you move away in order to counteract perspective which would otherwise change their size in comparison to the sky.

Here are some pics to demonstrate:

Photo taken close up - all rings fin in the view.

Further away - the outer ring no longer fits in view.

Further away still - only the centre ring fits in view now.

Cheers,

Chris

[spaceboy]

Now I know your havin me at it

[tom714uk]

Cool! Very illuminating pictures. Thanks Chris...

[spaceboy]

Forgive me cgarry you are correct :o A lens magnifies the reticle onto the finder plate, so yes the projected circle dose physically grow in size the further away you are. When a10ken asked "what the optimum distance from eye to glass would be" I considered this to mean under normal use. Obviously going to the extremes that the outer rings of the telrad circle are no longer within there visible boundaries then this would surely go with out saying your not at an optimum distance?? (hence thinking your on a wind up ) Under normal use the telrad circles "should" remain in perspective to the sky behind.

SPACEBOY

[tom714uk]

I suspect it can't be much to do with the lens. Two people looking at the plate from different distances, at the same time, must see different parts of it lit up, hence my hypothesis about the nature of the glass. It would be a similar sort of thing to a hologram (if I'm correct) so maybe they use holographic techniques to make it.

[a10ken]

I thought that's what I did

you did spaceboy, and that was what i thanked you for!

[great_bear]

I suspect it can't be much to do with the lens. Two people looking at the plate from different distances, at the same time, must see different parts of it lit up, hence my hypothesis about the nature of the glass. It would be a similar sort of thing to a hologram (if I'm correct) so maybe they use holographic techniques to make it.

Er... Nope - it's just an LED with a picture of circles in front, a lens, and a square of plain old plastic.

No rocket science here.

A Telrad will cost around £5 coming off the production line.

[tom714uk]

Me trying to be too clever as has been known. Thanks for the correction...

Cheers,

Tom

[spaceboy]

I thought that's what I did

you did spaceboy, and that was what i thanked you for!

SORRY!! My bad!!! With drawl symptoms making me a bit edgy It was easier to pack in fags than it is not getting out under the stars.

[a10ken]