4" f/10 Tal Refractor

[KaStern_Former_Member]

Hello Folks,

did anyone take his Tal Refractor apart?

I am interested to know ich lens is directed towards the object.

Is it the double-convex? Thank you for an answer.

The background for my question is:

I see a dealer advertise the Tal as a "Steinheil" type scope.

In that case the flint type lens is directed towards the object,

whereas in a Fraub´nhofer type achromat the double-convex

crown lens is the one directed towards the object.

Thank you for your answers, Karsten

[Peter Drew]

Hello Karsten.

My Tal 100RS objective has the the double convex towards the object so would appear to be the Fraunhofer design. The less curved of the convex surfaces should also be on the outside towards the object.

[Capricorn]

Seem to recall this being asked some time back, try CN.

The answer was that the TAL has the "standard" arrangement of Fraunhofer.

Someone emailed TAL and asked,

Not sure where the Steinheil notation came from, perhaps they may have made them to other way once but I have the impression that it was reported as just Franhofer.

If the post about this was here on SGL someone may be able to locate it, otherwise I would guess I read it on CN.

[Capricorn]

The less curved of the convex surfaces should also be on the outside towards the object.

Are you sure of this?

Usually the most curved side is towards the object to reduce spherical aberation, ratio of radai being around 1:6.

Equally I seem to recall something about the TAL lens and maybe that is it.

[KaStern_Former_Member]

Hello Peter,

thank you for your answer. Double-convex towards the objekt, that has to be a Fraunhofer achromat.

But the steeper curved side should too be directed towards the object. So writes Rutten / van Venrooij

in the book "telescope optics" in chapter 6 and they show it in fig 6.2

Thank you for your answer, Capricorn. Like you write the radii should differ 1:6

I hope that others who looked inside can confirm what Peter told about the orientation of the lenses.

Greetings, Karsten

[Astro_Baby]

Andy H used his contacts at TaL to answer this some time back...its a Fraunhofer according to the TAL factory and they should know I guess.

[AndyH]

Hi.

After seeing the APM page, I posted a question on the Tal/NPZ forum and it was confirmed all 3 Tal achro's are of the Fraunhofer design.

Details here.......

http://npzoptics.ru/forums/index.php?showtopic=7&st=135

Cheers,

Andy.

[KaStern_Former_Member]

Hi Andi,

thank you for that link. It confirms that it is a Fraunhofer type refractor.

Strange but it confirms too that Petern Drew is right:

The drawing shows that the lesser curved side points towards the object.

What puzzles me is the APM telescope advertisment.

According to them it should be a Steinheil type refractor.

Thanks to all, Karsten