Is there an easy way to work out the focal length of a 'scope?

[Kaptain Klevtsov]

My new Tal 200K has been the subject of much Googling and other web-based reading. I've read the manufacturers website, the importers stuff, FLO stuff, Cloudy Nights etc and they say its an F10. Now that its here and I'm waiting for some dark to point it at, I actually looked in the manual that came with it, and it says, in black and white, that its f/8.5!

I'm not all that bothered what it is, its longer than the Newt., but I'm wondering just how long it is.

Kaptain Klevtsov

[SteelRat]

It's quite common that Tal scopes are sent with the wrong destructions. The 250k is 8.5.

The NPZ site definately has the 200 as 2000mm length f10. You got the wrong book mate?

[Kaptain Klevtsov]

Probably the wrong book as it says that the focuser is R&P and that I can stiffen it up by twisting the knobs in opposite directions. That doesn't even start to work, they're solidly fixed to each other. We'll call it f/10 then.

Kaptain Klevtsov

[AndyH]

Hey KK.

If i remember correctly, the original handful of 200K's were F8.5 . Thereafter the scopes were changed to F10. I could be wrong, but it does ring a few bells. I'm sure this was mentioned over on the Talscopes yahoo site. Perhaps the instructions haven't been upgraded. I reckon the scope IS f10 and the change is just to the focuser. I think the new focuser is the same that's on the biggie KMT.

Just my thoughts.

Andy.

PS: just to forward info on the aforementioned yahoo site, never ever touch the adjustment screws on the meniscus, only adjust(if needed) the primary mirror. Apologies if you already know this info, but they bang on about this all the time.

[SteelRat]

Perhaps the instructions haven't been upgraded.

That sounds pretty typical. Never knew the 1st lot were 8.5 though. Nice one Andy 8)

[GordonCopestake]

Ruskies for you! Where are all the pictures though!?

[Astroman]

You can measure the distance from the objective lens to the field stop on your eyepiece when an object is focused and determine the focal length. Divide this number by the aperture diameter and you get the focal ratio, measured as F/xx. The field stop on the ep is NOT the final lens, it is an anular disk with a hole in it visible through the inside looking toward the final lens. (Basically looking in the same direction as light travels from the objective.) It's generally close to where the 1.25" barrel screws on, or a little farhter in.

HTH

[The Warthog]

In the literature you referred us to, Tal said that one of the problems with other catadioptric telescopes was the f/10 ratio, so I assumed it would be less for the Klipklop scope. If it is f/10, and produces a nice image, in what way is it superior to an SCT?

[The Warthog]

Try this:

Put an ep with a known AFOV in the focuser, and look at a star about the celestial equator. Move the scope so the star is at the eastern edge of the FOV, and time it crossing the FOV. That star will move at 1 degree in four minutes, so you can get the actual FOV from dividing the number of seconds it took the star to cross by 240.

So, if it takes 120 seconds for the star to cross, your actual FOV is 120/240 = .5 degrees. If you are using a Plossl with a AFOV of 52 degrees, your magnification is 51/0.5 = 104x. If it is an 18mm Plossl, your focal length is 18 x 104 = 1872, for an f/ratio of about 9.375.

That should do it.

[Kaptain Klevtsov]

If it is f/10, and produces a nice image, in what way is it superior to an SCT?

40% cheaper better WH.

thanks for the tips guys, I'll try the tape measure first, as that doesn't need stars, I should be able to see which f/ ratio is possible within the focus travel as I think its one or t'other. If we get any stars to point at, I'll check the timing thing, as that sounds more definitive and lets me play with the cross hair thingy.

Kaptin Klipklop

[themos]

Put a ToUcam in it and in another scope of known FL and point them at the same landmark. The ratio of size in pixel terms should be the ratio of the FLs.

I think.

[The Warthog]

I don't think the tape measure will work in a catadioptric, since the scope is optically longer than it is physically. This is the purpose of the curved secondary mirror.

[Kaptain Klevtsov]

I know what you mean WH, but the difference between f/8.5 (1700mm) and f/10 (2000mm) is 300mm, or 100mm per pass in the three pass system. This is 100mm, or 4" in old money, so I should be able to tell if its one or t'other.

Kaptain Klevtsov

[The Warthog]

Yeah, but.

The secondary mirror is a convex mirror, with an effect similar to a negative lens, such as the one in a Barlow. So the path is much less than 2 metres, but optically equal to two metres, as if you had put a 2x Barlow in a 500mm scope, effectively increasing the f/l to 1000mm. Your measure won't tell you what you want to know.

Besides, I'm not sure that the cat isn't a two pass system, as the primary is still the mirror. The glass in the front is a 'corrector plate.' I think the f/l should be measured from the primary mirror.

[Kaptain Klevtsov]

You're obviously correct, as the 'scope is nowhere near a metre long, and as there is no front corrector plate its a two pass system.

Kaptain Klevtsov