correction lens ????

[steelfixer]

i have decided to start a new thread on my focus tube problem.

appolagies to all who have tried to help but the further i go into this the more confusing it becomes.

For my sins i purchased a 6 inch Seban and the image and focus quality is apauling..

it has a 1400mm focal length from a main tube which is approximately 700mm in length.

it apparantly achieves this focal length by using a lens at the base of the focus tube.

I have just removed this lens to see what it is.

This lens is a 15mm thick piece of glass that is concaved on both sides it nither magnifies or reduces an object placed behind it from either direction.

Can anyone tell me what it does??? or how this system works.

The lens is of such poor quality it resembles lightly frosted glass or maybe this is an effect caused by its shape I just dont know.

thanks

graham

[Ags]

You have my sympathy - those Seben's have quite a reputation!

If the glass is concaved on both sides it must act as a negative lens. A crude barlow effectively.

Post some photos to show us what this thing looks like!

If I understand what I have read correctly the Seben has a spherical mirror which produces a poor image. The defects are made less apparent by adding a barlow element to increase the focal length and stretch out the defects (you could say it magnifies the central portion of the image which is least appalling). Removing the barlow element (the frosted glass thing, I guess) will make the defects worse.

Perhaps put it back together and get your money back? But it does sound as if you are having some fun tearing this beast to pieces.

If you remove the barlow element entirely I guess you could replace it with a better one like the TAL X2 or TAL X3 from FLO, although I am not sure how you would bring it all to focus. At least you would have a good barlow!

[Peter Drew]

I have had experience of just one Seben 6" of the same design. Although the performance fell short of those telescopes usually recommended on SGL, with a bit of TLC (Tinkering Like Crazy) it was at least reasonably useable. It's essential to use a laser collimator to align the optics as the corrector/focal length extender lens in the drawtube makes other methods somewhat tricky. I suspect that this lens may be intended to add a little more correction than just a Barlow so an alternative one might not have the desired improvement. In any event, the lens should be crystal clear and not "frosted".

[steelfixer]

hi all

fun is not the phrase i would use.

i have a laser collimator but i have found the only way i can use it is to remove this stupid lens because it caused the laser to scatter and you end up with a large blob on the primary insted of a dot.

very interested in this spherical mirror part though.

please could you explain the difference between a spherical mirror and a parabolic.

thanks

graham

[Naz]

Parabolic reflector - Wikipedia, the free encyclopedia

Basic Telescope Optics

http://tinyurl.com/3pt5vnq

Have a read, may answer some of your questions.

[steelfixer]

thanks for the link i am off to have a read

[haitch]

hi all

fun is not the phrase i would use.

i have a laser collimator but i have found the only way i can use it is to remove this stupid lens because it caused the laser to scatter and you end up with a large blob on the primary insted of a dot.

very interested in this spherical mirror part though.

please could you explain the difference between a spherical mirror and a parabolic.

thanks

graham

A barlow will spread the light so that is normal and aligning the secondary will need to be done with the lens removed but you can still align the primary with it in. In fact you can do it more accurately with it in as the spread out beam will reflect the centre spot within the spread out light. This is a more precise alignment because it won't be affected by slight misalignment of the secondary or play in the focusser tube. You'll need something like a piece of card with a hole in to let the beam through at the end of the focusser tube to do this otherwise you won't see the reflected image. If I'm rambling and getting confusing look up barlowed laser collimation.

HTH

Paul

[steelfixer]

hi Naz

well that is some reading matter.

I would not have believed such a simple thing as a mirror could be so complicated.

[steelfixer]

hi paul thanks for the infomation. Its back to the refence books again.

who ever thought a telescope could get to be so involved.

good fun though beats the tele anyday.

[steelfixer]

Nice one Paul thanks I will try that one out tomorrow.