can't focus!!!!!!!!!!!

bartman (Bart) · #1 29-10-2009, 11:13 PM

I am at wits end!
What am I doing wrong?????

Arrrggghhh
Surely it must be something simple.
Skywatcher ED80 Pro (gold tube), SW 28 mm EP and/or 2X barlow.

I just can not focus on Jupiter or the Moon.............
I literally have to take the eye piece out of the focuser or barlow about 10-20 mm to get it to focus at full drawn out focus tube (ie furthermost away from the objective)

The finder scope gives a better picture than the main scope!

Shouldn't this be a plug'n'play situ?

Go on.....tell me I did something stu-pid!

Bartman

BTW I just bought this scope to compliment my 10" BD SW as a guide scope/ wateva in the future as the price was good!
(maybe I shouldn't have?)

wasyoungonce (Brendan) · #2 29-10-2009, 11:28 PM

You need an extension tube or a diagonal.

The ED80, like many scopes is built with the need to use a diagonal to achieve focus.

If you don't use a diagonal you will at least need a tube extension that is about the equivalent length of a diagonal...approximately 50mm.

More here.

bartman (Bart) · #3 30-10-2009, 12:43 AM

mmmm k,havn't yet gotst da diagonal....just got the OTA.
Cool , so Ill source a good Diagonal.

So I guess it was a simple answer...

Maybe I should enroll in a Optics course of some sort

Nah, I should just think a bit more logical!

Btw.... would a focal reducer help? Logical part of the brain not active ATM!
Maybe....a pictorial Opticts 101 would be greatly accepted ( or Troypiggo - Troy - you could help me out with one of your Wonderful Google Toolman directions hehehehe BTW no offence taken!)

Bartman

wasyoungonce (Brendan) · #4 30-10-2009, 11:09 AM

A focal reducer will bring the apparent focal plane inwards. The ED80 was more than likely designed with it's current set-up, aka needs an extension tube to achieve focus as it can be adapted for use in a variety of ways:
As a prime focus lens;
With an eyepiece;
With a focal reducer/multiplier.

Each of these requires a differing position of the focuser and the best way to achieve this (range) was probably to make it with the focal plane just outside ...thus it needs an extension tube for most applications.

Have a look here and here.

Both links have calculators that can give you an estimate on how much a focal reducer will bring the apparent focal plan inwards.

Terry B · #5 30-10-2009, 01:47 PM

The problem with using a focal reducer is that it will make the already short focal length even shorter. If you want to look at planets as you stated then you will only be seeing a little dot. You can use very short focal length eyepieces to compensate but they are not easy to use. You will also get more image degredation from the lenses compared to a diagonal- more surfaces etc.

bartman (Bart) · #6 31-10-2009, 08:34 PM

Coool thanx guys

I'll put you're suggestions to good use!
Bartman