Is something misalligned?

[timelord]

Greetings to all--I have recently fitted a crayford style focusser to my 10" newt and centred the secondary, collimated the scope and performed star test all seems well.
My problem is I can see the edge of the focusser draw tube as a thin crescent from 12 o'clock to 6 o'clock primary mirror side.
It is really bugging me as it catches your eye everytime I look down the focusser. Sould it be visible as a thin circle all the way round? /not at all? Or is something out of allignment?
Maybe I,m worrying about nothing as everything else is ok. :help:

[Tannehill]

You see a reflection of the focuser drawtube when you look at the secondary mirror thru the drawtube? Sorry, I'm not able to visualize what you are describing. Depending on light conditions you might be seeing some internal reflection which - if you wanted to be real particular - you could eliminate by baffling your focuser drawtube...but I confess my scope is downstairs in my storage locker and can't easily look in to see what might correspond to your description. Can you give more hints as to what you see where? Or even try taking a digital pic thru the drawtube if you have a digital camera?

scott

[Garyh]

Hi Timelord,
sounds ok to me!!!....:thumbsup:

This page might help understand the reflections on sees.

http://www.fpi-protostar.com/bgreer/collim.htm

A thin crescent from the inside of the focusers drawtube (on primary mirror side) sounds right and I get the same in my newt..

cheers Gary

[Tannehill]

Which image in Greer's page there is the crescent you see? I ran down and peeked into mine and can't figure out what we're talking about..... Could be lack of caffeine, tho....

scott

[timelord]

Thanx guys the web link shows exactly what I mean and has put me at ease.
Alex.

[Garyh]

Hi Scott, the pic doesn`t show the crescent shape but in the last image on the page that shows the eye in the drawtube that is offset from the centre, on some newts you will see a thin crescent shape from the edge of the bottom of the drawtube making a thin crescent...not sure if this is because the edge is not blackened and having a larger secondary? But mine shows the same....I attached a copy with where the crescent is seen...colored red..
cheers

[Tannehill]

Ah-hah! Thank you very much. Now I see what timelord meant. That's stray light from beyond front of tube directly impacting the other side (primary side) of the inner aspect of the shiny metal focuser tube.

For tinkering enthusiasts and highly discriminating photon gourmets, I guess that's another place to apply mods to reduce contrast robbing stray light...but then I'm a glutton not a gourmet....wife says I should change my avatar to Light Pig, since I keep acquiring bigger and bigger dobs...

Thank you again for the effort to post that pic, I appreciate it.

Bryan Greer's sites and articles have never failed to educate me. He's quite good with explanations....an engineer with specialty in optics....and he posts his email on his site for people like us to ask him questions.

I've copied his mirror fan recommendations also used some of his flocking papers....very top quality gear, his stuff...

thanks

scott

[Ingo]

Timelord, do you happen to like high powered lasers?

[timelord]

Ingo why do you ask?

[Ingo]

Just Wondering.