completely screwed my collimation

andrew2008 · #1 23-12-2008, 05:13 PM

Just tried to fix secondary mirror and can't get it. Just watched Andyshotglass again and although i can get the mirror centred in the view finder using a cheshire am finding it impossible to adjust the tilt of the mirror so that all the clips on the primary are in the image at the same time.

This is frustrating to say the least

Took it back to purchaser and they said that this was just to do the focal length (F5 skywatcher 10") of the scope and wasn't necessary. Is this right as everything ive read has said they should be in view?

Would really appreciate any help and clarification on this problem

bmitchell82 (Brendan) · #2 23-12-2008, 05:29 PM

Take the cheshire out. and look you will see all the clips around the outside but make sure your eye is flus up agains the focusser.
If you have fixed the secondary, i found using a laser to get the angle of the dangle is the easiest way and the cheshire to get the primary lined up
shouldnt take you more than 5 min unless your padantic and want it 100%

erick (Eric) · #3 23-12-2008, 05:30 PM

My understanding is that if you get the tilts of the mirrors right (and the rotation of the secondary) then you should get sharp image which is the first thing you want. Any probably that is the most important outcome you want, to start with. If, however, you are achieving this without seeing the full primary mirror reflected in the secondary, then you are likely losing some of the light that is being reflected off the primary. The effect of this is to slightly reduce your limiting magnitude??

Maybe this is too simplistic an understanding, but I think, initially, you want sharp images. Trying to get every photon that is captured by the primary and excluding every unwanted photon (with flocking and light shields) can be secondary activities.

In the latest Oz Sky & Telescope, there is a review of a secondary mirror holder where the pivot point is almost at the back of the secondary mirror. In the GSO, for one, the pivot point is cm behind the secondary, so tilting the mirror can move it significantly. That is probably what is happening. I understand then that one should follow an iterative process - tilt/position/tilt/position/tilt...

andrew2008 · #4 23-12-2008, 05:36 PM

So it's not necessary for the cheshire to be in when you see all the clips on the secondary?

andrew2008 · #5 23-12-2008, 05:40 PM

Quote:

Originally Posted by erick

My understanding is that if you get the tilts of the mirrors right (and the rotation of the secondary) then you should get sharp image which is the first thing you want. Any probably that is the most important outcome you want, to start with. If, however, you are achieving this without seeing the full primary mirror reflected in the secondary, then you are likely losing some of the light that is being reflected off the primary. The effect of this is to slightly reduce your limiting magnitude??

Had wondered if this was the case. Unfortunately all the images im seeing in secondary look sharp to me.

erick (Eric) · #6 23-12-2008, 05:54 PM

Quote:

Originally Posted by andrew2008

Had wondered if this was the case. Unfortunately all the images im seeing in secondary look sharp to me.

The main test is the image in your eyepiece. If this is sharp, I'd be cheering! That's 90% of what you want.

bmitchell82 (Brendan) · #7 23-12-2008, 05:55 PM

. they will always seem clear to you becauase your eye is focussing on what ever it is in the secondary

you need to get the secondary mirror pointing to the center of the primary, then adjust the primary to focus the light cone into the middle of your ep. this is what the cheshire does. And why eric was saying. It might take many iterations of moving the secondary, then adjusting the primary. to get it right. A laser collminator does the angle of the secondary regardless of the tilt its only when the laser is returned the same place that it left is when you know the mirror tilt is right.

Getting the center mark on the SW mirror lined up with the dot on the cheshire is the goal.

Its very easy once you figure out what the light path is doing. and will give you beautiful clear pictures.!

andrew2008 · #8 23-12-2008, 06:56 PM

Thanks Brendan and Eric.
Had a lightbulb moment and have got it (well believe so). Was very close at 1 point but trying to push it just ever so better kinda messed it again. Secondary is in centre of primary and everything looks nice and concentric.

Woohoo even have clear skies tonight to see what views are like.

RobF (Rob) · #9 23-12-2008, 07:02 PM

Perhaps take a picture (from eyepiece position) with focuser screwed half way out for the gurus here to comment on Andrew.

On the odd occasion I've taken the secondaries of my scopes completely out of whack just to force me to learn how to get things aligned properly. At the rough early stages of alignment/collimation, I also find it can help to forget the 3 secondary screws and just use your hand to get a feel for "where you to go" before getting back into it with the Allen key.....
(Caution: Rob enterprises can not be held legally responsible for the total destruction of your hard won collimation...!)

andrew2008 · #10 23-12-2008, 07:13 PM

Used my hand a little to do the same thing

Found it helped to do this then hold the position while checking EP to see the difference each movement of my had changed the image to get a better idea of which way i needed to move the 3 screws to tilt the secondary.

bmitchell82 (Brendan) · #11 23-12-2008, 07:29 PM

If im having a crack at it from zero, ill sometimes run it with my hand and try and hold it as close as i can and then screw the main screw up. then have another look at it.

Once you get the hang of it, things are a piece of p155 to do before i get into photography with the dob ill be investing in Cats eye equipment or the like so i can get it 100%