Collimation questions

[Michael1971]

Hi,

I have been busy trying to solve my elongated stars problem. Its getting at a hair pulling stage now..

I discovered a fact that can't be solved without buying a new coma corrector.

It seems that a GSO type 2" CC can only cover a chip of max 20mm. Since my DSLR chip is bigger this can't be solved. Wished I figured that out before...

But since most of my stars look not round I am pretty sure it has also to do with a misalignment of the secondary mirror.

My procedure:

With color cards for contrast, I center the secondary in the focus tube (I use a webcam and Mire de collimation to put it exactly centered.)

Then I put a collimation cap on the focusser and center the secondary to the primary. (so that the 3 mirror clips are all seen)

Then I insert my cheshire and center the primary.. But if I look in the cheshire, I can see only 1 mirror clip and thats confusing since I can see all 3 of them with my collimation cap. (i can actually see the bottom of the foucusing tube..)Should I ignore the clip(s) seen in the cheshire?

Thanks

Mike

[Moonshane]

hi Mike

this thread might help.

with such a relatively large secondary you might need to use an extension tube with the cheshire and pull it quite far out of the focuser to centre and round the secondary to the drawtube. when doing this ignore the mirror clips, just get the secondary round and central. this is done only by turning the secondary on its axis and / or loosening / tightening the centre bolt of the secondary to move the secondary up or down the OTA.

then using the cross hairs in the cheshire, centre the donut on the primary to the centre of the cross hairs on the cheshire (sight tube). you do this with the three adjusters on the secondary. doing this naturally centres the secondary tilt / primary alignment to the drawtube.

then finally use the cheshire face to centre the primary. using these methods you will note that the mirror clips are irrelevant; some scopes don't even have them.

all that said, I don't know if this will make stars out of round. my knowledge of imaging fits on a stamp.

[Michael1971]

Hey Moonshane,

Thanks for this clear write up. I had some ah! moments so thats good! lol.

CS mate!

[Moonshane]

I should point out that once set up, unless you knock it heavily, you should only ever have to adjust the primary alignment. check the others too but they rarely need adjustment.

another tip it to tighten the primary collimation bolts all the way in and then back them off about half a turn or so. this tightens the springs so less chance of movement when slewing. also, don't bother with the locking bolts. take them out. they can cause collimation to shift when tightened carelessly.

[Michael1971]

Thats another good tip, I allready tightened the primary collimation bolts but maybe I should tighten it more.

I also noticed that if you use the central bolt to rotate the secondary that it drops down centre, probably because its heavy and that makes it even more fiddling

At this point I always get confused which bolts I should use to correct it again..

I will try your method as described in your first reply.

hope i am able to do it..

[Moonshane]

sure you can. the secondary should be only just loose enough to turn

[Michael1971]

I have been busy today for hours to check it again.

I dont have a sight tube,dont know what it is really..., only a short cheshire.

Centering seemed ok

Step 2 however, centering the donut on the cheshire crosshairs also works but step 3 isnt needed then anymore..since this step already centers the donut in the croshairs..

Am I misunderstanding something, or could it be that my primary was already aligned?

Could you also explain what you mean with chishire sight tube and cheshire face?

Thanks

[laser_jock99]

.... also, don't bother with the locking bolts. take them out. they can cause collimation to shift when tightened carelessly.

If you have a GSO type mirror cell, throw away the 'locking bolts' and replace with a set of 'passive' springs. The 'six spring mirror mod' is huge boost to collimation stability http://stargazerslounge.com/topic/188976-8-f4-newtonian-mirror-cell-mods/

[Michael1971]

Hi,

Had to come back to this topic again since I dont understand somethings that are going on with colimation.

Centering secondary under the focuser: If I do this with unscrewing the middle screw, the mirror will drop down from center. it will not go straight left or right.

I have to center the secondary by adjusting one of the tilt bolts, I guess the secondary needs that support because its to heavy

If I then want to adjust secondary to primary you have to do this with the 3 tilt bolts, guess what..if I turn these, even slightly..the secondary goes of center and it is not in the middle of the focuser.

Can someone tell me how this is done or if you know these kind of troubles.

I am in a vicious circle..

Thanks

Mike

[Moonshane]

you have to ensure you are looking at the secondary not the reflected primary in the secondary. it's not possible to move the secondary by adjusting the primary . if you hold a piece of paper between the secondary and the primary this blocks the primary reflection and allows you to see the secondary. leave in place to the end of point 4) below

to centre the secondary under the drawtube you have to check one or all (possibly having to adjust all, none or some) of the following :

1) check that each pair of opposite secondary vanes are the same length. this ensures the secondary stalk is central in the tube. it probably is.

2) if you really want to check your focuser is totally square to the tube (I never do this and it probably is).

3) loosen the centre bolt (a little - not a lot and ensure it always has plenty of thread left) to allow you to move the stalk up the tube (to move the secondary 'left' or down to move 'right'

4) rotate the secondary so it presents a round face

5) adjust the tilt of the secondary to centralise the cross hairs (the fuzzy ones) with the primary donut. use the three adjusters. it's possible you may need to repeat step 4) if this proves difficult then repeat step 5)

6) adjust the primary using the three adjusters on the cell so the centre dot is central

you are likely in reality once done properly to only adjust point 6) on a regular basis unless you bump the scope.

[Michael1971]

Hi

When centralizing the secondary i always use colored cardboard to block the reflections.

I cant centralize it by just using the middle bolt, its to high or to low and not in the middle.

Only way to correct this would be by using the tilt screws, spider vanes, or adjusting the focuser.

Since the vanes has been measured I rather not do that. So today i tried moving the focuser slightly.

Since I have flocked the scope some time ago, all parts went of and maybe somethings are not so squared as the were in the beginning..

When I take the cards out I tried centering the donut but I can't see the fuzzy cross of the Cheshire, probably because I have a short Cheshire.

So I try a laser to do this. My laser is only not good colimated itself I noticed..

After this I use the colimation bolts to centralize the donut to the Cheshire.

If I look back in the focuser now, the secondary is again to low or to high and not in the middle..

That's the problem I am having..

[Moonshane]

when you say too high / too low not in the middle, do you mean (looking down the focuser with the primary to the right) that the secondary is to the left or the right? or is it slightly up or down?

it might pay to check the side view of the secondary adjusters. they should all be approximately the same length. it's possible you have over-extended one of them which is producing excess tilt on the secondary.

[Michael1971]

Yes, when I look through the focusser with primary to the right its slightly down, well obviously down..

This was also the case with the tilt srews completly out. I then gave them all the same turns to make sure they were about even.

Was busy with it today and I got an impovement, altough I just tested it on Vega and intra focus looked good and extra focus not.

But this might has something to do with the focusser itself although no idea what..

[Moonshane]

any chance of a pic down the focuser? this might allow a little more guidance to be given. it should be as central as you can get it.

[Michael1971]

Actually I do, I got it a bit better..dont know how exactly but anyway. Shot some images last night there is still something wrong with the stars, maybe that another problem.

[Moonshane]

that does not look too bad. I cannot see the edge of the secondary but you can see all the primary so it won't be a big problem. are you using a coma corrector for imaging? your shapes look like coma to me.

[Michael1971]

yes, thats why it was puzzling me. I got a 2" coma corrector, looks like the GSO one on the net.

I recently got a m48 bayonet with proper extension tube, should be the right distance.

One thing that might be an issue is that I read that these GSO are not really suitable for sensors bigger then 20mm and my dslr sensor is bigger..

Have to say that mine is not a GSO, it just looks like one.

Maybe this is not the best CC afterall.