I know it's a cheek but...

[Astro_Baby]

The secondaries are offset on their holder by the factory and collimation will cancel out offset in most cases.

The 'short' light path is most likley created by primary to secondary distance or the focuser drawtube being too short.

I'd try the 10mm Plossl before much else and see what that does because some EPs can be weird like this.

[TopHouse]

Total guesswork but.... follow me on this, the primary mirror is at a roughly 45 degree angle to the focusser & secondary, the centre screw on the primary holder not only allows the primary to rotate to get the sweet spot, but it also, in effect moves the primary up and down the tube slightly depending on the amount it's screwed in/out.

The centre screw doesn't 'technically 'tighten' the primary in place, that is achieved by it's positions level of tension against the 3 adjusting screws.

So, in effect you have a small degree of movement up or down the tube, does this also not have an effect on the position of focus on the eyepiece?

If the above is correct, and as I say, I'm guessing, couldn't this then be used to correct the issue of eyepieces not achieveing focus even at the full extent of travel of the focussing tube?

[yeti monster]

Total guesswork but.... follow me on this, the primary mirror is at a roughly 45 degree angle to the focusser & secondary, the centre screw on the primary holder not only allows the primary to rotate to get the sweet spot, but it also, in effect moves the primary up and down the tube slightly depending on the amount it's screwed in/out.

The centre screw doesn't 'technically 'tighten' the primary in place, that is achieved by it's positions level of tension against the 3 adjusting screws.

So, in effect you have a small degree of movement up or down the tube, does this also not have an effect on the position of focus on the eyepiece?

If the above is correct, and as I say, I'm guessing, couldn't this then be used to correct the issue of eyepieces not achieveing focus even at the full extent of travel of the focussing tube?

close, except for confusing the nomenclature of the mirrors.

The Primary mirror is the HUGE "dished" one at the back, which collects and focuses the incoming light. The secondary turns this reflected light through 90 degrees, up into the focuser, where the focal point of the incoming light will lie. This is what you EPs are trying to focus on, hence the "movingupanddownwardness" of the focuser.

Stight travel of the secondary for correct positioning below the focuser should make nominal difference to the focal point of the primary mirror within the focuser tube.

[TopHouse]

OK, I was nearly there then ;o) but I've got a good excuse, i'm new, am now going looking up what nomenclature means.

[yeti monster]

He he!

Whilst you're at it, look up movingupanddownwardness too..........

[TopHouse]

now that one I know, it's in with eyepiecescrewything

[yeti monster]

See, you're picking this up very well now. Soon you will know all about tescelopes and astonomology.

[Frankenstein]

I was in bed early but I'm up early. Some more info in response to the questions raised by other members:

Astro Baby

1. The supplied 10mm ep no longer reaches focus at mid-point. It used to but not now that I have made my adjustments.

2. I've tried gently pulling the focuser out further but it won't budge. It would be interesting to see if anyone else with a 250 dob can move their focuser out more than mine. I don't want to pull any harder.

3. The scope is brand new. I bought it from FLO about a week ago - so it's something that I've done.

4. I'm quite certain that the secondary mirror is at its correct position in relation to the primary - but I could be wrong. Viewing with the collicap shows a circle with the centre lined up with the focuser centre. I have not tried to adjust the position of the secondary mirror in relation to the primary. This is the central screw at the rear of the secondary mount and I have not touched it.

Yeti Monster;

The 4 vanes which hold the secondary mirror were not of equal length when I originally got the scope, and this is what prompted me to make adjustments. Unfortunately I didn't take a note of the four lengths. At the moment all 4 vanes measure 42.5 mm. From what I can remember prior to my adjustments, 1 vane was about 41mm and the corresponding vane on the opposite side was 44mm - but I can't remember which pair of vanes.

Anyone else;

If anyone else has got a Sky Watcher 250 dob, then I would be grateful if they could post a picture of their focuser fully racked out, or measure the amount of travel that there is on their focuser - I have 1.5" of travel on mine. I don't want to try to adjusting mine any further just yet.

A NEW THOUGHT - OPINIONS PLEASE

I mentioned earlier that I spent about 6 hours adjusting the secondary mirror. Is it possible that during this period I have steadily pushed the secondary mirror forward with the 3 adjusting screws - they are currently recessed at about 1/4". If so then does this recessed distance account for a larger linear travel on the focuser - greater than 1/4" perhaps?

[room16]

Moving the primary up the tube has more of an effect on the focus point than the secondary i should imagine. I have the 250px as i mentioned earlier but it had the old R&P focuser which i swapped out for a moonlite.

If you have moved the primary upwards then this will change the focusing point for that 10mm and as such you would require more outward focus.

Now, if you adjust the secondary it will have a small effect on focus, but i shouldn't imagine a great deal if the secondary is actually located central in the collicap.

Don't yank the focuser out, chances are as that is a lowpf crayford, it probably has little outward travel and you dun wanna break it lol.

[Doc]

It sounds like you have moved your primary to far up the tube...

Undo all the primary locking screws and move the primary as low down the tube as you can. When it's in it's lowest position, turn all collimation screws 1 turn to add a little tension and then collimate as normal.

[Frankenstein]

Doc, you may have provide me with a clue with the word 'tension'.

I think that I may be about to be nominated astro bozo of the year

On page 11 of my instruction manual the following is written:

"There are 3 sets of screws on the back of your telescope. The larger screw of the set is the set is the locking screw (Fig.v. - ref to diagram in manual); the smaller is the adjusting screw...

However looking at the diagram (which you can't see) it labels the larger screw as the adjusting screw, and it labels the smaller screw as the locking screw.

I HAD TO CHOOSE ONE SCREW TYPE TO ADJUST AND THE OTHER SCREW TYPE TO LOCK - I'VE PROBABLY CHOSEN INCORRECTLY

I have taken a couple of shots of the base of my scope. Here are the 3 pairs of screws:

http://i818.photobucket.com/albums/zz102/frankystein/Base1.jpg

Now a photo of the base with a screw removed.

http://i818.photobucket.com/albums/zz102/frankystein/Base2.jpg

I decided that the diagram was correct, and I have been adjusting with the larger spring loaded screw, and locking with the other - the screw shown removed.

Well - did I choose wrongly?

[Doc]

You adjust with the screw which contains the spring and lock down with the one that contains NO spring.

[kniclander]

on the solid tube the larger screw is the adjusting screw. tbh, i'd be a bit surprised if you'd moved your primary that far without realising and form your pic it doesn't look like it has moved very far (btw on my solid tube there is certainly more than 1.5 inches focuser travel - about 3 IIRC).

i hope you sort it out - it is an interesting thread but I can imagien pretty frustrating for you.

[Kit-Fox]

Just read through all this, after realising my scope had become slightly out after recieving a nasty knock against the door jam.

God I hate collimation. Despite having done it once before, i still am a nervous wreck doing it thinking i'm getting it all wrong.

[Astro_Baby]

Yes your right - the secondary mirror (which is the small one opposite the focuser) can be moved up and down the tube by using the central screw.

Absolutely make sure the scope is horizonatl when messing with this and dont loosen the screw too much or else the secondary will fall off.

You can adjust the secondary mirror up and down the tube using the central screw BUT it should be perfectly ecntral to the focus tube.

The primary mirror at the back of the scope - the big one - can also be moved up and down the scope tube using its adjusters. There are two sets of knobs at the back of the primary. One set adjusts the mirror the second set locks it.

Normally the larger of the two screws is the adjuster and there is normally a spring between the mirror and the adjuster. My best is the primary has moved too far up the tubne in all the messing about. Do as suggested back a few posts. Losoen the lock screws all the way. Now wind the adjusters in and pull the mirror as far back down the tube as you can go and start from there.

DONT apply force to the drawtube of the focuser - if a gentle pull doesnt move it then its really at the end of its travel. Simalrly all the collimation adjusters at the back of the scope shouldnt eb put under stress - a gentle touch is required.

When you get perfect adjustment and all is well the lock screws are wound in just until they contact the mirror. Afterwards recheck collimation to make sure you havent caused the main mirror to move.

[Frankenstein]

Well it's a relief to know that I was using the correct set of screws!

I have pulled the primary mirror all the way back as far as it can possibly go several times now, but I always end up with same results.

It looks like I may have to just live with the situation. I'll keep checking this thread for suggestions. If I do find a solution then I'll let everyone know. I am really grateful for everyones help, thanks!

[Astro_Baby]

Ok - is there anyone near you like an astro society. Hantsastro used to do a telescope amnesty thing at Winchester - thats quite near you I think. Not sure if Hantsastro is still going though.

[Frankenstein]

Nah! I haven't got any friends - I'm a sad old git really.

[AndyH]

We're going to have to scratch our heads and find a fix , or your young ones are going to auction you on ebay

Andy.

[Frankenstein]

If I can get a sharp focus with my high mag eps then I'm not too worried about having to move the eps out a little. It's just annoying me that the scope isn't operating as it's designed to, and I'll always have a nagging feeling that it could perform better. As soon as the skies clear I'll report on its performance. If I can't get Saturn's rings then I'm going to have to come up with a solution one way or another.

[Frankenstein]

I got there in the end!

The problem was, as most of you suspected' caused by the primary mirror being too far 'up' the tube. I just couldn't understand why this kept happening. The reason was very simple, and here is why...

In order to collimate, essentially from scratch, I loosened all of the locking screws correctly. Then I pulled the primary mirror all of the way back using the adjustment screws. I turned each adjustment screw methodically a few turns at a time until I felt that each screw was finger tight - only finger tight because I was wary of stripping threads. However one screw was tighter than than the other 2, and so part of the primary mirror wasn't set back far enough. I should have done a visual check of the position of the backplate rather than rely on the 'feel' - DOH!

Well I now have a perfectly collimated scope and I'm ready for Saturn's rings!

I now understand my scope and how to collimate, it is very easy so I hope that other beginners are not put off by my antics.

I mentioned earlier that I have worked out a technique to ensure that the elliptical shape of the secondary mirror is a PERFECT circle when viewed down through the focuser. Now that I've got my scope sorted, I feel more confident in my abilities and I will add a thread later about it.

......MY THANKS TO EVERYBODY......

[AndyH]

Good to hear that it's sorted.

Cheers,

Andy.

[LukeSkywatcher]

Thankfully its sorted.

Well I now have a perfectly collimated scope and I'm ready for Saturn's rings!

Shame they aint ready for you. I think they are only about 4 degrees open "tilted" right now. Still................it gives us all a chance to see Saturns moons.

[Astro_Baby]

LOL Paul