Question on collimation ( sigh :( )

[beamish]

All you experienced chaps and chapesses,

Is it at all possible to have a star in the centre of FOV with perfect concentric Airy rings and then,

a) have stars elsewhere looking odd ? ( though I don't appear to have this now)

when one looks down the focuser, items "do not appear aligned" ?

In short I've been sooo close to smashing this bl**dy thing ! If I try to align the mirrors etc as in the txt books with a cheshire, the resultant appearance of the primary in the secondary appears clipped by the bottom edge of the secondary mirror. ( I don't ever recall seeing this when I first had it), this also results in mishapen Airy rings.

So after many different ways of getting the same result I tried something a little different. I opted to centre the reflection of the primary visually in the "centre" of the 2ndry and then tweak from there before moving to tweaking the primary.

This resulted in what I recall it looking like when I initially got it, no clipping ! The object (that I made careful note to remain in the centre of the FOV) now has XLNT Airy rings "just "either side of focus and focus is sharp ! BUT as I said earlier it doesn't follow txt book appearance, and I am no doubting what my eyes and intelect tell me.

Does this/can this be ??

My only possible solution to this is that there is an offset at work but with the casing holding the 2ndry in the centre of the optical flat glass at the front of the scope, the only way the offset can be achieved would be this apparant "tilting".

Any ideas ??

[barkis]

Offset does enter into the equasion Karlo, especially in a fast instrument. The secondary must be offset slightly away from the focuser, and toward the main mirror. Some imaging scopes have a larger secondary, which may make this offset unnecessary, though if you cannot align the optical elements properly, then the offset most likely needs to be done. Is it not possible to adjust the spider position in the OTA.

Here are a couple of Images. They may help, or not, but I post them anyway.

Ron.

[beamish]

Hya Ron,

No the 2ndry holder cannot be adjusted away from the centre.It's fixed to the centre of the optical flat window at the front of the scope. hence all the 2ndry adjustments are restricted to just the 2ndry mirror within the holder, if that makes sense.

I have got perfect Airy rings with the object (art.Star) in the centre of the FOV though and this occurs right down to just a gnats hair either side of focus. I have the reflection of the underside of the mirror polished colly cap appearing centred too, my concerns are that the crosshairs of the cheshire are not- is this the result of offset ?

Karlo

PS I made sure that the colly cap sat at the focal plane of the scope

[brianb]

the resultant appearance of the primary in the secondary appears clipped by the bottom edge of the secondary mirror.

That's wrong. You need to move the secondary towards the primary. Probably by lossening (or tightening) all the secondary adjusting screws the same amount.

You will not get good performance unless you're using the whole of the primary, and if you can't see the whole edge of the primary in the secondary when using a "peep sight" then something is wrong.

Get the secondary aligned first, ignoring the primary altogether. A peep sight is the best tool for aligning the secondary. Then start aligning the primary, leaving the secondary alone (whatever the temptation). You should not have to touch the secondary more than once in the scope's life, unless you give the scope a real hard bash or dismantle it to refresh the coatings.

[acey]

Items "not aligned" does indeed sound like offset, which is as things should be.

Offset away from the focuser (i.e. making the secondary not central in the tube) is not essential, it's an option that most people avoid.

Offset along the optical axis (ie up or down the scope) happens automatically when you do the standard procedure with a sight-tube/laser/cheshire etc., making things concentric as you go. (It happens at stage one, when you make the secondary *look* central under the focusser). You find when you finish that the reflection of your eye is "off-set" towards the top of the tube.

Yes, it is possible in a well-collimated scope to have perfect diffraction rings in the centre of field, and distortion (coma) away from the centre. In fact it's quite normal. Fast scopes are particularly prone to it, but it could also be coma due to the eyepiece. Stars outside the centre of field will all appear slightly elongated, the axis of elongation being towards the central point.

Sounds to me like you're fine. Certainly, if the scope is for visual use then there's no problem, since you've got central stars looking as they should, and the centre is what you're looking at.

[acey]

PS (Your follow-up cross-posted).

If you can't see all of the primary it might be because:

You're doing something wrong

Your secondary is too small

Your primary is too far up the tube.

Start by checking number 3: loosen all the collimation springs to get the primary as far down the tube as possible. In my f4 scope I found I had to do this - fast scopes are an issue in this respect.

Once the primary is about as far as it will go, try to get it in full view and centred in the secondary. If you can't then the secondary is too small (some scopes are said to have this design flaw). It's not the end of the world, just means you're losing a bit of light. The solution would be to change the primary or live with it.

But this would have nothing to do with the other issue of coma which ,as I said, sounds like it is not really an issue.

[beamish]

Thanks Brian, the point being that this would be/has been the result IF I were to follow the txt book methods of collimation. What I have right now is the whole of the primary clearly in the 2ndry and the centres aligned as described above, but when you look into the cheshire, the centre of the crosshairs are offset to one side of the centre of the everything else-and I'm beginning to doubt my own sanity since the resultant image ( Airy Rings ) tell me that this is correct.

I can ONLY move the 2ndry backwards/fwds, and tilt I can't move it away from the focuser due to the holder/optical glass as described.

In my mind, this would all make sense if there is an offset required.

[acey]

It sounds fine!

If the cross-hairs are to one side of the spot then it might be that the secondary is rotated slightly about the optical axis (ie it's own optical axis is not in line with the focusser.) The solution is to give the secondary a twist. But even this error wouldn't make a significant difference - it just leads to a slight loss of light, not aberration.

Your centre-of-field star-test is fine: you've got nothing to worry about.

[beamish]

Thanks ACEY, this sounds like it.

In the past I had collimated it as per txt book and thought the centre objects were sharp, Kevin (BeyondVision) suggested that collimation may be an issue since outer stars were mishapen. Having checked again everything as per txt book collimation seemed right, hence my battles yesterday.

Now I have hopefully cracked it , the reflection of the primary in the 2ndry is no where near the bottom edge of the 2ndry, well cntered in the 2ndry, but appears offset in the focuser if that makes sense with respect to the cheshire.

I desperately need a clear night !!!

[beamish]

PS the learning curve isn't steep !

It's vertical !!

[Milamber]

Incidentally, have you checked the cheshire? Not seeing the edge of the primary is not a problem since this is likely to be the place (if any) that the mirror is not figured to the optimum (hence the reason for turned down edges). As has been said, a fast scope will display odd things near the edge of the field, caused by the above or by eyepieces. Best thing is to not look at the edge of the field... if it doesn't bother you that you get come at the edge of a dSLR image then why get bothered when you see it visually?

Arthur

[kniclander]

karlo,it's funny but I had exactly the same problem with the x-hairs of the cheshire n0t appearing to be centred in the primary. However, everything else was ok and the star test was pretty good so I managed to resist the temptation to start all over again from scratch and just used the scope. tbh, if it hadn't been a clear night I would have started again form scratch:D

Is one possibility that the cheshire x-hairs aren't centred? Would that throw the whole thing out?

Dan

[beamish]

I'm sure it would. I've gone with everything else Dan. The centre of FOV star is showing XLNT rings, the primary reflection is wholly in the 2ndry, the underside of the polished colly cap is centered within, blah blah so I'm sticking with that.

When conds. allow I'll check with a star (real one), but so far I'm hopeful. I just wish there was another owner of one of these instruments I could talk to

[acey]

Incidentally, have you checked the cheshire?

This is a good point, regarding the issue of seeing the whole of the primary (though it sounds like that's sorted).

The right length for a Cheshire or sight-tube is dictated by the f-ratio. If a scope is very fast then the Cheshire may be a little long for it, so that all of the primary won't be visible. I encountered this with my own scope. But I knew that the primary really was too far up the tube, because with some eyepieces I couldn't even bring stars to focus!

The proof of the pudding will indeed be a star test.

[barkis]

Karlo, I'm sorry, I took it for granted the scope in question was a Newtonian:iamwithstupid:

Ron.