Collimating the Secondary?

[johnkirkpatrick]

Ok my secondary looks perfectly circle through the home made coll cap when focuser racked in as far as it can go, but the further out I rack the focuser the perfect circle is no longer perfect? Does this matter?

Ill try take a pic with my phone so they wont be great....

The top 2 pics are focuser all the way in.

The 3rd is fully out and the 4th is half way..

[johnkirkpatrick]

Anyone?? This is really bothering me lol

[barkis]

If your focuser is not at 90 degrees to the light path, it may look fine racked in, but deviate as it is racked out.

It's all I can think of, as the secondary and main mirror are fixed.

Is the Colli cap accurately made, in both fit, and hole centralisation?

Ron.

[johnkirkpatrick]

The hole in the coll cap is centred. how do I set the focuser to 90 degrees?

[perrin6]

John, Is there a screw that tightens the drawtube in the focuser. I am wondering if there is a little play in it as this might manifest more as the drawtube is racked out ?

[perrin6]

In pic. 3 I'm thinking gravity is pulling the drawtube down v. slightly.

[johnkirkpatrick]

Perrin6, Do you mean the screw than tightens the focuser so in doesnt move? My scope has the standard focuser supplied. Im still not sure how to fix it. I have measured the spider vains and are all equal (about 11cm each from the center screw on the secondary holder). The secondary is circle till I focus all the way out.. I have never touched the secondary since buying the scope 3months ago. But have moved the primary once in a while.

[barkis]

It is unusual for the focuser to be out of kilter, but I have seen it before.

Before you attempt anytrhing drastic, why not run through Astro_Baby's Collimation tutorial, it has helped many members sort their scope optical alignment out.

Ron.

[perrin6]

Perrin6, Do you mean the screw than tightens the focuser so in doesnt move? My scope has the standard focuser supplied. Im still not sure how to fix it. I have measured the spider vains and are all equal (about 11cm each from the center screw on the secondary holder). The secondary is circle till I focus all the way out.. I have never touched the secondary since buying the scope 3months ago. But have moved the primary once in a while.

Oh - it's a Crayford isn't it? - I was imagining a conventional focuser-type that has a tightening screw on the top which, if loose would allow gravity to make the drawtube point slightly upwards when racked out. I don't know if such a thing can happen with a Crayford so perahps it is more of an alignment issue.

[Moonshane]

I don't profess to being an expert but to me, the secondary is not as important as the primary for collimation (for visual observing anyway) as long the shape is round, it is roughly centralised under the drawtube and you can see the whole of the primary in the secondary. looking at the pics your collimation of the secondary is almost 'good enough' (perfection of collimation is not possible with the average set-up but you almost certainly cannot tell the difference with your eyes) but if you look at the top two pics, there's a little room to move the secondary 'up and right a little' using the three secondary bolts to get it more central. I think if you do this your bottom images will also improve and you'll just see the edges of all of the mirror clips. in reality, I don't think you'll affect the visual image image greatly (one way or the other) if you leave the secondary as it is. If you now centre the primary, you'll be OK. personally. I'd just tweak the secondary though.

[Jason D]

johnkirkpatrick, your secondary mirror looks OK -- not perfect but OK. If I were you, I would forget about it and accept what you have.

For a perfectly centered secondary mirror, the primary reflector edge will coincide with the secondary mirror edge when you place the camera lens along the focuser axis and rack out the focuser. See attachment.

By the way, as you rack the drawtube in and out for a perfectly positioned secondary mirror, the primary mirror edge reflection will shift slightly with respect to the secondary mirror edge.

The impact of a non-optimally positioned secondary mirror under the focuser is the shift of what is referred to as the "100% illumination field". However, this alignment is more critical for imaging as opposed to visual observation. Based on your photos, I would find it hard to believe your views will be noticeably impacted. Just enjoy your scope and accept what you have.