collimating a f3.9 newtonian

[alacant]

Hi everyone. I just took delivery of a 208mm f3.9 with an 85mm secondary. The secondary looked good through the collimation cap so I just adjusted the primary to centre everything. It looks like the enclosed picture: the reflection is centred. What's this thing about displaced reflection for fast f ratios? The views look OK edge to edge on non-stellar objects however for obvious reasons, no star test will be possible any time soon.  TIA.

[Peter Drew]

The offset of a secondary mirror is to allow for the fact that the light cone from the primary is wider at the low point where the secondary intercepts it than the corresponding high point. With a secondary as large as 85mm it should be enough to easily cover the cone with the collimation as shown in the diagram. 

[alacant]

Hi Peter. Thanks for the info. In fact, blocking the primary mirror and looking through the Cheshire, all I see is mirror; I can only see the physical secondary by removing the Cheshire. The secondary looks dead centre through the collimation cap. I wonder if it's worth moving the secondary toward the primary a few mm. Or is life too short...? TIA.

**edit: just tried it. Moved the secondary down as far as the adjusters would allow -about 2-turns anti-clockwise of the central adjuster-. Still centres centrally without displacement. Is it the large secondary causing this perhaps? On my 150 f5 with a 55mm secondary, there is a noticeable displacement...

[alacant]

Hi. Found it. The secondary wasn't centred; I was looking at the primary reflection rather than the mirror itself, DUH. Now I have the secondary properly centred, it looks as expected. Must say that the spring loaded knobs on the secondary make this much easier than having to use a screwdriver. I followed the same procedure as with my f5. Here are the views through the collimation cap and the Cheshire. I can't see any difference from my f5. Maybe I should be doing something different for the 3.9(?) TIA.

[happy-kat]

I want to follow this for your replies.

My rambles, only take them as rambles not fact. This is me just taking out loud,  looking at your picture irrespective of secondary reflection being offset in a fast telescope (which yours is showing) the outer shape of the secondary mirror shouldn't that still be centred and concentric to the primary.

Please don't adjust anything I'm just sharing what I thought and hope to read your replies so I can understand more.

[alacant]

Hi. Dunno, but I think that the view through the collimation cap shows it is.

1. I got the secondary circular in the centre of the collimation cap.

2. I got the centre 'donut' reflection from the primary centred on the crosshairs of the Cheshire using the secondary adjusters.

3. I adjusted the primary so that the reflection from the Cheshire's 45º face coincided with the crosshairs and so the centre of the donut.

That's it. Why guides on the matter extend to 10 pages I do not understand, the very reason I think there must be more to it.

I believe there's a unique position that this can happen and you have no control over where it is. But of course, that's just me doing what seems right. It's the same as I do with my f5 reflector, so it can't be right, can it?  Here's a snap -not an idealised diagram- from astrobaby. Do tell me if I need to do something else. TIA.

[barkis]

Your fast f3.9 is primarily an Imaging telescope, and the last thing you want is Vignetting by the secondary, I
would think the semi major axis dimension of 85mm is easily enclosing the reflection from the primary.
If that wasn't the case, I think you would by necessity, have to displace the secondary according to convention.
An oversized secondary light loss effect in not all that critical for Imaging, but might be perceptible visually.
 

 

[alacant]

According to the ES blurb, it already is as it comes with back-focus to spare even with a dslr. Even so, there's still a concentric annulus of spare secondary.