Diagonal offset....again

[Davidv]

I know there have been some excellent posts on this subject but I haven't spotted what I need to know so I wonder if anyone can help. I’ve decided to rebuild my homemade 8.75” Newtonian and will be replacing the single vane diagonal support with a four vane spider. This got me thinking about diagonal offset. From what I’ve read I gather it’s important with large fast mirrors but what about an 8.75” F7.2? I’ve calculated the offset would be less than 2mm.

If I had the diagonal further away from the primary so the cone of light comfortably sat within the diagonal, would this be an acceptable alternative to trying to introduce less than 2mm offset, which sounds a bit awkward to say the least.

[BinocularSky]

You are unlikely to notice an enormous amount of difference in the visual image with your 8.75" f/7.2 if you don't offset. However, if you are rebuilding it, why not do it "properly"? If you think in terms of the optical centre of the secondary coinciding with the optical axis of the light path, then offset is automagic and not awkward at all!

If you have the diagonal further away from the primary, then the Newtonian focus will be closer to the side of the tube. Have you ensured that you will still be able to reach focus with all your eyepieces, etc? If you do move it further away so that, as you say, the light cone sits within the (non-offset) diagonal, then you are using a bigger central obstruction than you need. Again, you will probably be unlikely to notice the difference visually.

As for it being awkward to offset, I have found that it is no more awkward than precisely setting the physical centre of the secondary to the optical axis -- in fact, if anything, it is less awkward.

[Davidv]

However, if you are rebuilding it, why not do it "properly"?

Thanks Steve. I must concede you've probably got a point there. That aside, I understand why the diagonal needs to move away from the focusser but I'm not clear on why it also needs to be offset towards the primary???

[gliderpilot]

Sorry to move the conversation away from the original post but why does a secondary need to be offset and how does one calculate it?

I haven't used an offset on my 8.5" f5 and can't notice the difference although I don't doubt it can be measured.

By inspection, I would have thought that aligning the secondary with the optical axis was all that is required?

Mark

[BinocularSky]

Given the old adage that 1 piccie = 1 kiloword, here is a piccie from my book AstroFAQs:

[BinocularSky]

Sorry to move the conversation away from the original post but why does a secondary need to be offset and how does one calculate it?

See diagram in my post above. The physical centre of an ellipse is not on the axis of the cone of which it is a section.

The approximate calculation (good enough for practical purposes) is:

Offset = (minor axis of secondary) / (4 * focal ratio), so a 36mm ma diagonal with an f/5 scope will have:

Offset = 36 /(4*5) = 36/20 = 1.8mm

[gliderpilot]

Thanks for that, Steve, very succint and instantly understandable. I always thought the centre of the secondary aligned with the optical axes due to symmetry so I am off to read my Euclidean geometry!

Mark

[Davidv]

Thanks Steve. As Mark says, the drawing makes it very clear.

Mark, no apology necessary, I'm off to read my Euclidean geometry when I find out what it is!

Dave

[robbieince]

Fascinating this,

I have a f3.8 ag8 and if I read this right, the optical axis will line up with the spider vanes but the actual centre of the secondary will not?

Looking from the eyepiece point of view, the transverse offset will mean that the spider vanes should not align with the actual central of the secondary but will with the apparent centre and they will also align with the optical axis and therefore the centre of the primary - is this right?

[BinocularSky]

Robbie, I don't know the scope or what adjustments are available to you, and I am not clear what yuo are describing, but I would be surprised if a combination of the explanations on Nils Olof's superb page and the discussion and diagrams on Bryan Greer's page don't enable you to answer your questions. Of course, given that your scope is an astrograph, if you are getting consistently offset vignetting, your diagonal is not offset.