Which method off Secondary Offset to use?

[marcopolo]

Another question about my 12in F3.3 scope!!!

Is there an accepted preferable method of offsetting the secondary mirror. For an F3.3 there is going to be quite an appreciable offset according to the charts.

So should I use 'method A' of moving the secondary away from the focuser and towards the primary, or 'method B' of moving the secondar closer to the primary and then tilting the primary?

Given the very fast ratio I would be inclined not to use B as the tilt angle, I would have thought, is becoming quite appreciable!

I should probably say i'll be using the catseye collimation system from Jim Fly in the US.

cheers,

Marco.

[Jason D]

I would start with what you referred to as method 'B'. The name we use here is "New Method." This method works as long as:

1- You do not have DSC installed as this method would impact DSC accuracy

2- Your front aperture (UTA/OTA front opening) is wide enough to avoid vignetting. That is, as long as you do not see any part of the UTA/OTA front rim reflection via the primary mirror when looking through the middle of the focuser at the focal plane.

If you decide to go to what you referred to as method ‘A’ (“Classic Offset”) then it is advisable that the secondary mirror is installed with the proper offset. Some would attempt to use the spider vanes to move the whole secondary mirror away from the focuser, assuming the secondary was installed without an offset, but doing so will get one set of spider vanes out-of-line which might worsen the diffraction spikes.

Jason

[callump]

Thanks Jason, those are really useful diagrams.

Callum

[marcopolo]

Many thanks. So presumably any axis tilt in the new method doesn't introduce significant aberrations?

[Jason D]

Many thanks. So presumably any axis tilt in the new method doesn't introduce significant aberrations?

The New Model method introduce "NO" aberrations whatsoever.

Many think my tilting the primary mirror, the focal plane will tilt at the eyepiece as shown in the first attachment. This is FALSE. In both methods, the focal plane will remain perpendicular though it will be pushed out by few millimeters when the New Model method is used (second attachment -- figures A vs. .

Jason