You also miss out on on-axis light too, it's not just the wide angles. This can be quite a lot if your secondary is 'traditionally' sized. I upped my 3.1 secondary to a 3.5 and offset it. The view was much better!
Quote:
Originally Posted by ausastronomer
Hi Markus,
You are really over complicating your own learning curve by worrying about this. Secondary offset really only becomes important under 2 scenarios.
1: For visual observers who are using a minimalist sized secondary mirror by design and regularly using eyepieces with a large field stop. ie 31mm Nagler, 35mm or 41mm Panoptic, 40ml Pentax XW and a few of the new longer focal length ES eyepieces. That in itself is counterproductive because you wouldn't design the telescope with a minimalist sized secondary mirror if you intended a lot of use with such eyepieces.
2: For imagers who are intending very long exposures and who need super accurate tracking.
Secondary offset is designed to place the axis of the light path at the optical centre of the secondary mirror and it is also intended to make the optical and mechanical axes of the telescope coincidental. Even with either partial or full offsets on any telescope under 18" aperture, the observable differences are pretty small.
You will also notice if you run some numbers through the formula that the amount of offset required away from the focuser and towards the primary mirror increases as the aperture of the telescope increases and its F ratio decreases. With your 16"/F4.5 the offset would be about 4mm and with your 8"/F6 about 1.5mm. However, both of these scopes (as are all mass produced scopes) are supplied with generously sized secondary mirrors. I don't think you are going to achieve a single thing (other than confusing yourself) worrying about full or partial offsets with either of these telescopes.
If you position your secondary mirror using your sight tube you will achieve a partial offset (toward the primary mirror) in any case.
I don't worry about secondary offset in any of my telescopes which all have marginally sized secondary mirrors by design and they all work pretty well. I used to offset the secondary in my 18"/F4.5 Obsession which uses a borderline sized secondary @ 3.1" (probably smaller than the secondary in your 16"), but ultimately I figured it was a good bit of extra work for no observable gain.
Cheers,
John B
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