Question on recent "Collimation" post

[pat]

I'm confussed about a comment made : When centering the secondary beneath the focuser. It can either be round, or central, but not both (according to Astrobabys guide).

When I read the guide part, it says that the secondary is directly under the focuser tube and appearing as a round circle. As mine appears.

Do these two statements have the same meaning??

I don't want to seem dense. Pat

[Parus major]

Yes. The secondary is cylindrical but cut at a 45° angle so from 90° to the facing side it should appear perfectly round.

[pat]

Thanks James, It just seemed that that these two versions contridicted each other. Pat

[umadog]

You should e-mail AB if the guide is not consistent or is confusing. Then she can amend it if needed. Here are more details should you need them: Telescope Reviews: Useful info about secondary mirror alignment

[pat]

Thanks- How do I e-mail AB? Pat

[umadog]

Like this!

Stargazers Lounge - View Profile: Astro_Baby

[jimmyjamjoejoe]

This is about my post - seems you're confused about it. The problem i was having was that i couldn't achieve a round and central secondary no matter what i did, even though astrobabys guide says thats what you need.

http://stargazerslounge.com/beginners-help-advice/173520-collimation.html#post2134983

It's apparent in the first stage of collimation (according to astrobabys guide), when just centering the secondary beneath the focuser. It can either be round, or central, but not both.

I meant "The problem is with the first stage of astrobabys guide), not that astrobaby says it should be round or central etc etc...

It turned out the focuser wasn't square.

[Jason D]

Yes. The secondary is cylindrical but cut at a 45° angle so from 90° to the facing side it should appear perfectly round.

Here is an interesting mathematical trivia. An elliptic shape cut from a cylinder at 45 degree angle will not appear as a perfect circle when viewed at 45 degree angle (mathematically speaking). Only an elliptic shape cut from a cone at 45 degree angle will appear as a perfect circle only when viewed from a 45 degree angle and from the apex point of the parent cone. In real life, a human eye will not see the difference.

Jason

[Parus major]

More so when using one eye the appearance is on a 2D flat plain.