collamating question dob 200p

[hoot37]

think i did it right but doudting myself.

i made a collamating cap from the eye piece holder cover and when looking through it i could see 2 mirror clips, but if i turned the focus wheel in till the tube appeared on the left the 3rd clip also came into view so left it at that.

then today my cheshire arrived so when looking through it the main mirror was out by about a inch, and also i could only see 2 clips through it so adjusted the 2nd mirror so i could all 3 clips spaced equal as seen in astro babys guide.

but the strange thing is now everything lines up spot on without adjusting the primary mirror.

so cut to the chase is it ok to use the cheshire to align both mirrors or should i use a cap for 1 and cheshire for other (sorry for long post )

[jimmyjamjoejoe]

Well, it sounds fine. If you can see all 3 clips and everything lines up central, then all the light is coming from the primary to the secondary, and then to your eye, so the optical train is intact.

But if it really bugs you, use astrobabys trick of paper in the tube between the secondary and primary, and take a look down the cheshire to check if the secondary is central and round. If it is (it probably is) then everything is tickity boo. If it isn't, well, adjust it, then go through the rest of the process again.

[umadog]

The cap isn't a great tool. Let me explain the differences between the tools.

If your tool has cross-hairs then it's not "a Cheshire", it's a combination tool comprise of a Cheshire and sight-tube. The cross hairs an long tube constitute the sight-tube. The 45 degree bit constitutes the Cheshire. A Cheshire doesn't have to include a 45 degree thing, which is only there to help light the surface. For example, a "collimation cap" with a shiny internal surface is a Cheshire. It is important to get the names right because different tools have different uses and there ends up being great confusion as to which tool does what. The Cheshire is used to adjust the tilt of the primary. Unless you're desperate, that's all it should ever be used for. The sight-tube is for adjusting the tilt of the secondary. It will also help you to round the secondary in the focuser and to centralise the secondary in the focuser. It does this because the long tube helps to better frame the secondary.

The differences between the tools are shown here, along with a good visual demonstration of why you shouldn't rely on a "collimation cap" to adjust the tilt of the secondary (scroll down for that):

Telescope Reviews: My collimation is that bad!

Finally, if your scope star-tests well then you're good to go. Remember that the tight collimation tolerances you see on the web relate to high-power viewing. Often you're not going that high.

[hoot37]

ok thanks for replys just when you think you cracked it it gets confusing

[umadog]

It's ok, I think you're probably on course!