Collimation - Is this normal?

[pvaz]

Hello.

I was just telling Abernus on another thread that I did my collimation manually and it worked fine. So I rechecked it and still have something bothering me a bit:

My manual says I should see the 1st image attached. But I actually see a more oval shapped inner circle (black "frame" holding the secondary to the spider, check 2nd image atached). Other than that, the secondary is directly under the focuses and looks centered, the primary is well centered and it passes the star test. I get perfect circles when I get a centered star out of focus.

Is it normal to see that shape around the secondary or did I do something wrong?

PS-> I exaggerated the image a bit...

[Lester]

Hi Pvaz, Looks to my like you may have the secondary tilted, and have some of its side visible from the focuser. When looking down the focus tube you should not be able to see the sides of the secondary holder.

All the best.

[Taff]

This is a great webpage on collimation. http://www.astro-baby.com/collimation/astro-babys collimation guide.htm.Mark

[Taff]

Try' Astro-Baby Astronomy Website. Mark

[pvaz]

Thanks guys. The secondary did have a slight tilt. All perfect now.

[Astro_Baby]

I was going to say it could be the secondary excessively tilted BUT it could also be completely normal. Depending on the focal ratio you will tend to see an elipse shape around the returned secondary image much like depicted. Fast scopes tend to show this.

[pvaz]

I was going to say it could be the secondary excessively tilted BUT it could also be completely normal. Depending on the focal ratio you will tend to see an elipse shape around the returned secondary image much like depicted. Fast scopes tend to show this.

It's possible, I read your tutorial (great work BTW) and sow it could be the reason.

I toke a while adjusting the secondary and the best position I can put it in still show a bit of an elipse, but much less then before, it's almoust a perfect circle now.

[Astro_Baby]

If you do the secondary alignment and it shows a true circle under the focuser, with all three mirror clips showing (which your drawing suggests is the case) then you align the primary so that the primary centre dots are aligned and true to the Cheshire Crosshairs - then you must be correct. A fast scope will show a fair bit of 'elipse' as shown in my guide.

There is an alterantive which is to have perfectly concentric circles by moving the secondary down the tube. This has a downside of unequal field illumination which isnt a problem BUT its also harder to do. Hence the offset style alignment.

If star testing is showing good stars, airy disks or good diffraction spikes then all is well.