collimation qu of the day..................

[kniclander]

a couple of qus -

1. when I check that my 2ndary is centred with a home made collicap, I see all mirror clips but they are not exactly the same size, but pretty close. Hence, my 2ndary is not perfectly centred. My understanding is that this will result in a slight loss of brightness but not affect contrast resolution. Is that right? Does is matter if the hole in my collicap is not precisely centred (when I rotate the focuser tube the clips move a little bit, ie the one that looked slightly bigger changes). It's slightly out however I turn the focus tube. Is that just because the hole is not perfectly centred?

2. when I use the cheshire for the primary, there are three crosses which I think are the spider, the cheshire cross-hairs and the reflection of the cheshire crosshairs. The cheshire crosshairs do not line up exactly with the spider, ie when I rotate it so that are all lined up. Does this mean that the x-hairs in my cheshire are in the wrong place and does that matter? Also, the cheshire and the reflection of the cheshire x-hairs don't completely line up.

3. there is a tiny threaded hole in the side of my cheshire near the top - what's it for?

thanks

[boboco]

Hi,

Regarding Q1 - are you looking through the hole from a distance? I find that if I am too close then the image is definitely off because my eye is slightly off centre. Standing back helps. What is the view like with the focuser open (i.e. no eyepiece in it) - stand a long way back and see if the view is still correct. From what I have read, unless the secondary is REALLY off, it does not matter. If your really concerned about slop and being off centre then albedo0.39 has recommended a self centering adaptor which is here (flo don't stock it).

Q2 - try to ignore the spider and don't try to line up with it. Your looking for the reflections in the very centre of the image. This guide has some quite nice clear images (Collimating Newtonian Optics) in particular this image (http://www.oarval.org/c-start.jpg) and notice that the spider is not mentioned anywhere. Just the cheshire cross hairs and the reflection of the cross hairs are important. [Although the author does mention centering the secondary - others with more experience than me have said its not really important unless its way off].

Q3 - if its the same as mine its a grub screw to keep the "mirror" part of the cheshire in its tube, seems to do nothing else.

Cheers,

Karl.

[kniclander]

thanks Karl and thanks for the links. Very interesting.

Dan