Jump to content


Cheshire cross-hair visibility in Newtonian

Recommended Posts

Hi everyone. I'm having difficulty focusing on the crosshairs in my Cheshire collimator. My eyesight is reasonable and whilst the refection of the cross-hairs back from the primary is clear, the cross-hairs themselves are not; they appear out of focus. This is particularly annoying when aligning the centre mark of the primary on the cross-hair intersection during secondary adjustment [1]. I've found a partial solution since my phone focuses perfectly on the cross-hairs whilst retaining a clear view of the reflections -example attached-, but I'd like to be able to do this in real time. I'm wondering if this is a mind over matter thing and I must instruct my brain to focus on the cross-hairs, rather than the reflections. Any thoughts? TIA.

[1] I have the 45º Cheshire face covered during this process.


Link to comment
Share on other sites

The cross hairs are much closer to the eye than the virtual images from the mirrors - I can't focus on mine either. It doesn't matter to me since I never use them. For secondary alignment I use a laser, looking down the top of the tube and adjusting the secondary to get the red dot centred on the primary. If I were using the crosshairs I think I'd just tolerate a blurred view and get things reasonably centred: the primary is more critical. If the primary is correctly aligned with respect to the secondary then the only significant "risk" is that the secondary is rotated slightly with respect to the focuser, resulting in a tiny loss of illumination at the eyepiece. If in doubt then do things iteratively: secondary-primary-secondary-primary... until satisfaction, boredom or madness ensue. I wouldn't use a camera to do it for fear of misalignment but if it works for you the problem is already solved. If you really want to see your Cheshire crosshairs in focus then the only solution may be backing away from the viewing hole or else use reading glasses. We all need them eventually....

Incidentally, the "f-ratio" of a Cheshire (length to width) needs to be in reasonable correspondence with the f-ratio of the scope, in order that the whole primary will be visible through it. For a long-focus Newtonian you can use a long Cheshire, and the cross-hairs at the bottom end will be far enough from the eye to be seen in focus. Fast Newtonians (such as my f4.9 Skywatcher 12") necessitate short Cheshires, hence the eye strain.

Edited by acey
  • Like 2
Link to comment
Share on other sites

1 minute ago, acey said:

reading glasses. We all need them eventually

Thanks. Yes, of course. The most obvious I hadn't thought of. The problem with the phone is that it's not in real time so you can't see which way your adjustment is going and the live view doesn't show the cross-hairs at all.  

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.