7 replies to this topic

[Princess Leah]

I sometimes read on CN that collimation is less critical at longer focal lengths. I am trying to envisage why this is.

 

I have a cheap TS optics 60m F15 refractor, that is useful for quick looks of the moon and the gas giants.

 

Previously it was a little out of collimation. After loosening the plastic lens cell retainer and shaking the air spaced doublet, I was able to get it collimated.

However at 160X I don't see any difference to the eyepiece image post/pre collimation.

 

Thanks Leah.

[deSitter]

Slow refractors have very large regions around the optical axis where the performance is maximum. In this region the Airy pattern will appear the same as on axis. Somewhat farther off, you'll still get good performance but the Airy pattern will have rings visibly brighter on one side. The farther from the optical axis you get, the more distorted the pattern until performance suffers even at low power.

 

Fast telescopes are the opposite - the region around the optical axis with top performance is very small. On my 10" f/4.5 Newtonian, the diffraction-limited region is about 2mm wide! On this telescope, final collimation is done by critical examination of a star field in a wide angle eyepiece.

 

The limit of slow is the pinhole camera, with a focal ratio of infinity. Every point on the object is directly connected to every point on the image by one line, so performance is the same everywhere in the image plane.

 

-drl

[ngc7319_20]

as @deSitter says, at long focal lengths (at least for refractors) the region of good image quality is larger.  So a little tilt error in the lens collimation has less effect.  Another effect is that the glass curves in a refractor lens are more shallow -- so in terms of wavelengths of error, any spacing tilt or centering error will have less effect at longer F/ ratios.

 

If you want to dig in to the collimation business, I suggest reading on Star Testing.  Dick Suiter's book is a fantastic reference on this, if you can find a copy. 

https://www.amazon.c...n/dp/0943396441

A green filter and 5mm or 6mm eyepiece on a bright star will help to see what is going on.  The light patterns just inside and outside best focus contain a lot of information about the collimation.

 

There is also the Aberrator software for simulating collimation errors. 

https://www.softpedi...Aberrator.shtml

 

A so-called Cheshire eyepiece (used in the lab) will also help in studying the alignment of the lens internally, and relative to the telescope tube.  Each lens surface makes a reflection, and looking at these reflections will tell you about the alignment and collimation of the lens.  Celestron used to include a cheap one with their refractors.

https://en.wikipedia...eshire_eyepiece

https://www.highpoin...on-eyepiece-cce

Edited by ngc7319_20, 12 November 2024 - 09:31 AM.

[Sebastian_Sajaroff]

I sometimes read on CN that collimation is less critical at longer focal lengths. I am trying to envisage why this is.

 

I have a cheap TS optics 60m F15 refractor, that is useful for quick looks of the moon and the gas giants.

 

Previously it was a little out of collimation. After loosening the plastic lens cell retainer and shaking the air spaced doublet, I was able to get it collimated.

However at 160X I don't see any difference to the eyepiece image post/pre collimation.

 

Thanks Leah.

The larger the ratio, the bigger the focus region. So, it's easier to position your lenses or mirrors to be reasonably well aligned.

Also, on cheap telescopes collimation errors are hidden by bigger optics defects (chromatic aberration, coma, spherical aberration, astigmatism, etc.) 

[Princess Leah]

Slow refractors have very large regions around the optical axis where the performance is maximum. In this region the Airy pattern will appear the same as on axis. Somewhat farther off, you'll still get good performance but the Airy pattern will have rings visibly brighter on one side. The farther from the optical axis you get, the more distorted the pattern until performance suffers even at low power.

 

Fast telescopes are the opposite - the region around the optical axis with top performance is very small. On my 10" f/4.5 Newtonian, the diffraction-limited region is about 2mm wide! On this telescope, final collimation is done by critical examination of a star field in a wide angle eyepiece.

 

The limit of slow is the pinhole camera, with a focal ratio of infinity. Every point on the object is directly connected to every point on the image by one line, so performance is the same everywhere in the image plane.

 

-drl

Youve described that beautifully. I now have an image in my mind. 

Thanks again Leah.

[Tony Flanders]

Another way of looking at this is that the whole point of collimation is to place the center of the field of view at the spot where coma is smallest. Coma is directly proportional to the distance from the optical axis, and inversely proportional to the cube of the focal ratio. So at any given distance off-axis an f/8 scope has one-eighth the coma of an f/4 scope.

Edited by Tony Flanders, 13 November 2024 - 01:53 PM.

[deSitter]

Another way of looking at this is that the whole point of collimation is to place the center of the field of view at the spot where coma is smallest. Coma is directly proportional to the distance from the optical axis, and inversely proportional to the cube of the focal ratio. So at any given distance off-axis an f/8 scope has one-eighth the coma of an f/4 scope.

That's my "secret sauce" - I make the coma at the edge of an ultra-wide the same all around. You can artificially induce coma with a tunable Paracorr and use that if you have a hard time seeing it.

 

-drl

[Princess Leah]

That's a great idea!