collimation

[speckofdust]

I am sure there is a good reason, but why is it not possible to make a reflector that has all the relevant optical elements firmly fixed in place so that collimation is not needed from time to time? Curiosity....thanks in advance.

[ONIKKINEN]

No such thing exists as fixed in place in practical terms. Not even for refractors if we are talking about same sized apertures and not an 80mm frac and a 200mm newton (very unfair comparison). Machining tolerances would need to be beyond all reason for every part of the telescope and imaging train to be "fixed in place" and even then a slight bump might undo it all. Its much simpler to allow adjustments in the design.

Some cheap refractors cannot be collimated but that does not mean they dont need to be as you occasionally read in forums when a bad copy is sold.

[michael8554]

I'd say because of conflicting requirements.

If the primary mirror edge retainers are wrongly fitted they can cause pinched optics.

Too loose and the mirror can move.

Secondary mirror spiders robust enough to firmly hold the secondary will have large diffraction spikes.

Too skinny and the secondary can move.

Only tiny amounts of flex will show.

I'm sure there are other conflicts.

Michael

 

[NGC 1502]

15 hours ago, speckofdust said:

I am sure there is a good reason, but why is it not possible to make a reflector that has all the relevant optical elements firmly fixed in place so that collimation is not needed from time to time? Curiosity....thanks in advance.

AstroSystems of Luton got close to this.  The primary was glued to thin felt, the felt was glued to a plate that was part of the mirror cell.  This did not stress the glass because the felt effectively allowed expansion and contraction of the glass and the cell in different amounts without affecting the other.  Simple and  brilliant idea!  And no mirror clips protruding onto the optical surface.

The secondary was siliconed to its mount.  Some versions had a full tube diameter support, some had a shorter single vane stalk.  Whilst these used thicker metal than usual, once set up they did not shift.  They were a bit of a faff to adjust but you only needed to do it once!

I once owned an AstroSystems 8.5” f5 Newtonian on an alt-az pillar mount. It was brilliant. And then I STUPIDLY SOLD IT!!

If anyone has one, I urge you to be TOTALLY STUPID AND SELL IT TO ME😁

Ed.

[speckofdust]

Come to think of it, unlike a refractor, the need to clean the primary mirror at least once in a while and the process of remounting it will also introduce errors in alignment.

 

[NGC 1502]

On 06/06/2023 at 07:25, speckofdust said:

Come to think of it, unlike a refractor, the need to clean the primary mirror at least once in a while and the process of remounting it will also introduce errors in alignment.

 

Indeed, with most reflectors that’s true.  Not so however with AstroSystems Newtonians.  The whole primary cell with mirror attached to the tube with tiny precision screws.  It was a simple job to remove, clean the mirror and reattach with collimation unaffected.  Tape could be applied around the edge of the mirror and cell so the felt between mirror and cell did not get wet.

Simple, effective, brilliant 👍 ……..sadly they don’t make them like that now!

Ed.

[johnturley]

25 minutes ago, NGC 1502 said:

Indeed, with most reflectors that’s true.  Not so however with AstroSystems Newtonians.  The whole primary cell with mirror attached to the tube with tiny precision screws.  It was a simple job to remove, clean the mirror and reattach with collimation unaffected.  Tape could be applied around the edge of the mirror and cell so the felt between mirror and cell did not get wet.

Simple, effective, brilliant 👍 ……..sadly they don’t make them like that now!

Ed.

That's similar to my 14 in Newtonian, I've never had to re-collimate the primary, just the secondary.

John 

[AstroMuni]

On 07/06/2023 at 09:07, johnturley said:

I've never had to re-collimate the primary, just the secondary.

You would hope its the other way around as the secondary is harder to collimate 😞