RCOS collimation

[raymo]

I have often wondered why RCOS scopes seem to be a pain in the rear end
regarding collimation. Having a fixed primary mirror, why are they so
much worse than a Newt for example?
raymo

[Bassnut (Fred)]

Im guessing you mean Ritchey–Chrétien rather than just the brand name RCOS (which used Ritchey–Chrétien mirrors).

Newts and RCs both have pros and cons. Collimation is much more difficult generally on RCs (Hyperbolic mirrors) compared to Newts (Parabolic mirrors), others will explain why.

RCOS was an unfortunate term to use, RCOS made scopes so stable and robust, that the need for re-collomation was rare compared to other RC brands. Ive done it once on my RCOS.

[raymo]

Thank you Fred, sorry about the mistake.I realise that they are an RC scope, but everybody seems to comment on their collimation troubles with their
RCOSs, so I used that name.
raymo

[glend (Glen)]

In my RC neither mirror is fixed, both mirrors have adjustment screws like a newt. As was pointed out the problem arises from the optical design of the mirrors compared to a newt. RC owners can easily get into a cycle of adjustment that 'chases' collimation. There are tools and techniques that make it easier but it can not be as simple as a newt. The refusal to accept the need for special tools is often the cause of building frustration by the novice owner. Sure you can collimate an RC with a Cheshire but you have to know exactly what your seeing and how the surfaces inter-relate. Taking a newt collimation skills set and assumptions into RC can result in frustration. I admit i was a slow learner.

[raymo]

Thanks Glen, when I said fixed, I meant mounted and adjustable, as
opposed to SCTs and Maks whose primaries move for focusing.
raymo

[RickS (Rick)]

One of the benefits of the corrected Dall Kirkham design (as used by Planewave, Ceravolo, etc.) vs Ritchey Chretien is that the secondary is spherical so it doesn't need to be precisely centred and good collimation is less difficult to achieve. You can, of course, get brilliant results with either type of scope...

Cheers,
Rick.

[raymo]

Thanks everyone.
raymo