Longs Focal Length scopes 16" RC vs. CDK 12.5 vs. C14 Edge HD

I’d ask the same question. What are you looking for that the RC doesn’t provide but the alternatives do?

Thanks Yuxuan , Reg and Tiff for your replies. Our GSO 16” RC was recently moved between locations and collimation is proving difficult for unknown reasons. The stars start out round but go egg-shaped after several hours. We’ve tried multiple attempts at collimation but none are succeeding now. Collimation was hard in the past, but not impossible. The primary was recently reseated as it would pinch and give triangular stars at temps below 0C. Now the mirror is free floating with a tiny paper thin gap to the edge clamps.

We have an ASI6200MM camera and M54 spacers in the imaging train, so we have no vignetting (FL 3250). I always thought the C14 was a way to go, but not with the issues mentioned. A C14 or a used CDK 12.5 may be about the max for our budget. We are into deep sky, not planetary. We’d like to stay at around 2500mm FL or larger as smaller galaxies and nebulae are in our interest.

Any further thoughts appreciated. Thanks.

John

That camera is worthless when using an Edge. I have an Edge11 and because I’m a stickler for data quality it uses a QHY533M camera rather than a full frame.

Get the CDK if you have the budget. It’s a decision you will never regret. After a very bad collision between my CDK and my Paramount MX+, collimating the CDK took less than 30 minutes.

The CDK I have now, again, collimated after a different problem in a matter of minutes.

Those GSO scopes can be a real trap when it comes to collimation. It really kills all the fun to spend hours trying to get it just right.

Reg Pratt · May 13, 2026, 04:08 PM

So the RC was fine until you moved it? If that’s the case I would exhaust all options in getting your RC sorted before spending money on an entirely new optical system. How were you collimating in the past? RCs aren’t difficult to collimate if done with the right process. Unless some mechanical component was damaged during the move you should be able to get it back into good collimation.

If you have already exhausted all options then then CDK is definitely the way to go. I would just hate to lose aperture if it isn’t strictly necessary.

Thanks Reg. That’s the dilemma. We have tried 3 or 4 different collimation tools including a newly purchased Takahashi collimation scope. We can get an alignment but then it drifts over time. The scope is about 20 years old and something may be going south with age. Despite the efforts of several qualified individuals the problem has not been resolved. The CDK seems like an option if we can find one at the right price.

John

I agree, it could be very much worth it to have the primary mirror cell and secondary support looked at. I know the scope is “old” but honestly, these things are not that complicated and the engineering is well understood.