Tommy Mastro · Dec 10, 2025, 12:17 AM
Tobiasz · Dec 8, 2025 at 08:43 AM
Hi,
there is the Sharpstar 2508 0.8x Reducer Corrector designed for RCs. Many shops are selling rebrands of that reducer like the TSRCRED from TS or the Omegon Pro Reducer. I think an Artesky rebrand is there, also. You should check that out.
Reaching focus depends on your focuser, what have you currently installed?
Tommy Mastro · Dec 8, 2025, 12:14 AM
When I set it inside the tube, my sensor was too close.
What do you mean by that? Why not drive out the focuser to the desired focus position?
A RC with a 2.5” reducer always ends up with a short 3 inch focuser, where the reducer is inserted into the focuser tube. Depending on the setup you may also need extension tubes for the reducer INSIDE the focuser tube which eats up precious backfocus.
Hot take: Did you verify your mirror spacing with a ronchi eyepiece? My RC provided a lot more backfocus after fixing the mirror spacing with the ronchi.
Regards
Tobi
I’m new to RCs so I’m not familiar with the Ronchi method. I will Google it.
I’m not sure what you mean by drive out the focuser to the desired position. I am using the stock factory focuser. When I set the reducer into the focuser so the rear of the reducer is connected to the end of the focuser tube (instead of the front of the reducer screwing on to the focuser tube) it is too close and doesn’t reach focus.
I will definitely look for the Sharpstar options. Thanks!
The guide I used can be found here. https://lambermont.dyndns.org/astro/rc/Ronchi-testing-the-focal-length.pdf
It is short and straightforward and can be done with your eyes against a star or with a photographic ronchi. Both are offered by Gerd Neumann Shop for example. I use the classic ronchi eyepiece without a camera against a star. Please beware that fiddling with the mirrors will mess up the collimation which will be visible in the Ronchi eyepiece. I always collimate the secondary with a laser after every change, so that the effect of the error is small. It does not have to be perfect only when you're done to verify.
My mirror spacing was messed up at first and my changes worsened it so I needed two evenings to fix that (first time with a Ronchi). But for now it looks very good. My focal length increased to 2040mm, which is around f/8.2.
Tommy Mastro · Dec 10, 2025, 12:17 AM
When I set the reducer into the focuser so the rear of the reducer is connected to the end of the focuser tube (instead of the front of the reducer screwing on to the focuser tube) it is too close and doesn’t reach focus.
So it is the other way around? You would need more in focus travel to reach focus, even with only the focuser attached and no extension tubes? If so, that is unusual and I think your mirror spacing might be really off here. Without a reducer, how long is your backfocus distance from the back of the tube to the focal plane? AFAIK it should be around +-240mm depending on the tube design.
For comparison, my RC backfocal distance is somewhere around 270-280mm with the fixed spacing.
Regards
Tobi
