RC8 GSO collimation summary. Generic equipment discussions · GALASSIA 60 · 1/5/2022 · 5 · 692 · 0

The literature on this problem is very vast and dispersed, and having recently purchased an RC optic and faced collimation problems for the first time, I ask for some advice. I don't want to spend a fortune on items or devices for collimation and let's say that I have available to do it: a) Reego collimator 2) Collimable laser collimator 3) Tilter ring 4) mirror. What I have found is that to collimate the primary and see the ring I have to screw the fok directly to the breech, because I am not going to unscrew the last part of the internal hood. So I would proceed like this:
1) Complete optical train with spacers: I insert the laser and with the mirror I check that the laser centers the circle: to do this I act on the screws of the tilter 2) By acting on the secondary mirror I try to re-center the laser beam on the reference of the laser collimator. 3) When the two previous operations have been done I unscrew everything and directly screwing the fok on the breech through the Reego I try to center the shadow of the secondary at the center of the primary (anellus). As the primary interacts with the axis of the fok: 4) Control and fine collimation with the laser and tilter. Is this a fair procedure or am I doing something wrong? Thanks in advance and clear skies.

I've requested help about this not too long ago: https://www.astrobin.com/forum/c/astrophotography/equipment/yet-another-rc-collimation-thread/

I think your reego (assuming that's the same as TS) should be enough to collimate your telescope. I am not familiar with laser collimators when it comes to RC so I cannot add much here. You start with the primary, then do the secondary, then go back and forth few times to get them both on point.

TS has a video using that collimation tool on an RC6, it's a good video. I think they made a mistake of telling people to loosen the secondary scew... don't do that. You also need to remove the baffle extension to be able to see the primary and collimate it. Don't forget to reinstall it I was also told in my forum post that not only the extension can be removed but also the whole thing, I think that's preferable.

My current routine is: remove baffle tube extension, insert autocollimator (something I had from newt days) and center primary, insert tak collimation scope and center secondary, repeat until satisfied, put back the baffle tube extension, double check secondary with tak, move on. This was before I was told that the whole baffle tube can be unscrewed, so my next attempt I will only need the tak.

Final tuning should be done on the stars, using the DSI method (seems to be the most preferable method out there). Something I have yet to master...

Here's my method of RC8 collimation using only a Cheshire and a bright flashlight:

https://www.ancientphotonsastro.com/post/bench-collimating-a-gso-rc8

Many RC8 users complain of the mechanical vs optical alignment being an issue with these GSO RC's. While it may be true to a smaller degree than many imply, I believe starting with a very solid mechanical alignment with a simple tool like a Cheshire will get you very close. Solid optical alignment shouldn't be too far off from solid mechanical alignment. The problem is the methods people use to mechanically align. In fact, the telescope manual itself is incorrect!!

Lasers are problematic. Rarely do they register properly into the focuser, and often they are mis-collimated themselves leading to an apparently proper mechanical alignment but poor optical alignment. So, with using a laser being one of the most popular methods of RC collimation, there are many complaints of "my scope is perfectly mechanically collimated but my stars still look terrible! It must be the discrepancy in mechanical vs optical alignment!". In reality, the laser has misled the collimator.

The other big issue is the focuser on the RC8 being directly attached to the primary mirror cell. First of all, this means any adjustment to the primary mirror inherently tilts the focuser. But even more important than that: the weight of the imaging train pulls on the primary mirror cell causing collimation shifts at different altitudes in the sky. This misleads people as well. They perform a solid mechanical alignment on the bench, point their scope to a target at 60° altitude and realize it's "out of collimation". But, if only they had pointed it to a starfield near zenith (say above 75°) instead, they would have seen really good collimation. Again, misleading the collimator. The focuser issue is a much larger issue overall, especially for those who have a heavier imaging train. Someone on CN machines a custom decoupler so that the two are not attached. It's a huge improvement in terms of ease of aligning the focuser to the secondary and eliminating flexure of the primary mirror cell.

Anyway, I've managed to finally get my RC8 dialed in after mitigating some issues and finding a very easy, repeatable bench collimation method. Here are a couple of results:

Leo Triplet - RC8

IC434 - LRGB - RC8

Chad