Collimating these GSO RC8's can be a real pain in the a**. But, if you know what you're looking at, you might just be able to easily do it from the bench using only a Cheshire eyepiece. By "bench collimation", I mean collimating the telescope during the daytime without having to tweak it further under the stars (and wasting imaging time!). I've managed to get mine almost perfectly collimated with the following method.
A couple of important points:
-AFTER COLLIMATING ON THE BENCH, YOU MUST LET YOUR RC PROPERLY ACCLIMATE TO OUTDOOR TEMPERATURES BEFORE CHECKING COLLIMATION ON THE STARS. These RC tube telescopes require quite a bit of time to acclimate to outdoor temps, especially in the winter. I usually put mine outside about two hours before I test to see if my collimation is good - sometimes it takes longer.
-when you check your collimation under the stars, choose a fairly crowded starfield near zenith. Why? Because the RC8 is prone to flexure caused by the imaging train pulling on the primary mirror cell. This means any FOV below about 70° altitude may appear mis-collimated as the imaging train pulls on the primary mirror cell causing it to slightly shift. If you think you have this problem, see my previous post about GSO RC8's.
The Method
For this method, you will need a decent Cheshire and a reasonably bright flashlight. I use this Cheshire from Agena. You should remove the crosshairs from the end of it for our purposes.
If you have purchased the optional custom de-coupler plate or focuser collimation ring, I'll go over over a way to align the focuser at the end of this blog. For now, to keep things simple, let's get the mirrors aligned and figure out what to look for in the Cheshire.
Step 1: Position the telescope so that it is parallel to the floor and pointing at a slightly illuminated white wall. Reach in front of the tube between the spider veins and unscrew the baffle extension tube. If you have an older model, yours might not have one - in which case proceed to the next step. The extension tube is only about 3 or 4" long.
Step 2: Remove all of the extensions from the back of the OTA, and connect your focuser directly to the threads of the primary mirror cell. Rack your focuser all the way in. Insert the Cheshire into the focuser (you will need to use a 1.25" adapter). Be sure it seats correctly and is not tilted. Don't over-tighten the locking screws, just make them snug enough to hold everything in place.
Step 3: Shine the bright flashlight into the cut-out of the Cheshire. If you look through the Cheshire hole, you should see something like this:
Make sure your flashlight is bright enough to illuminate everything. If it's not bright enough, you won't see the "rings" necessary to align the primary. Here's what it will look like if your light is not bright enough:
There's not enough info here to adjust the primary, so be absolutely sure to use a fairly bright light source.
Here's the black ring you need to make concentric to have a properly-aligned primary mirror. The image pictured is OUT of collimation:
Step 4: Make adjustments to the primary mirror screws to make the black ring concentric around the silver ring. The small screws on the primary mirror cell are "locking" screws, while the larger ones actually adjust the mirror cell (push/pull). Unlock each locking screw by about 1/2 turn. What I've found is that adjusting the large screw closest to the "fat" part of the black ring works best. Loosening it usually makes the ring thinner on that side, while tightening it makes it fatter. Essentially, you'll need to experiment with adjusting the three larger screws until the black ring is as perfectly concentric as possible. You may also have to loosen the locking screws further if you mirror is way out of alignment. Here's a tight crop of what it should look like once it's properly aligned:
Once you've achieved concentricity, don't tighten the locking screws just yet - you need to adjust the secondary first and then come back to check the primary again.
Step 5: Point the flashlight away from the Cheshire cut-out. You should now be able to see the secondary dot. It needs to be centered inside the ring as pictured below (well-collimated):
If it is not centered, adjust the three allen screws on the front of the secondary mirror. Make very small adjustments, one screw at a time. If one becomes loose, you may have to slightly tighten the other two. Trial and error will get you centered. Be as precise as possible. Just ensure you don't over-tighten or over-loosen any of the screws. They should feel like they are holding in place, but not wrenching down. LEAVE THE CENTER PHILLIPS SCREW ALONE!!!!
Step 6: If you had to adjust the secondary mirror, shine the bright flashlight into the Cheshire cut-out again and look at the primary alignment. It may have shifted if you made a substantial adjustment to the secondary mirror. If it has indeed shifted, adjust it so that is concentric again. Then recheck your secondary mirror, and adjust it again if it changed. Iterate back and forth between primary and secondary adjustments until they converge as simultaneously aligned. For me, I only had to adjust one extra iteration before they converged.
Once you are satisfied with both mirrors, you need to tighten the primary locking screws. BE CAREFUL. Over-tightening the locking screws can, and will, cause the primary mirror to shift. You must watch closely in the Cheshire as you tighten to make sure you don't alter the primary alignment. Tighten one of the locking screws very lightly, then move clockwise doing the same for the other two locking screws. Then, tighten a little more and continue around. They don't need to be extremely tight - just snug enough to avoid any shifting. Once you've tightened them to your liking, triple check everything to ensure you haven't caused any misalignment.
Your mirrors are now aligned. You can re-install the baffle extension and add your focuser extensions to test your collimation under the stars. Make sure you orient your focuser in the same way it was oriented when collimating. Rotating it may cause a shift in alignment. Don't forget, when testing for the first time - let your telescope acclimate to the outdoor temperature for a couple of hours and choose a star field near zenith.
Aligning the Focuser to the Tube/Secondary Mirror
As previously mentioned, there is no way to align the focuser separately from the primary mirror unless you buy the collimation ring or have the custom de-coupler plate installed. Without one of these add-ons, every time you adjust the primary mirror, the focuser moves with it.
The collimation ring allows to make adjustments to the focuser tilt separately, but will still shift anytime the primary is adjusted - which will usually require re-adjusting the focuser. The more-advanced de-coupler plate modification allows to align the focuser once, then collimate your mirrors without having to go back and adjust the focuser again.
The best way I've determined to tell if your focuser is mis-aligned to the tube/secondary, is by the thin outline of light around the entire circle in the Cheshire view. I check after aligning the mirrors. Again, the baffle extension needs to be removed to see this, and the focuser needs to be directly attached to the back of the tube (no extensions):
As seen here, the focuser is not aligned to the tube/secondary. This tells me that on my RC8, when the mirrors were properly aligned, the focuser was not. It's an inherent design flaw of the GSO RC8. Some owners might get lucky and have one that has an aligned focuser when the mirrors are also aligned. I think for a majority of owners, this is unfortunately not the case.
If you don't own a collimation ring or the de-coupler plate and are unable to properly align the focuser, you might see a couple of issues. An unbalanced field of stars is likely, meaning the best star shapes will be off-center, and off-axis aberrations will be worse in one or more corners than the rest. Another issue I noticed on my telescope was internal reflections caused by misalignment of the baffle/focuser/tube (only noticeable with LRGB). My collimation improved quite a bit once I was able to get the focuser aligned, and the reflections disappeared.
Since the collimation ring adds a bit of backfocus to the focuser, you won't be able to see the thin outline of light around the Cheshire view when it's installed. This is why you see a lot of people using a laser to align the focuser. I'm not a fan of lasers for this, because they are often mis-collimated themselves or don't properly register into the focuser - causing inaccurate pointing of the laser and therefore misalignment of the focuser when adjusting.
I have a de-coupler plate installed on my scope, so here's my method:
Step 1: With the scope parallel to the ground and pointing at a white wall, remove the ENTIRE baffle tube from the telescope and set it down inside the tube (it simply unscrews). Install all of the extension tubes you normally use during imaging to the rear of the telescope along with the focuser. Rack the focuser all the way OUT.
Step 2: Inset the Cheshire eyepiece and look through it. You should be able to see the outline of light around the Cheshire, except now it will be a much wider ring due to removing the baffle tube. Racking the focuser all the way out makes the ring less wide in your view - which allows you to better see if it is non-concentric. Unfortunately, I didn't take a photo of this, but it should be obvious what to look for.
Step 3: Make adjustments to the de-coupler plate/collimation ring push/pull bolts to make the outer ring perfectly concentric. Once finished, ensure everything is snug - but not at the expense of shifting things.
If you have the de-coupler plate, congrats - your focuser is aligned. You may proceed to re-checking the alignment of the mirrors, and adjust if necessary.
If you have the collimation ring, you need to check alignment of the mirrors again. If they are out, adjust them so they are aligned. Go back and check the focuser alignment and adjust if needed. Iterate adjusting the focuser and mirrors until converged.
Results of Method
Here is an in-focus starfield after using this method to bench collimate my RC8. The camera is a 4/3 sensor (ZWO ASI294MM Pro), 60s exposure, without a flattener installed:
And a mosaic to compare corners/edges:
Here's a slightly out-of-focus field to analyze symmetry and overall collimation:
On an RC8, it's possible to achieve a mostly flat field (minimal off-axis aberration) on a camera with a 4/3 sensor. With a larger APS-C camera (ZWO ASI2600MM Pro), I use a Hotech SCA flattener to improve the off-axis aberration visible in the corners and around the edges. Here's a quick 21-minute RGB integration of the M37 star cluster (7 x 60s exposures each filter). This is uncropped to view stars to the edges (pardon the stacking artifacts). The Hotech flattener does a great job flattening the field:
Here's the first - and relatively short - project I completed after receiving the custom de-coupler plate, using this method to collimate. Bortle 7 backyard. Gear: iOptron RC8, ZWO ASI2600MM Pro, Hotech SCA Flattener, ZWO AM5, Optolong LRGB filters. This is about 3.5 hours of L (120s subs) and 1 hour each of RGB (120s subs) on the Leo Triplet (M65, M66 and NGC 3628):
And here's a very short integration/test of IC434 in LRGB. Bortle 7, same gear as above. 1.5 hours of L (120s subs), 30 mins each of RGB (120s subs). Taming Alnitak is no easy task at this focal length, but I gave it my best shot. This project could use many more hours of integration to improve contrast and bring out more detail, but this result looks promising to my eyes:
Summary
In my experience, the easiest and most effective way to collimate a GSO RC8 is by using a Cheshire eyepiece to view alignment. I've tried the DSI method of collimation under the stars, and it has also worked. But, it takes time, is complex, and requires decent seeing conditions. I've also tried following the basic instructions in the telescope manual - but realized the flaw of the primary mirror cell being attached directly the focuser renders the manual's method as incorrect (unless you're one of the lucky few that has a perfectly aligned focuser!).
GSO RC8's suffer from sag/flexure caused by the weight of the imaging train pulling directly on the primary mirror cell. This can cause collimation changes when pointing the telescope to different parts and altitudes of the sky. For this reason, it's important to check your collimation on a crowded starfield near zenith. It's also important to allow the telescope to fully acclimate to the outdoor temperature for about two hours before checking collimation. Otherwise, tube currents will wreak havoc on your images, making it impossible to analyze collimation.
It's possible to get "usable" collimation without having a focuser collimation ring or the custom de-coupler plate. For the best result, though, I recommend one of those upgrades. I also recommend avoiding using a laser to collimate the focuser unless you are sure the laser itself is well-collimated and it can properly register into the focuser without being tilted.
As I have never tested my method on an RC8 other that mine, I'll be very interested to hear if others try this method with success. I think it will work quite well for most who have a GSO RC telescope that is not substantially flawed.
Chad