263 replies to this topic

[nebulachadnezzer]

If you still have the stock GSO focuser you can prove to your self that there was nothing wrong with it.

I'm sorry, but we disagree on the quality.

The goofy compression-ring attachment method is just the most often cited problem with them. Yes, you can stand on your head and overcome that, but it shouldn't be necessary when the focuser should just screw onto the flange directly.

Beyond that, the Crayford bearing kind of sucks, and the draw tube is kind of wonky in a way that can cause collimation problems of its own under a heavy imaging train load.

The Moonlites are over-engineered in comparison and heavy as rocks but they do the job well.

[nebulachadnezzer]

3 inches of extension but it matters not a bit.  If I remove the entire focuser stack and leave the threads at the rear of the primary assembly naked, the appearance does not change a bit.  Remember, the first large bright reflection and the 2nd bright crescent are serial reflections of the secondary viewed on the surface of the primary.  Thus the size of the secondary is the only parameter that matters.

You mentioned the open end of the focuser, and it's clear in your photo above you're back-lighting it, so that's why I asked. I may give this a try with just the M90x1 extensions on it. I have all three on there at the moment just for safe keeping.

It does seem to help seeing the HoM effect quite a bit. Up to now I've been trying to see it with only ambient light entering the front of the OTA. My focusers are generally capped when they don't have a camera attached.

[MikeECha]

I'm sorry, but we disagree on the quality.

The goofy compression-ring attachment method is just the most often cited problem with them. Yes, you can stand on your head and overcome that, but it shouldn't be necessary when the focuser should just screw onto the flange directly.

Beyond that, the Crayford bearing kind of sucks, and the draw tube is kind of wonky in a way that can cause collimation problems of its own under a heavy imaging train load.

The Moonlites are over-engineered in comparison and heavy as rocks but they do the job well.

It was a joke. Does your screen show the emoji at the end? I find that emoji to be one of the funnier things I have ever seen. That emoji should be made the mascot for out hobby. It totally embodies us.

[nebulachadnezzer]

It was a joke. Does your screen show the emoji at the end? I find that emoji to be one of the funnier things I have ever seen. That emoji should be made the mascot for out hobby. It totally embodies us.

LOL. I saw the "smashing head into a brick wall" emoji (also a favorite of mine, used quite often at work in fact) but I wasn't sure if you were conveying sarcasm or frustration with people maligning the GSO focuser.  

Sorry for the misread.

[MikeECha]

LOL. I saw the "smashing head into a brick wall" emoji (also a favorite of mine, used quite often at work in fact) but I wasn't sure if you were conveying sarcasm or frustration with people maligning the GSO focuser.  

Sorry for the misread.

I apologize too. I was secretly traying to amend the some what harsh answer I gave you on the other thread. I was not dismissing your collaboration. I was trying to avid going off on a tangent as commonly happens on these forums on thread that was not mine.

 

All good here.

 

Have a good night.

[dg401]

I tell you, the more I work with this scope the better opinion I have about the design of these modest but good scopes.

These scopes can be very frustrating because it seems to take a bit of bloody-minded dedication to fight past the point where you're certain your scope is a steaming POS.  If you do work past this point, and start to figure the thing out, it is a nice scope for a crazy low price.

 

Unfortunately, there is no shortage of self-appointed experts whose advise only contributes to the steaming POS cycle of frustration.  To be fair, some of them know what they're talking about, but they lack the ability to communicate clearly and be an effective teacher.  Then there is technically sound information that requires an appetite for painful self-flagellation (I'm looking at you, DSI).   I read technical documents written in German all the time... and I don't read German... and yet I can't follow this beast of a procedure, technically correct as it may be.  If anybody finds it simple or straightforward, well, you're clearly smarter than I am.  Then there is the bunch who have no idea what they're talking about but that fact doesn't dissuade them from jabbering away, only adding noise to the conversation.  Then there's the laser and holographic disk and gadget peddlers... May they lead interesting lives.

 

It's funny... sometimes you return to basic principles and find what you're looking for.

 

I fought the collimation, and the collimation painted my backside red for something like 7 weeks.  Then the crazy old retired guy from Seattle came along and like all the others, I said, "Yea, I've got some time to waste on this thing...".  My first lousy result was better than I'd gotten in all that time trying everything and parting with more cash all along the way.  So I kept going...and just like the crazy old guy claimed, I collimated my RC6 with a Cheshire and my right eye to the point where I wouldn't even bother with a star adjustment to try to get any better.
 

I also have a better and better opinion of these modest but good scopes, but I've also got the scars inflicted by the journey.

[dg401]

You mentioned the open end of the focuser, and it's clear in your photo above you're back-lighting it, so that's why I asked. I may give this a try with just the M90x1 extensions on it. I have all three on there at the moment just for safe keeping.

It does seem to help seeing the HoM effect quite a bit. Up to now I've been trying to see it with only ambient light entering the front of the OTA. My focusers are generally capped when they don't have a camera attached.

Back-lighting for the win. Absolutely.  I wouldn't collimate without it.

 

[Edit] When you think about it, the backside hardware isn't seen... No, it can't be seen by the secondary.  The primary baffle tube sees to that. The only thing that the back end presents directly to the secondary mirror is a primary baffle tube sized cylinder of light.

Edited by dg401, 07 August 2021 - 01:00 AM.

[MikeECha]

These scopes can be very frustrating because it seems to take a bit of bloody-minded dedication to fight past the point where you're certain your scope is a steaming POS.  If you do work past this point, and start to figure the thing out, it is a nice scope for a crazy low price.

 

Unfortunately, there is no shortage of self-appointed experts whose advise only contributes to the steaming POS cycle of frustration.  To be fair, some of them know what they're talking about, but they lack the ability to communicate clearly and be an effective teacher.  Then there is technically sound information that requires an appetite for painful self-flagellation (I'm looking at you, DSI).   I read technical documents written in German all the time... and I don't read German... and yet I can't follow this beast of a procedure, technically correct as it may be.  If anybody finds it simple or straightforward, well, you're clearly smarter than I am.  Then there is the bunch who have no idea what they're talking about but that fact doesn't dissuade them from jabbering away, only adding noise to the conversation.  Then there's the laser and holographic disk and gadget peddlers... May they lead interesting lives.

 

It's funny... sometimes you return to basic principles and find what you're looking for.

 

I fought the collimation, and the collimation painted my backside red for something like 7 weeks.  Then the crazy old retired guy from Seattle came along and like all the others, I said, "Yea, I've got some time to waste on this thing...".  My first lousy result was better than I'd gotten in all that time trying everything and parting with more cash all along the way.  So I kept going...and just like the crazy old guy claimed, I collimated my RC6 with a Cheshire and my right eye to the point where I wouldn't even bother with a star adjustment to try to get any better.
 

I also have a better and better opinion of these modest but good scopes, but I've also got the scars inflicted by the journey.

You read my thoughts don't you?  And this "...Then there is technically sound information that requires an appetite for painful self-flagellation.." is just hilarious.

 

There is a lot of repetition of anecdotes in completely wrong contexts. Things that may have been true at one point in time, due to technology changes are no longer true or effective or necessary. Reasonable accuracy in hole location and machined surfaces for example are things that for the most part can be assumed with high certainty to be correct these days no matter where they come from. The mechanical and optical axes of the secondary not being the same argument makes my head explode.

 

On other cases the reasons for doing or not doing things have been lost over time. Case in point: "..do not touch the center screw or loose collimation.." when in the context of adjusting the focal length. Yes, but the real reason is because that is not how the fl is adjusted. That fine thread with the retaining ring on the secondary holder is screaming at you "adjust me here" 

 

Anyway, I love these technical discussions.

[dg401]

Well, gents, I made a mistake... a very fortunate and precise mistake...

 

And I couldn't be more pleased.

 

I've been doing the Hall of Mirrors with the focuser tube open and pointing straight downward.  Something was wrong this time... the crescent slivers were missing on a couple vanes...and I could barely see them on the other two vanes... at least compared to what I've become accustomed to.

 

Duh... you left the 2" to 1.25" adapter in the focus tube you silly prat.  Waitaminute... you silly prat... you silly, lucky, brilliant, fine young man, you!

 

The adapter being left in the focus tube was shadowing a bit of the back-lighting and causing the bright crescent shaped 2nd reflection of the secondary to be extremely abbreviated.. and able to be balanced over the 4 vanes such that each vane showed the barest, most minute sliver.   Just as too small a Cheshire bore will result in too small a center dot and make judging the centering versus the secondary donut more difficult, having the crescent shaped slivers appearing with the focus tube open resulted in slivers that were larger and harder to judge precisely.  By making the slivers smaller, anything but a PERFECT Hall of Mirrors alignment would make one or even two of the slivers disappear.

 

In short... serendipity.   My Hall of Mirrors was already pretty precise.  Now I'd say it's an order of magnitude more precise.

 

Of course, we must see if it's an order of magnitude more precise on the correct orientation, or an order of magnitude more precise but precisely off center a bit?  Cloudy tonight.  Oh well.

[dg401]

"..do not touch the center screw or loose collimation.."

 

If I was a Youtube video making type (and I am not), I think I might start a video with my RC6 stripped to a pile of parts, the only assembled ones being the secondary cell, the secondary mirror, and it's threaded retaining ring.  I'd also leave the vanes attached to the central hub... but everything else... a pile of parts.  Then I'd start assembling, It would be maybe ten minutes to have a superficially assembled scope.  Then I'd adjust the three turns on the primary screws.  4 turns on the fine threaded secondary cell ring.  8.5 turns on the secondary central screw.  Tighten down the 3 secondary screws... I dunno, maybe 5 more minutes.  Now we're up to 15 minutes.  Then TinySpecks's procedure using everything I've learned through repetition and study.  Probably a 30 minute video.  Which is still shorter than some of these videos that will leave you wishing at the end of your days that you could have the time back that you wasted watching those videos...

 

Then at the end, flashed across the screen:  "No lasers were used in the making of this video"

Edited by dg401, 07 August 2021 - 04:16 PM.

[dg401]

Has anyone taken apart the GSO provided focuser for the RC6/8?

 

I bought my Moonlite, and tossed it aside.  I finally got around to taking it apart today, and what's this?

 

 

That's an angled compression flange.  So much for the conventional wisdom that you need to buy a tilt-ring or a Moonlite or whatever to get focuser tilt in case your focuser isn't coaxial with your primary.

 

So I shove an M3 washer that's about 0.5mm thick into the gap, and tighten things down...

 

 

And we have tilt!

 

 

If one was going to do this, I suspect a small curved shim maybe an inch long would be the way to go.  It might need to be somewhat thinner than 0.5mm, that's actually a lot of tilt with the 0.5mm washer.

 

Seems rather an awkward way to get tilt, and would require experimenting with shim thickness and positioning... but, and here's the but... it can be tilted.

 

 

[MikeECha]

 

Has anyone taken apart the GSO provided focuser for the RC6/8?

 

I bought my Moonlite, and tossed it aside.  I finally got around to taking it apart today, and what's this?

 

 

 

That's an angled compression flange.  So much for the conventional wisdom that you need to buy a tilt-ring or a Moonlite or whatever to get focuser tilt in case your focuser isn't coaxial with your primary.

 

So I shove an M3 washer that's about 0.5mm thick into the gap, and tighten things down...

 

 

 

And we have tilt!

 

 

 

If one was going to do this, I suspect a small curved shim maybe an inch long would be the way to go.  It might need to be somewhat thinner than 0.5mm, that's actually a lot of tilt with the 0.5mm washer.

 

Seems rather an awkward way to get tilt, and would require experimenting with shim thickness and positioning... but, and here's the but... it can be tilted.

 

 

That is the old style that does not have the conic flange mine has. 

 

And now I see why the old focuser might actually have the problem it is blamed for. If you tighten one set screw before the rest unevenly, you can shift the focuser to one side. Thus the "tilt" which is more like a displacement. You can check that by loosening all the set screws and see if there is play side to side. That design is used in many nose piece holders and accessories that may be good for visual but not so much for imaging in my opinion.

 

My Celestron OAG has those flanges on both ends but the angle is a lot sharper leaving no chance for the thumb screw to push the assembly in any other direction but forward. 

 

But now that you know it, you could manage it. And that is the key: if one understands where the misalignments come from, they can be fixed.  

 

The flange on my focuser is machined as part of the focuser body and it is not removeable. It is also the linear bearings type. When I got it with my scope, one of the bearings was broken and the balls were rattling inside the tube. I collected them all and with a lot of patience I put them back, re-greased it, reassembled the focuser, it and to this day no problems.

[dg401]

With this collimation, I've been consistently seeing this on-axis:

 

 

Not quite right on-axis.  It's out of focus on the 9 o'clock/3 o'clock axis.  And consistently a bit more corner aberration maybe than an APSC sensor has a right to have as well.

 

So I tested with putting an offset on the Cheshire center dot.  Relative to the Vixen Rail at the bottom of the OTA, and looking into the Cheshire, I decided to butt the Cheshire dot up against the secondary donut at 6 o'clock, 9 o'clock, 12 o'clock, and 3 o'clock.

 

First I did 6 o'clock and checked the tri-bahtinov result, same not quite right tri-bahtinov image.  Then 12 o'clock.  Still no luck.  Then 9 o'clock.  Same problem.  Then I tried offsetting the Cheshire center dot up against the secondary donut at 3 o'clock.

 

and I saw this:

 

 

Nice!  It's a dimmer star than the earlier image, so not as bright.  On-axis tri-bahtinov mask is right down the middle, or nearly enough so.  It's quite a bit better than what I've been seeing.

 

So I tweaked the Hall of Mirrors since moving the secondary created a small but perceptible change in the HOM.

 

And just as it was getting into morning twilight, I got this image in Cassiopeia. 

 

 

This, finally, is what I want (or very nearly so) out of my RC6 with an APSC sensor.  Obviously, a full frame sensor would show field curvature, and to use the full frame, a flattener would be needed, but for a crop sensor, this is as good as I could hope for.  No, it's not perfect and it's not going to be.  But this is so much better than anything I've achieved previously.  If you're seeing anything but round corner stars on APSC, it's not field curvature, it's mis-collimation.  I'll have to duplicate it when it's not so close to sunrise and the image is less noisy (I pushed the black point to darken the sky).  If any corner is worse than the others, it would be bottom right and it's not really all that bad.  Still, you can tell the other three corners are round to "off-round", but the lower right corner stars are oval.  The Cheshire dot offset was an arbitrary distance toward 3 o'clock on the secondary donut, and a perfectionist could continue to tweak it from there and perhaps beat the last bit of corner aberration out. 

 

So, Hall of Mirrors is deadly accurate when pushed to the limit.  The Cheshire centering is not (and now I'm wondering if that sloppy "find the optical center of the secondary" test I did might have been a better test than I gave it credit for?) However, if you're seeing more corner aberration than you should, or not quite perfect on-axis, just offset the Cheshire dot toward the secondary donut until you find the offset direction that gives a superior result to the others.

 

I've been staring at this so long, I may have myself convinced that I've gotten better results than I really have.  Anybody care to put a fresh eye on this and let me know if I've upped my game, or am I laboring under a cloud of wishful thinking?

 

And if you've never considered a tri-bahtinov mask, I can't praise the item enough.

 

Edited by dg401, 08 August 2021 - 04:29 PM.

[dx_ron]

Is your RC6 tri-bahtinov mask commercially available, or something you 3D printed?

[TinySpeck]

With this collimation, I've been consistently seeing this on-axis:

...

Beautiful results, and what excellent experimental technique!  I think what this shows is that your secondary optical axis is slightly off from the donut, and you found the actual axis by experiment.

 

The trick now is to find the secondary optical axis, not necessarily centered in the donut, without a star (real or artificial), and modify the procedure accordingly.  If this is possible...  Was there anything in the Cheshire image you noticed "lining up" better when the dot was at its 3 o'clock orientation?

 

Or there may be a way to use the Hall of Mirrors alone to do an indoor collimation.  I don't know how to discriminate between primary and secondary adjustments with only the one test though.

[quilty]

Or there may be a way to use the Hall of Mirrors alone to do an indoor collimation.  I don't know how to discriminate between primary and secondary adjustments with only the one test though.

 

 

 

Or there may be a way to use the Hall of Mirrors alone to do an indoor collimation.  I don't know how to discriminate between primary and secondary adjustments with only the one test though.

Good point, me neither but I hope so

[dg401]

Is your RC6 tri-bahtinov mask commercially available, or something you 3D printed?

Optcorp has them from Artesky.  12 different sizes from 60 - 250mm.

[dg401]

Beautiful results, and what excellent experimental technique!  I think what this shows is that your secondary optical axis is slightly off from the donut, and you found the actual axis by experiment.

 

The trick now is to find the secondary optical axis, not necessarily centered in the donut, without a star (real or artificial), and modify the procedure accordingly.  If this is possible...  Was there anything in the Cheshire image you noticed "lining up" better when the dot was at its 3 o'clock orientation?

 

Or there may be a way to use the Hall of Mirrors alone to do an indoor collimation.  I don't know how to discriminate between primary and secondary adjustments with only the one test thoug

There was nothing in the Cheshire view to tip me off.

 

It's possible we're bumping up against the limit of what can be accomplished with a Cheshire and an eye. 

 

The Hall of Mirrors on an RC6 with a 2" to 1.25" adapter installed in the focuser tube is extremely precise using the crescent sliver of the 2nd bright reflection of the open end of the focuser tube.  Without the adapter installed in the focuser tube, the crescent slivers are much larger and not as precise.  We still need an RC8 user to check whether this also works on an RC8.  This is just my take on Hall of Mirrors.  Other users might find something else in the HOM view just as precise or even more precise, but this is what I've settled on.

 

But the secondary mirror... there's our fly in the ointment.  As long as the Cheshire and adapter ring can rotate freely without the dot wandering, then they're good.  At the very least, we can get the center field sweet-spot near enough to perfect to satisfy many (but not all) situations.

[TinySpeck]

It's possible we're bumping up against the limit of what can be accomplished with a Cheshire and an eye. 

You could see that the dot wasn't quite central in the donut when you pushed it to 3 o'clock for your final test though, so I don't think we're hitting the limit of eye/Cheshire.

 

The Hall of Mirrors on an RC6 with a 2" to 1.25" adapter installed in the focuser tube is extremely precise using the crescent sliver of the 2nd bright reflection of the open end of the focuser tube.  Without the adapter installed in the focuser tube, the crescent slivers are much larger and not as precise.  We still need an RC8 user to check whether this also works on an RC8.  This is just my take on Hall of Mirrors.  Other users might find something else in the HOM view just as precise or even more precise, but this is what I've settled on.

HOM is indeed very precise, but either primary or secondary adjustments will affect it so you can't use it alone.  I think you need to keep HOM for the primary and Cheshire for the secondary.

 

It might be that for this procedure to be fully general, you go through it as originally described and then tweak the secondary using a single central star and perhaps a tri-Bahtinov.  That would accommodate situations where the secondary donut isn't exactly on the optical axis.  A one-star tweak like that isn't too bad, and could even be done shortly after sunset on a bright star.  No lost imaging time.

[dg401]

You could see that the dot wasn't quite central in the donut when you pushed it to 3 o'clock for your final test though, so I don't think we're hitting the limit of eye/Cheshire.

 

HOM is indeed very precise, but either primary or secondary adjustments will affect it so you can't use it alone.  I think you need to keep HOM for the primary and Cheshire for the secondary.

 

It might be that for this procedure to be fully general, you go through it as originally described and then tweak the secondary using a single central star and perhaps a tri-Bahtinov.  That would accommodate situations where the secondary donut isn't exactly on the optical axis.  A one-star tweak like that isn't too bad, and could even be done shortly after sunset on a bright star.  No lost imaging time.

I started the night with the dot centered.  It's pretty easy to see any deviation from center due to an adjustment, so I'm not sure I follow your point.

 

Actually, and looking at it closely in the light of day it's more like 1:30.  Again, when looking through the Cheshire, the Vixen rail is at 6:00.  The Vanes are at 1:30, 4:30, 7:30, and 10:30, and the dot lines up better with the 1:30 vane.  I did the testing last night in the dark with an SQM reading of 21.3 at the zenith, so it was pretty dim (and I was standing on a folding wooden chair just asking to fall off).

 

I'm convinced that neither the primary nor the secondary can compensate for the other.  As MikeECha busted my nuggets for back on page 6, there's one coaxial positioning for primary and secondary with the design of these scopes.  Any other positioning will give at best parallel mirrors but not coaxial mirrors.  The HOM can't tweak out a secondary alignment issue, and the secondary can't tweak out an HOM issue.

 

Your last paragraph sums it.  And since it's your baby, I'm pleased that you've begun to consider the tri-bahtinov mask as perhaps an item that really should have a seat at the table in the final procedure.  Just for reference, OPT has then all the way up to 250mm.  I think my 150mm version was $32 and the 250mm version goes for $48.  Try to find a laser (that won't do you any good) for that price. 

 

I'm not 100% certain that bumping the central dot from the center to the donut edge really makes a significant change to the HOM.  I imagined last night that it did, but I'd have to confirm that before I would claim it to be fact.  I DO know that you can make all your test adjustments to the central dot position without worrying about the HOM.  So, no problem finding the best position for the central dot and then making a final fine HOM tweak if it's needed.

 

It looks like clouds on and off tonight, so I'll set up in the back yard.  It's not the 21.3 zenith from last night, but 19.1 will do... and I don't have to drive 1.25 hours for it.  Anyway, it was a waste of effort/time getting to the darker sky since all I really did was test.

Edited by dg401, 08 August 2021 - 07:55 PM.

[TinySpeck]

I started the night with the dot centered.  It's pretty easy to see any deviation from center due to an adjustment, so I'm not sure I follow your point.

...

Maybe I misunderstood you!  We're both saying the same thing here.  You can see the smallest adjustment by eye, so it doesn't seem that we're limited by that, but if your donut is not exactly on the secondary optical axis you may have to do the final tweak with a star.

 

I'm convinced that neither the primary nor the secondary can compensate for the other.  As MikeECha busted my nuggets for back on page 6, there's one coaxial positioning for primary and secondary with the design of these scopes.  Any other positioning will give at best parallel mirrors but not coaxial mirrors.

...

I'm not sure I agree with him though.  The primary and secondary are well fixed in the OTA, that's true, but that's not necessary to make them coaxial.  Using the tilt adjustments on both mirrors you can point them at one another (make them perfectly coaxial) even if they're not on the mechanical axis of the OTA.

 

The HOM can't tweak out a secondary alignment issue, and the secondary can't tweak out an HOM issue.

...

I guess that's true!  That ties the primary and HOM together, as are the secondary and Cheshire.  It's good to have a justification for those pairings.

 

I'm not 100% certain that bumping the central dot from the center to the donut edge really makes a significant change to the HOM.  I imagined last night that it did, but I'd have to confirm that before I would claim it to be fact.  I DO know that you can make all your test adjustments to the central dot position without worrying about the HOM.  So, no problem finding the best position for the central dot and then making a final fine HOM tweak if it's needed.

...

I found that adjusting the HOM did not seem to affect the Cheshire dot position, which makes sense.  The dot-in-the-donut relies only on secondary tilt.  For the VERY final adjustment, though, after making the indoor collimation as exact as possible, it should be the secondary on a star.  That will have the effect of moving the dot away from the donut center and onto the secondary optical axis.

 

Do you have a recipe for how to adjust the secondary to have a certain effect on the tri-Bahtinov?  This whole thing is pretty canned except for that.

[dg401]

Do you have a recipe for how to adjust the secondary to have a certain effect on the tri-Bahtinov?  This whole thing is pretty canned except for that.

I don't have a recipe, not yet.

 

An artificial star would be the way to go for running this to ground.  The TBM spike pairs are spaced 120° as are the 3 secondary adjustment screws.  I would line up a TBM spike with each of the 3 screws (This isn't my original idea, but after you've read 20 different sources, it starts to blur together and you can't remember where to give the credit for the idea).  Then it would be an exercise in figuring out how to make the un-centered spike pair move toward center.

[Hondo]

This is an excellent thread, thanks to all that have contributed.

 

 

Hall of Mirrors - Precise 2nd Bright Reflection of Open End of Focus Tube

 

This works perfectly for an RC6.  It would be good if an RC8 user could verify that a similar bright and very thin 2nd crescent moon shaped reflection of the open end of the focus tube is visible on that scope as well.

I have been using the HoM technique for a few years now on my Mallincam VRC-8, most likely a GSO variant.  I just checked my scope and I can confirm the 2nd crescent moon shaped reflections are visible.  They are however, extremely thin and at first I could not see it.  I also tried with the 1.25" adapter in as suggested but it was no longer visible.  I made a slight adjustment to centre the crescents and I anxiously await for clear skies to see if I have any improvement in collimation.

[TinySpeck]

An artificial star would be the way to go for running this to ground.  The TBM spike pairs are spaced 120° as are the 3 secondary adjustment screws.  I would line up a TBM spike with each of the 3 screws (This isn't my original idea, but after you've read 20 different sources, it starts to blur together and you can't remember where to give the credit for the idea).  Then it would be an exercise in figuring out how to make the un-centered spike pair move toward center.

For me, targeting a high bright star just after sunset is easier than an artificial star.  I'm jealous of those of you who can use an artificial star whenever you darn well please...

 

That's a great start, to line the tri-Bahtinov mask up with the secondary adjustment screws.  Let me know what tweak has what effect and I'll incorporate it into the upcoming (easily-readable, summarized) blog version of this method.  I'm not gonna take full credit for this group effort either!

[dg401]

This is an excellent thread, thanks to all that have contributed.

 

I have been using the HoM technique for a few years now on my Mallincam VRC-8, most likely a GSO variant.  I just checked my scope and I can confirm the 2nd crescent moon shaped reflections are visible.  They are however, extremely thin and at first I could not see it.  I also tried with the 1.25" adapter in as suggested but it was no longer visible.  I made a slight adjustment to centre the crescents and I anxiously await for clear skies to see if I have any improvement in collimation.

It is the extreme thinness of the 2nd crescents that makes their use so precise.  Seems yours are already razor thin on an 8" without adding the 2" to 1.25" adapter.  Ideally, you should be able to move your head in a circle around the OTA and see the thin crescent move around the OTA symmetrically as it follows your head motion.