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RC collimation with no laser or stars

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#126 dg401

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Posted 05 August 2021 - 05:43 AM

The clouds permitted me a couple star-fields.

 

The collimation for this one was done after my somewhat silly "How many licks does it take to get to the optical center of a tootsie roll" experiment.  I was attentive to the collimation, but I didn't get into obsessive/compulsive mode.  I think these are 120 second exposures at ISO1600.  A good bit noisy as I've not yet rubbished my DSLR for a dedicated/cooled camera (I can only collect so many toys in a timeframe of X before my wife gets all "wifey" on me).

 

Really nice sharp & rounds stars all over (if you don't look too closely at the upper right corner; and now that I've mentioned it, I know you will...)

 

Somewhere between Sadr and Fawaris:

 

Cygnus Star-Field
 
Same image at image center:
 
Image Center

 

Vulpecula near M27:
 
Vulpecula Star-Field
 

Yea, that's a defect of sorts in my sensor far left of center.  Don't know what or why it is.  It usually gets cropped out for DSOs so I can live with it.  I also missed a mote of dust on the sensor in the upper left corner.  I have had worse.


Edited by dg401, 05 August 2021 - 07:19 AM.

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#127 MikeECha

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Posted 05 August 2021 - 06:05 AM

The center spot on my RC6 is at mechanical dead center (referenced against the mirror locking ring).

Yes, ... and so is the optical axis. If it was not, one could not adjust the focal length with that ring.

 

And I meant to write "...inaccuracy in marking..." 



#128 dg401

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Posted 05 August 2021 - 12:33 PM

Yes, ... and so is the optical axis. If it was not, one could not adjust the focal length with that ring.

 

And I meant to write "...inaccuracy in marking..." 

Perhaps too much is made of the whole optical versus mechanical center issue.

 

The following recalls a mention I made in a post here a few days ago about troubleshooting at customer sites.  I work with ion optics... applying voltage to sets of ion lens pairs along an axis to affect the x/y displacement of positive ion fragments as they are accelerated down an axis toward the guts of the instrumentation... If it's not working properly when set "right down the center".  Start offsetting voltages in different directions and play with the voltage symmetry between the ion optic lens pairs... You might not have the sweet spot "right down the middle", but you can sure find out where the sweet spot is if it's offset elsewhere.  It shouldn't take much imagination at all to grasp the point I'm driving at here.

 

I will test (you knew I'd say that) the photographic result of making a subtle change of the Cheshire dot position to the four corners (x) and to the four sides (+).  If there is anything to it, maybe 3 of the 8 offset positions will makes things better, and 5 of the 8 offset positions will make things worse.  If something definite does show up (and can be repeatably and consistently reproduced), it also gives me the information I need to make an indexing mark on the edge of the secondary cell to let me know, "bump the Cheshire dot slightly this way".
 


Edited by dg401, 05 August 2021 - 12:37 PM.

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#129 TinySpeck

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Posted 05 August 2021 - 01:12 PM

... "I will test (you knew I'd say that) the photographic result of making a subtle change of the Cheshire dot position to the four corners (x) and to the four sides (+).  If there is anything to it, maybe 3 of the 8 offset positions will makes things better, and 5 of the 8 offset positions will make things worse.  If something definite does show up (and can be repeatably and consistently reproduced), it also gives me the information I need to make an indexing mark on the edge of the secondary cell to let me know, "bump the Cheshire dot slightly this way".

 

Your careful testing and empirical data are very welcome.  We can theorize all day, but eventually it comes down to evidence.


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#130 dg401

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Posted 05 August 2021 - 01:24 PM

Your careful testing and empirical data are very welcome.  We can theorize all day, but eventually it comes down to evidence.

Comes from 25 years in an analytical instrument laboratory before leaving to go on the road fixing the da*n things when the wheels fall off.



#131 dg401

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Posted 05 August 2021 - 02:52 PM

For reference, here is the Cheshire view of the secondary.  The center dot will vary with Cheshire aperture.  I rather like this one (It's from Celestron, or at least they slapped their name on it...)  relative to the secondary donut as the ring of light between the Cheshire shadow and the donut is tight.  Having too big a central dot makes it harder to judge centering.  This one is just right.  Also, it's very easy to just kiss the edge of the center dot right up the edge of the donut in the corners (x) and at the sides (+) to test for possible directional bias away from from the apparent center.  Getting a decent shot with an Iphone 8 through a Cheshire isn't easy.  This took awhile!

 

Cheshire View to Secondary

Edited by dg401, 05 August 2021 - 03:40 PM.

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#132 dg401

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Posted 05 August 2021 - 03:23 PM

Visiting Stellarium for a moment...

 

This is one of the biggest apparent size DSOs that we can arguably fit in a 1370mm focal length field of view with an APSC sensor.  And it can still fit without having to use the corners.

 

(Arguable because the full extent of the Triangulum Galaxy goes on a bit wider than the frame, but if you can't really see it...)

 

M33 arguably fits into RC6/APSC field of view

 

Most of the time, we'll be imaging targets considerably smaller than this.

 

Yes, there are times I want/need those fiddly corners, but usually, I crop 'em and forget 'em.

 

It is possible to sometimes make too much of field curvature/corner aberration.

(I think I hear the large format sensor guys clearing their throats for rebuttal...)


Edited by dg401, 05 August 2021 - 03:30 PM.


#133 TinySpeck

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Posted 05 August 2021 - 03:29 PM

For reference, here is the Cheshire view of the secondary.  The center dot will vary with Cheshire aperture.  I rather like this one relative to the secondary donut as the ring of light between the Cheshire shadow and the donut is tight.  Having too big a central dot makes it harder to judge centering.  This one is just right.  Also, it's very easy to just kiss the edge of the center dot right up the edge of the donut in the corners (x) and at the sides (+) to test for possible directional bias away from from the apparent center.  Getting a decent shot with an Iphone 8 through a Cheshire isn't easy.  This took awhile!

 

Nice idea!  Did you drill out your Cheshire?  I'd want to be very careful about that to make sure the hole stays central, like chucking it in a good lathe and choking way up on the drill bit so it stays stiff.

 

Good point about rarely needing star perfection to the corners, too.  Most of my images are cropped to some extent too.  If you want to make mosaics though (like of Andromeda with your setup) you might want better corners.


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#134 dg401

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Posted 05 August 2021 - 03:39 PM

Nice idea!  Did you drill out your Cheshire?  I'd want to be very careful about that to make sure the hole stays central, like chucking it in a good lathe and choking way up on the drill bit so it stays stiff.

 

Good point about rarely needing star perfection to the corners, too.  Most of my images are cropped to some extent too.  If you want to make mosaics though (like of Andromeda with your setup) you might want better corners.

Drill out my Cheshire?  Heavens no!

 

I have a big enough pile of useless crap made useless by my not leaving well enough alone.

 

That's the central dot as is out of the box.  Recall, it's not the size of the peep-hole, but the size of the hole drilled through the 45 degree external light deflection wedge that sets the dot size.  That's an elliptical hole that appears round when viewed from above or below.

 

*I don't have any elliptical drill bits.  lol.gif

 

*Yes, a circle is an ellipse, but don't anyone try to get cute about that,


Edited by dg401, 05 August 2021 - 03:55 PM.


#135 TinySpeck

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Posted 05 August 2021 - 04:19 PM

That's the central dot as is out of the box.  Recall, it's not the size of the peep-hole, but the size of the hole drilled through the 45 degree external light deflection wedge that sets the dot size.  That's an elliptical hole that appears round when viewed from above or below.

 

D'oh!  Of course!  I've been missing that all along, but you're quite right.  Now, where did I put that elliptical drill bit so I can enlarge mine...?


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#136 dg401

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Posted 05 August 2021 - 04:33 PM

D'oh!  Of course!  I've been missing that all along, but you're quite right.  Now, where did I put that elliptical drill bit so I can enlarge mine...?

Would it help if I reminded you that drilling straight down into the flat top of the wedge assembly below the peep-hole will (of course) create a round hole, but due to the 45° angle cut, the hole on the plane of the reflective surface will be elliptical due to the fact that it's the same thing as cutting a cone at a 45° angle (thus the whole terminology around "conic sections")?

 

So my comment, "That's an elliptical hole that appears round when viewed from above or below." is wrong.  That's a round hole that appears elliptical on the face of the 45° cut.  And now my brain's getting all hurty because of conic sections versus cylindrical sections and not having thought about the latter since about forever.

 

Edit:  I think I got it, you can cut elliptical, parabolic, and hyperbolic sections from a cone.  You can only cut elliptical sections from a cylinder.  I really should knock this off and go play fetch with the dog.


Edited by dg401, 05 August 2021 - 04:54 PM.

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#137 dg401

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Posted 06 August 2021 - 12:41 PM

From a discussion with the OP that he suggested I post here in case it's of interest:

 

I performed three different tests.  First, I performed a Farpoint laser collimation.  Then I performed a hybrid laser/hall of mirrors collimation.  Then I performed the OPs collimation.

 

1. Laser only collimation.

 

Endpoint was a) laser directly on secondary center, b) laser reflection off secondary directly back onto source.

 

Result as starting point versus OPs procedure:

 

Cheshire off-center over the donut.  Hall of mirrors superficially OK at a glance but poor under any sort of critical inspection.

 

It didn't take much wrench turning at all with the OPs procedure to fix the deviations.  BUT.. and this is a huge BUT...  It doesn't matter if you're 5 or 6 wrench turns off perfect if you have no idea what 5 or 6 wrench turns to make.  You might as well close your eyes and guess.  The endpoint was easy to quick to reach but the results were garbage.

 

2.  Hybrid Collimation.

 

Endpoint was a) laser directly on secondary center, b) hall of mirrors.

 

Result as starting point versus OPs procedure:

 

I was able to get the hall of mirrors fairly close but I just couldn't get it perfect.  Cheshire dot was half on the donut, half in the central circle.

 

This was clearly a better collimation that using the laser alone.  It took even fewer and smaller adjustments to line up HOM and the Cheshire dot,  but this method is still blind to the needed fine tweaks.  Interestingly, it took make iterations to converge to an endpoint.

 

3.  OPs collimation:

 

Endpoint was a) center dot centered in donut, b) hall of mirros

 

Result: Visual symmetry for both HOM and Cheshire dot.  Fast convergence but exacting to get final exact centering.


Edited by dg401, 06 August 2021 - 12:42 PM.

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#138 nebulachadnezzer

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Posted 06 August 2021 - 01:53 PM

From a discussion with the OP that he suggested I post here in case it's of interest:

 

I performed three different tests.  First, I performed a Farpoint laser collimation.  Then I performed a hybrid laser/hall of mirrors collimation.  Then I performed the OPs collimation.

You're a saint. Thanks for being so thorough with your testing!


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#139 Sky King

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Posted 06 August 2021 - 02:03 PM

"My Cheshire is a 1.25" model in a self-centering 2" adapter from AstroMania."  I found the short version of the Cheshire, but not the 2" self-centering adapter. Anyone have a link? Thanks!



#140 dg401

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Posted 06 August 2021 - 02:20 PM

You're a saint. Thanks for being so thorough with your testing!

Thanks, no problem.

 

Based on the Cheshire and Hall of Mirrors visuals after the Farpoint collimation, the Farpoint (and perhaps any laser for that matter) can get you close.  But with the hyperbolic mirror pair of the RC, close isn't good enough.  By "visuals", I mean the centering of the Cheshire dot and the symmetry of the Hall of Mirrors when treated as the starting point for the OPs collimation procedure.
 

I'm not suggesting we all sell our lasers.  They're good and useful for a lot of applications.  But collimating an RC isn't one of them.



#141 dg401

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Posted 06 August 2021 - 02:26 PM

"My Cheshire is a 1.25" model in a self-centering 2" adapter from AstroMania."  I found the short version of the Cheshire, but not the 2" self-centering adapter. Anyone have a link? Thanks!

Mine is pretty generic (Celestron) without any special self-centering.  I find that the secret sauce is to put the scope on the mount and orient the focuser pointing straight upward.  Gravity is your friend and it will level the 2" to 1.25" adapter and and 1.25" Cheshire as well.  No need to tighten any screws to activate a nasty compression ring.  Those compression rings are a curse when it comes to collimation.

 

The Celestron Cheshire I have (you could probably buy the same Cheshire from any of a dozen vendors who slapped their name on the thing just after it got off the boat from China) has a wire crosshair pair built in.  Rip that useless thing out.  It blocks the view of the center dot.


Edited by dg401, 06 August 2021 - 02:28 PM.

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#142 TinySpeck

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Posted 06 August 2021 - 03:32 PM

"My Cheshire is a 1.25" model in a self-centering 2" adapter from AstroMania."  I found the short version of the Cheshire, but not the 2" self-centering adapter. Anyone have a link? Thanks!

This is the one I used.  "Self centering" seems to be the search phrase to use, but I don't see many of them out there.  If you can find a native 2" Cheshire that would probably be better, but I couldn't.


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#143 dg401

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Posted 06 August 2021 - 07:19 PM

This is the one I used.  "Self centering" seems to be the search phrase to use, but I don't see many of them out there.  If you can find a native 2" Cheshire that would probably be better, but I couldn't.

My experience is that when using GIYF* during collimation, I can repeatedly remove and gently drop the 2" to 1.25" adapter into the loosened focuser and the Cheshire into the loosened adapter, and the location of the center dot stays the same to within the limit of my visual acuity (which is very sharp with reading glasses... other than being old and presbyopic, I can still see the wrinkles on a flea's backside at distance).  With the main force on the adapter and Cheshire being directly downward, I can drop them in and they take care of themselves.  The whole centering discussion may be much less critical with the vertical orientation than a horizontal orientation due to the fact that with horizontal orientations, GINYF as it's pulling the adapter and the Cheshire toward the bottom-most point of the tubes producing a small but directed bias away from concentricity.  This bias must be dealt with, and dealing with it opens a can of worms.  With vertical, there is no bottom-most point and no directional bias in the X/Y plane to play silly buggers with concentricity... every point around the perimeter is bottom-most.  I say drop the adapter and the Cheshire in, get your collimation, and then feel free to remove and insert the adapter/Cheshire ten times, and I defy anyone to see a bit of difference in centering on nine of them (and you're imagining it on the tenth).

 

Now if you have tubes with an excessive amount of slop, then you are leaving an important variable with too much wiggle room (literally).  A snug fitting adapter and Cheshire takes care of most of the work for you.

 

And if I really want to be an insufferable terd, I can point out that a "self-centering" device will center, but to what?  A mechanical center or a center coaxial to the optics?  We can recursively worry about every aspect of a system that has a theoretical central axis around which everything fits in perfect concentricity... and we do.  But in the end, we have to stop somewhere.

 

*Gravity is Your Friend


Edited by dg401, 06 August 2021 - 07:29 PM.


#144 dg401

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Posted 06 August 2021 - 09:27 PM

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

 

This is my secret sauce for doing Hall of Mirrors on a GSO RC6.  The 2nd bright reflection of the open end of the focus tube can be adjusted to identical "crescent moons" at each of the 4 vanes (and anywhere else around the OTA perimeter). No crescent should be wider than any other. 

 

With careful adjustment of the primary screws, the view can be made identical at all 4 vane.   The primary is now dead coaxial to the secondary.  Even a very tiny adjustment to any of the three primary screws will throw something out on the views.

 

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.

 

Each vane  and it's reflection can also be inspected against the corresponding crescent.  Each vane lined up with it's reflection should precisely bisect the crescent.  The physical vane closest to your eye will be very blurry since you can't focus at a couple inches, but you still can line it up to it's more distant and easier to focus upon reflection.

 

Here are the views at each vane.  It was difficult to get precise position or focus; the eye can do much better.  Once you know what to look for and you train your brain to it, it's easy to know when you're done.

 

It takes a lot of work to get everything perfect, but this effort is very well spent.

 

Hall of Mirrors - Vane 1

 

Hall of Mirrors - Vane 2
 
Hall of Mirrors - Vane 3
 
Hall of Mirror - Vane 4

 

Note: Some of the bisected crescent moons don't look symmetrical around their vane reflections (1st and 3rd).  This is just sloppy camera work.  Again, the eye/brain can do far better judging in real time.


Edited by dg401, 06 August 2021 - 09:36 PM.

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#145 MikeECha

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Posted 06 August 2021 - 10:39 PM

 

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

 

This is my secret sauce for doing Hall of Mirrors on a GSO RC6.  The 2nd bright reflection of the open end of the focus tube can be adjusted to identical "crescent moons" at each of the 4 vanes (and anywhere else around the OTA perimeter). No crescent should be wider than any other. 

 

With careful adjustment of the primary screws, the view can be made identical at all 4 vane.   The primary is now dead coaxial to the secondary.  Even a very tiny adjustment to any of the three primary screws will throw something out on the views.

 

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.

 

Each vane  and it's reflection can also be inspected against the corresponding crescent.  Each vane lined up with it's reflection should precisely bisect the crescent.  The physical vane closest to your eye will be very blurry since you can't focus at a couple inches, but you still can line it up to it's more distant and easier to focus upon reflection.

 

Here are the views at each vane.  It was difficult to get precise position or focus; the eye can do much better.  Once you know what to look for and you train your brain to it, it's easy to know when you're done.

 

It takes a lot of work to get everything perfect, but this effort is very well spent.

 

 

 

 
 
 
 
 

 

Note: Some of the bisected crescent moons don't look symmetrical around their vane reflections (1st and 3rd).  This is just sloppy camera work.  Again, the eye/brain can do far better judging in real time.

 

Have you checked if your primary is coaxial when the collimation screws are are all the way in which is the mirror pulled all the way back? My 6" is. 

 

BTW I respectfully disagree with your graph on your post #111. The tube cannot be in that position and the mirrors still be in collimation. That is because the tube is attached between the front and back casted rings which is where the mirrors are attached. Those in turn, are attached to at least one very straight and solid dovetail. In my case two of them, one on the top and one on the bottom.

 

You could check that right now that you are collimated. Just put the scope face down on a flat surface, remove the focuser and check with a level. The only error in level will be what you did with the collimation screws. And I bet that If you screw the collimation screws all the way in (what I call reset) you might find all is leveled again and  therefore coaxial too. Which may indicate that adjusting the primary on these scopes is not even needed for collimation. It is all about the secondary. 

 

At least I have come to that conclusion working on my copy of the scope.

 

I know talking theoretically makes some people roll their eyes, but that is actually the starting point in the design and troubleshooting thought process: "...what if..."



#146 dg401

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Posted 06 August 2021 - 11:21 PM

Have you checked if your primary is coaxial when the collimation screws are are all the way in which is the mirror pulled all the way back? My 6" is. 

 

BTW I respectfully disagree with your graph on your post #111. The tube cannot be in that position and the mirrors still be in collimation. That is because the tube is attached between the front and back casted rings which is where the mirrors are attached. Those in turn, are attached to at least one very straight and solid dovetail. In my case two of them, one on the top and one on the bottom.

 

You could check that right now that you are collimated. Just put the scope face down on a flat surface, remove the focuser and check with a level. The only error in level will be what you did with the collimation screws. And I bet that If you screw the collimation screws all the way in (what I call reset) you might find all is leveled again and  therefore coaxial too. Which may indicate that adjusting the primary on these scopes is not even needed for collimation. It is all about the secondary. 

 

At least I have come to that conclusion working on my copy of the scope.

 

I know talking theoretically makes some people roll their eyes, but that is actually the starting point in the design and troubleshooting thought process: "...what if..."

Now I gotta think...  Assume we have a secondary, an optical tube, and a primary/focuser unit (moves as one) on a GSO RC all perfectly coaxial.  Arbitrarily assign one point on the OTA as "north".  If we tilt the north of the primary/focuser unit 1° toward the secondary via adjustment of a single pull screw, with no movement to the east or west, then could we tilt the north of the secondary 1° away from the primary and re-establish a coaxial state between the primary/focuser and secondary?  Or would the mirrors end up parallel but not coaxial because we've moved the centers onto different axes?

 

Yea, I think you're right... I could line up the Cheshire dot, but I'll bet it would be impossible to get the hall of mirrors right.  Generally speaking an OTA can be independent, but in this design, you're probably right that it can't.  We'd need some wiggle room to shift either mirror slight toward one side or the tube or the other to re-establish the common axis, and this scope doesn't provide that.

 

I've developed the habit of tightening the primary pull screws all the way down if I fear I'm losing track of where the primary is sitting, then loosening each of them exactly 3 turns (2.4mm).  Locked down, it's roughly but not exactly in line.  I could probably collimate with one of the 3 screws locked down and the other two slightly loosened appropriately, but that would remove a degree of freedom and make for quite a challenging exercise.  But I've standardized on three turns from tight and go from there.  Similarly, when I do this, I know I can screw the secondary 4 turns into it's ~3 inch frame and then give the center screw 8.5 turns from the point where it first catches, and then equally tighten down the 3 screws and end up at 1370mm give or take a few.   At least that's where I start a collimation from a pile of parts.


Edited by dg401, 06 August 2021 - 11:38 PM.


#147 MikeECha

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Posted 06 August 2021 - 11:39 PM

Now I gotta think...  Assume we have a secondary, an optical tube, and a primary/focuser unit (moves as one) on a GSO RC all perfectly coaxial.  Arbitrarily assign one point on the OTA as "north".  If we tilt the north of the primary/focuser unit 1° toward the secondary via adjustment of a single pull screw, with no movement to the east or west, then could we tilt the north of the secondary 1° away from the primary and re-establish a coaxial state between the primary/focuser and secondary?  Or would the mirrors end up parallel but not coaxial because we've moved the centers onto different axes?

 

Yea, I think you're right... I could line up the Cheshire dot, but I'll bet it would be impossible to get the hall of mirrors right.  Generally speaking an OTA can be independent, but in this design, you're probably right that it can't.

Yes, parallel but not coaxial. That is why it is very important start out with the primary as centered as possible on the tube and with a coaxial focuser/mirror unit. That is a no brainer. The design of these scopes provides that facility. And as you said before, if you do not do it, you star out with already an strike against you.

 

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


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#148 nebulachadnezzer

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Posted 07 August 2021 - 12:10 AM

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'll try to do it later this weekend. I don't currently have a focuser on my RC8, because I transferred the Moonlite CSL to my truss RC10. I have the RC6/RC8 flange for a Moonlite CS so I can swap that over from another scope in case I want to run both RCs on the same night. 

It seems the extension tubes would have some impact on this as well. How many much if any extension was on your RC6 when you shot those images?



#149 MikeECha

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Posted 07 August 2021 - 12:18 AM

I'll try to do it later this weekend. I don't currently have a focuser on my RC8, because I transferred the Moonlite CSL to my truss RC10. I have the RC6/RC8 flange for a Moonlite CS so I can swap that over from another scope in case I want to run both RCs on the same night. 

It seems the extension tubes would have some impact on this as well. How many much if any extension was on your RC6 when you shot those images?

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



#150 dg401

dg401

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Posted 07 August 2021 - 12:19 AM

I'll try to do it later this weekend. I don't currently have a focuser on my RC8, because I transferred the Moonlite CSL to my truss RC10. I have the RC6/RC8 flange for a Moonlite CS so I can swap that over from another scope in case I want to run both RCs on the same night. 

It seems the extension tubes would have some impact on this as well. How many much if any extension was on your RC6 when you shot those images?

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.


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