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My take on GSO Ritchey-Chrétien collimation

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#51 TinySpeck

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Posted 10 July 2021 - 08:10 PM

Have you seen this https://youtu.be/a3UOGDUaq6o

Yes, I liked the idea but I think it's too complicated.  Plus, I'm now leery of lasers.  Everyone loves them (because what's cooler than a laser?) but they need to be mounted and aligned exactly and their resulting dot (or hologram) is fuzzy and shimmery, not rotationally symmetrical, and difficult to interpret.  A diode laser source is not a point but a bar, it needs to be stopped down to approximate a point, and it's just not that precise.  Especially after it's been reflected off a convex secondary.

 

What!? You're just going to leave us hanging there?

Hah!  There it is, the clamor I was looking for!  Even for an audience of one I'm a pushover.  I'll be gone for a few days, but I'll put something together after that and post it as a new thread.


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#52 nebulachadnezzer

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Posted 10 July 2021 - 08:39 PM

I’m definitely not a fan of the Howie Glatter concentric ring hologram. I’ve got one. It was the most expensive, efficient way to make my nearly collimated RC8 into an uncollimated mess. I need to sell the HG. I have found that nothing beats iterating the DSI method under the stars. Lasers and even get Tak score just don’t give you enough of a simulation of infinity focus.
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#53 nebulachadnezzer

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Posted 10 July 2021 - 08:39 PM

I’m definitely not a fan of the Howie Glatter concentric ring hologram. I’ve got one. It was the most expensive, efficient way to make my nearly collimated RC8 into an uncollimated mess. I need to sell the HG.

 

I have found that nothing beats iterating the DSI method under the stars. Neither lasers nor the Tak scope give you enough of a simulation of infinity focus.

 

i should sell the Tak scope, too.


Edited by nebulachadnezzer, 10 July 2021 - 08:42 PM.

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#54 TinySpeck

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Posted 11 July 2021 - 11:16 AM

I’m definitely not a fan of the Howie Glatter concentric ring hologram. I’ve got one. It was the most expensive, efficient way to make my nearly collimated RC8 into an uncollimated mess. I need to sell the HG.

I totally agree.  I would also throw the DSI under-the-stars method under the bus, though.  I'll explain all when I post my spiffy new method in a week or so.



#55 teashea

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Posted 11 July 2021 - 08:58 PM

TOOLS REQUIRED

 

• Laser collimator, dot only (like a Howie Glatter)
• Headset magnifier (like an OptiVisor)
• Inspection mirror (angled mirror on a stick)
• Small diffuse or side-illuminating flashlight
• Cheshire
• Binoculars
• 2.5, 3, and 4 mm hex keys for the scope, also whatever you need for focuser tilt
• #2 Phillips screwdriver (to set focal length if necessary)

 

PROCEDURE

 

1. Set focal length (if necessary).  This scope is widely spec'd as 1625 mm focal length, but it is actually 1600 mm.  See post here: https://www.cloudyni...rs-for-gso-rc-8 .  The OPT website and some others have it correctly now.  When you set the focal length to 1600 you also get the spec'd back focus of 250 mm.

 

It is important to have the correct focal length for an RC scope.  This generally needs to be done under the stars with a plate solver.  If you decrease the mirror to mirror distance by x, you increase the focal length by at least 10x.  Note that increasing the focal length will increase the back-focus, too, and vice-versa.

 

Adjust focal length by moving the secondary incrementally in or out using the center screw while keeping the three tilt adjustment screws snug.  To move the secondary, the center screw is pull: CW pulls the secondary mirror away from the primary and reduces the focal length.  The three hex screws are push: CW on them pushes the secondary mirror toward the primary and increases the focal length.  Loosen the push before pulling and vice-versa.

 

2. Reset the primary (if necessary), maintaining focal length.  Do this if you know your primary is way out of whack.  Fully loosen all the small black primary screws, tighten the large silver screws all the way (counting 1/6 turns by flats on the hex keys), then loosen them the same number of turns each (the average of the tightening turn counts) and tighten the black screws.  This maintains the focal length you set in the previous step.

 

3. Reset the focuser tilt (if necessary).  For a Moonlite CS: loosen the focuser rotation lock thumbscrew and all four focus tilt setscrews.  The focuser is now loose on its flange.  Pulling the focuser away from the OTA, tighten the tilt adjust setscrews until they all just make contact with the same snugness.  Focuser rotation will drag slightly at this point.

 

4. Adjust focuser tilt so the laser dot points directly at the secondary donut.  Insert the laser snugly into the focuser.  You can clearly see the secondary donut by looking in from the OTA open end and using the headset magnifier, inspection mirror, and a diffuse or side-illuminating flashlight hung on a hook inside the OTA a few cm.  The magnifier lets you see the donut in good detail from close up.  The flashlight illuminates the secondary donut from inside the OTA so you can see it more easily, and it eliminates the visual dazzle of the laser dot.  The inspection mirror is an angled mirror on a stick so you can look back at the secondary mirror from the open tube end.

 

5. Adjust secondary tilt until the reflected laser dot is centered on the laser source.  Use binoculars from the OTA open end to see a reflection of the laser source face in good detail.  Yes, good old field glasses!  I was amazed that this worked, but the effective distance to the laser source image is far enough that you can use binoculars.  This brings the laser source image up close and you can make a good judgment on how well centered the reflected dot is.  The reflected "dot" is actually smeared out by the secondary with many diffraction rings visible, but you can center it well by eye this way.

 

6. Adjust the primary tilt using a Cheshire.  Install the Cheshire snugly in the focuser.  Look through the Cheshire "pinhole" and center the dot (the reflection of the pinhole) in the secondary mirror donut.  This is hard to explain but easy to do with precision once you see it.

 

7. Iterate steps 4 – 6 until converged. 

 

8. Fine tune the primary tilt with a star near zenith (if necessary).  I used this primary fine-tuning method from Vixen.  Place an out-of-focus bright star in the center of the view.  Move the scope so the star donut moves in the direction of the narrowest part of the donut.  Adjust the primary in the direction to bring the star back toward center.  This eliminates guesswork over which primary screw to adjust.  My scope only required about 1/16 turn on one primary screw.
 

attachicon.giftrim.png

 

(continued)

Very intelligently written.  


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#56 dg401

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Posted 12 July 2021 - 01:51 PM

I'm new here, so hello everyone.

 

I wonder if anyone can answer a question tangential to the topic of this thread.

 

I received my RC6 a few weeks ago, and the first thing I did was check the focal length via pixel scale analysis at astrometry.net.  I got 1412 mm for the focal length.  This made sense because the scope is spec'ed at 1375 mm and most users find focus for a DSLR camera using the 2 inch and a single 1 inch extender.  I have to use the 2" and both 1" extenders to achieve DSLR focus.

 

So what do I make of the 1412 mm focal length?  Should I consider it wrong or could it be correct for this primary/secondary pair? The word from folks smarter than I am is that a hyperbolic RC primary/secondary pair have a single correct focal length.  So how much deviation is too much and what aberration is aggravated by having the wrong focal length on an RC? 

 

The collimation looks fairly good and de-focused donuts look decent on and off axis, but there is that extra inch and a half of back-focus on an already back-heavy OTA.    I've got the know-how and equipment to reset the focal-length and re-collimate, but it's not something I really want to do just because.  Advise is welcomed!


Edited by dg401, 12 July 2021 - 06:04 PM.


#57 Rasfahan

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Posted 12 July 2021 - 06:29 PM

I'm new here, so hello everyone.

 

I wonder if anyone can answer a question tangential to the topic of this thread.

 

I received my RC6 a few weeks ago, and the first thing I did was check the focal length via pixel scale analysis at astrometry.net.  I got 1412 mm for the focal length.  This made sense because the scope is spec'ed at 1375 mm and most users find focus for a DSLR camera using the 2 inch and a single 1 inch extender.  I have to use the 2" and both 1" extenders to achieve DSLR focus.

 

So what do I make of the 1412 mm focal length?  Should I consider it wrong or could it be correct for this primary/secondary pair? The word from folks smarter than I am is that a hyperbolic RC primary/secondary pair have a single correct focal length.  So how much deviation is too much and what aberration is aggravated by having the wrong focal length on an RC? 

 

The collimation looks fairly good and de-focused donuts look decent on and off axis, but there is that extra inch and a half of back-focus on an already back-heavy OTA.    I've got the know-how and equipment to reset the focal-length and re-collimate, but it's not something I really want to do just because.  Advise is welcomed!

If stars look good (you‘re the judge) don‘t touch anything. Seriously.

 

A wrong FL leads to spherical aberration, so a loss of resolution and „big, round stars“.


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#58 dg401

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Posted 13 July 2021 - 04:48 AM

If stars look good (you‘re the judge) don‘t touch anything. Seriously.

 

A wrong FL leads to spherical aberration, so a loss of resolution and „big, round stars“.

 

Thank you.

 

"Decent" photographic results aside, I know I'm going to have to collimate as I'm seeing this with a tri-bahtinov mask.  manual rotation of a regular bahtinov mask also shows the same behavior.  I wish I was better at interpreting what I'm seeing here.  I only know this isn't right.

 

GSO RC6 - Out of box


#59 TinySpeck

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Posted 17 July 2021 - 03:46 PM

...  I'll explain all when I post my spiffy new method in a week or so.

Well, the numbers on my new collimation procedure didn't come out as good as I hoped.  The results look good, but I think I can do better, so I'll put off posting anything about it till I do.



#60 dg401

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Posted 18 July 2021 - 10:59 PM

Well, the numbers on my new collimation procedure didn't come out as good as I hoped.  The results look good, but I think I can do better, so I'll put off posting anything about it till I do.

I'll be looking forward to anything that might move the dial forward toward taming the collimation on these GSOs.

 

It seems obvious by now that lasers and cheshires can only get us close with these scopes, and close isn't good enough due to a likely difference in physical versus optical center of the small hyperbolic secondary along with certain lack of uniformity to the primary & secondary baffles.  It's a recipe for a very unforgiving collimation as we are all painfully aware.

 

When I align my Moonlite CS with the primary mirror, I aim my scope straight down at a matte white target on the floor with the secondary assembly removed.  This way, the focuser compression ring can remain loose and the Farpoint laser is held evenly in place by gravity pulling it straight down against the focuser without any nasty side force to throw things out of whack. The tilt mechanism on the focuser is then adjusted to coincide the laser spot on the paper with it's dimmer reflection.  Unscrewing the beam narrowing aperture from the laser gives a brighter spot and a brighter reflection.  The reflection can be de-focused and used as a target onto which the laser spot is centered.  IMO, this is the only step that can be accurately done with a laser on these scopes.  If the laser spot and it's reflection coincide, the focuser is perfectly aligned with the primary mirror.  Period.  Any later adjustments relative to the secondary will benefit from a correctly aligned focuser/primary mirror since they move as a pair.

 

Unless I've missed something...


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#61 MikeECha

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Posted 19 July 2021 - 06:48 AM

If you understand where misalignments are coming from and you find solutions for them, a well collimated laser is the best tool you have short of an optical bench. Surface curvature should not matter. Make the angle of incidence of the beam equal to zero on the spot you want (the center of the secondary) and it will reflect back at zero. 

 

I put a sample sub in my gallery (https://www.cloudyni...ion-with-laser/) It shows a sub collimated with a laser and I did not do any star test. From my table to imaging. 

 

Are the stars on my image round enough?



#62 VegasAl

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Posted 19 July 2021 - 07:18 AM

MikeECha,

I clicked your link and it says no images to display. I'm really interested in this topic and woouldlove to be able to see 

what your image looks like. 

 

Thanks 

Al



#63 MikeECha

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Posted 19 July 2021 - 09:44 AM

Hello AI

I posted a sub on my CN gallery from my AT6RC @ 1365fl, native no corrector. I think the stars look round enough. I would like to hear what you and others think.

It is public image so I think you could go to my gallery. I just do not how a user would get there. The link worked for me but I guess it is because it is my account.

#64 dg401

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Posted 19 July 2021 - 10:47 AM

If you understand where misalignments are coming from and you find solutions for them, a well collimated laser is the best tool you have short of an optical bench. Surface curvature should not matter. Make the angle of incidence of the beam equal to zero on the spot you want (the center of the secondary) and it will reflect back at zero. 

 

I put a sample sub in my gallery (https://www.cloudyni...ion-with-laser/) It shows a sub collimated with a laser and I did not do any star test. From my table to imaging. 

 

Are the stars on my image round enough?

The link turns up no image.

 

If you are able to simply point a laser at the center dot on your secondary, reflect it back onto the laser point in the focuser, and get no on-axis coma or off-axis astigmatism without any star testing or addition tweaking, you are very fortunate.  That is not the normal case with these small GSO RCs.



#65 MikeECha

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Posted 19 July 2021 - 11:40 AM

The image is in my gallery because is a 39 meg sub. I think you can get to it.

Of course I did not get to that point by chance. I did a lot of work on it to understand where the misalignment was coming from and fixed them before collimation.

My point is that only when you get the laser to fall back on its own reflection your can say your secondary is aligned with the focuser and there is no point in starting to collimate the primary until that point. You fix the secondary and the rest is pice of cake.

That is independent of the surface curvature.

I am at work. I will try to get the picture available when I get home

#66 nebulachadnezzer

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Posted 19 July 2021 - 11:53 AM

The image is in my gallery because is a 39 meg sub. I think you can get to it.

There's your problem. :)

CN has a limit of 500KB images. To share a 39MB sub you'd be better off trying to link to the file on Google Drive or something similar.



#67 MikeECha

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Posted 19 July 2021 - 05:28 PM

MikeECha,

I clicked your link and it says no images to display. I'm really interested in this topic and woouldlove to be able to see 

what your image looks like. 

 

Thanks 

Al

AI

 

Here is the picture

 

PI_ABERRATION_INSPECTOR.JPG

 

PI_ABERRATION_INSPECTOR.JPG



#68 dg401

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Posted 19 July 2021 - 05:33 PM

The image is in my gallery because is a 39 meg sub. I think you can get to it.

Of course I did not get to that point by chance. I did a lot of work on it to understand where the misalignment was coming from and fixed them before collimation.

My point is that only when you get the laser to fall back on its own reflection your can say your secondary is aligned with the focuser and there is no point in starting to collimate the primary until that point. You fix the secondary and the rest is pice of cake.

That is independent of the surface curvature.

I am at work. I will try to get the picture available when I get home

Considering the almost universal frustration encountered while collimating these scopes, perhaps you can share your insights about where the miscollimation is coming from? What have you figured out that we have not such that a good single dot laser does the trick?

There are finite degrees of freedom we’re dealing with here. One the z-axis, we have primary to secondary distance (focal length) as well as focuser to primary/secondary distances (focus). On the x and y axes we have focuser to primary, focuser to secondary, and primary to secondary alignment/centering versus a perfect 0,0.

Let’s assume for a moment the primary and secondary are properly spaced (verifiable by pixel scale analysis). This will give a focal length of 1375mm for an RC6 and 1600mm for an RC8.

if my focuser/primary mirror are throwing a correctly aligned laser dot reflection right back onto the laser dot viewed on a white surface at an appropriate distance. the primary and focuser are aligned in x and y.

if the primary/focuser pair hits the center dot of the secondary with a laser beam, then the focuser/primary are aligned with the center of the secondary in x and y.

finally if the secondary then throws the laser reflection right back onto the laser, then both the secondary center and the secondary tilt is aligned with the focuser/primary in x and y.

EXCEPT… It doesn’t work and many of us are ripping out all our hair, tossing our RCs in a closet, spending some months growing new hair, bringing our RCs back out of the closet for another go… lather, rinse, repeat.

If anyone has a method that works and are able to explain so that others may reproduce the results, that’s the holy grail. So far, we’ve got a lot of talk but no holy grail.

Edit: Just thinking, tilt is actually in z. One side goes in, the other out.

Edited by dg401, 19 July 2021 - 05:42 PM.


#69 MikeECha

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Posted 19 July 2021 - 05:35 PM

For those interested in checking the star roundness of the image I attempted to provide this morning here it is

 

  https://www.dropbox....10.00.fits?dl=0

 

To the OP, sorry if it looks like I am hijacking this thread. It is not my intention at all.



#70 nebulachadnezzer

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Posted 19 July 2021 - 05:38 PM

Can't really tell from that. There aren't enough stars visible because of the bright one in the center affecting your stretch.

Do you have an out-of-focus stars image, full frame?


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#71 dg401

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Posted 19 July 2021 - 05:52 PM

OP seems pretty consistent in supporting any discussion that moves the bar forward. We’d love to hear more about how you achieved your results because we’d also love to take a crack at reproducing them.

Edited by dg401, 19 July 2021 - 05:53 PM.


#72 TinySpeck

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Posted 19 July 2021 - 06:42 PM

Yes, OP approves.  grin.gif   

 

@MikeECha -- @nebulachadnezzer is right, star quality over the whole field can't be determined from your AberrationInspector mosaic.  We need to see a full field of stars, and while you're at it do FWHMEccentricity on a focused field to get contours of FWHM and eccentricity.  That's an even more rigorous test, and will show imperfections that are not apparent by looking at the image.



#73 MikeECha

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Posted 19 July 2021 - 07:44 PM

Considering the almost universal frustration encountered while collimating these scopes, perhaps you can share your insights about where the miscollimation is coming from?  What have you figured out that we have not such that a good single dot laser does the trick?

 

There are finite degrees of freedom we’re dealing with here.  One the z-axis, we have primary to secondary distance (focal length)   as well as focuser to primary/secondary distances (focus). On the x and y axes we have focuser to primary, focuser to secondary, and primary to secondary alignment/centering versus a perfect 0,0. 

Let’s assume for a moment the primary and secondary are properly spaced (verifiable by pixel scale analysis).  This will give a focal length of 1375mm for an RC6 and 1600mm for an RC8.  

 

if my focuser/primary mirror are throwing a correctly aligned laser dot reflection right back onto the laser dot viewed on a white surface at an appropriate distance. the primary and focuser are aligned in x and y.  
 

if the primary/focuser pair hits the center dot of the secondary with a laser beam, then the focuser/primary are aligned with the center of the secondary in x and y. 
 

finally if the secondary then throws the laser reflection right back onto the laser, then both the secondary center and the secondary tilt is aligned with the focuser/primary in x and y.  
 

EXCEPT… It doesn’t work and many of us are ripping out all our hair, tossing our RCs in a closet, spending some months growing new hair, bringing our RCs back out of the closet for another go… lather, rinse, repeat.  

 

If anyone has a method that works and are able to explain so that others may reproduce the results, that’s the holy grail.  So far, we’ve got a lot of talk but no holy grail. 
 

DG401

 

I di not want to hijack the thread but if it is of interest to you or somebody else, I will try to explain what I have learned.

 

This video https://youtu.be/a3UOGDUaq6o from my post #49 above gave me the idea. The trick is to collimate and fix the secondary first and use it as a fixed reference for the primary.

 

But that is the main problem with collimating (my) RC: the lack of a fixed reference to stablish the optical axis of the scope. By that I mean a line that is orthogonal to the center of the secondary and coaxial with the focuser. I did a lot of cheshire collimation and always ended up with perfect circles and bad pictures. So that was the first thing I had to solve. 

 

In the video, a tilt plate, the focuser and a laser is used to align the focuser and secondary together first. But I do not have a tilt plate and I do not want one as collimation with it fixes your camera rotation at that angle. At any other angle the sensor will be tilted.

 

To align the focuser, I used a 1.25 HG laser and put it thru the center hole on the secondary holder. However, in the video you also see that the first step is to align the edge of the primary with the scope tube. As you know, on these RCs the GSO focuser is attached to the back plate. So I "reset" the primary all the way back. I actually took the scope apart and checked that this would put the plane made by the edge the primary perfectly parallel with the back of the M91 thread where the focuser is attached. I put the primary assembly on its back on top of the most flat and leveled surface in my house (the kitchen counter)  and took measurement. Since the threads are made in a lathe (probably CNC) I know I can trust perpendicularity/parallelism. In doing this, when I align the bright outer ring of the primary with the tube, the laser should go thru the center hole of the secondary ... and guess what: it did. But even if it did not, I would have tilted the primary a little bit to get the laser thru the center hole of the secondary. That is the reference I need to get the actual reference (the secondary) spot on in place. Done with focuser for now.

 

For the secondary now.

 

The first thing I did not  like about the RC designs is that the secondary which requires tiny little adjustments is mounted on springs that gives an infinite amount of DoF. It makes no sense to me to have all that freedom and then have to control it with coarse thread screws. I needed to make a flat surface to butt the secondary against. That surface will also put the secondary very close to centered on the mechanical axis of the scope which will also make it easy to align with the focuser now aligned with the hole that supports it. But also, that surface had to allow for the tiny little adjustments and only tiny no matter what while still firmly holding the secondary. A rubber surface of some sort would meet the requirements. It acts as springs keeping positive tension on the bolts and still a flat. Rubber washer about 1" OD x 1/8 thick from the hardware store to the rescue. My scope (bought it used) had not spring on the center screws. Great! that encouraged me to "fix it". So, with the washer in place, I retracted the secondary collimation screws and snug the secondary against the washer. I put the laser in the focuser and with my hand/fingers I directed the beam on the secondary back to the laser while peeking from the front. Then adjusted the collimation screws to keep the beam on on the laser. When the two bright spot fall on top of each other, the secondary is collimated and as the video says you do not touch it again. 

 

And yes, I can hear the reader asking "how are you going to adjust the FL". Well, in my opinion based on what I have learned, the right way to adjust the FL on these RCs is actually the big, fine thread and the retaining ring you see there where the secondary mirror assembly screws on to. When you use that you can change the distance of the secondary very accurately (fine thread) without disturbing your collimation. That is the same mechanism on guide scopes such as the Orion Mini and many others where the front element moves up and down.  

 

At this point, I was ready to collimate my primary which now requires very little adjustment. I did not use the HG laser concentric circles here. The accuracy of those methods that use visual assessment of the roundness/concentricity depend too much on visual acuity that I do not have. So I use an iterative combination of reflection of the laser laser beam for the primary and cheshire for the focuser. For the focuser I only care to about putting the center spot of the cheshire back on the center donut of the secondary. For the primary, laser beam fall back on its own. 

 

I again hear the reader saying "what about the GSO focuser issue". Well, in my opinion based on what I have learned, my GSO focuser has no issues. The problem is how you sit the focuser on the scope flange. Mine has a conical flange end and that is a "built in" tilt plate. By design or otherwise? I do not know. But in order to put the cheshire dot back in the center on this step, I place the scope face down, loosen the M91 collar, put a flashlight on the 45 deg cutout and center carefully while retightening the collar. When tightened and centered, I go back to the laser for the primary. Then I repeat until I can not improve the situation anymore.

 

An the result is what you see on the picture I uploaded. I went outside and the first out of focus start field was very clear to me I could no improve the results. So the picture you see went from my table to capture. No other adjustment were done.

 

Another very important thing to fixe before collimation is nosepiece tilt and that is well known. I bought a HG Paralizer with a T tread (not the one for eyepieces with a srew) and that works great.

 

Sorry for this long post but I do not how to explain it any shorter. The devil is always in the details. 

 

I hope this helps you or someone else with the "universal frustration" . lol

 

I personally would not like this hobby without its complexity.

 

Let me know if you have questions.



#74 MikeECha

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Posted 19 July 2021 - 07:50 PM

Yes, OP approves.  grin.gif   

 

@MikeECha -- @nebulachadnezzer is right, star quality over the whole field can't be determined from your AberrationInspector mosaic.  We need to see a full field of stars, and while you're at it do FWHMEccentricity on a focused field to get contours of FWHM and eccentricity.  That's an even more rigorous test, and will show imperfections that are not apparent by looking at the image.

Hello guys.

 

I do not have out of focus. I have been under clouds or bad clarity sky for over a month now. I had it in mind to show it get an out of focus field.

 

How about the fits image itself on my link. Are you able to download. 

 

https://www.dropbox....10.00.fits?dl=0



#75 dg401

dg401

    Explorer 1

  • -----
  • Posts: 94
  • Joined: 11 Jul 2021

Posted 19 July 2021 - 08:41 PM

DG401

 

I di not want to hijack the thread but if it is of interest to you or somebody else, I will try to explain what I have learned.

 

<snipped to not repeat long paragraphs>

 

Let me know if you have questions.

A lot to unpack there.  Hoping there's something there that clicks!


Edited by dg401, 19 July 2021 - 08:52 PM.



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