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RC collimation with no laser or stars

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#76 dg401

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Posted 02 August 2021 - 05:19 PM

grin.gif   I know!  We see what we want to see, and I was struggling to keep that under control.  I did know where to look, but I swear I did see a slight deviation.  Since I corrected it, saw that it looked better, and the measurements confirmed it, I figured it had to have been there.

 

But yeah, the trick is to see the imperfection first, you're right.

Speaking of imperfection...

 

I've improved my cheshire dot considerably in the past few minutes.  The secret?  Don't try any of this when you or your eye is fatigued.  It's a recipe for failure.

 

As for lighting up the cheshire, a bright flashlight is an awful idea, the glare induces even more eye fatigue (see above).  Bright indirect light such as daylight provides a much clearer and glare-free illumination.

 

And if you're one of us over 50 types and you've got presbyopia, for goodness sake, use the correct strength reading glasses when peeping through the cheshire.  I thought I could focus my eye at the distance from the cheshire to the secondary, but with a low powered (1.75) pair of reading glasses, the dot was smaller and the secondary donut was finer.  I can easily discern centering defects that were imperceptible previously.

 

I'm coming around to the OPs way of looking at room for improvement.  Getting the meatware optimized is just as important as the hardware.


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#77 TinySpeck

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Posted 02 August 2021 - 05:42 PM

Speaking of imperfection...

 

I've improved my cheshire dot considerably in the past few minutes.  The secret?  Don't try any of this when you or your eye is fatigued.  It's a recipe for failure.

 

As for lighting up the cheshire, a bright flashlight is an awful idea, the glare induces even more eye fatigue (see above).  Bright indirect light such as daylight provides a much clearer and glare-free illumination.

 

And if you're one of us over 50 types and you've got presbyopia, for goodness sake, use the correct strength reading glasses when peeping through the cheshire.  I thought I could focus my eye at the distance from the cheshire to the secondary, but with a low powered (1.75) pair of reading glasses, the dot was smaller and the secondary donut was finer.  I can easily discern centering defects that were imperceptible previously.

 

I'm coming around to the OPs way of looking at room for improvement.  Getting the meatware optimized is just as important as the hardware.

Hah!  I was going to include warnings about when to use your reading glasses, blinking to clear "floaters", and keeping both eyes open at the Cheshire, but I didn't want to sound like an old fart.  I should have realized that many of us are in the same old boat.  I do remember standing up, shaking my head, blinking, walking around, and looking again at a crisper image.  Ya do what ya gotta do.

 

Yes, a flashlight can ruin the image. It can be deceiving too -- it can make the Cheshire dot look off-center when it isn't.  Sometimes I work a little flashlight around at various glancing angles to get the best illumination.

 

Glad to hear you've found a quick, free method for image enhancement!  I'm eager to get the pinhole Cheshire-cam going -- that should help a lot.  I'm thinking that mounting a variable intensity diffused white LED directed into the cutout would be helpful too.


Edited by TinySpeck, 02 August 2021 - 05:46 PM.


#78 dg401

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Posted 02 August 2021 - 06:29 PM

 I'm eager to get the pinhole Cheshire-cam going -- that should help a lot.  I'm thinking that mounting a variable intensity diffused white LED directed into the cutout would be helpful too.

How are you going to make sure the pinhole Cheshire-cam axis is coaxial to the Cheshire? lol.gif

 

I'll show myself out.


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#79 TinySpeck

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Posted 02 August 2021 - 07:03 PM

How are you going to make sure the pinhole Cheshire-cam axis is coaxial to the Cheshire? lol.gif

 

I'll show myself out.

Once you've collimated the collimator, you'll need to collimate the collimator-collimator, ad infinitum...  


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#80 nebulachadnezzer

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Posted 02 August 2021 - 09:04 PM

Suggestion for indoor or night time Cheshire use: If you have a light panel for making flats, it can also be a good, diffuse light source for a Cheshire. I agree flashlights are a terrible way to light them.


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#81 dg401

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Posted 02 August 2021 - 09:49 PM

Hah!  I was going to include warnings about when to use your reading glasses, blinking to clear "floaters", and keeping both eyes open at the Cheshire, but I didn't want to sound like an old fart...

You've got your mug posted as your profile photo... I'm afraid the old fart boat has already sailed.


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#82 dg401

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Posted 03 August 2021 - 02:08 AM

I performed another collimation per the author's procedure, this time wearing reading glasses, and I'm certain I've achieved a considerably more precise collimation than my previous efforts.

 

I tested with an artificial star.  1000 micron @ 60 meters (3.4 arcsec).  Considerably above Dawes/Rayleigh limits but no worse than the seeing on most East Tennessee nights which average out at fair to poor seeing.  Nice and bright so was able to take each exposure at 0.1 sec.

 

I am impressed and I'm beginning to be a believer.  I have been on the losing side of a pitched battle with this RC6 since mid June.  Image center is round (could be a bit more round; defocus shows fatter to the left).  Corners show aberration but they're well balanced (distortions all point toward image center).  As a starting point goes, this is much better than I achieved previously and I could probably do some passable imaging with this if I steer clear of the corners.

 

Image center:

 

Center
center defocus
 
Top left corner:
 
Top left
Top left defocus
 
Bottom left corner:
 
bottom left
bottom left defocus

 

Top right corner:

 

Top right
Top right defocus

 

Bottom right corner:

 

bottom right
bottom right defocus

 

I should be able to get better corners.  Perhaps someone can tell me how much better on an APSC sensor?

 

I made up a sensor diagram showing APSC versus full frame as they relate to the image circle.  Just something to help me visualize field curvature.  The circle in the center represents what I consider the sweet spot at the center of the image circle.  It's about 37 arcsec in diameter on an RC6 at 1370mm with an APSC sensor.

 

sensors

 


Edited by dg401, 03 August 2021 - 10:00 AM.

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#83 nebulachadnezzer

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Posted 03 August 2021 - 09:42 AM

 

I performed another collimation per the author's procedure, this time wearing reading glasses, and I'm certain I've achieved a considerably more precise collimation than my previous efforts.

That looks really quite good! For a bench collimation that's astonishingly good!

 

You could image with that collimation and get quite satisfactory results. The on-axis coma would be measurable with software but it wouldn't be highly visible in the images. You are down to the often-described "1/20th of a turn" remaining required adjustment on your primary, and perhaps that ends up driving a similar need on your secondary.

Very nice work!

To correct your on-axis coma at this point you need to tighten the primary pull screw that corresponds to the narrower side of your center star image. Remember, though, that you're working with a reflector and a camera that is most likely producing images that are rendered upside-down. You will want to test your camera orientation before making adjustments. Usually what I do is take an exposure with my hand roughly masking one quarter of the scope. The easy way to avoid confusion here is to move the mask to cover the narrower section of the star image. Once you've found that, you know which side of the primary needs to be pulled back (the side that corresponds to where you placed your hand/mask). Again, you are 1/10th to 1/20th of a turn from perfect.

I think your off-axis astigmatism is close to balanced. You'd have to judge it again after tweaking the primary.

Unfortunately the RC6 isn't going to produce a perfectly flat image on an APS-C sensor (without a corrector like the TS RCFlat2). I don't recall the spot diagram, though. It looks like your corner stars are ~1.5X the diameter of your center star on the long axis, at least with your artificial star here. That seems like a lot. Could this be caused by inter-mirror distance?


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#84 TinySpeck

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Posted 03 August 2021 - 10:17 AM

 

I performed another collimation per the author's procedure, this time wearing reading glasses, and I'm certain I've achieved a considerably more precise collimation than my previous efforts.

...

@nebulachadnezzar said it all: very nice work!  waytogo.gif   The "pointyness" of the out-of-focus stars in the corners is all generally toward the center of the image, which could be just the residual field curvature of the RC.  A focal length which isn't quite what your mirrors demand will contribute to that too (as @neb said: inter-mirror distance, which sets your focal length).

 

If you want to improve this even further, I'd suggest a Ronchi test to get your focal length exact, then a field flattener.  I bet you'd get close to perfection after that.


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#85 dg401

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Posted 03 August 2021 - 10:25 AM

That looks really quite good! For a bench collimation that's astonishingly good!

 

You could image with that collimation and get quite satisfactory results. The on-axis coma would be measurable with software but it wouldn't be highly visible in the images. You are down to the often-described "1/20th of a turn" remaining required adjustment on your primary, and perhaps that ends up driving a similar need on your secondary.

Very nice work!

To correct your on-axis coma at this point you need to tighten the primary pull screw that corresponds to the narrower side of your center star image. Remember, though, that you're working with a reflector and a camera that is most likely producing images that are rendered upside-down. You will want to test your camera orientation before making adjustments. Usually what I do is take an exposure with my hand roughly masking one quarter of the scope. The easy way to avoid confusion here is to move the mask to cover the narrower section of the star image. Once you've found that, you know which side of the primary needs to be pulled back (the side that corresponds to where you placed your hand/mask). Again, you are 1/10th to 1/20th of a turn from perfect.

I think your off-axis astigmatism is close to balanced. You'd have to judge it again after tweaking the primary.

Unfortunately the RC6 isn't going to produce a perfectly flat image on an APS-C sensor (without a corrector like the TS RCFlat2). I don't recall the spot diagram, though. It looks like your corner stars are ~1.5X the diameter of your center star on the long axis, at least with your artificial star here. That seems like a lot. Could this be caused by inter-mirror distance?

Thanks for your feedback.

 

I'm familiar with hand-masking to find which screw(s) best correspond to whichever side of a de-focused star.  It's generally the first step in the process of tweaking to make things worse.  In this case, it's very minor and I might actually make things better for a change.

 

Interesting that my tri-bahtinov mask sniffed out the on-axis coma.  There it is on the 10:00 - 4:00 axis.

 

On axis coma
 
You refer to inter-mirror distance and I'll take that to mean focal length.  I'm at the manufacturer specification of 1370mm but it was set by the manufacturer at 1413mm.  Maybe I take it back up to 1413mm and see if that gives me better corners?  It's going to have to be significant though since the back-focus is already ridiculous on these things and another 43mm (and another extension ring) doesn't excite me.

Edited by dg401, 03 August 2021 - 10:43 AM.

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#86 dg401

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Posted 03 August 2021 - 10:42 AM

@nebulachadnezzar said it all: very nice work!  waytogo.gif   The "pointyness" of the out-of-focus stars in the corners is all generally toward the center of the image, which could be just the residual field curvature of the RC.  A focal length which isn't quite what your mirrors demand will contribute to that too (as @neb said: inter-mirror distance, which sets your focal length).

 

If you want to improve this even further, I'd suggest a Ronchi test to get your focal length exact, then a field flattener.  I bet you'd get close to perfectin after that.

Rather than a Ronchi test and the associated cost of the device, I can try different focal lengths.  Maybe image the artificial star at 1390mm and 1410mm, see which one looks better, and then image at 10mm focal length to either side of that?

 

I tell you, it was the reading glasses (5 pack for $15 on Amazon) and the Celestron Cheshire ($39.95 on Amazon).  As for all the expensive stuff I bought that didn't do jack-spit to get collimated, at least my dog enjoys chasing the laser beam...

 

The only flattener that I currently have is a WO Flat 6AIII (0.8 reducer) for my 81mm f6.9.  Maybe I play adapter-ring-roulette and see if I can't make it play nice with my RC6.  Ultimately, I'm not a fan of reducers.  If I wanted a scope with 80% of the focal length, I'd buy it that way.

 

But seriously, have yourself a victory lap... It's not perfection without lasers or stars, but it's awfully close.  The hardest part is having the patience (and stubbornness) to fight it through.


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#87 nebulachadnezzer

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Posted 03 August 2021 - 10:49 AM

 

Thanks for your feedback.

 

I'm familiar with hand-masking to find which screw(s) best correspond to whichever side of a de-focused star.  It's generally the first step in the process of tweaking to make things worse.  In this case, it's very minor and I might actually make things better for a change.

 

Interesting that my tri-bahtinov mask sniffed out the on-axis coma.  There is is on the 10:00 - 4:00 axis.

 

You refer to inter-mirror distance and I'll take that to mean focal length.  I'm at the manufacturer specification of 1370mm but it was set by the manufacturer at 1413mm.  Maybe I take it back up to 1413mm and see if that gives me better corners?  It's going to have to be significant though since the back-focus is already ridiculous on these things and another 43mm (and another extension ring) doesn't excite me.

 

I think you would definitely find you could tweak that primary the tiny bit it needs and reach perfection for on-axis coma. I'm still quite impressed you got it this close on the bench.

I can see the effect in your tri-bahtinov image as well. Is that still using your artificial star? My read of your out-of-focus central star image says your needed adjustment is at 3 o'clock.

Yes, my reference to inter-mirror distance was in regard to focal length. However I don't know if you'll see improvement over what you have. I do think you're at least approaching the limits of the RC6. However since you've been adjusting the focal length I thought it might be something you could try. It may be diminishing returns, though.

I believe that for an APS-C sensor you're still going to need a flattener on an RC6. The TS RCFlat2 seems to be a good one. The downside is that it needs 109mm of back focus. That said, you can insert it fairly far into your focuser so it's not like you're adding 109mm after the focuser.


Edited by nebulachadnezzer, 03 August 2021 - 10:51 AM.

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#88 TinySpeck

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Posted 03 August 2021 - 10:52 AM

Rather than a Ronchi test and the associated cost of the device, I can try different focal lengths.  Maybe image the artificial star at 1390mm and 1410mm, see which one looks better, and then image at 10mm focal length to either side of that?

 

I tell you, it was the reading glasses (5 pack for $15 on Amazon) and the Celestron Cheshire ($39.95 on Amazon).  As for all the expensive stuff I bought that didn't do jack-spit to get collimated, at least my dog enjoys chasing the laser beam...

 

The only flattener that I currently have is a WO Flat 6AIII (0.8 reducer) for my 81mm f6.9.  Maybe I play adapter-ring-roulette and see if I can't make it play nice with my RC6.  Ultimately, I'm not a fan of reducers.  If I wanted a scope with 80% of the focal length, I'd buy it that way.

 

But seriously, have yourself a victory lap... It's not perfection without lasers or stars, but it's awfully close.  The hardest part is having the patience (and stubbornness) to fight it through.

Thank you, sir!  This is really great news, for both of us.  Glad your dog likes the laser, too, although that's a pretty expensive dog toy....

 

@neb's post just came in, and he said what I was going to.  Note that you'll need to re-do collimation after each focal length adjustment, so the results might vary a little and it would be hard to pick the best one.


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#89 dg401

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Posted 03 August 2021 - 10:54 AM

I think you would definitely find you could tweak that primary the tiny bit it needs and reach perfection for on-axis coma. I'm still quite impressed you got it this close on the bench.

I can see the effect in your tri-bahtinov image as well. Is that still using your artificial star? My read of your out-of-focus central star image says your needed adjustment is at 3 o'clock.

Yes, my reference to inter-mirror distance was in regard to focal length. However I don't know if you'll see improvement over what you have. I do think you're at least approaching the limits of the RC6. However since you've been adjusting the focal length I thought it might be something you could try. It may be diminishing returns, though.

I believe that for an APS-C sensor you're still going to need a flattener on an RC6. The TS RCFlat2 seems to be a good one. The downside is that it needs 109mm of back focus. That said, you can insert it fairly far into your focuser so it's not like you're adding 109mm after the focuser.

Rather than tighten at 3:00, maybe I play the contrarian and loosen at 9:00? 

 

I'll show myself out again.  lol.gif
 



#90 nebulachadnezzer

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Posted 03 August 2021 - 11:04 AM

The only flattener that I currently have is a WO Flat 6AIII (0.8 reducer) for my 81mm f6.9.  Maybe I play adapter-ring-roulette and see if I can't make it play nice with my RC6.  Ultimately, I'm not a fan of reducers.  If I wanted a scope with 80% of the focal length, I'd buy it that way.

Unfortunately your WO reducer/flattener almost certainly won't give you good results with an RC. Refractors typically require 3-element correctors that are tweaked to the needs of a particular 3-element lens objective. RCs only need a two-element flattener.

 

Rather than tighten at 3:00, maybe I play the contrarian and loosen at 9:00? 

 

I'll show myself out again.  lol.gif
 

Same difference, except of course which way you're moving the mirror does also impact inter-mirror distance. That said, if it's the difference between tweaking one screw vs two, and you're more comfortable with that, there's no other reason you couldn't do it that way.



#91 TinySpeck

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Posted 03 August 2021 - 11:04 AM

I tell you, it was the reading glasses (5 pack for $15 on Amazon) and the Celestron Cheshire ($39.95 on Amazon).

...  The hardest part is having the patience (and stubbornness) to fight it through.

I should include a test verification step: if you make a 1/20 turn adjustment and you DON'T see an effect, you're not looking hard enough.  Unfortunately the edit window on my original post has closed so people will need to wade this far through the thread to find this pearl.  So I repeat: 

 

If you make a 1/20 turn adjustment and you DON'T see an effect, you're not looking hard enough.


Edited by TinySpeck, 03 August 2021 - 11:07 AM.

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#92 dg401

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Posted 03 August 2021 - 11:14 AM

Thank you, sir!  This is really great news, for both of us.  Glad your dog likes the laser, too, although that's a pretty expensive dog toy....

 

@neb's post just came in, and he said what I was going to.  Note that you'll need to re-do collimation after each focal length adjustment, so the results might vary a little and it would be hard to pick the best one.

I'm thrilled all to bits that I can finally delete that overly-complex DSI abomination from my desktop.

 

I think the point that bears repeating is that with your procedure, you and some schmo have both surpassed all the lasers and holographic projections and gotten all the way to just a single 1/20th turn micro-tweak from perhaps as good as it gets.

 

But here's the thing, people are a stubborn lot.  The market for collimation lasers and other expensive gadgets is not going to crash anytime soon.  Folks will continue to insist it takes $$$ and a sympathetic glance from the deity of optics to get these GSO RCs collimated.  But, as we say in the South, "I'm here to tell you, t'aint so."


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#93 dg401

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Posted 03 August 2021 - 11:17 AM

Unfortunately your WO reducer/flattener almost certainly won't give you good results with an RC. Refractors typically require 3-element correctors that are tweaked to the needs of a particular 3-element lens objective. RCs only need a two-element flattener.

 

Same difference, except of course which way you're moving the mirror does also impact inter-mirror distance. That said, if it's the difference between tweaking one screw vs two, and you're more comfortable with that, there's no other reason you couldn't do it that way.

Two elements?  You mean an RC requires a less expensive flattener than other scopes?

 

For once it's not the more expensive option!


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#94 dg401

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Posted 03 August 2021 - 11:25 AM

I should include a test verification step: if you make a 1/20 turn adjustment and you DON'T see an effect, you're not looking hard enough.  Unfortunately the edit window on my original post has closed so people will need to wade this far through the thread to find this pearl.  So I repeat: 

 

If you make a 1/20 turn adjustment and you DON'T see an effect, you're not looking hard enough.

With these screws, 1/20th of a turn is often more a case of the screw "jerking" than turning smoothly.  A very sparing application of lubricant to help screws turn smoothly can really help.  I use "Tri-flow" lubricant on all the over-priced moving parts when I PM customer instruments.  Good stuff.  Just go easy, it works well on screws, not so much on mirrors.


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#95 Brent Campbell

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Posted 03 August 2021 - 11:48 AM

Don’t want to sound snobby here.  But for all the time put into this just get the hotech sct collimator.  It also works for any cases scope.   It has a process for centering the device on the mirror.  It’s expensive at $545 but it solves the issue..  it also saves observing time because you start out collimated.  



#96 TinySpeck

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Posted 03 August 2021 - 11:55 AM

Don’t want to sound snobby here.  But for all the time put into this just get the hotech sct collimator.  It also works for any cases scope.   It has a process for centering the device on the mirror.  It’s expensive at $545 but it solves the issue..  it also saves observing time because you start out collimated.  

There has been 10x the time put into this thread than the actual procedure requires.  It's really a simple process.

 

I tried the Hotech collimator some time ago and didn't have good results.  Partly because the laser holograms are fuzzy (see the very first post in this thread) and partly because it was extremely sensitive to alignment and vibration.  Talk about time-consuming!  I'm glad it worked for you though.



#97 nebulachadnezzer

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Posted 03 August 2021 - 11:59 AM

Two elements?  You mean an RC requires a less expensive flattener than other scopes?

 

For once it's not the more expensive option!

Yep. The RC produces an inherently flatter image than I believe any other uncorrected design in a consumer telescope. The downside is how hard they are to collimate (once you figure out how to correctly grind the mirrors -- ask the HST development team about that).

Maybe if someone starts producing a consumer grade three-mirror anastigmat like the JWST the RC can be beat, but I bet collimation of that would make RCs look like a walk in the park. :)



#98 dg401

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Posted 03 August 2021 - 12:03 PM

Don’t want to sound snobby here.  But for all the time put into this just get the hotech sct collimator.  It also works for any cases scope.   It has a process for centering the device on the mirror.  It’s expensive at $545 but it solves the issue..  it also saves observing time because you start out collimated.  

My thought is this.  The time is investment.  The payoff is that I don't need a $545 gadget to collimate a $500 scope.

 

My experience was that I performed this procedure and got crappy results... so I performed it again and again... each time I got better at it and worked out more bugs..  For me, the magic bullet was a $3 pair of reading glasses.

 

And now I use my Farpoint laser to entertain my dog.  And I'm glad that it does entertain my dog.  Because you know what it can't do?  Collimate a Ritchey–Chrétien.


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#99 dg401

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Posted 03 August 2021 - 12:05 PM

Yep. The RC produces an inherently flatter image than I believe any other uncorrected design in a consumer telescope. The downside is how hard they are to collimate (once you figure out how to correctly grind the mirrors -- ask the HST development team about that).

Maybe if someone starts producing a consumer grade three-mirror anastigmat like the JWST the RC can be beat, but I bet collimation of that would make RCs look like a walk in the park. smile.gif

The HST team knew how to grind a mirror.  Assembling a null-corrector properly, not so much.
 


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#100 nebulachadnezzer

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Posted 03 August 2021 - 12:08 PM

Don’t want to sound snobby here.  But for all the time put into this just get the hotech sct collimator.  It also works for any cases scope.   It has a process for centering the device on the mirror.  It’s expensive at $545 but it solves the issue..  it also saves observing time because you start out collimated.  

 

There has been 10x the time put into this thread than the actual procedure requires.  It's really a simple process.

 

I tried the Hotech collimator some time ago and didn't have good results.  Partly because the laser holograms are fuzzy (see the very first post in this thread) and partly because it was extremely sensitive to alignment and vibration.  Talk about time-consuming!  I'm glad it worked for you though.

The Hotech SCT collimator isn't a hologram but a target-based laser reflector device. That said, I'm not a fan. And yes, a big part of the problem with it is the imprecision of diode lasers.

I like to buy and try these things, so as a result I own the Hotech SCT collimator. I can't agree that it's of any use at all with an RC, and I'm not convinced it's even a superior method for a SCT. The only advantage I see for it for SCTs is that it can be used in close proximity (vs an SCT needing an artificial star to be placed at a fairly large distance, like 60-100m away).

However, I've never gotten a good result with mine on my C11 XLT. Yes I went through all of the procedures to align it to the scope and triple-checked everything because I couldn't believe the scope was so bad from the factory. In all likelihood it was fine.

The resulting collimation was trashed. I fixed the SCT under the stars in five minutes. SCT collimation is a no-brainer, really. Corrector alignment isn't, but that's a different story.


Edited by nebulachadnezzer, 03 August 2021 - 04:43 PM.

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