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Diffraction rings not visable?

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#1 qns

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Posted 27 February 2021 - 09:34 PM

Greetings,

Question on my new Meade LX85 6" f/10 ACF UHTC Catadioptric Telescope (OTA.

I'll keep my question short and sweet.

No matter how long outside, the star or the power all I see going thru focus is a nice

round doughnut with absolutely no diffraction rings. Is hard to imagine nothing I have

tried produced only a single doughnut.  I see tons of pics with multiple rings and some

like mine with none.  I have read plenty and don't know what to do.  Don't want to send

it back if there is something basic and I just don't get it, but it seems to me I should

have seen a few nice round concentric rings.

Thanks for any thoughts on this.

Joe

 

 



#2 dustyc

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Posted 27 February 2021 - 09:38 PM

Use the highest power eyepiece you have. When I tried to do the same with my old C8 I had to use an 11mm plossl to get a good pattern.


Edited by dustyc, 27 February 2021 - 09:39 PM.


#3 decep

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Posted 27 February 2021 - 09:40 PM

In 2 years, I have never seen diffraction rings either.  Even with a 12" SCT.

 

I think it comes down to "seeing".  I do not think I have ever had anything other than average seeing.  The air is never stable enough to see the diffraction rings where I live (GA).


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#4 Jon Isaacs

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Posted 27 February 2021 - 09:42 PM

Seeing diffraction rings requires stable seeing and a scope that is thermally stable.  And the larger the scope, the more difficult it is because everything is concentrated into a smaller area.  Seeing diffraction rings in a 60mm is not difficult.  

 

Jon


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#5 Stellar1

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Posted 27 February 2021 - 11:07 PM

Same here, have never seen them in my CPC1100, I gave up trying after many attempts and just figured my model didn’t come with those lol, views were great anyway.


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#6 maroubra_boy

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Posted 27 February 2021 - 11:22 PM

The only way to see diffraction rings is with sharp focus of a bright star at high magnification.

 

Think about it, as you defocus, the energy of the star is being spread out, and the larger the doughnut, the more and more this energy is being spread out.  With the diffraction rings, as they are much fainter than the star at the centre, the energy is diluted even faster as you defocus.  You will not see diffraction rings around a defocused star if the doughnut is large and especially if the optical quality of the scope isn't great.

 

With my C8 I was only able to ever see one diffraction ring at most.  With my 9" Deluxe Russian Mak, I can see as many as 8 rings around Sirius.  The difference being entirely optical quality - the better the photons are controlled to go where they are supposed to, the faint energy of the outer diffraction rings will also fall into place where they should.  With my 7" Deluxe Mak, I could see up to 7 rings.  A friend's standard Russian 6" Mak only shows 3 diffraction rings.  When defocused, what use are the defuse diffraction rings anyway?  It is the central doughnut we want.  A large doughnut for course/gross collimation, and then a small, tight doughnut to do the collimation fine tuning as with a large doughnut the energy is spread out a large amount, and this will also cancel any remaining small mis-collimation that may still be present.  A small doughnut will reduce this error and show up whatever misalignment remains.

 

If the scope is not at equilibrium or insulated, the rings will still be there, but disrupted.  Before I wrap my 9" Mak, I am able to see the rings without problem, but all broken up and shimmering.

 

Alex.


Edited by maroubra_boy, 27 February 2021 - 11:33 PM.


#7 Dave Ponder

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Posted 28 February 2021 - 12:00 AM

I think 25-30X per inch is a good start for magnification.  Like others have stated, “seeing” is very important as well as thermal equalization.  Then there is collimation.  If you want to see the rings, make an artificial star.  Take a flashlight, cover the end with aluminum foil, make the tiniest hole you can with a fine needle, put the flashlight 40 ft or more away if possible inside and see what you get.  You should see a nice diffraction pattern.  You may see that your collimation needs work. (I am not starting a discussion as to the best separation distance between the scope and the artificial star).  I remember thinking something was wrong with my first SCT because I could not see the rings.  My scope ended up being just fine after I collimated the scope, allowed the scope time to “cool down”,  used high power on a really good night of seeing.

Clear skies!


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#8 maroubra_boy

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Posted 28 February 2021 - 12:06 AM

Yes, I forgot to mention how important good, stable seeing conditions are as well.  Poor seeing will mean the diffraction rings will be disrupted and broken up.

 

Collimation needs to be done at high magnification too in order to be sure that errors are eliminated.

 

If you collimate using a devise of some kind, laser or whatever, you should still verify by star testing, always.  THIS is the final proof of the pudding with all scopes.  Professional instruments will have initial collimation done with a laser, but always verified by star testing.  And star testing is done with a small doughnut, not large.  Remember, a large doughnut will even out whatever small error may still be present.  A small doughnut means a much tighter image and shows up these wee errors need tweeking.


Edited by maroubra_boy, 28 February 2021 - 12:11 AM.

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#9 maroubra_boy

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Posted 28 February 2021 - 12:17 AM

There was one C8 I looked through that showed up to 5 diffraction rings!  Absolutely stunning unit!  There are some mighty fine mass production SCT's out there, but it means looking through a lot of them to find them.  This particular C8 I was pitting it against a 7" Intes Mak to see which I was going to keep.  The Intes won by a bee's willy.  If I hadn't come across that Russian Mak, I would have been totally happy with that C8.

 

In that shoot out, both scopes were collimated so tightly you could just about hear them squeal.  I still know where that C8 calls home - good to keep tabs on where the good scopes are! grin.gif


Edited by maroubra_boy, 28 February 2021 - 12:19 AM.


#10 qns

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Posted 28 February 2021 - 01:58 AM

Thank you all for the comments, CN is a great resource. I'm feeling encouraged to keep

up the good fight.  I have two weeks to work on this so I'll try one of those DIY artificial stars at about 100', jack up the magnification far as I can while paying attention to those other parameters and take a pic of what I'm seeing. 

The search for the mysterious and elusive diffraction rings continues.  I did read someplace where a poor main mirror finish could result in a "busy" round doughnut with no diffraction rings. So much has been written about them it is natural to believe one has a problem when they are not showing up. 

Thanks again,

Joe



#11 Jon Isaacs

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Posted 28 February 2021 - 02:22 AM

The brightness of the diffraction rings also depends on the size of the central obstruction. SCTs are generally between 0.3D and 0.4D.  0.0D would be a refractor, A newtonian could be anywhere from 0.14D on up but is generally around 0.2D-0.25D. 

 

https://www.telescop...obstruction.htm

 

central_obstruction_PSF.PNG

 

 

https://www.telescop...obstruction.htm

 

Jon


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#12 maroubra_boy

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Posted 28 February 2021 - 03:34 AM

The search for the mysterious and elusive diffraction rings continues. I did read someplace where a poor main mirror finish could result in a "busy" round doughnut with no diffraction rings. So much has been written about them it is natural to believe one has a problem when they are not showing up.
Thanks again,
Joe

Mate, FORGET THE DOUGHNUT!!

The only way to see whatever diffraction rings your scope can show is with the star in sharp focus.

You have been proving to yourself all this time that the faint diffraction rings cannot be seen with the star as a doughnut all along. Make sure your scope is well collimated, focus the bright star sharp, have your scope acclimated or insulated and under good seeing conditions whatever rings are to be seen will be shown to you.

These rings are of low energy. Defocus the bright star & all you are doing is disperse into nothing the little energy that these feeble rings have.

Edited by maroubra_boy, 28 February 2021 - 03:45 AM.


#13 luxo II

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Posted 28 February 2021 - 04:09 AM

OP you need to focus sharply - no donuts - and use a 6-7mm eyepiece. It is also possible that you won’t see diffraction rings- just a “ circle of least confusion”; I’ve owned two SCTs like that before.


Edited by luxo II, 28 February 2021 - 04:11 AM.


#14 PETER DREW

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Posted 28 February 2021 - 07:13 AM

There is a common confusion between extrafocal rings and diffraction rings.  Best to define which before seeking advice.


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#15 qns

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Posted 28 February 2021 - 12:09 PM

Very helpful Pete very helpful.

#16 btschumy

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Posted 28 February 2021 - 01:16 PM

There is a common confusion between extrafocal rings and diffraction rings.  Best to define which before seeking advice.

Yes, I'm not sure what maroubra_boy is referring to when he says he sees 8 diffraction rings around Sirius.  I've never heard of seeing so many diffraction rings.  The number (and brightness) of rings is related to the size of the central  obstruction, as Jon referenced in his post above.  I would think you'd have to have a massive obstruction for 8 rings to be visible.  I wouldn't consider this a good thing.


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#17 maroubra_boy

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Posted 28 February 2021 - 04:48 PM

Hi Bill,

 

What I have noticed with scopes that are of a known quality, such as these Intes Maks, is the better the quality, the more diffraction rings are seen around the very brightest of stars.  This is because the photons are all going where they are supposed to, so the energy of these outer rings would will all be going exactly to the position that they should.  And this too depends on the prevailing seeing conditions.

 

The central obstruction of Maks tends to also be smaller than that of mass production SCTs, so it not the size of the central obstruction that's at play.  Most of these SCT's struggle to show one diffraction ring.  But with these Russian made optics, the standard 1/6th wave scopes can show up to four rings, and my 1/8th wave scopes have shown 7 rings (a 7" Mak I had) or 8 rings (my current 9" and a friend's 10") with the brightest of stars, and the central obstruction of these scopes is between 26% and 29%.  With the few really good mass production SCT's I have encountered, these too showed multiple diffraction rings, but these scopes are very few and far between.  Mass production Maks can also show one or a few diffraction rings, again depending on the optical quality of each individual instrument.

 

And yes, this can be a problem with particularly bright stars, like Sirius, no denying that.  And thankfully this is not a problem often encountered.  Nor is it a problem with the Moon or planets as these are extended objects and not point sources of light, so there is no diffraction pattern associated with these.

 

Alex.


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#18 AJK 547

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Posted 04 March 2021 - 02:46 PM

The only way to see diffraction rings is with sharp focus of a bright star at high magnification.

 

Think about it, as you defocus, the energy of the star is being spread out, and the larger the doughnut, the more and more this energy is being spread out.  With the diffraction rings, as they are much fainter than the star at the centre, the energy is diluted even faster as you defocus.  You will not see diffraction rings around a defocused star if the doughnut is large and especially if the optical quality of the scope isn't great.

 

With my C8 I was only able to ever see one diffraction ring at most.  With my 9" Deluxe Russian Mak, I can see as many as 8 rings around Sirius.  The difference being entirely optical quality - the better the photons are controlled to go where they are supposed to, the faint energy of the outer diffraction rings will also fall into place where they should.  With my 7" Deluxe Mak, I could see up to 7 rings.  A friend's standard Russian 6" Mak only shows 3 diffraction rings.  When defocused, what use are the defuse diffraction rings anyway?  It is the central doughnut we want.  A large doughnut for course/gross collimation, and then a small, tight doughnut to do the collimation fine tuning as with a large doughnut the energy is spread out a large amount, and this will also cancel any remaining small mis-collimation that may still be present.  A small doughnut will reduce this error and show up whatever misalignment remains.

 

If the scope is not at equilibrium or insulated, the rings will still be there, but disrupted.  Before I wrap my 9" Mak, I am able to see the rings without problem, but all broken up and shimmering.

 

Alex.

Alex, thanks for the explanation... I’ve learned a lot in this thread!

 

FWIW,  I have a 2019 replacement C8-A that the Celestron QA team selected from their QA’d inventory stash,  then Celestron bench tested... collimated... and then sent it me for testing and collimation refinement.   All I can say is this particular C8-A can REALLY take mag., has presented me with thrilling visual views of targets like the Vallis Alpes rille, Ina IMP, the small ‘Craterlets’ next to Aristarchus peak, etc.  Totally happy with this particular unit.  

 

My question is ‘What star should I select (Sirius?) to get an idea on the optical quality of this C8-A with respect to the diffraction ring development”.  I’ve side-by-side tested this C8-A against a friends hand built Planetary Newt 8” (Null tested and is ~1/8 wave), and my C8-A easily develops more solid... progressively fainter diff rings around the Airy disk during an evening of excellent stable conditions.  After numerous nights of side-by-side testing, I honestly couldn’t detect where his dedicated planetary Newt out performed the C8-A in resolving capabilities.  The only area I could see where the 8” newt performed a bit better was in slightly better contrast due to less CO size.

 

Any further insight is much appreciated.

 

*** Honestly, I’ve always thought the less # of diffraction rings ... the better the optics!  Dumb me! smirk.gif

 


Edited by AJK 547, 04 March 2021 - 07:20 PM.


#19 maroubra_boy

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Posted 04 March 2021 - 05:29 PM

The "best" star is the brightest one closest to zenith.  Sirius for me comes very close to zenith.  What is most important though is seeing conditions.  They must be as good as possible because you need to rip high magnification for this exercise.  So if seeing is good, then chose the brightest star you can that is say at least 45deg above the horizon.  The higher the better.

 

You will also need to be dark adapted to give your eyes every chance.  The outer rings will be very faint, and even require averted vision to see the outer most.

 

Don't beat yourself up over this.  This is starting to get into the realm of nerds within nerds that is astro.  Until I started to see for myself that mass production SCT's are not created equal I would not have bothered investigating the why and then having the veil lifted off my eyes to what exceptional optics can actually do... bigshock.gif

 

When star testing, no matter what scope you are using, keep the star in the centre.

 

Alex.

 

PS:  You MUST have the collimation of your scope really tight to the point that it is squealing.  Mirror flop in some scopes makes this a near impossible task.  If your scope shows mirror flop, you must use a very disciplined method to focus your scope by winding out a little in just ONE direction, and then slowing wind in to achieve focus.  If you go too far then wind out again and  then in again to focus.  Do not try to focus by winding back out if you have gone too far.  This exact same pattern you must do with collimation.  Wind the focuser out to just past where you want the donut's size to be, and then wind in that small touch.  NOW is when you collimate.  Because with this discipline when you are focusing your scope this action will set the mirror back into its optimal position with collimation too.  Not follow this regime of focus and collimation, and mirror flop will always put the optics out of whack.

 

Also, if your scope shows mirror flop, do not attempt to collimate with the scope set horizontal.  It MUST be pointed up to at least 45/60deg elevation as this is the working position of the mirror.  Having the scope horizontal is not putting the same load on the primary mirror and it does not move in the same way.

 

Alex.


Edited by maroubra_boy, 04 March 2021 - 05:55 PM.

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#20 AJK 547

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Posted 04 March 2021 - 05:58 PM

Thanks for responding Alex!

 

Clear,  steady nights.



#21 AJK 547

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Posted 04 March 2021 - 07:42 PM

The "best" star is the brightest one closest to zenith.  Sirius for me comes very close to zenith.  What is most important though is seeing conditions.  They must be as good as possible because you need to rip high magnification for this exercise.  So if seeing is good, then chose the brightest star you can that is say at least 45deg above the horizon.  The higher the better.

 

...

 

PS:  You MUST have the collimation of your scope really tight to the point that it is squealing.  Mirror flop in some scopes makes this a near impossible task.  If your scope shows mirror flop, you must use a very disciplined method to focus your scope by winding out a little in just ONE direction, and then slowing wind in to achieve focus.  If you go too far then wind out again and  then in again to focus.  Do not try to focus by winding back out if you have gone too far.  This exact same pattern you must do with collimation.  Wind the focuser out to just past where you want the donut's size to be, and then wind in that small touch.  NOW is when you collimate.  Because with this discipline when you are focusing your scope this action will set the mirror back into its optimal position with collimation too.  Not follow this regime of focus and collimation, and mirror flop will always put the optics out of whack.

 

Also, if your scope shows mirror flop, do not attempt to collimate with the scope set horizontal.  It MUST be pointed up to at least 45/60deg elevation as this is the working position of the mirror.  Having the scope horizontal is not putting the same load on the primary mirror and it does not move in the same way.

 

Alex.

Great point Alex.  This particular C8-A shows almost no ‘mirror flop’ during intra/extra focus, and I also only ‘fine collimate’ during an evening of VG+ or > seeing conditions.  I’ve used the final focusing step of moving the primary mirror ‘against gravity’ with all my SCT’s to insure the mirror sled is precisely repositioned for final focus or fine collimation.  I never collimate on any target close to horizon.

 

 FWIW, if I have a rare night of excellent seeing, I will ‘touch up’ my fine collimation in the same elevation/altitude as my intended targets for that evening.  I believe I read this last ‘trick’ in an article by Damion Peach on ‘tuning’ his SCT’s prior to an AP session.

 

*** I’m stoked to observe just how many diff. rings this C8-A produces! 
 

Again, thanks mate! 
 

Clear, steady nights


Edited by AJK 547, 04 March 2021 - 09:00 PM.

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#22 luxo II

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Posted 05 March 2021 - 01:50 AM

*** I’m stoked to observe just how many diff. rings this C8-A produces! 

Unfortunately that isn't necessarily a "A Good Thing".

 

Seeing 2, maybe 3 is one thing. Four, maybe five perhaps, on Sirius. Any more and I'd say the scope isn't fully corrected for SA.

 

I'd collimate near the zenith, and leave it at that.


Edited by luxo II, 05 March 2021 - 01:59 AM.

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#23 AJK 547

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Posted 05 March 2021 - 08:20 AM

luxo II, thanks for the response!

 

 I spent quite a bit of time comparing my C8-A against the dedicated Planetary Newt. (over a period of a few observing weeks).  What I noticed with the C8-A was that I could easily discern three (3) diffraction 'rings' around the airy disk.  The 1st ring was 'solid + sharp',completely formed with a little less energy than the airy disk...the 2nd diff. ring was again fully formed but with significantly less energy... the 3rd diff. ring was fully formed and once again sharp, but barely detectable. I did not use Sirius as the target star during those summer tests.  I believe I used Altair at that time...

 

 

***I find the discussion on this optical artifact simply fascinating, and I will report back (if anyone cares) when I can get the scope out again this spring under excellent 'seeing' conditions. 

 

No one will believe this, but one of my C6's also has similar relative optical performance as the C8-A.  Ex. one evening with VG+ seeing and no jet stream, this particular Evo6 gave me views of 4 - 5 elongated dark brown 'barges' (of extremely sharp definition) in the NEB.  It also is refractor like in that it has literally NO mirror shift during focus.  An astronomer friend had the chance to look through this Evo6 a while back, and within 5 sec. of viewing he was shocked of the 'visuals' and surprised it had no 'mirror shift'.  In another session, when I shifted my attention to Saturn, Cassini was razor sharp and I repeatedly saw what looked like the Encke gap/division (not minima) as a VERY FEINT dark line in the far outer edge of 'A' ring.   I'm thinking I'll put this little Evo6 also up to the diffraction ring test!

 

To all... Clear and Steady nights.


Edited by AJK 547, 05 March 2021 - 12:10 PM.


#24 charlesgeiger

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Posted 05 March 2021 - 07:44 PM

What I have read is the larger the secondary obstruction, the brighter the first diffraction ring will be.  So Jon's information above nails it.  So more energy is poured into the first diffraction ring and if the optics are great in an obstructed scope, you can actually get more point source resolution as the airy disk is smaller.  But many obstructed telescopes blend the airy disk with the first diffraction ring which lessens the resolution and produces more light scatter.

But the more diffraction rings you may see can be caused by spherical aberration.  What you should see with a great optical scope is the first through third diffraction ring on bright point sources.  I have Sutier's book but it is in storage so I can't reference it.  Again, I have read that more rings on bright point sources can be caused by spherical aberration and I have read that in different sources.  Maybe these references are incorrect but I would suspect many diffraction rings can cause reduction in contrast.  YMMV

Charlie


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#25 AJK 547

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Posted 05 March 2021 - 10:14 PM

Charlie, thanks for the explanation.  I have Suiter’s book and I’ll have to carefully gleen (as much information as I can understand) to get a better handle on this topic.  All I can say is the ‘visuals’ between the C8-A and my friends Planetary newt were so equivalent I couldn’t detect a clear winner.  As noted earlier, his 8” newt did possess a ‘hint’ better contrast (probably based on smaller CO) ...but on pure visual resolving capabilities (Luna/Planetary/tight doubles, etc) it was a dead tie... over many nights of side-by-side comparison under varying seeing conditions.

 

Further testing this spring should be fun!

 

Clear, steady nights.


Edited by AJK 547, 05 March 2021 - 10:17 PM.



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