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#26 TOMDEY

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Posted 22 June 2019 - 08:50 AM

Hey, question for the optical experts - Does it matter where one places the Mask? I mean does it matter if the mask is near the primary mirror versus near the top of the Dob? Could that alter how sharp the image is?

Generally not. It only introduces a tiny amount of field-dependent differential astigmatism.

 

It matters only slightly. What it does is to make the footprint on the PM somewhat field dependent. In a Newtonian, that dependence is given by my cartoon down there.

 

The equation is approx. Angles are in radians. The field-dependent clocking and parity of the projection displacement is pretty obvious.

 

e.g. For a 6-inch off-axis stop on the mouth of a 16-inch F/4 Newt looking at a star half a degree out in the field... the offset of the footprint on the PM is sheared about 9%, around half an inch, half a degree out in the field.

 

It's OK to ignore that stuff... almost entirely academic.    Tom

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  • 100 Newtonian Mask Aperture Stop Shear at PM.jpg


#27 Darren Drake

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Posted 22 June 2019 - 02:17 PM

It's best to place the aperture stop furthest away from the observer so as to minimize thermal noise from the observer's body heat 



#28 slavicek

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Posted 22 June 2019 - 11:14 PM

Generally not. It only introduces a tiny amount of field-dependent differential astigmatism.

 

It matters only slightly. What it does is to make the footprint on the PM somewhat field dependent. In a Newtonian, that dependence is given by my cartoon down there.

 

The equation is approx. Angles are in radians. The field-dependent clocking and parity of the projection displacement is pretty obvious.

 

e.g. For a 6-inch off-axis stop on the mouth of a 16-inch F/4 Newt looking at a star half a degree out in the field... the offset of the footprint on the PM is sheared about 9%, around half an inch, half a degree out in the field.

 

It's OK to ignore that stuff... almost entirely academic.    Tom

Thanks Tom for the explanation - as always bow.gif . I've been building a contraption (=aperture mask holder) which will place the aperture mask about 1/2" from the primary mirror. I went by my "feel". "Feel" told me that the closer the mask to the mirror the sharper the image. Your equations say otherwise so I am likely wrong - I hope I will be able to test my masks soon on the planets. Problem is that there are so many contraptions to test, but not enough time... lol.gif



#29 TOMDEY

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Posted 23 June 2019 - 07:40 AM

Thanks Tom for the explanation - as always bow.gif . I've been building a contraption (=aperture mask holder) which will place the aperture mask about 1/2" from the primary mirror. I went by my "feel". "Feel" told me that the closer the mask to the mirror the sharper the image. Your equations say otherwise so I am likely wrong - I hope I will be able to test my masks soon on the planets. Problem is that there are so many contraptions to test, but not enough time... lol.gif

The nice thing is that this kinda stuff is quick, easy, cheap, and non-destructive.

 

I stuck them on the front of my ~field-use~ telescopes , like a loose cap, so I could spin them around while looking thru the eyepiece. That lets one do an in-the-blind empirical experiment to ferret out the ~best image/wavefront~. And then look to see what clocking produces the most pleasing rendition. Turns out that arm-waving theory and practice are not always well-correlated!

 

Other things to experiment with are hole size and radial location. At work, I even put a GIANT iris there and expand/contract while looking at the very high magnification image of a point source, coming from a GIANT collimator!

 

I've always been both a theorist and experimentalist. Analyze the heck outa something and only then run lab and field experiments to see what really happens!    Tom

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  • 104 Lab Dahl Collimator annotated.jpg


#30 Vla

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Posted 24 June 2019 - 09:14 AM

To take a closer look at the mask effects, here's what it looks like in OSLO, with a 560mm f/3.6 mirror. Tom was, of course, right that there is some astigmatism added with the mask at the top vs. mask at the mirror (roughly 10%), but another interesting effect is that the best image field is now tilted in the plane telescope axis and mask's diameter, perpendicular to the central ray coming through the mask. In this case it is 5 degrees, which is quite a bit. At 17.5mm off axis it throws best focus 1.5mm out of the central point's plane. If we take twice lower height, so that it can fit into a 10mm ultrawide eyepiece, it is still 0.75mm off, i.e. requiring about 7.5 diopters accommodation. Since it is nearly 1/9 of the eye f.l. of 59 diopters, it corresponds to accommodation required from infinity to an object at 9 times the eye physical f.l. of 23mm, or about 8 inches. That is better than the average eyesight, so, depending on individual accommodation limit, the diffraction image can appear as any of the shapes between fully accommodated and no accommodation (top). Field along the perpendicular radius has no tilt, but does have tilt component built into the wavefront, which makes it look more like coma (in either case, astigmatism is the dominant aberration but, since it originates in the mirror's coma, it changes with the field radius, not with the square of it, as does regular astigmatism).

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  • msk.png

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#31 TOMDEY

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Posted 24 June 2019 - 10:16 AM

To take a closer look at the mask effects, here's what it looks like in OSLO, with a 560mm f/3.6 mirror. Tom was, of course, right that there is some astigmatism added with the mask at the top vs. mask at the mirror (roughly 10%), but another interesting effect is that the best image field is now tilted in the plane telescope axis and mask's diameter, perpendicular to the central ray coming through the mask. In this case it is 5 degrees, which is quite a bit. At 17.5mm off axis it throws best focus 1.5mm out of the central point's plane. If we take twice lower height, so that it can fit into a 10mm ultrawide eyepiece, it is still 0.75mm off, i.e. requiring about 7.5 diopters accommodation. Since it is nearly 1/9 of the eye f.l. of 59 diopters, it corresponds to accommodation required from infinity to an object at 9 times the eye physical f.l. of 23mm, or about 8 inches. That is better than the average eyesight, so, depending on individual accommodation limit, the diffraction image can appear as any of the shapes between fully accommodated and no accommodation (top). Field along the perpendicular radius has no tilt, but does have tilt component built into the wavefront, which makes it look more like coma (in either case, astigmatism is the dominant aberration but, since it originates in the mirror's coma, it changes with the field radius, not with the square of it, as does regular astigmatism).

Very good! Doug Sinclair was my advisor at the U of R way back when. ~RIP~ I wrote some software for him. He was one of the first to develop good user-friendly optical design and evaluation software. Nice to see it is still being used and developed.



#32 Vla

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Posted 24 June 2019 - 01:51 PM

What was really nice is making that program available for free use as a quite extensive version, more than anyone else's I know of.



#33 slavicek

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Posted 25 June 2019 - 04:57 PM

To take a closer look at the mask effects, here's what it looks like in OSLO, with a 560mm f/3.6 mirror. Tom was, of course, right that there is some astigmatism added with the mask at the top vs. mask at the mirror (roughly 10%), but another interesting effect is that the best image field is now tilted in the plane telescope axis and mask's diameter, perpendicular to the central ray coming through the mask. In this case it is 5 degrees, which is quite a bit. At 17.5mm off axis it throws best focus 1.5mm out of the central point's plane. If we take twice lower height, so that it can fit into a 10mm ultrawide eyepiece, it is still 0.75mm off, i.e. requiring about 7.5 diopters accommodation. Since it is nearly 1/9 of the eye f.l. of 59 diopters, it corresponds to accommodation required from infinity to an object at 9 times the eye physical f.l. of 23mm, or about 8 inches. That is better than the average eyesight, so, depending on individual accommodation limit, the diffraction image can appear as any of the shapes between fully accommodated and no accommodation (top). Field along the perpendicular radius has no tilt, but does have tilt component built into the wavefront, which makes it look more like coma (in either case, astigmatism is the dominant aberration but, since it originates in the mirror's coma, it changes with the field radius, not with the square of it, as does regular astigmatism).

I have pretty much the mirror you used in your analysis. So, as an amateur building an aperture mask, what lesson shall I take from your analysis?

I will have the mask near the PM (my goal is about 0.5" above it with adjustable mask tilt) which means that the light beam will pass thru it twice. First going down to the PM and then it will pass it again on it's way up to the secondary mirror - then the beam of light will be under slight angle. Is that what you were talking about?

So, ideally one should place the mask right on the mirror...shocked.gif



#34 Orion68

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Posted 11 July 2019 - 09:57 PM

I am NOT an optical expert so this may be all wrong and your mileage may vary.

 

But, I made an aperture mask for my Nexstar 11 GPS to reduce turbulence in the airy disk when seeing is not very good.  Normally, the airy disk is a smeared mess and it is hard to tell how close the collimation is. The mask seems to help quite a bit with overcoming the poor seeing and smoothing out the image. I used a thin piece of Styrofoam and cut out a roughly 3" circle. With the mask in place I can easily see if my collimation is off (unless I'm way off base).

 

So, it seems like an aperture mask can be useful in a number of situations.



#35 dave brock

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Posted 13 July 2019 - 05:54 PM

If the seeing is so bad that you can't judge the collimation I'd suggest it doesn't really matter.
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#36 Vla

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Posted 14 July 2019 - 10:28 PM

I have pretty much the mirror you used in your analysis. So, as an amateur building an aperture mask, what lesson shall I take from your analysis?

I will have the mask near the PM (my goal is about 0.5" above it with adjustable mask tilt) which means that the light beam will pass thru it twice. First going down to the PM and then it will pass it again on it's way up to the secondary mirror - then the beam of light will be under slight angle. Is that what you were talking about?

So, ideally one should place the mask right on the mirror...shocked.gif

No, I didn't think about double passage. In this particular case, at 1/2 inch separation, the reflected beam would get trimmed off about 0.5mm at the very bottom, but that wouldn't have practically any effect. The reason why larger mask separation increases off axis aberration is that the chief ray (central ray of the ray bundle passing through the opening) of oblique incoming light hits the mirror at a slightly higher point, and the entire pencil of light hits a higher, more strongly curved section of the mirror. But, as Tom already said, the difference is pretty much negligible.



#37 slavicek

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Posted 16 July 2019 - 09:18 PM

No, I didn't think about double passage. In this particular case, at 1/2 inch separation, the reflected beam would get trimmed off about 0.5mm at the very bottom, but that wouldn't have practically any effect. The reason why larger mask separation increases off axis aberration is that the chief ray (central ray of the ray bundle passing through the opening) of oblique incoming light hits the mirror at a slightly higher point, and the entire pencil of light hits a higher, more strongly curved section of the mirror. But, as Tom already said, the difference is pretty much negligible.

So, I finally got the mask apparatus working. I did attached the 4" mask about 1/2" from the primary mirror and compared the views of Jupiter with the views thru 100 mm TAK. The image was as good in the DOB as it was in the TAK! The only difference was that the image was "upside down". As we all know when using mask one gets an off axes viewing (= no central obstruction) and there are no diffraction spikes from the secondary mirror spider. I am very exited about it. Next new moon I am taking the Dob to dark sky location and I will be using 7" mask instead of the 4". I hope to report something even more exciting...


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#38 Conaxian

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Posted 22 July 2019 - 12:10 PM

Having the mask up on front of the tube takes care of any dew forming on the secondary.

It has that going for it.  Stray light is also greatly reduced. It is easier to install, with much less chance of touching either mirror.

I liked it up on the front.



#39 Sarkikos

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Posted 23 July 2019 - 12:53 PM

Your result will be something like the biggest spider vanes ever smile.gif

Exactly.  This idea has come up many times before.  The correct answer is always the same:  It is equivalent to having very thick spider vanes.  Don't do it!  grin.gif

 

AFAIK, the only aperture masks that do any good on a Newtonian are maybe an apodizing mask (in my 10" f/4.8 Dob), or maybe an annular mask if the primary has an edge defect, like a TDE (this helped an 8" f/6 I used to own). 

 

If a wider annular mask (narrower opening) improves the image, the problem is really thermals.  Use fans behind the primary to acclimate, maybe fans blowing across the front to disrupt the boundary layer.

 

An off-set mask will eliminate the spider vanes and central obstruction, but you are reducing the effective aperture by over 75%.  I guess if you have a lot of aperture to burn, it might be worthwhile.  grin.gif

 

Mike


Edited by Sarkikos, 24 July 2019 - 07:57 AM.


#40 Sarkikos

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Posted 23 July 2019 - 12:54 PM

Waste of time IMO. I have a 10” LB with a very good mirror set. I also have excellent 100 and 120 mm ED refractors. If seeing is equal, the 10” reflector slaughters the excellent refractors in planetary detail.

 

Right now I’m not doing a lot of planetary viewing due to the planets being very low on the horizon. Not sure how it is at your location.

Exactly right!  waytogo.gif

 

Whoever says a 120mm ED beats a 10" Dob on the planets, never looked through a decent 10" Dob ... or their seeing is very, very bad.

 

Mike


Edited by Sarkikos, 23 July 2019 - 12:55 PM.


#41 Sarkikos

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Posted 23 July 2019 - 01:01 PM

If the seeing is so bad that you can't judge the collimation I'd suggest it doesn't really matter.

Yep.  Either just do deep sky that night or go inside and watch TV.

 

Mike



#42 Ed D

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Posted 24 July 2019 - 07:21 AM

Allan, here is an idea for you to consider.  Why not make an aperture mask with two holes, spaced exactly 180 deg and exactly the same distance from the center of the tube to the center of the holes.  Aperture masks like this have been used a lot in imaging.  What it does is that when you merge the two images you have exact focus.  I have made and used several masks (focusing aids) like this and used them visually, too.  It works.  The main thing is to have fun doing this.

 

Ed D


Edited by Ed D, 24 July 2019 - 07:22 AM.


#43 Sarkikos

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Posted 24 July 2019 - 08:01 AM

Having the mask up on front of the tube takes care of any dew forming on the secondary.

It has that going for it.  Stray light is also greatly reduced. It is easier to install, with much less chance of touching either mirror.

I liked it up on the front.

A better method for preventing dewing on the secondary and for blocking ambient light, is a dew/light shield extension on the front of the OTA.  I know this is true for solid tube Newts.  And you don't reduce the effective aperture.

 

Mike



#44 Daniel Mounsey

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Posted 25 July 2019 - 08:17 AM

Generally not. It only introduces a tiny amount of field-dependent differential astigmatism.

 

It matters only slightly. What it does is to make the footprint on the PM somewhat field dependent. In a Newtonian, that dependence is given by my cartoon down there.

 

The equation is approx. Angles are in radians. The field-dependent clocking and parity of the projection displacement is pretty obvious.

 

e.g. For a 6-inch off-axis stop on the mouth of a 16-inch F/4 Newt looking at a star half a degree out in the field... the offset of the footprint on the PM is sheared about 9%, around half an inch, half a degree out in the field.

 

It's OK to ignore that stuff... almost entirely academic.    Tom

 

Hi Tom,

I think it’s nice that you are sharing your expertise in optics, however, from a hands-on experienced perspective, there are several issues you are not addressing. Then when I scrolled down to see you were using an SCT, this sheds even further issue into the matter. Is there anything you would like to add regarding off axis placement before I say anything?

Regards


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#45 TOMDEY

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Posted 25 July 2019 - 11:35 AM

Hi Tom,

I think it’s nice that you are sharing your expertise in optics, however, from a hands-on experienced perspective, there are several issues you are not addressing. Then when I scrolled down to see you were using an SCT, this sheds even further issue into the matter. Is there anything you would like to add regarding off axis placement before I say anything?

Regards

Hi, Daniel; Wax lyrical, please!

 

My intent was to only and briefly answer the topic-starter guy's fundamental question, with a 16-inch Newt as an example... and then later show a different example of fiddling with variable mask to scrutinize the sources I had designed and built for a test-lab collimator. My use of the word ~differential astigmatism~, in the Newt example is paramount there. All he wanted to know was if the longitudinal placement of an off-axis stop (on the object-space side of the system) changes things much  --- and the answer remains resoundingly "No.", regardless of the configuration of the telescope. That is, stop at entrance port vs stop at first significantly-powered element... will not noticeably change the effect of stop / no stop. What mostly changes is the stopped-down enlarged Airy Disc... little else. I didn't want to mushroom that into a detailed theoretical discussion --- at least for the basic question.

 

PS: Geometric Spot Diagrams are great, but the full-blown coherent solve is where the action's at, for off-axis stops. Thankfully, we can generate those quite nicely now-a-days. The off-axis-stop impact is almost entirely diffractive.

 

[My career was building imaging satellites, including Hubble-class and even more ambitious. I was frequently in friendly battles with the Systems Engineers, who would (understandably) analyze the bejesus out of every concept... but frequently present the shops with unbuildable designs. So, we would bounce back and forth, until we wound up with things that could go to production. Similar comment regarding testing, deployment and operational use.]

 

The Executive Summary answer to the original question, in context... is --- "No."

 

But, I'm sure you have experience regarding other things that should be considered in the use of stops. And those would be of great interest to the CN audience!    Tom


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#46 NHRob

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Posted 25 July 2019 - 12:01 PM

You can try moving the aperture stop around to try the different quadrants.  It's possible that one quadrant of the mirror has a better figure than the others.


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#47 Daniel Mounsey

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Posted 25 July 2019 - 02:54 PM

Tom,

Thanks for putting matters into context. I appreciate your thoughts and you certainly have an impressive background. waytogo.gif



#48 TOMDEY

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Posted 25 July 2019 - 04:29 PM

Tom,

Thanks for putting matters into context. I appreciate your thoughts and you certainly have an impressive background. waytogo.gif

Thanks, Daniel! But, I frequently remind myself that what I don't know far exceeds what I (think I) know. And, alas, I gave up pole vaulting and the violin, when I broke my wrist --- had it not been for that tragic accident, the World of Optics would have never seen...  Aw, who am I kidding, other than myself?!    Tom

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  • 273 Toms Demolished Wrist.jpg


#49 Daniel Mounsey

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Posted 26 July 2019 - 08:50 AM

So sorry to see this Tom. It’s odd how health comes up to bite us and it changes our lives completely, then it took you another direction. They said if Chicxulub never hit, we wouldn’t be here. So much irony in life. 




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