Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Aperture Mask

  • Please log in to reply
36 replies to this topic

#1 Allan Wade

Allan Wade

    Soyuz

  • *****
  • topic starter
  • Posts: 3789
  • Joined: 27 Jan 2013
  • Loc: Newcastle, Australia

Posted 12 June 2019 - 03:25 AM

I am going to make an aperture mask for a specific observation I'm planning in the 32". A crazy thought came to mind, that I have never seen put into practice before. Rather than make one cutout, is there a reason why you can't make 4 cutouts in between all the spider veins. I'm sure there's a reason why it won't work, as I have never seen anyone do it that way before. I plan to start with the one hole anyway, which will turn my dob into a 12" f/10 apo.



#2 Allan Wade

Allan Wade

    Soyuz

  • *****
  • topic starter
  • Posts: 3789
  • Joined: 27 Jan 2013
  • Loc: Newcastle, Australia

Posted 12 June 2019 - 03:53 AM

A friend has helped me answer the question, thanks Colin. It seems obvious now but with 4 holes the space in the centre of the holes acts like a big secondary obstruction and the gaps between each hole like a huge, thick spider vein. So lots of diffraction effects.

 

One hole it shall be!


  • Asbytec and happylimpet like this

#3 ManuelJ

ManuelJ

    Apollo

  • -----
  • Posts: 1408
  • Joined: 19 Dec 2005
  • Loc: Madrid, Spain

Posted 12 June 2019 - 03:53 AM

Your result will be something like the biggest spider vanes ever :)



#4 siriusandthepup

siriusandthepup

    Viking 1

  • *****
  • Posts: 996
  • Joined: 14 Feb 2006
  • Loc: Central Texas, USA

Posted 12 June 2019 - 03:56 AM

Well, you can do that. But you already have a telescope with 4 cutouts - with your normal spider. Making them circular instead of 1/4 of the pie sections doesn't improve the diffraction effects. And, more importantly, this doesn't help you "cut through" the seeing with a smaller aperture. You are back to looking through the seeing cells with full aperture - kinda defeats the whole purpose.


  • Allan Wade likes this

#5 TOMDEY

TOMDEY

    Soyuz

  • *****
  • Posts: 3980
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 12 June 2019 - 05:58 AM

Allan, I think your original epiphany up at the top remains valid! I've played with various aperture stops (more accurate to call them ~subtractive pupil stops~) for decades. By far the most common are:

 

>stopping down a telescope with an abrupt round external centered stop

>apodizing down a telescope with a feathered symmetric external centered stop

>off-axis unobstructed round stop

 

But those are just a few from an infinite menu. No reason not to at least try some others... go crazy with some cut-outs and just see what happens!

 

Back in Physical Optics 201, British Professor Emeritus Bryan Thompson handed us a bunch of amplitude stop cartoons and the corresponding Impulse Responses... all mixed up. It was part of our assignment to match them up... based solely on what he had taught us regarding the nature of Fourier Transforms. Computations were not needed, just an abiding knowledge and intuition of how the FT manifests. It was a really fun puzzle!

 

Years later, Alan Wertheimer and I came up with even fancier pupil masks comprising both amplitude and phase alterations. That allowed us to properly phase segmented Space Telescopes with on-board minimalist instrumentation. So, Thompson's puzzle paid off for us, decades later! The entire gizmo is the size of a coffee cup and seamlessly/continuously phases ~Space Telescope~ Class Imagers. We patented the technology. US Patents #5120948, #5146072, #7764385.

 

Anyway, I would suggest that you take any convenient smallish telescope and just play around with a bunch of masks and see what happens. It should be a lot of fun!    Tom

Attached Thumbnails

  • 040 Wertheimer Dey Patent.jpg
  • 041 Wertheimer Dey Patent.jpg

  • Allan Wade, howardcano and Earthbound1 like this

#6 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 77148
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 12 June 2019 - 06:24 AM

I am going to make an aperture mask for a specific observation I'm planning in the 32". A crazy thought came to mind, that I have never seen put into practice before. Rather than make one cutout, is there a reason why you can't make 4 cutouts in between all the spider veins. I'm sure there's a reason why it won't work, as I have never seen anyone do it that way before. I plan to start with the one hole anyway, which will turn my dob into a 12" f/10 apo.

 An APO is a refractor.. by definition.  

 

A 32 inch Dob with a 12 inch aperture mask trades the relatively small diffraction effects of the CO for the much larger diffraction effects of the 12 inch aperture.  

 

With the aperture mask, the Airy Disk is 2.7 times larger.. Contrast is concentrated energy.

 

Jon


  • Oberon likes this

#7 Vla

Vla

    Ranger 4

  • *****
  • Posts: 370
  • Joined: 08 Mar 2014

Posted 13 June 2019 - 03:15 PM

Optimum aperture contrast/resolution-wise is determined by seeing. For the averaged seeing, it is two to three times the Fried coherence length, r0. In 2 arcsec seeing, r0 for the green light is nearly 3 inch, twice as much in 1 arcsec seeing. In the first case, optimum aperture would be 6-9 inch, in the second twice larger. Since in the actual field conditions there are always moments of better than average seeing, 3xr0 is probably a better bet. This as long as aperture and seeing are the only significant factors.



#8 phonehome

phonehome

    Vostok 1

  • *****
  • Posts: 156
  • Joined: 19 Dec 2010

Posted 13 June 2019 - 10:45 PM

One thing for sure: with a mask the speed of focus will change dramatically.  This may/may not help depending on the target but it is a good exercise in understanding focusing speed/tolerance.  Example: at f-2.75 (my native) an over-sized focusing knob + the focuser's 10-to-1 reduction is required to achieve good results.  But when masked down to about f-7 the small direct-drive focusing knob works just fine.  The difference is very obvious.


  • izar187 likes this

#9 Allan Wade

Allan Wade

    Soyuz

  • *****
  • topic starter
  • Posts: 3789
  • Joined: 27 Jan 2013
  • Loc: Newcastle, Australia

Posted 17 June 2019 - 02:19 AM

One thing for sure: with a mask the speed of focus will change dramatically.  This may/may not help depending on the target but it is a good exercise in understanding focusing speed/tolerance.  Example: at f-2.75 (my native) an over-sized focusing knob + the focuser's 10-to-1 reduction is required to achieve good results.  But when masked down to about f-7 the small direct-drive focusing knob works just fine.  The difference is very obvious.

That's something interesting I will look for Ed.

 

I bought some material to make the aperture mask. I will end up with a 12" unobstructed aperture. All this for another attempt at Amalthea next week. After almost 2 years of unsuccessful attempts at this little moon I need to try something different. I believe only a handful of people have seen Amalthea since Edward Barnard discovered it in 1892, and almost all the observations were through big refractors.



#10 DavidC

DavidC

    Surveyor 1

  • *****
  • Posts: 1613
  • Joined: 24 Nov 2005
  • Loc: Mesa, Arizona

Posted 18 June 2019 - 09:41 PM

I am making an off axis aperture mask for my 10 inch lightbridge, but using a single 4 inch hole. I got the idea from san francisco sidewalk astronomers, but they had it as plans for a solar filter. I'm making it for planets and double stars. I've been told by stepping the aperture down to 4 inches, planets won't be as bright, therefore I can use more power on them. At 1270 mm focal length, I'm hoping for impressive views on planets by using more power. Am I thinking this correctly?

 

Thanx, David



#11 phonehome

phonehome

    Vostok 1

  • *****
  • Posts: 156
  • Joined: 19 Dec 2010

Posted 18 June 2019 - 09:49 PM

Allan,

 

Which quadrant of the secondary holder are you planning on using for your mask?  The one between the focuser and secondary usually offers the largest clear aperture obtainable by taking advantage of the secondary offset.  It also usually doesn't have the equivalent of thermal "tube currents" in the path.

 

Ed



#12 Darren Drake

Darren Drake

    Soyuz

  • *****
  • Posts: 3832
  • Joined: 09 Oct 2002
  • Loc: Chicagoland

Posted 18 June 2019 - 11:14 PM

I just made a 7 inch aperture stop today for my 18.  Worked great tonight.  I've made them many times before but it's been a while.  Seeing tonight was so good the better views were at full aperture..

Attached Thumbnails

  • IMG_20190619_40580.jpg

  • stevew and Orion68 like this

#13 Allan Wade

Allan Wade

    Soyuz

  • *****
  • topic starter
  • Posts: 3789
  • Joined: 27 Jan 2013
  • Loc: Newcastle, Australia

Posted 19 June 2019 - 01:02 AM

I am making an off axis aperture mask for my 10 inch lightbridge, but using a single 4 inch hole. I got the idea from san francisco sidewalk astronomers, but they had it as plans for a solar filter. I'm making it for planets and double stars. I've been told by stepping the aperture down to 4 inches, planets won't be as bright, therefore I can use more power on them. At 1270 mm focal length, I'm hoping for impressive views on planets by using more power. Am I thinking this correctly?

 

Thanx, David

My experience is to always use the largest aperture possible for planets. My lifetime top 3 views include Jupiter in a 32” at over 900x. That’s just under a 1mm exit pupil which is sweet if the sky can handle it. But despite what you’ll often read, there’s no rule book to observing so I would build your mask and see what you think.



#14 Allan Wade

Allan Wade

    Soyuz

  • *****
  • topic starter
  • Posts: 3789
  • Joined: 27 Jan 2013
  • Loc: Newcastle, Australia

Posted 19 June 2019 - 01:07 AM

Allan,

 

Which quadrant of the secondary holder are you planning on using for your mask?  The one between the focuser and secondary usually offers the largest clear aperture obtainable by taking advantage of the secondary offset.  It also usually doesn't have the equivalent of thermal "tube currents" in the path.

 

Ed

Hey Ed. You know I’ve got John’s secondary system in the 32” with the offset spider veins. So the gap at the 3 and 9 position is a lot more than the 12 and 6 positions. I was going to put it in the quadrant furtherest away from me to get it away from any body heat.



#15 Allan Wade

Allan Wade

    Soyuz

  • *****
  • topic starter
  • Posts: 3789
  • Joined: 27 Jan 2013
  • Loc: Newcastle, Australia

Posted 19 June 2019 - 01:08 AM

I just made a 7 inch aperture stop today for my 18.  Worked great tonight.  I've made them many times before but it's been a while.  Seeing tonight was so good the better views were at full aperture..

Hey Darren. I think your experience will be the typical answer for David in the post just above.



#16 SteveG

SteveG

    Cosmos

  • *****
  • Posts: 7602
  • Joined: 27 Sep 2006
  • Loc: Seattle, WA

Posted 19 June 2019 - 01:31 PM

I am making an off axis aperture mask for my 10 inch lightbridge, but using a single 4 inch hole. I got the idea from san francisco sidewalk astronomers, but they had it as plans for a solar filter. I'm making it for planets and double stars. I've been told by stepping the aperture down to 4 inches, planets won't be as bright, therefore I can use more power on them. At 1270 mm focal length, I'm hoping for impressive views on planets by using more power. Am I thinking this correctly?

 

Thanx, David

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.



#17 TOMDEY

TOMDEY

    Soyuz

  • *****
  • Posts: 3980
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 19 June 2019 - 08:57 PM

I've done this on Newts >>> I build it out of black Styrofoam art-board and slip it over the mouth of the telescope so I can rotate it to any quadrant while looking thru the eyepiece. That way, all the theory and guesswork drop right out of the discussion. You just rotate it to get the best image! Could also make various sizes, etc. It's nice to theorize, but sometimes empirical is the way to go!    Tom

Attached Thumbnails

  • 82 clockable off-axis stop while looking.jpg

  • izar187, Starman81 and starzonesteve like this

#18 Oberon

Oberon

    Mercury-Atlas

  • -----
  • Posts: 2862
  • Joined: 24 Feb 2013
  • Loc: Hunter Valley NSW Australia

Posted 20 June 2019 - 05:17 AM

Irrespective of any potential, real or imaginary benefits offered by an aperture mask, any mask that has rougher cut edges than the usually clean edge of the mirror will introduce new and unwanted aberrations. So when making a mask, make it with a clean cut thin edge.


  • izar187, happylimpet and starzonesteve like this

#19 Vla

Vla

    Ranger 4

  • *****
  • Posts: 370
  • Joined: 08 Mar 2014

Posted 20 June 2019 - 08:55 AM

Smooth edges have more of a cosmetic effect. Rough edges don't induce aberrations, because they don't affect wavefront shape, and unless the edge is ridiculously rough, the diffraction effect will be negligible. As an illustration, effect of a 2-inch focuser protruding into the light path of a 200mm diameter mirror. As much as 1 inch into the light path will take only about 1% of the energy out of the central maxima (which, expectedly, becomes somewhat elongated, because the vertical mirror diameter is effectively shorter).

Attached Thumbnails

  • pro.png

  • TOMDEY and howardcano like this

#20 TOMDEY

TOMDEY

    Soyuz

  • *****
  • Posts: 3980
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 20 June 2019 - 12:58 PM

Smooth edges have more of a cosmetic effect. Rough edges don't induce aberrations, because they don't affect wavefront shape, and unless the edge is ridiculously rough, the diffraction effect will be negligible. As an illustration, effect of a 2-inch focuser protruding into the light path of a 200mm diameter mirror. As much as 1 inch into the light path will take only about 1% of the energy out of the central maxima (which, expectedly, becomes somewhat elongated, because the vertical mirror diameter is effectively shorter).

Yes indeed! The effects are diffractive and tiny, not what we optics guys call aberrations. I also like your focuser signature there... Fourier Transform (impulse-response) says it all.

 

Masks roughly-cut with scissors or a knife are perfectly fine. The one thing to try to avoid is long straight edges. Those will give noticeable spikes. The three straight edges of the focuser there... do a little bit of that.

 

On the tech/theory side... there are infinitely many wavefronts that will produce the same impulse response. That's because the sensor (eye or camera) detects only amplitude, but not phase. So you can't inverse-transform back to the wavefront by processing on the one image of a star... unless you use two or more (ideally many) focus positions' images. And that is what we call ~phase diversity analysis~ (what was used to assess Hubble's flaw). And what is implicitly involved in the various casual ~Sar Tests~ that we often talk about here.    Tom



#21 Vla

Vla

    Ranger 4

  • *****
  • Posts: 370
  • Joined: 08 Mar 2014

Posted 20 June 2019 - 08:48 PM

Tom, you are an optics guy. Me, I only pretend. Thinking about back-transforming the wave/phasefront from either phase or amplitude (since there is underlying relationship between the two), I can see how there can be infinitely many possible outcomes for any given wavefront point, or a small patch, but with enough of sample points for the entire surface, it should boil down to only a many. That (Fourier) relation between the space and frequency is something I haven't quite wrapped my mind around yet, but it's beautiful nevertheless. Like hinting that many seemingly different faces could actually be One. Have we strayed from the topic? Neah - faces, masks, that's pretty close...



#22 Oberon

Oberon

    Mercury-Atlas

  • -----
  • Posts: 2862
  • Joined: 24 Feb 2013
  • Loc: Hunter Valley NSW Australia

Posted 21 June 2019 - 06:49 AM

Yes indeed! The effects are diffractive and tiny, not what we optics guys call aberrations. I also like your focuser signature there... Fourier Transform (impulse-response) says it all.

 

Masks roughly-cut with scissors or a knife are perfectly fine. The one thing to try to avoid is long straight edges. Those will give noticeable spikes. The three straight edges of the focuser there... do a little bit of that.

 

On the tech/theory side... there are infinitely many wavefronts that will produce the same impulse response. That's because the sensor (eye or camera) detects only amplitude, but not phase. So you can't inverse-transform back to the wavefront by processing on the one image of a star... unless you use two or more (ideally many) focus positions' images. And that is what we call ~phase diversity analysis~ (what was used to assess Hubble's flaw). And what is implicitly involved in the various casual ~Sar Tests~ that we often talk about here.    Tom

Happy to be corrected, and its quite possible I'm recalling advice more specific to a spectrograph slit. That said, I was thinking in terms of a scissor or knife being a reasonably clean cut, in contrast to a hole cut in styrofoam.



#23 TOMDEY

TOMDEY

    Soyuz

  • *****
  • Posts: 3980
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 21 June 2019 - 11:31 AM

Happy to be corrected, and its quite possible I'm recalling advice more specific to a spectrograph slit. That said, I was thinking in terms of a scissor or knife being a reasonably clean cut, in contrast to a hole cut in styrofoam.

Oh yeah, I would sure make a spectrograph slit nice and accurate! Also, I actually make the holes if foam masks... using good "hole saws" or a router. They come out nice and round.

 

Decades ago I even wrote a (casual) paper on the "thick knife edge", where I derived the effect of non-zero thickness knife on the impulse-response (Fourier Transform real part) as seen by the detector array (or eye). It's quite interesting, and something that virtually none of us take into account. (The context was imaging stellites, onboard alignment suite... where such things matter!)   Tom



#24 CrazyPanda

CrazyPanda

    Surveyor 1

  • *****
  • Posts: 1772
  • Joined: 30 Sep 2012

Posted 21 June 2019 - 01:29 PM

I've been told by stepping the aperture down to 4 inches, planets won't be as bright, therefore I can use more power on them.

Not sure I follow that logic. Using more power makes the view dimmer, so you actually want *more* aperture to use more power, not less aperture to use more power.

 

But at the end of the day, the atmosphere has the final say. Garbage seeing will obscure resolvable details just as strongly in 4" as in 10".


Edited by CrazyPanda, 21 June 2019 - 01:33 PM.


#25 slavicek

slavicek

    Mariner 2

  • *****
  • Posts: 211
  • Joined: 01 Aug 2017
  • Loc: Massachusetts

Posted 21 June 2019 - 08:17 PM

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?




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics