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

What is an apodizer?

  • Please log in to reply
16 replies to this topic

#1 hokkaido53

hokkaido53

    Viking 1

  • *****
  • topic starter
  • Posts: 963
  • Joined: 07 May 2012
  • Loc: New Mexico

Posted 22 August 2019 - 09:39 AM

First...Greetings from New Mexico!

IN the CN forums,I have seen several drawings of the planets that were done using an "apodizer."

After some googling, I found a number of detailed technical explanations of how the device reacts with light, but without any reference to amateur observing at the telescope.

Can someone point me toward a good, not-too-technical explanation of this device? Also, is it used only for the planets, or is it helpful with observing nebulae, galaxies, etc.?

Thanks, and clear skies,

Roy



#2 sg6

sg6

    Fly Me to the Moon

  • *****
  • Posts: 5693
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 22 August 2019 - 10:35 AM

Not sure of the term "Apodizer".

I assume they mean an "apodization filter" and most are not.

 

Simplest apodization filter is a simple aperture mask. Make aperture smaller, longer f number, removes the "poor" spherical edges of the lens and get less CA.

 

An apodization filter is in effect a variable ND filter. The blocking being a function of a position on the lens. So the extreme edge may be 99% block and the center 0%. Could be a straight line function, or a parabolic, or any other function. And there are lots. Gaussian is common. If I recall a simple straight line, edge 100% block and center 0% block, is pretty good.

 

The idea is that the final image has better resolution - reduction of the airy disk. I did some work on them far too many years ago - Super Resolution by Non-Uniform Optics. Great fun, basic rule is you cannot win. As I removed the airy disk the main center became weaker and weaker. With the airy disk out of the way there was in effect not central bit. All the light was in the removed airy disk.

 

You need to delve into Fourier transforms really. Usually the result of that is something close to insanity.

 

My arguement is: There is no "Apodization Filter".

If there were then none of us would buy a $2,000 triplet, we would buy a $300 achro and add a $100 apodization filter to the front. So getting an Apo scope at $400 not an Apo triplet scope at $2,000.

 

Did have a link to examples but now on a little used machine.

 

Remember that as an ND filer you may see some form of improvement but you have again as with a filter removed light. So half the incident light is lost and therefore is that an improvement?


  • alstarjoey likes this

#3 Joe1950

Joe1950

    Cosmos

  • *****
  • Posts: 9404
  • Joined: 22 Aug 2015

Posted 22 August 2019 - 11:19 AM

This is 1/1000 as technical as sg6 wrote. But I think it is along the same lines.

 

Many years ago, maybe in the 1970s, there was a do-it-yourself project to make an Apodizing Screen. It was literally made of blackened window screen. 

 

I had a Celestron 8 and to make such a screen, you would cut 3 pieces of window screen a diameter a little larger than the front corrector, to fit the front of the tube. Say 8-1/2”.

 

Next, you would cut holes in each disk. I can’t recall the exact numbers, but the holes cut would be say 60% of the diameter (8”), 70% and 80%. You ended up with essentially 3 rings of window screen with different size center apertures cut out.

 

You would then lay them together, tilting the angle of the threads of each so that no two angled alike. It was affixed together and placed over the front of the scope.

 

It darkened the outside areas of the scope in steps and was supposed to take light from the diffraction rings, not the Airy disk. Centering a star or planet like Jupiter, there was a pattern of artifacts that surrounded the central real image that looked odd.

 

Did it work?  I don’t know. It made the image less bright, but I can’t say if it was more detailed or not. Some thought it was, some thought it wasn’t.

 

In the long run, most didn’t use them due to the pattern of annoying reflected artifacts. 

 

 

Here is an example of an article describing the construction and use.


Edited by Joe1950, 22 August 2019 - 11:22 AM.

  • Doug Culbertson and alstarjoey like this

#4 Doug Culbertson

Doug Culbertson

    Cosmos

  • *****
  • Posts: 7572
  • Joined: 06 Jan 2005
  • Loc: N. Florida

Posted 22 August 2019 - 11:47 AM

This is 1/1000 as technical as sg6 wrote. But I think it is along the same lines.

 

Many years ago, maybe in the 1970s, there was a do-it-yourself project to make an Apodizing Screen. It was literally made of blackened window screen. 

 

I had a Celestron 8 and to make such a screen, you would cut 3 pieces of window screen a diameter a little larger than the front corrector, to fit the front of the tube. Say 8-1/2”.

 

Next, you would cut holes in each disk. I can’t recall the exact numbers, but the holes cut would be say 60% of the diameter (8”), 70% and 80%. You ended up with essentially 3 rings of window screen with different size center apertures cut out.

 

You would then lay them together, tilting the angle of the threads of each so that no two angled alike. It was affixed together and placed over the front of the scope.

 

It darkened the outside areas of the scope in steps and was supposed to take light from the diffraction rings, not the Airy disk. Centering a star or planet like Jupiter, there was a pattern of artifacts that surrounded the central real image that looked odd.

 

Did it work?  I don’t know. It made the image less bright, but I can’t say if it was more detailed or not. Some thought it was, some thought it wasn’t.

 

In the long run, most didn’t use them due to the pattern of annoying reflected artifacts. 

 

 

Here is an example of an article describing the construction and use.

 

That apodizing screen was my first thought when I read the thread title. I always wanted to try one, but I was just to lazy to make one. 


  • Joe1950 likes this

#5 David Gray

David Gray

    Surveyor 1

  • *****
  • Posts: 1655
  • Joined: 06 Aug 2012
  • Loc: Ower Yonder 01.5ºW. 55ºN (UK)

Posted 22 August 2019 - 12:31 PM

Yes they do work (for planets and double stars....not galaxies, nebulae etc) – speaking as one who used one for near 3 decades now.

 

A discussion developed on one of my Sketching Forum threads recently, following on from here....

 

https://www.cloudyni...s-am/?p=9372762

 

Extract from Post #16 there..........

 

“Also to expand on Apodizing I should perhaps add........

I should have clarified *mid-scale seeing*: being that where there is still a discernible/steadier central disk as with Pickering 4-7 but still some significant disturbance of the rings.  In short no clear (over-agitated) central disk {then] no real apodizing gain; and better than Pickering 7 little further improvement other than as a neutral filter – which at the other end might also help in some types of poor seeing.

To get the gist see Damian Peach’s, now classic, seeing animation........   
http://www.damianpea...m/pickering.htm 
 

What that has to do with planets is that the scope renders them as a multitude of overlapping Airy Disks.  In the main the device does not work well with the Moon as there is too much overlap of apodizing diffraction effects from the disparate brighter regions in the field reducing contrast – think of having a dozen Jupiters in the field........!  But I have determined that the ring suppressing is in effect by noting the apparent stilling-degree of the seeing albeit offset by the aforesaid contrast loss.  Tho’ regions on the terminator or crescent cusps (isolated mountains etc.)  have been seen sharpened to advantage with the device.

I suspect not taking account of the type seeing explains why some have dismissed the gain as imagined or whatever: it's not an omnipotent magic cure!  Of course some just do not like the surrounding diffraction 'mess'........I'm too concentrated on eking out planetary detail to care or even notice it......If I want to see a pretty/'truer' scene (or faint moons) I take it off - easy as it just drops-in/lifts-out-of the end of the scope - no affixing needed and weighs a few ounces.

As I have said before: the guys who do the numbers are missing something somewhere - learned dissent or no
t.......”

 

Extract from Post #18..........

 

“The guys missing something in their critique I suspect is possibly failure to evaluate over a range of seeing.  As I found it usually starts to fail in poor/very poor seeing and offers decreasing gain in the better end of the scale.  It’s that mid-scale where I find it works most effectively. I say *mid-scale* to avoid the more ambiguous *average seeing* which would vary location to location.  As it happens, in my location mid-scale is the average seeing..........!! 

As a Secondary School (UK) Non-Diploma drop-out I do not possess the science of those guys – purely empirical/suck-it-and-see being my approach.

A history of my apodizing experience..........

 

1963 (serendipity!): looking at Jupiter (3” scope) from the stair-landing window the wind blew the hitched-back net curtain across the scope and I was astonished/intrigued by the sharpened view. As the curtain fluttered the *rainbow* Diffraction ‘Mess’ varied from striking to gross but as long as Jupiter was  bright enough it remained tack-sharp.

 

1991: that curtain effect being in my mind to follow up on all those years then one night with Mars & Jupiter out of sheer whim I hung the fireguard over the end of the 16.3” D-K and WOW!!  So a piece cut out of the guard (we had a spare) became the apodizer for a few years. 

 

Then an edge-damaged 18” (perfect drop-in fit!) stainless steel flour sieve (see attached pic) got years of use till the proper 3-layer one I still use.

Shortly after the fireguard experience I sent BAA veteran Alan W. Heath (then Saturn Section Director) a piece of net curtain and he swore it seemed to tame the seeing and still uses it to date on his 10” Newt.

His successor David Graham, on a visit, got a look through the D-K at Saturn with the apodizer (flour sieve) applied and immediately exclaimed “It works!!!”  Both are very seasoned critical observers of planets – Alan started when I was still at school (1953)!”

DK Apods.jpg

 

 

 

 

 


Edited by David Gray, 22 August 2019 - 02:04 PM.

  • Joe1950 likes this

#6 gundark

gundark

    Messenger

  • *****
  • Posts: 469
  • Joined: 07 Jun 2015
  • Loc: Southern California

Posted 22 August 2019 - 02:11 PM

You know what I could use for my images?

 

An APOD-izer.

 

It would make every image an Astronomy Picture Of the Day.

 

roflmao.gif

 

Now back to our scheduled program…

 


  • Fernando134 and Joe1950 like this

#7 Joe1950

Joe1950

    Cosmos

  • *****
  • Posts: 9404
  • Joined: 22 Aug 2015

Posted 22 August 2019 - 02:48 PM

I should try one again. Seeing here has been abysmal.

 

Did I read the % diameter of each ring? I’ll find it.

 

Ring 1:    90%

Ring 2:    78%

Ring 3:    55%   Of full objective diameter.

 

All screens offset 30o relative each other. 

 

Use darkened window screen.


Edited by Joe1950, 22 August 2019 - 03:10 PM.


#8 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 24417
  • Joined: 01 Feb 2006

Posted 22 August 2019 - 04:23 PM

An apodizing mask works by suppressing the energy in the outer diameter of the Airy Disk and the energy in the diffraction rings. 

When seeing is redistrubuting some of this energy in the area around the AD and inner rings, the mask can reduce it enough to regain at least some of the contrast that would be lost to seeing. 

 

I would say that if you are in so-so seeing, an apodizing mask is better than a sub aperture mask. I have used both types and the apodizing mask is (in my opinion) much better, but that being said, it is not a cure for seeing though it does provide some small relief. 

Me personally?  Once you have done sufficient high resolution observing, using any kind of mask on a night of so-so seeing is still leaving you with a much less than thrilling view.  On such nights, I learned to point my telescope at other things.  I don't want to spend an hour looking at a degraded view. 

 

On nights when seeing is exceptional, if there is a planet to see, I swing my scope to it.   If the seeing is not all that good, I just look at other things.  Seeing a high resolution view of Jupiter, Mars, or Saturn in a big scope with good optics is such a treat!  Seeing it in less than its full splendor is to me, a disappointment.   An apodizing mask can help some though, and again, I think it is a much more effective solution than stopping a big aperture down.  That is such a waste. 


  • Usquebae and Joe1950 like this

#9 Pat Rochford

Pat Rochford

    Mariner 2

  • *****
  • Posts: 203
  • Joined: 30 Sep 2008

Posted 22 August 2019 - 07:10 PM

What is an apodizer?

 

Something a person with a head cold has while their entre' is being prepared.

 

I'll be here all week.


  • lphilpot, Fernando134, Joe1950 and 1 other like this

#10 Joe1950

Joe1950

    Cosmos

  • *****
  • Posts: 9404
  • Joined: 22 Aug 2015

Posted 22 August 2019 - 07:43 PM

Rim_Shot.gif



#11 lsfinn

lsfinn

    Mariner 2

  • *****
  • Posts: 200
  • Joined: 28 Jan 2013
  • Loc: Santa Fe, NM

Posted 22 August 2019 - 08:11 PM

An apodizer is an optical filter, placed over the aperture of a scope, whose purpose is to change the distribution of optical intensity at the focus. 

 

An apodizer is generally used to reduce the energy in the diffraction rings of a point source. Done right this can increase contrast, though at the cost of overall resolution. 

 

Apodizers generally taper the transmission across the aperture, with full transmission toward the enter and reduced transmission toward the periphery. How one tapers determines, for a point source, the fraction of optical intensity in the diffraction rings and the size of the central spot. 

 

A technical discussion can be found at <https://www.telescop...izing_mask.htm>


  • Joe1950 likes this

#12 PatNois

PatNois

    Lift Off

  • *****
  • Posts: 16
  • Joined: 08 Feb 2014

Posted 22 August 2019 - 09:01 PM

I remember this was a hot topic back in the day. Early 70s maybe. Can’t remember exactly. But I do remember it cleaning up an image suffering from poor seeing. Since they are easily made - try one and see.


  • Joe1950 likes this

#13 hokkaido53

hokkaido53

    Viking 1

  • *****
  • topic starter
  • Posts: 963
  • Joined: 07 May 2012
  • Loc: New Mexico

Posted 22 August 2019 - 09:32 PM

You know what I could use for my images?

 

An APOD-izer.

 

It would make every image an Astronomy Picture Of the Day.

 

roflmao.gif

 

Now back to our scheduled program…

I always thought an "apodizer" was a snack before the main course of a meal.

But seriously...Thanks for the quick replies.

- Roy


  • Fernando134 and Joe1950 like this

#14 Joe1950

Joe1950

    Cosmos

  • *****
  • Posts: 9404
  • Joined: 22 Aug 2015

Posted 22 August 2019 - 09:51 PM

Brought back fond memories of the 70s, Roy. And I may run over to the Home Store tomorrow for a roll of screening and give it a try.



#15 skywolf856

skywolf856

    Messenger

  • -----
  • Posts: 493
  • Joined: 25 Jan 2008
  • Loc: SE Michigan

Posted 24 August 2019 - 01:27 AM

Ok, here's some stuff from my archives.

 

 

Attached Files


  • Joe1950 likes this

#16 Bowlerhat

Bowlerhat

    Ranger 4

  • -----
  • Posts: 328
  • Joined: 05 Jun 2019
  • Loc: Melbourne, Australia

Posted 24 August 2019 - 06:08 PM

Hmm, so this is for SCTs only?



#17 Joe1950

Joe1950

    Cosmos

  • *****
  • Posts: 9404
  • Joined: 22 Aug 2015

Posted 24 August 2019 - 06:54 PM

I didn’t think so. 

 

But, SCTs and MCTs, by virtue of their relatively  large secondary obstructions, move more light from the central Airy disk to the diffraction rings. The principal behind the Apodizing Mask is to reduce the light in the diffraction rings, so SCTs and MCTs would likely benefit more from the mask  than other designs.




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