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Aperture masks for achros-do they wor

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#1 TexasSky

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Posted 01 March 2015 - 10:20 PM

Hi all,

im a big fan of achros, love their price/performance, crisp views, and just plain cool........I know, I know, they are not an apo.....but heck I still like'em lots......I have several of them from 50mm up to 150mm...........

now, as we all know CA is a "side effect" of Achros.....it seems to bother some more than others.....but what I see a lot of people say is....

"mask your large aperture achro down to get a much longer effective focal length and reduce the CA on brighter objects such as jupiter and the moon etc...."

now, as mentioned, I own lots of achros, but honestly have never tried it......I seem to use the "right sized" scope instead......for planets, I'll use my 4" f15......for wide field sweeping, my 4" f7......

ok, lots of rambling, so I'll cut to my question.....do aperture masks on large achros really work to reduce CA?......are there any drawbacks to this?.....does using the center "thicker" portion of a large masked achro give different views than an equivalent full aperture design?

 

of course, I will have to experiment with this on my own.....but was wondering what you all think or have experience in here......

bob


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#2 bandazar

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Posted 01 March 2015 - 10:46 PM

Course they work, at least to a certain extent.  Don't expect miracles though.  The drawback is that you work with smaller aperture.  And there of course is possible light bleed off with internal reflections if you don't have things all matt black inside.  I did it when I had an 80mm short tube and now I'm doing it on my 150mm xlt omni refractor.  It also depends on the object you are looking at too and what magnification you are looking at.  Higher magnification is more sensitive to issues with chromatic abberation than lower magnification.  Masking my 150mm f/5 to a 120mm f6.25 cuts down on CA to about a 3rd or even 1/4th (some of this of course is due to less light coming in which cuts down on CA as well, not only the increased focal length).  Looking at Jupiter at low power in f/5 mode is very annoying, but to me is doable in 120mm mode.

On paper, I guess you could say that a 150mm f/5 stopped down to 120mm f6.25 would give an equivalent view of a regular 120mm f6.25 scope.  But in the real world, larger scopes have to deal with more cooldown issues, for example.  And it is harder to optically figure a fast lens than a slow one so the larger scope might not have as good optics as the equivalent smaller scope.  And you might have to deal with extra internal reflection issues of course if the dewsheild/aperture mask used allows stray light in and also allows for internal reflections.

Gravity may even play a role if the lens is large enough.  But I'm guessing the lens would have to be very large for this to even be a consideration.


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#3 x935418

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Posted 02 March 2015 - 02:46 AM

Hi TexasSky

 

my previous scope was 4" F5 and i did alot of experience with CA and masking effect on it.

 

there is a simple formula about CA that identify your errorless aperture in achros:

 

SQRT(8*FocalLen) : (focal len in mm)

 

if your aperture is larger than calculated in formula, so you HAVE TO stop down it , if you want to get a sharp picture in maximum magnification

for example let to calculate your 4" F15 scope:

 

Focal len: 15*100mm    = 1500mm
CA free  : SQRT(8*1500)= 110mm

 

so your 4" F15 dos not need to stop down as it has just 100mm aperture

and it can use all of it, give you 200X

 

but let me show my fast achro :

 

Focal len: 5*100mm    = 500mm
CA free  : SQRT(8*500)= 63mm

 

So i have to stop down my scope and on 50X/Inch rule : i can get 126X magnification, any magnification above this, will result blurry picture.

 

Important note:

the formula used is Sidgwick law and it may be used just

for high contrast objects, but if you want to see low contrast objects

such as Jupiter or sunspots, it is better to use Conrady formula :

 

SQRT(5*FocalLen) : (focal len in mm)

 

in this case you 4" F15 :

CA free  : SQRT(5*1500)= 87mm

 

So in this case you have to stop down your scope to 87mm and
eliminate about 13mm (100-87)  if you want to get best image for low contrast

objects, and it convert youe scope to a 87mm F17 !

and it will give you something about 175X (maximum)

 

At the end, some of my maximum magnification:

 

Saturn  :125X      - as Sidgwick formula
Sunspots:70-80X - less then even Conrady formula !
Moon    :166X      - more then Sidgwick formula, as it has huge contrast

 

hope this help


Edited by x935418, 02 March 2015 - 02:54 AM.


#4 TexasSky

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Posted 02 March 2015 - 07:07 AM

Excellent! Thanks! This is great information!...im anxious to make some masks and give it a try!

bob



#5 russell23

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Posted 02 March 2015 - 07:18 AM

Yes.  Using aperture masks definitely reduces CA.   With my Vixen 140NA I had a 107mm aperture mask that in combination with the Baader Fringe Killer gave nice CA free views of the Moon around 100x.  Jupiter still showed some CA with that combination.  But if I then used an 80mm Aperture mask  (and the FK) I really could not see CA when looking at Jupiter with that scope.

 

The aperture masks had a significant positive effect because there was a lot more CA with the filter but no aperture mask.

 

Dave



#6 eklf

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Posted 02 March 2015 - 07:42 AM

.....does using the center "thicker" portion of a large masked achro give different views than an equivalent full aperture design?

 

 For average optics using just this thicker portion has advantages as these central "shallower" curves induce less aberrations than the sharper edge curves of a smaller aperture. 


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#7 Element79

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Posted 02 March 2015 - 01:16 PM

Both the Conrady and the Sidgwick criteria will still give you some CA.  If you want truly APO performance in an Achro use the formula:

 

A (in mm) = SQRT ( 3.4 * FL (in mm) )

 

This is the formula that makes the color blur smaller than the Airy disk and when that happens the telescope becomes color free (for visual light).


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#8 AlphaGJohn

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Posted 02 March 2015 - 02:10 PM

For another option, see my comments on this thread: the Baader Semi-APO Filter.

 

John



#9 russell23

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Posted 02 March 2015 - 02:32 PM

For another option, see my comments on this thread: the Baader Semi-APO Filter.

 

John

And you can do both.  I found a 107mm Aperture mask combined with the Fringe Killer for my Vixen 140NA resulted in acceptable color correction for lunar observations.

 

Dave



#10 Jon Isaacs

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Posted 02 March 2015 - 02:46 PM

 

Course they work, at least to a certain extent.  Don't expect miracles though.  The drawback is that you work with smaller aperture.

 

:waytogo:

 

There are no free lunches, the reduction in chromatic aberration is accompanied by a corresponding loss of aperture which means resolving power and some measure of contrast.  I have done a fair amount of experimentation with aperture masks, mostly with 80mm F/5 and 100mm F/6 refractors but with others as well.  It always seemed like it was a wash, the view was aesthetically more pleasing but the level of detail was not significantly altered one way or the other.  

 

During the day, for birdwatching, I find a mask quite useful.  An ST-80 with only the center cap removed is a 42mm F/9.5 and higher magnification views much sharper than with the full aperture.  And the phases of Venus are also much improved, resolution is not an issue, fine scale contrast is not an issue, only the overwhelming chromatic aberration from the very bright planet. 

 

Jon


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#11 GShaffer

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Posted 02 March 2015 - 03:39 PM

I regularly stop down my 6" AR6 to 4" when viewing the moon and planets......major improvement.



#12 Vondragonnoggin

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Posted 02 March 2015 - 03:52 PM

Hi all,

im a big fan of achros, love their price/performance, crisp views, and just plain cool........I know, I know, they are not an apo.....but heck I still like'em lots......I have several of them from 50mm up to 150mm...........

now, as we all know CA is a "side effect" of Achros.....it seems to bother some more than others.....but what I see a lot of people say is....

"mask your large aperture achro down to get a much longer effective focal length and reduce the CA on brighter objects such as jupiter and the moon etc...."

now, as mentioned, I own lots of achros, but honestly have never tried it......I seem to use the "right sized" scope instead......for planets, I'll use my 4" f15......for wide field sweeping, my 4" f7......

ok, lots of rambling, so I'll cut to my question.....do aperture masks on large achros really work to reduce CA?......are there any drawbacks to this?.....does using the center "thicker" portion of a large masked achro give different views than an equivalent full aperture design?

 

of course, I will have to experiment with this on my own.....but was wondering what you all think or have experience in here......

bob

Sounds like you already have the issue solved by the use of your different sized achros.

 

I have never used an aperture mask for my achros. I have used filters, but i made the decision awhile ago that if i want color corrected views of planets, I either pay for an apo or use a reflector. I honestly have never been bothered by CA on planetary with a filter on the achro though, but generally stick to using my MCT on brightest targets that need high power. i buy scopes for the aperture and all my scopes are small to start with. Biggest is 6" achro. I don't want to mask something already small to start with.


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#13 x935418

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Posted 04 March 2015 - 02:42 AM

Both the Conrady and the Sidgwick criteria will still give you some CA.  If you want truly APO performance in an Achro use the formula:

 

A (in mm) = SQRT ( 3.4 * FL (in mm) )

 

This is the formula that makes the color blur smaller than the Airy disk and when that happens the telescope becomes color free (for visual light).

 

yes , it coincidence with my experience on Jupiter and other low contrast objects ,

in this case, it is virtually equal with an apo



#14 Jon Isaacs

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Posted 04 March 2015 - 07:34 AM

 

Both the Conrady and the Sidgwick criteria will still give you some CA.  If you want truly APO performance in an Achro use the formula:

 

A (in mm) = SQRT ( 3.4 * FL (in mm) )

 

This is the formula that makes the color blur smaller than the Airy disk and when that happens the telescope becomes color free (for visual light).

 

yes , it coincidence with my experience on Jupiter and other low contrast objects ,

in this case, it is virtually equal with an apo

 

 

The difficulty with this approach to an aperture mask is that it does not result in an optimal aperture mask for a given focal length... For a 1200mm focal length, one is left with only 63mm of aperture.   While the color blur is hidden behind the Airy disk, it is in part because the blur is smaller but in part because the Airy disk is larger.  

 

If one has a 150mm F8 with a 1200mm focal length, there are a variety of choices.  Hiding the blur behind the Airy disk suggests 63mm aperture but Sidgwick's criteria suggests a 4 inch F/12 which I have to think would be a superior planetary scope.. One might even find a larger aperture was the better compromise.

 

Jon



#15 Galicapernistein

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Posted 04 March 2015 - 09:04 AM

My SkyWatcher 100 ED came with an aperture mask built into the objective lens cover, with its own removeable cap. Does anyone know what this was designed to be used for?



#16 Peter Besenbruch

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Posted 04 March 2015 - 01:57 PM

My SkyWatcher 100 ED came with an aperture mask built into the objective lens cover, with its own removeable cap. Does anyone know what this was designed to be used for?

 

It was designed with the purpose of sticking a finger inside, and pulling. ;)


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#17 Galicapernistein

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Posted 04 March 2015 - 02:40 PM

 

My SkyWatcher 100 ED came with an aperture mask built into the objective lens cover, with its own removeable cap. Does anyone know what this was designed to be used for?

 

It was designed with the purpose of sticking a finger inside, and pulling. ;)

 

Then I guess it's a good thing that my finger is shorter than the dew shield. I guess if I accidentally super glue the lens cover to the dew shield, it might come in handy. It would be nice if it were threaded, though. I would like to be able to screw on a solar filter. It would be interesting to see what sunspots look like at f20, even with a reduced lens.



#18 brettecantwell

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Posted 05 March 2015 - 02:02 AM

Masking the aperture down works as you are slowing the F ratio down (decreasing aperture, so you lose resolution and brightness.)



#19 x935418

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Posted 05 March 2015 - 05:38 AM

Masking the aperture down works as you are slowing the F ratio down (decreasing aperture, so you lose resolution and brightness.)

 

"Decreasing aperture result lose in resolution"  is correct
just in a lens that have no CA error (like an apo or masked achro)

in this case each mm of aperture , participate in making resolution
and when you decrease aperture, you lose details

that provided by masked part of lens.

 

But other wise in fast achro, masking apreture, eliminate TROUBLOUS parts of lens (outer parts) that are responsible of softness in image (details disappear in blurry image)

 

so it result increasing of resolution  IN COMPARE with unmasked  achro lens .

 

If you stop down your ED 80 scope, every millimeter of stopping down , result decrasing in resolution, because you are eliminate good parts of lens that can be used to make extra pixel for you, but in achro, it is better for us  

to eliminate outer part as they destroy details.

 

is it clear ?

 

 

For example in my 100mm F5 achro:
I have very blurry image at 166X (6mm eyepiece + 2X barlow) no details are visible on moon.

 

But when i stop down it to F8 (63mm aperture) i get a very sharp image,
a lot of details that i wouldn't see if i don't mask it.

 

 

Of course as you said there is lose in brightness, but as we are speaking about
CA that appear for bright object(like moon), it is not important for us.

 

I never stop down my scope when I look to ring nebula (or other DSO).

In DSO, you will use lower magnification , and in low power , CA is not a big problem, just  there is some rainbow around bright star

(that is not important , a least for visual)

 

 

At the end I want to say a subject that I found about CA filters:

 

there is two effect of CA:

 

1- in low power:
It make some rainbow around bright object, but it doesn't effect
on details (not noticeable), it doesn't make image blurry(not noticeable), and all rainbows can be eliminate easily with a CA filter (I use orion vblock)

 

2- in high power:
There is no rainbow, but instead CA make softness in picture and cause to loosing details and sharpness.

In this case , using filter is not useful, it doesn't make picture sharp
I found when i want to see maximum details on moon, a filter doesn't help me
it just eat my voluble pixel and make picture a little soft

(in low power it not important as we have a lot of pixel but in high power we have a little pixels provided by correct part of  lens),

 

in this case I use some additional stop down inside scope!

by using two diaphragm before eyepiece, I can get 66X/Inch extra sharp picture on moon !

but with filter it is impossible to reach this level of magnification

So highest possible magnification in a achro can be 

achieved just by masking (in or out) but not by CA filter.


Edited by x935418, 05 March 2015 - 05:51 AM.

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#20 Jon Isaacs

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Posted 05 March 2015 - 05:57 AM

 

 

For example in my 100mm F5 achro:
I have very blurry image at 166X (6mm eyepiece + 2X barlow) no details are visible on moon.

But when i stop down it to F8 (63mm aperture) i get a very sharp image,
a lot of details that i wouldn't see if i don't mask it.

 

:question:

 

I wonder why you are getting a blurry image at 166x with no details visible? Certainly chromatic aberration softens the image but no details visible, that doesn't quite ring true to me.  Having owned a 102mm F/5 Achromat and currently owning 80mm F/5 and 100mm F/6 Achromats, I see plenty of detail at 150x and 200x, it may not be apo sharp but there is plenty to see.  I was out Monday night with the 100mm F/6 and was seeing significant detail at 150 and 225x, I was in no way bored, there was more than enough to keep me interested..  

 

Jon



#21 x935418

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Posted 05 March 2015 - 06:04 AM

 

 

Both the Conrady and the Sidgwick criteria will still give you some CA.  If you want truly APO performance in an Achro use the formula:

 

A (in mm) = SQRT ( 3.4 * FL (in mm) )

 

This is the formula that makes the color blur smaller than the Airy disk and when that happens the telescope becomes color free (for visual light).

 

yes , it coincidence with my experience on Jupiter and other low contrast objects ,

in this case, it is virtually equal with an apo

 

 

The difficulty with this approach to an aperture mask is that it does not result in an optimal aperture mask for a given focal length... For a 1200mm focal length, one is left with only 63mm of aperture.   While the color blur is hidden behind the Airy disk, it is in part because the blur is smaller but in part because the Airy disk is larger.  

 

If one has a 150mm F8 with a 1200mm focal length, there are a variety of choices.  Hiding the blur behind the Airy disk suggests 63mm aperture but Sidgwick's criteria suggests a 4 inch F/12 which I have to think would be a superior planetary scope.. One might even find a larger aperture was the better compromise.

 

Jon

 

 

i agree with you that there is no optimal aperture for a given focal len

but each formula is useful for spacial target

 

it depend to contrast of your target, for high contrast you can use a little more

and for low contrast must use a little less  apreture

 

i use Sidgwick for moon and Saturn

Conrady is good for Jupiter and sunspots



#22 Jon Isaacs

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Posted 05 March 2015 - 06:18 AM

 

 

i agree with you that there is no optimal aperture for a given focal len
but each formula is useful for spacial target

it depend to contrast of your target, for high contrast you can use a little more
and for low contrast must use a little less  apreture

i use Sidgwick for moon and Saturn
Conrady is good for Jupiter and sunspots

 

It still has to depend on the scope one is starting with.. Both Sidgwick and Conrady are design criteria based on a fixed aperture.

 

In my experiments viewing Jupiter, stopping my 100mm F/6 down to a Sidgwick 70mm did not show me more detail, I am doubtful that stopping it down further to a Conrady 55mm would have improved things.  

 

Jon



#23 x935418

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Posted 05 March 2015 - 06:31 AM

 

 

 

For example in my 100mm F5 achro:
I have very blurry image at 166X (6mm eyepiece + 2X barlow) no details are visible on moon.

But when i stop down it to F8 (63mm aperture) i get a very sharp image,
a lot of details that i wouldn't see if i don't mask it.

 

:question:

 

I wonder why you are getting a blurry image at 166x with no details visible? Certainly chromatic aberration softens the image but no details visible, that doesn't quite ring true to me.  Having owned a 102mm F/5 Achromat and currently owning 80mm F/5 and 100mm F/6 Achromats, I see plenty of detail at 150x and 200x, it may not be apo sharp but there is plenty to see.  I was out Monday night with the 100mm F/6 and was seeing significant detail at 150 and 225x, I was in no way bored, there was more than enough to keep me interested..  

 

Jon

 

 

 

Ya ISS

 

i think i didn't explain my means about word "detail"

when i say "i don't see any details"  my means

is not something like "i don't see Plato"

Because Plato is not a detail, it is big feature on moon
I can see Plato even with closed eye! :D

 

when i say "no detail" my means is : no details around a big features like Plato

 

my means is :no good view from its outer walls

my means is :no good view on Teneriffe

and so on ...

 

whenever i see soft image i say : NO DETAIL :mad:


Edited by x935418, 05 March 2015 - 06:35 AM.


#24 x935418

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Posted 05 March 2015 - 06:41 AM

 

 

 

i agree with you that there is no optimal aperture for a given focal len
but each formula is useful for spacial target

it depend to contrast of your target, for high contrast you can use a little more
and for low contrast must use a little less  apreture

i use Sidgwick for moon and Saturn
Conrady is good for Jupiter and sunspots

 

It still has to depend on the scope one is starting with.. Both Sidgwick and Conrady are design criteria based on a fixed aperture.

 

In my experiments viewing Jupiter, stopping my 100mm F/6 down to a Sidgwick 70mm did not show me more detail, I am doubtful that stopping it down further to a Conrady 55mm would have improved things.  

 

Jon

 

 

yes , Jupiter is a hard target

 

as i said above, even conrady formula suggest too big aperture for Jupiter

Element79  formula:SQRT ( 3.4 * FL (in mm) ) is better for jupiter

with this formula , you have to stop down your scope about 45mm

and in this case it give you 90X magnification and it would be maximum

magnification of your scope for Jupiter

 

my F5 give me maximum 70-80X



#25 x935418

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Posted 05 March 2015 - 06:58 AM

I forget to say another subject:

 

in low contrast target like Jupiter and sunspot, it seems that stopping down ,

dos not give us higher magnification.

 

i remember on sunspots,  i tried a lot but i never could get more then 70X,

no difference that i stop down my scope or not.

 

but it effects on Saturn, without masking it was impossible to get

more then 100X without softness, but by masking

i could get 120-130X sharp image

 

And on moon , it effect even more , without masking anything over 100X was

blurry but with masking , there was no problem on 166X

 

So i conclude that for low contrast objects, this formulas

just specify maximum magnification that you can get (by specifing your CA free

aperture size) and  there is no difference that you stop down your scope or not !


Edited by x935418, 05 March 2015 - 07:10 AM.



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