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"Stopping down" aperature

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#1 DouglasPaul

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Posted 04 May 2021 - 03:09 PM

What is the purpose? I read that it makes a longer focal length, to what end? And are there different methods?


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#2 TOMDEY

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Posted 04 May 2021 - 03:32 PM

No, it doesn't make a longer focal length.

 

There are a few circumstances where intentionally masking the outside edge (or more) of the aperture can help.

 

>if the image is overpoweringly bright

>if the primary mirror has a ~turned edge~

>to increase depth of focus

>to reduce aberrations (image actually gets sharper)

>to unstress a lenticulated imaging array

>to reduce atmospheric degradations

 

But most all of those rationalizations assume a crummy optical system to start with... which, admittedly is the majority of amateur telescopes. If your telescope is a superior premium instrument --- its performance will be spectacularly best when used wide-open.    Tom


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#3 Jon Isaacs

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Posted 04 May 2021 - 04:24 PM

What is the purpose? I read that it makes a longer focal length, to what end? And are there different methods?

 

Douglas:

 

The focal length is unchanged, the aperture is reduced so that focal ratio, (focal length / aperture) is increased. This can reduce aberrations.

 

For example, chromatic aberration of a refractor depends on both focal ratio and aperture. Reducing the aperture reduces the CA, increasing the focal ratio reduces the CA.

 

Thus stopping down the aperture can dramatically reduce the CA. With this comes a loss of resolution and light gathering. Sometimes it's worth it.

 

Point an 80 mm F/5 achromat at Venus and it will be surround in purple haze. Remove only the center lens cap, it's aperture is now about 40 mm, the focal ratio is F/10 and  there's very little CA to be seen.

 

Jon


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#4 Steve Dodds

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Posted 04 May 2021 - 04:44 PM

Usually stopping down is done off axis, providing a unobstructed, high f/ratio refractor like performance.


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#5 Jon Isaacs

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Posted 04 May 2021 - 06:06 PM

Usually stopping down is done off axis, providing a unobstructed, high f/ratio refractor like performance.

 

That would be a Newtonian. 

 

Of course obstructing the view with an off-axis mask reduces the fine scale contrast and reduces the resolving power..  indeed just like a refractor.. ;)

 

Jon


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#6 RLK1

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Posted 04 May 2021 - 06:20 PM

That would be a Newtonian. 

 

Of course obstructing the view with an off-axis mask reduces the fine scale contrast and reduces the resolving power..  indeed just like a refractor.. wink.gif

 

Jon

My astrosystems mirror cover comes with a port for off axis use but I've used it just a few times to try it out. It's supposed to turn my 16" f4.5 dob into an unobstructed 6" f12 scope or something along those lines. However,  If your mirror is really good there's no need for it. If your mirror quality isn't great, then using a portion of the mirror minus the edge can be helpful. Fortunately, I've got a great mirror...


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#7 Steve Dodds

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Posted 04 May 2021 - 06:31 PM

My astrosystems mirror cover comes with a port for off axis use but I've used it just a few times to try it out. It's supposed to turn my 16" f4.5 dob into an unobstructed 6" f12 scope or something along those lines. However,  If your mirror is really good there's no need for it. If your mirror quality isn't great, then using a portion of the mirror minus the edge can be helpful. Fortunately, I've got a great mirror...

The other reason to do it is if your seeing conditions are not great, like here in Utah, I put a mask on every time I look at planets.  Bad seeing is magnified by aperture. 


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#8 RichA

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Posted 04 May 2021 - 09:45 PM

No, it doesn't make a longer focal length.

 

There are a few circumstances where intentionally masking the outside edge (or more) of the aperture can help.

 

>if the image is overpoweringly bright

>if the primary mirror has a ~turned edge~

>to increase depth of focus

>to reduce aberrations (image actually gets sharper)

>to unstress a lenticulated imaging array

>to reduce atmospheric degradations

 

But most all of those rationalizations assume a crummy optical system to start with... which, admittedly is the majority of amateur telescopes. If your telescope is a superior premium instrument --- its performance will be spectacularly best when used wide-open.    Tom

True.  Good telescopes aren't like camera lenses with mediocre optics and dozens of elements that often benefit from stopping down to control uncontrolled aberrations.  Only recently have camera lenses reached a point where using them wide open produces results comparable to stopping down one stop.  A diffraction-limited scope shouldn't benefit from it.  However, I have seen scopes with good optics and sub-standard baffling that do improve contrast when stopped down.


Edited by RichA, 05 May 2021 - 11:37 AM.

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#9 ccaissie

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Posted 05 May 2021 - 07:15 AM

Our club 16" f/4.5 is sensitive to atmospheric effects (New England is not known for Antoniadi I or II seeing) and occasional tube and cooldown issues.  Though it star tests well at full aperture, masking it down to a 6" f/12 off-axis Newtonian creates amazingly sharp planetary views...Which in the North are often through lots of atmosphere.  The idea of eliminating the 25% CO (Central Obstruction) shows up here as well.  Reducing the aperture with a concentric mask increases the relative CO.

 

Tight collimation, high altitude objects, and steady seeing allows GREAT full-aperture operation.

 

We have a 6" f/10 Jaegers that has a chipped lens.   Blacking out the chip helps, but a concentric mask gives us 5" f/12, and it compares with a D&G 5f12 I had.  


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#10 MKV

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Posted 05 May 2021 - 10:28 AM

Stopping down an image-forming optical system affects mostly longitudinal spherical aberration, LSA and its chromatic component, by a factor y²/8f, and sagittal coma by a factor hy²/4f² or h/(16N²).*

 

where:

y = aperture radius (half of aperture diameter D/2)

f = paraxial focal length

h = image height off-axis.

N = focal ratio or f-number of the system, i.e. f/D or f/2y.

 

In axially well corrected, systems (y²/8f ≈ 0) stopping down an aperture has no effect on spherical aberration, but will affect coma by the inverse square of the focal ratio, and directly (linearly) by the image height off-axis.

 

Example: for an f/5 Newtonian (N = 5), at h = 1, sagittal coma = 0.0025 units in length. Stopping it down to f/8 reduces its sagittal coma to 0.001 units.

 

_____

*Tangential coma = 3 times longer than the sagittal coma, or 3h/(16N²), but it's invisible except with very bright stars and/or long exposure because it is much dimmer than the sagittal component.


Edited by MKV, 05 May 2021 - 01:38 PM.


#11 RLK1

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Posted 05 May 2021 - 12:22 PM

Stopping down an image-forming optical system affects mostly longitudinal spherical aberration, LSA and its chromatic component, by a factor y²/8f², and sagittal coma by a factor hy²/4f² or h/(16N²).*

 

where:

y = aperture radius (half of aperture diameter D/2)

f = paraxial focal length

h = image height off-axis.

N = focal ratio or f-number of the system, i.e. f/D or f/2y.

 

In axially well corrected, systems (y²/8f² ≈ 0) stopping down an aperture has no effect on spherical aberration, but will affect coma by the inverse square of the focal ratio, and directly (linearly) by the image height off-axis.

 

Example: for an f/5 Newtonian (N = 5), at h = 1, sagittal coma = 0.0025 units in length. Stopping it down to f/8 reduces its sagittal coma to 0.001 units.

 

_____

*Tangential coma = 3 times longer than the sagittal coma, or 3h/(16N²), but it's invisible except with very bright stars and/or long exposure because it is much dimmer than the sagittal component.

So, say if a mirror has a turned edge, and you use an off-aperture port that picks up the 70% zone or thereabouts, it's more substantial in reducing an edge-induced aberration than what you've stated above, correct? And are you also saying it doesn't have an effect on seeing?



#12 KBHornblower

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Posted 05 May 2021 - 12:53 PM

Our club 16" f/4.5 is sensitive to atmospheric effects (New England is not known for Antoniadi I or II seeing) and occasional tube and cooldown issues.  Though it star tests well at full aperture, masking it down to a 6" f/12 off-axis Newtonian creates amazingly sharp planetary views...Which in the North are often through lots of atmosphere.  The idea of eliminating the 25% CO (Central Obstruction) shows up here as well.  Reducing the aperture with a concentric mask increases the relative CO.

 

Tight collimation, high altitude objects, and steady seeing allows GREAT full-aperture operation.

 

We have a 6" f/10 Jaegers that has a chipped lens.   Blacking out the chip helps, but a concentric mask gives us 5" f/12, and it compares with a D&G 5f12 I had.  

My bold.  With perfectly figured optics and perfect seeing, the 16" as is has subtle reduction in small-scale contrast on a planet compared to an unobstructed 16" scope.  It will far outperform a 6" on planet details under these hypothetical conditions.  Poor seeing will have worse effects on the 16" most of the time, but I have seen tantalizing moments of good seeing in which the large scope could show its stuff.


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#13 MKV

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Posted 05 May 2021 - 04:38 PM

My bold.  With perfectly figured optics and perfect seeing, the 16" as is has subtle reduction in small-scale contrast on a planet compared to an unobstructed 16" scope.  It will far outperform a 6" on planet details under these hypothetical conditions.  Poor seeing will have worse effects on the 16" most of the time, but I have seen tantalizing moments of good seeing in which the large scope could show its stuff.

A turned edge doesn't contribute to LSA or coma. It simply scatters light and ruins image contrast. Small apertures perform better in less than optimal seeing because of the size of equithermal air cells, which are between 3 ad 4 inches. This means that small apertures will "see" steadier air at any time compared to bigger apertures. Oh by the way, LSA coefficient is y²/8f, or simply D²/32f. My original post has y²/8f². It was a cut-and-paste error on my part.


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#14 RLK1

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Posted 05 May 2021 - 05:04 PM

A turned edge doesn't contribute to LSA or coma. It simply scatters light and ruins image contrast. Small apertures perform better in less than optimal seeing because of the size of equithermal air cells, which are between 3 ad 4 inches. This means that small apertures will "see" steadier air at any time compared to bigger apertures. Oh by the way, LSA coefficient is y²/8f, or simply D²/32f. My original post has y²/8f². It was a cut-and-paste error on my part.

So, in other words, your initial post in this thread doesn't take into account that one of the reasons for using the off-aperture port is to obviate the deleterious effects of a turned-edge. And, as stated elsewhere in the thread, it's also useful for mitigating the effects of poor seeing. Oh, and thank you for correcting your cut and paste. Was that your own cut and paste or was it from an uncited source? 



#15 MKV

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Posted 05 May 2021 - 09:25 PM

So, in other words, your initial post in this thread doesn't take into account that one of the reasons for using the off-aperture port is to obviate the deleterious effects of a turned-edge. And, as stated elsewhere in the thread, it's also useful for mitigating the effects of poor seeing. Oh, and thank you for correcting your cut and paste. Was that your own cut and paste or was it from an uncited source? 

I was simply responding to comments about "stopping down aperture" in general. Using an off-axis mask is a different story.

 

I wrote out the equations in an equation editor and erroneously copied and pasted the one containing aperture and focal length squared (for coma). Noticed it later. The equations are K. Schwarzschild's well known work dating back to the mid 19th century.

 

Any kind of stop that blocks a turned edge will improve performance. 

 

Astronomical seeing consists equithermal air cells. Equithermal cells have uniform density and range in size from 3 to 4, or maybe even 5 inches or so. Smaller telescopes have the advantage over larger ones in that these cells fill their entrance pupil with steady and uniform air, which results in a more stable and clear view at the eyepiece end. Telescopes with larger apertures see different cells at once and hence present a more unstable views on usual nights. This is why a well made 4-inch APO will often outperform a much bigger reflector or catadioptric. So, aperture size is definitely related to seeing as a factor in image quality.


Edited by MKV, 05 May 2021 - 09:26 PM.

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#16 dave brock

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Posted 06 May 2021 - 01:47 AM

I've never seen an excellent small telescope outperform an excellent larger telescope, in 50 years of observing no matter what the seeing.
Bad seeing stops a bigger scope from reaching IT'S potential more than a smaller scope from reaching IT'S potential but in my experience it doesn't stop the bigger scope from performing at least as well as a smaller one.
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#17 MKV

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Posted 06 May 2021 - 12:52 PM

I've never seen an excellent small telescope outperform an excellent larger telescope, in 50 years of observing no matter what the seeing.
Bad seeing stops a bigger scope from reaching IT'S potential more than a smaller scope from reaching IT'S potential but in my experience it doesn't stop the bigger scope from performing at least as well as a smaller one.

It all depends what you mean by outperform. If I can not get a single glimpse of an Airy disc due to scintillation, that to me is unacceptable performance.  

 

In theory a bigger telescope always outperforms a smaller telescope. In reality, on special and rare nights of "perfect" seeing, a bigger telescope should  outperform a smaller one. On average, most common, or very bad seeing, the opposite is true. Looking at stars with a bigger one on such nights is like looking at something through the water. A smaller one shows much steadier or even very steady images. It doesn't mean it is capable of resolution better than the larger one, just that it's images are steady, better defined, and more pleasant to look at. 

 

I can say with utmost certainty that my first reflector, an Edmund 3-inch f/10 spherical mirror, was useable throughout the year. I didn't even know what "seeing" is. Every night I used it, its the images were rock steady. Of course, the same can't be said of an 8 or 10 inch telescope, or an excellent C14, no matter how good it is, and never mind a 20-inch.  

 

https://www.youtube....h?v=SlY56hIdeuE

 

The effects of "seeing" are not as dramatic for extended objects, although the object may appear to be "swimming" as if submersed in water, and resolution always suffers.


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#18 Argonautt

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Posted 06 May 2021 - 03:23 PM

It might be irrelevant to looking through an atmosphere thousands of meters high, but I wonder what would change if you made the OTA much, much longer, giving you perhaps a dozen metres of air you could regulate.



#19 dave brock

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Posted 06 May 2021 - 06:43 PM

It all depends what you mean by outperform.


I'm talking about planetary detail.

#20 Oberon

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Posted 07 May 2021 - 11:06 PM

It might be irrelevant to looking through an atmosphere thousands of meters high, but I wonder what would change if you made the OTA much, much longer, giving you perhaps a dozen metres of air you could regulate.

More is worse.
 

Roughly speaking, professionals break the causes of seeing down as follows:

 

1. mirror seeing

2. tube seeing

3. dome seeing

4. local seeing

5. atmospheric (usually upper) seeing

 

Basically, more is worse. A bigger mirror, tube or OTA is most likely going to have bigger effects. More atmosphere is worse (mountains are better because less atmosphere, space is best).


Edited by Oberon, 08 May 2021 - 07:27 AM.

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#21 Jeff B

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Posted 09 May 2021 - 12:22 PM

Here are a couple of good reasons to stop down an aperture, both associated with a bum edge.

 

The first set of DPAC images is from an APM 152ED at full, then reduced to 5" aperture.  Visually the images of Jupiter and Saturn at full aperture were lacking in sharpness with a somewhat "vague" focus and a "terrible" stellar airy disk.  At reduced aperture, the images improved significantly now being really very sharp despite the residual over-correction.

 

The second set is from an Orion XT8 Plus.  Again, at the full 8" aperture the images of Jupiter and Saturn were soft but at focus, the airy disk actually looked "OK".  But the give away was the soft images at focus.  At 7" aperture, the visual performance was, again, notably sharper with a clear focus position, despite the residual zoning.

 

But you also have to be careful with stops.

 

For Newtonians, the effective central obstruction gets larger (though the fully illuminated FOV gets larger)

 

For refractors, I've found a hefty portion of them have mild to moderate zoning in the middle.  That's where it does the least optical damage but again, stopping the aperture increases the amount of real estate any center zoning may have relative to the reduced aperture.

 

But experimenting with stops is soooooo quick and easy (requiring just paper, scissors and tape), that I highly recommend just experimenting with it if you're curious or suspect you have an optical issue.

 

Jeff

Attached Thumbnails

  • APM 152ED, Green, Full Aperture, Inside.jpg
  • APM 152ED, Green, 5 Inches, Inside.jpg
  • 8XT, Full Aperture, Outside.jpg
  • 8XT, 7 in, Outside.jpg

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#22 Jon Isaacs

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Posted 09 May 2021 - 10:53 PM

It all depends what you mean by outperform. If I can not get a single glimpse of an Airy disc due to scintillation, that to me is unacceptable performance.  

 

In theory a bigger telescope always outperforms a smaller telescope. In reality, on special and rare nights of "perfect" seeing, a bigger telescope should  outperform a smaller one. On average, most common, or very bad seeing, the opposite is true. Looking at stars with a bigger one on such nights is like looking at something through the water. A smaller one shows much steadier or even very steady images. It doesn't mean it is capable of resolution better than the larger one, just that it's images are steady, better defined, and more pleasant to look at. 

 

I can say with utmost certainty that my first reflector, an Edmund 3-inch f/10 spherical mirror, was useable throughout the year. I didn't even know what "seeing" is. Every night I used it, its the images were rock steady. Of course, the same can't be said of an 8 or 10 inch telescope, or an excellent C14, no matter how good it is, and never mind a 20-inch.  

 

https://www.youtube....h?v=SlY56hIdeuE

 

The effects of "seeing" are not as dramatic for extended objects, although the object may appear to be "swimming" as if submersed in water, and resolution always suffers.

 

A lot of vague terms, "average , most common seeing" that all depends on where you live. 

 

Seeing the airy disk is unnecessary for a larger aperture scope to outperform a smaller one. Rock steady images are not necessary for a larger scope to seriously outperform a smaller one.   You may never see the Airy disk in a 20 inch scope, the spurious disk is about 0.25" in radius but you can still split doubles and provide planetary contrast far greater than a 4 inch scope or even a 10 inch.  

 

I don't think Dave Brock was talking about theory, he was talking about what he had seen in 50 years of observing.  What one sees at the eyepiece is not theory...  My experience is similar to Dave's.  I have two main observing sites, one in the high desert where the skies are dark but the seeing is so-so, that means 2" is pretty good, 1" is rare and it can be worse.  I am able to observe there 100-120 nights a year with an average of about 4 hours per night.  My normal equipment is a 4 inch Apo refractor and either a 16 inch or 22 inch Dob.

 

My main objects of interest are deep sky but I will look at double stars and the planets. Regardless of the seeing, I never seen more planetary detail or split closer doubles in the 4 inch than in the larger scopes but of course the opposite is true.  

 

The other site is my home in San Diego where the skies are bright but the seeing is generally quite good, often very good. This is more like 60-80 nights a year.  In San Diego, 2" is poor, 1" is decent, less the 1" quite good.  Under these conditions, my 10 inch and 13.1 inch Dobs out perform my 3, 4 and 5 inch refractors.

 

Jon


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#23 MKV

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Posted 10 May 2021 - 05:26 AM

I don't think Dave Brock was talking about theory, he was talking about what he had seen in 50 years of observing.  What one sees at the eyepiece is not theory...  

You're quite right there. In theory a Newtonian will give a dimensionless dot for a star image. :o)

 

I've lived on the East Coast since I was a kid and our summers were always hot and humid. I lived in Arizona near Flagstaff for two years with low humidity and cool nights. I didn't like being out at night because of creepy crawlies and other critters. Florida is the same, not counting mosquitoes. Virginia, North Carolina and New York were better especially in winter time.

 

I've had a variety of telescopes from Edmund's small 3-inch f/10 spherical mirror to am 8inch Classical Cassegrain, two C8s, a Unitron 4-inch refractor, a Jaegers 3 and 5 inch f/5 refractors, A 6-inch f/4 Houghton, two 6-inch f/8s, and one 12.5-inch f/6 Newtonian. I had a chance to look through an 8-inch Clarke refractor on Long Island, NY, and a C14 in Miami, Fla. The C14 was the absolute worst experience ever.

 

In the end, it all depends on an individual. The sky where I currently reside is too bright and the seeing is almost never to my liking. That's why I eventually shifted from observing with optics to testing optics as a hobby. Additionally, I stopped being amazed by the same objects, same doubles, same lunar craters, same fuzzy DSOs. Large observatory images taken of the same objects are way better than anything I was ever able to see in a swimming layer of air while holding my breath. 


Edited by MKV, 10 May 2021 - 05:27 AM.


#24 dave brock

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Posted 10 May 2021 - 05:42 AM

Large observatory images taken of the same objects are way better than anything I was ever able to see in a swimming layer of air while holding my breath.


Hmmm.... do you also prefer looking at postcards rather than being there taking your own photos?

#25 MKV

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Posted 10 May 2021 - 06:00 AM

Hmmm.... do you also prefer looking at postcards rather than being there taking your own photos?

If the postcard pictures are better...yup. Why go through all the trouble (and expense) to get something inferior? 




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