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"Stopping down" aperature

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#26 Sketcher

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Posted 10 May 2021 - 09:27 AM

There's not any one purpose that applies to all people and all telescopes.  But probably the most common reasons are to reduce (in some cases, diagnose) the negative effects of poor optics and/or poor seeing conditions -- regardless of one's opinions on the effectiveness of reducing the aperture for such purposes.

 

But there can be other reasons as well:

 

Some may want to see what can be accomplished with a smaller aperture telescope, or what is the minimum aperture necessary to see this or that, or how gradually increasing one's aperture from 1cm up to the dull aperture of whatever telescope one is using affects what one can see.

 

Some may find it beneficial as well as economical to use sub-aperture, white-light solar filters.

 

Then there's the simple joy of making your own aperture stops and experimenting with them.

 

I've masked apertures of Newtonian reflectors, SCTs, and refractors -- and not always for the same reasons.  I've experimented with reduced apertures while using an essentially perfect apochrmat refractor and I've experimented with the effects of masking on telescopes (regardless of type) with poor, or less-perfect optics.

 

I once made a hexagonal mask to experiment with its use in double star observation.  I've experimented with the use of a full-aperture apodizing mask (or screen).

 

Sub-aperture masks can be used to intentionally reduce the amount of detail one can see.  This can be a less overwhelming way of getting started in the world of astronomical sketching -- particularly with the moon.

 

I made a series of careful Saturn observations using a very high quality 130mm apochromat refractor -- starting with a 1-cm aperture and moving up in 1-cm increments until I got to the telescope's full aperture.  I took notes and made sketches.  I started small so I wouldn't be tempted into thinking I'm seeing something that can no longer be seen -- which would be more likely if I had started at the full 130mm aperture and worked my way down.

 

I determined the smallest aperture at which I was able to see, just barely, the first hint of the existence of Cassini's Division (and I made the mistake of posting the result to an online forum where a so called "expert" claimed that I could not have possibly made that observation.)

 

So the "purpose" is all over the place.  The purpose is determined by the individual.  And for some (many? most?) there's no purpose (for them) to use an aperture mask.


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#27 Steve Dodds

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Posted 10 May 2021 - 04:30 PM

Hubble pics are certainly pretty, but pale to seeing a object with my own eyes, through a telescope I built.  There are astrophotography guys the will disagree with me, but they are plain wrong.


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#28 MKV

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Posted 10 May 2021 - 10:11 PM

Hubble pics are certainly pretty, but pale to seeing a object with my own eyes, through a telescope I built.  There are astrophotography guys the will disagree with me, but they are plain wrong.

You don't need a Hubble for spectacular images. An 8-inch CDK or RC Cassegrain, even a good 4-inch APO, will give you jaw dropping results -- it's a lot of work and it's also "being there" of sorts. There are countless images posted on the internet and NASA sites to enjoy for detail and clarity no earthbound telescope will give "live.". You can even sign up for a professional telescope online usage and capture these images yourself without going out.

 

But I understand the feeling you're trying to convey. Several years ago I was on a cruise ship lounging on our cabin balcony when, suddenly, with my bare eyes, even at my age, I spotted the glimpse of the Andromeda galaxy. It was a small, faint grayness in a seemingly jet-black sky. Last time I saw it was many decades ago when I was 14. It's still there

 

Two million years ago its light left on its journey towards us, reaching us today. When it left, the earliest humans were just emerging. When you see the object and think how far it is and the fact that you're looking back in time, it's not the detail that counts so much as what you're witnessing in that moment, and it's awe-inspiring. When you see a picture it's not the same experience, sort of like watching a game on TV vs being there in the stadium. Both have their place and worth.


Edited by MKV, 10 May 2021 - 10:13 PM.


#29 dave brock

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Posted 11 May 2021 - 12:06 AM

Think of the poor photons that travelled all that distance only to land on the boat's deck beside you,

having just completed a totally wasted journey.


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#30 dave253

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Posted 11 May 2021 - 01:43 AM

Sopping my 14” down to effectively a 50mm F/33 provides great lunar views! 


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#31 MKV

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Posted 11 May 2021 - 01:44 AM

Think of the poor photons that travelled all that distance only to land on the boat's deck beside you,

having just completed a totally wasted journey.

"Wasted" implies there's was another purpose, or even a purpose to begin with. Science tells me the universe is made up of rocks and gas that go around in circles. :o) Purpose? Hmmm.



#32 dave brock

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Posted 11 May 2021 - 02:04 AM

Wasted because (at least to my knowledge) a boat deck is unable to process a photon.


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#33 Oberon

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Posted 11 May 2021 - 04:16 AM

"Wasted" implies there's was another purpose, or even a purpose to begin with. Science tells me the universe is made up of rocks and gas that go around in circles. :o) Purpose? Hmmm.

In astronomy, photons alone inform the science. A photon lost is indeed a photon wasted. But if no one sees the photon, is it even light?


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#34 MKV

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Posted 11 May 2021 - 05:38 AM

In astronomy, photons alone inform the science. A photon lost is indeed a photon wasted. But if no one sees the photon, is it even light?

Perhaps we can agree that photons inform the scientist. There's no science without a scientist. Regardless, a photon will be exactly what it is -- a packet of radiomagnetic energy.



#35 cuzimthedad

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Posted 11 May 2021 - 10:07 AM

Back on topic please



#36 RLK1

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Posted 11 May 2021 - 11:48 AM

I see a significant benefit of using an aperture reducing port, other than those already stated elsewhere in the thread, being the ability to have a convertible telescope without the need for changing secondaries as in some convertible scopes, especially if you have the aperture to do so. In my case of a 16",  all I have to do to convert it to a 6" f12 or thereabouts is to lift up a knob on a round wooden port cover located on the mirror cover of my astrosystems dob. The mirror cover remains in place, thus protecting the unexposed surface of the primary while only the desired area is exposed for viewing. There's ample distance between the mirror cover and the primary to allow for air circulation.  Literally an instantaneous conversion without the need for making an aperture mask and fitting it to the mirror or, in the case of a dedicated convertible scope, for swapping out a secondary. 


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#37 DouglasPaul

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Posted 17 May 2021 - 11:05 PM

Lots of things to think about and thanks for all the input. 



#38 Jeff B

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Posted 18 May 2021 - 07:52 AM

I have access to some large aperture refractors and I routinely use aperture stops especially with the 11" F12.3 achromat (9.5", 8.5", and 7.0").  There are several reasons for their use and, for example, with the 11":

 

1. The stops mask the edge of the objective during cooling.  Objectives cool from the edge inwards resulting in mild to modest undercorrection depending on the degree of cooling going on.  Masking the edge gives a better overall figure during the cooling process allowing me to use the scope almost immediately at high power.  The 8.5" stop is the one I use the most.  As the objective stabilizes thermally, the stops may come off unless....

 

2.  ....the seeing sucks, in which case, for high power use, the stop may well stay on.  This allows me to "modulate" the aperture to "match" the prevailing seeing conditions.  I like steady views and I'm willing to reduce aperture to get them.

 

3.  The color correction (both spherochromatism, which is low to begin with at F12.3, and longitudinal CA, which is the difference in focus between the red, blue and green) improves, especially with achromats.  With the 11" F12.3 (135" FL) and the 9.5", 8.5" and 7" stops, the Conrady ratios are 1.1, 1.5, 1.9, and 2.8 respectively.  

 

4.  Related to number 3, is the fact that I have a suite of Chromacor color correctors, with varied strengths, and placed at specific points in the optical path, depending of the stop used.  For example, with the 11" stopped to 9.5" (F14) and using my bino-viewers (with no barlow), I place a Chromacor U1 (which adds undercorrection) about 1.5" to 2" in front of the 2" diagonal nose piece.   The resulting color correction reminds me very much of a 3" F15 achromat, but at a 9.5" aperture.  And it gets even better at 8.5" and 7.0" aperture.

 

I also use stops (7.0", 6.5" and 6.0") for my 8" F9 achromat as well for exactly the same reasons. 

 

So in the large achromat world, which is admittedly a small one, aperture stops are of great value to me.

 

Jeff


Edited by Jeff B, 18 May 2021 - 07:54 AM.

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#39 dan chaffee

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Posted 18 May 2021 - 12:04 PM


3.  The color correction (both spherochromatism, which is low to begin with at F12.3, and longitudinal CA, which is the difference in focus between the red, blue and green) improves, especially with achromats.  With the 11" F12.3 (135" FL) and the 9.5", 8.5" and 7" stops, the Conrady ratios are 1.1, 1.5, 1.9, and 2.8 respectively.  

 

Good reasons for stopping down that beast, but I'm pretty sure acceptable color correction is

relative to the viewer's tolerance levels of unfocused secondary spectrum.   A classic

8" f/15 Fraunhofer has a color blur 5 times the size of the diffraction disk and is still too

much for my viewing tastes.  Full 11 inch f/12.3 is considerably worse than that without a color

corrector. Personally, a defocused color blur of 3 times the diffraction disk is the limit

of what I want to observe with (ie 6 in. f/15). but then I spent years with newtonians

before using refractors.


Edited by dan chaffee, 19 May 2021 - 03:33 AM.


#40 starcruiser

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Posted 19 May 2021 - 08:03 PM

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

It's to simulate an unobstructed primary mirror (if you have a large reflector) and (supposedly) provide sharper, better planetary views without the large secondary getting in the way. I first read about it in magazines back in the early 80s. I tried it briefly on the 13.1" Coulter Odyssey 1 with the recommended 5" off-axis aperture. I wasn't impressed. It made everything, even bright Jupiter look like being viewed thru a solar filter after being accustomed to observing at full aperture. I never saw anything that looked "better" with the 5" off axis vs full 13", only dimmer...much dimmer. From my limited experience in this hobby, I'd say that aperture is still the single biggest factor in determining how much you get to see.


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#41 Roel

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Posted 21 May 2021 - 02:42 PM

A good reason for stopping down the aperture of a big telescope is to be able to use much lower powers.

Huh?

That's right! For example, using a 40mm 70 degree eyepiece in a 16" f/4 is pretty useless with an 10mm exit pupil. Background brightness is washing out every detail and stars at the edge are blurry seagulls.
Stop it down to 10" (f/6.3) and a relatively cheap 40mm eyepiece gives a nice exit pupil of 6.35mm and will perform much better at the edge.

This way I can comfortably use my 16" f/4 for true fields of view up to 1,75 degrees - the physical limit for a 2" eyepiece at this focal length.


Edited by Roel, 21 May 2021 - 02:50 PM.


#42 RalphMeisterTigerMan

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Posted 22 May 2021 - 02:40 PM

Percival Lowell was a big "fan" of stopping down the aperture of the 24" to suit the "seeing conditions". That is why he had a metal, manual controlled steel "aperture mask" which could be closed all the way to 6-inches. It has a fancy term whose name currently escapes me.

 

Clear skies and keep looking up!

RalphMeisterTigerMan


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