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How reducing the aperture can help with planetary observation?

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#1 jromev87


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Posted 11 September 2019 - 03:52 PM

Hi everyone! I have 2 telescopes, one is a reflector D=355mm F=1650mm, the other one is a refractor D=120mm F=600mm. I was reading something that I don't quite understand in an article about buying a first telescope. It said that


"adding a cap (word is "cache" in french) can improve planetary observation at high magnification as it will increase the effective focal lenght".


I know that my telescopes each have a front cap of the same diameter as the telescope that also have a smaller holes covered by a smaller cap. I always thought that it was used for solar observation so the telescope would not receive to much light and heat, but I wonder if it can be used in an other way. Does it have something to do with turbulence and how a large aperture can be affected by it?


Is using, let's say, my 355mm telescope with it's big front cap without the smaller cap would improve my planetary observation. And what about the 120mm?


Would love to make some tests outside, but it's cloudyTOnightS...

#2 Barlowbill


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Posted 11 September 2019 - 04:07 PM

Take the small cap off.  Make sure the small opening does not lie over one of your spider vanes.  Must lie in between the vanes.  Now look at Jupiter.  It should not be as bright but you very well may be able to see detail better.  It is called "masking down".  I made a mask out of cardboard since my tube cap does not have a built-in small cap.  Look up Bahtinov mask on the web.  I could see more detail but I found I can see about the same with a blue color filter.  Fun experiment.

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#3 Nicole Sharp

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Posted 11 September 2019 - 04:14 PM

I've been reading about this too, especially for the upcoming Mercury transit.  For daytime use, apertures larger than 5 inches (sometimes cited as 6 inches) are usually warned as not improving the resolution of Solar observation due to atmospheric turbulence.  The larger aperture just makes the turbulence more visible, whereas the reduced resolution from a smaller aperture can smooth out the turbulence for better visibility.  For photography, a larger aperture means you need shorter exposures and faster framerates to lucky-image through the turbulence.


I've also read that atmospheric turbulence occurs in cells averaging about 4 inches to 6 inches in diameter, which is why telescopes with apertures larger than that have more issues with seeing under poor atmospheric conditions.  Basically, an 8-inch telescope might be looking through two different atmospheric cells, which may actually have different refraction indices, so that can mess things up.  A famous historical example of this phenomenon is with Percival Lowell, who actually stopped the aperture of his 24-inch telescope down to a mere 3 inches in order to better observe Venus in the daytime.

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#4 sg6


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Posted 11 September 2019 - 04:20 PM

Optically if perfect optics the bigger should be better. The catch being the optics are not perfect. The edges are generally the poor bits and thay add sufficent aberrations in that this has to be accounted for. Easy thing is that that parabolic mirror is not parabolic, it will be a bit of a hybrid shape.


The faster the primary then the harder it is to get a good curve, so fast optics can result in a less distinct image. And that is not what you want. All you are doing is reducing the front aperture and so using (hopefully) the better part of the objective and so getting a better resultant image.

#5 aa6ww



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Posted 11 September 2019 - 04:44 PM

Basically you are increasing the focal ratio of your scope by reducing the aperture. Usually this helps when skies cant support larger apertures because you are trying to see through a larger area of a turbulent sky. More area under mediocre seeing conditions usually doesn't help. This is just one of many reasons why sometimes you hear of people boasting how their smaller scopes seem to out perform a larger scope.

Super planetary telescopes are generally high focal ratio scopes. You would get darker back ground skies, and sharper optics. Its not night and day difference but it all helps. The Classic refractor group can talk circles about this because we love the high focal ratio scopes.

This is an APO myth which comes from conditions which favor smaller apertures due to turbulent or mediocre conditions, not really because of optics. Another reason could be because the larger scope takes longer to acclimate to outside conditions then smaller scopes so quick grab and go sessions usually favors smaller scopes.

Back to your question:

Focal ratio (Focal number) = Focal length / diameter of your aperture

so in your case:

1650mm / 355mm aperture = F4.6

so that's your focal ratio.

If you make an aperture "mask" which I think is the same as your description of a cap and make a cover so you have 100mm of opening, (Aperture) your focal ratio would be:

1650 / 100mm = F16.5

In the refractor world, people would ogle over an F16.5 100mm telescope.

You would have to make an offset hole, so you block out the secondary mirror, and even position the hole between the spider vanes.

It may sharpen things up though you will loose a large amount of light and some resolution. Experiment with different sizes, just make it out of cardboard and place it over the opening of your scope. You can tape it down so it holds itself in place.

I use aperture masks to tone down the light when looking at the moon instead of using Neutral density filters.
The masks help noticeably when splitting double stars when you try and push the magnification and your stars could get a little puffy. So it could help sharpen up your planetary views.

I've made aperture masks for my 6" APO, 1200mm FL. I made a 100mm mask (F/12) and an 80mm mask (F/15) in less then ideal seeing conditions. The results are subtle but it helps and its fun also. Its noticeable on puffy stars cleaning them up and making them sharper in turbulent conditions.

I hope this helps.


Edited by aa6ww, 11 September 2019 - 04:51 PM.

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#6 Asbytec


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Posted 11 September 2019 - 04:50 PM

Having observed Jupiter with a 6" for many years in nice tropical seeing, I agree with Nicole. Seeing does favor smaller apertures. There is a lot to be said for a steady image when observing planets. However, resolution is reduced. So, there is a trade-off between a very appealing steady image for the resolution of a larger aperture. 


But, all is not lost for larger aperture. A smaller aperture presents a larger Airy disc, but an aperture about 3 times the diameter of the "seeing cell" actually produces the smallest seeing induced blurry star images even though the star images are not as well defined as in the smaller aperture. So, with a "seeing cells" of about 100mm in good seeing will benefit apertures up to about 300mm. See bottom of figure 79 in the link below. In fact, the entire section on seeing is worth reading. 




Now, Airy discs and the resolution limit set by Rayleigh and Dawes do not apply to extended objects like the moon and planets, but the idea of higher resolution does. So, you can stop down an aperture and get a pleasing steady image, or you can use aperture to get higher resolution given the seeing conditions. Many observers simply use magnification where the seeing effects are not readily visible. 


If you use a mask, I'd recommend the largest aperture you can fit over the primary mirror and between the spider vanes to avoid the obstruction. Not the little ~2" hole in the front cap. By the way, the obstruction does have some effect, but it's generally overblown. A larger obstructed aperture will always(sic) have a higher level of contrast at all spatial frequencies than a smaller aperture. What you're really gaining with an aperture mask is a pleasing image. Again, a pleasing image is, well, pleasing to look at. Nothing wrong with that.  

Edited by Asbytec, 11 September 2019 - 04:59 PM.

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#7 vtornado



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Posted 11 September 2019 - 05:05 PM

On paper aperture should win

1) the  theoretical resolution scales with aperture

2) by gathering more light you activate the cone cells of your eye which specialize in color and fine detail.


However ...

1) In a fast achromat detail will be smeared by unfocused red/blue light.  Reducing the aperture while

maintaining the same focal length increases the focal ration and lowers the chromatic ratio.

Perhaps lowering this is more effective than the loss of resolution.  Dawes ratio increases linearly with

diameter the chromatic ration falls by the square.


2) in low cost optics it is often the edge of the optic that has the most abberations.  By masking you

eliminate the edge.  I assume you also reduce zonal errors of the optic.


3) most big optics are fast optics.  Generally it is harder to craft high fidelity large fast optics than slow small optics.

I have not seen any  dud $100, f10 refractors.  I have seen dud  $500 10 inch f5 reflectors.


4) this i'm not sure of.  Many say that a large aperture does not perform well in turbulent seeing.  This may be a

fallacy of expectations.  In bad seeing a 12 inch dob will be limited in its potential. For example suppose seeing is limited to 100x by the atmosphere.  The dob  has a potential of  300x, so you are disappointed.  A good 60 mm refractor will also be limited to 100x on this night but that might be its limit on ANY night.

So you have wheeled out this huge instrument, waited an hour for it to cool, collimated it , and it is no better than a 60mm refractor just plopped down on the ground.

Edited by vtornado, 11 September 2019 - 05:18 PM.

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#8 cookjaiii


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Posted 11 September 2019 - 05:09 PM

My 100mm f/6 achromatic refractor does so-so on Jupiter, with lots of purple haze and poor visibility of surface details.  When I mask it down to f/11, the purple disappears and details pop.  You don't lose too much detail on Jupiter at the smaller aperture because it is so bright.  

Edited by cookjaiii, 11 September 2019 - 06:37 PM.

#9 jromev87


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Posted 11 September 2019 - 05:19 PM

Thank you all for your answers.


So adding a mask will increase the focal ratio, not the focal lenght as it was written in the article I had read. By doing so, I should see a dimmer object with less resolution, but that can be more steady. This added steadiness could mean mean a better image and potentially help me to use higher magnification when the seing is not so good. Am I right so far?

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#10 fcathell



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Posted 11 September 2019 - 06:05 PM

Norme pretty much covered it, but one other thing that may affect planetary viewing with larger scopes is the glare from the planet due to the large aperture. The glare itself can stop down your eye's pupil. Using neutral filters or stopping down the scope's aperture to a reasonable level should help this.



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#11 Asbytec


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Posted 11 September 2019 - 07:50 PM

"Many say that a large aperture does not perform well in turbulent seeing."

It may not perform well in terms of imaging perfect diffraction patterns as a smaller aperture can, however the resolution of the larger aperture still produces a smaller disturbed point source image. For example, a disturbed 8" star image can fit inside the Airy disc of a 4" image. The 8" image will also be better defined at the center of that smaller blur prividing higher resolution. Only when the larger aperture begins to bloat does the image blur begin to approximate that of a smaller aperture which will also be somewhat affected by the same adverse seeing conditions.
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#12 gregj888



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Posted 29 September 2019 - 01:26 PM

Don't assume you have seeing cells (Ro) of 4" (100mm) or greater unless you are in a pretty good (high) location and have good seeing.   I've gotten one report of 20" Ro in visual at ESO but for most of us, in visual it's likely << 4".  I figure 1"-3" here  in Portland OR. on most nights.


"To" also matters photographically, how fast the cells move.


Also note:

     1) the Ro is inversely proportional to the wavelength

     2) the seeing effect is two dimensional so decreasing Ro by 2 effects the seeing by 4 (square)... 

     3) narrowing the band pass helps reduce the blur...  so a V filter may show more than no filter and a low pass filter

          won't be quite as good a band pass filter within reason.

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#13 stevereecy


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Posted 13 October 2019 - 10:00 PM

Ok, this is fascinating. I have a 4-inch Mak-Cass that will be used mostly for planets.  It has a focal length of 1250mm.   I'm considering moving up to a 6-inch Mak-Cass that has a focal length of 1500mm.  From what I'm reading, I won't have much benefit by moving u in aperture....especially in light of the fact that the purpose of the scope is grab and go and a larger scope will have longer cooling times.  So maybe I should spend my money on some premium eyepieces instead?

#14 j.gardavsky


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Posted 14 October 2019 - 12:47 PM

Masking down the aperture on achromatic refractors makes an improvement on 2 ugly issues:

1. The spherochromatism is reduced

2. The classic CA fringes loose their brightness, in another words their brightness fades.


You'll loose some of the diffraction limited resolution, but the chromatic blur becomes reduced, so you will see more details on Monn and planets, and some close binaries will become cleaner separated.

The visiblity of the Airy diffraction pattern will also improve, and you can use it for testing your high mag eyepieces.


When you have a good reason not to mask the aperture, than take the pass-band filters.

The narrow pass-band green Solar Continuum filter kills both the spherochromatism and the CA.

The wide passband 470nm blue filter at least reduces these both ugly aberrations, and the resolution on the shorter (blue) wavelength is slightly enhanced. (The diffraction limit resolution scales inversly with the wavelength.)


I have both, the mask and the filters, as above.

The green filter is super on the Moon as soon as you get frienly with the green Moon.

The blue filter shows more details around the Great Red Spot on Jupiter.




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#15 Miranda2525


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Posted 18 October 2019 - 07:57 AM

When the seeing is supreme, the larger telescope will always have the best resolving power to see small details...as long as the figure is excellent, cooling is optimal, etc.

Edited by Miranda2525, 18 October 2019 - 07:58 AM.

#16 Asbytec


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Posted 18 October 2019 - 08:58 AM

When the seeing is supreme, the larger telescope will always have the best resolving power to see small details...as long as the figure is excellent, cooling is optimal, etc.

Agreed. Though, there is something to be said about the appeal of a steady image in a smaller aperture (and a large one, for that matter), even if it is not performing to that of a larger one. My old 6" was rock steady most nights, even though it was not showing the resolution and color of a larger aperture. So, I do appreciate the steadiness of a nice image even in a smaller aperture. 

#17 ItsMeTony


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Posted 18 October 2019 - 10:57 AM

 I have the Orion 120ST.  I really do enjoy in the pure simplicity of sweeping through the skies and getting that "wow" factor I can only really get with my eyeball to the eyepiece.  When  I want to check out the planets or the moon I  put on the objective lens dust cap with the small center cap removed. The center cap hole is 52mm. My scope is slowed down from f/5 to f/11.5.  The CA now, for me, nearly negligible.  The planets and the moon are noticeably dimmer but very pleasant to observe.

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