40 replies to this topic

[Aperture Mask]

Can an apo use more powerful eyepieces than an achro? Thanks to you all.

[Mitrovarr]

Sort of. Apos are generally considered to be better at high power, partly because they lack CA to obscure details and reduce contrast and partly because they're usually higher quality. But, details about the individual scopes matter (a really good, slow achro might beat a cheap apo of equivalent aperture), and the diffraction-based limitations of both are the same.

[Ernest_SPB]

main difference between apochromatic and achromatic refractors is difference in F-number (with the same or similar level of CA control) - apochromatic ones could be made more fast (short focal)

so to get the same diameter of exit pupil (e.g. 0.5 mm) and same level of diffraction influence apochromatic refractors require shorter in focal length ("more powerful") eyepieces

[sg6]

Usually they seem to be able to handle it better. The reason is likely a combination, as in lack of CA, reduced (none) SA and depth of field/focus.

[PXR-5]

Generally yes in my experience, But if its a long focal achro you will be fine. For example a ST80 at f5, No, stick with low power, that's what it's made for. At high power CA becomes very visible, at low power it offers very wide field views

[daquad]

Usually they seem to be able to handle it better. The reason is likely a combination, as in lack of CA, reduced (none) SA and depth of field/focus.

Reduced depth of focus with a low focal ratio scope, APO or not, but why reduced spherical aberration (SA)?  Given equal optical quality, I should think a high focal ratio scope would show less SA than a low one.

[Aperture Mask]

Generally yes in my experience, But if its a long focal achro you will be fine. For example a ST80 at f5, No, stick with low power, that's what it's made for. At high power CA becomes very visible, at low power it offers very wide field views

I stated in several post that "an aperture mask increases my focal ratio, which allows me to use more powerful eyepieces". However, a lot of people disagree with that.

[PXR-5]

Generally yes in my experience, But if its a long focal achro you will be fine. For example a ST80 at f5, No, stick with low power, that's what it's made for. At high power CA becomes very visible, at low power it offers very wide field views

I stated in several post that "an aperture mask increases my focal ratio, which allows me to use more powerful eyepieces". However, a lot of people disagree with that.

Well, the problem becomes you now have less aperture to support the increased magnification. Personally I would use full aperture and try filtering.

[Wildetelescope]

 

Generally yes in my experience, But if its a long focal achro you will be fine. For example a ST80 at f5, No, stick with low power, that's what it's made for. At high power CA becomes very visible, at low power it offers very wide field views

I stated in several post that "an aperture mask increases my focal ratio, which allows me to use more powerful eyepieces". However, a lot of people disagree with that.

 

Increasing your focal ratio by reducing your aperture will reduce the amount of CA that you see at a given magnification.  BUT, you will loose native resolution because you have reduced your aperture.  Have done the test myself with my 120 F8.35 celestron.   On a good night, I will see more, sharper, detail on jupiter with a full aperture(CA and all) than I do with a stopped down aperture.  However, on a bad atmosphere night, I might be able to do a better job focusing with the stopped down aperture, and pull a LITTLE more detail out than with a full aperture, for the conditions on that night.  But, it will be no where near what resolution you can get with a full aperture on a stable night.   Now, if I compare an 80 mm F7 scope to an 80 mm f15 scope, THEN I will be able to more consistently use higher magnifications with the F15 scope, independent of sky conditions.  At least that is my experience.  

 

In the end, you need to use what you have at hand, and if you are happy with what you are looking at through the eyepiece(masked or not masked..) then go for it.  Enjoy yourself and experiment!  That is what this is all about.  You do not have to take my word or that of anyone else.   Point the scope at Jupiter one evening and try out a series of masks and see for your self how contrast and what features you resolve change.  Try drawing what you see.  It is a great experiment that is pretty simple to do and helps you understand the physics behind what you are seeing.   If you do it on different nights, you can also see how different amounts of atmospheric instability also effect what you see. 

 

Hope this helps.

 

JMD

[Aperture Mask]

 

 

Generally yes in my experience, But if its a long focal achro you will be fine. For example a ST80 at f5, No, stick with low power, that's what it's made for. At high power CA becomes very visible, at low power it offers very wide field views

I stated in several post that "an aperture mask increases my focal ratio, which allows me to use more powerful eyepieces". However, a lot of people disagree with that.

 

Increasing your focal ratio by reducing your aperture will reduce the amount of CA that you see at a given magnification.  BUT, you will loose native resolution because you have reduced your aperture.  Have done the test myself with my 120 F8.35 celestron.   On a good night, I will see more, sharper, detail on jupiter with a full aperture(CA and all) than I do with a stopped down aperture.  However, on a bad atmosphere night, I might be able to do a better job focusing with the stopped down aperture, and pull a LITTLE more detail out than with a full aperture, for the conditions on that night.  But, it will be no where near what resolution you can get with a full aperture on a stable night.   Now, if I compare an 80 mm F7 scope to an 80 mm f15 scope, THEN I will be able to more consistently use higher magnifications with the F15 scope, independent of sky conditions.  At least that is my experience.  

 

In the end, you need to use what you have at hand, and if you are happy with what you are looking at through the eyepiece(masked or not masked..) then go for it.  Enjoy yourself and experiment!  That is what this is all about.  You do not have to take my word or that of anyone else.   Point the scope at Jupiter one evening and try out a series of masks and see for your self how contrast and what features you resolve change.  Try drawing what you see.  It is a great experiment that is pretty simple to do and helps you understand the physics behind what you are seeing.   If you do it on different nights, you can also see how different amounts of atmospheric instability also effect what you see. 

 

Hope this helps.

 

JMD

 

Cool

[Mitrovarr]

Generally yes in my experience, But if its a long focal achro you will be fine. For example a ST80 at f5, No, stick with low power, that's what it's made for. At high power CA becomes very visible, at low power it offers very wide field views

I stated in several post that "an aperture mask increases my focal ratio, which allows me to use more powerful eyepieces". However, a lot of people disagree with that.

Well, the problem becomes you now have less aperture to support the increased magnification. Personally I would use full aperture and try filtering.

Yep, as you lose aperture you lose theoretical resolving power. Refractors typically don't have a lot of aperture to throw away, and the fast ones don't have much focal length to work with. So you go from a telescope that is limited from CA to one that is limited because of small aperture. It can still be worth trying because aperture masks can easily be made out of cardboard and such, but I don't usually find it helpful.

[rmollise]

main difference between apochromatic and achromatic refractors is difference in F-number (with the same or similar level of CA control) - apochromatic ones could be made more fast (short focal)

so to get the same diameter of exit pupil (e.g. 0.5 mm) and same level of diffraction influence apochromatic refractors require shorter in focal length ("more powerful") eyepieces

 

It's not just a matter of focal length/focal ratio. An apochromats will by its nature usually handle more power than an achromat, especially current achromats which tend to come in at focal ratios no higher than f/10 - 11.

[Mitrovarr]

main difference between apochromatic and achromatic refractors is difference in F-number (with the same or similar level of CA control) - apochromatic ones could be made more fast (short focal)
so to get the same diameter of exit pupil (e.g. 0.5 mm) and same level of diffraction influence apochromatic refractors require shorter in focal length ("more powerful") eyepieces

It's not just a matter of focal length/focal ratio. An apochromats will by its nature usually handle more power than an achromat, especially current achromats which tend to come in at focal ratios no higher than f/10 - 11.

I think a key component of the superior apo performance is that almost all achromats made now are fairly low end, while all apochromats made are at least mid-end, and some are very high. This translates not just to better optical quality, but also a much better focuser, lens cell, baffling, and so on than a current achromat. All of that stuff helps put the apo ahead.

Some classic achromats are very, very good because they were high end instruments at the time. They compete better with apos than modern ones do (that said from what I've heard they still usually lose to them).

[Jon Isaacs]

Now, if I compare an 80 mm F7 scope to an 80 mm f15 scope, THEN I will be able to more consistently use higher magnifications with the F15 scope, independent of sky conditions.  At least that is my experience.

 

 

Have you made the comparison between an 80 mm F/7 apo and a longer focal length achromat?  In my experience,  my 80 mm F/7 Williams Optics Megrez Il consistently out performed the longer focal length 80 mm achromats I owned. 

 

To answer the original question:

 

Chromatic aberration means that the various colors do not come to a common focus.  Some colors are in focus,  some colors are out of focus.  This obviously affects the contrast and sharpness of the image.  The amount of chromatic aberration depends on a number of factors but in an achromat, the glasses used are standardized and so the amount of chromatic aberration is determined by the aperture and the focal ratio,  the slower the focal ratio,  the less the CA,  the smaller the aperture,  the less the CA. 

 

This is why masking the aperture of an achromat reduces the CA.  An 80 mm F/5 achromat shows a great deal of false color on brighter objects but with a 40 mm mask it becomes an 40 mm F/10 and there is very little CA. 

 

But this comes at a heavy price.  The resolving power of a telescope,  the ability of the scope to see fine details,  is proportional to the aperture.  Whereas the 80mm F/5 can resolve the 2.3 arc-second "double double", it's beyond the reach of a 40 mm scope.  

 

Apochromats use special glasses and they dramatically reduce the chromatic aberration,  essentially eliminating it.  This means they can operate at full aperture and bring all the colors to focus at the same point.  Additionally,  the care put into manufacturing an apo telescope both optically and mechanically is likely to be superior. 

 

Better optics,  better mechanical, very good color correction,  these all allow for a cleaner sharper image that can be more highly magnified. 

 

Apochomats are expensive,  particularly in the larger large apertures.  Object the commonly available designs,  reflectors are free of chromatic aberration since mirrors reflect all the colors identically.  This means that a good quality reflector with a large aperture can be free of false color and yet provide the resolution and fine scale contrast while still being affordable. These Scopes have issues of their own.. 

 

Jon 

[t.r.]

I think the OP is simply hung up on magnifying power being the end result desired when really it is the size of the aperture and the quality of it that will determine what "useable" magnification one can obtain. Hence the reason why a 60mm refractor at 120x shows less resolution of details than a 120mm refractor at the same 120x! The larger scope will show new details not seen in the smaller at the same magnification. Magnification alone does not determine what one will see it is the resolving power of the objective!!! Everyone is dancing around this clearly...but I think the OP thinks magnification determines what is seen! It does not! It only helps.

Edited by t.r., 18 December 2017 - 03:49 PM.

[Wildetelescope]

John Issacs: "Have you made the comparison between an 80 mm F/7 apo and a longer focal length achromat?  In my experience,  my 80 mm F/7 Williams Optics Megrez Il consistently out performed the longer focal length 80 mm achromats I owned."

 

That is a good question John.  I have not done a side by side comparison, but at one time I DID own an Orion Eon 80, which had an f ratio of ~6.3 or so If I remember correctly. The sample I had was very good optically.  Based on memory, I will relate the following observations.

 

The orion ED scope provided (REALLY) excellent contrast at low powers (50-100x) with a dark background and pin point stars. M45 was great to observe.   Because of its fast nature, it took really small focal ratio EP's or a Barlow to push things up over 150X, but its performance on Jupiter in the 150-180 range was very good as well, and I did not notice any CA.  At that scale contrast was limited only by seeing, and the one or two jupiter images I captured looked fine with no false color.  However, if I went north of 200X, the image deteriorated rapidly with a lot of false color.  With my f15 Towa, I get nice contrast and sharp images of Jupiter, little or no CA even at magnifications over 200X.  That said, at more reasonable mags(<200) I would probably have to give a slight edge to the ED scope.   I suspect that a well executed F7 or F8 ED scope would be a very clear improvement over my sample 80 mm f15 achromat.

 

I HAVE compared the 80 mm Towa to my TV102 on Jupiter.  As expected, the TV102 shows both better contrast and detail, AND better color rendition than the smaller Achromat.  However, on less stable nights, I WILL see more  induced false color around the planet during moments of INSTABILITY in the TV102, than I will in the Towa.

 

These are my observations.  Is the f15 Towa optically equivalent to a good 80 mm ED doublet?  Probably not, but it IS surprisingly close.   For my own observing habits, the longer focal length suites me.  In my area, light pollution is so bad, that I really need 4 inches or better to make me happy looking at Messier objects, which is why I sold the orion and bought the TV102.  However, the F15 Towa is a nice piece of history and is nice for quick looks of the moon and planets, when I feel the urge.   Certainly there is a collector's component to my purchase.  It is a nice example of what is possible with that design, and it did not cost me much. 

 

One of these days, I will take it to an outreach event with my TV102 and my AP triplet and let everyone see the difference in designs, etc...   I would like to do the same with my Mak, my C8 and my Dob to compare reflector designs.  

 

Cheers!

 

JMD

[gezak22]

If anybody knows how to analytically relate a fixed defocus to resolution, for a given aperture, focal length, wavelength, I would love to see it. Together with the Dawes criterion, it would then be pretty simple to plot resolution vs aperture for various values of defocus.

[photoracer18]

John Issacs: "Have you made the comparison between an 80 mm F/7 apo and a longer focal length achromat?  In my experience,  my 80 mm F/7 Williams Optics Megrez Il consistently out performed the longer focal length 80 mm achromats I owned."

 

That is a good question John.  I have not done a side by side comparison, but at one time I DID own an Orion Eon 80, which had an f ratio of ~6.3 or so If I remember correctly. The sample I had was very good optically.  Based on memory, I will relate the following observations.

 

The orion ED scope provided (REALLY) excellent contrast at low powers (50-100x) with a dark background and pin point stars. M45 was great to observe.   Because of its fast nature, it took really small focal ratio EP's or a Barlow to push things up over 150X, but its performance on Jupiter in the 150-180 range was very good as well, and I did not notice any CA.  At that scale contrast was limited only by seeing, and the one or two jupiter images I captured looked fine with no false color.  However, if I went north of 200X, the image deteriorated rapidly with a lot of false color.  With my f15 Towa, I get nice contrast and sharp images of Jupiter, little or no CA even at magnifications over 200X.  That said, at more reasonable mags(<200) I would probably have to give a slight edge to the ED scope.   I suspect that a well executed F7 or F8 ED scope would be a very clear improvement over my sample 80 mm f15 achromat.

 

I HAVE compared the 80 mm Towa to my TV102 on Jupiter.  As expected, the TV102 shows both better contrast and detail, AND better color rendition than the smaller Achromat.  However, on less stable nights, I WILL see more  induced false color around the planet during moments of INSTABILITY in the TV102, than I will in the Towa.

 

These are my observations.  Is the f15 Towa optically equivalent to a good 80 mm ED doublet?  Probably not, but it IS surprisingly close.   For my own observing habits, the longer focal length suites me.  In my area, light pollution is so bad, that I really need 4 inches or better to make me happy looking at Messier objects, which is why I sold the orion and bought the TV102.  However, the F15 Towa is a nice piece of history and is nice for quick looks of the moon and planets, when I feel the urge.   Certainly there is a collector's component to my purchase.  It is a nice example of what is possible with that design, and it did not cost me much. 

 

One of these days, I will take it to an outreach event with my TV102 and my AP triplet and let everyone see the difference in designs, etc...   I would like to do the same with my Mak, my C8 and my Dob to compare reflector designs.  

 

Cheers!

 

JMD

I have done some things like that. During Mars Opposition 2005 I setup a 6" F15 Jaegers and my early C-9.25 side-by-side outside Frederick. There was hardly any difference up to a least 400x due to the extraordinary conditions at the time. I pushed both to around 600x to see how they broke down and surprisingly some detail was still visible thru both. There was more color shift in the refractor but not much else was different.

[Jon Isaacs]

I HAVE compared the 80 mm Towa to my TV102 on Jupiter.  As expected, the TV102 shows both better contrast and detail, AND better color rendition than the smaller Achromat.  However, on less stable nights, I WILL see more  induced false color around the planet during moments of INSTABILITY in the TV102, than I will in the Towa.

 

 

That's likely due to the atmospherics and the larger aperture. 

 

In any event I've used my 80 mm F/7 FPL-53 doublet at 200x plus on the planets etc without visible CA.  I think F/6.25 is pushing it a bit for an 80 mm doublet.. 

 

Jon

[Wildetelescope]

 

I HAVE compared the 80 mm Towa to my TV102 on Jupiter.  As expected, the TV102 shows both better contrast and detail, AND better color rendition than the smaller Achromat.  However, on less stable nights, I WILL see more  induced false color around the planet during moments of INSTABILITY in the TV102, than I will in the Towa.

 

 

That's likely due to the atmospherics and the larger aperture. 

 

In any event I've used my 80 mm F/7 FPL-53 doublet at 200x plus on the planets etc without visible CA.  I think F/6.25 is pushing it a bit for an 80 mm doublet.. 

 

Jon

 

Agreed!

 

jmd

[Jeff B]

As others have said, F ratio is king even with APO's but try to find an F10 APO bigger than 4" that's in ready production.  One saving grace with long FR achromats is the use of filters to isolate specific portions of the spectrum.

[Mitrovarr]

As others have said, F ratio is king even with APO's but try to find an F10 APO bigger than 4" that's in ready production.  One saving grace with long FR achromats is the use of filters to isolate specific portions of the spectrum.

Really? I've heard that really excellent apos are equally good for high power work whether fast or slow, which is why it's basically impossible to find slow apos anymore.

[Peter Besenbruch]

 

As others have said, F ratio is king even with APO's but try to find an F10 APO bigger than 4" that's in ready production.  One saving grace with long FR achromats is the use of filters to isolate specific portions of the spectrum.

Really? I've heard that really excellent apos are equally good for high power work whether fast or slow, which is why it's basically impossible to find slow apos anymore.

Define "slow." At f7.5, my 80mm ED scope is rather slow in today's market. Unlike Wildetelescope's shorter doublet, the extra length helps the scope easily cruise past 200x, much like his longer, f15 achromat. Still I tend to wish my scope were a little longer, if only to eliminate any residual field curvature I can see.

[Jon Isaacs]

As others have said, F ratio is king even with APO's but try to find an F10 APO bigger than 4" that's in ready production.  One saving grace with long FR achromats is the use of filters to isolate specific portions of the spectrum.

 

With apos,  there are a lot more variables than aperture and focal ratio and certainly aperture is more important in terms of resolution and contrast.  Optical Quality and the various design factors like the number of elements and the glasses chosen also play important roles. 

 

With an achromat, a slower focal ratio translates directly into better color correction. With an apo,  the color correction at normal focal ratios can be essentially perfect so the slower ratio is unnecessary.. 

 

Jon

[Mitrovarr]

As others have said, F ratio is king even with APO's but try to find an F10 APO bigger than 4" that's in ready production. One saving grace with long FR achromats is the use of filters to isolate specific portions of the spectrum.

Really? I've heard that really excellent apos are equally good for high power work whether fast or slow, which is why it's basically impossible to find slow apos anymore.

Define "slow." At f7.5, my 80mm ED scope is rather slow in today's market. Unlike Wildetelescope's shorter doublet, the extra length helps the scope easily cruise past 200x, much like his longer, f15 achromat. Still I tend to wish my scope were a little longer, if only to eliminate any residual field curvature I can see.

It used to be common for many apos to be F10 or more. I think the invention of better glasses and fluorite made faster ones possible.