119 replies to this topic

[Princess Leah]

Living in the city I often have a thermal ceiling of 50--60X.

Due to this I have the dubious merit of gathering knowledge of what works best at this magnification.

Although 50-60X is low for planetary/lunar work; the planets and the moon are my friends and I miss them if I don't observe them.

 

What I have discovered that over time, by repeated practice, I am able to see more and more detail even at this modest magnification.

I have the same experience terrestrially, where there is a similar thermal limit.

 

Surprisingly some of the best scopes are traditional achomats.

 

Why is this?

What makes a lens sharp?

At low power CA is not a problem for these achromats. Do we then have to consider spherical aberration in an achromat? Or is there less scatter with these well figured lens? Does a lens with less scatter contribute to better sharpness (ie better planetary contrast).

 

My experience so far with refractors is that the following have the best viewing for planets/lunar at 50-60X

 

Carton 70mm F15

TS Optics 60mm F15 

Celestron/Vixen First Scope 80, (quite exceptional I thought).

Vixen F15 (older Japanese model- can't remember aperture).

Skywatcher 70/500 (old blue model).

Televue Pronto/Ranger

Skywatcher Evolux 62ED 

 

What other achromats should be added to this list?

What modern ED scopes are of equivalent sharpness.

Are any of a reasonable priced- skywatcher 100ED for example?

Is certain ED glass softer in composition and therefore harder to 'sharpen/figure?'

 

 

When on a budget is there a trade-off between sharpness and CA control? Does this explain why the Skywatcher Evolux 62ED (unknown glass) is very sharp, but has some CA - making it a better visual scope.

 

 

Obviously as magnification increases, CA in an achromat softens the image.

But for this discussion let us imagine we are viewing at moderate magnification where CA is not intrusive 

Edited by Princess Leah, 14 April 2024 - 12:11 PM.

[Echolight]

I often trade sharpness for resolution.

A sharp nothing is still nothing.

 

I could have started with a Evostar 100ED, but chose a C6R instead. And I believe it has showed more detail on the Moon and planets than would have been possible with the 100ED. And shows and splits fainter stars more easily. 

 

There is though a fine line that often starts and ends with the stability of the mount, and therefore the image. And as much as I have tried to love generic solid tube commercial dobs, I have a hard time getting along with the image instability created by the tube vibrations, and not as sturdy mounts as the dob crowd likes to preach. So in spite of their smaller aperture, true religion is often found in refractors.

However, I do love my 8 inch f4 for richfield. Little widefield refractors can't do what it does. Not much for high power sharpness so far though.

 

For this specific question:

"When on a budget is there a trade-off between sharpness and CA control?"

With the C6R, I've seen incredible amounts of details on the Moon and planets that I don't think a 100ED could match. A 120ED probably could one up the big 6 achro though... at a price.  And an in tune and acclimated 24 year old C8 with reducer/corrector trumps it on the Moon at mags up to 400. While the 6" f8 achromat shows a ton of detail along the terminator, as magnifications go north of 350x (which for me is where many rilles start to become obvious) the images are noticeably muddied due to CA compared to the bigger mirror.

But the big achro does very well on faint stars, more easily showing E and F.

Looking at very bright stars, such as Sirius, is where the CA is most intrusive in the C6R.

 

Aside from all that, I'm a bit surprised to not see your ED80 in the list. But a FirstScope 80 is there.

Edited by Echolight, 14 April 2024 - 07:12 AM.

[db2005]

I'd add the following to the list of very sharp optics:

 

Takahashi FC76Q

Zeiss Telementor 63/840 achromat

Takahashi FC-100

Vixen SD81S

 

In terms of sharpness and overall optical quality I'd rate my samples of Synta 100ED and 80ED at a tier below.

 

In my experience: slower scopes tend to perform noticeably better than faster scopes; more expensive scopes perform better than less expensive scopes; achromats costing the same as an equivalent-size ED tend to perform optically at least as well or better than the ED. The caveat is that there are very few well made achromats left on the market.

 

It's worth keeping in mind that ED glasses exist in different quality grades, and that the glass can be figured and finished to different standards depending on the skill and effort expended on the glass. The mating element chosen for the ED glass can also have a profound effect on the color correction that can be achieved.

 

The cheap scopes I've owned were limited by the optical quality of the scope, not the laws of physics or the night's seeing. In the high-end scopes I find that the limiting factor is never the scope's optical quality, but the seeing and the laws of physics pertaining the the scope's aperture.

Edited by db2005, 15 April 2024 - 12:18 AM.

[Princess Leah]

Very interesting 

 

I'd add the following to the list of very sharp optics:

 

Takahashi FC76Q

Zeiss Telementor 63/840 achromat

Takahashi FC-100

Vixen SD81S

 

In terms of sharpness and overall optical quality I'd rate my samples of Synta 100ED and 80ED at a tier below.

 

In my experience: slower scopes tend to perform noticeably better than faster scopes; more expensive scopes perform better than less expensive scopes; achromats costing the same as an equivalent-size ED tend to perform optically at least as well or better than the ED. The caveat is that there are very few well made achromats left on the market.

 

It's worth keeping in mind that ED glasses exist in different quality grades, and that the glass can be figured and finished to different standards depending on the skill and effort expended on the glass. The mating element chosen for the ED glass can also have s profound effect on the color correction that can be achieved.

 

The cheap scopes I've owned were limited by the optical quality of the scope, not the laws of physics or the night's seeing. In the high-end scopes I find that the limiting factor is never the scope's optical quality, but the seeing and the laws of physics pertaining the the scope's aperture.

Very interesting, thanks for sharing. I was surprised to find my old Synta 70/500 is sharper than my Synta 80ED scope.

However all my other Synta achromats are less sharp than my 80ED.

Edited by Princess Leah, 14 April 2024 - 07:19 AM.

[Princess Leah]

I often trade sharpness for resolution.

A sharp nothing is still nothing.

 

I could have started with a Evostar 100ED, but chose a C6R instead. And I believe it has showed more detail on the Moon and planets than would have been possible with the 100ED. And shows and splits fainter stars more easily. 

 

There is though a fine line that often starts and ends with the stability of the mount, and therefore the image. And as much as I have tried to love generic solid tube commercial dobs, I have a hard time getting along with the image instability created by the tube vibrations, and not as sturdy mounts as the dob crowd likes to preach. So in spite of their smaller aperture, true religion is often found in refractors.

However, I do love my 8 inch f4 for richfield. Little widefield refractors can't do what it does. Not much for high power sharpness so far though.

 

For this specific question:

"When on a budget is there a trade-off between sharpness and CA control?"

With the C6R, I've seen incredible amounts of details on the Moon and planets that I don't think a 100ED could match. A 120ED probably could one up the big 6 achro though... at a price.  And an in tune and acclimated 24 year old C8 with reducer/corrector trumps it on the Moon at mags up to 400. While the 6" f8 achromat shows a ton of detail along the terminator, as magnifications go north of 350x (which for me is where many rilles start to become obvious) the images are noticeably muddied due to CA compared to the bigger mirror.

But the big achro does very well on faint stars, more easily showing E and F.

Looking at very bright stars, such as Sirius, is where the CA is most intrusive in the C6R.

 

Aside from all that, I'm a bit surprised to not see your ED80 in the list. But a FirstScope 80 is there.

The first scope is one with the premium vixen lens.  I think it was called a firstscope?

 

Here it is https://www.cloudyni...-firstscope-80/

Edited by Princess Leah, 14 April 2024 - 07:27 AM.

[Echolight]

The first scope is one with the premium vixen lens.  I think it was called a firstscope?

 

Here it is https://www.cloudyni...-firstscope-80/

Yeah. I've seen those. Just surprised to see it rated higher than a China FPL-53 f7.5 doublet.

The ones I've seen for sale were priced as collector's items. Close to the price of a used 100ED, and likely needing an equivalently sized mount.

[gnowellsct]

Very interesting

Very interesting, thanks for sharing. I was surprised to find my old Synta 70/500 is sharper than my Synta 80ED scope.
However all my other Synta achromats are less sharp than my 80ED.

May be an issue of execution, not design. Lower quality and price don't usually mean that units have an across the board quality loss. Well it might. But often the decline is less control of variance. The Synta 70mm on or above spec and the 80mm below it.

[Echolight]

Very interesting 

 

Very interesting, thanks for sharing. I was surprised to find my old Synta 70/500 is sharper than my Synta 80ED scope.

However all my other Synta achromats are less sharp than my 80ED.

Were you using the same diagonal?

I think there's a problem with the comes with diagonal that came with my Evostar 80ED.

 

Point them both at Sirius, Vega, and the terminator of Moon, and push the x's to 150 or more. That might kill the romance.

Edited by Echolight, 14 April 2024 - 08:00 AM.

[gnowellsct]

When on a budget is there a trade-off between sharpness and CA control? Does this explain why the Skywatcher Evolux 62ED (unknown glass) is very sharp, but has some CA - making it a better visual scope.

Why would chromatic aberration make it a better visual scope?

[Princess Leah]

I have three 2 inch diagonals. William optics/Meade/skywatcher etc.

I also have tried several of these 80EDs and I retained the best one.

 

The Pronto is sharper in my eyes than any 80ED. The skywatcher 70/500 is almost as sharp as the Pronto.

Although we're talking about 70X magnification, before CA effects view too much.

[Princess Leah]

Why would chromatic aberration make it a better visual scope?

It wouldn't. However many say this has scope has too much colour for astrophotography.

[Princess Leah]

I'd add the following to the list of very sharp optics:

 

Takahashi FC76Q

Zeiss Telementor 63/840 achromat

Takahashi FC-100

Vixen SD81S

 

In terms of sharpness and overall optical quality I'd rate my samples of Synta 100ED and 80ED at a tier below.

 

In my experience: slower scopes tend to perform noticeably better than faster scopes; more expensive scopes perform better than less expensive scopes; achromats costing the same as an equivalent-size ED tend to perform optically at least as well or better than the ED. The caveat is that there are very few well made achromats left on the market.

 

It's worth keeping in mind that ED glasses exist in different quality grades, and that the glass can be figured and finished to different standards depending on the skill and effort expended on the glass. The mating element chosen for the ED glass can also have s profound effect on the color correction that can be achieved.

 

The cheap scopes I've owned were limited by the optical quality of the scope, not the laws of physics or the night's seeing. In the high-end scopes I find that the limiting factor is never the scope's optical quality, but the seeing and the laws of physics pertaining the the scope's aperture.

Is/was the Takahashi FS-78 not a little sharper?  Have you made this list in your order of preference? ie do you feel the Vixen is the least sharp.

[dnrmilspec]

The Skywatcher ED100 is a sleeper scope.  It is very sharp.  FPL-53 glass and virtually color free.  It is also a manageable size. 

 

+1 for the EQ6 and equivalents.  They punch above their weight when they are properly mounted, equipped with a good diagonal and eyepiece and used for their intended purpose.  For example.  Look at M42 and compare  them to a many thousands of dollars APO of the same size and the additional money starts to look like a ton to pay.  Then, consider that with one of these on a good mount, AND an 8" SCT for the same mount one can save a bunch of money over big money APO. 

[CHASLX200]

Is/was the Takahashi FS-78 not a little sharper?  Have you made this list in your order of preference? ie do you feel the Vixen is the least sharp.

Both of my FS78's were 100x per inch killers. Same for the AT80ED i got now that cost many times less.

[Jon Isaacs]

Yeah. I've seen those. Just surprised to see it rated higher than a China FPL-53 f7.5 doublet.

The ones I've seen for sale were priced as collector's items. Close to the price of a used 100ED, and likely needing an equivalently sized mount.

 

I have owned quite a number of the Celestron/Vixen 80mm F/11.3 achromats.  This one, I paid $50 for it and the owner was so happy to get it out of his garage that he delivered within the hour. 

 

 

I have no idea who rated these higher than a Synta 80mm F/7.5 FPL-53 doublet but it certainly would not have been me.  Good scopes for what they are.. 

 

Jon

[Terra Nova]

Want 100X per inch? Tak FC-76 F8!

Attached Thumbnails

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[Jon Isaacs]

I have three 2 inch diagonals. William optics/Meade/skywatcher etc.

I also have tried several of these 80EDs and I retained the best one.

 

The Pronto is sharper in my eyes than any 80ED. The skywatcher 70/500 is almost as sharp as the Pronto.

Although we're talking about 70X magnification, before CA effects view too much.

 

That is not sharpness.  Try 160x on Jupiter or Saturn.  

 

Jon

[Echolight]

 

I have owned quite a number of the Celestron/Vixen 80mm F/11.3 achromats.  This one, I paid $50 for it and the owner was so happy to get it out of his garage that he delivered within the hour. 

 

 

 

I have no idea who rated these higher than a Synta 80mm F/7.5 FPL-53 doublet but it certainly would not have been me.  Good scopes for what they are.. 

 

Jon

 

For $50 that's a good deal if you want a little scope.

Looks lighter than I remember. Of course an ED100 isn't all that heavy.

Edited by Echolight, 14 April 2024 - 08:38 AM.

[TOMDEY]

"Sharpness" is a common casual term that varies in meaning. It generally seems to blend comfort, resolution, contrast, and color correction ... when one perceives the image as pleasing, and ~natural~. When you enjoy all four of those combined --- that manifests as another casual term --- "snap focus", where you know you've nailed best focus without having to finesse the knob back and forth, never satisfied.     Tom

[Princess Leah]

I think the figure and polish (my idea of sharpness) of the lens is best looked at by comparing scopes side by side at low power and perceiving how much detail can be seen.

[Princess Leah]

The Skywatcher ED100 is a sleeper scope.  It is very sharp.  FPL-53 glass and virtually color free.  It is also a manageable size. 

 

+1 for the EQ6 and equivalents.  They punch above their weight when they are properly mounted, equipped with a good diagonal and eyepiece and used for their intended purpose.  For example.  Look at M42 and compare  them to a many thousands of dollars APO of the same size and the additional money starts to look like a ton to pay.  Then, consider that with one of these on a good mount, AND an 8" SCT for the same mount one can save a bunch of money over big money APO. 

What's the EQ6 scope?

[gnowellsct]

The op original list is not one that comes trippingly to the mind as a roster of sharp scopes. Db2005 provides a better one. And a poor mount can ruin a sharp optic as echolite indicates. I've seen a tec 140 give inferior images because poorly mounted. I wanted to have the Astro police arrest the owner.

Some discussions raise questions. If we are discussing mounts and someone chimes in that such and such a mount has only three second damping time and that it's much better than his other mounts, the information is that he's never had a really good mount. A discussion of sharpness where we have already conceded that top magnifications are not in play is somewhat the same.

The lack of sharpness is still there at medium powers it's just a question of how sensitive the observer is to it. The way to get sensitive is to spend time with one or more really sharp scopes. If the best Mount someone has ever used has a damping time of 3 seconds then that can become the definition of a stable mount, for that person. Because there are mounts out there that have 20 second damping times. It doesn't mean the three second damping time is a stable mount.

The way to experience sharp optics is to go to a star party where there are some on display, to have a friend who has one or two examples, or to pay the dough and see for yourself.

I will say it's not easy to find the premier optics at a star party. The income demographics of the nation work against it, and even people who have the income may have other spending priorities. Over 20 years I haven't seen even one TEC at our club events and the only Takahashi to appear is the one I used to own. We've got a fair sprinkling of astro-physics however.

Greg N

Edited by gnowellsct, 14 April 2024 - 09:07 AM.

[Echolight]

What's the EQ6 scope?

 One of these... Of course.

...on a slightly too light mount

Edited by Echolight, 14 April 2024 - 09:24 AM.

[gnowellsct]

I think the figure and polish (my idea of sharpness) of the lens is best looked at by comparing scopes side by side at low power and perceiving how much detail can be seen.

Well if you take a high resolution picture and look at it at low resolution you will see that more information is trying to be crammed into each pixel and the result is you lose detail.

There are Hubble pictures where you can play with this. You can also routinely play with it here on cloudy nights because the imagers often post links to high resolution versions of their files which are too big to upload here.

If you take a really excellent c14 picture of Jupiter, for example the ones by Damien peach, they look excellent at medium and low resolution. Now you take a Hubble image of Jupiter and a Damien peach picture of Jupiter and make them the same size and freeze them so that you can't zoom in or out to the point of pixelation, they will look substantially the same and you will wonder what's the deal here.

But if you start scrolling in, which is the same thing as high magnification, the information crammed into one pixel at low magnification spreads out. And if that information is there, because it is a high resolution instrument, you will get more details. But if the information is not there then you will get pixelation and fuzziness.

So when you zoom in on a Hubble shot of Jupiter, provided the file is high resolution, you will go in much deeper before pixelation then you can on a Damien Peach picture. And you discover that Hubble is immeasurably sharper.

When file sizes are limited this difference between instruments does not appear.

Greg N

Edited by gnowellsct, 14 April 2024 - 11:47 AM.

[Echolight]

I will say it's not easy to find the premier optics at a star party. The income demographics of the nation work against it, and even people who have the income may have other spending priorities. Over 20 years I haven't seen even one TEC at our club events and the only Takahashi to appear is the one I used to own. We've got a fair sprinkling of astro-physics however.

Greg N

Is an Astro-Physics not better than a TEak?