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Are Achros Sharp Planetary Scopes

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#151 Jon Isaacs

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Posted 08 April 2020 - 10:12 PM

There appears to be some revisionist history at work here.

 

Before the influx of low-cost apochromats, long-focus achromats were generally considered to be among the sharpest optical systems available. Forty or fifty years ago when I started observing, a 4 inch achromatic refractor was around $1,000 and many observers, myself included, were happy to pay this price at a time when 6 inch reflectors could be had for less than half this amount.

 

I am not contending in this post that achromats, even of long focus, are superior or even necessarily equal to today's better quality apos. I am responding to the OPs question as to whether achros are sharp planetary scopes. To this end it is clear that many authors and observers in the past thought they were. Not just myself and a some other posters on this thread.

 

So-o-o-o, of course long- focus achromats are sharp planetary scopes. Just because some modern day apos may arguably be even sharper doesn't mean this is no longer the case.

 

Steve

 

 

Steve:

 

I am not seeing any revisionist history, I am seeing the progress over the last 50 years being documented here. 

 

Planetary observation has progressed dramatically over the last 50 years.  Large Newtonians with thin quartz mirrors and near perfect optics with careful attention to thermal equilibrium were no where on the horizon.  SCTs and high end Maks were in their infancy.  The apo refractor was very uncommon and were not part of the consciousness of the amateur community.  Today's planetary scope is far more capable than those of 50 years ago.

 

And when it comes to slow achromats that people paid $1000 for, some were sharp, some weren't so sharp. That's the perspective of history. 

 

So, I think one can say that an achromat with a chromatic ratio near 5 can be provide reasonably contrasty, reasonable crisp views but as you say, in todays world, they won't be the sharpest nor will they be the most contrasty.

 

Jon


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#152 Rutilus

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Posted 09 April 2020 - 03:43 AM

Over the years I've done various tests of using my Chinese made Achromats with aperture masks. 

For example my 120mm f/8.3 meets Sidgwick at 90mm and Conrady at 70mm.

My 150mm f/8, meets Sidgwick at 100mm and Conrady at 75mm.

 

I also did many side by side comparison with three vintage Japanese scopes from the 1960s/70s.

These were 60mm f/15, 76mm f/16 and 80mm f/15. When using the Chinese scopes with relative aperture

mask, I could not detect any difference in the amount of planetary detail  visible in all the scopes.

The Chinese scopes were just as good as the vintage ones. The difference I noticed was slightly less light

scatter around the planets in the Chinese scopes, this I put down to more modern lens coatings.


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#153 Rutilus

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Posted 09 April 2020 - 03:55 AM

The detail visible in the image made with the FS-102, is typical of the detail that I see visually with my 150mm f/8 Achromat

and that scope only has a ratio of 1.35.  The Chinese  scopes, are not as sharp as a Apo of the same aperture size but they

are still capable of showing an observer plenty of planetary detail. 


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#154 sg6

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Posted 09 April 2020 - 04:05 AM

One aspect I find of an achro planetary scope is they are a good excuse to go buy a bit of a classic.

I grabbed a Tal 100 RS and it suits the needs well.

US could track down a D&G (think thats the name).

Maybe build an Istar - although a little expensive.

 

There are now a few long focal length ED's and both ED and achro are a crown and flint doublets so really what is the difference. Use the idea of an achro planetary scope as a potential reason for that slightly different scope.

 

Will still say there is really just the Moon, Jupiter and Saturn. Mars is more in the specialist scope area where magnifications in the region of 250x are required for possible surface detail, so in my thinking kind of excludes itself a bit. So 3 targets - not a lot.


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#155 daquad

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Posted 09 April 2020 - 08:47 AM

I would say they can be. 

 

The equation for the color blurr in an achromat is:

 

CB = (735 x D(mm)) / (1900 x FR)

 

A 60mm F/15 :  CB =(735 x 60mm)/(15x1900) = 1.55 This means the color blur is 1.55 times the diameter of the Airy disk. 

 

An 100mm F/5:  CB = (735 x 100)/(5x1900) = 7.7 The chromatic blur is 7.7 times the diameter of the Airy disk.  

 

A 100 mm F/9 FPL-53 doublet:  CB = (735 x 100)/(9x11000) = 0.742 times the diameter of the Airy disk. 

 

It is my understanding that the specific color balance can be adjusted somewhat, this is really playing with the spectral response of the eye, trying to put the chromatic aberration where it will be the least visible.  But the color blur is a result of the longitudinal focus error, 1 part in 1900 for an achromat and it can't be avoided and since the contrast is affected by colors that closer to the in focus colors, it's probably mostly an aesthetic gain and not a contrast gain.  

 

If you look at these three objectives, the 100mm F/5 has a color blur that is huge when compared to the Airy disk and obviously it affects the contrast in a major way.  The 60mm F/15 has a very small color blur, barely larger than the Airy Disk and scopes like this seem color free with high contrast.  This scope has a chromatic ratio of 6.35, well over the Conrady criteria.  

 

The 100mm F/9, essentially a Skywatcher ED-100, has very good color correction, the chromatic blur is smaller than the Airy disk, the colors are essentially i focus, all of them.  According to Vlad's work, these scopes have a polychromatic strehl of 0.98. 

 

Just some numbers.  

 

Can an Achromat be sharp on the planets?  I'd say yes.  Are achromats sharp on the planets.  Some are, some aren't.  

 

Jon

Jon, you said what I said, but with more detail.  Glad we are on the same page.

 

Dom Q.


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#156 Bomber Bob

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Posted 09 April 2020 - 02:23 PM

Over the years I've done various tests of using my Chinese made Achromats with aperture masks. 

For example my 120mm f/8.3 meets Sidgwick at 90mm and Conrady at 70mm.

My 150mm f/8, meets Sidgwick at 100mm and Conrady at 75mm.

 

I also did many side by side comparison with three vintage Japanese scopes from the 1960s/70s.

These were 60mm f/15, 76mm f/16 and 80mm f/15. When using the Chinese scopes with relative aperture

mask, I could not detect any difference in the amount of planetary detail  visible in all the scopes.

The Chinese scopes were just as good as the vintage ones. The difference I noticed was slightly less light

scatter around the planets in the Chinese scopes, this I put down to more modern lens coatings.

I've tested & reviewed 2 China-made achromatic refractors here on CN:  An ES FL-AR102 F10 and a Bresser AR-102L F13.  IMO, both performed well for the price & type / characteristics.  I was glad to see that the metal hardware has greatly improved over the decades -- the first China-made scopes I saw in person had castings that looked & felt like very light and brittle pot-metal  However, my vintage 4" F10 & F15 fracs out-performed the ES & Bresser, respectively.  Still, for a new F10 achromatic, the very low cost ES controlled false color well:

 

Explore Scientific FL-AR102 - Moon (Clavius) 20161209V10AS34.jpg

 

The F13 Bresser gives a taste of old frac views of double stars & star clusters -- but its price is about half that of budget EDs.


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#157 Jeff B

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Posted 09 April 2020 - 08:22 PM

Achromats can indeed be used for good quality, sharp lunar shots too.  Here is a heavily cropped and zoomed single frame shot of Clavius from prime focus of the 11" F12 D&G achromat at full aperture.  No manipulation of the color but I did sharpen it up some with a little contrast boost.  Not bad at all and visually it was much sharper as the seeing was very good.  A greatly reduced pixel count of the full shot is also included.  

 

Jeff

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#158 Jeff B

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Posted 09 April 2020 - 08:28 PM

Here is another go at the Clavius crop but I pumped up the brightness a bit to better show the purple tint seen visually.

 

Still pretty sharp for a single frame with my Canon 6D.  And with a Conrady ratio of 1.12 no less, which according to conventional wisdom, is completely useless.

 

Jeff

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#159 Jeff B

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Posted 09 April 2020 - 08:56 PM

Here is a single shot with a barlow, Canon 6D, 8.75" F12 achromat stopped to 6", again, no color manipulation, just some sharpening, though purple was indeed there in the brighter visual image.

 

This is another very sharp optic visually, especially stopped to 8".

 

Jeff

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#160 Dave1066

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Posted 10 April 2020 - 04:58 AM

That's some very nice photos you have there Jeff B. I'd expect a 11" F12 to have much worse chromatic aberration. Food for thought indeed. 



#161 roadi

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Posted 10 April 2020 - 08:57 AM

Anywho, 6 pages ago, the OP started this thread with:  A little bit loose ended let’s stick to observation made by the better branded achros and without criticizing their color performs how do they do in terms of sharpness on planetary objects visually.  

 

My answer is still Yes.  My advice to someone with 0 refractor experience who wants to try an achro:  Buy a quality 1950s / 1960s Japan-made 3" F15, a 1970s CZJ Telementor, or a 1960s Edmund 3" F15, and see for yourself.  It would be a thrifty experiment.

 

I don't sketch much as my art skills are primitive, but here's a Mars from almost 6 years ago.  The refractor was an awesome 1964 Hiyoshi 60mm F15 (Monolux Model 4380):

 

attachicon.gifM4380 - Mars 20140503U0330S.jpg

I would also add some japan made from 80´ties like Carton. The american made lenses of Jaeger, year of build I´m not sure, but two which I know of and had experienced my self that were about the equal of an Apo "visual" of same aperture are 60mm f15 Carton and 100mm f15 jaeger. Very sharp and contrasty.


Edited by roadi, 10 April 2020 - 08:59 AM.

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#162 Dave1066

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Posted 10 April 2020 - 10:14 AM

I would also add some japan made from 80´ties like Carton. The american made lenses of Jaeger, year of build I´m not sure, but two which I know of and had experienced my self that were about the equal of an Apo "visual" of same aperture are 60mm f15 Carton and 100mm f15 jaeger. Very sharp and contrasty.


Carton made some fine lens. My Skylight 60mm F16.7 is a Carton Lens. Very fine indeed.

David
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#163 Jeff B

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Posted 10 April 2020 - 12:55 PM

That's some very nice photos you have there Jeff B. I'd expect a 11" F12 to have much worse chromatic aberration. Food for thought indeed. 

Thanks.

 

Actually Dave there was indeed more "CA" visually with the 11", specifically a modestly brighter more of a yellowish purple skim over everything that the 6D camera just did not capture well, at least at the settings I used, which I've since forgotten.  However, visually, the live eye image was also much sharper than what is seen in the photo.   

 

Jeff


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#164 Rutilus

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Posted 11 April 2020 - 05:50 AM

80mm F/15 and 60mm ~F/16 are nearly Apochromatic because of the small aperture and long focal ratio.

 

Do you see any secondary spectrum/false color in the image?

Speaking for myself, I can always detect a tiny amount of false colour when looking at a bright object

like vega or Sirius (usually as a flash or flicker) in scopes that meet Conrady.

 

Last night I fitted a 80mm aperture mask to my 150mm f/8 Achromat and observed Venus in a dark sky,

there was a tiny amount of false colour, but I  had a wonderful of the planet without using any filtration.

The bright cusps were easily seen and shading visible. Along with a brighter spot on the limb of the planet.

I later used some colour filters which confirmed what I was seeing.  



#165 Rutilus

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Posted 11 April 2020 - 05:56 AM

Carton made some fine lens. My Skylight 60mm F16.7 is a Carton Lens. Very fine indeed.

David

I have a Carton 100mm f/13, which is extremely good. I remember early one morning observing the moon

along side my Tak TSA-102. I was using a magnification of around 100x and the views were the same in

both scopes. The tones and texture of the lunar surface were amazing. 


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#166 Rutilus

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Posted 11 April 2020 - 06:06 AM

This is a single frame shot made with my Canon 350D camera i-Star 150mm f/15 lens (I have always found single

frame shots a bit hit or miss) One day I hope to do some frame stacking as I think the lens will deliver some excellent

lunar images. 

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#167 Dave1066

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Posted 11 April 2020 - 06:55 AM

I have a Carton 100mm f/13, which is extremely good. I remember early one morning observing the moon
along side my Tak TSA-102. I was using a magnification of around 100x and the views were the same in
both scopes. The tones and texture of the lunar surface were amazing.

My Carton F16.7 only really shows chromatic aberration around Venus. Orange filter or Baader Semi Apo filter soon sorts that out. I have some "light" rated planetary filters coming through the post, which is what it needed for telescopes of 60-80mm aperture. My 80mm F15 and 101.6mm F15 show chromatic aberration around Venus. The only telescope to show chromatic aberration in violet on the moon is my 101.6mm but I have to really go looking for it, having said that when I used my 101.6mm F15 last week after a long hibernation of almost 2 years, I did not check collimation.

I'd like to get my hands on a 80mm F15 Carton or 100mm F13-F15 Carton. The only place that I can see Carton 80mm for sale is Moonraker telescopes. Haven't seen any Carton 80mm for sale secondhand for quite awhile. I would expect the Carton 80mm to best my Towa 80mm F15, having said that a Vixen 80mm F15 did not!

David

Edited by Dave1066, 11 April 2020 - 07:55 AM.

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#168 Jeff B

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Posted 11 April 2020 - 10:22 AM

My impression is that it's acceptable,and encouraged, to use filters on planets with your newt to bring out specific details and structures, however, use of the same filters with an achromat is some sort of band-aid to make a flawed scope work.  

 

Now if you really want to see just how intensely sharp a well made achromat can be, use a green filter, especially on the moon.

 

Jeff


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#169 Dave1066

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Posted 11 April 2020 - 11:07 AM

That's quite a unique perspective you have on filter use there Jeff. Certainly not one I've ever read anywhere either online in internet forums or books written by professionals.

Filters aren't about being paired with a certain design of instrument. Its about selecting the right type of filter to the size of the aperture of the instrument being used, and what it is you want to enhance on the planet you are looking at.

"Light" filters are for small instruments of between 50-100mm, "normal" are for between 100-150mm, and 150mm plus you should use "dark" filters.

David

#170 bobhen

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Posted 11 April 2020 - 11:19 AM

My impression is that it's acceptable,and encouraged, to use filters on planets with your newt to bring out specific details and structures, however, use of the same filters with an achromat is some sort of band-aid to make a flawed scope work.  

 

Now if you really want to see just how intensely sharp a well made achromat can be, use a green filter, especially on the moon.

 

Jeff

The moon is not the best target to use to judge optical quality in any telescope, including achromats. The moon is mostly a high contrast object with strong blacks and whites that can appear sharp in just about any scope.

 

Jupiter, with its more subtle markings, shadings and hues is a test target that better separates the men from the boys.

 

Bob


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