524 replies to this topic

[25585]

In respect of performance, what should you expect, and not expect, from an achromat?

 

Aperture and f-ratio will determine much, but what generally is allowable, given what achros are?

[StephenVP]

I expect to see CA in my Celestron 102GT and it is there to be sure, but I still for some reason grab it instead of my 102ED because the longer F ratio (f/9.8 vs f/7) gives me a really flat field with depth enough you want to reach into the eyepiece.

~Stephen.

[Stewc14]

I have 2 . An f5 and it has color till I install chroma or. I also have a 8” D&G and it’s f12. Add the chroma or and it’s apo like

[Jethro7]

Hello 25585,

I have two Achros left, an Altair Starwave 152 F/5.9 and a Altair Starwave Accent 102 ED F/11.22. I would expect and in fact receive wide fields of views and CA at high magnification from the short focal length faster Achro and smaller fields of view and no CA from the long focal length slow focal ratio Achro. The thing is using these Achros for the right job. The 152 F/5.9 works great for Star panning at low magnification and for night vision astronomy use on big Nebulae. The F/11.22 is a nice planetary and Lunar viewing scope. 

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

[SeattleScott]

As you are probably starting to see, what you can expect has a lot to do with the F ratio. A fast achro is more of a low power sweeper. A slow achro can be useful on lunar/planetary, although there will probably be noticeable CA, particularly on Jupiter.

[Astrojensen]

In respect of performance, what should you expect, and not expect, from an achromat?

 

Aperture and f-ratio will determine much, but what generally is allowable, given what achros are?

The question is so broad, it's like asking "what should you expect from a car?" without specifying whether you meant a Ford T from 1920, a Morris Mascot, or a F1 race car... 

 

What is generally allowable, you ask? That obviously differs enormously, depending on whether you use a 5" f/24 Lichtenknecker or a 5" f/5 Jaegers, a 150mm f/15 Zeiss E, or a Sky-Watcher 150mm f/5....

 

 

Clear skies!

Thomas, Denmark

[Jay_Reynolds_Freeman]

Achromats made with conventional glass types can be expected to work well for visual use -- no noticeable longitudinal chromatic aberration or spherical aberration and a reasonable field of view -- if they are well made and their focal ratio exceeds approximately 2.7 times the clear aperture in inches: Thus a good three-inch f/8.1, a good four-inch f/10.8, or a good six-inch f/16.2 should be a decent visual telescope. That formula is noted optical designer James G. Baker's assessment of his classic "Baker separated doublet" achromat, that uses conventional glasses.

 

Achromats made using an ED or fluorite glass for one element can be expected to work equally well for visual work at somewhat faster ratios, if they are well made; it is difficult to give a number for a formula because ED glass types have been improving, and there is perhaps a tradeoff between inexpensive glasses and short focal-ratio performance.

 

Achromats with shorter focal ratios are often used for wide-field, low-magnification instruments, because chromatic aberration and spherical aberration are less obvious at lower magnifications. For such purposes, a notable advantage of achromats is that they are lighter in weight -- often much lighter -- than better-corrected objectives that use three or more thick pieces of (heavy) glass.

 

None of this has much to do with imaging: Imagers make good use of wavelengths of light to which the human eye has little or no sensitivity -- designing a lens that works well enough to keep today's imagers happy is a whole different ball game, though visual achromats might do well if the reddish and violet ends of the spectrum were filtered out or not recorded.

 

Note that "achromat" is a type of design: A designer uses two pieces of glass, of different optical characteristics, to come up with an objective that has the same focal length for two wavelengths of light (for visual use, typically blue and yellow) and that corrects spherical aberration for one wavelength (for visual use, typically green). There is an occasional tendency to treat design types -- such as "achromat", "apochromat" and "superachromat" as if they were performance targets -- they aren't performance targets for telescopes any more than "flat-four", "inline-six" and "v-eight" are top-speed targets for automobiles.

 

Note also that "APO" is a marketing term for a high-performance refractor objective. There is lots of discussion in CloudyNights forums of what it ought to mean, but no generally agreed-upon precise definition of what it does mean.

 

 

Clear sky ...

[Russell Swan]

You should expect an amount of chromatic aberration depending on the focal ratio of the achromat.

You should expect to lose some contrast and detail on planets as a consequence of CA. The higher the power the greater the effect. 

You should expect a darker background sky than from other non-refractor optical configurations.

You should expect small, round stars.

You should expect excellent low to mid power performance on deep sky observation, with the limitation being aperture and sky conditions.  
You should expect good multiple star splitting capability, especially at higher focal ratios. 
You should not expect very good to excellent photographic capability. 

You should expect to pay a significantly lower price for all that compared to ED, semi-apo and apochromats. 

[CHASLX200]

In respect of performance, what should you expect, and not expect, from an achromat?

 

Aperture and f-ratio will determine much, but what generally is allowable, given what achros are?

Depends on speed.  

[Jon Isaacs]

 

Altair Starwave Accent 102 ED F/11.22.

 

In current parlance, an ED doublet is not an achromat. In this Astromart thread,  Roland Christen calls FPL-51 doublets, ED Doublet APOs.

 

An FPL-51 "ED Doublet APO" has 28% of the longitudinal color error of a standard crown-flint achromat. If a ED doublet is not an APO then neither is a Takahashi Fluorite doublet.  I consider them both APO's.

 

https://astromart.co...orite-etc-54455

 

I have owned quite a variety of crown flint achromats, larger and faster like a 102 mm F/5, large and pretty fast like my 120 mm F/8.3, small and fast like my Meade 60 mm F/5, small and slow like the 60 mm F/13.3 Asahi-Pentax's.  I once owned seven 80 mm F/11.3s simultaneously.

 

In terms of color correction, I subscribe to Sigdwick's 3x F/A criterion as tolerable CA for the planets and Conrady's 5x criterion as nearly/practically color free, they jive with my experiences.

 

Whereas an APO (doublet or triplet) can be quite versatile and perform well by providing wide, richest field views and near perfect high power planetary and double star views, with a standard achromat, you have to choose.  An ST-80 can provide wonderful wide field views but not stunning planetary views. An 80 mm F/15 can provide stunning (for an 80 mm) planetary views but not the sorts of wide fields a fast 80 mm can provide.

 

In the middle are 80 mm F/11.3's.. Decent but not perfect color correction combined with reasonable wide fields. With a 2 inch focuser they'll do 2.9°.

 

Achromats and refractors in general scale up poorly. A 60 mm F/7 meets the Sidgwick standard. For a 120 mm it has to be F/14 and and about 5.5 feet in length. 

 

Jon

Edited by Jon Isaacs, 23 March 2025 - 12:25 PM.

[vkhastro1]

Hello 25585,

I have two Achros left, an Altair Starwave 152 F/5.9 and a Altair Starwave Accent 102 ED F/11.22. I would expect and in fact receive wide fields of views and CA at high magnification from the short focal length faster Achro and smaller fields of view and no CA from the long focal length slow focal ratio Achro. The thing is using these Achros for the right job. The 152 F/5.9 works great for Star panning at low magnification and for night vision astronomy use on big Nebulae. The F/11.22 is a nice planetary and Lunar viewing scope. 

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

Jon beat me to it !

 

I have the same Starwave versions: 152 f/5.9 + 102 ED f/11

They are doublets.

The 152 f/5.9 is definitely an achromat

The 102 ED f/11 is definitely not an achromat.

 

ED glass in the case (probably FPL-51) is vastly different from a class crown/flint combo of an achromat.

Most ED doublets are around f/7 = somewhere between achromat and APO ~ called “semi-APO” 

IMO the f/11 ED is better than f/7 optically and closer to but not quite a true APO.

 

The imagery and contrast of my 102 ED f/11 is amazing !

Edited by vkhastro1, 23 March 2025 - 09:16 AM.

[Jon Isaacs]

 

IMO the f/11 ED is better than f/7 optically and closer to but not quite a true APO.

 

Oh good. Let's not discuss "true APOs."

 

Jon

[Refractor6]

 I'll stay clear of this one and just enjoy my last two remaining achromats from over 20 years back when I first joined the forum.  Back then any detailed topics about achromats turned into locked threads and heavily moderator edited ones.

 

 I'll define a achromat as the ones I enjoy using.....the end.

[Jethro7]

Jon beat me to it !

 

I have the same Starwave versions: 152 f/5.9 + 102 ED f/11

They are doublets.

The 152 f/5.9 is definitely an achromat

The 102 ED f/11 is definitely not an achromat.

 

ED glass in the case (probably FPL-51) is vastly different from a class crown/flint combo of an achromat.

Most ED doublets are around f/7 = somewhere between achromat and APO ~ called “semi-APO” 

IMO the f/11 ED is better than f/7 optically and closer to but not quite a true APO.

 

The imagery and contrast of my 102 ED f/11 is amazing !

Hello vkhastro1,

The Altair Starwave Accent 102ED F/11 is an improvement over the older Altair Fraunhofer design that I owned but even Achros can have optics made with "ED" glass. I did a lengthy bit of research a few years ago on the topic of what defines an Achro, Semi APO and APO.The demarcation lines between the classes are blurry. Basically in the simplist answer, it is the level of Chromatic Correction produced by the optics. Although highly unlikely in theory, even a simple Achro design could achieve these levels of Chromatic Correction. With that being said, The Altair 102ED F/11, was my first super scope for Lunar and Planetary viewing and I had many wonderful outings but this scope. However this scope has now reached a point of obsolescence for me. I guess I still keep that scope around for the fond memories. The Altair Starwave 152 F/5.9 still has a nitche amongst my premium APO's so there is still a place for these Achros. 

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

Edited by Jethro7, 23 March 2025 - 10:57 AM.

[vkhastro1]

Hi Jethro7,

 

It has been a long winter here in eastern Canada.

Hoping to “reconnect” to my Altair 102 ED f/11 shortly.

The views are definitely worth the wait !

[gnowellsct]

Based on my experience with the three I've owned I would expect an achromat to find another house to live in.

I have one of those 60 mm f / 11 Japanese department store scopes from the 1960s. I got it a real focuser during the brief interlude when a machinist was cranking out aftermarket focusers for scopes like that.

The focuser made the telescope much more usable but it also completely destabilized it on the 1960s Mount. I began to consider what kind of Mount to put it on And whether I want to tracking and what I should do about the finder, since the original was just a small tube with plastic lenses. Back of the envelope on further improvements was at least 500 bucks. We're talking rings, vixen dovetail, and a decent mount, etc. maybe more like a thousand bucks unless I just adapted it as an alternative telescope to use on top of one of my SCTs.

But I couldn't figure out why I would do that since I already have terrific telescopes in that function.

And then I said what the heck am I doing and put it down in the basement. Think of it as putting aside the money for the Pentax XW 16.5 and XW 23, though I had no idea at the time that those would ever exist.

Greg N

[PKDfan]

I've been desperately thinking back to my 13yo first telescope an achro 60mm Tasco of unknown focal length maybe 700mm and can't remember a single instance of detecting false colour with it and also back to a 4inch F/10 achro whose field was purple tinged and was returned the very next day.

As everyone has said. Its length to aperture dependent Richard and as Jay intimates (not so subtly) on how well its made.


CSS
Lance
P.s. you thinking of adding one to your plethora of scopes Richard ?

[Jon Isaacs]

Based on my experience with the three I've owned I would expect an achromat to find another house to live in.

I have one of those 60 mm f / 11 Japanese department store scopes from the 1960s. I got it a real focuser during the brief interlude when a machinist was cranking out aftermarket focusers for scopes like that.

The focuser made the telescope much more usable but it also completely destabilized it on the 1960s Mount. I began to consider what kind of Mount to put it on And whether I want to tracking and what I should do about the finder, since the original was just a small tube with plastic lenses. Back of the envelope on further improvements was at least 500 bucks. We're talking rings, vixen dovetail, and a decent mount, etc. maybe more like a thousand bucks unless I just adapted it as an alternative telescope to use on top of one of my SCTs.

But I couldn't figure out why I would do that since I already have terrific telescopes in that function.

And then I said what the heck am I doing and put it down in the basement. Think of it as putting aside the money for the Pentax XW 16.5 and XW 23, though I had no idea at the time that those would ever exist.

Greg N

 

I've owned many, many achromats. Nearly all were purchased used via Craigslist, a few via CN and AM.

 

The classics were typically small and slow on wobbly mounts that made them difficult to observe with by modern standards. 

 

Very likely the best scopes optically I've ever owned were two Asahi-Pentax 60 mm x 800mm from around 1960. At F/13.3, the Chromatic Ratio is 5.6, exceeding the Conrady standard. (Lance's 60 mm x 700 mm is at 4.95, essentially meeting the color free standard.)

 

What set these scopes apart were the mechanics of the OTA, the unique gear drive Alt-Az mount and the robust tripod. I frequently used it was th the 4.8 mm Nagler for about 170x. One ofy favorite observations was a double shadow transit of Jupiter.

 

The combination of the optics and the solid a I'll t-az mount made these really fun scopes.

 

 

In an episode of downsizing, both were given away. Prior to Covid, I purchased another one that I was supposed to pickup in Salt Lake City but that still hasn't happened.

 

Jon

[CHASLX200]

Sub F/10 sweeper peepers.  F/15 for planets 4" and smaller. Still too much color for a 6" at F/15 for my eyes.

[Sean Cunneen]

Let's assume you know your stuff and you have a very good collection of long achros and short APOs. Let's also assume your targets are bright planetary type targets.

The two things that are going to jump out at you right off the bat is image scale and a tiny bit of chromatic aberration on bright targets like the moon or Venus. The achros have a lot of built in magnification for any given eyepiece and the views feel better since you can use more comfortable longer eye relief designs and get great views.

Now, through the APOs the views are still sharp, but you have to have some very good Barlow's and select high power eyepieces to get the same detail. If you do, you'll notice there is way more low contrast fine details viewable in the apo and you think "yes, I have arrived" but then you step back and see your once elegant scope is a plumbing nightmare of Barlow's and adapters while the achro sits there so beautiful and elegant in its simplicity of form and function.

The later on you start thinking about how much glass there is between you and that planetary views and you do the math about how much light is lost going through each element and you start to wonder why they don't make planetary apos.... And you ask one of the very expensive telescope makers why they don't make one.. then they do you can't afford it so they don't make any more. Sigh.

Sean

[PKDfan]

I've owned many, many achromats. Nearly all were purchased used via Craigslist, a few via CN and AM.

The classics were typically small and slow on wobbly mounts that made them difficult to observe with by modern standards.

Very likely the best scopes optically I've ever owned were two Asahi-Pentax 60 mm x 800mm from around 1960. At F/13.3, the Chromatic Ratio is 5.6, exceeding the Conrady standard. (Lance's 60 mm x 700 mm is at 4.95, essentially meeting the color free standard.)

What set these scopes apart were the mechanics of the OTA, the unique gear drive Alt-Az mount and the robust tripod. I frequently used it was th the 4.8 mm Nagler for about 170x. One ofy favorite observations was a double shadow transit of Jupiter.

The combination of the optics and the solid a I'll t-az mount made these really fun scopes.



In an episode of downsizing, both were given away. Prior to Covid, I purchased another one that I was supposed to pickup in Salt Lake City but that still hasn't happened.

Jon

Such a fountain of wisdom !

Thanks Jon for confirming how my recollection makes sense optically.

I wish i knew what happened to it....i really hope it didn't get tossed into a tipple- for our UK members.



CSS
Lance

[CHASLX200]

Let's assume you know your stuff and you have a very good collection of long achros and short APOs. Let's also assume your targets are bright planetary type targets.

The two things that are going to jump out at you right off the bat is image scale and a tiny bit of chromatic aberration on bright targets like the moon or Venus. The achros have a lot of built in magnification for any given eyepiece and the views feel better since you can use more comfortable longer eye relief designs and get great views.

Now, through the APOs the views are still sharp, but you have to have some very good Barlow's and select high power eyepieces to get the same detail. If you do, you'll notice there is way more low contrast fine details viewable in the apo and you think "yes, I have arrived" but then you step back and see your once elegant scope is a plumbing nightmare of Barlow's and adapters while the achro sits there so beautiful and elegant in its simplicity of form and function.

The later on you start thinking about how much glass there is between you and that planetary views and you do the math about how much light is lost going through each element and you start to wonder why they don't make planetary apos.... And you ask one of the very expensive telescope makers why they don't make one.. then they do you can't afford it so they don't make any more. Sigh.

Sean

This is why i want a 2mm Delite..

[Maranatha]

I have found a decent achro can match an apo in sharpness up to 100x by simply masking the objective. I do the vast majority of my viewing under 100x so works great for me. I use my mak for the high mag stuff.

 

I will add that I do quite a bit of daytime and moon viewing, so CA is an issue which is how I came to learn about masking. In daytime use its like going from standard def to high def.

[Bomber Bob]

For F10 & Higher:

 

- Snap To! sharp lunar / planetary limb focusing at 50x per inch. [80x / inch = Excellent.  100x / inch = Outstanding.]

- Low to no false color, depending on Focal Ratio.

 

For F7 to F9:

 

- Same crisp views as above.

- False Color should not overwhelm the object.

 

For F6 & Lower:

 

- WIDE Fields!!

- False Color!

 

**  These are minimums -- not scopes I would necessarily keep!

Edited by Bomber Bob, 23 March 2025 - 01:31 PM.

[kmparsons]

I am fortunate to have an excellent achromat built by Richard Day of Skylight Telescopes. It is 100mm f/10 with an outstanding Vixen objective. I use it frequently, though i have triplet and doublet apos. I have seen CA, but it is generally negligible to undetectable. It provides excellent, contrasty views with pinpoint stars and a flat field. I was tempted by the Altair Planeta, but I really can't see it supplanting my beloved achro.