Where do achromatic refractors figure in today's world?

[Guest]

Today when you mention refractors you mostly hear about ED doublet objectives, triplet apo's or even quadrouplets. 

But, where do achromats fit in? There are several offered. I have in the past owned the older f15 80mm Towa, I now own a 40mm Tasco. This gave me my first view of Jupiter and it:s moons. Saturn's rings and Venus and phases. 

I have also owned 100mm, 120mm & 150mm Startravel refractors. They all performed very well for their designs. The Towa with it's very long focal length gave sharp views of the Moon and planets. 

The Startravel were great wide field instruments. In fact the best view I ever had of the double cluster was with an St 120mm mounted on an AZ4 mount observing from a dark sky in South Wales. 

But you don't seem to read many observing reports made with these telescopes. I enjoyed owning the 120mm & 150mm.

Edited May 28, 2019 by Guest

[Alfian]

That's an interesting question which in reality I don't think I have sufficient experience, (compared to many others here) to answer in any meaningful way. I have had a SW ED100 which should have been more than enough to tick my boxes very nicely, and indeed it was a great scope, but somehow I found myself, rightly or wrongly,  parting with it. I then acquired an old Tal100r and despite the Tal "mystique" and reputation I didn't expect it to perform as well as the ED100. Yes there is some colour but somehow the view through it are more absorbing and it makes me smile. I can't really explain that but I assume its down to the way Russian optics have that "something".  So to my mind achromats can still cut it - but there again I suppose it depends on what you demand of your optics and maybe I'm not as demanding as some.

[John]

A good achromat is a really enjoyable scope to use but for high power use they need to be long when the aperture exceeds around 100mm to allow false colour to be controlled to a level where it does not detract from the image and such long scopes are much more demanding on mounts. It's that mounting practicality that has moved me to ED doublet refractors where an F/7 can control CA as well as a tube twice as long, or more with an achromat.

The scope pictured below is a 150mm F/12 achromat with a well figured objective lens. It still showed a fair amount of CA around brighter targets and was also very challenging to mount adequately. My ED120 (shown next to the 150mm) is just so easy to mount and carry about and performs pretty much as well and shows practically no CA visually. The ED120 costs about the same as well.

 

[johninderby]

Achromats can be fine observing scopes and many today have never looked through one of classic achromats. Todays mass market achromats are the lower end of the market compared to the ED and APO as thry aren’t produced to the same standards. However some of the classic long focal length achromats were made to a very high standard indeed 

I remeber doing a head to head between my Carton 100 f13 and my WO FLT 98 APO on Jupiter and Saturn. The Carton simply produced better views.despite being an achromat.

Edited May 28, 2019 by johninderby

[vlaiv]

I guess it is the matter of shifting costs.

There were different periods in scope production history - when most of the cost went into manual labor of figuring the lens. Then it shifted to glass type. Nowadays I suspect that majority of costs is tied up again in manual labor, but this time it is related to scope assembly, handling/storage and shipping.

This somewhat levels the field between classical achromats and scopes with more exotic glass, and I guess that prices will continue to close the gap. This has important consequence - people just opt to go for ED / APO types of scopes because they resolve some of the difficulties associated with classical achromats - CA and mounting problems (for long fl scopes). Fact that manufacturers opted to shift achromats to entry segment - meaning less quality accessories - like focuser and tube does not help - it just makes justification for higher cost instruments.

This of course results in decreased demand for "entry" scopes like achromats, and in turn makes them less profitable. We are already at the point where high quality achromats are "oldtimers" of astronomy world. Soon all achromatic refractors will be in this class.

 

[Rob]

All above is a good reflection on the achromatic scope. And speaks the truth.

That said I'm a firm believer in that there's a place for every scope within our hobby. Its basically down to the 'one device does not fit all!'. The views through my 127L of lets say M57 or M27 provide a fabulous contrasty, almost 3D feel to observing these types of targets. The rich black back drop of the dark sky, along with trying to pull an edge of colour. I really like sweeping the milky way too.

I think there's a place!. Colour pull on Planets is also a wonderful thing.

Rob

Edited May 28, 2019 by Rob

[johninderby]

Long focal length achromats do have their place although admitedly not the best allrounders. Still there is something a bit magical about one of the old classic ones. 🙂

Perhaps it’s a bit like classic cars. I would love one of these. Of course just a tiny bit over my budget. 😁

Edited May 28, 2019 by johninderby

[michael.h.f.wilkinson]

One niche area where a good achromat can be really useful (especially if well corrected in terms of spherical aberration (SA)) is solar observing and imaging. In "white light" most use the green "Solar continuum" filters that have a narrow enough bandpass for chromatic aberration (CA) to be negligible. Likewise, if you insert a Quark, or a Ca-K module, CA is essentially absent. SA is still important, especially in the Ca-K case, as this is on the edge of the spectral range for which the scope will have been corrected.

[Guest]

59 minutes ago, John said:

A good achromat is a really enjoyable scope to use but for high power use they need to be long when the aperture exceeds around 100mm to allow false colour to be controlled to a level where it does not detract from the image and such long scopes are much more demanding on mounts. It's that mounting practicality that has moved me to ED doublet refractors where an F/7 can control CA as well as a tube twice as long, or more with an achromat.

The scope pictured below is a 150mm F/12 achromat with a well figured objective lens. It still showed a fair amount of CA around brighter targets and was also very challenging to mount adequately. My ED120 (shown next to the 150mm) is just so easy to mount and carry about and performs pretty much as well and shows practically no CA visually. The ED120 costs about the same as well.

 

It's monstrous indeed

[Saganite]

Well, here is one that is still very much in use, 5" f15 D&G. In the three and a half years of owning this scope I have had many wonderful observations and a good few have been posted on this forum, but for various reasons, since October last, it has had little use. The ground area is currently being redeveloped and I have erected a curved trellis around it with a detachable blackout curtain on the inside, so no extraneous light is visible whilst using it. It will be complete by the end of the summer and uninterrupted observing will resume....

Edited May 28, 2019 by Saganite

[mikeDnight]

I love long focal length refractors but my aging back doesn't. They can give superb definition and contrast, and can be as sharp as any ED if the lens is well figured. The biggest drawbacks would be the need for a hefty mount and the large observing arc the focuser will need, and housing it.

Short achro's have severe limitations as lunar and planetary scopes, but they make stunning rft's. The finest view of the Double Cluster I've ever had was through a SW 150 Star Travel. Rather than viewing the ST's as poor scopes, I consider them to be excellent, specialist rich field/comet seeker/deep sky refractors. I know I'm odd, but I'd rather have a 6" F5 ish achromat than a 10" reflector any day. But when it comes to the moon and planets, I'd much prefer the comfort of the shorter apo/ED.

 

[Ben the Ignorant]

If low-dispersion glass had been available since the beginning achromats would not exist, all refractors would be apos and this discussion would not be taking place, they are the byproduct of limited glass technology. There is no reason except nostalgia for long heavy cumbersome achromats today, and it is a valid reason for those who value it above convenience, but convenience is the mother of all inventions and changes in the way of life, so...

I bought an unexpensive 80mm f/7.5 achro several years ago, as a birthday gift for a friend, for his whole family to be more precise, because they own a rural home in southern Italy where they spend two months each year. The sky is super dark, M33 is visible to the unaided eyes; their simple achro with a low-power eyepiece and an AZ-3 mount is a great sky sweeper for these occasional stargazers.

At the same time I bought the same achro for myself to have a travel scope that I'm not afraid to tote around, 115€, so no fear. But a couple years later I unearthed a new 80 f/7 semi-apo sold for one third of the regular price because the lens was dirty and the store staff didn't want to clean it. 115€ for an achro but 184€ for an apo, so why keep the achro?

Funny that this thread arises now, since a couple of days I was just thinking of giving my achro to the same family because their year-long home is right at the edge of the city, so less light pollution, and their main window/patio happens to face South. The achro has not been used for at least a year. it made me realize I see no point in keeping it any more.

Of course it would be a different story if the scope was huge and expensive.

[Guest]

17 minutes ago, mikeDnight said:

I love long focal length refractors but my aging back doesn't. They can give superb definition and contrast, and can be as sharp as any ED if the lens is well figured. The biggest drawbacks would be the need for a hefty mount and the large observing arc the focuser will need, and housing it.

Short achro's have severe limitations as lunar and planetary scopes, but they make stunning rft's. The finest view of the Double Cluster I've ever had was through a SW 150 Star Travel. Rather than viewing the ST's as poor scopes, I consider them to be excellent, specialist rich field/comet seeker/deep sky refractors. I know I'm odd, but I'd rather have a 6" F5 ish achromat than a 10" reflector any day. But when it comes to the moon and planets, I'd much prefer the comfort of the shorter apo/ED.

 

I do feel for you. I recently sold my ED 120mm. It gave great views. But was a little too long for my viewing stance and comfort after breaking my ribs. I am considering an f7, 100mm ed refractor as it may offer better comfort being shorter and lighter. 

Edited May 28, 2019 by Guest
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[mikeDnight]

A 100mm F7 ED might be all the scope you'll need. They make great all round scopes that are easy to use and easy to mount, and the don't take up much room in the home. I've always been happy with smallish refractors and have thought that if i ever need to use a big scope, I'll take a look through someonelses. I've now been loaned a 10" F6.3 Dob, but so far I much prefer using my 100mm refractor.

[Guest]

2 minutes ago, mikeDnight said:

A 100mm F7 ED might be all the scope you'll need. They make great all round scopes that are easy to use and easy to mount, and the don't take up much room in the home. I've always been happy with smallish refractors and have thought that if i ever need to use a big scope, I'll take a look through someonelses. I've now been loaned a 10" F6.3 Dob, but so far I much prefer using my 100mm refractor.

I have an eight inch f 4.5 Newtonian which I keep in Dorset for the beautiful dark skies. 

[Saganite]

46 minutes ago, mikeDnight said:

I love long focal length refractors but my aging back doesn't. They can give superb definition and contrast, and can be as sharp as any ED if the lens is well figured. The biggest drawbacks would be the need for a hefty mount and the large observing arc the focuser will need, and housing it.

Short achro's have severe limitations as lunar and planetary scopes, but they make stunning rft's. The finest view of the Double Cluster I've ever had was through a SW 150 Star Travel. Rather than viewing the ST's as poor scopes, I consider them to be excellent, specialist rich field/comet seeker/deep sky refractors. I know I'm odd, but I'd rather have a 6" F5 ish achromat than a 10" reflector any day. But when it comes to the moon and planets, I'd much prefer the comfort of the shorter apo/ED.

 

 

Hi Mike,

The previous owner of this scope was, as I know you know, Dave (f15 Rules), and he reckoned that the sharpness, and contrast of this scope was almost on a level with his FS128,( high praise indeed) so good is the figure of this lens. I could have gone for a 5" Apo, but the elegance of this long tubed beauty really does it for me, and I feel no inconvenience, though mounting it satisfactorily has been a challenge....

[Paz]

I've had an ST120 for a few years and despite accumulating other scopes the ST120 still ranks joint first with my Maksutov as being my most used scopes. A ~4" achromat plus a ~4" maksutov cost less than one quality 4" apo and can cover all the bases admirably.

It does show colour on bright targets but filters can manage this well. It's a lot of aperture in a short tube and is low cost and so is not a worry to own.

I do aim to get an ed/apo in the future and it's possible when I do I will wish I did it sooner but I'm still enjoying what I've got and am not in a huge hurry.

[steveex2003]

54 minutes ago, mikeDnight said:

A 100mm F7 ED might be all the scope you'll need. They make great all round scopes that are easy to use and easy to mount, and the don't take up much room in the home. I've always been happy with smallish refractors and have thought that if i ever need to use a big scope, I'll take a look through someonelses. I've now been loaned a 10" F6.3 Dob, but so far I much prefer using my 100mm refractor.

This is very much my experience too. Of the scopes I have, I tend to have the little 80mm Frac and a 10" Dob set up to go in a room with a SSW facing window (the others remain packed when not in use). This has meant, despite its rather crude and lazy methodology, for the last few weeks I have been able to set an alarm, get up, open the window and point either scope at Jupiter/Saturn/Moon. I compare them everytime and you know I prefer the view of the Fracs. I know it's nice bright planetary targets, and the Dobs image is brighter. To my eye's, which are less than perfect, the image is so much sharper in either Frac and on such targets I really dont think i am missing any detail whatsoever between the 120mm and the 10". If anything I percieve there more detail in the sharpness of the image. The liitle 80mm is literally a one-handed affair, with a coffee in the other which is it's own joy. I havent even tried the 5" Mak recently, in a 3-way planetary shoot-out, so pleased with views I get with the Fracs (and some new EP's) but i will do so over the coming weeks.

Edited May 28, 2019 by steveex2003

[Vondragonnoggin]

My last three scope purchases were short achromats. I use a mak for high power views but occasionally point the 152mm F/5.9 achromat I have at high power targets and crank up the magnification. The other two are the 120ST and 150ST Skywatcher scopes at F/5.

 

planetary/lunar targets and double star targets have always been low priority targets for me even since my first scope which was an 8” dob. Just more interested in OC’s, GC’s, nebulae, galaxies, dense starfields, and rich hydrogen regions of the Milky Way.

Much like solar observation narrows to a small portion of red spectrum, I’m using Night Vision eyepieces and filtering for narrowband Ha in 3nm, 7nm, or 12nm or filtering for near IR in 610nm, 640nm, and 685nm. Galaxies generally filterless as well as planetary nebulae.

This is essentially all red spectrum viewing. CA problems are non-existent really. For globs, the high power views are quite revealing using longpass filters to cut light pollution and give a darker more contrasty view with the night vision eyepieces.

I bought the achromats to use with the night vision eyepieces and saw no point to buying apo’s if I’m filtering out all but red spectrum because the Intensifier response is strongest in red and all but non-existent in blue (GaAs Intensifier)

Generally it’s low power views though for nebulae, open cluster, sweeping the Milky Way and looking at dense star regions. The billowing hydrogen clouds of the Milky Way are revealed very well even in Bortle 7 zone LP. Night vision eyepieces are most often 27mm focal length oculars and I employ focal reducers to get brighter views and wider fields or barlows to get higher power globs, galaxies, and PN’s. A lot of times it’s just native focal length though so fast optics are brighter to start with.

Next year I’ll probably add a Quark Solar filter and get more use out of the achromats.

I still like the achromats with my regular eyepieces as well and still use those at times with UHC and OIII filters which also kills the CA. I don’t mind the field curvature or SA in them and I only have two AFOV’s in the night Vision stuff. 40° in my smaller gen 3 tube devices, and 65° in a much larger Gen 3 Intensifier tube device. The 40° view in particular really misses some edge aberrations or makes them more tolerable. Mostly the boost in what I’m seeing overpowers the aberrations so I find it a good compromise. The 152 F/5.9 probably has the best figure for all around use through medium and high powers.

 

If I’m Lunar/planetary viewing I tend to make a whole night of it and dedicate the time to it with my mak.

 

The achromats get a lot of flak on forums it seems, but can provide some very nice observing in my opinion. Even without the night vision I really liked them. Second scope was an AR127 F/6.5 and I enjoyed the views as much as my 8” dob. I also used it high power on planets sometimes taking it up to 240x and up. I liked it with a semi-apo filter for cutting CA a little and still having a proper color look as opposed to a yellow shift of some CA filters. A 495nm longpass is more effective at reducing CA but then shifts color to a lot of yellow. The AR127 was a little bit before I got into Night Vision astronomy but really influenced a love for refractors. The mak was bought after selling the AR127 to a friend of mine with a bunch of AP gear I wasn’t interested in anymore. He needed a good scope so I just made it a package deal. I bought the mak right after and that was seven years ago. All other achromats came after tha mak. I have only one ED lens scope. It is an AT72ED. Gets used a lot for the size.

 

Two Binocular Telescopes as well that are fast achromats. One 70mm F/6.2, and one that is 100mm F/5 but is listed as 100ED while claiming semi-apo performance but really no better than the F/6.2 70mm in CA control. Enjoy them up to 48x max for Milky Way sweeping.

Im a big fan of them for cost and cost/performance ratio. Not against apos or ED doublets, but think the achromat still has a place in observing and the market.

I should also mention that I’ve found I’m easily pleased by most equipment and tend to overlook a lot that the more discerning viewer might not be able to overlook. Mounts have to be really solid though.

Edited May 28, 2019 by Vondragonnoggin

[Ben the Ignorant]

41 minutes ago, Vondragonnoggin said:

Much like solar observation narrows to a small portion of red spectrum, I’m using Night Vision eyepieces and filtering for narrowband Ha in 3nm, 7nm, or 12nm or filtering for near IR in 610nm, 640nm, and 685nm. Galaxies generally filterless as well as planetary nebulae.

This is essentially all red spectrum viewing. CA problems are non-existent really. For globs, the high power views are quite revealing using longpass filters to cut light pollution and give a darker more contrasty view with the night vision eyepieces.

I bought the achromats to use with the night vision eyepieces and saw no point to buying apo’s if I’m filtering out all but red spectrum because the Intensifier response is strongest in red and all but non-existent in blue (GaAs Intensifier)

Good to know.

[paulastro]

On ‎28‎/‎05‎/‎2019 at 16:59, mikeDnight said:

 I know I'm odd,

 

Well Mike, it's not for me to disagree with you 🤣

[saarmason]

I have owned a few Apo refractors.  NP101 was the largest.  The views in the Televue NP101 were sharp and the wide field views awesome, but it is still only 101mm of glass.  I figured that since I enjoy refractors so much a little more aperture would be a good thing.  I saw a nice looking used AstroTelescope 152mm F5.9 Achromat on Astromart for a very low price and it has been my scope of choice for outreach programs and DSO observing   - under good clear skies-mounted on my WO EZ-mount (with counterweight installed) in my back yard.    The wide field views with 6 inches of aperture brought me into another dimension of viewing satisfaction while observing from the south shore of Long Island, NY.   The summertime Sagittarius area DSO's looked so much better with this scope than the smaller apo.  I didn't expect it to be that much of an improvement even without the WO minus violet filter fitted to the 2 inch diagonal.   I do own a very nice 10 inch Teleport newtonian with Zambuto mirror and viewing with that is totally enjoyable during club observing sessions - and dark sky sites - until my back gives out from contorting to suit the dob dips and sways locating objects, but for ease of use - sitting comfortably while observing -  the 6" achromatic refractor is amazing and comfortable - and that 6" of clear aperture pulls in almost as much light as an 8 inch dob - only sharper.  I guess my point is that a reasonably priced "well corrected" larger (rich field) achromat can provide amazing and immersive viewing experiences for a LOT less cost than a similarly sized APO.    This large achro will give me amazing viewing for the price of a couple of good 100 degree eyepieces.  That is why Achromats are still pretty cool astronomical instruments to enjoy on clear nights.  My dream scope was a large TAK, but I needed to buy a car instead 🙂.      Clear skies,  Geo.

[AdeKing]

My first foray into Fracs was a humble ST102 which I've since upgraded the focuser on and I still use it loads. It does have CA but personally I don't find it as obtrusive as others.  A semi-apo filter sorts most of the CA anyway. 

I have since acquired a Starwave Classic f/11 which is a cracking scope and due to the focal ratio the CA is incredibly well controlled. However the Starwave sees less use than other scopes in my collection as the length means that I have to make the effort to set up my EQ5 which isn't a task I relish as I'm very much an Alt-AZ fan. 

I also have two EDs an ED100 and Martin's former ED120 and a lovely little WO ZS66SD. 

The ST102 sees lots of outreach and WL solar use and is frequently used at home when I can't be bothered setting up one of the longer OTAs. 

I think that achros give great views for the price but in the case of short achros you just need to be aware that if used on high contrast targets such as lunar and planets that there will be some CA and whether you find it objectionable will be a personal thing. 

As Rob said above. 

On 28/05/2019 at 15:36, Rob said:

There's a place for every scope within our hobby. Its basically down to the 'one device does not fit all!'. 

[Ags]

I use my ST80 a lot. It acts as the widefield side of my dual scope DSO setup, and gives lovely views of the bigger clusters. As a travelscope it has excelled from dark sites. With some Baader solar foil it was a fun solar telescope. I've even used it as a telelens with my camera.

[Vondragonnoggin]

I was out last night with my Startravel 150 and a Williams Optics binoviewer. Highest I took it was 150x with a semi-apo in the diagonal and this was the first time I had tried that scope above 50x.

It was actually some great views with a pair of 9mm BST Planetary EP’s. I used the 1.8x GPC with the Binoviewers to achieve focus. The previous owner told me he hand picked the scope after testing three of them and said it was surprisingly a good figure for that model. He was right.

 

I usually use my mak or 6” F/5.9 achro, but the F/5 was actually putting up some stunning views in my opinion.

6 bands on Jupiter. Not large enough image scale to see the blue eddies in the bands, but what a steady view with no shimmer at all. Saturn was the same with several bands visible, Cassini visible but not Encke, 5 moons around Saturn although two of them I had to use averted vision to get them to blink in. Unbelieveable steady views though.

 

Maybe I’m just too impressed by anything I’m able to see where views are very steady and some amount of detail present. I even pointed at a few nebula with no filter and was able to make out the Lagoon, Triffid, Omega, and just barely a haze for the Eagle, a few globs, and Wild Duck cluster was stunning.

 

It surprised me. I normally steer people away from high power views with the short tubes, but maybe I’ve just been echoing advice from other members that don’t use short tubes and frequently debate best apos. I guess I’ll change the advice to “try it if you feel like it. The worst case is you’ll find you don’t like it at high powers”

Edited May 29, 2019 by Vondragonnoggin