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spaceboy

Is there a highly regarded achromat refractor ?

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I was just thinking, as you do. Is there a highly praised achromatic refractor out there ? With ED and APO refractors so far out the reach of the modest earning folk I would have thought there must be one high spec achromatic refractor that gives as good as it gets ?

Or does the result of all that refinement to make an achromat well corrected and tolerant of CA end up with a cheap ED refractor like Celestron's C100ED. I like the look of the Altair 152 but at £800+ again your getting very close to Chinese ED budget. I appreciate your not going to get the same aperture in an ED as you can a well corrected achromat but even so good glass does seem to compete very well often giving equal if not better planetary views than a larger achro.

From what I gather acromats can be just as good as ED refractors but at the expense of rather long focal lengths? I'm sure even if you could pick up a long frak at a fair price it would still end up fringing on Chinese ED due to mounting issues occurring from wielding such long refractors ?? 

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I use a AR127L f9.5 "old faithfull" it will hold its own with any ed doublet, there cheap as chips to buy second hand if you can find one. take a look at my flickr page in my sig for shots taken with it. it shows no more CA than my ed80, you don't need to spend huge amounts of money to get a good frac. charl.

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Well regarded achromats in the small range are anything by Vixen F10 and longer, also the venerable Tal 4" F10. Other 4" long focus achromats also have a good reputation. The common theme is long focus, however once you get up to 6" aperture and above, the potential performance is a trade off against the mounting problem. With the current popularity of APO's there is little incentive for manufacturers to service a niche market. I've made a number of 6" and 8" F20  folded achromat  refractors, they work very well but despite the relative compactness they still required a substantial mount.   :icon_biggrin:

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8 minutes ago, Peter Drew said:

 I've made a number of 6" and 8" F20  folded achromat  refractors, they work very well but despite the relative compactness they still required a substantial mount.   :icon_biggrin:

Well your little old 125mm push pull refactor does well too.  Now has a SW focuser from a 120 SW

Dont suppose you have an adapter ring to fit the 125 to the SW focuser, currently has a blue peter fix

 

 

 

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The Tal 100RS were always  well regarded but don't think they have been produeced for some time now so one would be used. After that on the new side it would be one of the Vixen's.

The problem is that there are several Flint glasses and several Crown glasses so one achro can/will change from another simply by the glass used and the resultant combination. That is likely the difference between a good Achro and a mediocre Achro. If the Crown component (I think it is Crown) has an Abbe numbers between a cheap Crown and FPL-51 then you still have an Achro but likely a good one.

One question it all leads to is: "What is an ED glass?" We talk of FPL-51 and FPL-53 but what about FSL-5 or BSL-7 (maybe BSL-7 is the Ohara equivalent of BK7). Looking at the Ohara chart it half looks like ED glass is an Abbe number of 80 and above, or maybe 75. Basically no specific definition - as best I know.

If you want a good achro you are likely looking at f/8 or slower and a make in which there is a degree of confidence that the glass used is as originally considered in the initial design.

The ES offering seem good and as they are seperated doublets there is better flexibility to design the 2 lens to operate better. On a cemented one two of the faces have to be the same radaii. Equally the ES ones seem to cost more, Bresser make very similar but they made cemented dioublets so R2=R3.

If I were to buy one (but I have a Tal100 anyway) then for cost the Bresser 102/1000, £212. I see that ES do not make an equivalent.

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2 hours ago, xtreemchaos said:

I use a AR127L f9.5 "old faithfull" it will hold its own with any ed doublet, there cheap as chips to buy second hand if you can find one. take a look at my flickr page in my sig for shots taken with it. it shows no more CA than my ed80, you don't need to spend huge amounts of money to get a good frac. charl.

I should have pointed out I'm not in the market for one. It's more a curiosity thing given most ED refractors are out of financial reach of many. I just wondered if there was one that really shone. Thinking about it now the Tal should have sprung to mind as I was typing the title. I've had an AR127L and I agree they are a very nice scope around if not a little on the heavy side. 

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1 hour ago, ronin said:

 

If I were to buy one (but I have a Tal100 anyway) then for cost the Bresser 102/1000, £212. I see that ES do not make an equivalent.

I think someone on the lounge has had a poor first experience with the bresser 102/1000. I think it was poor build quality why they felt the need to return the scope. I'm not sure if they did an optical review or not?

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if its one you want that shines as a ed and don't brake the bank it has to be sw ed120, I have the old gold one in the loft, its one what you feel like you want to keep hold of, mines a bit batterd but the optics are still great, keep meaning to get a new focuser for it because I think it will work well with my quark. charl.

Edited by xtreemchaos
missing letters

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6 minutes ago, xtreemchaos said:

if its one you want that shines as a ed and don't brake the bank it has to be sw ed120, I have the old gold one in the loft, its one what you feel like you want to keep hold of, mines a bit batterd but the optics are still great, keep meaning to get a new focuser for it because I think it will work well with my quark. charl.

I have an ED120 mate. I admit I'm after an achro but for a solar mod so already know which I want due to the specific focal length. This thread was more for interest (and to hopefully help those in the market) not to say I wouldn't be interested if I ever came across the crown jewel in achros if there was one??

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15 hours ago, spaceboy said:

I think someone on the lounge has had a poor first experience with the bresser 102/1000. I think it was poor build quality why they felt the need to return the scope. I'm not sure if they did an optical review or not?

I think it was lockie with a bresser 90.......i have been very tempted to try the Bresser 102 out of curiosity.

In the 4" achro i still say the TAL 100 RS MK1 (purple lens and 2" R anP focuser) is about the best achro but a Carton 4" F13 would beat it but just not available any more

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Interesting, I picked up an excellent 4" CARTON objective for my spectroheliograph.........

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On 9/5/2017 at 14:01, spaceboy said:

I think someone on the lounge has had a poor first experience with the bresser 102/1000. I think it was poor build quality why they felt the need to return the scope. I'm not sure if they did an optical review or not?

Hi, that was likely me, and it was the Bresser 90/1350 f/13 rather than the 102/1000 f/10. Basically Bresser's 100mm and above achros have good build quality with metal rings and metal hex focuser, but below 100mm (such as the 90mm) they use far too much plastic for my taste despite the great optics. 

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I have a Tal100RS and they are really crisp scopes. Sure there is some CA (as you would expect) on bright objects like Vega but for the price it was a bargain. Jupiter is stunning and at F10 its pretty kind on eyepieces.. 

As others have mentioned, second had might turn up a bargain as I believe they have stopped making them. The nearest equivalent is probably the Starwave F11 but they are more expensive and don't come with a finder like the TAL.

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The best achromats that I've owned are;

- Vixen SP102M 102mm F/9.8

- Tal 100R 100mm F/10

- Bresser Messier 127L 127mm F/9.4

I think the advent of the low cost ED doublets from Synta hit the demand for quality achromats though. When you can get an ED100 on the used market for £250-£300 I suspect most would choose that over a similarly priced F/10 4" achromat. Personally I'd tend to agree with them.

To compete with the ED100 doublet in terms of CA control, the 4" achromat is going to need to be around F/15 which introduces some challenging mounting issues.

Edited by John
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Mounting and other issues aside, I assume there comes a point where achromats start to come in to their own? or are bespoke larger refractors just expensive no matter what glass is in them? I see some "budget" 152 mm ED refractors for £3k so surely you'd get a lot of achromat for that money ??

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On ‎2017‎-‎09‎-‎05 at 11:42, spaceboy said:

I was just thinking, as you do. Is there a highly praised achromatic refractor out there ? With ED and APO refractors so far out the reach of the modest earning folk I would have thought there must be one high spec achromatic refractor that gives as good as it gets ?

Or does the result of all that refinement to make an achromat well corrected and tolerant of CA end up with a cheap ED refractor like Celestron's C100ED. I like the look of the Altair 152 but at £800+ again your getting very close to Chinese ED budget. I appreciate your not going to get the same aperture in an ED as you can a well corrected achromat but even so good glass does seem to compete very well often giving equal if not better planetary views than a larger achro.

From what I gather acromats can be just as good as ED refractors but at the expense of rather long focal lengths? I'm sure even if you could pick up a long frak at a fair price it would still end up fringing on Chinese ED due to mounting issues occurring from wielding such long refractors ?? 

There's this much published picture about CA in achromats

CA_in-Achromates.jpg.1d40b1a35aec47c0f53414489d0ff239.jpg

 

According to Telescope Optics by Rutten and van Venrooij, The empirical formula

Color Blur (CB)= 735 * aperture (in mm) * secondary spectrum / focal ratio

A CB of 1.0 or less is color free
A CB of 2.0 or so is a semi-apo
A CB of 3.0 or more is an achro

Heres's secondary spectrum for some typical glasses:

Fluorite  1/16000

FPL-53 is 1/10000

FPL-51 is 1/6000

BK7 is 1/2000

The empirical formula can vary by as much as 10% depending on the mating glass.

So the cheap f9 C100ED with FPL-53 would be:

CB = 735 * 100  * 1/10000 / 9 = 0.82, i.e. true apo if only CA is considered.

An achromat with BK7 will have a focal ratio 735 * 100  * 1/2000 /1=37 to reach CB=1, now that's a 3.7 meter long refractor.

With a FPL-51, you can cut down the focal ratio to a third (1/6000 vs 1/2000) for the same CB, considering the relatively low cost of FPL-51, there's not much saving in making a long achromat than a FPL-51 ED for median aperture refractors, IMHO.

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I get what you are saying but leading on from my last post, would a 10" APO be simply unrealistic price wise when you consider what you could achieve with an achromat for similar money. All totally hypothetical of course. I guess what I mean is at what size would the benefits of an APO outweigh the cost ??

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You'll have to work with me on this. I sort of know what I want to ask but I'm reading it and I can't get in a limited amount of text the question I think I'm trying to ask. 

Edited by spaceboy

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I think, and TBH sort of guessing a 150 F15 well corrected achro would MAYBE beat a say, 140 F7 ED APO and one would think the 150 F15 should be cheaper to produce, but it would come back to all the problems previously mentioned like trying to sort a mount for the big achro, a few years back TAL unveiled a 200RS, but it did not seem to go into full production, that would have been a dream scope, maybe i tell a lie re the big TAL, they list a 200 1800mm f/l apo

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My first serious aperture refractor was a 102mm F13 Vixen achromat which I bought from Peter Drew in the mid 1980's. It was a superb scope that gave terrific views of both the moon and planets, and brighter dso's. In my mind the F13 Vixen gave me the finest view of M82 that I've ever had, showing the galaxy as a slender mottled ghost with a dark lane bisecting the ghostly wedge of light. It might sound unimpressive but in reality it was jaw dropping, because it was so sharp and detailed. I can't ever remember noticing CA in that telescope, so it must have been very well controlled. One local astronomer booted his garden gate off its hinges and stormed off into his house after seeing the Vixen outperform his prized 8" Newtonian while looking at Saturn. 

I also bought a Helios 120mm achromat in 1999. First light through that scope revealed CA that was very well controlled, especially after considering the telescope was only F8.3 and as cheap as chips. For the money it was excellent! Looking at the moon on that first light night, the two things that stick in my mind are "where's the CA?" and "that's a very nice view!" The 6" version that i bought some time later was also good but the CA did become quite noticeable, as you might expect.

After owning a couple of fluorite fracs and also a number of ED fracs for a few years, I bought a 105mm Antares F14.3. I wanted to rekindle my love for the long focus refractor. The scope was beautiful and gave textbook star images, but it had a warmth to it that the apo's did not. The CA was essentially non existant but the length of the tube made it very awkward to mount and my comfort was compromised, so i couldnt relax fully while observing. I'd be very happy with the Antares 105 as my only scope but I'd have to mount it on a solid, permanent equatorial with Hargreaves strut to do it justice. Personally I feel a 4" F15 refractor is bordering on being nonportable, mainly because of the mounting issue, but otherwise definitely a great scope.

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11 minutes ago, mikeDnight said:

My first serious aperture refractor was a 102mm F13 Vixen achromat which I bought from Peter Drew in the mid 1980's. It was a superb scope that gave terrific views of both the moon and planets, and brighter dso's. In my mind the F13 Vixen gave me the finest view of M82 that I've ever had, showing the galaxy as a slender mottled ghost with a dark lane bisecting the ghostly wedge of light. It might sound unimpressive but in reality it was jaw dropping, because it was so sharp and detailed. I can't ever remember noticing CA in that telescope, so it must have been very well controlled. One local astronomer booted his garden gate off its hinges and stormed off into his house after seeing the Vixen outperform his prized 8" Newtonian while looking at Saturn. 

I also bought a Helios 120mm achromat in 1999. First light through that scope revealed CA that was very well controlled, especially after considering the telescope was only F8.3 and as cheap as chips. For the money it was excellent! Looking at the moon on that first light night, the two things that stick in my mind are "where's the CA?" and "that's a very nice view!" The 6" version that i bought some time later was also good but the CA did become quite noticeable, as you might expect.

After owning a couple of fluorite fracs and also a number of ED fracs for a few years, I bought a 105mm Antares F14.3. I wanted to rekindle my love for the long focus refractor. The scope was beautiful and gave textbook star images, but it had a warmth to it that the apo's did not. The CA was essentially non existant but the length of the tube made it very awkward to mount and my comfort was compromised, so i couldnt relax fully while observing. I'd be very happy with the Antares 105 as my only scope but I'd have to mount it on a solid, permanent equatorial with Hargreaves strut to do it justice. Personally I feel a 4" F15 refractor is bordering on being nonportable, mainly because of the mounting issue, but otherwise definitely a great scope.

I also owned an Antares 105 F15 (spirit of unitron), borderline ZERO CA, a total thing of beauty, BUT even an HEQ5 struggled to handle it to my liking, and it was just way to long...........to be able to own a scope like that again i would want an obsy with a bespoke mount to get the scope up high enough to be happy at the ep

 

If i were to look at another large scope it would be a Maksutov over a frac, just think how easy a 7" F15 Mak is to mount compared to a 7" F15 frac

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1 hour ago, spaceboy said:

Mounting and other issues aside, I assume there comes a point where achromats start to come in to their own? or are bespoke larger refractors just expensive no matter what glass is in them? I see some "budget" 152 mm ED refractors for £3k so surely you'd get a lot of achromat for that money ??

I don't think there is this point with regard to achromats.

Having owned refractors up to 6" f/12 (which was an achromat) my personal view is that there is an aperture where other scope designs start to make more sense than refractors. For 7" and above I think I'd look at a mak-cassegrain, mak-newtonian or a long newtonian with a small secondary.

Really big achromats can be purchased but because of the limited production and markets for them, they tend to be pretty expensive eg: this 10" F/12 is $32,000 :shocked:

http://www.eisci.com/ad10rr1.html

When you think about it, a Synta 6" F/8's, although they have their issues, are a lot of refractor if bought used for £200-£250 :smiley:

 

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I love the 8.5 inch f12.5 (I think) achromat that Peter Drew has up at TAC.  Peter built the tube, mount and housing and it's excellent.  Mind you, it would never fit in my garden! :smile:.

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Been mentioned a couple of times in the thread, here's mine - taken on a phone and the pics could be better (should've used the DSLR)...

For transport....with a fetching orange cap from Screwfix(!) which reduces the length to around 900mm...

 

WP_20170916_001.thumb.jpg.f41306ea4a88c24644d257958ae00c68.jpg

 

When up and running...

WP_20170916_002.thumb.jpg.fc3758463aa0be940c9ba28c591fd733.jpg

 

I used a Baader finder shoe and a dual finder attachment from FLO....simple red dot to get me in the area, RA finder to hone in...simples!

Stock TAL finder is very nice, but I can't get on with straight finders myself.  I've kept the TAL finder and shoe in a box for safe keeping.

WP_20170916_003.thumb.jpg.26b3f0210c69344ddf2dbdccd40d78ca.jpg

 

Humble 4" achromatic glass.   Very well figured though!  It's a great scope.  Perhaps one day they'll make them again...

 

WP_20170916_004.thumb.jpg.307bb9d3110e2d6dc50bc516cf2e00c8.jpg

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