Triplet or ed doublet

[andyo]

Hi all,

A question I have been thinking about is the advantage of a triplet over a good ED apochromatic doublet such as the skywatcher ed 80 evostar. I know that a good apochromatic triplets advantage is to eliminate colour aberration which is caused by the different wavelengths being focused at different points on the focal plane. While this advantage is clear for observing and imaging using a OSC camera my question concerns the use of imaging with a mono ccd camera. Is the problem of colour aberration applicable here  if you refocus for each of the different filters. Therefore is  there any reason to spend the considerable amount  of extra cash on a good triplet if the scope is only to be used for imaging with a mono ccd camera is it best to just go for a top quality ed doublet.

[Uranium235]

Good queston!

Generally, triplets hover around f6 - while good doublets are usually f7.5(ish). But the latter can suffer slightly in the blue channel (stars are slightly larger) - however that is fixable in processing by reducing the size of the stars in the blue channel slightly while processing. But, narrowband is unaffected whether its doublet or triplet (and some may argue you can do it with an achro!).

A good 80mm triplet however can cost close to 1k when you factor in the reducer/flattener (which is essential), while the humble 80ED is under half that value.

Perhaps you could kill two birds with one stone and plump for one of the ES tested Star71 telescopes, a great performer all-round.

[ronin]

If the idea is to use narrow band filters and refocus for each one/wavelength then you can likely just use a normal but reasonable achromat, and when you look at it an ED is just an achromat with a "better" glass element. Both define 2 wavelengths to focus at the same position relative to the optical centre of the objective. What a triplet does is allow 3 wavelengths to be focused at that plane. If 3 wavelengths are controlled then the remaining wavelengths between these 3 are unable to move far away from the defined "ideal" plane. In a doublet they are simply able to move further away before being dragged back.

One way to imagine it is 2 sticks 12 inches apart same height, hand a bit of string between them, consider these as red and blue to be the string on the sticks, the middle hangs down and that middle is Green, it hangs a fair bit off the top. Now add a 3rd stick in the middle at the same height and attach the entre of the string to that, Now we have R+G+B all at the same place and the 2 dangly bits are not ideal but are also less from the ideal then with just 2 sticks.

That is the difference between a doublet and a triplet. You control 3 wavelengths and the bits/wavelengths between the controlled wavelengths just cannot get away from the ideal as much. The better ED glass just allows you to tighten the string a bit more so a flatter bit of string when only 2 are involved.

 

[timwetherell]

For really critical applications such as planetary imaging, the problem is that filters have fairly wide colour bands say 300nm to 450nm for blue. Chromatic aberration is continuos and often rapidly increasing in the blue so that there might not be a single focus that would cover the entire blue band of the filter. However in my experience (admittedly primarily as a visual observer) the difference between a good ED doublet and triplet is minimal especially in apertures below about 5"

[gunfighter48]

The triplet refractor will have better resolution than an Achromatic Doublet because all three colors, RGB, focus at a single point. You will be able to see more and finer details. I had a ES AR152 Achro and then got a 130mm APO. The color is much better and no fringing around bright objects. I can see more and finer detail also. I saw 3-4 light colored bands on Saturn that I could never see before!

[andyo]

Thanks for the great replies guys good information there.Really nice explanation with the sticks Ronin.

22 hours ago, Uranium235 said:

Good queston!

Generally, triplets hover around f6 - while good doublets are usually f7.5(ish). But the latter can suffer slightly in the blue channel (stars are slightly larger) - however that is fixable in processing by reducing the size of the stars in the blue channel slightly while processing. But, narrowband is unaffected whether its doublet or triplet (and some may argue you can do it with an achro!).

A good 80mm triplet however can cost close to 1k when you factor in the reducer/flattener (which is essential), while the humble 80ED is under half that value.

Perhaps you could kill two birds with one stone and plump for one of the ES tested Star71 telescopes, a great performer all-round.

Interesting scope so these incorporate a flattener within the scope itself how does that work to get the right distance from the chip as this varies from camera to camera.

[Tim]

Great question

Doublets have another advantage over triplets, in that their cell design is simpler, less likely to distort during extreme temperature fluctuations, and they have a shorter cool down time.

If you only ever plan on imaging with a mono CCD, then a great doublet makes a lot of sense. But maybe you will want to use the scope visually at some point in the future? Or maybe with a DSLR or colour CCD in future. Never say never!

Incidentally, it is possible to get apochromatic results from a doublet, but they tend to cost more than an equivalent triplet!

[Uranium235]

41 minutes ago, andyo said:

Thanks for the great replies guys good information there.Really nice explanation with the sticks Ronin.

Interesting scope so these incorporate a flattener within the scope itself how does that work to get the right distance from the chip as this varies from camera to camera.

The beauty of it is, there is nothing to work out

When the telescope is focused, that simultaneously provides the flat field.

[andyo]

43 minutes ago, Tim said:

Great question

Doublets have another advantage over triplets, in that their cell design is simpler, less likely to distort during extreme temperature fluctuations, and they have a shorter cool down time.

If you only ever plan on imaging with a mono CCD, then a great doublet makes a lot of sense. But maybe you will want to use the scope visually at some point in the future? Or maybe with a DSLR or colour CCD in future. Never say never!

Incidentally, it is possible to get apochromatic results from a doublet, but they tend to cost more than an equivalent triplet!

Nice bit of info that Tim I never thought a doublet would have an advantage over a triplet except the price of course. I have been imaging now for a fair few years and initially bought my first scope a SW200p for observing never thought about imaging. However pretty much straight away got myself a spc900 lx modded webcam and was hooked on imaging.Soon got a dslr got that modded sold the 200p and got the SW Ed80 evostar pro.Then sold the modded dslr and got the Atik 314L+ mono which is the equipment I have had for about three or so years. In all this time I have only spent two evenings observing and that was when I got my first scope the 200p makes me smile when I think how the imaging bug took me down this path. I will be upgrading the camera to the Atik 460ex mono soon a much larger sensor than the atik 314L so may  consider a future move for a slightly more powerful scope such as the SW esprit 100 triplet which is what got me thinking about the question regarding triplets and doublets. Certainly not decided on anything yet as will be a while before I do upgrade the scope if I ever do. Funny how this hobby takes you in directions you didnt plan,yes  never say never

[Dude_with_the_tube]

ED is basically an improved acrhomat and to keep that colour correction at reasonable(improved) level they have to be as min as F7 as any shorter and it just doesnt work.Ideal focal length for ED is about F9 or perfectly it should be F12 then you will have literally APo performance.

APO with they 3 elements offer far better colour correction and as such dont need to have that long focal length as ED and usually will be shorter focal length.Obviously will cost more then the ED.

[Tim]

On 29/07/2016 at 21:47, andyo said:

 .................. may  consider a future move for a slightly more powerful scope such as the SW esprit 100 triplet .................

There are Triplets, and there are triplets. The Esprit's are Triplets.  Nice scope to look forward to. I love my Esprit 150ED

[andyo]

2 hours ago, Tim said:

There are Triplets, and there are triplets. The Esprit's are Triplets.  Nice scope to look forward to. I love my Esprit 150ED

Yes I do like the look of these scopes and If i decide to upgrade the scope it will more than likely be for one of these, in my experience with the 200p and  80ed pro they build decent scopes which offer real value for money

[MattJenko]

Some doublets are better than some other triplets, they are not always better...

[Skyline]

On 7/29/2016 at 21:05, Tim said:

Great question

Doublets have another advantage over triplets, in that their cell design is simpler, less likely to distort during extreme temperature fluctuations, and they have a shorter cool down time.

If you only ever plan on imaging with a mono CCD, then a great doublet makes a lot of sense. But maybe you will want to use the scope visually at some point in the future? Or maybe with a DSLR or colour CCD in future. Never say never!

Incidentally, it is possible to get apochromatic results from a doublet, but they tend to cost more than an equivalent triplet!

The colour correction on the skywatcher 80ED DS Pro is excellent, better then some other budget triplets equal to the same size, would you not agree ?

[andyo]

On 02/08/2016 at 23:10, Skyline said:

The colour correction on the skywatcher 80ED DS Pro is excellent, better then some other budget triplets equal to the same size, would you not agree ?

I have not had a triplet to compare but I certainly agree with the 80ED DS pro having excellent colour correction. I would like to know how the skywacher esprit 100 triplet compares to it as this would be a scope I may look at upgrading to at some point ,more aperture and faster scope with a similar focal length