Checking my facts - refractors, doublets and triplets

[matthasboldlygone]

Down the rabbit hole I go, it's deep and dark

So a standard Refractor has a single lens which results in Chromatic Aberration, the RGB light doesn't focus at the same point so you get a coloured haze around objects / stars. A reasonable scope for around £300 - £500

A doublet has two lenses and will focus the red and blue better, but the green will still be out a bit resulting in less Chromatic Aberration. But it still has CA - standard one £500 - £1000

A triplet has 3 lenses to focus all 3 together nicely, but these are rather sensitive and have to be handled with care. standard one £1500 - £2000 or upwards.

Is this correct?

A reflector doesn't have this problem because it uses mirrors not lenses (except eye pieces)? And appear considerably cheaper than a triplet.

What's the benefit of a triplet over a reflector?

Thanks for your advice 👍

[Davey-T]

12 minutes ago, matthasboldlygone said:

What's the benefit of a triplet over a reflector?

Traditionally refractors claim to have better contrast but a lot of stuff is based on scopes from years ago, modern computerised lens and mirror manufacturing has raised all standards considerably and bought prices down

Some reflectors are a pain to collimate and longer focal length refractors are " easier " to make.

Dave 

[IB20]

20 minutes ago, matthasboldlygone said:

 

What's the benefit of a triplet over a reflector?

Thanks for your advice 👍

A much lighter wallet. *quickly scarpers*

[John]

The most basic type that you can buy is an achromatic doublet - 2 lens elements. This would be your "standard refractor"

Nobody makes or sells a single element (non-achromatic) refractor.

The next step is an ED doublet refractor which uses a low dispersion glass element to reduce markedly the amount of chromatic aberration visible.

The next step is a triplet which if well executed can reduce chromatic aberration to virtually zero. 

Within each of the above types there are a range of qualities available and a wide range of pricing. The more expensive ED doublets can be somewhat more than a low to mid cost triplet for example.

The major advantage that the reflector has is that the cost per cm of aperture is a lot, lot lower because there are less optical surfaces to figure, polish and coat.

There are plenty of threads on the forum discussing the merits of the various scope designs. In my view they all have their strengths and weaknesses. Thats probably why many of us end up with a variety of them !

 

 

[matthasboldlygone]

20 minutes ago, Davey-T said:

Some reflectors are a pain to collimate and longer focal length refractors are " easier " to make.

 

9 minutes ago, John said:

The next step is an ED doublet refractor which uses a low dispersion glass element to reduce markedly the amount of chromatic aberration visible.

 

Thanks, it's starting to sink in now and make a bit more sense 👍

[Tenor Viol]

The 'logistics' of the choice have changed over the last 25 years or so. In the past the majority of amateur scopes would have been 6 or 8 inch Newtonians, then SCTs came along. Refractors were expensive and limited to lunar, planetary, and solar observers using f/12 to f/15 instruments with usually 3 or 4 inch lenses (75 - 100mm).

Computer design of optics plus computer controlled production has brought down the costs dramatically. There have also been developments in the making of specialist glass. This has changed the landscape.

For the price you used to pay for an 8 or 10 inch Newtonian, you can now have a 100mm short-focus apochromat with specialist glass providing highly corrected optics. The price of Newtonians has dropped dramatically so that entry level ones are very inexpensive. These are also faster now - often f/4.8 or so rather than the f/8 of 30 years ago. The use of Dobsonian mounts means that 12, 15, 18, and even 24 inch instruments are not rare. 

   

[mikeDnight]

Refractors give sharper images and produce more pleasing views. From rich star fields to high power, high definition views of the Moon, planets and binary stars, they are a great all round performers that are virtually maintenance free. They deliver star images that other scope designs try to attain but never quit reach. 

[Craney]

...... and on top of all this great advice regarding the scope itself, you have to consider the mount.

Comparing scopes of approximately equal cost  ( refractor vs reflector)  of a high quality, you can generally buy a much bigger aperture reflector.   Very tempting.

Alas, it will probably be a heavier, longer  item and offer a much larger area to be buffeted by the wind.   Hence...... you're gonna need a bigger mount.

Once you keep the quality threshold high..... ( why put the an observatory class instrument on a cheap, wonky piece of scrap metal ?)...... the investment becomes considerable, but necessary.

[Louis D]

53 minutes ago, John said:

The most basic type that you can buy is an achromatic doublet - 2 lens elements. This would be your "standard refractor"

Nobody makes or sells a single element (non-achromatic) refractor.

The next step is an ED doublet refractor which uses a low dispersion glass element to reduce markedly the amount of chromatic aberration visible.

The next step is a triplet which if well executed can reduce chromatic aberration to virtually zero. 

Within each of the above types there are a range of qualities available and a wide range of pricing. The more expensive ED doublets can be somewhat more than a low to mid cost triplet for example.

The major advantage that the reflector has is that the cost per cm of aperture is a lot, lot lower because there are less optical surfaces to figure, polish and coat.

There are plenty of threads on the forum discussing the merits of the various scope designs. In my view they all have their strengths and weaknesses. Thats probably why many of us end up with a variety of them !

And I would go further to subdivide ED and APO into four general categories:

1. ED Doublet of FPL-51/FCD-1 glass.

2. ED/APO Doublet of FPL-53/FCD-100 glass.

3. ED/APO Triplet of FPL-51/FCD-1 glass.

4. APO Triplet of FPL-53/FCD-100 glass.

As you progress into more elements and more expensive glasses that have lower dispersion, you get better chromatic correction.  Design points 2 and 3 above can flip flop depending on the level of design execution.  As discussed in another recent thread, the actual design choices for the objective and it's mating element(s) (curves, spacing, etc.) also make a difference in its ED/APO characteristics.

Then there's flourite "glass" which has properties slightly better than FPL-53 and is even more expensive.  It used to be the only game in town for ultra-low dispersion objectives.

[Stu]

7 hours ago, Louis D said:

And I would go further to subdivide ED and APO into four general categories:

1. ED Doublet of FPL-51/FCD-1 glass.

2. ED/APO Doublet of FPL-53/FCD-100 glass.

3. ED/APO Triplet of FPL-51/FCD-1 glass.

4. APO Triplet of FPL-53/FCD-100 glass.

As you progress into more elements and more expensive glasses that have lower dispersion, you get better chromatic correction.  Design points 2 and 3 above can flip flop depending on the level of design execution.  As discussed in another recent thread, the actual design choices for the objective and it's mating element(s) (curves, spacing, etc.) also make a difference in its ED/APO characteristics.

Then there's flourite "glass" which has properties slightly better than FPL-53 and is even more expensive.  It used to be the only game in town for ultra-low dispersion objectives.

Let’s not forget petzvals! There are quadruplet and quintuplet designs with doublet or triplet objectives and reducer/flattener doublets at the rear to flattener the field and reduce the focal length.

[Guest]

18 minutes ago, Stu said:

Let’s not forget petzvals!

🤔😂

[andrew s]

There are of course different reflectors. In addition to the Newtonian/Dobsonian we have the various Cassigrain flavours which include classical, Dall-Kirkham and Richey-Chretien as the  most common.

The also have a variety of correctors to remove various aberrations. 

Recently a range of small affordable CC and RC scopes have become available.

Regards Andrew 

[Louis D]

5 hours ago, andrew s said:

There are of course different reflectors. In addition to the Newtonian/Dobsonian we have the various Cassigrain flavours which include classical, Dall-Kirkham and Richey-Chretien as the  most common.

The also have a variety of correctors to remove various aberrations. 

Recently a range of small affordable CC and RC scopes have become available.

Regards Andrew 

And the various reflector designs to eliminate the central obstruction: Off-axis Newtonian, Yolo, Schiefspiegler, Stevick-Paul, Herschelian, etc.  Of course, these are mostly ATM builds.

[Louis D]

6 hours ago, Stu said:

Let’s not forget petzvals! There are quadruplet and quintuplet designs with doublet or triplet objectives and reducer/flattener doublets at the rear to flattener the field and reduce the focal length.

Oh, I remembered them, but I didn't know exactly how to classify them among the latest crop of ED/APO scopes coming out of China. 🤔

And yes, @merlin100, they're delicious. 😉

[Roy Challen]

Don't forget the Maks! And the Mak-Newts!

[matthasboldlygone]

Can someone just tell me what to buy 😂 🤷‍♂️

I guess I need 20 scopes all setup and have a look through them at the same thing. I have found a local observatory but all meetings cancelled till further notice. Maybe google images would be a good place, but with the processing and photoshopping (and blatant lies) I don't always trust what I'm being shown.

The search continues, but I am now thinking one scope won't fit everything I want. 

👍

[matthasboldlygone]

8 minutes ago, Roy Challen said:

Don't forget the Maks! And the Mak-Newts!

I found a Newt in my pond this morning 😛

[John]

Sorry if we have delivered too much info. It is a complex subject

On other forums threads on this topic have run for months !

 

[Craney]

11 minutes ago, matthasboldlygone said:

  I guess I need 20 scopes......

Yep....that just about nails it.

[Ricochet]

52 minutes ago, matthasboldlygone said:

Can someone just tell me what to buy

If you've got a garden and you don't have to carry the scope far, get an 8" dob, perhaps a 10" if you're happy carrying it in two trips.

If you don't have a garden and need something relatively easy to transport, try a 127 Mak, the AZGti mount looks like a nice option to me.

If it has to be a refractor then get a 100 or 120mm ED doublet. 

Assuming you want the telescope for visual observation that is.

[Louis D]

5 hours ago, matthasboldlygone said:

Can someone just tell me what to buy 😂

Well, you just changed the subject from your initial question.  If you're looking to buy, what is your budget, usage intention of visual vs. astrophotography, distance to carry or wheel-out scope to observing location, your personal weight lifting ability, portability requirements, desired objects to view/photograph, storage room available, manual vs. push-to vs. goto vs. tracking, etc.

[DeathWarpedUp]

Answer Louis's questions, thats my tip.

[matthasboldlygone]

1 hour ago, Louis D said:

Well, you just changed the subject from your initial question.  If you're looking to buy, what is your budget, usage intention of visual vs. astrophotography, distance to carry or wheel-out scope to observing location, your personal weight lifting ability, portability requirements, desired objects to view/photograph, storage room available, manual vs. push-to vs. goto vs. tracking, etc.

Well, yes the topic went quickly off-topic from the initial point. It was more a light hearted comment to the wealth of info coming my way. There's another topic about what I was planning to buy but thanks for your input.

[matthasboldlygone]

47 minutes ago, DeathWarpedUp said:

Answer Louis's questions, thats my tip.

Great tip 🤪 I wouldn't have thought of that...

I wonder if there's a way I can close this now, think I have what I need on the original topic

[x6gas]

19 minutes ago, matthasboldlygone said:

Great tip 🤪 I wouldn't have thought of that...

I wonder if there's a way I can close this now, think I have what I need on the original topic

Oh don't spoil our fun!

But it really does depend what you want to use it for.  Visual of faint fuzzies then it's difficult to look beyond a big, fast Newtonian.   I didn't realise when I started, but I got the bug for imaging and since I had to tear down and set up every session it quickly became apparent that collimating the scope and ensuring that the imaging gear was orientated exactly the same (so that the diffraction spikes were exactly the same every night) was too painful and I quickly settled on refractors (but not before I'd spent money on two Newtonians).