Which would you choose 6” Newt or 4” Achro ?

[dweller25]

I have been mulling this over (because it’s cloudy - again 😩)

6” F/8 Newtonian or 4” F/10 achromat.

What would you choose and why ?

[Naughty Neal]

I can't resist the looks of a nice looking achromat, no other reason needed in my books.

[dweller25]

3 minutes ago, Naughty Neal said:

I can't resist the looks of a nice looking achromat, no other reason needed in my books.

Agreed, but which would give the best views ?

[vlaiv]

11 minutes ago, dweller25 said:

Agreed, but which would give the best views ?

Of what?

I'm inclined to say that 6" F/8 newtonian of good optical figure would best 4" F/10 achromat on everything except wide field views.

[The60mmKid]

6" Newt! They're underappreciated! A good one is a wonderful instrument. Sharp, color-free images... enough aperture to keep one busy endlessly... a good balance of FOV and high-magnification capability... quick to cool... easy to mount... so comfortable to use as a dob... inexpensive... lovable.

Refractors are nice, too, though.

[dweller25]

16 minutes ago, vlaiv said:

Of what?

I’m keeping it deliberately vague 👍

[vlaiv]

3 minutes ago, dweller25 said:

I’m keeping it deliberately vague 👍

Well, here is one thing that you probably can't do with 6" F/8 newtonian:

It has a bit more focal length, but main issue is the size of secondary mirror and if it will illuminate 47mm of field

[Louis D]

I've extensively compared my 6" f/5 Newt to my 6" f/5.9 achromat over the past year, and it is no contest.  The Newt runs circles around the achromat.  It's color free, half the weight, one third the price, wider field of view, spherical aberration free, holds collimation exceptionally well, and is well corrected with a coma corrector.  I have spent countless hours and hundreds of dollars on various filters trying to make the achromat somewhat sharper without cutting out too much of the spectrum.  I honestly don't get the hype for large, fast achromats.  Either buy an APO refractor, or stick with a fast Newtonian.

[vlaiv]

2 minutes ago, Louis D said:

6" f/5.9 achromat

4" F/10 achromat is completely different instrument.

Mine, although cheap mass produced item (SW Evostar 102) showed me views of Jupiter and Saturn one evening that are probably in top 5 views of all times of these targets that I had with any scope.

[Peter Drew]

A top class 6" F8 Newtonian should give a 4" APO a good run for its money let alone a 4" F10 achromat.  The 6" F8 Newtonian will have an advantage in light grasp and resolution, the secondary obstruction would be too small to be a contrast issue.  Despite this, preference might be between the look and handling of the types, whether one can see past CA at high powers or whether one has nightmares about collimation.  Apples and apples would be a no brainer.  I would prefer the Newtonian.   🙂 

[Moonlit Night]

As a self confessed refractor fan I think you should buy the Newtonian. If you were talking about a fl7 with fpl53 or even fpl51 I would be tempted to tell you to buy the frac, but not a fl10 achro, even a TAL 100RS

Edited February 20 by Moonlit Night

[bosun21]

6" Newtonian would be my choice rather than the achro. Once collimated it would trounce the 4" achro. Some folk can tolerate CA but I can't. I've tried previously with all the filters etc to cancel the CA but still hated the yellow tint visible.

[dweller25]

Well the Newt seems to be winning, but those refractors do look good…..

Edited February 20 by dweller25

[vlaiv]

1 minute ago, dweller25 said:

Well the Newt seems to have the vote, but those refractors do look good…..

Out of interest, what is the purpose of this inquiry?

Are you looking to get yourself a new scope and wander which way to go or something else?

We have not discussed all the other things that go with owning a telescope - like size, mounting options, portability, storage and so on ... Are these relevant for this discussion?

[Moonlit Night]

They do, they do. You could always by one of these:  https://www.teleskop-express.de/en/telescopes-4/apochromatic-refractor-55/ed-refractor-222/ts-optics-102-mm-f-11-ed-refractor-with-2-5-rap-focuser-11262

[dweller25]

Just mulling things over @vlaiv on a cloudy day/week/month.

But thanks for the considerations 👍

 

[bosun21]

6 hours ago, vlaiv said:

Of what?

I'm inclined to say that 6" F/8 newtonian of good optical figure would best 4" F/10 achromat on everything except wide field views.

According to the calculator the Newtonian wins in the FOV both with a 30mm 70° eyepiece.

[vlaiv]

Just now, bosun21 said:

According to the calculator the Newtonian wins in the FOV both with a 30mm 70° eyepiece.

Only if you use F/5 newtonian instead of F/8.

Two different instruments.

[bosun21]

Just now, vlaiv said:

Only if you use F/5 newtonian instead of F/8.

Two different instruments.

Sorry I didn't see that he specified a F8 rather than the standard F5.

[John]

I have enough refractors so I'd go for the F/8 6 inch newtonian I think.

I always liked the classic looks of  the TAL 2M 🙂

 

[bosun21]

There's not a large difference in the FOV.

 

[vlaiv]

1 minute ago, bosun21 said:

There's not a large difference in the FOV.

Indeed, however, you will find that most 6" F/8 newtonians come with 1.25" focusers while most 4" F/10 achromats come with 2" focusers.

Limitation on 6" F/8 newtonian comes from the size of secondary mirror / central obstruction. To be general purpose scope, CO is often around 25%. That means 1.5" in diameter (a quarter of aperture size 6/4 = 1.5).

You can't effectively illuminate 2" field with 1.5" secondary mirror - you will get a lot of vignetting, and compromise is to use only 1.25" eyepieces.

Do another comparison between the two scopes - one using max field stop EP in 2" variety and other using something like 32mm plossl (which has largest field stop in 1.25" version).

[bosun21]

Yes I used to own the SW150PL which indeed had a 1.25" focuser. I bought it for the planets and the moon for which it was good. The achro will indeed beat it convincingly when using 2" eyepieces.

[bosun21]

 

Edited February 20 by bosun21

[Mr Spock]

19 minutes ago, vlaiv said:

Indeed, however, you will find that most 6" F/8 newtonians come with 1.25" focusers while most 4" F/10 achromats come with 2" focusers.

https://www.firstlightoptics.com/stellalyra-telescopes/stellalyra-6-f8-planetary-dobsonian.html