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

[Mr Spock]

4" apo yes, 4" achro no - the 6" f8 Newt is a clear winner.

[Moonlit Night]

43 minutes ago, John said:

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 🙂

 

I always wanted one, I still kinda do. One of the best, if not the best 6 inch Newtonian of its day, before the Chinese got there stuff together obviously. 

[vlaiv]

19 minutes ago, Mr Spock said:

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

Yet it has only 22% CO - meaning 33mm. If secondary is placed so that focus point is say 200mm away from focal plane secondary mirror (about 90mm to the edge of the tube and another 110mm for focuser and drawtube), then converging beam at secondary will be 25mm or comparable in size.

I think that around 19mm away from central axis illumination drop off will be 50%!

That is eyepiece with field stop of 38mm.

Even field of 1.25" is not fully illuminated. Intersection of two - converging beam and secondary mirror at edge of 1.25" field look like this:

 

 

Edited February 20 by vlaiv

[dweller25]

1 minute ago, vlaiv said:

Yet it has only 22% CO - meaning 33mm. If secondary is placed so that focus point is say 200mm away from focal plane (about 90mm to the edge of the tube and another 110mm for focuser and drawtube), then converging beam at secondary will be 25mm or comparable in size.

I think that around 19mm away from central axis illumination drop off will be 50%!

That is eyepiece with field stop of 38mm.

Even field of 1.25" is not fully illuminated. Intersection of two - converging beam and secondary mirror at edge of 1.25" field look like this:

 

 

OK for planetary though ??

[vlaiv]

Just now, dweller25 said:

OK for planetary though ??

Sure is.

We often use short FL eyepieces for planetary to get enough magnification. Rarely anyone uses less than say x70-x80 to view the planets.

That would mean lowest mag EP would be something like 15mm.

Even wide field 15mm EP will have relatively small field stop - like 20ish mm.

At 10 mm away from optical axis at focus point, illumination diagram looks like this:

Again not 100% - but that is closer to say 80% vignetting at lowest power wide field planetary eyepiece.

[bosun21]

What do you mean 50% drop off in illumination? I didn't get any vignetting with my 6" f8 using a 32mm 50° plossl. Also with a 40mm 40° plossl. I could clearly define the field stops. I'm confused 🤔.

[vlaiv]

19 minutes ago, bosun21 said:

What do you mean 50% drop off in illumination? I didn't get any vignetting with my 6" f8 using a 32mm 50° plossl. Also with a 40mm 40° plossl. I could clearly define the field stops. I'm confused 🤔.

Ok, so drop off in illumination is easiest to see in daylight or maybe at dusk or dawn. You also might be able to see it in light polluted night sky.

It actually depends on how bad it is.

Here is what it looks like (it was surprisingly hard to find good example):

So you will see field stop - being more or less sharp (red arrow) - that has nothing to do with vignetting, and then you will see a darker ring close to field stop (blue arrow) - where whatever is in the view will be a bit darker.

Now, if I say that there is drop off of 50% at the edge - this means that only 50% of the light will reach parts next to field stop. This however does not mean that image will be half as bright - because our vision is not linear - if there is 50% of light - we don't see it as being half as bright as 100% light - it's more like 80% (our vision is logarithmic in its nature).

We often don't notice the difference that is 7% or less - so if there is only 93% drop off at the edge - image will look normally to our eyes up to the edge.

In planetary viewing this won't be as important. In lunar it might be as it will spoil the view.

In DSO viewing it is very important as 50% light means ~0.75 magnitudes of difference.  If you have some star or object that is on threshold visibility in center of the field and you move it to the side - it will disappear and you won't be able to see it because of this.

It can also be annoying to have such vignetting if your skies are not pitch dark - you have some LP - because sky gradient will behave like sky in above image - it will be bright in center and get darker towards the field stop - which itself will be completely dark.

With your 6" F/8 and 1.25" eyepieces, you'll probably need to really look for it to be able to see it with those two eyepieces you mentioned.

 

[bosun21]

I understand what you mean now. Thanks 👍.

[John]

4 hours ago, bosun21 said:

I understand what you mean now. Thanks 👍.

Just don't try and look at the tip of a Herons beak with it ! 😁

Edited February 21 by John

[John]

6 hours 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...

 

The Skywatcher 150P F/8 dobsonian uses a 2 inch focuser. The 150PL OTA or EQ mount version uses 1.25 inch. An odd variation I always thought 🤔

[Franklin]

13 hours ago, dweller25 said:

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 ?

4" achro for me. Why? Because decades ago I was brainwashed by Patrick Moore into believing that a 6" reflector was about as good as a 3" refractor so obviously the 4" frac must be better. Also, I believe telescopes are devices for looking at things and looking into the side of scope at 90deg from the object I'm looking at just seems silly. Sorry Sir Isaac.

[Moonlit Night]

5 hours ago, Franklin said:

4" achro for me. Why? Because decades ago I was brainwashed by Patrick Moore into believing that a 6" reflector was about as good as a 3" refractor so obviously the 4" frac must be better. Also, I believe telescopes are devices for looking at things and looking into the side of scope at 90deg from the object I'm looking at just seems silly. Sorry Sir Isaac.

Back in the day that was true. My old Swift battered my friend Neil’s (very cool to be called Neil at the time) Newtonian. Silvering is in a different league these days not to mention parabolic mirrors which were absolutely not the norm. 

[vlaiv]

8 hours ago, John said:

The Skywatcher 150P F/8 dobsonian uses a 2 inch focuser. The 150PL OTA or EQ mount version uses 1.25 inch. An odd variation I always thought 🤔

Maybe they have different size secondary?

150PL being on EQ mount - more suitable for planetary work and thus having smaller secondary and 150P being general purpose scope - larger secondary.

[bosun21]

10 hours ago, John said:

The Skywatcher 150P F/8 dobsonian uses a 2 inch focuser. The 150PL OTA or EQ mount version uses 1.25 inch. An odd variation I always thought 🤔

Which 150P is F8? I have only seen them in the standard F5. As you said the 150PL is F8 with a 1.25" focuser.

[vlaiv]

26 minutes ago, bosun21 said:

Which 150P is F8? I have only seen them in the standard F5. As you said the 150PL is F8 with a 1.25" focuser.

https://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-150p-dobsonian.html

[mikeDnight]

It would depend on who made the 4" achromat!

I had a 4" F13 Vixen achromat back in the mid 1980's that was an absolute dream. One evening I took it with me when visiting a local astronomer (John Coates) and set it up alongside his 8.5" F6 Newtonian. We aimed the scopes at Saturn and compared the views. John, a life long and very experienced planetary observer, who is sadly nolonger with us, was in utter disbelief at just how good  the Vixen achromat actually was. I discerned this because of the barrage of colourful expletives he used in quick succession over a considerable length of time as he compared one scope with the other. All this finally culminated in John kicking his garden gate off its hinges and walking back into his house. I stood there for what seemed like ages wondering if he was coming out again, but he didn't. So I went into his house to see if he was ok, and all he could say, while making us both a brew was - "That's a bloody good telescope! Bloody good scope that is!!" I never let him look through my 4" after that!

Personally I would choose a "good", long focal length achromat over even an 8" reflector every time. Sadly they are hard to come by these days!

Edited February 21 by mikeDnight

[RobertI]

On 20/02/2024 at 11:21, dweller25 said:

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 ?

I have a 150PL (1.25” focuser) and Tal 100RS and my experience has been as follows:

- Newtonian has greater ability to split close doubles but achro has more aesthetically pleasing stars and airy discs

- Newtonian is better at revealing DSOs and resolving globs, but not so good at wide field

- Newtonian has more accurate colour rendition, the achro gives everything a yellow cast but does make yellows/oranges/reds stand out.

- Achro gives nice clean edges to planets and dark backgrounds, but more planetary detail visible in Newt

- Mars was not very good at all in the achro for some reason (perhaps someone can explain?).

- Newtonian was easier to use at or close to the zenith

- Achro was easier to mount and focus without vibration

 

Which is “better”? Of my two examples, the Newt is the better all rounder, but it does feel like comparing apples and pears! 

[vlaiv]

19 minutes ago, RobertI said:

- Mars was not very good at all in the achro for some reason (perhaps someone can explain?).

Maybe it is down to achro not being optimized in red.

We often focus on blue side of spectrum - because defocus of achromat is bigger on that side (violet halo part in particular), but optical designers have freedom on how to tilt this curve:

in above image we see three different ways to optimize achromat - blue, red and green (nothing to do with colors) - which bring different Fraunhofer lines into same focus.

Blue curve has the smallest violet bloat but on the other side - it has largest defocus in red part of the spectrum. Then there is also matter of spherochromatism.

All of that can cause red planet to loose contrast against black and white features.

[bosun21]

4 hours ago, vlaiv said:

https://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-150p-dobsonian.html

I thought we were talking about OTA's. I didn't realise that complete dobsonians were included.

[bosun21]

I have found that the majority of Newtonian owners settle for decent collimation and never get the collimation bang on. The difference can be seen immediately. The details on the planets are defined as is Luna. When I get a new Newtonian I spend the time on setting the secondary accurately. The primary is just an easy turn of the collimation screws. Once you see the difference for yourself you won't settle for near enough. I am now myself heading towards the apo camp. As much as I love the aperture of reflector telescopes I have fallen for apo refractor telescopes.

[dweller25]

2 hours ago, bosun21 said:

 As much as I love the aperture of reflector telescopes I have fallen for apo refractor telescopes.

That fall could will get expensive 😱

[bosun21]

Just now, dweller25 said:

That fall could will get expensive 😱

I know. I don't know what to do. I want a Tak and it will have to be one of.the 100mm models as they skyrocket as you increase the aperture. I could get a 120 mm triplet or even a 140mm Askar model. I'm also looking at the Stella Lyra 125. The snap to focus of the fracs has pushed me over the edge. Decisions, decisions 🙄. The Vixen I just sold is an excellent telescope but by being F9 it's a visual scope and I need something around F7 as I indulge in a bit of EAA from time to time.

[John]

I well and truly scratched my quality refractor itch back in 2016 when I bought a new Tak FC100-DL and a pre-owned TMB/LZOS 130 F/9.2 triplet within a couple of months of each other. First time I had needed to have anything astronomical as specified items in our house insurance 😬

No regrets though - nothing like trying things for yourself if you get the chance 🙂

 

 

[skyhog]

On 21/02/2024 at 01:16, Franklin said:

4" achro for me. Why? Because decades ago I was brainwashed by Patrick Moore into believing that a 6" reflector was about as good as a 3" refractor so obviously the 4" frac must be better. Also, I believe telescopes are devices for looking at things and looking into the side of scope at 90deg from the object I'm looking at just seems silly. Sorry Sir Isaac.

A burst of nostalgia reading that, the best example of false equivalence you are likely to come across. 

[Louis D]

On 20/02/2024 at 19:16, Franklin said:

4" achro for me. Why? Because decades ago I was brainwashed by Patrick Moore into believing that a 6" reflector was about as good as a 3" refractor so obviously the 4" frac must be better. Also, I believe telescopes are devices for looking at things and looking into the side of scope at 90deg from the object I'm looking at just seems silly. Sorry Sir Isaac.

In my experience, my 6" f/5 Newtonian runs huge circles around my 80mm (3.1") f/5 refractor.  To equate them in performance in any way is a complete insult to the Newtonian.

Edited February 22 by Louis D