Refractor aperture concerns

[flanker]

Hey folks

I've seen that in the Skywatcher line, a 120mm f8.3 weighs almost double that of a 102mm f9.8. Is the performance gain from the 120 really worth the doubling of weight? I'd be lugging it around the city, probably in a city bus. Will a 120 be manageable?

Thanks

Hari

[EnglishNightHawk]

Hopefully someone more experienced can share their opinion, but personally if I had to lug it around everywhere I would take the smaller scope.

[Rainmaker]

What are your targets, visual or imaging.  If visual will it be your main scope.  A 120mm F8.3 is a big scope and while portable it is not something I would want to cart around on public transport.  

Performance gain is definitely there, whether it is a worthwhile gain only you can answer that.  I went from from 120 F7.5 to 128 F8.1 and now I am back down at 106 F5 which, although a heavy scope, is at least more portable.  

I would recommend a 102 for its portability.  Don't forget you will also be carrying many accessories.

[jetstream]

Hello Hari, I have a SW120mm f7.5 that is light, but also long. In the SW case there is no way I consider mine portable for a bus trip unfortunately. The extra aperture of the 120mm over my 90mm is really noticeable, what do you want to view the planets/lunar or DSO/ everything? Maybe a Celestron C5 or C6 would work well for you?

[celestron8g8]

I don't have one but I know that is a lot of weight to carry around town on a bus .

[nightfisher]

There is no way i would be lugging an evostar 120 about on public transport, look at the skymax 127 for this purpose

[flanker]

My main purpose is visual. So I feel it's better to get a 120mm now and use my existing TS70 for portability till I upgrade to a better portable scope. What's your take on this?

Also, what makes an ideal main scope? My viewing interests are basically the moon, planets and brighter DSOs. Occasionally other DSOs, for which I need a bus trip. Should I get a fast or a slow main scope?

[x935418]

Hi
I have some experience with refractors (achro).

we can compare them in two way:

1- for magnification:

there is no difference between them.
as you know, in achros you can not use all of your diameter for magnification (in apo you can)
for example in my old scope (short tube 102mm F5) you may suppose to get maximum
200X magnification based on 50X for each inch(or two for each millimeter).
but i can just get 100X magnification , why?

because there is a formula that identify size of diameter that can be used
for high magnification.

this formula is: sqrt(5*F) where F is focal len in millimeter .
for my short achro it is: sqrt(5*500)=50mm
you see that just half of my lens can be used for high power
my scope has 100mm but just 50mm is for high power
so my maximum power would be : 50*2 =100X


for your scopes:

First     : scope focal len: 120*8.3=996mm => useable diameter for magnify: sqrt(5*996)=70mm
Second: scope focal len: 102*9.8=999mm => useable diameter for magnify: sqrt(5*999)=70mm
   
as you see in both scopes you have 70mm for high power that can give 140X for you.



2-for light gathering (DSO objects):

they are not same of course ,120mm has 38% more surface then 102mm and nothing prevent you to use it , but
i don't thing there is a big improvement that cost its double weight!

as you may know, a 5 inch is small for DSO, of course you can see near object well, but i don't thing it is far better from 4 inch.

for this reason i sold my 4 inch achro and go directly to 8 inch reflector !


hope this help.

 






 

[cotterless45]

You'll find more contrast with a 4" scope, in fact superb contrast. Neil English is the greatest fan of 4" achros.

I lug around a 150 f8, you don't really want to get on the bus with one and it'll frighten the horses,

Nick.

[flanker]

Where does that sqrt (5*F) formula come from? I tried looking it up but couldn't see it anywhere. The calculations for magnification seems sort of intuitively right, though.

Cotterless45 : do smaller scopes give better contrast?

[x935418]

you can see here :

http://www.gaherty.ca/rogers/achrefract.htm

i just converted it from inch to millimeter

[flanker]

Perfect! Thanks!

Still one query remains: does smaller aperture give better contrast?

[x935418]

Perfect! Thanks!

Still one query remains: does smaller aperture give better contrast?

i don't see any reason for this STRANGE opinion

[flanker]

You'll find more contrast with a 4" scope, in fact superb contrast. Neil English is the greatest fan of 4" achros.

I lug around a 150 f8, you don't really want to get on the bus with one and it'll frighten the horses,

Nick.

This is what I was referring to. Does a 4" simply give good contrast or is the contrast better than that of a 4.7?

[mark RD]

Perfect! Thanks!

Still one query remains: does smaller aperture give better contrast?

In short No, Better optics give better contrast. Some of the best veiws I have had  where though a Skywatcher ED80

[flanker]

Okay. Thanks!

[x935418]

OH

I think i found why cotterless45 say 4" will give better contrast !

1- As i said, you can use just a part of aperture for high power,
and it is IMPORTANT that you HAVE TO stop down scope (decreasing aperture size with a circle of cardbord).
You have to eliminate outer part of lens because it destroy the quality of picture in high power, otherwise

you can not reach the maximum magnification that specified by formula and you see just a blurry image.


2- there is two formula:
     Conrady : sqrt(5*FocalLen) - for excellent quality
     Sidgwick : sqrt(8*FocalLen) - for good quality


with Sidgwick formula both scopes (102mm and 120mm) can use about 90mm and give 180X magnification for good quality.

in 102mm you have to stop down and eliminate outer 10mm
in 120mm you have to stop down and eliminate outer 30mm

in 102mm you have just 10mm outer part that must eliminated by stop down
and if you don't do it, there is no big problem, but in 120mm , there is 30mm and it is important to stop down.

As cotterless45  mentioned he has a 6" (150mm) F8 archomat, that has 1200mm focal len.
it can use 97mm for high power (Sidgwick : 200X) and it has about 50mm outer part of lens that must be eliminated.

Now i guess that cotterless45  compare its own scope with a long tube 4" scope

in high power, i am sure in this situation , 4" give better quality
but if he stop down his scope to 100mm (with a card bord)
certainly he will see better image then 4"

I hope cotterless45 confirm this

[swamp thing]

. Some of the best veiws I have had  where though a Skywatcher ED80

Only in a refractor thread

[x935418]

Only in a refractor thread

I understand !

when i recently sold my small refractor  (4"-10kg)  and buy big reflector (8"-40KG),

my wife said: I think you are wrong, your new one is too heavy and bulky, is there any difference ?

But when i showed him Moon in 40X (extremely brilliant and contrasty), immediately she changed

her opinion and told me that it is the best view  that she has ever seen and said : YES THERE IS BIG DIFFERENCE !

[jetstream]

Perfect! Thanks!

Still one query remains: does smaller aperture give better contrast?

If seeing is poor a smaller aperture may give a better view, punching through the conditions. In this case a smaller scope could present a better visual image to you, looking like better contrast, of course any aperture comparison would be made with equal optics.

I think your question is a valid one to ask for sure.

[x935418]

If seeing is poor a smaller aperture may give a better view, punching through the conditions. In this case a smaller scope could present a better visual image to you, looking like better contrast, of course any aperture comparison would be made with equal optics.

I think your question is a valid one to ask for sure.

َ

As i know, poor seeing effects  on apreture larger then 6"

so when our choise is between 4" and 5" i think it is not important.

is't it?

[jetstream]

I think it depends on the objects viewed under the poor conditions, for solar my 120mm is much more affected by poor seeing than my 90mm. There are also times that the 90mm gives a steadier view of the moon, along the terminator, than the 120mm does.

If nighttime seeing is so bad that my SW120ED isn't performing, I usually go DSO hunting under clear skies- flailing away on the planets etc under lousy conditions is an exercise in frustration (for me). Any observing is better than no observing IMHO however.

We must also ensure the scope is cooled properly before we make any assumptions about sky conditions. It has been said that every inch of useable aperture gain is worth the extra resolution for lunar/planetary/solar.

What is your experience X9?

[x935418]

I think it depends on the objects viewed under the poor conditions, for solar my 120mm is much more affected by poor seeing than my 90mm. There are also times that the 90mm gives a steadier view of the moon, along the terminator, than the 120mm does.

yes , 120mm my suffer from bad seeing more then 90mm, but what is focal ratio of them ?

fast scops has less depth of focus and suffer from more from bad seeing.

a f/10 telescope has four times the depth of focus of a f/5 instrument.

if 90mm focal ratio is very far from 120mm, i think it may be rather about focal ratio insted of diamater

see below link:

http://www.fpi-protostar.com/bgreer/seeing.htm

If nighttime seeing is so bad that my SW120ED isn't performing, I usually go DSO hunting under clear skies- flailing away on the planets etc under lousy conditions is an exercise in frustration (for me). Any observing is better than no observing IMHO however.

agree

We must also ensure the scope is cooled properly before we make any assumptions about sky conditions.

we are speaking about refrecctors, is't int ?

It has been said that every inch of useable aperture gain is worth the extra resolution for lunar/planetary/solar.

yes but unfortunately our friend (flanker) can not use its extra inch

What is your experience X935418?

A little with a short tube 102mm F5 .

I tried to use each usable milimeter of it for high magnifiction.

maximum magnifiction with perfect quality i get was: 166X on moon,

i first stoped down it to F8, and with using two internal diafragm i could see very sharp image, it was great

recently buy a SW200p on EQ5, but not start using it seriously yet (i am a little busy now)

[jetstream]

I too thought that a longer f ratio may work better in poor seeing, but as Bryan says in your link http://www.fpi-protostar.com/bgreer/seeing.htm "Telescopes of equal aperture are affected the same by atmospheric turbulence, regardless of focal ratio." This is my experience with this as well.

My 90mm frac is f7 and my 120mm frac is f7.5, pretty close.  Cooling is needed equally for refractors,reflectors, SCT's or any scope- each scope will have a cooling "personality" I find. A refractors image in an uncooled scope is as bad as an uncooled reflector's, magnification and aperture considered, IMHO.

flanker might find that a C5 or C6 may work very well, however my suggestion could open up the obstructed vs unobstructed debate. If travel is an issue and the travel is to dark skies, aperture could well trump the smaller scope, if the scope is small enough (C5).

X9, you will love the 8" newt I think, it will give extremely good views of the planets/moon and most everything else. I hope this thread helps flanker with a good decision for himself, under his circumstances.

[John]

When you stop down the aperture of a scope the performance reduces to the stopped down aperture. With an F/5 achromatic refractor stopped down from 102mm to 63mm the focal ratio becomes F/8 and the amount of chromatic abberation is less as you would expect from a smaller aperture, slower achromat. The resolution limit also decreases to that of a 63mm scope - there is no "free lunch" 

To the original poster (Hari) I'd say, if portability is an important requirement a schmidt or maksutov cassegrain will give you aperture and focal length in a compact tube. A 6" SCT or Mak-Cassegrain is probably just as portable as a 4" F/10 refractor and will outperform the 120mm F/8.3.