Refractor aperture concerns

[x935418]

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.

shame to me !

I read somewhere that slow scopes suffer less from bad seeing, because of

their huge depth of focus, but i forget where it was,

yesterday i searched for it and found this page instead , but i didn't read it care fully !

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.

I am in love with my 8" already !

Waiting for Saturn, i saw it in 120X, hope to see at 240X.

[x935418]

When you stop down the aperture of a scope the performance reduces to the stopped down aperture.

your opinion is correct about APO or reflectors, that they have no CA.

as i told, in achromats, outer part of lens destroy image.

I said you can see better picture if you eliminate outer part of lens.

when i use 100X in 102 F5 without stopping down, it is very blurry image

but when i stop down it to 63mm F8, there is big improvement in resolution and contrast!

in other word i can say:

achromat before stop down is look like a old and severely damaged car ,

but when you stop it down, it convert to a good new bicycle.

surly bicycle is smaller and slower then a car, but if both were in same situation.

but here , your bicycle can go faster then the car !

there is no "free lunch"

yes i know, but i speak about cleaning lunch from unwanted  objects !

[flanker]

I missed a lot of conversations! Was out skywatching yesterday night. Took my TravelScope 70. Mostly will have the fruit of this discussion by the next time I go stargazing! Returned today morning and slept half the day

My main issue is this: I work in Bangalore and I don't have any vehicle around here. I stay in a heavily light polluted area. I plan to move back home to Hyderabad in a couple of years. Then I'll buy a 8" dob or some other piece of furniture .

As of now I need a good scope to start serious stargazing and something I can lug around in a bus here and when I shift home.

[flanker]

Why do APOs perform that well? Isn't it just about CA? If somehow my brain learns to cancel CA out, isn't an APO just as good as an achromat?

So isn't it worth it to buy short achromats? Like say, spending $1000 on a 150mm f5.

[Pig]

The only type of scope I have not looked through is a DOB, However, I have looked through a P200 newt and I must say the views were very impressive indeed, I believe they are very similar to a DOB.

To date the best contrasty and detailed views I have seen, through any scope, have come from my 76mm, 80mm refractors and the Lunt 60mm.

Does this mean that contrast and detail are in the eye of the beholder !!!!

[Rainmaker]

Chromatic aberration is not just about the coloured halo around objects, it is about loss of detail.  That halo is unfocused light which is to only distracting when viewing the object but it is information that is missed out on.  Apochromatic refractors have less of the colour because their lenses are able to focus that information which in turn increases the amount of detail or information our eyes can resolve.

[x935418]

Why do APOs perform that well? Isn't it just about CA? If somehow my brain learns to cancel CA out, isn't an APO just as good as an achromat?

So isn't it worth it to buy short achromats? Like say, spending $1000 on a 150mm f5.

if the problem was just halo , yes

but the main problem is degrade resolution , brain can not do any thing with it.

150mm F5 has 750mm focal len , so it gives you 61mm for high power (Conrady = excellent) or 77mm (Sidgwick = good)

if you buy a 4" F10, it give you more magnification.

of course it can show you great image at low power (for DSO), but again i say 150mm is small for DSO

why you don't think about 6" reflector ?  like below ?

[x935418]

excuse me , picture is too large

i couldn't find any EDIT link, how i can edit this ?

[flanker]

I wasn't planning to buy the 150mm refractor. It was just out of curiosity. I'll get a bigger reflector after I shift home in a year.

The CA funda is very clear to me now. Thanks folks!

So one final question : from what I understood out of the entire discussion is that a 102mm f10 is a better deal than a 120mm f8.3 for moon, planets and bright DSO. Right?

[x935418]

I wasn't planning to buy the 150mm refractor. It was just out of curiosity. I'll get a bigger reflector after I shift home in a year.

The CA funda is very clear to me now. Thanks folks!

So one final question : from what I understood out of the entire discussion is that a 102mm f10 is a better deal than a 120mm f8.3 for moon, planets and bright DSO. Right?

yes, since they have same  focal len, so they have same  magnification, but as jetstream said , 120mm is larger and  it may suffer more from bad seeing .

of course if you stop down them to 70mm (or 90mm), both will be equal and  there is no difference .

i think 4" F10 is excellent choice, my previous 4" was F5, if i had F10, i never would  sold it.

[swamp thing]

The only type of scope I have not looked through is a DOB, However, I have looked through a P200 newt and I must say the views were very impressive indeed, I believe they are very similar to a DOB.To date the best contrasty and detailed views I have seen, through any scope, have come from my 76mm, 80mm refractors and the Lunt 60mm. Does this mean that contrast and detail are in the eye of the beholder !!!!

Shaun the Dob is only a mounting buddy. You can't exactly look through a Dob, you look through whatever scope is mounted on it. I have seen Dob mounted Fracs and SCT before. .....pretty cool they were too

Anyway back to topic ahem (slaps own wrist)

[flanker]

Okay! Great. Thanks x935418. You clarified all my doubts!

[F15Rules]

Assuming you are talking about lower cost achro scopes, the 102" F10 will give sharper, more satisfying views in my opinion. AND it will be a whole lot easier to lug around your city on a bus!

I have found every 120mm achro I've looked through to be disappointing as a step up from a 102mm F10

Good luck with your choice.

Dave

[flanker]

Thanks, Dave.

[flanker]

Does the existence of a dual speed focusser need to be a criterion for selecting a long focal length refractor? I'm expecting that it shouldn't as it will have a greater impact on faster scopes than the slower ones.

But want you folks to clarify it!

[x935418]

Does the existence of a dual speed focusser need to be a criterion for selecting a long focal length refractor? I'm expecting that it shouldn't as it will have a greater impact on faster scopes than the slower ones.

But want you folks to clarify it!

yes you are right

[flanker]

I'm planning to mount the new 102 on a pier mount with a simple altaz mount. The pier has removable legs.

Got a deal from a local store for such a mount. He rates it's capacity at around 9kg and actually sells 120mm long tube refractors on it.

[F15Rules]

Does the existence of a dual speed focusser need to be a criterion for selecting a long focal length refractor? I'm expecting that it shouldn't as it will have a greater impact on faster scopes than the slower ones.

But want you folks to clarify it!

No, quite the opposite in fact. A longer focal length gives you more "depth of focus" than a short one, which is one of the appeals of them: they can take higher magnification and are easier to fine focus at such high magnification. That said, a nice crayford is still a worthwhile upgrade if your optics are worth the cost (I added a Moonlight to my F15 127mm). 

The other option is to source some larger focuser wheels which give you a bit more "leeway" as each partial turn moves smaller increments of focus..

Hope that helps,

Dave

[flanker]

Folks

Thanks to this forum, I'm also considering a Celestron C6. How would it compare to the 102mm achro on planets and bright DSOs?

[jetstream]

It just might blow the doors off the 102 achro, on everything. Cooling will be more of an issue with the C6 though, if your temps vary much there.

[flanker]

Okay. I heard that the high transmission rates of a refractor are impossible to achieve on an SCT. Would that matter?

And is an SCT environmentally sealed? Does the mirror oxidise and lose performance ?

[Moonshane]

you seem to be suffering from analysis paralysis. There is not an Astro scope of any type that is sealed all are open as soon as you remove the eyepiece.

All mirrors degrade slowly over time but the aperture gain over that time is substantial for the same cost.

There's no such thing as the perfect scope for everything not a free lunch. Somewhere you have to compromise.

I recommend attending a star party or club meeting before you buy

[flanker]

Star parties are quite rare in India. Also, I'm not trying to buy everything I ask for. There's simply no way I'd buy a dob at this point. I just wanted to clarify some lingering doubts.

I was trying to follow up on the comments I received about considering a CAT.

[John]

I've owned SCT's that are 10 years plus old and the mirrors have been pristine. Shane is right that the system is not completely closed but you don't get the dust and stuff on SCT / Mak-Cassegrain mirrors that you do on newtonians. Not that newtonians are difficult to clean though.

[Moonshane]

Sorry 

I was on my phone and could not see your location. Now I understand your wish to get it right first time.