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6" F6 or F8

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#1 Nebulae

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Posted 03 December 2012 - 11:50 AM

Hello everyone i am new to CN and am happy to be a part of this wonderful forum. I will rate 10/10. :cool: :jump:

Since 2 years i am using 4.5" F8, home made Newtonian telescope and i am fairly satisfied with its viewing, to see better viewing i am thinking to move to 6" home made Newtonian.

So please coach me in choosing correct focal length. Which one should i choose F6 or F8. It should be such way that i can see planets in details as well as Nebulae, double-star,galaxy and many more.

Can you tell me what can i see in details in 6" mirror?

Your help will be appreciated. :bow:

#2 bremms

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Posted 03 December 2012 - 12:11 PM

Other than tube length and weight, F8 is better for visual use. Less critical of eyepieces and alignment. F8 seems to be a nice sweet spot for compromise planetary/ Deep sky. I prefer F8 to F6 in a 6" scope. 40mm eyepiece gives you a 5mm exit pupil which is pretty much optimal for low power viewing.
F6 is still not too fast in such a small scope.

Personally I would go to an 8" F6. It is a much larger step up than a 6" which is not a big step from a good 4.5. 8"f6 is good at a lot of things and won't cost too much more than a 6" F8.

#3 Jeff Morgan

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Posted 03 December 2012 - 12:22 PM

There are a lot of factors to consider but in the end it all boils down to personal choice. Personally, I would go longer rather than shorter.

#4 bremms

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Posted 03 December 2012 - 12:51 PM

Here you go...
http://www.cloudynig...ct=70052&sor...

#5 Mark Harry

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Posted 03 December 2012 - 01:50 PM

8 inch aperture, you might as well double your weight, lack of convenience, and double the temp control issues. 6" F/8 has a lot of virtues, and has been the defacto standard for decades for a reason. Of course this is dependent on the area you view in.
F/8 has almost 1/2" diffraction limited area, where F/6 is around 1/4" It's much easier for planets with an F/8.
(what do I know??!!)

M.

#6 Ed D

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Posted 03 December 2012 - 02:36 PM

Of the two, I would strongly recommend the f/8, especially if you want to observe planets. I like the depth of focus, very tolerant collimation, and great performance with all but the worst eyepieces. A 6" f/8 with good optics is an incredably capable instrument, especially if you are an experienced observer and know how to see detail. There are plenty of sketches in the sketching forum that show planetary detail and color done by many observers with 6" scopes, as well as stunning detail visible in many DSOs (galaxies, clusters, etc).

Ed D

#7 John Jarosz

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Posted 03 December 2012 - 04:24 PM

Welcome to CN. Maybe you could tell us a bit about yourself, including where you usually observe from. That info might help others in forming their opinions.


Are you intending to build this new scope? Grind your own mirror? What kind of mount do you envision?

Since you've used a 4.5" scope for a couple of years you know more on the subject than most people that ask your type of question.

Are you looking for an interim scope, of a final solution? Difficult question I know. But if you think that you might go larger still in the future, then I would agree with those that say to move to an 8" F6 right now. You will see more, the scope is really not much larger than a 6" F8, and having the larger scope will forestall the need to move to a still larger scope in the immediate future. By the time you are ready to go beyond an 8" scope you will have a much better idea as to what kind of large scope to buy/build.


John

#8 Mike I. Jones

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Posted 03 December 2012 - 04:36 PM

+1 on the 8" f/6. Make the jump over 6".
Mike

#9 danjones

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Posted 04 December 2012 - 12:03 AM

Hi Nebulae, welcome to the forum. I'm also fairly new here, and these guys here know their stuff. I've been getting tons of great advice here while grinding my own mirror. They are terrific. I agree with the majority about the 8". It would be a nice jump from the original 4.75. If you are considering grinding your own mirror, and you do decide to go with a six, I got a couple of six inch plate glass blanks, 6" x 1" that I trepanned myself that are decent I'm wanting to get rid of. Cheap. I'm grinding a 10" out of the same glass as these, and just cut them for blanks for fun. I don't want them, and they would make good mirrors. Good luck on your adventure!

#10 Nebulae

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Posted 04 December 2012 - 01:29 AM

instead of 6" should i move to 8" then?

#11 pogobbler

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Posted 04 December 2012 - 02:28 AM

instead of 6" should i move to 8" then?


An 8" scope, should the extra weight not be a problem for you, would be a more noticeable increase in light gathering and resolution from your 4.5" as compared to a 6". An 8" f/6 would be the same length as a 6" f/8 and not really much heavier or bulkier, and an f/6 scope is still a slow enough scope that collimation isn't as critical, coma isn't a big issue, and reasonably priced eyepieces will still work okay.

That said, a 6" f/8 is still a workhorse of a scope and could provide years of excellent views. So I'd lean to an 8" f/6, but either is a good scope.

#12 Nebulae

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Posted 04 December 2012 - 02:38 AM

Thanks for detailed information.
Ok i would like to ask you one question. what is the difference between F8, F6 or F4.

According to me what i think is that on increasing or decreasing F number, focal length also increase or decrease.
So in field of viewing what actually happenes when we see through F8, F6 or F4 and what type of viewing does it provide.

Just for example : what can i see through 8" F6 and 8" F10.
8" F6 will be used for what purpose? and
8" F10 will be used for what purpose?

#13 Al8236

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Posted 04 December 2012 - 04:22 AM

In my way of thinking, and others may correct me if I am wrong, an 8" F6 is a good general purpose scope good for planetary with a high power EP and still can be used for deep sky with an low power EP.
The 8" F/10 becomes pretty much a planetary scope due to the limited field of view.
O.K guys let me have it! I'm sure someone will give a different point of view!

#14 StarStuff1

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Posted 04 December 2012 - 05:11 AM

One thing not being discussed enough is the actual viewing situation. If you take a newtonian out from a warm place to a colder viewing site there will be cool down issues. The 6-in will acclimate faster. An 8-in will be bothered more by seeing that is not steady.

It is hard for one scope to be best at all tasks. All sizes and focal ratios have their advantages and disadvantages. The 6-in f/10 in my avatar was a killer on planets, the Moon and double stars. It's high contrast images of DSOs was nice but I could still more details in planets, galaxies and nebulas in my 8-in f/6.

A 6-in f/8 would be easy to make and use. Then one day you come along and bump into a used 10-in f/5 at a price too good to resist and... :grin:

#15 Jon Isaacs

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Posted 04 December 2012 - 05:52 AM

instead of 6" should i move to 8" then?


First, let me welcome you to Cloudy Nights... :waytogo:

In my experience, either a 6 inch or an 8 inch will provide a significant upgrade from your 4.5 inch. If you are going to grind your own mirror, then a 6 inch F/8 would be easier and a reasonable place to begin. Otherwise, the added aperture of an 8 inch is definitely worthwhile.

But to really help you, the group needs to know more about where you are located, your resources, what you hope to look at, what kind of mount you are planning to use.

That said, here's a few things that I have found:

- In terms of manageability, ease of use, tube length is probably the most important factor. A 6 inch F/8 is about 48 inches long, a 8 inch F/6 is about 48 inches long, the 8 inch is heavier but still easily handled and so in terms of mounting, transporting and observing, they are very similar. An 8 inch F/10 is about 80 inches long and will require a stool or a short ladder even on a Dob mount.

- In terms of overall performance, aperture is more important than focal ratio. Aperture gains you better resolution, more light. The trade off is that the diffraction limited region is smaller, the mirrors are more difficult to make.

- F/6 is a nice place to be, a nice combination of a reasonably coma free field of view in a reasonably compact package.

- For a Dobsonian, a 48 inch focal length is pretty nice. Much shorter and even seated the eyepiece will be too low for comfortable observing. This is one reason why the common commercial dobsonians are the 6 inch F/8, the 8 inch F/6 and the 10 inch F/5, they are all about 4 feet long, provide a comfortable eyepiece height and fit across the backseat of most cars...

Jon

#16 Nebulae

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Posted 04 December 2012 - 06:27 AM

Thanks all my friend. I live in India.
It should be such way that i can see planets in details as well as Nebulae, double-star,galaxy and many more.

#17 ccaissie

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Posted 06 December 2012 - 11:41 PM

If you live in light polluted skies, the longer f/8 may give you a darker field of view. I just made a 6" f/8 mirror for a friend, and enjoyed using the scope when star testing it.

#18 ccaissie

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Posted 06 December 2012 - 11:47 PM

The 8" F/10 becomes pretty much a planetary scope due to the limited field of view. O.K guys let me have it!

What? You don't have a 3" Plossl?

#19 Jon Isaacs

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Posted 07 December 2012 - 08:26 AM

If you live in light polluted skies, the longer f/8 may give you a darker field of view. I just made a 6" f/8 mirror for a friend, and enjoyed using the scope when star testing it.


At the same magnification, a 6 inch F/6 and a 6 inch F/8 should have the same field brightness.

Jon

#20 Mike I. Jones

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Posted 07 December 2012 - 11:08 AM

Almost :) :

At any magnifications giving the same exit pupil diameter, a 6 inch F/6 and a 6 inch F/8 should have the same field brightness.

#21 KenScharf

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Posted 16 December 2012 - 03:48 PM

Are you making your mirror or looking to buy one already done? For many years the 6" F8 was the recommended "first" mirror for ATM's in the US. However on the other side of the pond it was an 8" F6. Some consider an 8" mirror easier to learn on than an 6". The Foucault shadows may be easier to "read" on the 8" F6 than on the 6" F8 (or so I've read). In any case the 48" focal length results in lots of usable magnification from "common" eyepieces, coma is well contained on either size mirror, and you don't need a ladder (unless you're a midget).

I split the difference on my current mirror project, a 7" F7. Now I need to get back to it and finish it!

#22 Datapanic

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Posted 17 December 2012 - 12:38 AM

An 8" f/7 is my favorite. Welcome to CN!






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