6" F6 or F8
Posted 03 December 2012 - 11:50 AM
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.
Posted 03 December 2012 - 12:11 PM
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.
Posted 03 December 2012 - 12:22 PM
Posted 03 December 2012 - 01:50 PM
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??!!)
Posted 03 December 2012 - 02:36 PM
Posted 03 December 2012 - 04:24 PM
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.
Posted 04 December 2012 - 12:03 AM
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.
Posted 04 December 2012 - 02:38 AM
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?
Posted 04 December 2012 - 04:22 AM
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!
Posted 04 December 2012 - 05:11 AM
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...
Posted 04 December 2012 - 05:52 AM
instead of 6" should i move to 8" then?
First, let me welcome you to Cloudy Nights...
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...
Posted 04 December 2012 - 06:27 AM
It should be such way that i can see planets in details as well as Nebulae, double-star,galaxy and many more.
Posted 06 December 2012 - 11:41 PM
Posted 06 December 2012 - 11:47 PM
What? You don't have a 3" Plossl?
The 8" F/10 becomes pretty much a planetary scope due to the limited field of view. O.K guys let me have it!
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.
Posted 07 December 2012 - 11:08 AM
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.
Posted 16 December 2012 - 03:48 PM
I split the difference on my current mirror project, a 7" F7. Now I need to get back to it and finish it!