Newtonian Focal Ratios - visual

[25585]

From purely visual & practical angles, are there any disadvantages or advantages between F4 or less, to F6 or more, in a Newtonian? 

F4 (or less) means shorter focal length so shorter tube but lower magnification. But what other pros & cons are there?

Quite a few scopes between F4 & F5, maybe the most

F5 up to F6 seems to be 2nd most common. 

What advice for buyers can be given from experience on best F ratio for 6, 8, 10 & 12 inch apertures in respect of optics features, maintenance & scope dynamics?

(My first Newtonian 8 inch was F8 - too long. Second 12 inch was just under F5 - only just portable. Current is 10 inch F5 - lighter but I miss the 12's solidity if not bulk). 

 

[Peter Drew]

I think you've answered most of the issues in your question.  For a good allround telescope I would go for the longest focal ratio you can handle comfortably.

[25585]

OK, say someone chose a scope because it was liftable & could put it in his car. But being so for the chosen aperture resulted in a F4 or less mirror. 

Had same person gone for a slightly smaller aperture, which would provide F5 but similar stowage dimensions, which would this beginner have been better off going for realistically?

You often don't know what you want to, or can see first buy. Going from binoculars is a very big step!

[cuivenion]

Coma takes up more of the field of view in the lower ratio scopes. You can use a coma corrector though.

[John]

I was attracted to my F/5.3 12" dob because it was unlikely to need a coma corrector and I really enjoy using ultra / hyper wide eyepieces. Thats proved fine in practice.

The 21% central obstruction and relatively fuss-free collimation demands were also factors.

The downside is that it's a "standing when observing" scope. Fine for me but shorter folks need a step or two to reach the eyepiece when the scope is pointing towards the zenith area which is a factor when I do outreach sessions with it.

The collimation "sweet spot" gets pretty small as the focal ratio drops below F/5. At F/4 it's just 1.4mm which might be a challenge for the novice collimator whereas at F/6 it's a relatively relaxed 4.8mm.

 

[Moonshane]

I love my 12" f4  and my 16" f4. Then again I also loved my 12" f5.3 and also love my 6" f11.  

The 12" f4 is for me the goldilocks scope.

[Moonshane]

Here it is 

[25585]

1 hour ago, Moonshane said:

Here it is 

Is that an Orion dobsonian mount or your own DIY make?

[Moonshane]

I made it ? but based it on their design although I'd argue I improved it

[25585]

2 hours ago, John said:

I was attracted to my F/5.3 12" dob because it was unlikely to need a coma corrector and I really enjoy using ultra / hyper wide eyepieces. Thats proved fine in practice.

The 21% central obstruction and relatively fuss-free collimation demands were also factors.

The downside is that it's a "standing when observing" scope. Fine for me but shorter folks need a step or two to reach the eyepiece when the scope is pointing towards the zenith area which is a factor when I do outreach sessions with it.

The collimation "sweet spot" gets pretty small as the focal ratio drops below F/5. At F/4 it's just 1.4mm which might be a challenge for the novice collimator whereas at F/6 it's a relatively relaxed 4.8mm.

 

Is there an equation showing what collimation "sweet spot" size is for different fl & fr?

More an issue for scaffold assemble-on-site scopes?

[25585]

1 hour ago, Moonshane said:

I made it ? but based it on their design although I'd argue I improved it

One of my next scope considerations is to get a new 12 inch OTA I can mount on my old Dark Star dob mount, which was a 12 inch. VX12L 1600mm fl or the VX12. How do you find the latter for collimation need & ease?

[Mr Spock]

You can get and excellent visual experience from any focal length providing it is well collimated and the optics are decent quality. Though it is true longer focal lengths are easier to collimate and shorter focal lengths are easier to manoeuvre.

You can also argue that a long focal length is better for planets due to the smaller secondary. I reckon a 10" f8 with an undersized secondary would be hard to beat in UK conditions. An 80" focal length might be a bit unwieldy though!

[Moonshane]

55 minutes ago, 25585 said:

One of my next scope considerations is to get a new 12 inch OTA I can mount on my old Dark Star dob mount, which was a 12 inch. VX12L 1600mm fl or the VX12. How do you find the latter for collimation need & ease?

Needs a tweak on the primary every use but so does every other newt I have ever owned. Not an issue.  I  use a paracorr but it's not absolutely essential.

 

[John]

2 hours ago, 25585 said:

Is there an equation showing what collimation "sweet spot" size is for different fl & fr?...

 

This Sky & Telescope article contains the maths at the 5th paragraph:

http://www.skyandtelescope.com/astronomy-resources/how-to-align-your-newtonian-reflector-telescope/

I do have a laser collimator but I prefer to use a cheshire eyepiece. I guess I just know what "collimated" looks like now though the cheshire so it's dead easy. I usually star test at the beginning of a session as well, just to be sure.