I see a fair number of threads dealing with folks pursuit of very fast f ratios. This is likely an obsession of the imagers, based on perceived time savings, but do the complexities of fast ratios justify the quest?
I have heard a fair number of experienced imagers, and visual guys, express a love for f5 as the perfect ratio. This is of course due in large part of the ease of Newtonuan collimation at f5 and slower ratios. Don't get me started on RCs, as a previous owner I have dealt with the precision required that always seemed to evade.
In the visual world, long focal lengths have historic preference, especially for double, triple star splitters, and planetary folks.
So what is your favourite ratio and why?
F3.54 is the perfect focal ratio if you’re using an 18” Dobsonian, it allows you to stay seated no matter what part of the sky you’re viewing, you just need to include the SIPS Paracorr in your focuser setup….
If, however, you’re addicted to refracting light instead of light bouncing off mirrors…. The you need to use F7.7 as the perfect focal ratio……because it’s the only ratio the mighty TOA130NFB comes in….
Depends on the target - very happy with F2 from the RASA with larger objects but the short FL does limit at times. I dont like/enjoy mono imaging, so I dont need to worry about changing filters . For the RASA8, tilt is a bit of a pain, but I can get reasonable results across most of the frame of APSC which is good enough. for me.
For longer focal lengths I do like the F5.3 on my Mak Newt.
Probably my ideal setup would be a RASA11, plus a 10inch F4 or F5 Newtonian at 1000-1200mm FL but might need a new mount as well.
F ratios in imaging got faster and faster in the last 10 years. F8 was common around 12 years or so ago.
Faster F ratios with larger apertures makes for more images given the opponents to making an image of clouds/rain, wind, time, moon, equipment problems. They all conspire to reduce the amount of exposure time.
But too fast and everything gets harder in setting up the gear. Add to that these "new" CMOS cameras with small pixels that show up defects easily and you have the other side of this that makes you want to have a longer focal ratio.
So the answer I think depends more on the quality of the gear. To achieve better than F5 is going to require better gear that is well setup. Below F4 is really demanding. Although I see a lot of excellent images from RASA setups at around F2.
So there is the need for speed and there is the rise of equipment problems slowing it.
I often thought F5 is good focal ratio. The FSQ is F5 and that makes a bright image quickly. F6 is still fine. F7 or slower is getting a bit slow.
So my answer is more of a range - F3.8 to F6 is a good imaging range with better gear needed to hit the faster F ratios otherwise F5/6 is practical and balanced between speed and practicality.
The fastest APO refractors are about F4.5 except perhaps a few exceptions like the FSQ with reducer. Large aperture scopes are generally slower and longer focal length.
Also keep in mind that using reducers often adds another layer of complexity and difficulty as exact spacing and the extra glass can add aberrations.
Obviously there is no one best f-ratio. For visual use I reason as follows:
I want my lowest power to give the max true fov without the exit pupil being too large for my pupil. [I would tolerate a small amount of vignetting in a large dobs at the lowest power to get a seated viewing position.] A young person has a max pupil diameter of 7mm but at my age I think it best to aim for <6mm. My lowest power is a WO 28mm UWAN with a afov of 85* but planning for a 31mm Nagler gives about the same numbers. I would be viewing with a paracorr which is also a 15% barlow.
Plugging in numbers gives Effective focal ratio = 28/6 = 4.67. True f-ratio = 4.67/1.15 = 4.06. Or more conservatively f-ratio = 28/5/1.15 = 4.87. My scope is f/4.5 so the exit pupil is 5.4mm which is pretty ideal, though an f/4.2 and 5.8mm exit pupil would also be acceptable.
F5 and f6 for me
Both visual and imaging
Scopes : Newtonian Reflectors
Imaging : 10” f5 Carbon fibre and 8” f5 Carbon fibre , 6” f6 steel tube
Visual : 12” f5 Goto Dob
Ease of collimation
Collimation holds well
Optical train minimal demand on equipment and user
Cost effective
Bang for buck
All rounder
F3.54 is the perfect focal ratio if you’re using an 18” Dobsonian, it allows you to stay seated no matter what part of the sky you’re viewing, you just need to include the SIPS Paracorr in your focuser setup….
Maaan that's a beautiful scope Matt....
I have a very good idea too, given my, poor mans, version (18" F4.1 Skywatcher). It's manageable but formidable and under great skies, delivers in spades.
F7 is the lucky number.
I have seen a couple of superb old Newtonians working at this ratio,
And it is the perfect number for modern APO's (I have one!)
I have a new Askar 130 f7.7 and it seems to image rather well and seems fast enough...my RASA 11 is nice but there are many annoying things, at least to me...I did get a great M42 with only an hour of 10 second exposures...but probably my best image came from my 80 mm at f5 mono camera...at the moment I run RASA 11, 115mm f5 triplet, 80mm f5 triplet and the Askar f7.7...each is great when everything works.
Alex
Cons:
-Very sensitive to tilt
-Very sensitive to collimation errors
-Very sensitive to backspacing issues
-Focus is critical
-There are only a couple of filters available on the market pre-shifted for f/3
-f/3 pre-shifter filters cannot be used with either f/2 or f/4 plus systems
Pros:
-Speeeeeeeeeeeed
-f/2 systems are optically faster but have front mounted cameras so you can't use a filter wheel. This means that either you are throwing away 2/3 of the light because of a Bayer filter for OSC or waking up in the middle of the night to switch out filters for mono.
Cons:
-Very sensitive to tilt
-Very sensitive to collimation errors
-Very sensitive to backspacing issues
-Focus is critical
-There are only a couple of filters available on the market pre-shifted for f/3
-f/3 pre-shifter filters cannot be used with either f/2 or f/4 plus systems
Pros:
-Speeeeeeeeeeeed
-f/2 systems are optically faster but have front mounted cameras so you can't use a filter wheel. This means that either you are throwing away 2/3 of the light because of a Bayer filter for OSC or waking up in the middle of the night to switch out filters for mono.
OR....
Buy another two such systems and stay in bed
I agree with John B, below, Newtonians with ratios of f6 and slower do not need Coma Correctors. My experience at f5 in a variety of newts have all been very positive, in terms of ease of collimation, no offset concerns, just simple collimation.
However, you cannot beat a good refractor for ease of use, low maintenance, etc.
F3.54 is the perfect focal ratio if you’re using an 18” Dobsonian, it allows you to stay seated no matter what part of the sky you’re viewing, you just need to include the SIPS Paracorr in your focuser setup….
