Upgrading a Newtonian, f/4 or f/5, 8" or 10"

[Chipela]

I've been observing for a year now and I'm considering to upgrade my gear. I currently have SW 150PDS and since I constantly feel it limits how much I can see of the deep sky objects, I'm considering to buying a bigger Newtonian. There just seems to be quite many options and I'd like to ask some info about a couple of tube parameters.

1) f number

Considering purely visual observation, what difference does the f number make (e.g. between 8" f/4 and 8" f/5 tubes)? I know that a lower f number is more demanding for eyepieces and correct collimation.

But is there anything else? Assuming that I adjust my eyepiece collection so that I get the same magnification (e.g. 50x would require 16mm EP for f/4 and 20mm EP for f/5 scope) is the resulting image similar? Or is it brighter or otherwise different with one of the tubes?

The reason I'm asking is that my Televues should be able to cope with f/4, and a 20-25 cm shorter tube would be very tempting, but I'd like to know if f/4 is inferior in some area.

2) Aperture

Does 8" give a considerably brighter image compared to my current 6" or should I jump to 10" to get a real visible improvement? 10" is not that much more expensive but it would mean that my EQ5 which should still work with 8" for visual observation would be too light to be used with 10" tube and suddenly I'd need to upgrade the mount too. (And dobsonian does'nt feel like a good option, equatorial breaks to small pieces and is easier to move around.)

Thanks!

 

[Peter Drew]

I think you've answered question 1 yourself in the main. Additionally for a given aperture, at the same magnifcation, the brightness stays the same regardless of F number visually.

Regarding question 2, the relative image brightness of 6", 8" and 10" apertures at the same magnification is  in the ratio of 9, 16 and 25 so there is quite a difference. A Dobsonian breaks down into two manageable components at these apertures and will be more stable than an EQ5.      

 

[spaceboy]

1) f number

Considering purely visual observation, what difference does the f number make (e.g. between 8" f/4 and 8" f/5 tubes)? I know that a lower f number is more demanding for eyepieces and correct collimation. The only real difference the f/ number makes is the length of the scope and the amount of coma the mirror produces. FOV and magnification with any given eyepiece will also differ. A 20mm eyepiece in an f/4 will show more sky over a 20mm ep in an f/6. A longer focal length slower scope often allows for higher magnifications with any given eyepiece. All this said the same FOV and magnifications can be achieved in both f/4 & f/6 just by varying ep selection. 

But is there anything else? Assuming that I adjust my eyepiece collection so that I get the same magnification (e.g. 50x would require 16mm EP for f/4 and 20mm EP for f/5 scope) is the resulting image similar? Or is it brighter or otherwise different with one of the tubes? It's not like F stop on a camera. The image to the eye will be no brighter for that you need a larger scope.

The reason I'm asking is that my Televues should be able to cope with f/4, and a 20-25 cm shorter tube would be very tempting, but I'd like to know if f/4 is inferior in some area. A common misconception is that good eyepieces will remove coma from a fast scope. Coma is a result of fast mirrors and not eyepieces. If you want to have a fast mirror free of coma you need a coma corrector. Premium eyepieces then reduce the risk of other aberrations being introduced, IE: astigmatism or pin cushioning.

2) Aperture

Does 8" give a considerably brighter image compared to my current 6" or should I jump to 10" to get a real visible improvement? 10" is not that much more expensive but it would mean that my EQ5 which should still work with 8" for visual observation would be too light to be used with 10" tube and suddenly I'd need to upgrade the mount too. (And dobsonian does'nt feel like a good option, equatorial breaks to small pieces and is easier to move around.)

Personally I don't feel an 8" Newtonian works on an EQ5. It will mount on an EQ5 but in practical use any breeze is going to cause vibrations. Avoiding vibration when focusing will become frustrating also and you'll end up wanting an auto focuser sooner or later. There will be some improvement in an 8" over a 6" but you would be far better getting a 10" dob. The dob is far more easier to move around than an EQ mount, far less hassle setting up and offer a very sturdy mounting solution at a fraction of the price required to mount on an EQ mount which for the record is a minimum of an EQ6. A 10" dob is going to offer you far more pleasing experience over the 8" EQ5 set up which will soon having you digging in your pocket again for a HEQ5 mount.