visual and AP performance between f/5 and f/10 scope

[ggodetucsamoht]

I still having doubts between the difference in visually seeing between a fast scope and slower one. Can anyone guide ? Thks.

[Gary]

You may find your answers here:

What is the focal ratio? How is it related to the f-stop?
http://www.celestron.com/c3/support3/in ... cleid=1650

Read the Focal Ratio segment in this article:
http://www.astropix.com/GADC/SAMPLE2/SAMPLE2.HTM

[ggodetucsamoht]

You may find your answers here:

What is the focal ratio? How is it related to the f-stop?
http://www.celestron.com/c3/support3/in ... cleid=1650

Read the Focal Ratio segment in this article:
http://www.astropix.com/GADC/SAMPLE2/SAMPLE2.HTM

Thanks Gary fot the link.

Actually i am looking at two model of 8" Newtonian for my ota upgrade. One specs at f/5 and the other f/3.9! I am curious to know what is the difference in seeing for this diff focal ratio. Threorically f/3.9 will have a little wider FOV as compare to the f/5 so seeing wise should not affect much. But for astrophotography wise it really make a huge difference.

But for a similar size of 8inch (203mm) but how can the manufacturer able to produce two different specs scope? I am actually comparing Orion 8" astrograph newtonian and celestron C8-NGT.

[Mariner]

A word of warning though. The faster (or lower F ratio) your scope, the better your optics need to be.

Slower scopes, though not as good for AP, tend to be more forgiving for visual in terms of eyepiece selection.

[Gary]

Actually i am looking at two model of 8" Newtonian for my ota upgrade. One specs at f/5 and the other f/3.9! I am curious to know what is the difference in seeing for this diff focal ratio. Threorically f/3.9 will have a little wider FOV as compare to the f/5 so seeing wise should not affect much. But for astrophotography wise it really make a huge difference.

For visual astronomy, some larger objects look aesthetically better when you can fit the whole object inside the same field of view or even with ample space to surround it/them - e.g. pleiades, andromeda galaxy, coat hanger, double clusters, ..etc. So even missing just a little bit of FOV may just "spoil" the view. An analogy will be a photo of 2 person close-up - one head is fully in the picture, the other with the outline of the hair style just going out of frame.

But for a similar size of 8inch (203mm) but how can the manufacturer able to produce two different specs scope? I am actually comparing Orion 8" astrograph newtonian and celestron C8-NGT.

If I understand your question correctly, the reason is that for the same aperture of primary parabolic mirror, one can be deliberately made more concave to obtain a faster focal ratio. This will generally require more accuracy in manufacturing and may increase the total cost.

[orly_andico]

the f3.9 scope will require much more precise collimation. and will have more coma and require better (read: expensive) eyepieces to get good edge-of-field performance.

for these reasons I think an f5 to f6 scope is a better bet if you can handle the length.

[cloud_cover]

For astrophotography while 3.9 is better than 5, its not a startling great difference because ultimately issues such as light pollution will probably be the limiting factor in your exposures as low contrast objects get drowned out in the overall sky brightness.
3.9 though will require much more precise collimation so mechanical factors come into play.
Another factor to consider is the size of the secondary mirror. Ensure it can fully illuminate your chip - some newts designed for visual may not.
Consider both scopes carefully and read reviews. At their price point, pay attention to the mechanical ease of use, stability of collimation and available in/outfocus as well as focuser rigidity

[ggodetucsamoht]

hi guys, really appreciate for your advice. The factor which lure me towards the f/3.9 scope is actually not the f/ratio but is actually on its focuser. It come with a crayford focuser with the OTA. Because of this focuser so i am enquiry what will be the difference for seeing with f3.9 and f5. As you guys also mention that it does need good optics to work in-line with such fast scope, well for this case i am not a televue supporter to get their EP at that price yet...I do have afew wide angle (66deg) eyepieces. In example, does Orion Expanse(66deg) 6mm consider to be good optics?

[orly_andico]

at f3.9 you pretty much need TV class optics to use the edge of field. Or, get a Paracorr, then you can use less-than-stellar eyepieces.

You can always retrofit a crayford focuser onto an f5 scope.

[ggodetucsamoht]

Hi Orly_andico

Noted on your suggestion.......thanks!!!